Rudolph's Pediatrics, 22nd Ed.

CHAPTER 138. Disorders of Sulfur-Containing Amino Acid Metabolism

Bernd Christian Schwahn

Sulfur-containing amino acids have various roles: They mediate the transfer of methyl groups for virtually all transmethylation reactions; they provide reactive thiol groups that are needed for detoxification of endogenous and exogenous substances; they help maintain the intracellular redox potential; and they are a source of sulphate. Thirteen disorders of sulphur amino acid metabolism have been consistently described, 11 of them potentially leading to disease.

HOMOCYSTINURIA

Severe hyperhomocysteinemia, defined as plasma total homocysteine (tHcy) concentrations above 100 μmol/L, is generally caused by single-enzyme deficiencies of homocysteine metabolism. When concentrations of free homocysteine exceed the plasma protein binding capacity, the disulfide homocystine forms nonenzymatically and is excreted in urine, hence causing homocystinuria. The term homocystinuria is sometimes used to indicate the most common form of the disease, which is caused by defective activity of the enzyme cystathionine β-synthase (CBS).1 Homocystinuria, however, results from a defect in CBS and from defects in the folateand cobalamin-dependent remethylation cycle (see Chapter 147).

CLASSICAL HOMOCYSTINURIA (CBS DEFICIENCY)

CLINICAL PRESENTATION

Classical homocystinuria presents as multisystemic disease with a dysplasia of connective tissue, with a predisposition to arterial and venous thromboembolism, and with mental retardation. Clinical variability is wide,2 but presentation is usually in the first decade with the exception of embolism, which occurs later. Homocystinuria is one of the few disorders of amino acid metabolism in which clinical manifestations tend to be progressive in adulthood, because many clinical manifestations result from arteriosclerosis and thrombotic complications.

The most characteristic feature of this disorder is subluxation of the ocular lens, which occurs in almost all untreated individuals until adulthood. Most patients have osteoporosis and skeletal abnormalities similar to those seen in Marfan syndrome, such as tall stature, scoliosis, genu valgum, pes cavus, arachnodactyly, and pectus carinatum or excavatum.3 In homocystinuria, however, the joints tend to be limited in mobility rather than hypermobile. Mental retardation is common, although it is often mild, and many individuals have psychiatric disturbances.4

METABOLIC DERANGEMENT, PATHOPHYSIOLOGY

The disturbance in metabolism resulting in homocystinuria is shown in Figure 138-1. CBS initiates the first step of homocysteine elimination. As a consequence of CBS deficiency, homocysteine, SAH, SAM, and methionine accumulate when methionine intake exceeds the residual transsulfuration and total remethylation activity. Moreover, high SAH inhibits many transmethylases, which increases accumulation of SAM and methionine. Increased homocysteine facilitates remethylation, which leads to further accumulation of methionine. The intermediate metabolites cystathionine and cysteine are decreased.

GENETICS

CBS deficiency (OMIM No. 236200) is an autosomal recessive trait. The CBS gene has been mapped to chromosome 21q22.3, and more than 100 different mutations have been identified, most of them being private.6,7 However, two mutations, I278T and G307S, account for 25% to 50% of all affected alleles, depending on the population. A few other mutations, including I278T, are usually associated with pyridoxine responsiveness. Antenatal diagnosis has been performed by assaying CBS activity in cultured amniocytes or cultured chorionic villi. Mutational analysis can be used for prenatal diagnosis once the mutation of the index case is known. The real incidence of CBS deficiency is not known but may vary between 1:20,000 and 1:200,000.8

DIAGNOSTIC TESTS, DIFFERENTIAL DIAGNOSES

Once hyperhomocysteinemia has been identified, usually in the range of 80 to 300 μmol/L, plasma amino acid chromatography can be used to search for hypermethioninemia and low cystine, cystathionine, and serine concentrations. Defects of folate- and cobalamin-dependent remethylation can be differentiated by a low or normal plasma methionine concentration and increased cystathionine. Increased urinary methylmalonic acid excretion and megaloblastic anemia suggest either concomitant cobalamin deficiency or a defect in cobalamin transport or metabolism. Severe folate or cobalamin deficiency should be ruled out by measuring serum vitamin concentrations (see Table 138-1 and Chapter 147). The diagnosis can be confirmed by measuring enzyme activity in fibroblasts or in phytohemagglutinin-stimulated lymphocytes.

TREATMENT, PROGNOSIS, AND LONG-TERM OUTCOME

Approximately half of all CBS-deficient individuals respond with clear biochemical improvement to supplementation with large doses of pyridoxine.11 Responsiveness is determined by the properties of the mutant apoenzyme and should be tested in every patient by administration of pyridoxine for at least a week (500 mg daily in children and 1000 mg per day in adults). Medical treatment aims to control the biochemical abnormalities to prevent complications or halt the progression of symptoms. Early treatment and maintenance of tHcy below 50 μmol/L prevents the patient’s exposure to excessive concentrations of free homocysteine and appears to be associated with a near-normal risk for thromboembolic events, improvement in behavior and IQ, and avoidance of lens dislocation.1 Those who do not respond sufficiently to pyridoxine supplementation need to be treated with a diet low in methionine and supplemented with L-cystine. In addition, the methyl donor betaine should be added to enhance remethylation. Individuals who do not restrict their methionine intake may develop extreme hypermethioninemia with betaine and are at risk for brain edema if methionine exceeds 1000 μmol/L.12

METHYLENETETRAHYDROFOLATE REDUCTASE DEFICIENCY

CLINICAL PRESENTATION

Like individuals with CBS deficiency, those with 5,10-methylenetetrahydrofolate reductase (MTHFR) deficiency are at risk for arteriosclerosis and thromboembolism. They have, however, fewer skeletal symptoms, and lens dislocation is not observed.15 Most suffer from gait disturbance, a dysmyelinating encephalopathy, seizures, and mental retardation. Psychiatric disturbances are very common. The phenotype of severe MTHFR deficiency ranges from early neonatal death to nearly asymptomatic adults and is dependent on genetic and environmental modifiers. The other two remethylation defects, namely methionine synthase (MTR) deficiency and MTR reductase (MTRR) deficiency (see Table 138-2), share similarities with MTHFR deficiency, as do three other primary defects in cobalamin metabolism (see Chapter 147). These are, however, less common and are associated with megaloblastic anemia due to methylfolate trapping, which is not a feature of MTHFR deficiency.

FIGURE 138-1. Pathways of sulfur amino acids and methyl group metabolism in man. Methionine adenosyltransferase (MAT) activates methionine to S-adenosylmethionine (SAM), the universal methyl group donor for numerous methylation reactions. These “transmethylation” reactions yield a methylated product and demethylated S-adenosylhomocysteine (SAH), which is readily hydrolyzed to homocysteine by SAH-hydrolase (SAHH), a highly reactive thiol compound. Homocysteine enters the transsulfuration pathway by condensation with serine and is catalyzed by cystathionine beta synthase (CBS), an enzyme requiring pyridoxal phosphate (B6) as cofactor. Cystathionine is catabolized to cysteine by cystathionase (CTH), another B6-dependent enzyme, and eventually to sulfite. Sulfite is oxidized to sulfate by sulfite oxidase (SOX). Cysteine is a precursor for the synthesis of glutathione via gamma-glutamylcysteine synthetase (gGCS) and glutathione synthetase (GSS). A considerable proportion of homocysteine is recycled by remethylation to methionine via the cobalamin (B12)–dependent enzyme methionine synthase (MTR). This reaction needs 5-methylfolate (CH3THF) as a methyl donor, which is provided by 5,10-methylenetetrahydrofolate reductase (MTHFR) from 5,10-methylenetetrahydrofolate (CH2THF), thus diverting one-carbon units from nucleotide synthesis to methylation. MTHFR requires riboflavin (B2) as a cofactor. The choline metabolite betaine provides a methyl group for an alternative remethylation reaction of homocysteine in liver and kidney. The flux through transsulfu-ration and remethylation pathways is regulated by the availability of SAM and the mitochondrial and cytosolic redox potentials. Apart from exogenous methyl groups that are ingested with methionine, choline, or betaine, the main endogenous source of one-carbon units is from serine. Serine is synthesized from glucose in a series of enzyme reactions involving 3-phosphoglycerate dehydrogenase (3-PGDH) and 3-phosphoserine phosphatase (3-PSPH). Serine is converted to glycine, forming methylenetetrahydrofolate (CH2). An excess of CH3 groups can be buffered by formation of sarcosine via glycine-N-methyltransferase (GNMT). Sarcosine is also a product of betaine catabolism to dimethylglycine, which is further demethylated by dimethylglycine dehydrogenase (DMGDH). Sarcosine can be converted to glycine by sarcosine dehydrogenase (SDH). GNMT and SDH form the sarcosine-glycine cycle, which regulates the ratio of SAM to SAH and thereby the flux through transmethylation reactions that use SAM as methyl donor.

METABOLIC DERANGEMENT, PATHOPHYSIOLOGY

MTHFR provides 5-methylfolate for the remethylation of homocysteine, a reaction that is catalyzed by the enzyme methionine synthase and requires cobalamin. MTHFR diverts one-carbon units from nucleotide synthesis to remethylation. Lacking MTHFR causes homocysteine accumulation in many extrahepatic tissues, including the brain. Alternative hepatic remethylation using betaine is enhanced, leading to betaine depletion. Transsulfuration is enhanced and cystathionine accumulates. Methionine and SAM are decreased. The latter may be responsible for the dysmyelination, resembling the one seen in cobalamin deficiency.

GENETICS

MTHFR deficiency (OMIM no. 236250) is an autosomal recessive trait caused by biallelic pathogenic mutations in the MTHFR gene on chromosome 1p36.3.  The common polymorphism c.677C>T does confer a thermolabile variant, and homozygotes have a decreased MTHFR activity when their folate status is poor. They may have mild hyperhomocysteinemia but no clinical symptoms. The c.677C>T polymorphism has gained much interest as a possible risk factor for cardiovascular disease and possibly for other multifactorial conditions in the general population.

DIAGNOSTIC TESTS, DIFFERENTIAL DIAGNOSES

Severe MTHFR deficiency can be suspected in the presence of moderate or severe hyperhomocysteinemia in combination with low-normal methionine and increased cystathionine in the absence of methylmalonic aciduria, megaloblastic anemia, and folate and cobalamin deficiency. The diagnosis must be confirmed with direct enzyme assay in fibroblasts. Molecular testing is available.

Table 138-1. Biochemical Differentiation of Severe Hyperhomocysteinemia >100 μmol/L

Severe hyperhomocysteinemia can be caused by genetic defects of homocysteine metabolism or by severe folate or cobalamin deficiency. High plasma methionine concentration points to homocystinuria due to cystathionine beta synthase (CBS) deficiency.

Low or low-normal methionine concentrations are found in all other conditions.

An increased urinary excretion of methylmalonic acid (MMA) reveals functional cobalamin deficiency, which is not found in disorders involving folate-dependent remethylation, such as methylenetetrahydrofolate reductase (MTHFR) deficiency, methionine synthase (MTR) deficiency, or methionine synthase reductase (MTRR) deficiency. The latter conditions are associated with intracellular accumulation of methylfolate (folate in erythrocytes), while in MTHFR deficiency, folate in erythrocytes is rather low.

Predominantly low serum folate concentrations point to nutritional folate deficiency or to a folate transport defect.

Increased urinary MMA together with increased plasma homocysteine and normal serum cobalamin points to a defect in intracellular cobalamin processing (Cbl C and Cbl D disorder).

Low serum cobalamin concentrations can result from defects in cobalamin uptake or in lysosomal trafficking (Cbl F). Lack of a response to parenteral cobalamin substitution requires further in vitro studies to confirm a Cbl F disorder.

TREATMENT, PROGNOSIS, AND LONG-TERM OUTCOME

There is no dietary treatment for MTHFR deficiency, but methionine supplementation can be beneficial.17 For alternative remethylation, an increased supply of betaine can partly compensate for the lack of folate- and cobalamin-dependent remethylation.18 Betaine supplementation is highly effective in preventing disease-associated symptoms when started early in newborns19 and in an animal model of this disease.20 Late-diagnosed patients show an improvement of myelination and neuropsychiatric symptoms on treatment with betaine, but their prognosis is generally poor.

Table 138-2. Other Rare Disorders of Sulphur-Containing Amino Acids

SULFITE OXIDASE DEFICIENCY AND MOLYBDENUM COFACTOR DEFICIENCY

The terminal step in the oxidative degradation of cysteine and methionine, the conversion of sulfite to sulfate, is catalyzed by the molybdenum-containing enzyme sulfite oxidase. Sulfite oxidase deficiency can be caused by a defect in the gene for the apoenzyme itself or by defects in the synthesis of the molybdenum cofactor required for its function. Molybdenum cofactor deficiency compromises the function of three enzymes and causes additional symptoms: xanthine oxidase deficiency leads to decreased urate production and urinary xanthine stones.

CLINICAL PRESENTATION

The classical presentation is that of a severe neonatal epileptic encephalopathy, muscular hypo- or hypertonia, progressive microcephaly, and developmental arrest. Early death is frequent. Lens dislocation is an early finding.32 A milder presentation has been described with mental retardation and regression and slowly progressive dystonic and choreoathetotic movement disorder.33,34

DIAGNOSTIC TESTS, DIFFERENTIAL DIAGNOSES

Increased urinary sulfite can be detected in fresh urine using commercial strip tests normally utilized for wine making.

TREATMENT, PROGNOSIS, AND LONG-TERM OUTCOME

Treatment is purely to ease symptoms and so far has not altered the early lethal course.

GAMMA-GLUTAMYL CYCLE

The synthesis and recycling of the sulfur-containing tripeptide glutathione involves a series of six enzymatic reactions termed the γ-glutamyl cycle. Glutathione has different important biological functions (eg, transporting amino acids, detoxifying free radicals and drugs, reducing reactions, and synthesizing other biomolecules). Deficiencies in four of the enzymes of the gamma-glutamyl cycle are associated with disease (see Table 138-2), two of which manifest with hemolytic anemia. The most frequently recognized and most severe disease is glutathione-synthetase deficiency.47

GLUTATHIONE SYNTHASE DEFICIENCY (PYROGLUTAMIC ACIDURIA)

CLINICAL PRESENTATION

Two clinically distinct forms of glutathione synthetase deficiency (GSSD) are known: a rare mild erythrocytic form that manifests as mild hemolytic anemia and that can be associated with splenomegaly,48and a more frequent generalized form that is associated with a variable degree of early neonatal metabolic acidosis, jaundice, and hemolytic anemia.

DIAGNOSTIC TESTS, DIFFERENTIAL DIAGNOSES

Urine organic acid analysis can readily detect 5-oxoproline in newborns with metabolic acidosis without ketosis, hypoglycemia, or hyper-lactic acidemia.