Sipple described pheochromocytoma associated with carcinoma of the thyroid gland in 1961.1 Seven years later, multiple endocrine neoplasia type 2 (MEN2) was recognized as a distinct entity from MEN type 1 (MEN1) after analysis of a family with cases of pheochromocytoma, medullary thyroid carcinoma (MTC), and parathyroid hyperplasia.

MEN2 is an autosomal dominant tumor susceptibility syndrome. There are several variants of MEN2, but they are all caused by an activating mutation in the rearranged during transfection (RET) proto-oncogene. The two main variants are MEN2A and MEN2B. More rare variants of MEN2 include familial MTC (FMTC), MEN2A with cutaneous lichen amyloidosis, and MEN2A or FMTC with Hirschsprung’s disease (Table 22-1).

MEN2 has been identified to date in about 500 to 1000 kindreds.2,3 All variants of MEN2 show a high penetrance for MTC. In fact, more than 90% of MEN2 carriers eventually show evidence of MTC or its precursor, C-cell hyperplasia.4,5 MEN2A, which accounts for 75% of MEN2 cases, is a syndrome of MTC in more than 90% of adult gene carriers, unilateral or bilateral pheochromocytoma in 50%, and multigland parathyroid tumors in 20% to 30%.3–5 MTC is the first neoplastic manifestation in most MEN2 kindreds because of its earlier and overall higher penetrance. MEN2B is characterized by MTC and pheochromocytoma, decreased upper-to-lower body ratio, a marfanoid habitus, and mucosal and intestinal ganglioneuromatosis. Hyperparathyroidism (HPT) does not occur in individuals with MEN2B.6 Patients with FMTC only develop MTC; they do not develop any of the other tumors associated with MEN2A and 2B.

To be classified as having FMTC, a kindred must have more than 10 carriers displaying MTC; multiple affected members or carriers older than age 50 years; and an adequate medical history to exclude pheochromocytomas or HPT, especially in older members.3 These rigorous criteria are used to avoid misclassifying small MEN2A kindreds whose members may not yet have developed pheochromocytomas or HPT. Ultimately, 20% to 25% of MTC cases are recognized as hereditary (see Chapter 6).

Mutations in the RET gene, located near the centromere on chromosome 10, cause MEN2, and the affected protein is a receptor tyrosine kinase.7 Its extracellular portion contains four cadherin-like repeats, a calcium-binding site, and a cysteine-rich domain. The intracellular portion contains a typical tyrosine kinase domain. RET disruption by germline mutations in humans and mice causes congenital aganglionosis of the gastrointestinal (GI) tract, leading to Hirschsprung’s disease. The RET protein is a subunit of a multimolecular complex that binds growth factors of the glial-derived neurotrophic factor family.8 Approximately 98% of MEN2 patients carry germline point mutations in RET.3,9 Most MEN2A mutations affect cysteines in the extracellular cysteine-rich domain of RET. MEN2A is associated most frequently with mutations of codon 634 (85%), particularly C634R. Most MEN2B patients carry the M918T mutation in RET kinase domain, but only a small fraction of patients harbor the A883F substitution. The mechanisms leading to RET oncogenic conversion in individuals with MEN2 depend on the site of the amino acid change. RET cysteine mutants form covalent dimers that display constitutive kinase activity; cysteine removal is believed to prevent the formation of intramolecular disulfide bonds, allowing free cysteine residues to form intermolecular bonds.8

Patients either present as an index case or as a member of a known MEN2 kindred. Index cases are most likely to present with MTC (i.e., a thyroid nodule), and the age of onset depends on the underlying RET mutations (see below). Occasionally, an index case may present with signs and symptoms of pheochromocytoma. It is unusual for MEN2A patients to present primarily with HPT.

MEN2 carrier determination may lead to highly effective clinical intervention and possible cure, particularly for MTC, by total thyroidectomy with possible lymphadenectomy.4 Additionally, DNA-based testing is now widely available, and only a limited number of mutations have been identified.3 Based on the specific RET mutation genotypes, many MEN2 carriers should undergo total thyroidectomy to prevent the expression of clinical MTC. In contrast to MEN1, phenotype–genotype correlations are seen in MEN2, which directs the management of these patients (see Management).3,9,10 The results of a genetic test thus may dramatically alter treatment, especially in presymptomatic patients.

Genetic Testing

Currently, 15 clinical laboratory improvement amendments (CLIA)-certified laboratories in the United States offer genetic testing for RET mutation. The standard screening test involves direct germline DNA sequencing using polymerase chain reaction for mutations in exons 10, 11, and 13 to 16. An RET germline mutation can be identified in more than 88% of FMTC, 95% of MEN2B, and 98% of MEN2A kindreds. After a mutation is identified in a family, screening at-risk relatives involves targeted DNA sequencing. If a mutation is not identified, the remaining 15 exons should be sequenced. This latter analysis is currently available only in research laboratories. If the extended RET mutation testing results are negative in the index case of a family, the family pattern of MEN2 is suspicious for an undiscovered RET mutation, then haplotype or genetic linkage testing of the RET locus should be considered.

Indications for RET Gene Mutational Analysis

Consensus guidelines for MEN2 genetic testing include confirmation of a clinical diagnosis in an index case, identification of asymptomatic at-risk relatives, and discontinuation of clinical screening in “at-risk” relatives with negative test results for a known mutation within the kindred.2 All first-degree relatives should be screened, and screening for second-degree relatives should be considered if testing of the first-degree link is not possible. The likelihood of an RET germline mutation in a patient with apparently sporadic MTC is 1% to 7%.11 An RET germline mutation is more likely to be identified in a patient with MTC diagnosed at an early age or with multifocal MTC. Given the critical implications of finding a RET mutation, all individuals with sporadic MTC should be tested for an RET germline mutation.2 Genetic testing also is beneficial for all children who have apparently sporadic cases of Hirschsprung’s disease and pheochromocytoma because of these disorders’ known association with MEN2. Genetic counseling issues are similar among all familial cancer syndromes. Before giving or authorizing blood or other tissue for genetic testing, children, adults, or the parents of children at risk should be counseled about the implications of genetic testing.