These syndromes have been described in nearly all ethnics group throughout the world. All three syndromes have autosomal dominant transmission patterns. Thus, children of affected individuals have a 50% chance of inheriting the genetic abnormality. The penetrance is virtually 100% by biochemical screening, however only 60-70% develop clinically apparent syndromes[3, 79]. About 25% of patients with medullary thyroid carcinoma have one of these familial syndromes. Because MTC is rare, the occurrence of two MTC's in a single family should raise suspicions of a familial syndrome. Up to 23% of pheochromocytoma patients also have a familial disorder, mainly either MEN-2 or von-Hippel-Lindau disease[80, 81].
Virtually 100% of patients with MEN-2A have C-cell hyperplasia or MTC, 50% have pheochromocytoma, and 20-30% have hyperparathyroidism. Some patients with MEN-2A develop Hirschsprung’s disease, which frequently precedes the development of MEN-2[82]. Patients with MEN-2B also have a 100% incidence of MTC, and frequently have pheochromocytomas. They also have a characteristic physical appearance at birth or shortly thereafter with mucosal neuromas of the eyelids, lips, and tongue, marfanoid habitus, and hyperflexible joints[83, 84]. They do not have the marfanoid lens or aortic abnormalities. Patients may also have abnormal gastrointestinal motility, due to wide spread ganglioneuromatosis of the gastrointestinal tract. Thus, even at a young age, patients with MEN-2B may have troubles with diarrhea, constipation, colonic dilatation, or even present with megacolon. A review of the literature shows that diverticulosis, diverticulitis, abscess and other complications have also been noted[85]. Patients with MEN-2B seldom have hyperparathyroidism. Patients with non-MEN familial medullary thyroid carcinoma have medullary thyroid carcinoma, but none of the other associated endocrine or neural-tissue involvement[86].
The major difference in patients with familial compared to the sporadic forms of these tumors is the multifocal nature of the lesions, the presence of hyperplastic states as precursors to frank tumors and different clinical behaviors.
Patients with genetic forms of MTC have diffuse bilateral C-cell hyperplasia of the thyroid, or frank MTC. The hyperplasia may not be apparent on routine histologic examination, thus immunostaining for calcitonin is essential in order to make the diagnosis. Detailed immunostaining may also reveal occult MTC. C-cell hyperplasia and occult MTC are consistently curable by total thyroidectomy, unlike clinically apparent MTC, where cure is infrequent[87, 88]. Patients with MEN-2A and 2B also may have adrenal hyperplasia preceding the development of pheochromocytoma. However, the clinical significance and management of adrenal hyperplasia in these patients is controversial.
The gene for all three diseases is the RET proto-oncogene, located at the centrometric region of chromosome 10 (10q11-2). The RET proto-oncogene gene is a transmembrane tyrosine kinase with a long extracellular domain, a single transmembrane region, and two cytosplasmic tyrosine kinase domains. The oncogene contains 21 exons spanning more than 60kb of genomic DNA. It is expressed in tissues of neural crest origin, and appears to have important function in cell migration and development. Ligands which bind to RET together with specific co-receptors include glial cell line-derived neurotrophic factor (GDNF), neurturin, artemin, and persephrin[89]. All MEN-2A families studied thus far have mutations of the RET proto-oncogene, but a few families with MEN-2B or non-MEN FMTC do not have such mutations. The vast majority of MEN-2A or FMTC patients will have a mis-sense mutation in the extracellular domain at a single codon (609, 611, 618, 620, or 634)[90]. Eighty-five percent of MEN-2A mutations occur at codon 634, whereas only 30% of mutations in non-MEN FMTC occur at codon 634[91]. Patients with MEN-2B typically have a point mutation in the intracellular kinase catalytic domain, most commonly a substitution of threonine for methionine at codon 918. A study of this mutation in transgenic mice revealed that it predisposes the mice to MTC[92]. RET mutations in MEN-2 typically result in constitutive activation. Different mutations lead to different levels of activation, which may have an affect on the clinical spectrum[93]. Mutations in the RET proto-oncogene are noted occasionally in patients with sporadic MTC, and rarely in patients with sporadic pheochromocytoma. Inactivating mutations in the extracellular domain of the RET proto-oncogene have been identified in patients with Hirschsprung's disease. Thus, the RET proto-oncogene appears to be associated with growth regulation of neuroendocrine cells. Mutations of the RET proto-oncogene act in a dominant fashion. Both peripheral leukocytes and tumors from patients with these syndromes contain a normal allele of the RET gene. Other genetic abnormalities identified in patients with MEN-2 and non-MEN FMTC include LOH at chromosome 1p, 22q, 17p or 3p. Thus, multi-step carcinogenesis may be applicable to patients with MEN-2.
Data are accumulating regarding genotype-phenotype correlations and mutations in various RET codons. For instance, the rare codon 768 and
804 mutations are associated with non-MEN FMTC, and these patients would not be expected to develop pheochromocytoma or hyperparathyroidism. However, current data are insufficient to determine the risk of pheochromocytoma in a given patient or family. Thus, all patients need to be carefully followed for the development of these tumors[89]. This may change in the future. A recent study showed that 20 of 21 patients with pheochromocytoma and MEN-2 had codon 634 or 918 mutations[94]. MTC aggressiveness may be related to the particular mutation in the codon[94]. The specific mutation in codon 634 in patients with MEN-2A may have an impact on tumor aggressiveness[95]. It is uncertain as to why the same point mutation can result in different clinical manifestations in the same family or in different families. This suggests that other genetic changes or modifiers are responsible for phenotypic differences. Alterations in the TGF-beta pathway, and increased expression of chondromodulin-1 may help explain the earlier onset of malignancy and skeletal abnormalities in MEN-2B compared to MEN-2A[96].
Biochemically, the tumors that arise in patients with these genetic syndromes are similar to those with the sporadic forms of the tumors. There are no distinct biochemical markers that allow identification of familial versus non-familial forms of the tumors.
Familial MTC tends to occur at a younger age than sporadic MTC. The clinical presentation and behavior differ with each of the clinical syndromes. Peak incidence of MTC is in the second and third decades for patients with MEN-2A. The cancer usually behaves in a relatively indolent fashion, even though it can metastasize early. Survival for patients with MEN-2A and MTC is somewhat better than for patients with sporadic MTC. A small subset of patients with MEN-2A have MTC which behaves very aggressively. Thus, there is a need for an aggressive surgical approach early on, to prevent widespread metastatic disease.
MTC in MEN-2B occurs in the first and second decades of life. It tends to be more aggressive and lethal[24, 97], and death from metastatic diseases has been reported in young children[98]. When MEN-2B is diagnosed, even in infants, surgical treatment for MTC should be performed as early as possible.
In non-MEN familial MTC, the peak incidences is in the fourth to fifth decade. MTC in this disorder behaves in the least aggressive fashion[86]. Although lymph node metastases are frequent, the disease usually follows an indolent course, and virtually never results in death.
Pheochromocytoma in MEN-2A and 2B has a peak incidence in the fourth and fifth decades. Although it may be detected at the same time as MTC, pheochromocytoma tends to develop at a later age than the thyroid disease[99]. Occasionally, pheochromocytoma is identified in childhood. These tumors are often bilateral and multiple. They are only occasionally malignant[100-102]. Classically, the ratio of norepinephrine to epinephrine is lower in the genetic forms of pheochromocytoma than in the sporadic form. Common early symptoms in these patients include palpitations, headaches, and anxiety. With aggressive family screening, some patients are identified who have no symptoms from the tumor. In patients in whom the disorder is not recognized, a hypertensive crisis may be induced by an operation or by childbirth.
Patients diagnosed with either MTC or pheochromocytoma should be studied for the possibility of MEN-2. A negative family history is not reliable, since about 40% of MEN-2A gene carriers do not develop clinically apparent disease. Thus, family members should be screened for both tumors. In a study of 174 patients with MTC screened for pheochromocytoma, 5 cases were identified in known MEN-2 families, and another 5 index cases were found, leading to the discovery of 5 new MEN-2 families[103].
In the past, members of MEN-2A and non-MEN FMTC families underwent yearly calcium/pentagastrin stimulation tests, starting before the age of five. However the identification of mutations of the RET proto-oncogene has virtually eliminated the need for calcium pentagastrin stimulation tests. Genetic screening with mutational analysis can identify gene carriers with extremely high accuracy. Multiple studies have confirmed that using molecular biology techniques, the index case of a new family, and every gene carrier in a known family of MEN-2A, can be identified with 100% accuracy[104-107]. The accuracy for identifying patients with MEN-2B and non-MEN FMTC is not quite as high, because of the small number of families that do not have the common RET proto-oncogene mutations. Members of MEN-2A families who are not gene carriers by genetic screening do not need to undergo biochemical testing. The accuracy of screening by direct analysis of RET proto-oncogene mutations for non-MEN FMTC still needs to be confirmed.
The biochemical diagnosis is similar to patients with sporadic disease. Patients with clinical MTC have elevated serum calcitonin. Pheochromocytoma is identified by elevated urinary epinephrine, norepinephrine, or VMA. Measurements of plasma normetanephrine and metanephrine may be more sensitive at detecting pheochromocytoma than urinary testing[108]. Patients with subclinical MTC usually do not have elevated basal serum levels of calcitonin. However, patients with C-cell hyperplasia or subclinical MTC increase calcitonin levels in response to calcium and/or pentagastrin. The combination test described by Wells and colleagues result in less false negatives than tests performed with either of the two agents alone[109]. This test consists of a 50 second infusion of calcium gluconate (2mg of elemental calcium/kg) followed by a 10 second bolus of pentagastrin (Peptavalon, Ayerst, 0.5mg/kg). Calcitonin levels are measured at 0, 2, 3.5, and 5 minutes, and generally peak at the 2 or 3.5 minute points. Pentagastrin is however, no longer available and reliance has to be placed on calcium infusion alone. Physicians screening patients for MTC must be thoroughly familiar with the particular calcitonin assay being used, as normal ranges vary somewhat. In general, normal females do not have a stimulated calcitonin level greater than 29 pg/ml, and in males the normal limit is 106 pg/ml. False positive results are uncommon. They occur in a small number of patients with normal thyroid glands, including the 5% of normal patients who have C-cell hyperplasia[87, 88, 110]. Provocative tests becomes abnormal at an average age of about 8-9 in gene carriers of MEN-2A.
Patients with MEN-2 syndromes must be screened for possible pheochromocytomas, before undergoing thyroidectomy for MCT. If pheochromocytoma is identified, surgery for this should always take precedence over any neck procedure, to avoid a potential hypertensive crisis following induction of anesthesia. Once a diagnosis of pheochromocytoma is established biochemically, non-invasive localization and the subsequent surgical approach is carried out as previously described for sporadic pheochromocytoma. Pheochromocytomas in MEN-2 patients almost always occur bilaterally, however they often do not occur synchronously. The contralateral adrenal lesion can develop many years later. Fortunately, pheochromocytomas in these patients are usually benign. Thus, surgery can be reserved for those patients with demonstrated elevated catecholamine secretion and with a discrete tumor or tumors, or symptoms. Useful tests in these patients include MIBG scans, CT scans, or MRI. Imaging studies usually reveal an abnormality when biochemical testing is abnormal[104]. Patients who present with bilateral disease should undergo bilateral adrenalectomy. Patients with unilateral disease should undergo unilateral adrenalectomy. Cortical-sparing adrenalectomy is an option in patients who require bilateral adrenal resection, resulting in 65% of patients corticosteroid independent, and a low (approximately 10%) recurrence rate[111]. These patients need to undergo yearly biochemical monitoring, to detect recurrent pheochromocytoma in the remaining adrenal gland. Interestingly, only one third to one half of these patients need a second surgery for recurrent pheochromocytoma in the remaining adrenal gland.
The primary management for genetic forms of MTC is total thyroidectomy. The goal is to do surgery before there are any clinical signs of disease, thus resulting in a high rate of cure. Patients with C-cell hyperplasia and microscopic MTC are consistently curable by total thyroidectomy, whereas patients with higher stage disease are not[109, 112]. Because of the field defect in the entire thyroid, and the multifocal nature of MTC in the genetic syndromes, total thyroidectomy is essential. Every effort must be made to leave no thyroid tissue or capsule behind. Failure to remove all thyroid tissue inevitably leads to recurrence[113, 114]. This point cannot be overemphasized. In addition to total thyroidectomy, central compartment lymph nodes should be excised, similar to the procedure done in patients with sporadic disease. In MEN-2 patients diagnosed by provocative testing, there are usually no palpable thyroid abnormalities, and the central lymph nodes are usually free of disease. For patients with involved central compartment lymph nodes, ipsilateral or bilateral modified neck dissection should be considered.
The timing of surgery depends on the particular syndrome. Patients with MEN-2A with positive biochemical screening should undergone total thyroidectomy no later than age 6. MTC in MEN-2B occurs at a younger age and is much more aggressive than in MEN-2A. As soon as the typical phenotype is recognized, total thyroidectomy should be performed, preferably before age 2, even if calcitonin levels are normal[115]. Two large European studies suggest that the particular codon affected can be used to help determine the timing of thyroidectomy[116, 117]. More data undoubtedly will come out in the future to help optimally time surgery.
The advent of genetic screening of first degree family members of those with newly discovered MTC has led to earlier diagnosis and treatment. Patients operated on who have C-cell hyperplasia or microscopic MTC have greater than a 90% cure rate. Provocative testing has been particularly useful in MEN-2A family members who lack the characteristics phenotypic features found in MEN-2B patients. Unfortunately, some patients do have distant metastases when first diagnosed, thus preventing any chance for cure[97].
Hyperparathyroidism is present about 20-30% of patients with MEN-2A, however it rarely occurs in MEN-2B patients. Surgery for MEN-2A parathyroid disease usually takes place during total thyroidectomy for MTC and is similar to that for patients with MEN-1 parathyroid disease. Subtotal parathyroidectomy or total parathyroidectomy with autotransplantation into the forearm muscle can be performed in hypercalcemic patients[106, 118]. Recurrence rarely occurs, even with long term follow-up. The surgical treatment of normocalcemic patients is controversial. One or two parathyroid glands may be found to be abnormal in normocalcemic MEN-2A patients. In patients whose family history reveals little hyperparathyroidism, more conservative surgery consisting of removal of the enlarged glands only can be considered. It should be noted that hyperparathyroidism rarely occurs after total thyroidectomy for C-cell hyperplasia detected by screening, even when the parathyroid glands are left in place.
The morbidity and mortality in patients with MEN-2 is due to recurrent medullary thyroid carcinoma. A study of 104 patients with sporadic and hereditary MTC revealed that patient age at presentation and tumor stage were the only independent predictors of survival[119]. The treatment of recurrent disease in patients with the genetic syndromes is similar to the sporadic counterparts. A study from the French Calcitonin Study Group involving 226 sporadic and heredity MTC patients showed that patients with preoperative calcitonin levels <50 pg/ml were more likely to normalize postoperatively[120]. If calcitonin levels remain elevated postoperatively, an aggressive approach may be considered in selected patients with disease confined to the neck. Prior to considering neck re-exploration, the evaluation should be thorough to exclude distant disease. In addition to CT or MRI of the neck, chest, and abdomen, laparoscopy has been advocated to exclude occult liver metastases[121]. Alternatively selected venous sampling can be performed for calcitonin. When elevated levels are isolated to one or both sides of the neck, modified unilateral or bilateral neck dissection is then performed. Such surgery can be performed by experienced groups with low morbidity[122]. There appears to be little role for the use of external beam radiation therapy in these patients[123]. There are few data on the use of combination chemotherapy in patients with systemic disease[124-127]. A variety of combinations including CVD have had limited success. Further study is clearly required in this area.