David G. Gardner MD
A group of heritable syndromes characterized by aberrant growth of benign or malignant tumors in a subset of endocrine tissues have been given the collective term multiple endocrine neoplasia (MEN). The tumors may be functional (ie, capable of elaborating hormonal products that result in specific clinical findings characteristic of the hormone excess state) or nonfunctional. There are three major syndromes: MEN 1 is characterized by tumors involving the parathyroid glands, the endocrine pancreas, and the pituitary; MEN 2A includes medullary carcinoma of the thyroid gland, pheochromocytoma, and hyperparathyroidism; and MEN 2B, like MEN 2A, includes medullary carcinoma of the thyroid, multiple neuromas, and pheochromocytoma, but hyperparathyroidism is typically absent.
MULTIPLE ENDOCRINE NEOPLASIA TYPE 1 (MEN 1)
MEN 1, also known as Wermer's syndrome, is inherited as an autosomal dominant trait with an estimated prevalence of 2–20 per 100,000 in the general population. Approximately 10% of MEN 1 mutations arise de novo. The term “sporadic MEN 1” has been applied to this group. MEN 1 has a number of unusual clinical manifestations (Table 22-1) that occur with variable frequency among individuals within affected kindreds.
Hyperparathyroidism is the most common feature of MEN 1, with an estimated penetrance of 95–100% over the lifetime of an individual harboring the MEN 1 gene. The diagnosis of hyperparathyroidism is usually made through a combination of clinical and laboratory criteria similar to those used in the identification of sporadic disease (see Chapter 8). It is typically the first clinical manifestation of MEN 1, though this varies as a function of the patient population being examined. Hyperparathyroidism in MEN 1 is due to hyperplasia of all four parathyroid glands (or more, if supernumerary glands are present). However, involved glands may undergo metachronous enlargement, and selective resection of these glands often results in sustained clinical remissions. MEN 1 is a rare cause of hyperparathyroidism, accounting for only 2–4% of cases in the general population.
Enteropancreatic tumors in MEN 1 can be either functional (ie, capable of producing a secreted product with biologic activity) or nonfunctional. Gastrinomas, frequently associated with Zollinger-Ellison syndrome, represent approximately 40–60% of the enteropancreatic tumors associated with this syndrome. Of equal importance, roughly 25% of patients with Zollinger-Ellison syndrome are found in MEN 1 kindreds. Insulinomas constitute approximately 20% of the islet cell tumors while the remainder represent a collection of functional (eg, glucagon- or vasoactive intestinal peptide-producing tumors) and nonfunctional tumors. It is noteworthy that the gastrinomas of MEN 1 are often small, multicentric, and ectopically located outside the pancreatic bed, most often in the duodenal submucosa. This latter feature can have a major impact on the therapeutic
approach to these patients (see below). Gastrinomas in MEN 1 are frequently malignant, as are their sporadic counterparts; however, for reasons that are only poorly understood, the biologic behavior of these tumors is less aggressive than that found in sporadic disease.
Table 22-1. Clinical manifestations of MEN 1.
The diagnosis of gastrinomas is based on demonstration of hypergastrinemia in the presence of gastric acid hypersecretion. This latter criterion excludes other more common causes of hypergastrinemia (eg, achlorhydria). When the diagnosis is in question, the secretin stimulation test, which stimulates gastrin secretion from gastrinomas but not from normal tissue, may be employed. (Note: The availability of secretin for parenteral administration is now quite limited.) Others have advocated measurements of multiple gastrointestinal hormones following a standardized mixed meal as the most efficient means of detecting the presence of neuroendocrine tumors in MEN 1.
Owing to their small size, gastrinomas can be difficult to localize in MEN 1. Computed tomography and MRI may be useful in identifying larger lesions but are typically not helpful in identifying smaller ones. These imaging procedures are useful, however, in demonstrating hepatic metastases when present. The most promising localization techniques studied to date include endoscopic and intraoperative ultrasound, selective arterial secretin injection (followed by hepatic vein sampling for gastrin), and radiolabeled octreotide scanning. Each of these has been employed successfully to identify tumors in studies involving small groups of patients. It is important to recognize, however, that almost half of gastrinomas are not found with preoperative localization studies.
Insulinomas in MEN 1 are detected using conventional biochemical testing (see Chapter 18). They can also be difficult to localize given their potential for multicentricity. Endoscopic ultrasound and selective arterial infusions of calcium with hepatic vein sampling (for insulin) have been used successfully to identify lesions in small groups of patients.
Pituitary adenomas occur in approximately 25% of patients harboring the MEN 1 gene. The majority secrete prolactin, with or without secretion of excess growth hormone, followed by those secreting growth hormone alone, nonfunctional tumors, and those secreting excessive amounts of ACTH (Cushing's disease). A prolactinoma variant of MEN 1 has been described (Burin variant). This variant is characterized by an increased frequency of prolactinomas, carcinoids, and hyperparathyroidism and infrequent appearance of gastrinomas in affected kindreds. It does not appear to be associated with a specific mutation of the MEN 1 gene. Pituitary tumors in MEN 1 are rarely malignant, but recent studies suggest that they may be larger and more aggressive than their sporadic counterparts. Diagnosis and management are similar to that of their sporadic counterparts (see Chapter 5).
Adrenal adenomas, including cortisol-producing adenomas, are seen in MEN 1, theoretically making the differential diagnosis of Cushing's syndrome in this setting complex (ie, adrenal adenoma versus basophilic adenoma of the pituitary gland versus ectopic ACTH secretion from a carcinoid tumor, which is also commonly associated with this syndrome). Empirically, most hypercortisolemia in this setting is due to pituitary disease. Adrenal adenomas are often found together with islet cell tumors in affected patients, and at least in some series they appear to lack the MEN 1 genetic defect. This has led to the suggestion that they represent a secondary rather than a primary manifestation of the underlying genetic defect. This, however, remains controversial. Thyroid disease has been said to be more common in MEN 1; however, with the possible exception of thyroid adenomas, this link remains obscure. Subcutaneous lipomas, skin collagenomas, and multiple facial angiofibromas are seen in 30–90% of family members in affected kindreds. Though clinically of little importance, when present, they may prove useful in identifying affected individuals within a kindred and lead to more effective screening (see below). Carcinoid tumors are seen with increased frequency in MEN 1. They are almost exclusively foregut carcinoids and may be found in the thymus, in the lung (bronchial carcinoids), or in the gastric mucosa. For unclear reasons, thymic carcinoids appear more commonly in males, bronchial carcinoids in females. They occasionally secrete hormonal products (eg, ectopic ACTH), are often malignant, and may behave aggressively. Leiomyomas and, rarely, pheochromocytomas have also been described in MEN 1.
MEN 1 is inherited as an autosomal dominant trait. Traditional linkage studies localized the defective gene
to the long arm of chromosome 11q13. Parallel analyses of DNA from endocrine tumors taken from MEN 1 patients demonstrated allelic loss in this area, frequently resulting from large DNA deletions. This raised the possibility that the defective gene was a tumor suppressor gene involved in the control of cellular growth. In this paradigm (Figure 22-1), the inherited defective allele is silent in the presence of a normal, functioning allele on the second chromosome. A subsequent somatic mutation (often a deletion that removes the normal allele) results in a null genotype in which the suppressor gene locus is either absent or defective on both alleles. The high frequency with which such deletions occur is thought to account for the dominant nature of this particular genetic defect. Release of the tumor suppressor gene's growth regulatory activity results in a hyperplastic growth response in cells harboring the somatic mutation. This promitogenic state probably provides the substrate for subsequent somatic mutations that result in acquisition of a true malignant phenotype, as occurs in gastrinomas associated with Zollinger-Ellison syndrome. Recent studies have succeeded in identifying the gene, termed the menin gene, which appears to be responsible for MEN 1 (Figure 22-2). Mutations have been identified throughout the entire 610-amino-acid length of the menin coding sequence and include nonsense mutations, missense mutations, and deletions. Almost 300 independent mutations have been described in MEN 1 kindreds to date. Menin is a nuclear protein whose precise cellular function has yet to be determined. It has been shown to interact with JunD, a constitutively expressed member of the extended jun/fos gene family. Menin suppresses JunD-dependent transcriptional activation in the intact cell, though it is unclear if and how this accounts for its growth regulatory activity since JunD is typically associated with inhibition of cell growth. It is of note that menin does not interact with other members of the Jun/Fos family.
Menin also interacts with SMAD 3, a transcription factor involved in the TGF-β signaling pathway.
Figure 22-1. Allelic loss in MEN 1. Afflicted patient's germ cells (shown on left) harbor both a normal and defective gene at the MEN 1 locus. Each of these is detected using selective restriction enzyme digestion and Southern blot analysis of the genomic DNA. Affected somatic cells (eg, parathyroid chief cells) undergo a second mutation, typically a deletion of the normal allele, resulting in detection of only the mutant allele by Southern analysis. Sporadic disease is thought to follow sequential mutation or deletion of each MEN 1 allele in the somatic cell. Solid bar (dark blue) with asterisk represents radiolabeled probe used in Southern analysis.
Figure 22-2. Some of the more common germline menin mutations from fifty-six MEN 1 kindreds and somatic mutations from twenty-four endocrine tumors. Mutations above the bar cause protein truncations through stop codons or frameshifts leading to premature stop codons; two cause splice errors. Mutations below the bar cause missense or single-amino-acid codon changes. (Reproduced, with permission, from Marx S et al: Multiple endocrine neoplasia type 1: Clinical and genetic topics. Ann Intern Med 1998;129:484.)
The MEN1 gene is also the gene most frequently mutated in a variety of sporadic endocrine tumors. Parathyroid adenomas (21%), gastrinomas (33%), insulinomas (17%), bronchial carcinoids (36%) and pituitary tumors (5%) often harbor somatic mutations at the MEN1locus. It is important to note that screening for germline mutations at the menin locus fails to detect mutations 10–20% of the time despite evidence for loss of heterozygosity at 11q13. It is thought that the relevant mutations may be in regulatory regions surrounding the menin gene. Haplotype testing or genetic linkage analysis can be useful in these cases to identify relevant MEN 1 carriers. Unlike MEN 2 (see below), there is no clear association between specific menin gene mutations and the nature or extent of endocrine gland involvement.
Therapy of hyperparathyroidism in MEN 1 is directed toward surgical extirpation of hyperplastic parathyroid tissue, typically with resection of three and one-half glands. This leaves one-half gland in situ in an attempt to preserve residual parathyroid function and avoid hypoparathyroidism. Alternatively, patients may be subjected to total parathyroidectomy with transplantation of the“most normal-appearing” tissue to the nondominant forearm. Prophylactic near-total thymectomy is typically performed at the time of neck exploration to cover the possibility of intrathymic parathyroid glands as well as thymic carcinoid tumors.
Both persistence and recurrence of hyperparathyroidism occur more frequently in MEN 1 than in sporadic disease. Persistence of hyperparathyroidism, defined as a failure to normalize serum calcium and
parathyroid hormone (PTH) levels following the initial surgery, occurs in 38% of cases of MEN 1. Recurrent disease, defined as reappearance of hyperparathyroidism following at least 3 months of normocalcemia, is seen in 16% of cases, and this may rise to 50% 8–12 years following surgery. The high frequency of persistent hyperparathyroidism in MEN 1 probably reflects the high frequency of supernumerary glands and ectopically located parathyroid tissue in patients carrying this gene. The increased frequency of recurrent disease is thought to result from the continued presence of the underlying mitogenic stimulus that drives parathyroid gland growth in this syndrome.
Therapy of gastrinomas in MEN 1 remains controversial. Suppression of gastric acid production with proton pump inhibitors (eg, omeprazole) remains a mainstay of therapy. Conservative medical management of these tumors has been predicated on their assumed low-grade malignant behavior, the dominance of complications related to gastric acid hypersecretion in contributing to morbidity and mortality, and the failure of most attempts at surgical resection to alter the natural course of the disease. More recently, recognition of the potential for more aggressive behavior in some of these tumors—in a recent study, 14% of these tumors demonstrated aggressive growth—and the fact that many of these tumors are “ectopically” located in duodenal submucosa rather than the pancreatic bed has renewed interest in the possibility of surgical cure. A number of small studies have reported encouraging results when measures to both localize and remove gastrinoma tissue in the pancreas and duodenum have been used. While the nature of the underlying genetic lesion and the multicentricity of these tumors may place limits on the prospects for cure of this disease in most patients, the slow growth characteristics of these tumors permit long periods of symptom-free survival following reduction of the tumor burden. For patients with liver or other metastatic disease, symptoms related to hypergastrinemia may be controlled with the proton pump antagonists, as described above. More conventional cancer therapy (eg, systemic chemotherapy, radiation therapy, or selective chemoembolization of hepatic metastases) is palliative and reserved for advanced stages of the disease.
It is important to remember that calcium stimulates gastric acid secretion. This may occur through gastrin-dependent and gastrin-independent pathways. In MEN 1 patients with both hyperparathyroidism and Zollinger-Ellison syndrome, correction of the hyperparathyroidism and attendant hypercalcemia frequently results in a reduction in both basal and maximal acid output and a decline in serum gastrin levels. Secretin stimulation tests often normalize following parathyroidectomy. More importantly, there is a reduction in the dose of medication (eg, H2 blocker) required to control symptoms of Zollinger-Ellison syndrome following parathyroid surgery in approximately 60% of patients.
Unlike gastrinomas, insulinomas are rarely localized outside the pancreatic bed. Therefore, a more aggressive approach can be directed toward the pancreas in planning surgical resection. Enucleation of the identifiable lesions in the pancreatic head and blind resection of the pancreatic body and tail are often more successful in correcting hyperinsulinemia than in restoring normal gastrin levels in patients with Zollinger-Ellison syndrome. Nonsurgical candidates (eg, those with serious coexisting disease or those in whom candidate tumors cannot be identified) can be managed with conventional medical therapy (eg, diazoxide or verapamil). (See Chapter 18.)
MEN 1 accounts for less than 1% of all pituitary tumors, 2–4% of cases of primary hyperparathyroidism, and about 25% of all gastrinomas. Thus, while routine screening for menin gene mutations is not indicated for sporadic cases of hyperparathyroidism or patients with pituitary tumors, screening of all cases of Zollinger-Ellison syndrome is likely to be cost-effective in identifying carriers. The frequency of germline mutations in the menin gene in patients with tumors thought to be sporadic based on family analysis is abut 5% for gastrinomas and 1–2% for other manifestations (hyperparathyroidism, prolactinomas, etc). Screening for MEN 1 within affected kindreds should be limited to those individuals for whom the index of suspicion for the syndrome is high (eg, familial history of endocrine tumors or hypersecretory states; history of multiple endocrine tumors or multigland involvement in the propositus). Identification of the carrier state should be done with the intent of acquiring information that will allow the clinician to focus screening on the relevant patient population (eg, members of an affected kindred who do not share the menin gene mutation need not be subjected to follow-up screening). Unlike the situation in MEN 2 (see below), carrier analysis should not be used to support a major therapeutic intervention. Such interventions (eg, pancreatic exploration) may be associated with significant morbidity, and there is no evidence that they prolong patient survival. Patients with hyperparathyroidism should be screened for MEN 1—even in the absence of a positive family history or any history of multiple endocrine tumors—if parathyroid hyperplasia is identified at the time of parathyroidectomy or if there is a history of recurrent hyperparathyroidism following parathyroidectomy. Patients with Zollinger-Ellison syndrome should be screened for MEN 1 given its high frequency (about 25%) in such individuals. Patients with isolated pituitary lesions have a low probability
of having coexistent MEN 1 and probably do not require screening unless there are other clinical features suggesting the syndrome.
While most clinicians agree that screening for MEN 1 in high-risk groups is worthwhile, details of the individual screening protocols vary considerably—from more focused, cost effective approaches to broad-based screening designed to detect occult disease. One example of a more cost-effective protocol is presented in Figure 22-3. In individuals within affected kindreds, ionized (or albumin-corrected) calcium and parathyroid hormone levels should be checked at yearly intervals. Serum gastrin levels should also be determined annually—or more frequently if Zollinger-Ellison syndrome is a prominent component of the phenotype in the affected kindred. In kindreds in whom disease is particularly aggressive, the secretin stimulation test (if available) can lend additional diagnostic sensitivity. Determination of fasting glucose, insulin, and proinsulin levels may prove useful, particularly if symptoms of hypoglycemia are present. Routine screening for other functional or nonfunctional islet cell tumors is probably not justified without clinical findings (eg, watery diarrhea, hypokalemia). As noted above, these lesions are typically multifocal and may be very small, which makes detection difficult even with sophisticated imaging studies. The same holds true for the pituitary lesions. In the absence of obvious clinical findings (eg, evidence of a hyper- or hyposecretory state or symptoms referable to a mass lesion in the sella turcica), routine screening should be confined to periodic measurements of serum prolactin and perhaps IGF-I. The former has been found to be useful in identifying pituitary disease in females harboring the MEN 1 gene defect. Imaging studies (eg, MRI of the pituitary and CT scan of the abdomen) should be performed at presentation and repeated at 3-year intervals.
Penetrance of MEN 1 is greater than 95% by age 45. Screening should be continued at periodic intervals at least to age 45. If there is no evidence of typical endocrine organ involvement by that age, screening frequency might be reduced. It is important to note, however, that the risk is not reduced to zero at age 45. A minority of patients will present with their first manifestation of the syndrome well after age 45. Surgical resection of diseased tissue (eg, parathyroidectomy) should be followed with continued screening looking both for recurrent disease and involvement of other organ systems.
MULTIPLE ENDOCRINE NEOPLASIA TYPE 2 (MEN 2)
MEN 2 is an autosomal dominant disorder with an estimated prevalence of 1–10 per 100,000 in the general population. It can be subdivided into two independent syndromes: MEN 2A (Sipple's syndrome) and MEN 2B. Manifestations of MEN 2A include medullary carcinoma of the thyroid, pheochromocytoma, and hyperparathyroidism. MEN 2B includes medullary carcinoma of the thyroid, pheochromocytoma, and a number of somatic manifestations (Table 22-2; Figure 22-4), but hyperparathyroidism is rare. Penetrance of MEN 2 is greater than 80% in individuals harboring the defective gene.
Medullary carcinoma of the thyroid is the most common manifestation of MEN 2 and often represents the first clinical presentation in individuals with multiorgan involvement. It also dominates the clinical course of patients affected with the disease. Eighty to 100 percent of individuals at risk will develop medullary carcinoma of the thyroid at some point during their lifetime. The classic thyroid lesion of MEN 2 is hyperplasia of the calcitonin-producing parafollicular cells, which typically serves as the precursor of medullary thyroid carcinoma. These tumors in MEN 2 patients tend to be multicentric and concentrated in the upper third of the thyroid gland, reflecting the normal distribution of parafollicular cells.
As much as one-fourth of all medullary carcinoma of the thyroid is genetic in origin. Roughly 45% of the heritable fraction is attributable to MEN 2A; 50% occurs as an isolated entity (isolated familial medullary carcinoma of the thyroid), and 5% is found in MEN 2B kindreds. The disease tends to behave more aggressively in MEN 2B than with either MEN 2A or familial medullary carcinoma, with earlier presentation (often before age 5) and more rapid progression.
Biochemical diagnosis depends heavily on the calcitonin-producing properties of the hyperplastic parafollicular cells or MCT. These lesions respond to pentagastrin or calcium infusions with significant increments in plasma calcitonin levels (see Chapter 7). Occasionally, immunohistochemical staining of poorly differentiated thyroid tumors for calcitonin will reveal the identity of the malignancy. The presence of extracellular amyloid is also one of the identifying features of these tumors. This material reacts with anti-calcitonin antisera, suggesting that it includes aggregated hormone released from neighboring tumor cells. MCT spreads initially within the thyroid bed and to regional lymph nodes. Distant metastases to liver, lung, and bone occur late in the course of the disease.
Pheochromocytomas develop in approximately 50% of individuals harboring the MEN 2 gene. They are usually located in the adrenal bed, often are bilateral, and are rarely malignant. Diagnosis is based on standard clinical criteria (eg, hypertension, presence of headaches, palpitations, diaphoresis), elevations in plasma or urine catecholamines or catecholamine metabolites (eg, urinary or plasma metanephrine or
normetanephrine) and demonstration of an adrenal mass on conventional abdominal imaging. As noted above for medullary carcinoma of the thyroid, pheochromocytomas in MEN 2 are preceded by a hyperplastic phase (adrenal medullary hyperplasia), although, unlike parafollicular cell hyperplasia, the adrenal precursor lesion can be difficult to detect with conventional biochemical testing.
Figure 22-3. Screening for MEN 1. Testing is targeted to individuals with a high probability of harboring the MEN 1 gene (eg, first-degree relatives of affected kindred members). If genetic testing is available and specific mutation is defined, screening might be limited to those individuals harboring the defective gene. Extension of screening to other pancreatic (eg, insulinomas) or pituitary tumors (eg, somatotroph adenomas) is based on the prevalence of the specific lesion in the affected kindred or the presence of signs or symptoms suggesting a particular lesion.
Figure 22-4. Patient with MEN 2B syndrome. Note the multiple neuromas on the lips and tongue and the marfanoid facies.
Table 22-2. Clinical manifestations of MEN 2.
As in MEN 1, hyperparathyroidism in MEN 2 is due to hyperplasia of the parathyroid glands. It is seen in about 25% of patients harboring the MEN 2A gene and is rarely seen as part of MEN 2B. The disease is usually less aggressive than its counterpart in MEN 1 and approximates more closely the behavior of sporadic disease. It responds well to surgical management.
There are a number of other phenotypic features associated with the MEN 2 syndromes. Cutaneous lichen amyloidosis is a pruritic erythematous skin lesion that is seen coincident with or often preceding the development of medullary carcinoma of the thyroid in MEN 2. Amyloid in these lesions is composed of keratin rather than calcitonin, as seen in medullary carcinoma. While the origin of the skin lesion is unknown, it has been noted more frequently in association with specific mutations of the MEN 2 gene (specifically Cys634 to Tyr634). In a second variant, MEN 2A or familial MCT is associated with Hirschsprung's disease (congenital megacolon; see below). This is most frequently found with RET mutations involving Cys609, Cys615, and Cys620. The intestinal ganglioneuromatosis, the presence of mucosal neuromas, marfanoid habitus and medullated corneal nerves seen in MEN 2B appear to be related to the underlying genetic defect (Figure 22-5). The intestinal lesions can disrupt gut motility, resulting in periods of severe constipation or diarrhea.
Figure 22-5. Structure of GDNFR-α/RET receptor complex. GDNF identifies the ligand, glial cell line-derived neurotrophic factor, and GDNFR-α the GDNF receptor. TKD, tyrosine kinase domain.
The pathogenesis of the MEN 2 syndromes has been worked out in elegant detail. Traditional genetic linkage studies localized the defective genes in MEN 2A, MEN 2B, and familial medullary carcinoma of the thyroid to the pericentromeric region of chromosome 10. Subsequent refinement in these analyses indicated that the defective gene was either closely linked to or identical with the ret proto-oncogene. RET is a single transmembrane domain, tyrosine kinase-linked protein which forms part of the receptor for the glial cell line-derived neurotrophic factor (GDNF) (Figure 22-5). This receptor, GDNFRα-1, is a glycosyl phosphatidyl inositol-linked cell surface protein that has also been shown to bind additional ligands, neurturin and artemin. As depicted schematically in Figure 22-6, its most striking structural feature is a series of cysteine residues clustered just outside the membrane-spanning segment. These cysteine residues are thought to exert a tonic inhibitory control on RET activity in the normal cell. RET also has a cadherin-like domain in that portion of the molecule projecting into the extracellular space and a tyrosine kinase-like domain in the intracellular portion of the molecule. RET is expressed endogenously in a variety of cells of neural crest origin, and it appears to play an important role in development. Knockout of the RET gene locus in mice results in the absence of myenteric ganglia in the submucosa of the small and large intestine and a variety of genitourinary anomalies, implying an important role in renal development.
Characterization of the RET gene in patients with MEN 2A demonstrated a number of mutations in affected kindred members which were not present in their normal counterparts. The mutations were clustered in the cysteines located in RET's extracellular, juxtamembrane domain (Figure 22-6). Structurally, these cysteines are encoded by nucleotides in exons 10 and 11 of the RET gene. A number of these mutations were simple missense mutations, while others involved deletion or insertion of small segments of DNA, but in each case one of the aforementioned cysteines proved to be involved. Selective mutation of one of these six cysteines has now been shown to account for more than 97% of all RET mutations associated with MEN 2A. The most frequently mutated residue is Cys634. This amino acid is mutated to arginine (Arg), phenylalanine (Phe), serine (Ser), glycine (Gly), tyrosine (Tyr), or tryptophan (Trp) in approximately 84% of affected MEN 2A kindreds. It has been suggested that mutation of Cys634 to Arg634 is associated with the phenotypic expression of hyperparathyroidism, while mutation of Cys634 to any of the amino acids indicated above is linked to pheochromocytoma. It should be noted that the Cys-to-Arg mutation at position 634 is also the most common mutation at this position, accounting for about 64% of all codon changes at this location. Interestingly, a rare but unique mutation involving a four-amino-acid insertion between Cys634 and Arg635 has been described that results in MCT and a high incidence of hyperparathyroidism but not pheochromocytoma.
Figure 22-6. Structural schematic of wild type RET, including amino acids that have been shown to be mutated in different disease states. MEN 2A is associated with mutations of Cys609, Cys611, Cys618, Cys620, Cys630, Cys634, and, rarely, Leu790 and Tyr791. Familial medullary carcinoma of the thyroid is frequently associated with mutations at the same Cys residues with the exception of Cys634, as well as mutations at Glu768, Leu790, Tyr791, Val804, and Ser891. MEN 2B mutations typically involve Met918 or, more rarely, Ala883, Ser922, or Val804/Ser904. Hirschsprung's disease is associated with multiple mutations or deletions extending over the full length of the RET molecule.
Ironically, patients with familial medullary carcinoma of the thyroid display many of the same cysteine mutations identified in MEN 2A, implying the existence
of independent modulatory genomic factors which restrict the effects of the RET mutation to the parafollicular cells of the thyroid in familial medullary carcinoma of the thyroid. The exception is the mutation of Cys634 to Arg634, which is almost uniformly associated with MEN 2A. Several additional mutations have been identified that may be more specific for familial medullary carcinoma (Figure 22-6). The observation that mutations of Cys634 have higher transforming potential than mutations at these other residues has led to the suggestion that MEN 2A represents the more severe phenotype (versus familial MCT) along a spectrum of disease resulting from RET activation. In fact, the noncysteine mutations, which are more common in familial MCT (as high as 60% of familial MCT in one series), are associated with delayed onset of parafollicular C cell disease, but other clinical features (tumor size, bilaterality, the presence of nodal metastases) were not different in patients harboring cysteine versus noncysteine mutations.
Interestingly, patients with MEN 2B do not harbor mutations in the cysteine residues affected in MEN 2A and familial medullary thyroid carcinoma. Instead, the majority possess a single-point mutation involving conversion of Met918 to Thr918. This mutation is found in more than 95% of cases of MEN 2B. A minority of patients have been shown to harbor an independent mutation of Ala883 to Phe883, Ser922 to Tyr922, or a mono-allelic combination of Val804 to Met804 plus Ser904 to Cys904. MEN 2B arises from spontaneous de novo mutations in as many as 50% of affected individuals. For unknown reasons, these mutations are found almost exclusively on the paternal allele.
A number of the germline mutations identified in MEN 2A and familial medullary carcinoma of the thyroid and the Met918 mutation in MEN 2B have been demonstrated as somatic mutations in sporadic medullary carcinoma of the thyroid (about 30–40%) and pheochromocytoma (less than 10%). Sporadic parathyroid disease due to these mutations appears to be rare if it occurs at all. The presence of the Met918 mutation, in particular, is associated with a less favorable clinical outcome in sporadic medullary carcinoma.
Independent studies have shown a close linkage between the RET gene locus and Hirschsprung's disease, a disorder characterized by failure of myenteric ganglia to develop normally in the hindgut of affected individuals. This leads to impaired gut motility and, in severe cases, megacolon (a phenotype similar to that reported for the RET knockout mice). Examination of RET coding sequence in Hirschsprung kindreds revealed a variety of mutations in both the intracellular and extracellular domains, some of which (eg, deletions) would be predicted to eliminate normal expression of the RET gene. This, together with the findings in the RET, GDNF, and GDNFRα-1 knockout mice alluded to above, suggests that Hirschsprung's disease represents the null phenotype for the RET locus. Interestingly, several patients have been described who possess features of both MEN 2 and Hirschsprung's disease. The RET mutations in these cases have involved conversion of Cys609, Cys618, or Cys620 to Arg. These mutations, while promoting dimerization and increasing tyrosine kinase activity in the RET protein (see below), also appear to have difficulty trafficking to and accumulating in the plasma membrane at the cell surface. It is conceivable that predominance of one or the other of these mechanisms in different cell types could result in a phenotype characterized by both activation (eg, MEN 2) and suppression (eg, Hirschsprung's disease) of RET activity in the same individual.
By inference, the defect in RET function in MEN 2 or familial medullary thyroid carcinoma arises from increased or altered activity of the RET tyrosine kinase. In the case of RETmen2a, the increase in activity appears to arise from interference with the tonic inhibition of RET tyrosine kinase activity by the clustered cysteine residues in the extracellular domain. This leads to increased dimer formation, autophosphorylation, and tyrosine kinase activity in the mutant RET molecules. In the case of RETmen2b, there appears to be a change in substrate specificity of the tyrosine kinase that contributes to the phenotype. The activity of RETmen2b—rather than being restricted to conventional RET substrates (RET substrates are similar to those recognized by the epidermal growth factor receptor)—is capable of phosphorylating substrates normally recognized by members of the Src and Abl families of cytoplasmic tyrosine kinases, signaling pathways which are closely identified with the regulation of cell growth. Thus, it appears that RETmen2b has acquired the capacity for activation of a potent mitogenic pathway in the expressing endocrine cells merely by altering its selection of substrates for phosphorylation. RETmen2b also potentiates phosphorylation of Tyr1062 more effectively than RETmen2a. This tyrosine serves as a docking site for multiple effector proteins, including Shc and PI-3K, implying that RETmen2b may be more effective in triggering downstream signaling pathways.
Treatment of heritable medullary carcinoma of the thyroid should include total thyroidectomy with at least central lymph node dissection. Given the multicentric nature of the disease, subtotal thyroidectomy predictably results in recurrent disease. Basal or stimulated calcitonin levels are used in the postoperative setting to evaluate the presence of residual disease. The precise timing of surgery in patients with subclinical disease—ie,
positive by genetic testing but without clinical or laboratory abnormalities—is controversial (see below), but most clinicians would agree that in kindreds with MEN 2B or clinically aggressive medullary thyroid carcinoma, patients should undergo surgery as soon as the genetic defect is demonstrated. Typically, this is before age 6 months in MEN 2B and before 5 years in MEN 2A. Foci of microscopic MCT are common, and metastatic disease has been described in the first year of life in patients with MEN 2B. Patients should always be screened for the presence of pheochromocytoma before undergoing neck exploration. Surgery for metastatic disease is palliative and targeted at reducing tumor burden rather than cure. Localization techniques (eg, MRI or selective venous sampling for calcitonin) can be helpful in identifying foci of malignant tissue. Radiation and chemotherapy are of limited utility and are largely confined to later stages of the disease.
Treatment of pheochromocytomas in MEN 2 is similar to that for sporadic pheochromocytomas (see Chapter 11). Alpha- and (occasionally) beta-adrenergic blockade is used to control blood pressure and associated hyperadrenergic symptoms and to restore normal intravascular volume in preparation for surgical resection of the tumor. Given the propensity for bilaterality of pheochromocytomas in this disorder, some have favored bilateral adrenalectomy at the time of initial surgery. However, since the incidence of bilaterality is well under 100% and because these tumors are rarely malignant, the most prudent strategy in the face of unilateral adrenal enlargement would appear to be unilateral adrenalectomy at the initial surgery with careful attention at follow-up looking for the presence of disease in the contralateral adrenal gland. This vigilant approach has the advantage of minimizing morbidity from recurrent pheochromocytoma while sparing the patient the risks associated with lifelong adrenal insufficiency.
Genetic screening for MEN 2A, MEN 2B, or familial medullary carcinoma of the thyroid is routinely carried out using polymerase chain reaction (PCR)-based tests designed to identify specific mutations in the RET coding sequence (Figure 22-7). Known RET mutations account for more than 95% of all instances of multiple endocrine neoplasia, and selected mutations (eg, Cys634 to Arg634 in MEN 2A) account for a disproportionate number of affected individuals. Individuals lacking any of the known RET mutations can be tested using conventional haplotype analysis if informative genetic markers and affected family members are available. Biochemical testing using basal or stimulated plasma calcitonin levels has been largely supplanted by genetic screens. The biochemical tests remain useful, however, in identifying residual disease after thyroidectomy.
In view of the fact that a high proportion of cases of MCT are familial to begin with and as much as 6% of patients with apparently sporadic MCT harbor germline RET mutations, genetic testing for RET germline mutations is probably indicated for all patients presenting with MCT. Controversy persists, however, in terms of what should be done for patients once the mutation has been identified. Some investigators citing incomplete “clinical” penetrance (according to published data, 40% of gene carriers do not present symptomatically prior to age 70) have argued that employing solely genetic criteria in making the decision for operative intervention subjects a small minority of patients to premature thyroidectomy. They argue that genetic testing should be used to identify those patients who require close clinical and biochemical surveillance to assist with the timing of surgery. Ideally, such biochemical testing (eg, pentagastrin stimulation) should be performed on an annual basis. Exceptions to this general approach might include patients with MEN 2B or a particularly aggressive form of familial medullary carcinoma of the thyroid where the potential for significant morbidity and mortality would justify operation in any patient harboring the genetic defect regardless of the physical or biochemical manifestations of the disease.
The more widely shared view is that the true penetrance of medullary carcinoma of the thyroid—combined clinical and preclinical disease—in MEN 2A is closer to 100%. This, when coupled with the high degree of sensitivity and specificity of the PCR-based genetic screens, difficulties encountered in obtaining adequate long-term patient follow-up and biochemical screening, and the potential for false-positive pentagastrin stimulation tests—even within MEN 2 kindreds—has led to the recommendation that total thyroidectomy should be performed in all individuals harboring an MEN 2-associated RET mutation. This argument has now been supported by several clinical studies in which parafollicular cell hyperplasia as well as early medullary carcinoma of the thyroid have been identified in operative specimens taken from genetically affected individuals despite normal pentagastrin stimulation tests. False-positive biochemical tests are also a concern. There are several reports in the literature of patients in affected kindreds who have undergone total thyroidectomy following positive pentagastrin stimulation tests but did not, in fact, harbor the MEN 2 gene mutation. Histologic examination of excised tissues revealed parafollicular cell hyperplasia, presumably unrelated to MEN 2, but no medullary thyroid carcinoma. Collectively, these findings point out the relative deficiencies
of biochemical versus genetic testing and offer a compelling argument for early operation as a means of reliably eradicating the disease.
Figure 22-7. Screening for MEN 2A. Genetic screening has largely supplanted biochemical testing in identifying individuals at risk. Details of decisions regarding treatment are discussed in the text. Preoperative plasma calcitonin levels are potentially useful for follow-up but should not supplant genetic testing in assessing the need for surgery. (See also Figure 7-56.)
OTHER DISORDERS CHARACTERIZED BY MULTIPLE ENDOCRINE ORGAN INVOLVEMENT
Carney complex is an autosomal dominant disorder characterized by cardiac, endocrine, cutaneous, and neural tumors. Myxomas of the heart, breast, and skin are seen frequently in this disorder, as is spotty pigmentation of the skin (lentiginosis). Endocrine tumors include primary pigmented micronodular adrenocortical hyperplasia (an ACTH-independent form of Cushing's syndrome), follicular thyroid carcinomas, adrenocortical carcinoma, somatotroph adenomas of the pituitary gland, and large-cell calcifying Sertoli cell tumors of the testes. In approximately half of the known Carney complex kindreds, the genetic lesion maps to 17q22–24, a locus that harbors the type 1 regulatory subunit of protein kinase A (PKA). This gene functions as a classic tumor suppressor, and loss of heterozygosity at this locus is associated with the Carney phenotype. A second locus, presumably accounting for the remaining approximately half of Carney complex kindreds, is located at 2p16. The nature of the genetic lesion at this locus remains unknown at present; however, the phenotype
is indistinguishable from that found with the 17q22–24 mutations alluded to above.
Neurofibromatosis Type 1
Neurofibromatosis type 1 (Recklinghausen's disease) is an autosomal dominant genetic disorder characterized by a variety of skin manifestations, including café au lait spots, subcutaneous neurofibromas, and axillary and inguinal freckles as well as neural gliomas (eg, optic nerve) and hamartomas of the iris (Lisch nodules). In addition, patients may have a number of endocrine neoplasias, including pheochromocytoma, hyperparathyroidism, medullary carcinoma of the thyroid, and somatostatin-producing carcinoid tumors of the duodenal wall. The genetic lesion in neurofibromatosis type 1 is located at 17q11.2, a locus that harbors the neurofibromin gene. Neurofibromin is a homolog of the p21 Ras-dependent GTPase-activating proteins and is thought to function in a tumor suppressor mode through regulation of Ras-dependent signaling activity.
Von Hippel-Lindau Disease
Von Hippel-Lindau disease is a heritable autosomal dominant disorder characterized by retinal and cerebellar hemangioblastomas, renal cell carcinoma, islet cell tumors, pheochromocytomas, and renal, pancreatic, and epididymal cysts. The presence of pheochromocytomas and most of the islet cell tumors is confined to the type 2 variant of the disease, which accounts for 25–35% of affected kindreds. The genetic lesion has been localized to 3p25. The VHL protein, which is normally encoded by this locus, participates in the formation of a multiprotein complex involved in the regulation of hypoxia-induced genes, transcriptional regulation, fibronectin matrix assembly, and ubiquitin ligases.
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