6.12 Multiple endocrine neoplasia type 2

Multiple endocrine neoplasia type 2 (MEN 2) is a rare cancer susceptibility syndrome which has at least three distinct variants: MEN 2A, MEN 2B, and familial medullary thyroid carcinoma (FMTC). The syndrome was first described by John Sipple in 1961 (1). The features of MEN 2A and its clinical variants are outlined in Box 6.12.1. Medullary thyroid carcinoma (MTC) is seen in all variants of MEN 2A and is frequently the earliest neoplastic manifestation, reflecting its earlier and overall higher penetrance. MEN 2 is due to the autosomal dominant inheritance of a germline missense mutation in the ‘hot-spot’ regions of the rearranged during transfection (RET) (OMIM 164761) proto-oncogene (2, 3). MEN 2 has an estimated prevalence of 1:30 000, with MEN 2A accounting for more than 75% of cases. The introduction of RET screening in family members of affected individuals has significantly altered the clinical outcome of MEN 2, by allowing prophylactic surgery for MTC, and screening enabling early intervention for phaeochromocytoma (4, 5). Prior to the availability of genetic screening, more that half of MEN 2 affected individuals died before or during the fifth decade from metastatic MTC or cardiovascular complications from an underlying phaeochromocytoma.

MEN 2A is characterized by MTC, unilateral or bilateral phaeochromocytoma, and primary hyperparathyroidism, due to parathyroid cell hyperplasia or adenomas. Rare variants include MEN 2A with cutaneous lichen amyloidosis, a pruritic cutaneous rash over the upper back, and MEN 2A associated with Hirschsprung’s disease, characterized by the absence of autonomic ganglion cells within the distal colonic parasympathetic plexus. In FMTC, MTC is the only clinical manifestation and this diagnosis requires more than 10 carriers in the kindred, multiple carriers or affected members over the age of 50 years, and an adequate medical history, especially in family members (4). These strict criteria attempt to prevent incorrect diagnosis of FMTC rather than MEN 2A and hence avoid the potentially catastrophic effects of failing to screen for a phaeochromocytoma. MEN 2B is the most aggressive MEN 2 variant and is characterized by MTC and phaeochromocytomas, but not primary hyperparathyroidism. Affected individuals may exhibit mucosal neuromas (lips, tongue, gastrointestinal tract) and skeletal abnormalities including a marfanoid habitus and kyphoscoliosis. In contrast to patients with Marfan’s syndrome, MEN 2B patients do not exhibit lens or aortic abnormalities.

The RET gene is situated on the pericentromeric region of chromosome 10 and has 20 exons. It encodes for the transmembrane RET receptor tyrosine kinase, expressed by cells derived from the neural crest. This receptor comprises an extracellular region which includes four cadherin-like domains, a calcium-binding site and a cysteine-rich domain, a transmembrane region, and an intracellular component containing at least two tyrosine kinase domains (Fig. 6.12.1). The extracellular domain is important for receptor dimerization and cross-phosphorylation whereas the intracellular tyrosine kinase domains affect adenosine triphosphate binding. Several functional ligands of RET have been identified, including glial cell line-derived neurotrophic factor (GDNF). These ligands, in association with the extracellular protein GDNF receptor α-1 (GFRα-1), bind to the extracellular RET receptor domain, inducing a homodimerization of RET molecules and a specific activation of the intracellular tyrosine kinase domain.

Whereas RET mutations associated with nonsyndromic Hirschprung’s disease arise from loss of function mutations, RET gain of function mutations in tyrosine signalling are associated with MEN 2 (6). The exact sequence of molecular events directing the transition from normal to hyperplasia to tumour is unclear. Oncogenic activation of the RET receptor due to germline mutations of RET are likely to initiate events as an inherited ‘first hit’, resulting in C cell and adrenal medullary hyperplasia. Progression to MTC and phaeochromocytoma requires second somatic ‘hits’ in activated C cells and adrenal medullary cells. The higher penetrance of MTC compared to phaeochromocytoma or parathyroid hyperplasia/ adenoma within MEN 2 suggests increased susceptibility of C-cell RET activation compared to adrenal medullary or parathyroid cells (7).

Unlike MEN 1, strong genotype–phenotype correlations exist within MEN 2 such that there are clear associations between mutations at specific codons and MEN 2 subtypes (4). Mutations clustered in the cysteine-rich extracellular domain (codons 609, 611, 618, 620, 630, and 634) are the primary causative factor in approximately 98% of cases of MEN 2A. Since these highly conserved cysteines are important for receptor dimerization, mutations result in ligand-independent dimerization and activation of the RET receptor complex (7). Mutations in the intracellular tyrosine kinase domain (codons 768, 790, and 804) are less common, traditionally associated with FMTC, and rarely associated with other MEN 2A-related tumours (5). Ninety-five per cent of MEN 2B cases involve a single point mutation leading to the substitution of methionine 918 for a threonine altering the substrate recognition pocket of the catalytic core of the receptor. Reports of the MEN 2A variant associated with cutaneous lichen amyloidosis all describe mutations in codon 634. MEN 2A-Hirschprung disease variants are associated with mutations in codons 609, 618, and 620 (6).

MTC arises from the parafollicular C cells of the thyroid. These neuroendocrine cells are derived from the neural crest and secrete calcitonin. Most patients with MTC have sporadic (nonfamilial) disease and 25–30% of cases of MTC are associated with MEN 2. In patients with MEN 2, MTC is usually bilateral and multifocal and C-cell hyperplasia represents the premalignant precursor of MTC. The aggressiveness of MEN 2-associated MTC depends on the variant of MTC, with MEN 2B being associated with the most aggressive forms. This variability reflects the underlying mutated RET codon. Presentation of MTC may be with a neck mass, or symptoms from distant metastases in association with elevated calcitonin (diarrhoea, flushing, weight loss, or bone pain). Circulating calcitonin concentrations, either basal or stimulated following pentagastrin administration, may be used as a tumour marker to detect MTC or to monitor disease progression or recurrence following surgery. However, biochemical screening for diagnosis of MTC in MEN 2 by measuring basal or stimulated calcitonin is largely obsolete due to the widespread availability and high diagnostic accuracy of genetic screening for RET mutations (4). Cross-sectional imaging of MTC using CT or MRI may be useful when planning surgery and metastatic disease can be detected using radioisotopes including ¹³¹I-metaiodobenzylguanidine (MIBG) and pentavalent ^99mTc-dimercaptosiuccininc acid (8).

Surgery represents prevention or cure in MTC and timing of prophylactic thyroidectomy is dictated by the underlying RET mutation. The major prognostic factor is tumour stage at presentation and, hence, early surgical intervention before cervical lymph node metastases appear is necessary to improve survival. Predictive genetic testing of members of MEN 2 families has enabled presymptomatic individuals at risk of developing MTC to be identified and a prophylactic thyroidectomy to be performed. RET mutations have been categorized as highest, high, and least risk in terms of guiding appropriate timing of thyroidectomy (4, 5). Patients with the highest risk mutations (codons 883, 918, or 922) should have a total thyroidectomy with central compartment node dissection performed between 6 and 12 months of age. In those with high-risk RET mutations, in codons 609, 611, 618, 620, 630, and 634, prophylactic thyroidectomy should be performed by age 5 years. The least risk is associated with RET codon mutations 768, 790, 791, 804, and 891, where surgery should be performed between age 5 and 10 years. MTC is not radiosensitive and standard chemotherapy regimens are of limited benefit. A recent development is targeted oncoprotein-specific therapy in the form of tyrosine kinase inhibitors and clinical trials are underway to ascertain their efficacy (9).

Phaeochromocytomas occur in approximately 50% of MEN 2A or 2B patients. These are usually benign, but are bilateral in up to 80% of cases and invariably arise from the adrenal medulla. In 25% of cases, phaeochromocytoma is the first clinical manifestation of MEN 2, compared to MTC in 40%. The highest risk of developing phaeochromocytoma is associated with RET mutations in codons 634 or 918 (10). Although certain RET mutations are not associated with phaeochromocytomas, these data describe only a small number of patients (11) and therefore, periodic biochemical screening of all RET mutation carriers for phaeochromocytoma should be performed from age 5–10 years.

Screening should take the form of urine or plasma metanephrine or catecholamine measurements. Following confirmation of the diagnosis biochemically, localization of the tumour can be carried out using cross-sectional imaging and MIBG scintigraphy (8). Surgical removal is the mainstay of treatment and the first-line choice is a laparoscopic approach if the tumour(s) is amenable to this technique. Some centres advocate the use of cortical sparing adrenalectomy for bilateral disease to reduce the increased mortality associated with bilateral adrenalectomy, which results from adrenal cortical insufficiency. Regular screening postoperatively should be undertaken to assess for recurrence or the development of a contralateral phaeochromocytoma in unilateral disease. Patients should be prepared for surgery using α-blockade, initially using intravenous phenoxybenzamine, and subsequent β-blockade if there are concerns regarding tachycardia. Prior to thyroidectomy, biochemical screening for phaeochromocytoma should be performed to prevent an intraoperative hypertensive crisis secondary to an undiagnosed phaeochromocytoma.

Primary hyperparathyroidism is a feature in approximately 30% of patients with MEN 2 and results from hyperplasia of the parathyroid glands. Adenomas may develop on a background of hyperplasia. Compared to MEN 1, parathyroid disease in MEN 2A is usually milder and has a later onset. Diagnosis is made by demonstrating hypercalcaemia in the context of an inappropriately normal or elevated parathyroid hormone level. In view of multigland involvement, a common surgical approach is removal of three and a half parathyroid glands.

In view of the clear associations between specific RET mutations and the potential risk for local and distant metastases from MTC at an early age, RET mutation testing allows guidance regarding timing of prophylactic thyroidectomy (4, 12) (see above, Clinical Management of Medullary Thyroid Cancer in MEN 2). RET mutational analysis should be initially performed in a family member known to have MEN 2 to determine the specific RET mutation for that family. Ninety-eight per cent of MEN 2 index cases have an identifiable RET mutation. Therefore, following the likely identification of the RET mutation in the index case, all members of that family of unknown RET status should be subsequently definitively genotyped. RET genotyping requires only a small volume of blood and can therefore be performed at birth or shortly after.

Approximately 98% RET mutations predisposing to MEN 2A and of 80% of FMTC are confined to exons 10 and 11. The majority of MEN 2B cases are associated with mutations in exon 16. However, as genetic analysis has become more common place in screening for RET mutations, cases of FMTC–MEN 2A have also been described associated with mutations in exons 8, 13, 14, and 15. Therefore, if a RET mutation in a family is unknown, it is important that if exons 10, 11, and 16 are negative, sequencing of exons 8, 13, 14, and 15 should be performed.

Since the discovery more than a decade ago that germline mutations in the RET proto-oncogene are associated with MEN 2, the subsequent introduction of genetic screening for RET mutations has had a significant impact on the clinical outcome of MEN 2. The clear genotype–phenotype relationship which exists in MEN 2 has enabled the risk stratification of RET mutation carriers following genetic screening. This, in turn, has allowed early intervention and potential cure in MTC by guiding prophylactic thyroidectomy and regular screening for development of phaeochromocytoma, once both major causes of death in these individuals. It is hoped that the future will see targeted molecular therapies to improve outcome in those previously unscreened individuals who are diagnosed with disease manifestations of MEN 2.