Brenner and Rector's The Kidney, 8th ed.

CHAPTER 40. Inherited Disorders of the Renal Tubule

Alain Bonnardeaux   Daniel G. Bichet



Inherited Disorders Associated with Generalized Dysfunction of the Proximal Tubule (Renal Fanconi Syndrome), 1390



Inherited Disorders of Renal Amino Acid Transport, 1399



Inherited Disorders of Renal Phosphate Transport, 1403



Inherited Disorders of Urate Transport, 1405



Inherited Disorders of Renal Glucose Transport, 1406



Inherited Disorders of Acid-Base Transporters, 1406



Bartter and Gitelman Syndromes, 1409



Inherited Disorders with Hypertension and Hypokalemia, 1411



Familial Hyperaldosteronism Type II, 1414



Pseudohypoaldosteronism, 1414



Inherited Disorders of Renal Magnesium Handling, 1415



Diabetes Insipidus, 1417

Considerable progress has been achieved in the past few years in understanding the molecular basis of several inherited renal tubule disorders. These advances have served several purposes. First, they have allowed the identification of several genes expressed in the renal tubule that encode proteins whose function has been investigated ( Table 40-1 ). Second, by providing natural variations in gene function, they have increased our knowledge of basic renal physiology. Third, they have increased our understanding of the diseases themselves and allowed us to make phenotype-genotype correlations. Fourth, it is now possible to offer natal and prenatal diagnosis and initiate research on gene therapy. Mutant proteins can lose function, gain function, or become dominant negative, interfering with the function of a normal protein produced by the other chromosome in a heterozygous situation, thus producing a phenotype that is worse than that of a null allele. In addition, mutant proteins can be misfolded inducing an unfolded protein stress response that can eventually trigger cell dysfunction and death.[1]

Most transport disorders discussed in this chapter are inherited as autosomal recessive traits. Thus, “private” mutations frequently produce the disease in each kindred and the frequency of the disease increases in populations with a high frequency of consanguineous matings. Although such diseases are rare and previously restricted to pediatric nephrology, recent progress in therapy is increasing longevity for many patients, thus confronting the adult nephrologists with new challenges. Each disorder is referenced with its OMIM (Online Mendelian Inheritance in Man) number. OMIM is a database containing a catalog of human genes and genetic disorders.

TABLE 40-1   -- Impact of DNA Variation on Protein Function


A mutation that reduces or abolishes a normal physiologic function (likely to be recessive)


A mutation that increases the function of a protein (likely to be dominant)

Dominant negative:

A mutation that dominantly affects the phenotype by means of a defective protein or RNA molecule that interferes with the function of the normal gene product in the same cell





The renal Fanconi syndrome is a generalized dysfunction of the proximal tubule with no primary glomerular involvement. It is usually characterized by variable degrees of phosphate, glucose, amino acid, and bicarbonate wasting by the proximal tubule. The clinical presentation in children is usually rickets and impaired growth. In adults, bone disease is manifested as osteomalacia and osteoporosis. In addition, polyuria, renal salt wasting, hypokalemia, acidosis, hypercalciuria, and low molecular weight proteinuria can be part of the clinical spectrum.

There are hereditary and acquired forms of the Fanconi syndrome (see review in Ref 2). Acquired forms in adults are usually associated with abnormal proteinurias such as in paraproteinemias or the nephrotic syndrome, with residual cases being secondary to tubular damage caused by toxic (reviewed in Ref 3) or immunological factors. Most hereditary forms of the Fanconi syndrome occur as part of the manifestations of readily identifiable inborn errors of metabolism or as sporadic or rare familial forms. Inherited disorders are classified into forms that primarily affect the proximal renal tubule (idiopathic) and forms that derive from the accumulation of toxic metabolic products in the kidneys (secondary) ( Table 40-2 ). Such metabolic disorders give rise to the renal Fanconi syndrome as part of a particular clinical spectrum.

TABLE 40-2   -- Causes of Inherited and Acquired Fanconi Syndrome






Idiopathic (AD)



Dent disease (X-linked hypophosphatemic rickets, X-linked recessive nephrolithiasis)






Cystinosis (AR)



Tyrosinemia type I (AR)



Galactosemia (AR)



Glycogen storage disease



Wilson disease (AR)



Mitochondrial diseases (cytochrome-c oxidase deficiency)



Oculocerebrorenal syndrome of Lowe



Hereditary fructose intolerance






Paraproteinemias (multiple myeloma)



Nephrotic syndrome



Chronic tubulointerstitial nephritis



Renal transplantation






Exogenous factors



Heavy metals (cadmium, mercury, lead, uranium, platinum)



Drugs (cisplatin, aminoglycosides, 6-mercaptopurine, valproate, outdated tetracyclines, methyl-3-chrome, ifosfamide)



Chemical compounds (toluene, maleate, paraquat, Lysol)


AD, autosomal dominant; AR, autosomal recessive.




Other inherited disorders of the proximal tubule can result in isolated anomalies of transport. Isolated or partial defects of the proximal tubule result in the selective wasting of amino acids, phosphate, bicarbonate, or glucose, and are described in other sections of this chapter.


The proximal tubule is responsible for reclaiming almost all the filtered load of bicarbonate, glucose, and amino acids, as well as most of the filtered load of sodium, fluid, chloride, and phosphate. The renal proximal tubule exhibits a very extensive apical endocytic apparatus consisting of an elaborate network of coated pits and small coated and non coated endosomes (see Chapter 2 ). In addition, the cells contain a large number of late endosomes or prelysosomes, lysosomes, and so-called dense apical tubules involved in receptor recycling from the endosomes to the apical plasma membrane. This endocytic apparatus is involved in the reabsorption of molecules filtered in the glomeruli. The process is very effective as demonstrated by the fact that although several grams of protein are filtered daily in the human glomeruli, human urine is virtually devoid of proteins under physiologic conditions. Reabsorption of solutes by proximal tubule cells is achieved by transport systems at the brush border membrane that are directly or indirectly coupled to sodium movement, by energy production and transport from the mitochondria, and by the Na+, K+-ATPase at the basolateral membrane. The Na+, K+-ATPase lowers intracellular Na+ concentration and provides the electrochemical gradient that allows Na+-coupled solute entry into the cell. A second pathway, the paracellular route, is responsible for reclaiming up to one half of the sodium and most of the water through tight junctions.

Multiple transport anomalies characterize the renal Fanconi syndrome. In addition, amino acids, glucose, phosphate, and bicarbonate are transported by multiple carriers. Therefore, defects responsible for the Fanconi syndrome lead to a general dysfunction of the proximal tubule cell. Examples of genetic defects associated with the renal Fanconi syndrome are storage diseases thought to affect proximal tubule cells in addition to other cells (hepatocytes). Specific organelle dysfunction of proximal tubule cells is thought to explain cystinosis (lysosomes) and Dent disease (endosomes).

The Fanconi syndrome, even when genetically transmitted, can be reversible, occurring following prolonged exposure to the noxious substance. For example, the defect is reversed after dietary restriction of tyrosine and phenylalanine in tyrosinemia,[4] fructose in hereditary fructose intolerance,[5] and galactose in galactosemia.[6] The duration of exposure is also important for the disorder to be expressed and is protracted in cadmium intoxication,[7] or short in fructose intolerance following a fructose load.[8]

Clinical Presentation


All amino acids are filtered by the glomerulus, and more than 98% are subsequently reabsorbed by the proximal tubule. Multiple transporters are responsible for the reabsorption. In the renal Fanconi syndrome, all amino acids are excreted in excess. Amino acids in the urine are usually quantified by one of several chromatographic methods at specialized centers. The excretion of amino acids parallels the physiologic excretion but to an elevated degree, particularly for the amino acids that have the highest levels of excretion physiologically (histidine, serine, cystine, lysine, and glycine). Clinically, amino acid losses are relatively modest, do not lead to specific deficiencies, and there is no need to supplement affected patients.

Phosphaturia and Bone Disease

Phosphate wasting is a cardinal manifestation of the Fanconi syndrome. Serum phosphate levels are usually decreased and tubular reabsorption of phosphate (TRP and TmP/GFR) is systematically reduced. Rickets and osteomalacia are produced by the increased urinary losses of phosphate as well as by impaired 1α-hydroxylation of 25-hydroxy vitamin D3 by proximal tubule cells. Rickets manifests itself by a bowing deformity of the lower limbs with metaphyseal widening of the proximal and distal tibia, distal femur, the ulna, and the radius. Bone manifestations in patients with adult-onset renal Fanconi syndrome are severe bone pain and spontaneous fractures.

Renal Tubular Acidosis

Acidosis is a frequent finding and is caused by defective bicarbonate reabsorption by the proximal tubule. Hence, renal acidification by the distal tubule is normal, as demonstrated by the ability to acidify urines at a pH below 5.5 when plasma bicarbonate is below the threshold. The metabolic acidosis is hyperchloremic and is also known as type II. Large doses of alkali may be necessary to correct serum bicarbonate levels.


Glucosuria is the fourth manifestation of the renal Fanconi syndrome. Serum glucose is normal and the amount of glucose lost in the urine varies from 0.5 to 10 g/24 hours. Massive glucosuria (and hypoglycemia) may be seen in glycogenosis type I.[9]

Sodium and Potassium Wasting

Renal sodium losses can be important in the Fanconi syndrome and lead to hypotension, hyponatremia, and metabolic alkalosis. Supplementation with sodium chloride is indicated and leads to improvement of symptoms. Potassium losses are secondary to increased delivery of sodium to the distal tubule and activation of the renin-angiotensin system secondary to hypovolemia. Potassium supplementation is indicated to correct serum potassium levels.


Polyuria, polydipsia, and dehydration can be prominent features of the Fanconi syndrome. There is a decreased concentrating ability of the kidney that could be related to abnormal tubule function of the distal tubule and collecting duct, possibly caused by hypokalemia.


Low molecular weight proteinuria is almost always present with the renal Fanconi syndrome, and is usually low to moderate in amounts. The dipstick test is frequently positive because of the presence of albuminuria. Beta2-microglobulin excretion rates can be measured to identify “tubular proteinuria”. The molecular mechanism of proximal tubule protein reabsorption, which is defective in renal Fanconi syndrome, includes a crucial role for endosomal acidification-machinery proteins, including the V-ATPase, CLC-5 chloride channels, and the endocytotic receptors megalin and cubilin.[10]


Hypercalciuria is a frequent finding in patients with the renal Fanconi syndrome. The pathogenesis is not known or multifactorial, but could be attributed to abnormal recycling of proteins involved in calcium reabsorption by the proximal tubule. Hypercalciuria is rarely associated with nephrolithiasis in the Fanconi syndrome, possibly because of the polyuric syndrome that frequently occurs.

Dent Disease, X-Linked Recessive Hypophosphatemic Rickets, and X-Linked Recessive Nephrolithiasis


Dent disease, X-linked recessive hypophosphatemic rickets, and X-linked recessive nephrolithiasis (OMIM 300009)[11] represent the same inherited disorder caused by mutations in the CLCN5 gene (chromosome Xp11.22) encoding a renal chloride channel, ClC-5. CLC-5 may function as an electrogenic Cl-/H+ exchanger in endosomes, implicated in acidification or membrane fusion. [12] [13] This X-linked recessive disease is associated with a primary renal Fanconi syndrome. ClC-5 function is not at the plasma membrane but rather in intracellular compartments, subapical endosomes of the proximal tubule, where it colocalizes with the V-type H+-ATPase and with reabsorbed proteins. ClC-5 is a vesicular channel important for renal endocytosis.[14] Because several hormones or their precursors are endocytosed from the primary urine, loss of function of the CLCN5 gene leads to secondary changes in calciotropic hormones and changes in phosphate excretion. The clinical spectrum includes varying degrees of low molecular weight proteinuria, hypercalciuria with calcium nephrolithiasis, rickets, nephrocalcinosis, and renal failure. The molecular basis for Dent disease offers important mechanistic insight into the role of organelle dysfunction ( Fig. 40-1 ).



FIGURE 40-1  Schematic representation of ClC channels with many helices that go only partway across the membrane.[332] Expression of ClC-5 is highest below the brush border membrane in a region rich in endocytic vesicles. ClC-5 is essential for proximal endocytosis by providing an electrical shunt necessary for the efficient acidification of vesicles in the endocytic pathway, explaining the proteinuria observed in Dent disease.[14] In healthy adults, the daily glomerular filtration rate is 150-180 liters and therefore the daily filtered load of albumin is in the range of 3300-5760 mg.[333] Because of its size and the tubule-to-blood concentration ratio, albumin cannot be reabsorbed passively on the paracellular route across the tight junctions; therefore the only mechanism able to mediate albumin reabsorption is endocytosis. Adsorptive or receptor-mediated endocytosis is responsible for the reabsorption of albumin, PTH, and Vit D3. The polyspecific receptor megalin binds a variety of ligands including vitamin-binding proteins, carrier proteins (e.g., transthyretin), lipoproteins (e.g., apolipoprotein B), hormones (e.g., PTH), drugs (e.g., aminoglycosides), enzymes (e.g., lipoprotein lipase), immune-related proteins (e.g., light chains), and myoglobin. Megalin also acts as a membrane anchor for the peripheral membrane protein cubilin and the two form a scavenger receptor complex. During the process of endocytosis, small plasma membrane invaginations are formed and the endocytic invaginations detach from the membrane to form endocytic vesicles. Once albumin, PTH, Vit D3 has been taken up by receptor-mediated endocytosis, their final destination is the lysosome and the common mechanism triggering receptor-ligand dissociation is the drop in pH in different endocytic compartments. Albumin-binding proteins megalin and cubilin recycle back to the plasma membrane and are not directed to lysosomes.



In the proximal tubule, endocytosis of many proteins is mediated by megalin, a recycling receptor of the low-density lipoprotein (LDL) family. After internalization, acidification of the endosomes is required for receptor-ligand interactions and cell-sorting events. Inhibition of the acidification interferes with cell-surface receptor recycling. ClC-5 may play a role in proximal tubular (early) endocytosis, probably by providing an electrical shunt to enable efficient pumping of the H+-ATPase. ClC channels are believed to be dimers that have two largely independent pores (reviewed in Ref 15). These pores can be gated individually or can be closed together by a common gate. ClC-5 is an endosomal chloride channel of 746 amino acids (see Fig. 40-1 ). It is weakly homologous to other CLC channels, but is more homologous to ClC-3. ClC-5 messenger RNA (mRNA) is predominantly expressed in the kidney, but is also present in liver, brain, testis, and intestine and colon. ClC-5 is highly expressed in all three segments (S1-S3) of the proximal tubule and in alpha-intercalated cells of the distal tubule of the rat kidney.[16] ClC-5 expression is highest below the brush border membrane in a region rich in endocytotic vesicles and colocalizes with the proton pump and with internalized proteins early after uptake.[16] In vivo endocytosis of a fluorescently labeled filtered protein revealed that ClC-5 colocalizes with the internalized protein at early (2 minutes), but not late (13 minutes), time points of uptake. ClC-5 was present in human kidney membrane fractions that also contained rab5, rab4, and the 31-kDa subunit of the H+-ATPase.[17] In transfection studies with ClC-5, the cells induce outwardly rectifying Cl- currents on whole-cell configuration that are measurable only at voltages greater than +20 mV. Because these positive voltages seem unphysiologic, it is unclear whether there is an additional, unknown β-subunit or another regulatory mechanism that may alter the voltage dependence. As a first step to identify sorting signals in ClC-5, a PY motif was found to be important for the internalization from the plasma membrane.[18] This was ascribed to an interaction with the WW domain containing ubiquitin protein ligases. This resembles the model proposed for the regulation of the epithelial Na+ channel (ENaC), whose internalization and degradation is triggered by the PY motif-dependent ubiquitination by a WW domain containing ubiquitin protein ligase.[19]

Acidification of the endosomes is required for receptor-ligand interactions and cell sorting events and inhibition of the acidification interferes with cell-surface receptor recycling (see Fig. 40-1 ).

The pathophysiology of Dent disease has been elucidated by knockout mouse models. [20] [21] [22] The knockout of ClC-5 led to low molecular weight proteinuria, [21] [22] and, depending on the mouse model, also to hyperphosphaturia[21] or hypercalciuria.[22] The proteinuria is the result of a cell-autonomous defect in endocytosis, which extends to fluid-phase endocytosis, receptor-mediated endocytosis, and the endocytosis of integral plasma membrane proteins such as de Na-PO4 cotransporter NaPi-IIa or the Na/H exchanger NHE3.[21] Endocytosis, however, is not totally abolished, but strongly reduced. The amount of the endocytotic receptor megalin, which mediates the uptake of a wide variety of proteins and other substrates, was significantly reduced, and its presence in the brush border appeared to be reduced.[21] This observation probably indicates a role of ClC-5 in recycling endosomes. A strong reduction of megalin in the brush border was also revealed by immunoelectron microscopy.[23] A reduction of megalin plasma membrane expression is expected to reduce receptor-mediated endocytosis even further. Renal cortical endosomes, which are predominantly derived from proximal tubules, had a lower rate and extent of acidification than wild-type endosomes in vitro. [20] [21] This strongly supports the hypothesis that the Cl- conductance provided by ClC-5 is needed to dissipate the voltage created by the electrogenic H+-ATPase, thereby enabling efficient acidification in the endosomal pathway. The link between endosomal acidification, which in turn leads to a defect in endocytosis and kidney stones is now better understood. Several hormones, including parathyroid hormone (PTH) and vitamin D3, are filtered into the primary urine. After binding to megalin, these hormones are normally endocytosed by proximal tubular cells. In the absence of ClC-5, the reduced endocytosis of PTH is expected to result in a progressive increase of luminal PTH concentration along the length of the proximal tubule, whereas serum concentrations of the hormone remain unchanged.[21] The increased luminal levels of PTH will stimulate apical PTH receptors, which in turn enhance the endocytosis of the apical Na-PO4 cotransporter NaPi-IIa. Indeed, immunocytochemistry revealed that the majority of NaPi-IIa had shifted to intracellular vesicles in knockout mice, whereas it resided in the brush border of wild-type proximal tubules.[21] This PTH-dependent decrease of NaPi-IIa in the plasma membrane readily explains the hyperphosphaturia that was observed in the knockout[21] and was found in patients with Dent disease.[24]

Parathyroid hormone is also known to stimulate the transcription of the enzyme α-hydroxylase, the enzyme that converts the inactive precursor 25(OH)-VitD3 to the active hormone 1,25(OH)2-VitD3 in proximal tubular cells. As expected from the increased luminal concentration of PTH, mRNA levels of α-hydroxylase and its enzymatic activity were increased in ClC-5 knockout mice, [20] [21] and many patients with Dent disease have slightly elevated serum concentrations of active VitD3. However, a large part of the precursor 25(OH)-VitD3 is taken up into proximal tubular cells through megalin-dependent apical endocytosis. Therefore, there are two opposing effects (up-regulation of the activating enzyme and loss of substrate) that may lead to an increase or decrease of active VitD3. The outcome will depend on many factors (including genetic and dietary ones), and may explain the clinical variability of Dent disease. Such a variability was even observed between the two ClC-5 knockout mouse models: Whereas the knockout mouse generated in Jentsch's laboratory has decreased serum levels of 1,25(OH)2-VitD3 and no hypercalciuria, [20] [21] the model from Guggino's laboratory has slightly elevated level of the active hormone and displays hypercalciuria. [22] [25] Thus the two opposing effects described previously explains the complex and variable symptoms of Dent disease through changes in calciotropic hormones that stem from defects in proximal tubular endocytosis, which in turn result from a defective acidification of endosomes. [20] [21]

Clinical Presentation

The clinical spectrum of CLCN5 mutations includes hypercalciuria with calcium phosphate nephrolithiasis, rickets, nephrocalcinosis, low molecular weight proteinuria, and renal failure (see reviews in Refs 24, 26). The same mutation can induce different phenotypes in different families,[27] probably because of genetic or environmental modifiers or both. The disease affects males predominantly but females frequently have an attenuated phenotype. However, only males develop renal failure.[27]

The excretion of low molecular weight proteins in the urine, such as albumin, β2-microglobulin, and α1-microglobulin, is thought to be the most reliable marker for the disease. It is not a specific finding because it can be seen in tubulointerstitial diseases as well. It is much more pronounced in males (frequently above 1 g/day) and sufficient to give a positive Labstix test. The degree of proteinuria is relatively constant and amounts to 0.5 to 2.0 g/day in adults and up to 1 g/day in children.[24] The nephrotic syndrome does not occur and albumin excretion represents less than half of the proteins excreted. Affected males usually excrete β2-microglobulin in amounts that are more than 100-fold the upper limit of normal. Female carriers can also have low molecular weight proteinuria, but this is usually less pronounced than in males, and sometimes absent. Several studies have suggested that an attenuated form of the disease with low molecular weight proteinuria as the only or predominant feature might be prevalent in Japan.[28] Further clinical studies have shown that these patients have most of the features of Dent disease, with hypercalciuria and declining renal function, and carry inactivating mutations in the CLCN5 gene.

Hypercalciuria is also a hallmark of this disorder and is present in most cases, beginning in childhood. It is usually overt and predominant in males (>7.5 mmol/day in males). Females are also frequently hypercalciuric but the values are usually closer to the upper limit of the normal range. Kidney stones are present in 50% of males investigated in several pedigrees, and are composed of calcium phosphate or a mixture of calcium phosphate and oxalate.[24]Multiple episodes starting during the teenage years are frequent. Radiologic nephrocalcinosis of the medullary type is seen in most affected males and occasionally females. Serum phosphate levels are usually below normal values or at the lower limit of the normal range. TmP/GFR is decreased, indicating defective reabsorption by the proximal tubule. Rickets is also a frequent event in children. It is cured by the administration of pharmacologic doses of vitamin D. Osteomalacia occurs in adults, and is also corrected following administration of vitamin D. Serum levels of 1,25(OH)D3 are normal or slightly raised, whereas 25(OH)D levels are normal.

Systemic acidosis is usually not seen before renal function deteriorates significantly. Males have urinary acidification defects detectable by an ammonium chloride load but renal acidification abnormalities are not a consistent feature of the phenotype. Spontaneous hypokalemia is common in males and there is an inability to concentrate urine maximally after vasopressin injection. Aminoaciduria and glucosuria are also frequent. Half the males of four pedigrees had raised serum creatinine with progressive renal failure. End-stage renal failure occurred at 47 ± 13 years. Renal biopsy specimens show a pattern of a chronic interstitial nephritis with scattered calcium deposits.[24] The glomeruli are normal or hyalinized; there is prominent tubular atrophy with diffuse inflammatory infiltrate composed of lymphocytes, and foci of calcification around and within epithelial cells.


Treatment is largely supportive. Renal stones and hypercalciuria are treated with supportive measures (and in particular, increasing fluid intake). Dietary restriction of calcium reduces calcium excretion but is not recommended because it might contribute to bone disease.[26] Thiazide diuretics may be given in small doses, but it is important to remember that these patients tend to have a salt-losing nephropathy and seem to respond with an excessive diuresis and decrease in blood pressure after the administration of diuretics.[24] Rickets is treated with small doses of vitamin D, but this treatment should be given with caution because it might increase urine calcium excretion and the risk of nephrolithiasis. Verifying urine calcium excretion before and after vitamin D therapy might be appropriate.[26]

Idiopathic Causes of the Renal Fanconi Syndrome

Idiopathic renal Fanconi syndrome occurs in the absence of known inborn errors of metabolism or acquired causes of the Fanconi syndrome (see Table 40-2 ). Most cases are sporadic, although familial cases associated with progressive renal failure have been reported and transmitted as an autosomal dominant trait.[29]



Cystinosis (OMIM 219800) is a rare autosomal recessive disease (incidence 1 in 100,000 to 200,000) of lysosomal transport of the disulphide amino acid cystine (reviewed in Refs 30, 31). Lysosomes are intracellular organelles containing enzymes responsible for the digestion of macromolecules that are optimally active at low pH. The byproducts of hydrolytic digestion exit the lysosome through specific transporters. Cystinosis is caused by inactivating mutations in CTNS, encoding an integral lysosomal membrane protein termed cystinosin.[32] CTNS has 12 exons and a 2.6-kb mRNA encoding a 367-amino-acid putative cystine transporter with seven transmembrane domains. Cystinosin possibly represents a novel H+-driven transporter that is responsible for cystine export from lysosomes. The impaired transport of cystine out of the lysosome leads to cellular accumulation and crystallization that destroys tissues, causing renal failure and a variety of other complications.

Clinical Presentation

Cystinosis is the most frequent cause of the Fanconi syndrome in children. The clinical presentation is variable (reviewed in Refs 30, 33) encompassing classic nephropathic cystinosis, a rare “adolescent” variant, and a mild adult-onset variant. Classic nephropathic cystinosis usually presents in the first year of life with failure to thrive, increased thirst, polyuria, poor feeding, and hypophosphatemic rickets. In whites, affected subjects frequently have blond hair and blue eyes, and are more lightly pigmented. Renal wasting of sodium, calcium, and magnesium is frequently seen, as well as tubular proteinuria. Progressive renal damage usually culminates to end-stage renal failure by the end of the first decade. The disease does not recur in the donor kidney.

Cystinosis can affect multiple organs including ocular, endocrine, hepatic, muscular, and central nervous system tissues ( Table 40-3 ). In the cornea, crystal deposits are absent at birth and appear by the end of the first year of life. They can be seen by slit-lamp examination as pathognomonic fusiform crystals involving the anterior third of the central cornea and the full thickness of the peripheral cornea. Eventually, these deposits progress to develop a characteristic haziness. Deposits can also be found in the irides and conjunctiva, as well as in the retina with consequent development of a peripheral retinopathy. Other features of cystinosis include hypothyroidism from cystine crystallization in the follicular cells of the thyroid gland. It is present in more than 70% of patients after age 10. Insulin-dependent diabetes mellitus can result from pancreatic longstanding cystine crystal accumulation, particularly after renal transplantation.[34] Hepatomegaly and splenomegaly with little clinical impact occur in more than 40% of subjects after age 10. A distal vacuolar myopathy is also a late frequent finding in 25% of cystinotic patients, with wasting in the small hand muscles, with or without facial weakness and dysphagia. In a previous study,[35] muscle biopsies revealed marked fiber size variability, prominent acid phosphatase-positive vacuoles, and absence of fiber type grouping or inflammatory cells. Crystals of cystine were detected in perimysial cells but not within the muscle cell vacuoles. The muscle cystine content of clinically affected muscles was markedly elevated. Central nervous system involvement has been described in the late stages of the disease (reviewed in Ref 33) with cystine crystal accumulation.

TABLE 40-3   -- Age-Related Clinical Characteristics of Untreated Nephropathic Cystinosis


Symptom or Sign

Prevalence of Affected Patients (%)

6–12 mo

Renal Fanconi syndrome (polyuria, polydipsia, electrolyte imbalance, dehydration, rickets, growth failure)


5–10 yr



8–12 yr



8–12 yr

Chronic renal failure


12–40 yr

Myopathy, difficulty swallowing


13–40 yr

Retinal blindness


18–40 yr

Diabetes mellitus


18–40 yr

Male hypogonadism


21–40 yr

Pulmonary dysfunction


21–40 yr

Central nervous system calcifications


21–40 yr

Central nervous system symptomatic deterioration


From Gahl WA, Thoene JG, Schneider JA: Cystinosis. N Engl J Med 347:111–121, 2002.




The diagnosis is usually made by measuring the cystine content of peripheral leukocytes or cultured fibroblasts. Cystinotic patients usually have values higher than 2 nmol of half-cystine per milligram of protein (normal < 0.2 nmol of half-cystine per milligram of protein). Alternatively, the diagnosis can be made by recognizing the characteristic corneal crystals on slit-lamp examination. Cystinosis can be diagnosed in utero by cystine measurements in amniocytes or chorionic villi, or at birth by cystine measurements on the placenta.


Early diagnosis, and appropriate treatment with cysteamine, dialysis, and renal transplantation has improved the outcome of patients with nephropathic cystinosis and many patients are now reaching adulthood. Symptomatic treatment involves rehydration, particularly during episodes of gastroenteritis. Replacement of bicarbonate losses with citrate or bicarbonate-containing salts is frequently indicated. Phosphate losses are replaced by phosphate salts and oral vitamin D therapy. Indomethacin has been used for decreasing the renal salt and water wasting syndrome. Recombinant human growth hormone can also be given to increase growth and does not increase the rate of progression of renal failure. [36] [37]

The cystine-depleting drug cysteamine is now widely used for cystinosis ( Fig. 40-2 ). It has been shown to slow the rate of progression of renal failure, and increase growth.[36] Kidney function stabilizes upon initiation of therapy and even allows glomerular function to improve if begun in the first year or two of life.[37] The growth rate becomes normal but there is no catching up. Topical cysteamine eyedrops can be used to treat ocular complications of cystinosis, and results in dissolution of corneal crystals.[38]



FIGURE 40-2  Mechanism of cystine depletion by cysteamine. In normal lysosomes (A), cystine and lysine freely traverse the lysosomal membrane through specific transporters (rectangular shape for the lysine transporter, cup shape for the cystine transporter). In cystinotic lysosomes (B, please note the absence of specific cystine transporters), lysine can freely traverse through specific transporters the lysosomal membrane, but cystine cannot, and it therefore accumulates inside the lysosome. In cysteamine-treated lysosomes (C), cysteamine combines with half-cystine (i.e., cysteine) to form the mixed disulfide cysteine-cysteamine, which uses the lysine transporter to exit the lysosome.  (Modified with permission from Gahl WA, Thoene JG, Schneider JA: Cystinosis. N Engl J Med 347:111–121, 2002. Copyright © 2002 Massachusetts Medical Society. All rights reserved.)




Transplantation is routinely performed and most patients do well. Kidneys from heterozygous donors are widely accepted because there is no evidence for cystine accumulation in kidney transplants. However, progression of other features of the disease (see Table 40-3 ) occurs in a significant number of renal transplant recipients.[30]

Glycogenosis Type 1 (Von Gierke Disease)

Glycogen storage diseases are inherited disorders that affect glycogen metabolism ( Fig. 40-3 ) (reviewed in Refs 39, 40). This section discusses type I glycogen storage disease because it is the only form associated with primary renal involvement. Type V glycogen storage disease (McArdle disease) as well as other rare glycogenoses are associated with rhabdomyolysis, myoglobinuria, and acute tubular necrosis, but will not be discussed further.



FIGURE 40-3  A, Left part of the figure: Major pathways of synthesis and breakdown of glycogen in liver. The broken line indicates that several enzymes have been omitted between pyruvate and fructose-1,6-P2. GLUT, glucose transport protein; UDP, uridine diphosphate; UDPG, uridine diphosphate-glucose. Right part of the figure: Schematic working model of the hepatic microsomal glucose-6-phosphatase system. E, the catalytic subunit of glucose-6-phosphatase; ER, endoplasmic reticulum; GLUT7, the microsomal glucose transport protein; SP, the stabilizing protein, a regulatory Ca2+-binding protein; T1, a microsomal glucose-6-phosphate transport protein; T2a, a microsomal Pi transport protein; T2b, a microsomal series: Pi, PPi, and carbamoylphosphate transport protein. This model is not meant to represent the actual topology of the six proteins in the membrane. B, Glucose-6-phosphatase is anchored in the endoplasmic reticulum by nine transmembrane helices. The amino terminus faces the ER lumen and the carboxyl terminus faces the cytoplasm.  A (Modified with permission from Chen WM, Deng HW: A general and accurate approach for computing the statistical power of the transmission disequilibrium test for complex disease genes. Genet Epidemiol 21:53–67, 2001.); B (Modified with permission from Pan C-J, Lei K-J, Chen H, et al: Ontogeny of the murine glucose-6-phosphatase system. Arch Biochem Biophys 358:17–24, 1998.)





Glycogen storage disease type 1 or von Gierke disease (GSD-1), refers to a group of autosomal recessive metabolic disorders caused by deficiencies in the activity of the glucose-6-phosphatase system that consists of at least two membrane proteins, glucose-6-phosphate transporter (G6PT) and G6Pase (reviewed in Ref 41). Mutations in the gene encoding glucose 6-phosphatase on chromosome 17 have been identified and shown to cause type 1a,[42] the most frequent form, whereas type 1b is caused by a deficiency in a microsomal glucose 6-phosphate transporter on chromosome 11.[43] Other variants include types 1c and Id (defect in microsomal glucose transport), but the molecular basis remains to be identified.[44]

Clinical Presentation

GSD-1 manifests functional G6Pase deficiency characterized by growth retardation, hypoglycemia, hepatomegaly, kidney enlargement, hyperlipidemia, hyperuricemia, lactic acidemia, and seizures. Muscle cramps and weakness, exercise intolerance, and fatigue are other clinical manifestations. Hypoglycemia occurs because of impaired gluconeogenesis, glycogenolysis, and recycling of glucose through the glucose 6-phosphate to glucose system. Accumulation of glucose 6-phosphate leads to increased glycolysis and lactic acidosis. Hyperuricemia and gout are a consequence of increased activity of hepatic AMP-deaminase and adenine nucleotide production, thus increasing uric acid production. Hyperuricemia also results from decreased renal excretion because urate competes with lactate for secretion. Dyslipidemia is caused by increased synthesis of VLDL, and LDL, and decreased lipolysis. Fatty infiltration of the liver is a frequent finding. Easy bruising and epistaxis result from prolonged bleeding time as a consequence of impaired platelet adhesion and aggregation. GSD-1β patients also suffer from chronic neutropenia and functional deficiencies of neutrophils and monocytes, resulting in recurrent bacterial infections as well as ulceration of the oral and intestinal mucosa.

A renal Fanconi syndrome occurs in GSD-1 with aminoaciduria, low molecular weight proteinuria, phosphaturia, and bicarbonaturia. Renal disease is common in adult patients with untreated GSD-1, evolves slowly, and is a late finding (reviewed in Ref 39). In children, increased kidney size, hyperfiltration, and moderate proteinuria are common.[45] Distal renal tubular acidosis, hypocitraturia, hypercalciuria, nephrocalcinosis, and calcium nephrolithiasis can be variably associated.[46] Virtually all patients studied have impaired distal tubular acidification.[46] This might be secondary to decreased ammonia excretion. The most common renal finding in GSD-1 is focal and segmental glomerulosclerosis with tubulo-interstitial atrophy. Glomerular changes include thickening, lamellation, and glycogen deposition in the glomerular basement membrane.[47]

A glucagon test with 1 mg given intramuscularly can be used to screen for glycogenosis and is frequently abnormal (rise in blood glucose < 4 mmol/L, usually at 30 minutes); 48-hour fasting blood glucose levels are frequently normal.[48] The definitive diagnosis of glycogenosis type 1 requires a liver biopsy with measurement of glucose 6-phosphatase activity or one of the three microsomal translocase systems. The liver histology is characterized by hepatocyte distension from glycogen and fat with large lipid vacuoles. There is no fibrosis. Abnormally high glycogen levels are noted in liver biopsy samples. Electron microscopy shows moderate to large excesses of glycogen in the cytoplasm, often displacing the organelles in the hepatocytes. Sequencing of the defective gene can be used to avoid liver biopsy.


Life expectancy in GSD-1 has improved considerably. The treatment goal is to maintain normoglycemia to avoid the metabolic complications that are secondary to hypoglycemia and lactic acidosis. Guidelines for the management of GSD-1 published by the Collaborative European Study on Glycogen Storage Disease I[49] include preprandial blood glucose higher than 3.5 to 4.0 mmol/L (60 to 70 mg/dL); urine lactate/creatinine ratio lower than 0.06 mmol/mmol; serum uric acid concentration in high normal range for age; and venous blood bicarbonate > 20 mmol/L (20 meq/L); serum triglyceride concentration < 6.0 mmol/L (531 mg/dL); normal fecal α1-antitrypsin concentration for GSD-1β; body mass index within two standard deviations or normal. Normoglycemia can be accomplished at night with nasogastric feeding of glucose[50] or with orally administered uncooked cornstarch. A single dose (1.75 to 2.5 g/kg) of uncooked starch at bedtime will maintain serum glucose concentrations for 7 hours or longer in most young adults.[51] Because hypoglycemia and lactic acidosis occur in adults as well, treatment might also be indicated after growth.[52] Kidney transplantation has been successfully performed, but does not correct the hypoglycemia.



Hepatorenal tyrosinemia (tyrosinemia type 1) is a rare autosomal recessive disorder (reviewed in Ref 53) affecting principally the liver, kidney, and peripheral nerves. Mutations in the gene encoding fumarylacetoacetate hydrolase on chromosome 15q23-q25 are responsible for tyrosinemia type 1.[54] The hepatic toxicity is caused by fumarylacetoacetate accumulation,[55] apparently inducing the release of cytochrome c, which in turn triggers activation of the caspase cascade in hepatocytes of affected animal models.[56] It causes chromosomal instability and mutagenesis, endoplasmic reticulum stress, cytotoxicity, and apoptosis.[57]

Clinical Presentation

The disorder is characterized by severe liver disease, which either causes liver failure in infancy or may take a more protracted course, with death often occurring during childhood or adolescence because of hepatoma development. Worldwide, the incidence is 1 in 100,000. It is particularly prevalent in the genetically isolated region of Saguenay-Lac-Saint-Jean in Quebec where the carrier rate is 1 in 20 and the incidence is 1 in 2000. Initially, liver dysfunction often affects coagulation factors, even before other signs of liver failure appear. In fact, jaundice and elevated liver enzymes are rare in the early stages of tyrosinemia. A common presentation mode is the “acute hepatic crisis” in which ascites, jaundice, and gastrointestinal bleeding are precipitated by an acute event such as an infection. Acute hepatic crises usually resolve spontaneously, but on occasion progress to complete liver failure and encephalopathy. Cirrhosis eventually develops in most patients with the disease. Hepatocellular carcinoma is frequent in tyrosinemic subjects with chronic liver disease.[58] It is believed that toxic metabolites that accumulate in tyrosinemia such as fumarylacetoacetate are mutagenic and contribute to the elevated rate of liver carcinoma.[55] Serial ultrasounds and CT scans are routinely performed. Neurologic crises are acute episodes of peripheral neuropathy with painful paresthesias and eventually autonomic dysfunction.[53]

Renal involvement is almost always present in tyrosinemic subjects[53] and is probably caused by succinylacetone toxicity.[59] It ranges from mild tubular dysfunction to renal failure. Glomerular filtration rate is frequently decreased. Hypophosphatemic rickets is the principal sign of tubular dysfunction, and acute decompensation can exacerbate the dysfunction. Generalized aminoaciduria is frequent. Nephrocalcinosis and nephromegaly can often be seen on renal ultrasound.[60] Glucosuria and proteinuria are usually mild. Tubular defects respond to diet but may be irreversible in chronic cases.


Dietary intervention with restriction of phenylalanine and tyrosine together with supportive measures can ameliorate the symptoms. Pharmacologic treatment with nitisinone, a peroral inhibitor of the tyrosine catabolic pathway, offers an improved means of treatment.[61] However, longer follow-up periods are needed to establish the role of this drug in ultimately protecting patients from end-stage organ involvement and hepatocellular carcinoma. Orthotopic liver transplantation has been used for several years in tyrosinemia type 1 (see review on management in Ref 53). The decision to perform liver transplantation depends on the patient's liver status and neurological symptoms. Stable patients with adequate liver function and the absence of nodules on CT scan can be treated conservatively with a low phenylalanine, low-tyrosine diet. The renal dysfunction may persist after transplantation because the renal enzyme is still defective.



Classic galactosemia is an autosomal recessive disorder caused by the deficiency of galactose 1-phosphate uridyltransferase,[62] which results in an inability to metabolize lactose. The genetic defects responsible for galactosemia are a deficiency of galactose 1-phosphate uridyltransferase,[63] galactokinase, or uridine diphosphate galactose 4-epimerase (reviewed in Ref 62). These enzymes catalyze the reactions in the unique pathway converting galactose to glucose. The consequences are abnormally high levels of galactose and its metabolites in plasma and body fluids.

Clinical Presentation

Clinical manifestations appear after exposure to galactose and can produce cataracts, failure to thrive, vomiting, inanition, liver disease, and developmental delay. The clinical spectrum ranges from cataracts for galactokinase deficiency, to important toxicity syndromes resulting from galactose exposure in galactose 1-phosphate uridyltransferase and uridine diphosphate galactose 4-epimerase deficiency. Vomiting, diarrhea, jaundice, hepatomegaly, and ascites occur in transferase deficiency. Tubular proteinuria, generalized aminoaciduria, and bicarbonaturia may occur and can quickly disappear after withdrawal of galactose. Most individuals with classic galactosemia have intellectual deficits. Although the potentially lethal, neonatal hepatotoxic syndrome is prevented by newborn screening and galactose restriction, long-term outcome for older patients with galactosemia remains problematic.

The diagnosis is suggested by elevated galactose or galactose 1-phosphate in serum, or galactose in the urine. The definitive diagnosis is made by the demonstration of the enzyme deficiency in blood cells, cultures skin fibroblasts, or other tissues.[62]


Elimination of dietary lactose from the diet is the mainstay of therapy for galactosemia. In infants, human milk or formula based on bovine milk is discontinued and a soy-based formula is given.

Lowe Oculocerebrorenal Syndrome


The oculocerebrorenal syndrome of Lowe (OMIM 309000)[11] is an X-linked recessive multisystem disorder characterized by congenital cataracts, mental retardation, and renal Fanconi syndrome (reviewed in Ref 64). The OCRL1 gene [66] [67] encodes a 105-kD Golgi protein with phosphatidylinositol (4,5) bisphosphate 5-phosphatase activity (OCRL1) that controls cellular levels of a critical metabolite, phosphatidylinositol 4,5-bisphosphate, involved in the inositol phosphate signaling pathway. We still know relatively little about what the OCRL1 protein actually does inside the cell and how it causes the disease. However, it is believed that it can influence membrane traffic and actin dynamics.[67] OCRL1 could regulate Golgi and endosomal trafficking pathways in addition to trans-Golgi network/endosome cycling (i.e., endocytic recycling) especially in the case of megalin and cubilin in the proximal tubule.[68] [69]

Clinical Presentation

Renal dysfunction (Fanconi syndrome) is a major feature and occurs in the first year of life, but the severity and age of onset vary. It is characterized by proteinuria (0.5 to 2 g of urinary protein per square meter of body-surface area per day), generalized aminoaciduria (100 to 1000 mmol of urinary amino acid per kilogram of body weight per day), carnitine wasting (mean fractional excretion, 0.05 to 0.15), phosphaturia, and bicarbonaturia.[69] Glucosuria is usually not present. Linear growth decreases after 1 year of age. Glomerular function also falls with age, with renal failure predicted between the second and fourth decade of life.

Neurological findings include infantile hypotonia, mental retardation, and areflexia. Prenatal development of cataracts is universal and other ocular anomalies include glaucoma, microphthalmos, and corneal keloid formation. Visual acuity is frequently decreased. Mental retardation is very common but not universal. Cranial magnetic resonance imaging show mild ventriculomegaly and cysts in the periventricular regions. Status epilepticus is also frequent. Death usually occurs in the second or third decade from renal failure or infection.

In the absence of reliable biochemical tests or a confirmed family history, the diagnosis is made clinically. It depends on the cardinal ocular, renal, and neurologic manifestations. Carrier detection by slit-lamp examination has high but not absolute sensitivity. Concentrations of the muscle enzymes creatine kinase, aspartate aminotransferase, and lactate dehydrogenase, as well as of total serum protein, serum α2-globulin, and high-density lipoprotein cholesterol, are elevated. Carrier detection can be performed by slit-lamp examination or by mutation detection or linkage analysis of markers when the mutation is unknown.


Treatment is supportive and includes taking care of ocular (cataract extraction, treatment of glaucoma), neurologic (anticonvulsants, speech therapy), and renal complications. Bicarbonate therapy is usually given at a dose of 2 to 3 mmol/Kg/day every 6 to 8 hours. Sodium or potassium phosphate can be given in amounts 1 to 4 g per day for phosphate depletion and vitamin D may be added if unsuccessful.

Wilson Disease


Wilson disease (OMIM 277900)[11] is an autosomal recessive disorder in which biliary excretion of copper and incorporation into ceruloplasmin is impaired, leading to liver damage from copper accumulation. The frequency of the disease is approximately 1 in 100,000 live births. The disease-associated gene encodes a copper-transporting P-type ATPase, the WND protein, [71] [72] [73] that is targeted to the mitochondria. This suggests that its role for copper-dependent processes takes place in this organelle.[73] Accumulation in the brain, kidney, and eyes leads to loss of coordination, proximal tubular dysfunction, and the characteristic corneal rings.

Clinical Presentation

Most patients with Wilson disease present with liver dysfunction, neurologic symptoms, or a combination. Liver symptoms can take multiple forms (i.e., chronic and acute liver failure). The biliary excretion defect leads to accumulation of copper in the liver, with progressive damage, and overflow to the brain. This causes accumulation of copper in the central nervous system, manifested as dysarthria and coordination defects of voluntary movements. This is frequently accompanied by involuntary movements. Pseudobulbar palsy is frequent and a common mode of death in unrecognized cases. The Kayser-Fleisher ring is the most important sign of Wilson disease. It is a yellow-brown (dull-copper-colored) granular deposit on Descemet membrane at the limbus of the cornea usually seen earliest at the upper and lower poles.

Affected adults often show most features of the Fanconi syndrome with aminoaciduria, bicarbonaturia, phosphaturia, glucosuria, and low molecular weight proteinuria.[74] Children do not have renal manifestations usually. Hypercalciuria is frequent, and kidney stones and nephrocalcinosis have been described in several cases.[75] Ultrastructural findings on renal biopsies include electron-dense deposits in the tubular cytoplasm.[76]

Wilson disease should be suspected in all subjects with acute or chronic liver dysfunction. The diagnosis of Wilson disease can be made by identifying Kayser-Fleisher rings, or by finding increased levels of serum or urine copper and reduced serum ceruloplasmin levels. Increased liver copper (>300 mg/g dry weight) is a reliable finding. Lack of incorporation of 64Cu or 67Cu into ceruloplasmin over 48 hours is the most definitive test available.


Chelation of copper with D-penicillamine is the treatment of choice for Wilson disease, is very effective, and has dramatically changed the course of the liver disease.[77] Adults usually require 1 g per day of D-penicillamine given in two doses. In patients, 24-hour urinary excretion of copper should be monitored to achieve copper losses of 2 mg per day. Doses of D-penicillamine can be decreased after 1 or 2 years to achieve urinary losses of 1 mg per day. There are several problems with D-penicillamine that include toxicity and increased serum copper levels initially, which can worsen the neurologic symptoms. Alternative treatments include zinc salts, which block intestinal copper absorption by inducing metallothionein synthesis in the mucosal intestinal cells. Tetrathiomolybdate appears to be an excellent form of initial treatment in patients who present with neurologic symptoms and signs. In contrast to penicillamine therapy, initial treatment with tetrathiomolybdate rarely allows further, neurologic deterioration.[78] Orthotopic liver transplantation can be used in cases with severe hepatic decompensation.[79]

Hereditary Fructose Intolerance

There are several disorders of fructose metabolism, secon-dary to deficiencies in aldolase B, fructose 1-phosphate aldolase, and fructokinase, respectively (reviewed in Ref 80). Hereditary fructose intolerance caused by fructose 1-phosphate aldolase deficiency (fructose-1,6-bisphosphate aldolase, EC is an autosomal recessive disorder characterized by vomiting shortly after the intake of fructose (reviewed in Ref 81). The disease can be associated with proximal tubule dysfunction (aminoaciduria, bicarbonaturia, and phosphaturia) and lactic acidosis. Kidney biopsy shows discrete findings. Liver dysfunction, hepatomegaly, cirrhosis, and jaundice appear from prolonged exposure. Hypoglycemia is unfortunately frequently absent. Continued ingestion of noxious sugars leads to hepatic and renal injury and growth retardation. The most common mutation has a prevalence of 1.3%, suggesting a frequency of 1 in 23,000 homozygotes.[82]

The pathophysiology of the renal Fanconi syndrome is not clear, but could be related to vacuolar proton pump dysfunction in the proximal tubule, as a direct binding interaction between V-ATPase and aldolase was demonstrated for the regulation of the V-ATPase.[83] This study showed that aldolase B was abundant in endocytosis zones of the proximal tubule, a subcellular domain also abundant in V-ATPase.[83] Vacuolar H+-ATPases (V-ATPases) are essential for acidification of intracellular compartments and for proton secretion from the plasma membrane in kidney epithelial cells and osteoclasts. Perhaps the release of nonfunctional aldolase B in response to fructose ingestion impairs the coupling of the V-ATPase to glycolysis. Thus, mechanistic similarities in organelle dysfunction between Dent disease and hereditary fructose intolerance are apparent.

The management of hereditary fructose intolerance involves withdrawal of sucrose, fructose, and sorbitol from the diet.


Because amino acids are not significantly bound to proteins in the plasma (except for tryptophan, which is 60% to 90% bound), they are freely filtered by the glomerulus. However, the proximal tubule will reabsorb 95% to 99.9% of the filtered load; thus the excretion of more than 5% of the filtered load of an amino acid is abnormal. Aminoaciduria occurs when a renal transport defect of the proximal tubule decreases the reabsorptive capacity for one or several amino acids, or when the threshold for reabsorbing an amino acid is exceeded by elevated plasma levels as a result of a metabolic defect (“overflow aminoaciduria”). Such inherited diseases of amino acid metabolism will not be discussed in this section. Theoretically, renal aminoacidurias can be secondary to defects in brush border or basolateral transporters and intracellular trafficking of amino acids. They are usually detected by newborn urine screening programs in several Western countries, and the most frequent abnormalities identified (apart from phenylketonuria, now normally detected by blood screening) are cystinuria, histidinemia, Hartnup disease, and iminoglycinuria.[84]Clinically, the most significant renal aminoaciduria is cystinuria ( Table 40-4 ). Most of the other disorders are rarely symptomatic.

TABLE 40-4   -- Classification of the Aminoacidurias



Amino Acids


Basic amino acids

Cystine, lysine, ornithine, arginine

Lysinuric protein intolerance


Lysine, arginine, ornithine

Isolated cystinuria






Hartnup disease

Neutral amino acids

Alanine, asparagine, glutamine, histidine, isoleucine, leucine, phenylalanine, serine, threonine, tryptophan, tyrosine, valine

Blue diaper syndrome





Glycine, proline, hydroxyproline










Proteins ingested in the regular diet and degraded in the intestine are absorbed by the intestinal mucosa as amino acids and small oligopeptides. Apical and basolateral transporters carry the amino acids into the blood, where they will be used for metabolic needs, but will also be freely filtered by the kidneys. Renal amino acid reabsorption occurs in the proximal tubule through a variety of transporters. Most amino acids are reabsorbed by more than one transporter and almost completely reclaimed, except for histidine, which has a fractional excretion of 5% (reviewed in Ref 85). Amino acids can share transporters with low affinity but high transport capacity, and have a specific transporter for one amino acid that has a high affinity and low maximal transport capacity. Because most amino acid carriers have not been cloned, a more detailed discussion is not possible at this stage. Common carriers have been divided into five groups and transport neutral and cyclic amino acids, glycine and imino acids, cystine and dibasic amino acids, dicarboxylic amino acids, and β-amino acids (reviewed in Ref 85). The transport of amino acids is coupled to the sodium gradient established by the basolateral Na+-K+-ATPase.


Cystinuria (OMIM 220200)[11] is the most frequent and best known of the aminoacidurias (reviewed in Ref 86). It is an autosomal recessive disorder associated with defective transport of cystine, and the dibasic amino acids ornithine, lysine, and arginine. It involves the epithelial cells of the renal tubule and gastrointestinal tract ( Fig. 40-4 ). The formation of cystine calculi in the urinary tract, potentially leading to infection and renal failure, is the hallmark of the disorder. Cystine is the least soluble of the naturally occurring amino acids, particularly at low pH. Worldwide, the prevalence is approximately 1 in 7000. Thus, it is one of the most frequent Mendelian disorders. The prevalence varies according to geographic location and has been estimated at 1 in 15,000 in the United States,[87] 1 in 2000 in England,[88] 1 in 4000 in Australia,[89] and 1 in 2500 in Jews of Libyan origin.[90] Newborn screening programs worldwide now help identify cases.



FIGURE 40-4  Model for the reabsorption of different amino acids in the proximal tubule cell opossum kidney cell. Transepithelial flux of amino acids through the cell is ensured by the presence of different transport systems at the apical and basolateral membrane. A tertiary active transport mechanism accounts for the reabsorption of dibasic amino acids (AA+) and cystine (CssC): an apical Na+-dependent neutral amino acid transport system (B0AT1 (SLC6A19) defective in Hartnup disorder) accounts for the high accumulation of neutral amino acids (AA0) in the cell, which provide the driving force for the entry of cystine and dibasic amino acids through system b0,+ (rBAT-b0,+AT). Dibasic amino acid and cystine influx are favored by the negative membrane potential and the rapid reduction of cystine to cysteine, respectively. Net efflux of dibasic amino acids is accounted by exchange with neutral amino acids plus sodium via system y+L (4F2hc-y+LAT-1) at the basolateral membrane. The pool of intracellular neutral amino acids (including cysteine) can be exchanged with the extracellular neutral amino acid pool via the basolateral system L (4F2hc-LAT-2). As long as this exchange is 1:1, the neutral amino acid individual pools, but not the total pool, will change depending on the concentrations of the different amino acids at either side of the basolateral membrane and on the intrinsic assimetry of the transporter. Therefore, a facilitative neutral amino acid transporter (T) must be present at the basolateral membrane to explain net transport of these amino acids. [336] [337]  (Modified from Chillaron J, Roca R, Valencia A, et al: Heteromeric amino acid transporters: Biochemistry, genetics, and physiology. Am J Physiol Renal Physiol 281:F995–1018, 2001.)






Cystinuria is genetically heterogeneous and caused by mutations in either of two genes implicated in dibasic amino-acid transport by the proximal tubule: SLC3A1 and SLC7A9. SLC3A1 encodes rBAT, and SLC7A9 encodes bo,+AT, a subunit that associates with rBAT to form the active transporter (reviewed in Ref 91). By itself, the subunit bo,+AT is sufficient to catalyze transmembrane amino acid exchange (i.e., the exchange of dibasic amino acids for neutral amino acids).[92] A model for the reabsorption of different amino acids in the proximal tubule is represented in Figure 40-4 .

Type A cystinuria (also called type I, OMIM 220100)[11] is caused by mutations in both alleles of SLC3A1 (chromosome 2). It is a completely recessive disease in which both parents excrete normal amounts of cystine (0 to 100 mmol/g creatinine). Jejunal uptake of cystine and dibasic amino acids is absent and there is no plasma response to an oral cystine load. The risk of nephrolithiasis is very high. SLC3A1 gene encodes the renal proximal tubule S3 segment and intestinal dibasic rBAT amino acid transporter (see Fig. 40-4 ).[93] Over 103 different mutations have been reported to date.[94] The most common point mutation, the M467T and its relative M467K, have been expressed in vitro in oocytes.[95] The amount of rBAT protein was similar in normal and mutant-injected oocytes. However, most of the M467T and M467K proteins were located in an intracellular compartment, contrary to the wild-type protein, and were endoglycosidase H- sensitive, suggesting longer residence time in the endoplasmic reticulum. These data indicate impaired maturation and transport to the plasma membrane of the mutants.

Type B cystinuria (also called non-type I cystinuria, OMIM 600918)[11] is an incompletely recessive form in which both parents excrete intermediate amounts of cystine (100 to 600 mmol/g creatinine). Parents may also have a normal pattern as demonstrated in 14% of cases.[96] The SLC7A9 gene encoding BAT1 and located on chromosome 19q13 causes cystinuria type B and 66 mutations have been identified.[94] BAT1 is a subunit linked to the rBAT via a disulfide bond. It belongs to a family of light subunits of amino acid transporters, expressed in the kidney, liver, small intestine, and placenta. Co-transfection of bo,+AT and rBAT brings the latter to the plasma membrane, and results in the uptake of L-arginine in vitro.

Type AB cystinuria is caused by one mutation in SLC3A1 and one mutation in SLC7A9 and this digenic inheritance is an exception. This type would involve the offspring of one parent carrier of a mutation on chromosome 2 and of another parent with a mutation on chromosome 19. Interestingly, the observed prevalence of AB patients is much lower than expected. Considering a similar frequency of mutations in SLC7A9 and SLC3A1, we would expect one third of the patients to suffer from type A disease, one third from B disease, and one third from AB disease. Indeed, the prevalence of type A disease is similar to that of type B disease; however, type AB is extremely rare.[96] Thus, type AB patients may suffer from a mild phenotype and therefore, in most cases, escape detection, or alternatively carry two mutations in SLC7A9 (which was not detected) and a coincidental carrier state for an SLC3A1 mutation.[96]

Renal Transport Defect

The existence of cystinuria has been recognized since 1810, first suspected from two patients with bladder stones, hence the name cystic oxide and cystine to characterize the chemical composition of the stones. [98] [99] Garrod later suspected that the disorder was due to a defect of cystine metabolism.[99] This turned out not to be the case as cystinuria is caused by a transporter defect.

Intestinal Transport Defect

Cystinuria also presents with a defective intestinal absorption of dibasic amino acids, which implies that there is a transporter defect in the gut similar to renal proximal tubule cells. This was first suggested from the increased urinary excretion of decarboxylation products such as putrescine and cadaverine,[100] which originate from the bacterial degradation of lysine and arginine, respectively. Why intestinal amino-acid transport defect does not lead to more serious metabolic problems is not known, but could be explained by the ability of the intestine to absorb small (di and tri) peptides.[101] It is also likely that there are other transporters of dibasic amino acids, as isolated cystinuria without lysinuria, argininuria and ornithinuria,[102] and dibasic amino aciduria without cystinuria have been described.[103]

Clinical Presentation

The only known manifestation of cystinuria is nephrolithiasis. Clinical expression of the disease frequently starts during the first to third decade but may occur from the first year of life up to the ninth decade. The disease occurs equally in both sexes but males tend to be more severely affected than females. Cystine stones are made of a yellow-brown substance, are very hard, and appear radiopaque on roentgenograms, due to sulfur molecules. Stones are frequently multiple, staghorn, and tend to be smoother than calcium stones. Magnesium ammonium phosphate and calcium stones can also form as a result of infection.

Diagnosis can be made by the analysis of a simple urine sample where typical hexagonal crystals will appear. Acidification of concentrated urine with acetic acid can also precipitate crystals not visible initially. Diagnosis is ultimately made by measurement of cystine excretion in the urine. This is usually performed in specialized centers using chromatographic methods. Quantitative ion-exchange chromatography is a frequently used method.[104]Quantitation of cystine can also be performed with a colorimetric method after reduction to the thiol.[105] Methods based on spectrophotometry involving oxidation of cysteine by thallium have been described.[106] High performance liquid chromatography methods are also used.[107] The cyanide-nitroprusside test has been widely applied as a qualitative screening procedure. This method is particularly useful for the detection of homozygotes.[108]False positives include homocystinuria and patients with acetonuria.


A regularly followed medical program based on high diuresis and alkalization with second line addition of thiols slows down or markedly decreases stone formation, and precludes the need for urological procedures in more than half of the patients.[109] Patients poorly compliant with hyperdiuresis remain at risk for recurrence.


Cystine production arises from the metabolism of methionine. Attempts at reducing methionine in the diet have been tried in the past but are both uncomfortable and of limited usefulness. [111] [112] Reducing sodium in the diet results in lower urine cystine. [113] [114] [115]

Decreasing Urine Cystine Saturation

This is usually accomplished with a combination of increased fluid intake and increasing urine pH. Increasing fluid intake should ideally reach 4L per day because many cystinurics excrete 1 g or more of cystine each day. At least, daily urine output of 3L seems necessary.[109] It is also important to drink at bedtime and during sleep to prevent supersaturation during periods of reduced urine output.[115] Cystine solubility can be increased by alkalinization of the urine with potassium citrate or bicarbonate but the solubility of cystine does not increase until the pH reaches 7.0 to 7.5. Citrate is the preferred method because alkalinization lasts longer. The requirements for alkali often reach 3 to 4 mmol/Kg.


Patients who are unable to comply with a regimen of high fluid intake and urine alkalinization or who fail despite adequate treatment may be given d-penicillamine in doses of 30 mg/Kg/day up to a maximum of 2 g. Through a disulfide exchange reaction, d-penicillinamine can form the disulfide cysteine-penicillamine, which is much more soluble than cystine. Several studies have reported that d-penicillamine is generally well tolerated in cystinurics, [117] [118] although frequent side effects such as rash, fever, and more rarely arthralgias and medullary aplasia have been reported (reviewed in Refs 118, 119). Other reactions include proteinuria and membranous nephropathy, epidermolysis, and loss of taste. Inhibition of pyridoxine by d-penicillamine is also a potential side effect.[120] Another drug that may be useful in cystinuria is mercaptopropionylglycine.[121] Its mechanism of action is identical to d-penicillamine and is as effective as d-penicillamine in reducing urine cystine excretion. Side effects are similar to d-penicillinamine and include skin rash, fever, nausea, proteinuria, and membranous nephropathy.[122] Finally, captopril has been advocated as a potential treatment for cystinuria, but its efficacy is controversial.

Surgical Management

Symptomatic stones that do not pass spontaneously often require surgical treatment.[123] The introduction of extracorporeal shock wave lithotripsy has not been of great benefit to cystinuric patients. Cystine stones are hard and have proven difficult to pulverize. Consequently, percutaneous lithotripsy is more effective. Recent progress in urological treatment of kidney stones has decreased the need for open surgery.[124] Urinary alkalinization as well as direct irrigation of the urinary tract with d-penicillamine, N-acetylpenicillamine, or tromethamine to form disulfide compounds, has resulted in the dissolution of stones. This approach often requires irrigation for several weeks with a risk for potential complications of catheterization.

Transplantation is sometimes necessary for patients with terminal renal failure from chronic obstruction or infection (or both). A kidney from an unaffected donor will not form cystine stones.

Lysinuric Protein Intolerance


Lysinuric protein intolerance (LPI) (OMIM 222700)[11] is a very rare recessively inherited dibasic amino acid transport disorder, mostly reported in Finland (reviewed in Ref 125). The disease is caused by defective basolateral membrane efflux of the cationic amino acids lysine, arginine, and ornithine in intestinal, hepatic, and renal tubular epithelia. Inactivating mutations in a novel transcript, SCL7A7, have been found in several families with LPI. [127] [128] [129] SCL7A7 cDNA encodes a 511 amino acid protein, y+LAT-1, predicted to harbor 12 membrane-spanning domains, with both amino and carboxy termini located intracellularly. This protein is thought to be part of the y+L multimeric unit. Cationic amino acid transport occurs through five different systems: y+, y+L, b+, b0,+, and B0,+ (reviewed in Ref 129). Defective system y+L transport explains the abnormality in cationic amino acid transport. System y+L mediates sodium-independent high-affinity transport of cationic amino acids and the transport of zwitterionic amino acids with low affinity. It is responsible for renal reabsorption and intestinal absorption of dibasic amino acids at the basolateral membranes. y+L transport is induced by a cell surface glycoprotein heavy chain (4F2hc) that represents the heavy chain subunit of a disulfide-linked heterodimer.[130]

Clinical Presentation

The amino acid deficiency leads to impaired urea cycle and postprandial hyperammonemia. The urinary excretion of lysine and all cationic amino acids is increased and the plasma levels are decreased. Clinical findings include protein aversion, nausea, jaundice, hyperammonemia, coma, and metabolic acidosis. Micronodular cirrhosis of the liver occurs from protein malnutrition, and pulmonary alveolar proteinosis is an occasional finding.[131]Hyperammonemia is induced by low levels of ornithine, which provides the carbon skeleton for the urea cycle.[132] Affected subjects are symptom-free while on breast feeding but fail to thrive on weaning. Various renal disorders including IgA nephropathy have been described.[133] Lysinuric protein intolerance can also present as childhood osteoporosis.[134] Various immunological abnormalities have been described[135] and are possibly secondary to low arginine levels, the substrate for nitric oxide synthase and NO production.[136]


The current treatment of lysinuric protein intolerance involves moderate protein restriction with supplementation with 3 g to 8 g of citrulline daily during meals and lysine.[137] Citrulline is transported by a different pathway than dibasic amino acids and can be converted to ornithine and arginine in the liver. Lysine cannot be made from citrulline.

Hartnup Disease


Described initially in 1956 in the Hartnup family, the Hartnup disorder (OMIM 234500)[11] is an autosomal recessive, and usually benign condition, consisting of excessive urinary excretion of monoamino, monocarboxylic (neutral) amino acids alanine, asparagine, glutamine, histidine, isoleucine, leucine, methionine, phenylalanine, serine, threonine, tryptophan, tyrosine, and valine (reviewed in Refs 138, 139). Its incidence has been estimated at 1 in 26,000 in newborn screening programs. Hartnup disorder is caused by mutations in the neutral amino acid transporter B(0) AT1 (SLC6A19). [141] [142] The transporter is found in kidney and intestine, where it is involved in the resorption of all neutral amino acids. Most newborns identified prospectively by genetic screening programs have been completely asymptomatic. In most affected individuals there is also a decreased intestinal absorption of neutral amino acids, particularly tryptophan.

Clinical Presentation

The clinical features of this disorder, if any, are due to nicotinamide deficiency, which is partly derived from tryptophan. These include a photosensitive erythematous skin rash (pellagra-like) clinically identical with niacin deficiency, intermittent cerebellar ataxia, and rarely mental retardation. Emotional instability, psychosis, and depression have been rarely noted, particularly during episodes of ataxia. Although the Hartnup family had several cases with mental retardation, most affected subjects described subsequently have not had mental retardation. Hartnup disease should be suspected in all subjects with pellagra and unexplained intermittent ataxia. Siblings of affected patients should be screened as well. Clinical manifestations can be triggered by periods of inadequate dietary intake or increased metabolic needs. For example a young woman presenting with pellagra precipitated by prolonged lactation and increased activity[142] was diagnosed with Hartnup disorder.

The diagnosis is easily made by performing a urinary amniogram and shows increased excretion of neutral amino acids, but not glycine, cystine, dibasic, dicarboxylic, and imino amino acids. Thus, any confusion with the renal Fanconi syndrome is avoided by performing a complete evaluation of amino acids in the urine using one of several chromatographic methods. The pattern of amino acid excretion, rather than the total amount, is the determining factor.[138] The reabsorption defect involves 12 amino acids and most patients with Hartnup disease have the same pattern of aminoaciduria. The levels of amino acids in the monoamino, dicarboxylic group (such as glutamic acid and aspartic acid), and basic group (lysine, ornithine, arginine) are normal or slightly increased. The excretion of proline, hydroxyproline, and cystine are also normal. Despite the defect, substantial renal tubular transport of the involved amino acids remains and renal clearances are inferior to GFR. Because amino acids might be reabsorbed by high capacity low affinity and low capacity highly specific transporters, residual renal reabsorption of neutral amino acids might occur through specific transporters, passive diffusion, and partial activity of the defective transporter.[138]


Treatment of symptomatic cases involves the administration of nicotinamide in doses of 50 to 300 mg/day. The value of treating asymptomatic cases is not known, but given the harmlessness of the treatment, this might be a rational choice.


Familial iminoglycinuria (OMIM 242600)[11] is a benign autosomal recessive disorder with no clinical symptoms whose main interest is that it suggests the presence of a common carrier for the imino acids proline and hydroxyproline, as well as glycine. [144] [145] The molecular defect underlying iminoglycinuria has not yet been identified. However, two transporters, the proton amino acid transporter PAT1 (SLC36A1) and the IMINO transporter (SLC6A20) appear to play key roles in the resorption of glycine and proline. Iminoglycinuria was discovered after the application of chromatographic methods to the investigation of disorders of amino acid metabolism. The diagnosis is usually suggested from an increased urinary excretion of imino acids and glycine. Newborns and infants usually excrete detectable amounts of imino acids and glycine up to 3 months (see review in Ref 145). Thus, the presence of increased urinary excretion of imino acids and glycine after 6 months can be considered abnormal. It can be part of a generalized defect of the proximal tubule (the Fanconi syndrome) or a selective defect.


This group refers to disorders that have in common persisting hypophosphatemia caused by a reduction in renal tubule reabsorption of Pi (reviewed in Ref 146). Each disorder is characterized by an increased fractional excretion of Pi (or decreased TmP/GFR) with frequent metabolic bone disease presenting as rickets in childhood and osteomalacia in adults.

Renal Phosphate Excretion

The determinants of phosphate homeostasis are ingestion, intestinal absorption, and renal excretion. Normal phosphate intake in adults varies from 800 to 1600 mg/day and the average serum phosphate levels remain normal over a wide range of intake. Contrary to active calcium absorption, which is greatest in the duodenum with a lower rate in the jejunum, ileum, and colon, active Pi absorption is highest in the jejunum and ileum with a lower rate in the duodenum and colon.[147] Intestinal phosphate absorption is regulated by active vitamin D metabolites, and vitamin D supplementation results in increased intestinal phosphate absorption in patients experiencing renal failure.[148]Inorganic phosphate (Pi) is filtered by the glomerulus and reabsorbed in the proximal tubule. The difference between the amount of Pi filtered and reabsorbed will determine the net appearance of Pi in the urine. Phosphate reabsorption by the proximal tubule occurs by a Tm-limited active process. The fractional reabsorption of filtered phosphate is usually estimated by the tubular reabsorption of Pi (TRP), and is a simple equation to assess the renal tubular phosphate transport:


Given a normal renal function and a normal diet, the TRP is usually above 85%. A more precise way of estimating the tubular reabsorption of Pi is to calculate the theoretical threshold:


In the normal state, moderate phosphate deprivation that leads to a marginal decrease in serum phosphate levels induces a reduction in urine excretion of phosphate and an increase in 1,25(OH)2D3 levels. Further reduction that leads to moderate decreases in serum phosphate levels induces a virtual disappearance of urine Pi.[149] This occurs by an increase in Pi reabsorption by the proximal tubule as demonstrated by animal and vesicle studies (reviewed in Ref 150).

Renal proximal tubular reabsorption of Pi is determined by a Na/Pi-cotransporter system. [152] [153] Several Na/Pi cotransporters have been identified (see also Chapter 16 ). The regulation and pathophysiological alterations of renal proximal reabsorption of inorganic phosphate can be ascribed to the net amount of the Na/Pi-cotransporter NaPi-IIa localized in the brush border membrane.[151] The net amount of NaPi-IIa appears to be the result of an endocytotic rate regulated by a complex network of different protein kinases. New approaches demonstrate that NaPi-IIa is part of heteromeric protein complexes, organized by PDZ proteins. Such complexes are thought to play important roles in the apical positioning and regulated endocytosis of NaPi-IIa. The Na/Pi type II cotransporter is down-regulated by PTH and phosphate overload and up-regulated by phosphate deprivation. A reduction in serum phosphate levels also leads to increased 1,25(OH)2D3 levels[153] from increased activity of the rate-limiting enzyme for the synthesis 1,25(OH)2D3, the 1α-hydroxylase. This mitochondrial enzyme is a member of the P-450 family.[154] The fall in serum phosphate levels also inhibits bone deposition and the increase in serum 1,25(OH)2D3 increases bone resorption, thus favoring a net shift of phosphate from bone. Higher serum levels of 1,25(OH)2D3 also increase intestinal phosphate and calcium absorption. As a consequence, serum calcium levels increase and inhibit PTH secretion. The reduction in PTH levels does not lead to a further increase in phosphate reabsorption by the kidney because the proximal tubule is insensitive to the action of PTH in states of phosphate deprivation. As a result, one should predict that a renal phosphate leak will lead to raised serum 1,25(OH)2D3 levels, decreased PTH and induce hypercalciuria.

X-Linked Hypophosphatemic Rickets


X-linked hypophosphatemic rickets (OMIM 307800)[11] is the most common inherited hypophosphatemic disorder, accounting for 80% of cases of familial phosphate wasting. It is an X-linked dominant disorder characterized by hypophosphatemia with reduced TmP/GFR, normal serum calcium and PTH levels, and inappropriately normal to low serum 1,25-(OH)2D3 levels. XLH is caused by mutations in the PHEX gene (Phosphate regulating gene with Homologies to Endopeptidases on the X chromosome). It encodes an M13 zinc metalloprotease whose native substrate has not yet been identified. Although it is not immediately apparent how loss of PHEX function leads to a decrease in renal Pi reabsorption, it has been suggested that PHEX is involved in the inactivation of a phosphaturic hormone or the activation of a Pi conserving hormone [156] [157] and that loss of PHEX function is associated with either an excess of phosphaturic hormone or a deficiency in the Pi conserving hormone. In either case, renal type IIa, type IIc, and perhaps other Na/Pi cotransporters would be down-regulated and Pi wasting would ensue. However, endogenous PHEX substrates have not yet been identified. PHEX is expressed in osteoblasts [158] [159] and in tumor tissue associated with the paraneoplastic syndrome of renal phosphate wasting.

Clinical Presentation

Patients demonstrate short stature (growth retardation), femoral and/or tibial bowing presenting early in life, and histomorphometric evidence of rickets and osteomalacia. Males are usually more severely affected than females and there is variable penetrance. Serum phosphate levels are usually lower than 2.5 mg/dl (0.8 mmol/l) and the TmP/GFR is lower than 1.8 mg/dl (0.56 mmol/l). The hallmark of X-linked hypophosphatemic rickets is normal 1,25(OH)2D3 levels. [160] [161] There is apparently no correlation between the serum levels of phosphate and the severity of the disease. Affected children tend to have higher serum phosphate and TmP/GFR levels compared to affected adults, as is the case with normal subjects. The earliest sign of the disease in children can be increased serum alkaline phosphatase levels.


Early therapy with 1,25(OH)2D3 (1.0 to 3.0 mg per day) and phosphate (1 to 2 g per day in divided doses) has a beneficial effect on growth, bone density, and deformations.[161] Nephrocalcinosis due to vitamin D and phosphate therapy can lead to deterioration of renal function.

Autosomal Dominant Hypophosphatemic Rickets

Autosomal dominant hypophosphatemic rickets (ADHR; OMIM 193100) is characterized by low serum phosphorous concentration, phosphaturia, inappropriately low or normal 1,25(OH)2D levels, and bone mineralization defects that result in rickets, osteomalacia with bone pain, lower extremity deformities, and muscle weakness.[146] Features of ADHR are similar to those of XLH. However, ADHR is far less common than XLH and is characterized by incomplete penetrance and variable age of onset. The gene responsible for ADHR encodes a new member of the fibroblast growth factor (FGF) family, FGF-23, a 251-amino-acid peptide that is secreted and processed to amino- and carboxy-terminal peptides at a consensus pro-protein convertase (furin) site, RHTR (ArgHisThrArg).[162] Missense mutations in FGF-23, identified in four unrelated ADHR families, involve the two R residues in this proteolytic cleavage site[163] and abrogate peptide processing.[164] FGF-23 expression is not readily detectable in normal tissues but it is abundantly expressed in tumors removed from patients with oncogenic hypophosphatemic osteomalacia, an acquired renal Pi wasting disorder with features of XLH and ADHR.[165]

Hereditary Hypophosphatemic Rickets with Hypercalciuria

Hereditary hypophosphatemic rickets (OMIM 241530)[11] associated with hypercalciuria is a rare autosomal disease initially described in a Bedouin family with multiple consanguineous matings.[166] The disease maps to chromosome 9q34, which contains SLC34A3, the gene encoding the renal sodium-phosphate cotransporter NaPi-IIc. Nucleotide sequence analysis has revealed a homozygous single-nucleotide deletion (c.228delC) in all individuals affected.[167] Sequencing of this gene in another study revealed disease-associated mutations in five families, including two frameshift and one splice-site mutation.[168]

Affected subjects appear to have a chronic renal phosphate leak with an appropriate response to hypophosphatemia. Hypophosphatemia leads to stimulation of renal 1α-hydroxylase causing increased synthesis and serum levels of calcitriol. As a result, intestinal absorption of calcium is enhanced, resulting in increased urinary excretion. Other characteristic features include rickets, short stature, normal serum calcium levels, and suppressed parathyroid function. Patients respond to administration of daily oral phosphate (1 g to 2.5 g per day). This leads to an increase in serum phosphate and decreased serum 1,25(OH)2D3, calcium and alkaline phosphatase. Growth rate is restored, and the clinical manifestations of rickets and osteomalacia disappear (e.g., bone pain, muscle weakness).

Familial Tumoral Calcinosis

Familial tumoral calcinosis (FTC; OMIM 211900) is a severe autosomal recessive metabolic disorder that manifests with hyperphosphatemia and massive calcium deposits in the skin and subcutaneous tissues. Affected individuals report recurrent painful, calcified subcutaneous masses of up to 1 kg, often resulting in secondary infection and incapacitating mutilation. Using linkage analysis, the gene was mapped to 2q24-q31 and sequence analysis of the gene GALNT3, which encodes a glycosyltransferase responsible for initiating mucin-type O-glycosylation, identified biallelic deleterious mutations in individuals with FTC.[169] A second gene for FTC encoding FGF23 has been found. A missense mutation in the FGF23 gene abrogates FGF23 function by absent or extremely reduced secretion of intact FGF23.[170]

Hereditary Selective Deficiency of 1α,25(OH)2D3

This rare form of autosomal recessive vitamin D responsive rickets (OMIM 264700)[11] is not a disease of tubule transport per se, but a 1α-hydroxylation deficiency, and is described in this chapter because the enzyme is specifically expressed in the proximal tubule ( Fig. 40-5 ). It results from inactivating mutations in the P450 enzyme 1α-hydroxylase. [172] [173] Vitamin D is metabolized by sequential hydroxylations in the liver (25-hydroxylation) and the kidney (1α-hydroxylation). Hydroxylation of 25-hydroxyvitamin D3 is mediated by 25(OH)D3 1α-hydroxylase in the kidney. Using an elegant strategy, Takeyama and associates cloned the P450 component of the enzyme 1α-hydroxylase.[154] The human gene maps to chromosome 12q14 and the structure has been subsequently determined.[173] Patients usually appear normal at birth and develop muscle weakness, tetany, convulsions, and rickets starting at 2 months of age. Serum calcium levels are low; PTH levels are high with low to undetectable 1,25(OH)2D3.[174] Serum levels of 25(OH)D3 are normal or slightly increased. Once recognized, this rare disorder is easily treated with physiological doses of 1,25(OH)2D3 and will result in healing of rickets and restoring of the plasma calcium, phosphate, and PTH levels.



FIGURE 40-5  Schematic representation of the molecular genetic basis of three inherited forms of rickets. Vitamin D-dependent rickets type I is secondary to mutations in the 1α-hydroxylase gene. This gene is responsible for the 1α-hydroxylation of 25-hydroxyvitamin D (25-OH) that occurs in the convoluted and straight portions of the renal proximal tubule. This 1α-hydroxylation is catalyzed by 25-hydroxyvitamin D-1α-hydroxylase (1α-hydroxylase), a mitochondrial cytochrome P450 enzyme that is subject to complex regulation by parathyroid hormone, calcium, phosphorus, and 1.25-dihydroxyvitamin D itself. This disorder is characterized by failure to thrive, muscle weakness, hypocalcemia, secondary hyperparathyroidism, and the bony changes of rickets. The hallmarks of the disease are the findings of greatly reduced serum concentrations of 1.25 (OH)2D despite normal or increased concentrations of 25-OHD, and the reversal of clinical and laboratory abnormalities by administration of physiologic amounts of 1.25 (OH)2D3.[337] Vitamin D-dependent rickets type 2, also termed hereditary vitamin D-resistant rickets, is due to mutations in the gene for the vitamin D receptor. X-linked hypophosphatemic rickets results from loss-of-function mutations in the PHEX gene (phosphate-regulating gene with homologies to endopeptidases, on the X chromosome). In this disease, serum concentrations of 1.25 (OH)2D are inappropriately low despite the hypophosphatemia.



Hereditary Generalized Resistance to 1α,25(OH)2D3

This rare autosomal recessive disorder (OMIM 277400)[11] is similar to selective deficiency of 1α,25(OH)2D3 with the salient features that serum levels of 25(OH)D3 and 1α,25(OH)2D3 are increased and the disease does not respond to doses of 1α,25(OH)2D3 and 1α,(OH)D3. In addition, approximately half of the cases described have alopecia. In a subset of affected kindreds, the disease is due to mutations in the vitamin D receptor gene. Premature stop codons in the vitamin D receptor gene are responsible for the phenotype, resulting in the absence of the ligand-binding domain.[175]

Resistance to Parathormone Action

Pseudohypoparathyroidism (PHP) is associated with biochemical hypoparathyroidism (i.e., hypocalcemia and hyperphosphatemia) due to parathyroid hormone resistance rather than to PTH deficiency. Patients with PHP type 1a have a generalized form of hormone resistance plus a constellation of developmental defects termed Albright hereditary osteodystrophy. Within PHP type 1a families some individuals will show osteodystrophy but have normal hormone responsiveness, a variant phenotype termed pseudo-PHP. By contrast, patients with PHP type 1b manifest only PTH resistance and lack features of osteodystrophy. These various forms of PHP are due to defects in the GNAS1 gene that lead to decreased expression or activity of the α-subunit of the stimulatory G protein (G(s)alpha). Tissue-specific genomic imprinting of GNAS1 accounts for the variable phenotypes of patients with GNAS1 defects (reviewed in Ref 176).


Familial Renal Hypouricemia

Renal hypouricemia is an autosomal recessive disorder characterized by impaired urate handling in the renal tubules. It is associated with exercise-induced acute renal failure and nephrolithiasis. [178] [179] Hyperuricosuria, associated with dehydration or exercise, results in acute uric acid nephropathy, and causes an obstructive acute renal failure. This can be prevented by forced hydration with bicarbonate or saline solutions. Based on clearance and micropuncture studies obtained in experimental animals, the major modes of renal urate handling have been divided into four components that include glomerular filtration, presecretory reabsorption, secretion, and postsecretory reabsorption (reviewed in Ref 179). Mutations in the urate transporter URAT1 (encoded by SLC22A12) have been found to cause the presecretory type of renal hypouricemia.[180] Affected subjects typically have very high urate fractional excretion (50% or higher) with parents having intermediate levels.[181] A post-secretory form of renal hypouricemia is not linked to SLC22A12[182] suggesting that another urate transporter is responsible for this disorder.


Under normal conditions, glucose is almost completely reabsorbed by the proximal tubule. Thus, very small amounts of glucose are present in the urine of most normal individuals. The appearance of a significant amount of glucose in the urine (500 mg or 2.75 mmol/day in adults) is most often due to hyperglycemia (overload glucosuria), and rarely, to abnormal handling of glucose by the kidney ( Table 40-5 ). Renal glucosuria may be part of a generalized defect of the proximal tubule (Fanconi syndrome), or present as an isolated defect.

TABLE 40-5   -- Causes of Glucosuria






Diabetes mellitus












Angiotensin I-converting enzyme inhibitors



Dextrose IV solutions



Total parenteral nutrition

Renal glucosuria

Idiopathic renal glucosuria

Glucose-galactose malabsorption

Fanconi syndrome


For discussion of angiotensin I-converting enzyme inhibitors, see Milavetz JJ, Popovtzer MM: Angiotensin-converting enzyme inhibitors and glycosuria.

Arch Intern Med 152:1081–1083, 1992.




Renal Glucosuria

Familial renal glucosuria (OMIM 233100)[11] or FRG is an inherited renal tubular disorder characterized by persistent isolated glucosuria in the absence of hyperglycemia. It is usually a benign clinical condition. FRG is transmitted as a codominant trait with incomplete penetrance. Homozygotes can show glucosuria of more than 60 g/d, evidence of renal sodium wasting, mild volume depletion, and raised basal plasma renin and serum aldosterone levels.[183]Some cases have been associated with selective aminoaciduria,[184] unlike the generalized aminoaciduria seen in Fanconi syndrome. Mutations in the sodium/glucose co-transporter SGLT2 coding gene, SLC5A2 are responsible for the disorder.[185] Some Japanese patients might have mutations in the GLUT2 glucose transporter.[186]

The definition of glucosuria is arbitrary and different investigators have proposed different guidelines to define abnormal from normal glucosuria. A currently accepted stringent definition of glucosuria proposes the following criteria:



The oral glucose tolerance test, the levels of plasma insulin and free fatty acids, and the glycosylated hemoglobin levels should all be normal.



The amount of glucose in the urine (10 g to 100 g per day) should be relatively stable except during pregnancy when it may increase.



The degree of glucosuria should be largely independent of diet but may fluctuate according to the amount of carbohydrates ingested. All specimens of urine should contain glucose.



The carbohydrate excreted should be glucose. Other sugars are not found (fructose, pentoses, galactose, lactose, sucrose, maltose, and heptulose).

Subjects with renal glucosuria should be able to store and utilize carbohydrates normally.

Glucose-Galactose Malabsorption

Initially described in 1962, glucose-galactose malabsorption (OMIM 606824)[11] is a rare congenital disease resulting from a selective defect in the intestinal transport of glucose and galactose. It is inherited as an autosomal recessive trait and is characterized by the neonatal onset of severe watery and acidic diarrhea that results in death unless these sugars are removed from the diet.[187] Normally, lactose in milk is broken down into glucose and galactose by lactase, an ectoenzyme on the brush border, and the hexoses are transported into the cell by the Na+-glucose cotransporter SGLT1. The disease occurs occasionally in adults. The acidic diarrhea results from bacterial metabolism of sugar in the stools and can be improved with antibacterial treatment. Important weight losses from hyperosmolar dehydration and metabolic acidosis are frequent. The disease is usually suspected from the clinical history and the presence of glucosuria despite normal serum glucose levels. Dramatic improvement occurs after withdrawal of glucose and galactose from the diet.

Mutations in the glucose-galactose transporter SGLT1 [188] [189] located on chromosome 22q13.1 cause glucose-galactose malabsorption. This results in the absence of the transporter in the intestine and the kidney because of decreased cotransporter trafficking to the plasma membrane.[188]


Renal tubular acidosis (RTA) is a clinical syndrome characterized by hyperchloremic (normal anion gap) metabolic acidosis secondary to abnormal urine acidification. This can be identified by inappropriately high urine pH, bicarbonaturia, and reduced net acid excretion. Proximal and distal forms of RTA are frequently accompanied by hypokalemia. Proximal RTA generally occurs as part of the renal Fanconi syndrome ( Table 40-6 ). Rare forms of hereditary proximal and distal RTA have been identified (reviewed in Refs 189-191), and will be discussed here.

TABLE 40-6   -- Classifications, Features, and Underlying Molecular Transport Defect in Inherited Renal Tubular Acidosis

Proximal RTA

Clinical Features



Autosomal recessive pRTA with ocular abnormalities

Band kerotopathy, glaucoma, cataracts, short stature, mental retardation, dental enamel defects, pancreatitis basal ganglia calcification



Autosomal recessive pRTA with osteopetrosis and cerebral calcification (Inherited carbonic anhydrase II deficiency)

Mental retardation, osteopetrosis, cerebral calcification



Autosomal dominant pRTA

Short stature, osteomalacia



Distal RTA

Autosomal dominant dRTA

Complete or incomplete dRTA, hypercalciuria, nephrocalcinosis, nephrolithiasis, hypokalemia, short stature, osteomalacia, rickets



Autosomal recessive dRTA

Complete or incomplete dRTA

H+-ATPase (A4 subunit)



Other features as above




Reported in Asian populations



Autosomal recessive dRTA with progressive nerve deafness

Complete or incomplete dRTA

H+-ATPase (B1 subunit)



As above but with late-onset nerve deafness

H+-ATPase (A4 subunit)


Adapted from Laing CM, Toye AM, Capasso G, et al: Renal tubular acidosis: Developments in our understanding of the molecular basis. Int J Biochem Cell Biol 37:1151–1161, 2005.

RTA, renal tubular acidosis.





Mechanisms of Renal Acidification

A typical Western diet generates an acid load of ∼1 mmol of mineral acid per Kg of body weight, which must be excreted by the kidney. In addition, the kidney filters ∼4000 mmol of bicarbonate daily and must reclaim most of the filtered load in order to maintain acid-base balance. Excretion of the ingested acid load and reabsorption of filtered bicarbonate are accomplished by complex processes requiring coordinated actions of transport and enzymatic activities in the apical and basolateral membranes ( Fig. 40-6 ).



FIGURE 40-6  Mechanisms of renal acidification.



In the proximal tubule, filtered bicarbonate (HCO3-) is almost completely reabsorbed by an indirect mechanism. H+ and HCO3- are generated by intracellular hydration of CO2 by carbonic anhydrase II. H+ secretion occurs across the apical membrane via the NHE3, the Na+/H+ exchanger, and a H+-ATPase while HCO3- is transferred via a basolateral Na+–HCO3- co-transporter. The secreted H+ ions react with filtered HCO3- to form H2CO3, which is rapidly converted to CO2 and H2O by another form of carbonic anhydrase (IV) present in the apical membrane. The CO2 and H2O then diffuse into the cell. This results in the removal of a filtered HCO3- and its replacement by another in the plasma but the process is neutral in terms of net urinary H+ excretion because the secreted H+ are used to reabsorb filtered HCO3-.

Carbonic anhydrases are zinc metalloenzymes that catalyze the reversible hydration of CO2 to form HCO3- and protons, according to the following reaction:

CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3-

The first reaction is catalyzed by carbonic anhydrase and the second reaction occurs instantaneously. Carbonic anhydrase IV is a membrane-associated enzyme anchored to plasma membrane surfaces by a phosphatidylinositol glycan linkage. It accelerates the formation of CO2 and H2O from H2CO3-.

Net urinary elimination of H+ depends on its buffering and excretion as titratable acid (mainly phosphate: HPO42-+H+↔ H2PO4-), and excretion as NH4+. The production of NH4+ from glutamine by the proximal tubule and its secretion generate new plasma HCO3-. This process is stimulated in metabolic acidosis.

The connecting segment and collecting duct type A intercalated cells ( Fig. 40-7 ) secrete H+ into the lumen via a vacuolar Mg2+-dependent H+ ATPase and possibly an exchanger, H+/K+-ATPase. The generation of H+ is catalyzed by carbonic anhydrase II and HCO3- is transported across the basolateral membrane through the AE1 HCO3-/Cl- exchanger. Luminal H+ is trapped by urinary buffers, including ammonium secreted by the proximal tubule and phosphate.



FIGURE 40-7  Autosomal dominant and autosomal recessive distal renal tubular acidosis. Dominant distal renal tubular acidosis is due to mutations in the gene SLC4A1 encoding the chloride-bicarbonate exchanger AE1. [202] [203] The AE1 gene (chromosome 17) encodes both the erythroid (eAE1) and the kidney (kAE1) isoforms of the band 3 protein.[338] Mutations in the gene (ATP6V1B1, 2p13) encoding the b1 subunit of H+-ATPase cause recessive distal renal tubular acidosis with sensorineural deafness.[204] Distal RTA with preserved hearing is secondary to mutations in AIP6V0A4, which encodes the α4 subunit of the proton pump.[199] Both H+ and H+/K+-ATPases are represented. The H+-ATPase is schematically represented according to the proposed structure of the F1-ATPase of the inner mitochondrial membrane.[339] F1 is represented as a flattened sphere 80Å high and 100Å across. The three α- and three β-subunits are arranged alternately like the segments of an orange around a central α-helix 90Å long. Mutations in the b subunit are causing autosomal recessive distal renal tubular acidosis. Autosomal recessive distal renal tubular acidosis has also been found, in a small kindred, for the SLC4A1 mutation G701D.[203]



Proximal Renal Tubular Acidosis (type II RTA)

Primary, isolated hereditary proximal RTA is an extremely rare disorder. Proximal RTA usually occurs as part of the spectrum of the Fanconi syndrome in which the excretion of glucose, amino acids, and phosphate is increased. The diagnosis of proximal RTA rests on an appropriately acid urine pH (pH < 5.5) in acidotic patients and a high fractional excretion of bicarbonate (>10%–15%) during intravenous loading with NaHCO3. Large amounts of bicarbonate must be given to correct the serum bicarbonate levels.

Sodium-Bicarbonate Symporter Mutations

Inactivating mutations in SLC4A4 (OMIM 604278),[11] the gene coding for the Na+-HCO3- (NBC1) symporter cause permanent isolated proximal renal tubular acidosis with various ocular abnormalities such as band keratopathy, glaucoma, and cataracts.[192] The Na+-HCO3- symporter has been found to be expressed in multiple ocular tissues,[193] thus explaining abnormalities. Pancreatitis can be associated with NBC1 mutations as it is expressed in the pancreas.[194]

Carbonic Anhydrase II Deficiency

Recessive mixed proximal-distal RTA accompanied by osteopetrosis and mental retardation (OMIM 259730)[11] is caused by inactivating mutations in the cytoplasmic carbonic anhydrase II gene.[195] The pathogenesis of the mental subnormality and cerebral calcification is poorly understood. More than 50 cases have been described, predominantly from the Middle East and Mediterranean region. The disorder is discovered late in infancy or early in childhood through developmental delay, short stature, fracture, weakness, cranial nerve compression, dental malocclusion, and/or mental subnormality. Typical radiographic features of osteopetrosis are present, and histopathologic study of the iliac crest reveals unresorbed calcified primary spongiosa. The radiographic findings are unusual, however, in that cerebral calcification appears by early childhood and the osteosclerosis and skeletal modeling defects may gradually resolve by adulthood. Patients are usually not anemic. A hyperchloremic metabolic acidosis, sometimes with hypokalemia, is caused by renal tubular acidosis that may be a proximal, distal, or combined type.[196] Bilateral recurrent renal stones, hypercalciuria, and medullary nephrocalcinosis have been described.[197] There is no established medical therapy, and the long-term outcome remains to be characterized. There is a mouse model for CA-II deficiency for which gene therapy has been transiently successful, using retrograde injections in the renal pelvis.[198]

Distal Renal Tubular Acidosis

Hereditary distal RTA is a genetically heterogeneous disorder with dominant and recessive forms caused by the dysfunction of type A intercalated cells (reviewed in Refs 190, 199 [see Fig. 40-7 ]). Affected transporters include the AE1 Cl-/HCO3- exchanger of the basolateral membrane and at least two subunits of the apical membrane vacuolar (v)H+-ATPase, the V1 (head) subunit B1 (associated with deafness) and the V0 (stalk) subunit A4. Clinical features include inability to acidify urine, variable hyperchloremic hypokalemic metabolic acidosis, hypercalciuria, nephrocalcinosis, and nephrolithiasis. Patients with recessive dRTA present with either acute illness or growth failure at a young age, sometimes accompanied by deafness. Dominant dRTA is usually a milder disease and involves no hearing loss.

Chloride-Bicarbonate Exchanger Mutations

Mutations in the SLC4A1 gene encoding the chloride-bicarbonate exchanger AE1 (OMIM 179800)[11] can lead to dominant or recessive distal RTA. AE1, the Cl-/HCO3- exchanger, is expressed in the erythrocyte and in type A intercalated cells of the kidney. The renal AE1 contributes to urinary acidification by providing the major exit route for HCO3- across the basolateral membrane. Distal RTA results from aberrant targeting of AE1.[200]

The dominant form is usually a mild disorder that can be discovered incidentally after a kidney stone episode. [202] [203] In a previous study of four kindreds, affected subjects had serum bicarbonate concentrations between 14 and 25 mmol/L,[201] and serum potassium levels between 2.1 and 4.2 mmol/L. Minimum urine pH following an acid load varied from 5.95 to 6.8 (normal < 5.30). Nephrocalcinosis and kidney stones were present in approximately 50% of subjects. Deafness was absent.

The recessive form of RTA (OMIM 109270)[11] is usually diagnosed at a younger age, often before one year. It is found in Southeast Asia (Thailand, Papua-New Guinea, and Malaysia) where it is associated with ovalocytosis.[203]Affected subjects present with vomiting, dehydration, failure to thrive, or delayed growth. Nephrocalcinosis, kidney stones, or both are frequent, and rickets can be present. Severe metabolic acidosis with serum pH<7.30 and serum bicarbonate < 15 mmol/L is frequent. Serum potassium levels are also lower than autosomal dominant distal RTA.

Proton ATPase Subunit Mutations

Mutations in ATP6V1B1, the B1-subunit of the apical proton pump ATP6B1 mediating distal nephron acid secretion (OMIM 267300),[11] cause distal renal tubular acidosis with sensorineural deafness in a significant proportion of families. [205] [206] In type A intercalated cells, the H+-ATPase pumps protons against an electrochemical gradient. Active proton secretion is also necessary to maintain proper endolymph pH. These findings implicate ATP6B1 in endolymph pH homeostasis and in normal auditory function, as nearly all patients with ATP6V1B1 mutations also have sensorineural hearing loss.[202]

Mutations in the ATP6V0A4 gene on chromosome 7 (OMIM 602722)[11] also give rise to recessive distal RTA,[206] but hearing is preserved. [200] [206] [208] ATP6V0A4 encodes a newly identified kidney-specific A4 isoform of the proton pump's 116-kD accessory a subunit.[206]

The treatment of distal RTA involves the correction of dehydration, electrolyte, and bicarbonate anomalies, which will improve symptoms. In adults, administration of 1 to 3 mmol alkali/Kg of body weight usually corrects the metabolic abnormality. In children, up to 5 mmol/Kg may be required. Potassium supplementation may be needed even after correction of the acidosis.


In 1962, Bartter and co-workers described two patients with hypokalemic metabolic alkalosis, hyperreninemic hyperaldosteronism, normal blood pressure, as well as hyperplasia and hypertrophy of the juxtaglomerular apparatus.[208] Since then, it has been recognized that familial hypokalemic, hypochloremic metabolic alkalosis is not a single entity but rather a set of closely related disorders (reviewed in Ref 209). Although Bartter syndrome and Bartter mutations are used commonly as a diagnosis, it is likely, as explained by Jeck and colleagues,[209] that the two patients with a mild phenotype originally described by Dr. Bartter had Gitelman syndrome, a thiazide-like salt-like salt-losing tubulopathy with a defect in the distal convoluted tubule. As a consequence, salt-losing tubulopathy of the furosemide type is a more physiologically appropriate definition for Bartter syndrome. Bartter syndrome is a genetically heterogeneous disorder affecting the loop of Henle, where 30% of the filtered sodium chloride is reabsorbed, typically presenting during the neonatal period and associated with hypercalciuria and nephrocalcinosis ( Fig. 40-8 ). In contrast, Gitelman syndrome ( Fig. 40-9 ) is a disorder affecting the distal tubule,[210] which is usually diagnosed at a later stage, and is associated with hypocalciuria, hypomagnesemia, with predominant muscular signs and symptoms.[211]



FIGURE 40-8  Schematic representation of transepithelial salt resorption in a cell of the thick ascending limb of the loop of Henle (TAL). Thirty percent of the filtered sodium chloride is reabsorbed in the TAL and most of the energy for concentration and dilution of the urine derives from active NaCl transport in the TAL. Filtered NaCl is reabsorbed through NKCC2, which uses the sodium gradient across the membrane to transport chloride and potassium into cell. The potassium ions are recycled (100%) through the apical membrane by the potassium channel ROMK. Sodium leaves the cell actively through the basolateral Na-K-ATPase. Chloride diffuses passively through two basolateral channels, ClC-Ka and ClC-Kb. Both of these chloride channels must bind to the b subunit of barttin to be transported to the cell surface. Four types of Bartter syndrome (types I, II, III, and IV) are attributable to recessive mutations in the genes that encode the NKCC2 cotransporter, the potassium channel (ROMK), one of the chloride channels (CIC-Kb), and barttin, respectively. A fifth type of Bartter syndrome has also been shown to be a digenic disorder that is attributable to loss-of-function mutations in the genes that encode the chloride channels CIC-Ka and CIC-Kb.[216] As a result of these different molecular alterations, sodium chloride is lost into the urine, positive lumen voltage is abolished, and calcium (Ca2+), magnesium (Mg2+), potassium (K+), and ammonium (NH4+) can not be reabsorbed in the paracellular space. In the absence of mutations, the recycling of potassium maintains a lumen-positive gradient (+8 mV). Claudin 16 (CLDN16) is necessary for the paracellular transport of calcium and magnesium. (Modified from Bichet DG, Fujiwara TM: Reabsorption of sodium chloride—lessons from the chloride channels. N Engl J Med 350:1281–1283, 2004, with permission from The New England Journal of Medicine.)






FIGURE 40-9  Gitelman syndrome: loss-of-function mutations of the thiazide-sensitive Na-Cl cotransporter.



Bartter Syndrome


Bartter syndrome (OMIM 601678, 241200, 607364, and 602522)[11] is an autosomal recessive disorder affecting the function of the thick ascending limb of loop of Henle, giving a clinical picture of salt wasting and hypokalemic metabolic alkalosis. It is caused by inactivating mutations in one of at least four genes encoding membrane proteins (Bartter syndrome I-IV), [213] [214] [215] [216] [217] respectively the Na+-K-2Cl- cotransporter (SLC12A1 encoding NKCC2), the apical inward-rectifying potassium channel (KCNJ1 encoding ROMK), a basolateral chloride channel (ClCNK encoding ClC-Kb), and BSND, a protein that acts as an essential activator β-subunit for ClC-Ka and ClC-Kb chloride channels (see Fig. 40-8 ). Gain-of-function mutations in the extracellular calcium ion-sensing receptor (CaSR) cause a variant of Bartter syndrome [218] [219] with hypocalcemia.

Clinical Presentation

Most cases with Bartter syndrome present antenatally or in neonates. Polyhydramnios and premature labor is a common finding. Polyuria and polydipsia are always present. Post natal findings include failure to thrive, growth retardation, dehydration, low blood pressure, muscle weakness, seizures, tetany, paresthesias, and joint pain from chondrocalcinosis.[219] In contrast to patients with Gitelman syndrome, those with Bartter syndrome are virtually always hypercalciuric and normomagnesemic. Nephrocalcinosis occurs almost universally in Bartter patients harboring NKCC2 (type I) and ROMK (type II) mutations, but in only 20% of those harboring ClC-Kb mutations.[213]This could be attributable to lower urine calcium excretion. Patients with ROMK mutations may show hyperkalemia at birth, which converts to hypokalemia within the first weeks of life.[220] Thus, they can be misdiagnosed with pseudohypoaldosteronism type I (see later). They do not need important K+ supplementation, contrary to other Bartter patients. This could be explained by the fact that ROMK, in addition to being required for sodium reabsorption in the thick ascending limb, is also expressed in the collecting duct. The type III Bartter syndrome (ClC-Kb) phenotype is highly variable and may present either as a typical antenatal variant or as a “classic” Bartter variant characterized by an onset in early childhood and less severe or absent hypercalciuria and nephrocalcinosis. Barttin mutations (type IV Bartter) are usually associated with an extremely severe phenotype with intrauterine onset, profound renal salt and water wasting, renal failure, sensorineural deafness, and motor retardation.[221] Sensorineural deafness is specific for Barttin (type IV) because it is an essential subunit of chloride channels in the inner ear.[222] The severity of type IV Bartter syndrome would be consistent with both ClC-Ka and ClC-Kb contributing to basolateral chloride exit in the TAL. A fifth type of Bartter syndrome also has been shown to be a digenic disorder that is attributable to loss-of-function mutations in the genes that encode the chloride channels ClC-Ka and ClC-Kb.[216]


The treatment of Bartter syndrome usually involves potassium supplements, spironolactone, and non-steroidal anti-inflammatory drugs. Indomethacin has been widely used, as elevated levels of urinary prostaglandin E2 has provided a rationale.[209] Angiotensin I-converting enzyme inhibitors have been used successfully in conjunction with potassium supplements. [224] [225] Therapy should lead to catch-up growth in infants. [226] [227] [228]

Gitelman Syndrome


Gitelman syndrome (OMIM 263800)[11] is a milder disorder compared with Bartter syndrome,[228] and is usually diagnosed in adolescents and adults. It is an autosomal recessive trait caused by inactivating mutations in the SLC12A3 gene encoding the thiazide sensitive Na-Cl cotransporter, or NCCT. [211] [230] This results in sodium and chloride wasting with secondary hypovolemia and metabolic alkalosis. Activation of the renin-angiotensin-aldosterone system from volume depletion, plus increased sodium load to the cortical collecting duct lead to increased sodium reabsorption by the epithelial sodium channel, counterbalanced by potassium and hydrogen excretion, resulting in hypokalemia and metabolic alkalosis. Enhanced passive Ca2+ transport in the proximal tubule rather than active Ca2+ transport in the distal convoluted tubule explains hypocalciuria. Hypomagnesemia remains unexplained. Down-regulation of the epithelial Mg2+ channel transient receptor potential channel subfamily M, member 6 (TRPM6) has been recently demonstrated.[230]

Clinical Presentation

Contrary to Bartter syndrome ( Table 40-7 ), Gitelman syndrome does not present symptomatically in the neonatal period. Cases are often discovered incidentally. Subjects have hypokalemic metabolic alkalosis but in contrast with Bartter syndrome, they are hypocalciuric and hypomagnesemic, and do not have signs of overt volume depletion.[211] Polyuria and polydipsia are not features of Gitelman syndrome either. Arthritis due to chondrocalcinosis in several joints has been described.[231] Urinary prostaglandin E2 levels are normal,[232] compatible with the poor response observed to prostanoid synthetase inhibition. The major differential diagnosis of Gitelman syndrome is diuretic abuse, laxative abuse, and chronic vomiting. A careful history as well as measurement of urinary chloride and detection of diuretics should help differentiate between these conditions.

TABLE 40-7   -- Clinical Differences between Bartter and Gitelman Syndromes


Bartter Syndrome Type I (NKCC2)

Type II (ROMK)


Type IV (Barttin)

Type V (CaSR)

Gitelman Syndrome








Failure to thrive







Growth retardation





















Muscle cramps/spasm





















Sensorineural deafness


















The treatment of Gitelman syndrome includes potassium supplementation and spironolactone.[233] Non-steroidal anti-inflammatory drugs are usually not helpful because prostaglandin levels are normal.


Most patients with hypokalemia and hypertension have essential hypertension associated with the use of diuretics, secondary aldosteronism from renal artery stenosis, or primary hyperaldosteronism from adrenal gland hyperplasia or adenoma. Hereditary causes of hypertension and hypokalemia include excess secretion of aldosterone or other mineralocorticoids and abnormal sensitivity to mineralocorticoids. They are characterized primarily by low or low-normal plasma renin, normal or low serum potassium and salt-sensitive hypertension, suggesting enhanced mineralocorticoid activity (reviewed in Refs 234, 235). The molecular basis for several of these traits has been elucidated.

Congenital Adrenal Hyperplasia

Inherited abnormalities in steroid biosynthesis cause hypertension in some cases of congenital adrenal hyperplasia. These autosomal recessive disorders arise from defici-encies of key enzymes of the steroid biosynthesis pathway (reviewed in Ref 236). The decrease in cortisol production causes an increase in ACTH secretion and subsequent hyperplasia of the adrenal glands. The phenotypes are determined both by deficiencies as well as overproduction of steroids unaffected by the enzymatic defect. Hypertension is observed in only two of the three major subtypes of congenital adrenal hyperplasia (11β-hydroxylase and 17α-hydroxylase deficiencies), as metabolic blockade distal to 21α-hydroxylase allows the formation of 21-hydroxyl groups necessary for mineralocorticoid precursor biosynthesis. Other clinical manifestations are dependent on the effects of the enzymatic defect on androgen biosynthesis with either an increase (11β-hydroxylase) or a decrease (17α-hydroxylase) in the production. In both deficiencies, overproduction of cortisol precursors that are metabolized to mineralocorticoid agonists or that have intrinsic mineralocorticoid activity induces volume and salt-dependent forms of hypertension. The elevated zona fasciculata deoxycorticosterone (DOC) produces mineralocorticoid hypertension with suppressed renin and reduced potassium concentrations. Aldosterone, the most important mineralocorticoid, regulates electrolyte excretion and intravascular volume mainly through its effects on renal distal convoluted tubules and cortical collecting ducts.

11β-hydroxylase Deficiency

Inactivating mutations in the gene encoding 11β-hydroxylase[237] cause the second most common form of congenital adrenal hyperplasia (OMIM 202010),[11] representing 5% of cases (90% are caused by 21-hydroxylase deficiency). This disease is associated with excess production of DOC, 18-deoxycortisol, and androgens. By virtue of the significant intrinsic mineralocorticoid activity of DOC, subjects harboring mutations in both alleles of the gene exhibit hypokalemic hypertension. Because the androgen pathway is unaffected, prenatal masculinization occurs in females and post natal virilization occurs in both sexes. The diagnosis of 11β-hydroxylase is made by the detection of increased levels of DOC and 18-deoxycortisol.[238] The treatment consists of exogenous corticoids that inhibit ACTH secretion. Correction of mild salt wasting from reduced mineralocorticoid production may be necessary.[239]

17α-hydroxylase Deficiency

17α-hydroxylase deficiency (OMIM 202110)[11] results in reduced conversion of pregnenolone to progesterone and androgens and absent sex hormone production. The absence of sex hormone formation in both the adrenal glands and the gonads causes hypogonadism and male pseudohermaphroditism, and is usually detected at adolescence because of failure to undergo puberty. Elevated glucocorticoid-suppressible levels of DOC and corticosterone as well as their 18-hydroxylated products are responsible for hypertension, hypokalemia, and renin and aldosterone suppression. The clinical features vary depending on the enzymatic activity affected.[236] In severe 17α-hydroxylase deficiency, both the 17α-hydroxylase and 17,20-lyase activities are reduced or absent. This results in excess mineralocorticoid activity, hypertension, and produces a female phenotype in all subjects caused by absent sex steroid production in both the adrenal and gonad. Partial 17α-hydroxylase deficiency leads to sexual ambiguity in males without hypertension. Corticosteroid replacement corrects ACTH levels and hypertension. Women usually require hormonal therapy. Genetic males reared as females also require estrogen replacement. Genetic males reared as males require surgical correction of their external genitalia and androgen replacement therapy.[236]

Liddle Syndrome


Liddle syndrome (OMIM 177200)[11] is an autosomal dominant form of hypertension characterized by hypokalemia and low levels of plasma renin and aldosterone, resulting from either premature termination or frameshift mutations in the carboxy-terminal tail of the epithelial sodium channel (ENaC) β- or γ-subunits.[234] The amiloride-sensitive epithelial Na+ channel is a tetramer formed by the assembly of three homologous subunits, a, b, and g, with the a subunit being present in two copies[240] ( Fig. 40-10 ). The NH2 and carboxy-terminal terminal segments are cytoplasmic and contain potential regulatory segments that are able to modulate the activity of the channel. Mutations in the b and g subunits of ENaC lead to channel hyperactivity by deleting or altering a conserved proline-rich amino acid sequence referred to as the PY-motif. SCNN1B β-subunit mutations or SCNN1G γ-subunit mutations could lead to an increase in the number of channels in the membrane or in their “openness”. The identification of specific binding domains for Nedd4 (for all subunits) and a spectrin (for the a subunit only) within the cytosolic carboxy-terminal region of the ENaC subunits suggests that interactions with cytoskeletal elements control the expression of the ENaC at the apical membrane. Therefore, Nedd4 and a spectrin appear to play a role in the assembly, insertion, and/or retrieval of the ENaC subunits in the plasma membrane.[241] Mutations affecting the Neddd4 binding domain also affect ubiquitination of ENaCs, thus favoring ENaC residence in the apical membrane and enhanced sodium reabsorption (reviewed in Ref 242).



FIGURE 40-10  The ENaC channel is composed of two a, one b, and one g subunit surrounding the channel pore.[240] Each subunit has two transmembrane domains with short cytoplasmic amino and carboxy termini and a large ectocytoplasmic loop. Mutations in subunits of ENaC cause either Liddle syndrome (b or g subunits) or the autosomal recessive form of pseudohypoaldosteronism-1 (a, b, or g subunits).[341] The autosomal dominant form of pseudohypoaldosteronism type 1 is secondary to mutations in the mineralocorticoid receptor gene.[263] These ENaC and mineralocorticoid receptor mutations recapitulate the main pathway for sodium reabsorption and potassium secretion accross the principal cell of the cortical and medullary collecting duct. Sodium transport in tight epithelia of the distal nephron is mediated by the epithelial sodium channel (ENaC) and the Na, K-ATPase. The ENaC is located at the apical membrane and constitutes the rate limiting step for electrogenic sodium transport, whereas the Na, K-ATPase, located at the basolateral membrane creates the driving form for this process. Note that only the a subunit is glycosylated. The mechanism of ENaC expression in an aldosterone-sensitive epithelial cell is also represented. A, In a resting state, few ENaCs, which facilitate sodium reabsorption in a rate-limiting fashion, are resident in the apical membrane. Factors known to enhance ENaC surface expression and activity are counterbalanced by retrieval of these channels from the membrane through the ubiquitination pathway mediated by Nedd4-2. B, Shortly after aldosterone exposure and binding to the MR, transcriptional stimulation of SgK1 leads to phosphorylation of Nedd4-2, which subsequently disrupts ENaC/Nedd4-2 interactions. In this situation, ubiquitination of ENaCs is reduced, thus favoring ENaC residence in the apical membrane and enhanced sodium reabsorption.



Clinical Presentation

Liddle syndrome is characterized clinically by inappropriate renal sodium reabsorption, blunted sodium excretion, and low-renin hypertension (see review by Ref 243). The features of this syndrome were described by Liddle and colleagues in 1963 in a large pedigree,[244] and were normalized by triamterene but not by spironolactone, a mineralocorticoid receptor antagonist. Affected subjects are at increased risk of cerebrovascular and cardiovascular accidents. Liddle syndrome can be differentiated from other rare Mendelian forms of low-renin hypertension with urinary/plasma hormonal profiles. Glucocorticoid remediable aldosteronism is associated with increased production of 18-hydroxy cortisol and aldosterone metabolites. Apparent mineralocorticoid excess (reviewed in Ref 245) is associated with increased urinary cortisol (tetrahydrocortisol) over cortisone (tetrahydrocortisone) metabolites ( Table 40-8 ).

TABLE 40-8   -- Urinary Steroid Profiles in Mendelian Forms of Low Renin Hypertension


Liddle Syndrome

Glucocorticoid-Remediable Aldosteronism

Apparent Mineralocorticoid Excess













18-OH F

Not detected


Not detected













Adapted from Warnock DG: Liddle syndrome: An autosomal dominant form of human hypertension. Kidney Int 53:18–24, 1998.

TH-Aldo, tetrahydroxyaldosterone.






Hypertension is not improved by spironolactone but can be corrected by a low-salt diet and ENaC antagonists (amiloride or triamterene).

Apparent Mineralocorticoid Excess


The syndrome of apparent mineralocorticoid excess (AME, OMIM 207765[11]) is a rare autosomal recessive disorder that results in hypokalemic hypertension, with low serum levels of renin and aldosterone (reviewed in Ref 246). AME is caused by a deficiency in 11β-hydroxysteroid dehydrogenase type 2 enzymatic activity (11β HSD2), responsible for the conversion of cortisol to the inactive metabolite cortisone, therefore protecting the mineralocorticoid receptors from cortisol intoxication. [248] [249] In AME, cortisol acts as a potent mineralocorticoid and causes salt retention, hypertension, and hypokalemia with a suppression of the renin-angiotensin-aldosterone system. A milder phenotype, or type 2 variant, also results from abnormal activity of the enzyme.[249] 11β HSD2 may be inhibited by licorice (glycyrrhetinic acid) explaining increased mineralocorticoid receptor activity and the subsequent hypertension.[250] Cushing syndrome and extremely high cortisol levels can overcome the ability of 11β HSD2 to convert cortisol to cortisone.

Clinical Presentation

Apparent mineralocorticoid excess is associated with severe juvenile low-renin hypertension, hypokalemic alkalosis, low birth weight, failure to thrive, poor growth, and nephrocalcinosis.[251] The urinary metabolites of cortisol demonstrate an abnormal ratio with predominance of cortisol metabolites (i.e., tetrahydrocortisol plus 5 α-tetrahydrocortisol/tetrahydrocortisone in the range 6.7–33, whereas the normal ratio is 1.0).[252] The milder form of AME (“type II”) lacks the typical urinary steroid profile (i.e., biochemical analysis reveals a moderately elevated cortisol to cortisone metabolite ratio).[249] The heterozygote state is phenotypically normal but associated with subtle defects in cortisol metabolism.


The treatment of AME is sodium restriction and either triamterene or amiloride. Spironolactone is not effective. Additional antihypertensive agents may be used as needed.

Autosomal Dominant Early-Onset Hypertension with Severe Exacerbation during Pregnancy

This is a rare autosomal dominant disorder described in a family that is associated with activating mutations in the mineralocorticoid receptor (OMIM 605115). [11] [254] By screening the mineralocorticoid receptor in 75 patients with early onset of severe hypertension, Geller and colleagues identified a 15-year-old boy with severe hypertension, suppressed plasma renin activity, low aldosterone, and no other underlying cause of hypertension, harboring a heterozygous missense mutation (S810L) in the mineralocorticoid receptor gene. Of 23 relatives evaluated, 11 had been diagnosed with severe hypertension before age 20, whereas the remaining 12 had unremarkable blood pressures. Two L810 carriers had undergone five pregnancies; all had been complicated by marked exacerbation of hypertension with suppressed aldosterone levels. The S810L mutation alters a conserved amino acid and results in constitutive and altered mineralocorticoid receptor activity, with progesterone and other steroids lacking 21-hydroxyl groups, normally mineralocorticoid receptor antagonists, becoming potent agonists. Spironolactone was also a potent agonist of L810, suggesting that this medication is contraindicated in L810 carriers.

Glucocorticoid-Remediable Hyperaldosteronism


This disease, also known as familial hyperaldosteronism type I (OMIM 103900),[11] aldosteronism sensitive to dexamethasone, glucocorticoid-suppressible hyperaldosteronism, and syndrome of ACTH-dependent hyperaldosteronism is an autosomal dominant hypertensive disorder caused by a chimeric gene duplication ( Fig. 40-11 ) arising from unequal crossover between aldosterone synthase and 11β-hydroxylase,[254] two highly similar genes with the same transcriptional orientation lying 45,000 base pairs apart on chromosome 8. Humans have two isozymes with 11β-hydroxylase activity that are respectively required for cortisol and aldosterone synthesis. CYP11B1 (11β-hydroxylase) is expressed at high levels and is regulated by ACTH, whereas CYP11B2 (aldosterone synthase) is normally expressed at low levels and is regulated by angiotensin II. In addition to 11β-hydroxylase activity, the latter enzyme has 18-hydroxylase and 18-oxidase activities and can synthesize aldosterone from deoxycorticosterone (reviewed in Ref 255). Thus, the unequal crossover between the two genes will result in the aldosterone synthase gene being under the control of regulatory promoter sequences of the 11β-hydroxylase. The chimeric gene product is expressed at high levels in both the zona glomerulosa and zona fasciculata and is controlled by ACTH. This leads to increased production of 18-hydroxy cortisol and aldosterone metabolites.



FIGURE 40-11  The chimeric gene duplication causing glucocorticoid-remediable aldosteronism (GRA). Both genes are linked on chromosome 8 and are separated by 45kb.[342]



Clinical Presentation

The phenotype of GRA is highly variable.[256] Affected individuals can have mild hypertension and normal biochemistry, and be clinically indistinguishable from patients with essential hypertension. However, some subjects have early onset severe hypertension, hypokalemia, and metabolic alkalosis. In a study of 376 patients from 27 genetically proven GRA pedigrees, 48% of all GRA families and 18% of all GRA patients had cerebrovascular complications, which is similar to the frequency of aneurysm in adult polycystic kidney disease.[257] The diagnosis is usually established by measuring 18-hydroxy or 18-oxocortisol metabolites in the urine or with the dexamethasone suppression test.[258] In addition, because they secrete aldosterone in response to ACTH, glucocorticoid administration can suppress excessive aldosterone secretion.[259] The dexamethasone suppression test is a variably reliable method for establishing the diagnosis. Cases without the disease (i.e., subjects with an aldosterone producing adenoma or with idiopathic hyperaldosteronism) can suppress aldosterone secretion.[260] The diagnosis of GRA can be definitively established by demonstrating the chimeric gene by molecular techniques.[254]


Simple glucocorticoid replacement is the treatment for GRA. Salt restriction combined with either spironolactone or ENaC inhibition, are also effective.


Familial hyperaldosteronism type II (FH-II, OMIM 605635[11]) is characterized by hypersecretion of aldosterone due to adrenocortical hyperplasia, an aldosterone-producing adenoma, or both. In contrast to familial hyperaldosteronism type I, FH-II is not suppressible by dexamethasone. Stowasser and colleagues[258] reported five families with this phenotype with a segregation pattern supporting dominant inheritance. Analysis of an extended kindred has found linkage between familial hyperaldosteronism type 2 and markers on chromosome 7p22.[261]


Pseudohypoaldosteronism Type I


Pseudohypoaldosteronism type I (PHA-I) is a rare disorder characterized by salt wasting, hypotension, hyperkalemia, metabolic acidosis, and failure to thrive in infants. There are two subtypes of PHA-I.[262] The autosomal recessive form (OMIM 264350 and 177735)[11] leads to severe manifestations that persist in adulthood and is caused by inactivating mutations in any of the three subunits (a, b, g) of the epithelial sodium channel (ENaC). The autosomal dominant form (OMIM 177735)[11] is associated with milder manifestations that remit with age and is caused by mutations in the mineralocorticoid receptor (MR) gene[263] resulting in haploinsufficiency or in dominant negative actions. Homozygous MR mutations are probably lethal in humans because knockout mice show an important salt wasting syndrome and die a few days after birth.[264]

Clinical Presentation

The clinical contrast between PHA-I due to ENaC or MR mutations is striking.[262] Autosomal recessive PHA-I presents neonatally or in childhood and is characterized by renal salt wasting, hypotension, hyperkalemia, metabolic acidosis, and on occasion, failure to thrive. Other biological features include hyponatremia, high plasma and urinary aldosterone levels despite hyperkalemia, and elevated plasma renin activity. Autosomal dominant PHA-I presents with milder manifestations with remission of the syndrome with age. Differential diagnosis must be made with aldosterone synthase deficiency, salt-wasting forms of congenital adrenal hyperplasia, and adrenal hypoplasia congenita, which all cause aldosterone deficiency, and are associated with hyponatremia, hyperkalemia, hypovolemia, elevated plasma renin activity, and sometimes shock and death (reviewed in Ref 265). Bartter syndrome II (ROMK gene mutations) can also present in the neonatal period with a similar (transient) clinical picture.


Treatment consists of salt supplementation, which can greatly improve hyponatremia, hyperkalemia, and growth. Administration of aldosterone, fludrocortisone, and deoxycorticosterone is not helpful. Patients with the recessive form usually need lifelong treatment for salt wasting and hyperkalemia, whereas in the dominant form, treatment can usually be withdrawn in adulthood.[234]

Pseudohypoaldosteronism Type II


Pseudohypoaldosteronism type II (PHA-II; OMIM 145260)[11] also known as familial hyperkalemia and hypertension or Gordon syndrome, is a volume-dependent low-renin form of hypertension characterized by persistent hyperkalemia despite a normal renal glomerular filtration rate.[266] Hypertension is attributable to increased renal salt reabsorption and the hyperkalemia to reduced renal K+ excretion. Reduced renal H+ secretion is also commonly seen, resulting in metabolic acidosis. The features of PHA-II are chloride-dependent, because they are corrected when infusion of sodium sulfate or sodium bicarbonate is substituted for sodium chloride.[267] In addition, these abnormalities are ameliorated by thiazide diuretics, which inhibit salt reabsorption in the distal nephron by the electroneutral Na-Cl cotransporter.

The disease is genetically heterogeneous and three loci have now been mapped to chromosomes 17, 1, and 12.[268] Two genes causing PHA-II have been identified on chromosomes 12 and 17.[269] Both genes encode members of the WNK (WNK, with no lysine [K]) family of serine-threonine kinases that localize to the distal nephron. WNK1 and WNK4 interact with other kinases, which are involved in the regulation of ion transporters.[270] WNK1 and WNK4 function as molecular switches, eliciting coordinated effects on diverse ion transport pathways to maintain homeostasis during physiological perturbation. In vitro, WNK1 inhibits ROMK by stimulating its endocytosis.[271]In PHA-II, mutations that appear to activate WNK1 and inactivate WNK4 have been proposed to result in increased TSC activity. On one hand is the overactivity of the thiazide-sensitive Na+-Cl- cotransporter (TSC), and on the other hand is the increased paracellular reabsorption of Cl-, also known as the “chloride shunt” hypothesis (reviewed in Ref 272). It is possible that WNK mutations behave as loss-of-function for the negative regulation of TSC and gain-of-function when it comes to ROMK channel endocytosis. WNK4 negatively regulates surface expression of NCCT, thus implicating loss of this regulation in the molecular pathogenesis of the disorder.[273] The action of these kinases may serve to increase transcellular or paracellular chloride conductance in the collecting duct, thereby increasing salt reabsorption and intravascular volume, while concomitantly dissipating the electrical gradient and diminishing K+ and H+ secretion. Recent data suggest that WNK1 plays a general role in the regulation of epithelial Cl- flux.[274]

Clinical Presentation

Pseudohypoaldosteronism-II is usually diagnosed in adults but can also be seen neonatally.[275] Unexplained hyperkalemia is the usual presenting feature and occurs prior to the onset of hypertension. The severity of hyperkalemia varies greatly and is influenced by prior intake of diuretics and salt intake. Causes of spurious elevation of potassium should be ruled out before this diagnosis is made. In its most severe form, it is associated with muscle weakness (from hyperkalemia), short stature, and intellectual impairment. Mild hyperchloremia, metabolic acidosis, and suppressed plasma renin activity are findings variably associated with the trait. Aldosterone levels vary from low to high depending on the level of hyperkalemia. Urinary concentrating ability, acid excretion, and proximal tubular function are all normal.


Thiazides reverse all biochemical abnormalities. Lower than average doses can be given if overcorrection is seen. Loop diuretics may also be used.


Magnesium is the second most abundant intracellular cation and plays an important role for protein synthesis, nucleic acid stability, neuromuscular excitability, and oxidative phosphorylation. Under normal conditions, extracellular magnesium concentration is maintained at nearly constant values. Hypomagnesemia results from decreased dietary intake, intestinal malabsorption, or renal loss.

Primary hypomagnesemia is composed of a heterogeneous group of disorders characterized by renal and intestinal Mg2+ wasting, often associated with hypercalciuria ( Table 40-9 ).[276] The genetic basis and cellular defects of a number of primary hypomagnesemias have been elucidated ( Table 40-10 ). These inherited conditions affect different nephron segments and different cell types and lead to variable but increasingly distinguishable phenotypic presentations.

TABLE 40-9   -- Hereditary Magnesium-Losing Disorders: Clinical and Biochemical Characteristics


Age at Onset

Serum Mg2+

Serum Ca2+

Serum K+

Blood pH

Urine Mg2+

Urine Ca2+


Renal Stones

Isolated dominant hypomagnesemia with hypocalciuria







Isolated recessive hypomagnesemia with normocalciuria








Familial hypomagnesemia with hypercalciuria/nephrocalcinosis




N or ↓





Hypomagnesemia with secondary hypocalcemia








Autosomal dominant hypoparathyreoidism



N or ↓




Antenatal Bartter syndrome/hyperprostaglandin E syndrome









Classic Bartter syndrome


N or ↓



N to ↑




Antenatal Bartter syndrome with sensorineural deafness






N to ↑



Gitelman syndrome






Adapted from Konrad M, Weber S: Recent advances in molecular genetics of hereditary magnesium-losing disorders. J Am Soc Nephrol 14:249–260, 2003.




TABLE 40-10   -- Inherited Disorders of Magnesium Handling




Gene Locus



Isolated dominant hypomagnesemia with hypocalciuria





γ subunit of the Na+-K+-ATPase






Isolated recessive hypomagnesemia with normocalciuria






Familial hypomagnesemia with hypercalciuria/nephrocalcinosis





Claudin 16, tight junction protein

Hypomagnesemia with secondary hypocalcemia





TRPM6, putative ion channel

Autosomal dominant hypoparathyroidism





CASR, Ca2+/Mg2+ sensing receptor

Antenatal Bartter syndrome/hyperprostaglandin E syndrome





NKCC2, Na+K+2Cl- cotransporter





ROMK, renal potassium channel

Classic Bartter syndrome





CLC-Kb, distal tubule chloride channel

Antenatal Bartter syndrome with sensorineural deafness





Barttin, chloride channel βsubunit

Gitelman syndrome





NCCT, Na+Cl cotransporter

Adapted from Konrad M, Weber S: Recent advances in molecular genetics of hereditary magnesium-losing disorders. J Am Soc Nephrol 14:249–260, 2003.




Familial Hypomagnesemia with Hypercalciuria and Nephrocalcinosis

The syndrome of renal hypomagnesemia with hypercalciuria and nephrocalcinosis (OMIM 248250)[11] is a rare autosomal recessive trait characterized by profound Mg2+ wasting that results in severe hypomagnesemia not corrected by oral or intravenous magnesium supplementation. The disorder is caused by mutations in claudin 16 (CLDN 16), previously known as paracellin-1, [278] [279] a protein located in tight junctions of the thick ascending loop of Henle, related to the claudin family of tight junction proteins (see Fig. 40-8 ). Renal calcium wasting leads to parenchymal calcification and renal failure. Other clinical findings include polyuria, polydipsia, ocular abnormalities, recurrent urinary tract infections, and renal colics with stone passage. Bilateral nephrocalcinosis is observed in all cases. Every patient shows hypomagnesemia with inappropriately high urinary Mg2+ excretion (Mg2+ fractional excretions of 16.2±7.1%). Hypercalciuria is present in every case except in those with advanced renal insufficiency. Serum parathormone levels are abnormally high. Serum calcium, phosphorus and potassium, and urinary excretions of uric acid and oxalate are normal. Neither chronic oral Mg2+ administration nor thiazide diuretics normalize serum Mg2+ levels or urinary Ca excretion. Renal function worsens in every case, with several patients requiring chronic dialysis. The progression rate of renal insufficiency correlates with the severity of nephrocalcinosis. After kidney graft, tubular handling of Mg2+ and Ca2+ is normal.

Familial Hypomagnesemia with Secondary Hypocalcemia

Familial hypomagnesemia with secondary hypocalcemia (OMIM 602014)[11] is an autosomal recessive disease that results in electrolyte abnormalities shortly after birth, caused by mutations in TRPM6 ( Fig. 40-12 ). [280] [281]TRPM6 protein is a new member of the long transient receptor potential channel (TRPM) family. It is similar to TRPM7, which encodes a protein with features of a calcium and magnesium permeable divalent cation channel. TRPM7 is particular in being both an ion channel and a serine/threonine kinase.[281] Affected individuals show severe hypomagnesemia and hypocalcemia, which lead to seizures and tetany. The disorder has been thought to be caused by a defect in the intestinal absorption of magnesium, rather than by abnormal renal losses. Restoring the concentrations of serum magnesium to normal values by high-dose magnesium supplementation can overcome the apparent defect in magnesium absorption and in serum concentrations of calcium. Life-long magnesium supplementation is required to overcome the defect in magnesium handling by these individuals.



FIGURE 40-12  Magnesium reabsorption in the distal convoluted tubule. In this seg-ment, magnesium is reabsorbed by an active transcellular pathway involving an apical entry step probably via a magnesium permeable ion channel and a basolateral exchange mechanism, presumably a Na+/Mg2+ exchanger. The molecular identity of this exchanger is still unknown. HSH, hypomagnesemia with secondary hypocalciuria; GS, Giltelman syndrome; IDH, isolated dominant hypomagnesemia; ADH, autosomal dominant hypoparathyroidism; BSND, antenatal Bartter syndrome with sensorineural deafness; cBS, classical Bartter syndrome.  (Modified from Konrad M, Weber S: Recent advances in molecular genetics of hereditary magnesium-losing disorders. J Am Soc Nephrol 14:249–260, 2003.)




Isolated Dominant Hypomagnesemia with Hypocalciuria

This is a rare autosomal dominant disorder (see Fig. 40-12 ) (OMIM 154020)[11] caused by a dominant negative mutation of the FXYD2 gene resulting in a trafficking defect of the g subunit of the Na+/K+ ATPase at the basolateral membrane of the distal convoluted tubule.[282] Defective activity of the Na+/K+ ATPase can lead to either depolarization, reduced intracellular K+, or increased intracellular Na+. Abnormal Na+/K+ ATPase activity in the distal convoluted tubule could explain reduced Mg2+ influx and Mg2+ wasting. Hypomagnesemic members have lower urinary excretion of calcium, presumably as a consequence of increased reabsorption in the loop of Henle.[283]

Ca2+/Mg2+-Sensing Receptor-Associated Disorders

An important regulator of magnesium homeostasis is the Ca2+/Mg2+-sensing receptor (CASR).[284] Activating mutations of the CASR gene were first described in families affected with autosomal dominant hypocalcemia (ADH). Affected individuals present with hypocalcemia, hypercalciuria, and polyuria, and about 50% of these patients have hypomagnesemia.[276]



The conservation of water by the human kidney is a function of the complex architecture of renal tubules within the renal medulla.[285] The principal cells of the renal collecting tubules are responsive to the neurohypophyseal antidiuretic hormone arginine vasopressin (AVP). The major action of AVP is to facilitate urinary concentration by allowing water to be transported passively down an osmotic gradient between the tubular fluid and the surrounding interstitium.

The first step in the antidiuretic action of AVP is its binding to the vasopressin V2 receptor (AVPR2 in Fig. 40-13 ) located on the basolateral membrane of collecting duct cells. This step initiates a cascade of events—receptor-linked activation of G protein (Gs), activation of adenylyl cyclase, production of cyclic adenosine-monophosphate (cAMP), and stimulation of protein kinase A (PKA)—that leads to the final step in the antidiuretic action of AVP, that is, the exocytic insertion of specific water channels, aquaporin 2 (AQP2), into the luminal membrane, thereby increasing the water permeability of that membrane. These water channels are members of a superfamily of integral membrane proteins that facilitate water transport.[286] Aquaporin 1 (AQP1, also known as CHIP, channel-forming integral membrane protein of 28 kDa) was the first protein shown to function as a molecular water channel and is constitutively expressed in mammalian red blood cells, renal proximal tubules, thin descending limbs of the loop of Henle, and other water-permeable epithelia.[287] Murata and colleagues[288] have described an atomic-model of AQP-1 at 3.8Å resolutions, and “real-time” molecular dynamics simulations of water permeation through human AQP1 have been obtained by de Groot and Grubmüller.[289] The latter have proposed that conserved fingerprint (asparagine-proline-alanine [NPA]) motifs form a selectivity-determining region; a second (aromatic/arginine) region is also proposed to function as proton filter. These data have thus solved a longstanding physiologic puzzle—how membranes can be freely permeable to water but impermeable to protons. At the subcellular level, AQP1 is localized in both apical and basolateral plasma membranes that may represent entrance and exit routes for transepithelial water transport. In contrast to AQP2, limited amounts of AQP1 are localized in membranes of vesicles or vacuoles. In the basolateral membranes, AQP1 is localized to both basal and lateral infoldings. AQP2 is the vasopressin-regulated water channel in renal collecting ducts. It is exclusively present in principal cells of inner medullary collecting duct cells and is diffusely distributed in the cytoplasm in the euhydrated condition, whereas apical staining of AQP2 is intensified in the dehydrated condition or after vasopressin administration. These observations are thought to represent the exocytic insertion of preformed water channels from intracellular vesicles into the apical plasma membrane (the shuttle hypothesis) (see Fig. 40-13 ). The short-term AQP2 regulation by AVP involves the movement of AQP2 from the intracellular vesicles to the luminal membrane; in the long-term regulation, which requires a sustained elevation of circulating AVP for 24 hours or more, AVP increases the abundance of water channels. This is thought to be a consequence of increased transcription of the AQP2 gene.[290] AQP3 and AQP4 are the water channels in basolateral membranes of renal medullary collecting ducts. In addition, vasopressin also increases the water reabsorptive capacity of the kidney by regulating the urea transporter UT-A1 expressed in the inner medullary collecting duct, predominantly in its terminal part.[291] AVP also increases the permeability of principal collecting duct cells to sodium.[292] In summary, in the absence of AVP stimulation, collecting duct epithelia exhibit very low permeabilities to sodium, urea, and water. These specialized permeability properties permit the excretion of large volumes of hypotonic urine formed during intervals of water diuresis. In contrast, AVP stimulation of the principal cells of the collecting ducts leads to selective increases in the permeability of the apical membrane to water (Pf), urea (Purea), and Na (PNa).



FIGURE 40-13  Schematic representation of the effect of AVP to increase water permeability in the principal cells of the collecting duct. AVP is bound to the V2 receptor (a G-protein-linked receptor) on the basolateral membrane. The basic process of G-protein-coupled receptor signaling consists of three steps: a hepta-helical receptor that detects a ligand (in this case, AVP) in the extracellular milieu, a G-protein (G) that dissociates into a subunits bound to GTP and bg subunits after interaction with the ligand-bound receptor, and an effector (in this case, adenylyl cyclase) that interacts with dissociated G-protein subunits to generate small-molecule second messengers. AVP activates adenylyl cyclase increasing the intracellular concentration of cAMP. The topology of adenylyl cyclase is characterized by two tandem repeats of six hydrophobic transmembrane domains separated by a large cytoplasmic loop and terminates in a large intracellular tail. The dimeric structure (C1 and C2) of the catalytic domains is represented (see text). Conversion of ATP to cAMP takes place at the dimer interface. Two aspartate residues (in C1) coordinate two metal co-factors (Mg2+ or Mn2+ represented here as two small black circles) that enable the catalytic function of the enzyme.[343] Adenosine is the large open circle and the three phosphate groups (ATP) are the three small open circles. PKA is the target of the generated cAMP. The binding of cAMP to the regulatory subunits of PKA induces a conformational change, causing these subunits to dissociate from the catalytic subunits. These activated subunits (C) as shown here are anchored to an AQP2 containing endocytic vesicle via an A-kinase anchoring protein (AKAP). The local concentration and distribution of the cAMP gradient is limited by phosphodiesterases (PDEs). Cytoplasmic vesicles carrying the water channel proteins (represented as homotetrameric complexes) are fused to the luminal membrane in response to AVP, thereby increasing the water permeability of this membrane. The dissociation of AKAP from the endocytic vesicle is not represented. Microtubules and actin filaments are necessary for vesicle movement toward the membrane. When AVP is not available, AQP2 water channels are retrieved by an endocytic process, and water permeability returns to its original low rate. AQP3 and AQP4 water channels are expressed constitutively at the basolateral membrane.



In nephrogenic diabetes insipidus (NDI), the kidney is unable to concentrate urine despite normal or elevated concentrations of the AVP. In congenital NDI, the obvious clinical manifestations of the disease, that is, polyuria and polydipsia, are present at birth and must be immediately recognized to avoid severe episodes of dehydration. Most (>90%) of congenital patients with NDI have mutations in the AVPR2 gene, the Xq28 gene coding for the vasopressin V2 (antidiuretic) receptor. In less than 10% of the families studied, congenital NDI has an autosomal recessive inheritance and mutations have been identified in the AQP2 gene (AQP2) located in chromosome region 12q13; that is, the vasopressin-sensitive water channel. One hundred eighty-three different putative disease-causing mutations in the AVPR2 gene have now been published in 284 unrelated families with X-linked NDI. When studied in vitro, most AVPR2 mutations lead to receptors that are trapped intracellularly and are unable to reach the plasma membrane. A minority of the mutant receptors reach the cell surface but are unable to bind AVP or to trigger an intracellular cAMP signal. Similarly, AQP2 mutant proteins are trapped intracellularly and cannot be expressed at the luminal membrane. This AQP2-trafficking defect is correctable, at least in vitro, by chemical chaperones. Other inherited disorders with mild, moderate, or severe inability to concentrate urine include Bartter syndrome (MIM601678),[219] cystinosis, and autosomal dominant hypocalcemia. [30] [277]

Clinical Presentation

Loss-of-Function Mutations of the AVPR2

X-linked NDI (OMIM 304800) is secondary to AVPR2 mutations, which result in a loss-of-function or dysregulation of the V2 receptor.[293]

Males who have an AVPR2 mutation have a phenotype characterized by early dehydration episodes, hypernatremia, and hyperthermia as early as the first week of life. Dehydration episodes can be so severe that they lower arterial blood pressure to a degree that is not sufficient to sustain adequate oxygenation to the brain, kidneys, and other organs. Mental and physical retardation and renal failure are the classical “historical” consequences of a late diagnosis and lack of treatment. Heterozygous females exhibit variable degrees of polyuria and polydipsia because of skewed X chromosome inactivation.[294]

The “historical” clinical characteristics include hypernatremia, hyperthermia, mental retardation, and repeated episodes of dehydration in early infancy.[295] Mental retardation, a consequence of repeated episodes of dehydration, was prevalent in the Crawford and Bode study,[295] in which only 9 of 82 patients (11%) had normal intelligence. Early recognition and treatment of X-linked NDI with an abundant intake of water allows a normal life span with normal physical and mental development.[296] Two characteristics suggestive of X-linked NDI are the familial occurrence and the confinement of mental retardation to male patients. It is then tempting to assume that the family described in 1892 by McIlraith[297] and discussed by Reeves and Andreoli[298] was an X-linked NDI family. Lacombe[299] and Weil[300] described a familial form of diabetes insipidus with an autosomal type of transmission and without any associated mental retardation. The descendants of the family originally described by Weil were later found to have autosomal dominant neurogenic diabetes insipidus (OMIM 192340), a now well-characterized entity secondary to mutations in the prepro-arginine-vasopressin-neurophysin II gene (AVP).[301] More than 50 AVP mutations segregating with autosomal dominant or autosomal recessive neurohypophyseal diabetes insipidus have been described (see for a list of mutations). Patients with autosomal dominant neurogenic diabetes insipidus retain some limited capacity to secrete AVP during severe dehydration and the polyuro-polydipsic symptoms usually appear after the first year of life, when the infant's demand for water is more likely to be understood by adults.

The early symptomatology of the nephrogenic disorder and its severity in infancy is clearly described by Crawford and Bode.[295] The first manifestations of the disease can be recognized during the first week of life. The infants are irritable, cry almost constantly, and although eager to suck, will vomit milk soon after ingestion unless pre-fed with water. The history given by the mothers often includes persistent constipation, erratic unexplained fever, and failure to gain weight. Even though the patients characteristically show no visible evidence of perspiration, increased water loss during fever or in warm weather exaggerates the symptoms. Unless the condition is recognized early, children will experience frequent bouts of hypertonic dehydration, sometimes complicated by convulsions or death; mental retardation is a frequent consequence of these episodes. The intake of large quantities of water, combined with the patient's voluntary restriction of dietary salt and protein intake, lead to hypocaloric dwarfism beginning in infancy. Affected children frequently develop lower urinary tract dilatation and obstruction, probably secondary to the large volume of urine produced. Dilatation of the lower urinary tract is also seen in primary polydipsic patients and in patients with neurogenic diabetes insipidus. [303] [304] Chronic renal insufficiency may occur by the end of the first decade of life and could be the result of episodes of dehydration with thrombosis of the glomerular tufts.[295]

In 1989, we observed that the administration of 1-desamino-8-D-arginine vasopressin (desmopressin acetate, dDAVP), a V2 receptor agonist, increased plasma cAMP concentrations in normal subjects but had no effect in 14 male patients with X-linked NDI.[304] Intermediate responses were observed in obligate carriers of the disease, possibly corresponding to half of the normal receptor response. Based on these results, we predicted that the defective gene in these patients with X-linked NDI was likely to code for a defective V2 receptor.[304] Since that time, a number of experimental results have confirmed our hypothesis: (1) The NDI locus was mapped to the distal region of the long arm of the X chromosome, Xq28; (2) the V2 receptor was identified as a candidate gene for NDI; (3) the human V2 receptor was cloned,[305] and (4) 183 putative disease-causing mutations have now been identified in the V2receptor and the list of new mutations is still expanding ( Fig. 40-14 ).



FIGURE 40-14  Schematic representation of the V2 receptor and identification of 183 putative disease-causing AVPR2 mutations. Predicted amino acids are given as the one-letter code. Solid symbols indicate missense or nonsense mutations; a number indicates more than one mutation in the same codon. The names of the mutations were assigned according to recommended nomenclature.[344] The extracellular, transmembrane, and cytoplasmic domains are defined according to Mouillac B, Chini B, Balestre M-N, et al: The binding site of neuropeptide vasopressin V1a receptor. J Biol Chem 270:25771–25777, 1995. The common names of the mutations are listed by type. Eighty-nine missense, 18 nonsense mutations, 45 frameshift, 7 inframe deletions or insertions, 4 splice-site, as well as 19 large deletions and 1 complex mutation have been identified.



Generally, X-linked NDI is a rare disease with an estimated prevalence of approximately 8.8 per million male live births in the Province of Quebec (Canada) during the 10-year period 1988 to 1997.[294] In defined regions of North America, however, the prevalence is much higher: we estimated the incidence in Nova Scotia and New Brunswick (Canada) to be 58 per million for the 10-year period 1988 to 1997.[294] It is assumed that the patients in these regions are progeny of common ancestors. An example is the Mormon pedigree, with its members residing in Utah (Utah families); this pedigree was originally described by Cannon.[306] The “Utah mutation” is a nonsense mutation (L312X) predictive of a receptor that lacks transmembrane domain 7 and the intracellular COOH-terminus.[307] The largest known kindred with X-linked NDI is the Hopewell family, named after the Irish ship Hopewell, which arrived in Halifax, Nova Scotia, in 1761.[308] Aboard the ship were members of the Ulster Scot clan, descendants of Scottish Presbyterians who migrated to the Ulster in Ireland in the 17th century and left Ireland for the New World in the 18th century. Whereas families arriving with the first emigration wave settled in northern Massachusetts in 1718, the members of a second emigration wave, passengers of the Hopewell, settled in Colchester County, Nova Scotia. According to the “Hopewell hypothesis”,[308] most patients with NDI in North America are progeny of female carriers of the second emigration wave. This assumption is mainly based on the high prevalence of NDI among descendants of the Ulster Scots residing in Nova Scotia. In two villages with a total of 2500 inhabitants, 30 patients have been diagnosed, and the carrier frequency has been estimated at 6%. Given the numerous mutations found in North American X-linked NDI families, the Hopewell hypothesis can not be upheld in its originally proposed form. However, among X-linked NDI patients in North America, the W71X (the Hopewell mutation) mutation is more common than another AVPR2 mutation. It is a null mutation (W71X; [308] [310]) predictive of an extremely truncated receptor consisting of the extracellular NH2-terminus, the first transmembrane domain, and the NH2-terminal half of the first intracellular loop. Because the original carrier can not be identified, it is not clear whether the Hopewell mutation was brought to North America by Hopewell passengers or by other Ulster Scot immigrants. One hundred eighty-three different putative disease-causing mutations in the AVPR2 gene have now been reported in 284 unrelated families with X-linked NDI (see Fig. 40-14 ). The diversity of AVPR2 mutations found in many ethnic groups (whites, Japanese, African Americans, Africans) and the rareness of the disease is consistent with an X-linked recessive disease that in the past was lethal for male patients and was balanced by recurrent mutations. In X-linked NDI, loss of mutant alleles from the population occurs because of the higher mortality of affected males compared with healthy males, whereas gain of mutant alleles occurs by mutation. If affected males with a rare X-linked recessive disease do not reproduce and if mutation rates are equal in mothers and fathers, then, at genetic equilibrium, one third of new cases of affected males will be due to new mutations. We and others have described ancestral mutations, de novo mutations, and potential mechanisms of mutagenesis.[294] These data are reminiscent of those obtained from patients with late-onset autosomal-dominant retinitis pigmentosa. In one fourth of patients, the disease is caused by mutations in the light receptor rhodopsin. Here too, many different mutations (approximately 100) spread throughout the coding region of the rhodopsin gene have been found.[310]

The basis of loss-of-function or dysregulation of 28 different mutant V2 receptors (including nonsense, frameshift, deletion, or missense mutations) has been studied using in vitro expression systems. Most of the mutant V2 receptors tested were not transported to the cell membrane and were thus retained within the intracellular compartment. Our group also demonstrated that misfolded AVPR2 mutants could be rescued in vitro[311] but also in vivo[312] by nonpeptide vasopressin antagonists acting as pharmacological chaperones. This new therapeutic approach could be applied to the treatment of several hereditary diseases resulting from errors in proteins folding and kinesis.[313]

Only three AVPR2 mutations (D85N, G201D, P322S) have been associated with a mild phenotype. [315] [316] In general, the male infants bearing these mutations are identified later in life and the “classic” episodes of dehydration are less severe. This mild phenotype is also found in expression studies: the mutant proteins are expressed on the plasma membrane of cells transfected with these mutants and demonstrate a stimulation of cAMP for higher concentrations of agonists. [316] [317]

Loss-of-Function Mutations of AQP2 (OMIM 107777)

The AQP2 gene is located in chromosome region 12q13. Males and females affected with congenital NDI have been described who are homozygous for a mutation in the AQP2 gene or carry two different mutations ( Fig. 40-15 ).[315] [318] The oocytes of the African clawed frog (Xenopus laevis) have provided a most useful testbed for looking at the functioning of many channel proteins. Oocytes are large cells that are just about to become mature eggs ready for fertilization. They have all the normal translation machinery of living cells and so they will respond to the injection of mRNA by making the protein for which it codes. Functional expression studies showed that Xenopus oocytes injected with mutant cRNA had abnormal coefficient of water permeability, whereas Xenopus oocytes injected with both normal and mutant cRNA had coefficient of water permeability similar to that of normal constructs alone. These findings provide conclusive evidence that NDI can be caused by homozygosity for mutations in the AQP2 gene. A patient with a partial phenotype has also been described to be a compound heterozygote for the L22V and C181W mutations.[318] Immunolocalization of AQP2-transfected CHO cells showed that the C181W mutant had an endoplasmic reticulum-like intracellular distribution, whereas L22V and wild-type AQP2 showed endosome and plasma membrane staining. The authors suggested that the L22V mutation was key to the patient's unique response to desmopressin. The Leucine 22 residue might be necessary for proper conformation or for binding of another protein important for normal targeting and trafficking of the molecule. More recently, we obtained evidence to suggest that both autosomal dominant and autosomal recessive NDI phenotypes could be secondary to novel mutations in the AQP2 gene. [320] [321] [322] [323] [324] Reminiscent of expression studies done with AVPR2 proteins, Mulders, Deen, Tamarappo, and Verkman and co-workers also demonstrated that the major cause underlying auto-somal recessive NDI is the misrouting of AQP2 mutant proteins. [325] [326] [327] [328] [329] [330] To determine if the severe AQP2-trafficking defect observed with the naturally occurring mutations T126M, R187C, and A147T is correctable, cells were incubated with the chemical chaperone glycerol for 48 hours. Redistribution of AQP2 from the endoplasmic reticulum (ER) to the membrane-endosome fractions was observed by immunofluorescence. This redistribution was correlated to improved water permeability measurements.[326] It will be important to correct this defective AQP2-trafficking in vivo.



FIGURE 40-15  A, Schematic representation of aquaporin-2 (AQP2) and identification of 35 putative disease-causing AQP2 mutations. Solid symbols indicate missense or nonsense mutations; a number indicates more than one mutation in the same codon. The locations of the NPA boxes and the PKA phosphorylation site (Pa) are indicated. The extracellular, transmembrane, and cytoplasmic domains are defined according to Deen PMT, Verdijk MAJ, Knoers NVAM, et al: Requirement of human renal water channel aquaporin-2 for vasopressin-dependent concentration of urine. Science 264:92–95, 1994. Solid symbols indicate the location of the missense or nonsense mutations. There are 25 missense, 2 nonsense, 6 frameshift deletion or insertion, and 2 splice-site mutations. B, A monomer is represented with six transmembrane helices, A-F. The asterisk indicates where the molecular pseudo-2-fold symmetry is strongest.[346]



Polyuria, Polydipsia, Electrolyte Imbalance, and Dehydration in Cystinosis

Polyuria may be as mild as persistent enuresis and as severe to contribute for death from dehydration and electrolyte abnormalities in infants with cystinosis who have acute gastroenteritis.[30]

Polyuria in Hereditary Hypokalemic Salt-Losing Tubulopathies

Patients with polyhydramnios, hypercalciuria, and hypo- or isosthenuria have been found to bear KCNJ1 (ROMK) and SLC12A1 (NKCC2) mutations. [212] [220] Patients with polyhydramnios, profound polyuria, hyponatremia, hypochloremia, metabolic alkalosis, and sensorineural deafness were found to bear BSND mutations. [216] [222] [223] [331] These studies demonstrate the critical importance of the proteins ROMK, NKCC2, and Barttin to transfer NaCl in the medullary interstitium and thereby to generate, together with urea, an hypertonic milieu (see Fig. 40-8 ).

Carrier Detection, Perinatal Testing, and Treatment

We encourage physicians who observe families with X-linked and autosomal recessive NDI to recommend mutation analysis before the birth of an infant because early diagnosis and treatment can avert the physical and mental retardation associated with episodes of dehydration. Diagnosis of X-linked NDI was accomplished by mutation testing of cultured amniotic cells (n = 6), chorionic villus samples (n = 7), or cord blood obtained at birth (n = 31) in 44 of our patients. Twenty-one males were found to bear mutant sequences, 16 males were not affected, and 5 females were not carriers. The affected patients were immediately treated with abundant water intake, a low-sodium diet, and hydrochlorothiazide. They never experienced episodes of dehydration, and their physical and mental development is normal. Gene analysis is also important for the identification of non obligatory female carriers in families with X-linked NDI. Most females heterozygous for a mutation in the V2 receptor do not present with clinical symptoms: few are severely affected (Ref 294 and Bichet, unpublished observations). Mutational analysis of polyuric patients with cystinosis or hypokalemic salt-losing tubulopathy is also of importance for a definitive molecular diagnosis.

All complications of congenital NDI are prevented by an adequate water intake. Thus, patients should be provided with unrestricted amounts of water from birth to ensure normal development. In addition to a low-sodium diet, the use of diuretics (thiazides) or indomethacin may reduce urinary output. This advantageous effect must be weighed against the side effects of these drugs (thiazides: electrolyte disturbances; indomethacin: reduction of the glomerular filtration rate and gastrointestinal symptoms). Many affected infants frequently vomit due to an exacerbation of physiologic gastroesophageal reflux. These young patients often improve with the absorption of an H2 blocker and with metoclopramide (which could induce extrapyramidal symptoms) or with domperidone, which seems to be better tolerated and efficacious.


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