* Objective.-To review the role of RET mutation analysis in the diagnosis of multiple endocrine neoplasia type 2 (MEN 2) and in presymptomatic screening for this disorder.
Data Sources.-Review of the medical literature and current clinical practice.
Conclusions.-RET mutation analysis is a sensitive and specific test for MEN 2. It plays a pivotal role in the diagnosis and management of patients and families with
MEN 2 and in the individual who presents with an apparently sporadic medullary thyroid carcinoma or pheochromocytoma. These disorders may first come to the attention of either the anatomic or clinical pathologist, who has the opportunity to see that appropriate testing is done. As with any familial disease, professional genetic counseling is an important part of the care of these patients.
(Arch Pathol Lab Med. 1999;123:1047-1049)
Genetic testing for familial cancer is controversial, particularly when it is used in a presymptomatic, predictive manner. In many cases the sensitivity and specificity of the tests are low, and the therapeutic options, if a test is positive, may be less than satisfactory or even non-- existent. This is not the case with multiple endocrine neoplasia type 2 (MEN 2). In the last decade diagnostic and predictive genetic testing has contributed enormously to the successful management of individuals and families with this disorder, helping to prevent serious disease and improving the quality of life.
THE MEN 2 SYNDROMES
The triad of medullary thyroid carcinoma (MTC), pheochromocytoma, and parathyroid hyperplasia/adenoma, now known as multiple endocrine neoplasia type 2A (MEN 2A), was described first by Sipple in 1961 and reported as an autosomal dominent familial disorder in 1962. In many families, lifetime clinical penetrance of MTC is near 60%, while biochemical testing will identify additional asymptomatic individuals to a level of more than 90% by the age of 40 years (see below).3 Approximately 50% may develop pheochromocytomas, and 10% to 15% parathyroid disease.4
Two additional variants of this syndrome are recognized today, familial MTC (FMTC) and multiple endocrine neoplasia type 2B (MEN 2B).4 As the name suggests, FMTC is characterized by MTC alone; affected individuals do not have pheochromocytomas or parathyroid disease. MEN 2B is characterized by early onset of often aggressive MTC and pheochromocytoma, but parathyroid disease does not occur. In addition, individuals with MEN 2B have multiple mucosal neuromas affecting the lips and gastrointestinal tract and a marfanoid habitus. All 3 of these disorders are due to mutations in the RET protooncogene and are inherited in an autosomal dominant pattern. New germline mutations leading to MEN 2A and FMTC are rare but account for approximately half of the cases of MEN 2B. (Note about nomenclature: MEN 2A, MEN 213, and FMTC refer to the specific syndromes. MEN 2 refers to any or all of these disorders.)
In the MEN 2 syndromes MTC development is preceded by generalized hyperplasia of the calcitonin-producing C cells in the thyroid, presumably the result of a proliferative stimulus by mutant RET. The C-cell hyperplasia can usually be demonstrated by careful study of the thyroid following immunohistochemical staining for calcitonin. In contrast, C-cell hyperplasia is usually not present in the thyroid glands of individuals with sporadic, nonfamilial MTC. This feature can be used to help distinguish between FMTC and nonfamilial MTC.5
THE RET PROTO-ONCOGENE
A decade of positional cloning activities was brought to a successful conclusion in 1993 with the identification of RET, a known receptor tyrosine kinase proto-oncogene, as the MEN 2 gene.6,7 Initial studies demonstrated that the vast majority of individuals with MEN 2A or FMTC carried a missense mutation in 1 of just 5 cysteine codons (609, 611, 618, 620, and 634) in exons 10 and 11 of the extracellular domain of RET.6-8 A different RET mutation was discovered in almost all individuals with MEN 213, a missense mutation in codon 918 (M918T) in exon 16, at the catalytic site of the tyrosine kinase domain.9 Interestingly, nearly 40% of sporadic, nonfamilial MTCs have been shown to have somatic M918T mutations identical to the germline MEN 2B mutation, while only a small number harbored somatic mutations identical to the common MEN 2A germline mutations.10 Later studies revealed a few additional rare germline RET mutations in FMTC and MEN 2A (see below).
BIOCHEMICAL TESTING FOR MEN 2
For several decades biochemical testing has been the principal means for detecting MTC, pheochromocytoma, and parathyroid hyperplasia/adenoma. Urine and/or plasma catecholamine measurements are well-known tests for pheochromocytoma, as are serum calcium and parathyroid hormone measurements for parathyroid disease. Testing for MTC deserves additional comment.
MTC is the most common expression of the MEN 2 disorders and potentially the most serious in the sense that it may progress to metastatic disease without noticeable symptoms or physical signs. However, since MTCs are derived from the calcitonin-producing C cells of the thyroid, an increased C-cell mass is reflected by an increased level of circulating calcitonin in the blood. Calcitonin is an excellent tumor marker, particularly when it is measured following its release from C cells by pharmacologic stimulation with calcium and pentagastrin.11 In many cases, it is sufficiently sensitive to identify C-cell hyperplasia in an MEN 2 patient prior to malignant transformation to MTC. Before RET mutation analysis was available, this test was essential for early identification of affected individuals in families at risk for MEN 2A or FMTC. It is still a useful test in many circumstances, particularly when dealing with families in which a RET mutation has not been found.
DISTINGUISHING SPORADIC AND FAMILIAL MTC BY HISTOPATHOLOGY
Sporadic, nonfamilial MTC is 3 times more common than MTC associated with the MEN 2 syndromes,so but even in the absence of a family history suggestive of familial disease, the possibility of familial disease must always be investigated. Useful information in this regard can be obtained by examining the resected thyroid for C-- cell hyperplasia or multifocal MTC, which are generally associated only with the familial disorder. Examination of the entire thyroid gland, with immunohistochemical staining for calcitonin to identify C cells, is often necessary.5
RET MUTATION ANALYSIS IN MEN 2
Fortunately the spectrum of RET mutations in MEN 2A and FMTC is limited to codons 609, 611, 618, 620 (exon 10), and 634 (exon 11) in MEN 2A and to these same 5 codons plus 2 additional rare mutations in codons 768 (exon 13) and 804 (exon 14) in FMTC. Only 1 RET mutation, M918T (exon 16), has been associated with MEN 2B.12 These mutations are analyzed easily by direct sequencing or by various screening techniques. Rare single cases of additional RET mutations associated with MEN 2 have been described. A useful source of information about RET mutations is available online from The Human Gene Mutation Database Cardiff, http://www.uwcm.ac.uk/ uwcm/mg/search/120346.html.
The likelihood that an individual (or family) with MEN 2A (that is, MTC plus pheochromocytoma and/or parathyroid hyperplasia/adenoma) will have 1 of these mutations is 90% to 95%. In the case of FMTC, the likelihood is slightly lower, 85% to 90%. Almost all cases of MEN 2B, 95% to 98%, are mutation positive.12 The sensitivity of mutation analysis, therefore, depends on the exact phenotype of the patient. The specificity of the tests) is virtually 100%.
Mutation analysis is useful to (1) identify at-risk individuals in known, mutation-positive MEN 2 families; (2) find a marker (that is, a RET mutation) for testing at-risk members of a family with clinically apparent MEN 2; and (3) determine if an apparently sporadic case of MTC or pheochromocytoma is actually the presenting lesion in a patient with MEN 2.
In a known, mutation-positive family, the advantages of testing at-risk individuals for the presence of the mutation are obvious: timely therapy and clinical monitoring for those who are affected and cessation of unnecessary anxiety and possible unnecessary clinical studies for those who are unaffected. In this setting, the test is virtually 100% sensitive and specific. A consensus is developing that early thyroidectomy, perhaps as soon as age 5 years, is a better course of action than periodic biochemical testing for C-cell hyperplasia/MTC.4 If that is the case, then RET mutation analysis should be done by the age of 5 years. Periodic evaluation for pheochromocytoma and parathyroid disease should continue indefinitely. While the scientific argument for predictive mutation analysis in this setting is strong, powerful emotional and ethical issues must be considered as well, particularly when testing is done on children. Professional genetic counseling should always be part of the care of these families.
In a family with clinically apparent MEN 2A, mutation analysis will be positive 90% to 95% of the time. In that case, the family may be managed as described above. If mutation analysis is negative, however, several courses of action should be considered. First, there should be a careful review of family medical records to document the presence of MEN 2: histologic proof of at least 2 cases of MTC or of MTC and pheochromocytoma. Second, periodic biochemical testing for MTC as well as for pheochromocytoma and parathyroid disease should be done until the age of 35 to 45 years.4,10 Finally, genetic linkage analysis with markers at the RET locus on chromosome 10q11.2 may be investigated as an alternate means of identifying gene carriers in large families with many affected individuals. Professional genetic counseling is essential in this setting.
Mutation analysis is also indicated in the evaluation of apparently sporadic MTC and pheochromocytoma. Because MEN 2A and FMTC are not fully penetrant, because of the small size of some families, and because family medical histories may be incomplete or inaccurate, many cases of apparently sporadic MTC are in fact familial. Approximately 2% to 4% of patients with apparently sporadic pheochromocytoma also carry a germline RET mutation.13,14 If a germline RET mutation is found, the situation is made clear, and the patient and his family should be managed as described above. If RET mutation analysis is negative, the likelihood of familial disease is significantly reduced. In this case Bayesian analysis is useful in determining the lowered risk of familial disease. For example, in an isolated case of MTC in a 60-year-old man, the initial likelihood of familial disease could reasonably be estimated at 20%. If the MTC is not multifocal and there is no apparent C-cell hyperplasia on careful examination of the thyroid, the risk may be lowered to less than 10%. Assuming that the mutation analysis will be positive in 90% of familial cases and is negative in the case in question, the risk of familial disease if further reduced to less than 1%. Although this is rarely done, sometimes useful information may also be obtained by examining the MTC tissue directly for a somatic M918T mutation. Since somatic M918T mutations may occur in up to 40% of sporadic MTCs and are rarely seen in familial MTCs, the finding of this mutation would further tip the balance in the direction of sporadic MTC. Professional genetic counselors, who are trained to make this type of risk assessment, should be consulted.
GENOTYPE/PHENOTYPE CORRELATION IN MEN 2A AND FMTC
The rare codon 768 and 804 mutations have only been associated with FMTC and, to the extent that one can draw conclusions from limited data, predict that individuals with these mutations will not develop pheochromocytoma or parathyroid disease. General inferences may also be drawn by examining genotype-phenotype correlations between the various codon 609, 611, 618, 620, and 634 missense mutations and the 3 MEN 2A variants (MTC plus pheochromocytoma, MTC plus parathyroid hyperplasia/adenoma, MTC plus pheochromocytoma plus parathyroid hyperplasia / adenoma) and FMTC. These correlations strongly suggest that codon 634 mutations are most likely to produce a phenotype that includes pheochromocytoma and/or parathyroid disease.12 But this information, which is still being accumulated and evaluated, is of little help in determining whether a given individual or family is at sufficiently low risk of developing pheochromocytoma to preclude careful monitoring for this tumor. Mutations at all of these codons have been associated with the full spectrum of MEN 2A tumors in some families, and at this time all mutation-positive individuals should be considered to be at high risk for developing pheochromocytoma and/or parathyroid disease.
RET mutation analysis is a sensitive and specific test for MEN 2. It plays a pivotal role in the diagnosis and management of patients and families with MEN 2 and in the individual who presents with an apparently sporadic MTC or pheochromocytoma. These disorders may first come to the attention of the anatomic or clinical pathologist, who has the opportunity to see that appropriate testing is done. As with any familial disease, professional genetic counseling is an important part of the care of these patients.
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2. Cushman P. Familial endocrine tumors: report of two unrelated kindred affected with pheochromocytomas, one also with multiple thyroid carcinomas. Am Med. 1962;32:352-360.
3. Easton DF, Ponder MA, Cummings T, et al. The clinical and screening age-- at-onset distribution for the MEN-2 syndrome. Am J Hum Genet. 1989;44:208215.
4. Sherman SI, Gagel RF. Disorders affecting multiple endocrine systems. In: Fauci AS, Braunwald E, Isselbacher KJ, et al, eds. Harrison's Principles of Internal Medicine. 14th ed. New York, NY: McGraw-Hill; 1998:2131-2135.
5. Block MA, Jackson CE, Greenawald KA, Yott JB, Tashjian AH. Clinical characteristics distinguishing hereditary from sporadic medullary thyroid carcinoma. Arch Surg. 1980;115:142-148.
6. Mulligan LM, Kwok JBJ, Healey CS, et al. Germ-line mutations of the RET proto-oncogene in multiple endocrine neoplasia type 2A. Nature. 1993;363:458460.
7. Donis-Keller H, Dou S, Chi D, et al. Mutations in the RET proto-oncogene are associated with MEN 2A and FMTC. Hum Molec Genet. 1993;2:851-856.
8. Xue F, Yu H, Maurer LH, et al. Germline RET mutations in MEN 2A and FMTC and their detection by simple DNA diagnostic tests. Hum Molec Genet. 1994;3:635-638.
9. Hofstra RMW, Landsvater RM, Ceccherini I, et al. A mutation in the RET proto-oncogene associated with multiple endocrine neoplasia type 213 and sporadic medullary thyroid carcinoma. Nature. 1994;367:375-376.
10. Ponder BAJ. Multiple endocrine neoplasia type 2. In:Vogelstein B, Kinzler KW, eds. Genetic Basis of Human Cancer. New York, NY: McGraw-Hill; 1998: 475-487.
11. Wells SA, Baylin SB, Linehan WM, Farrell RE, Cox EB, Cooper CW. Provocative agents and the diagnosis of medullary carcinoma of the thyroid gland. Ann Surg. 1978;188:139-141.
12. Eng C, Clayton D, Schuffenecker I, et al. The relationship between specific RET proto-oncogene mutations and disease phenotype in multiple endocrine neoplasia type 2: International RET Mutation Consortium analysis. )AMA. 1996;276: 1575-1579.
13. Hartmut PH, Neumann MD, Berger DP, et al. Pheochromocytomas, multiple endocrine neoplasia type 2, and von Hippel-Lindau disease. N Engl / Med. 1993;329:1531-1538.
14. Eng C, Crossey PA, Mulligan LM, et al. Mutations in the RET proto-oncogene and the von Hippel-Lindau disease tumor suppressor gene in sporadic and syndromic phaeochromocytomas. J Med Genet. 1995;32:934-937.
Accepted for publication May 21, 1999.
From the Department of Pathology, Dartmouth-Hitchcock Medical Center, Lebanon, NH.
Presented at College of American Pathologists Conference XXXIV, Molecular Pathology: Role in Improving Patient Outcome, Bethesda, Md, February 26-28, 1999.
Corresponding author: Walter W. Noll, MD, Department of Pathology, Dartmouth-Hitchcock Medical Center, One Medical Center Dr, Lebanon, NH 03756.
Copyright College of American Pathologists Nov 1999
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