Mutations of the RET protooncogene have been identified in most patients with MEN 2 (Fig. 50-3). RET encodes a tyrosine kinase receptor that in combination with a co-receptor, GFRα, normally is activated by glial cell–derived neurotrophic factor (GDNF) or other members of this transforming growth factor β–like family of peptides, including artemin, persephin, and neurturin. In the C cell there is evidence that persephin normally activates the RET/GFRα-4 receptor complex and is partially responsible for migration of the C cells into the thyroid gland, whereas in the developing neuronal system of the gastrointestinal tract, GDNF activates the RET/GFRα-1 complex. RET mutations induce constitutive activity of the receptor, explaining the autosomal dominant transmission of the disorder.
Naturally occurring mutations localize to two regions of the RET tyrosine kinase receptor. The first is a cysteine-rich extracellular domain; point mutations in the coding sequence for one of six cysteines (codons 609, 611, 618, 620, 630, and 634) cause amino acid substitutions that induce receptor dimerization and activation in the absence of its ligand. Codon 634 mutations occur in 80% of MEN 2A kindreds and are most commonly associated with classic MEN 2A features (Figs. 50-3 and 50-2); an arginine substitution at this codon accounts for half of all MEN 2A mutations. All reported families with MEN 2A and cutaneous lichen amyloidosis have a codon 634 mutation. Mutations of codon 609, 611, 618, or 620 occur in 10–15% of MEN 2A kindreds and are more commonly associated with FMTC (Fig. 50-3). Mutations in codons 609, 618, and 620 also have been identified in a variant of MEN 2A that includes Hirschsprung disease (Fig. 50-3). The second region of the RET tyrosine kinase that is mutated in MEN 2 is in the substrate recognition pocket at codon 918 (Fig. 50-3). This activating mutation is present in ~95% of patients with MEN 2B and accounts for 5% of all RET protooncogene mutations in MEN 2. Mutations of codon 883 and 922 also have been identified in a few patients with MEN 2B.
Uncommon mutations (<5% of the total) include those of codons 533 (exon 8), 666, 768, 777, 790, 791, 804, 891, and 912. Mutations associated with only FMTC include codons 533, 768, and 912. With greater experience, mutations that once were associated with FMTC only (666, 791, V804L, V804M, and 891) have been found in MEN 2A as there have been occasional descriptions of pheochromocytoma. At present it is reasonable to conclude that only kindreds with codon 533, 768, or 912 mutations are consistently associated with FMTC; in kindreds with all other RET mutations, pheochromocytoma is a possibility. The recognition that germ-line mutations occur in at least 6% of patients with apparently sporadic MTC has led to the firm recommendation that all patients with MTC should be screened for these mutations. The effort to screen patients with sporadic MTC, combined with the fact that new kindreds with classic MEN 2A are being recognized less frequently has led to a shift in the mutation frequencies. These findings mirror results in other malignancies in which germ-line mutations of cancer-causing genes contribute to a greater percentage of apparently sporadic cancer than was considered previously. The recognition of new RET mutations suggests that more will be identified in the future.
Somatic mutations (found only in the tumor and not transmitted in the germ line) of the RET protooncogene have been identified in sporadic MTC; 25–60% of sporadic tumors have codon 918 mutations, and somatic mutations in codons 630, 768, and 804 have been identified (Fig. 50-3).
TREATMENT: Multiple Endocrine Neoplasia Type 2
SCREENING FOR MULTIPLE ENDOCRINE NEOPLASIA TYPE 2 Death from MTC can be prevented by early thyroidectomy. The identification of RET protooncogene mutations and the application of DNA-based molecular diagnostic techniques to identify these mutations have simplified the screening process. During the initial evaluation of a kindred, a RET protooncogene analysis should be performed on an individual with proven MEN 2A. Establishment of the specific germ-line mutation facilitates the subsequent analysis of other family members. Each family member at risk should be tested twice for the presence of the specific mutation; the second analysis should be performed on a new DNA sample and, ideally, in a second laboratory to exclude sample mix-up or technical error (see www.genetests.org for an up-to-date list of laboratory testing sites). Both false-positive and false-negative analyses have been described. A false-negative test result is of the greatest concern because calcitonin testing is now rarely performed as a diagnostic backup study; if there is a genetic test error, a child may present in the second or third decade with metastatic MTC. Individuals in a kindred with a known mutation who have two normal analyses can be excluded from further screening.
There is a consensus that children with codon 883, 918, and 922 mutations, those associated with MEN 2B, should have a total thyroidectomy and central lymph node dissection (level VI) performed during the first months of life or soon after identification of the syndrome. If local metastasis is discovered, a more extensive lymph node dissection (levels II to V) is generally indicated. In children with codon 611, 618, 620, 630, 634, and 891 mutations, thyroidectomy should be performed before age 6 years because of reports of local metastatic disease in children this age. Finally, there are kindreds with codon 609, 768, 790, 791, 804, and 912 mutations in which the phenotype of MTC appears to be less aggressive. A clinician caring for children with one of these mutations faces a dilemma. In many kindreds, there has never been a death from MTC caused by one of these mutations. However, in other kindreds, there are examples of metastatic disease occurring early in life. For example, metastatic disease before age 6 years has been described with codon 609 and 804 mutations and before age 14 years in a patient with a codon 912 mutation. In kindreds with these mutations, two management approaches have been suggested: (1) perform a total thyroidectomy with or without central node dissection at some arbitrary age (perhaps 6–10 years of age) or (2) continue annual or biannual calcitonin provocative testing with performance of total thyroidectomy with or without central neck dissection when the test becomes abnormal. The pentagastrin test involves measurement of serum calcitonin basally and 2, 5, 10, and 15 min after a bolus injection of 5 μg pentagastrin per kilogram of body weight. Patients should be warned before pentagastrin injection of epigastric tightness, nausea, warmth, and tingling of extremities and reassured that the symptoms will last ~2 min. If pentagastrin is unavailable, an alternative is a short calcium infusion performed by obtaining a baseline serum calcitonin and then infusing 150 mg calcium salt IV over 10 min with measurement of serum calcitonin at 5, 10, 15, 30 min after initiation of the infusion.
The RET protooncogene analysis should be performed in patients with suspected MEN 2B to detect codon 883, 918, and 922 mutations, especially in newborn children in whom the diagnosis is suspected but the clinical phenotype is not fully developed. Other family members at risk for MEN 2B also should be tested because the mucosal neuromas can be subtle. Most MEN 2B mutations represent de novo mutations derived from the paternal allele. In the rare families with proven germ-line transmission of MTC but no identifiable RET protooncogene mutation (sequencing of the entire RET gene should be performed), annual pentagastrin or calcium testing should be performed on members at risk.
Annual screening for pheochromocytoma in patients with germ-line RET mutations should be performed by measuring basal plasma or 24-h urine catecholamines and metanephrines. The goal is to identify a pheochromocytoma before it causes significant symptoms or is likely to cause sudden death, an event most commonly associated with large tumors. Although there are kindreds with FMTC and specific RET mutations in which no pheochromocytomas have been identified (Fig. 50-3), clinical experience is insufficient to exclude pheochromocytoma screening in these individuals. Radiographic studies such as MRI or CT scans generally are reserved for individuals with abnormal screening tests or symptoms suggestive of pheochromocytoma (Chap. 51). Women should be tested during pregnancy because undetected pheochromocytoma can cause maternal death during childbirth.
Measurement of serum calcium and parathyroid hormone levels every 2–3 years provides an adequate screen for hyperparathyroidism, except in families in which hyperparathyroidism is a prominent component, in which measurements should be made annually.
MEDULLARY THYROID CARCINOMA Hereditary MTC is a multicentric disorder. Total thyroidectomy with a central lymph node dissection should be performed in children who carry the mutant gene. Incomplete thyroidectomy leaves the possibility of later transformation of residual C cells. The goal of early therapy is cure, and a strategy that does not accomplish this goal is shortsighted. Long-term follow-up studies indicate an excellent outcome, with ~90% of children free of disease 15–20 years after surgery. In contrast, 15–25% of patients in whom the diagnosis is made on the basis of a palpable thyroid nodule die from the disease within 15–20 years.
In adults with MTC >1 cm in size, metastases to regional lymph nodes are common (>75%). Total thyroidectomy with central lymph node dissection and selective dissection of other regional chains provides the best chance for cure. In patients with extensive local metastatic disease in the neck, external radiation may prevent local recurrence or reduce tumor mass but is not curative. Chemotherapy with combinations of adriamycin, vincristine, cyclophosphamide, and dacarbazine may provide palliation. Clinical trials with small compounds (tyrosine kinase inhibitors) that interact with the ATP-binding pocket of the RET, vascular endothelial receptor, and type 2 and epidermal growth factor receptors and prevent phosphorylation have shown promise for treatment of hereditary and sporadic MTC. A phase I trial of vandetanib has shown that 45% of patients have a 30% or greater reduction of tumor size and prolongation of progression-free survival by at least 11 months. Similar phase II results have been observed for XL184, sunitinib, tipifarnib, and sorafenib, and phase II trials of E7080 and pazopanib are under way. It seems likely that one or more of these compounds will be approved for treatment of metastatic MTC within the next few years.
PHEOCHROMOCYTOMA The long-term goal for management of pheochromocytoma is to prevent death and cardiovascular complications. Improvements in radiographic imaging of the adrenals make direct examination of the apparently normal contralateral gland during surgery less important, and the rapid evolution of laparoscopic abdominal or retroperitoneal surgery has simplified management of early pheochromocytoma. The major question is whether to remove both adrenal glands or remove only the affected adrenal at the time of primary surgery. Issues to be considered in making this decision include the possibility of malignancy (<15 reported cases), the high probability of developing pheochromocytoma in the apparently unaffected gland over an 8- to 10-year period, and the risks of adrenal insufficiency caused by removal of both glands (at least two deaths related to adrenal insufficiency have occurred in MEN 2 patients). Most clinicians recommend removing only the affected gland. If both adrenals are removed, glucocorticoid and mineralocorticoid replacement is mandatory. An alternative approach is to perform a cortical-sparing adrenalectomy, removing the pheochromocytoma and adrenal medulla and leaving the adrenal cortex behind. This approach is usually successful and eliminates the necessity for steroid hormone replacement in most patients, although the pheochromocytoma recurs in a small percentage.
HYPERPARATHYROIDISM Hyperparathyroidism has been managed by one of two approaches. Removal of 3.5 glands with maintenance of the remaining half gland in the neck is the usual procedure. In families in which hyperparathyroidism is a prominent manifestation (almost always associated with a codon 634 RET mutation) and recurrence is common, total parathyroidectomy with transplantation of parathyroid tissue into the nondominant forearm is preferred. This approach is discussed earlier in the context of hyperparathyroidism associated with MEN 1.