ANALYSIS OF RET PROTOONCOGENE POINT MUTATIONS DISTINGUISHES HERITABLEFROM NONHERITABLE MEDULLARY-THYROID CARCINOMAS

Background. The distinction of sporadic from inherited medullary thyro id carcinomas (MTCs) is of clinical importance because of the differen ces in prognosis, and the need for family screening for genetic counse ling required in the latter. Germline mutations in the RET protooncoge ne are associated with multiple endocrine neoplasia (MEN) type 2A, fam ilial medullary thyroid carcinoma (FMTC), and MEN type 2B. Somatic poi nt mutations in the same gene have been identified in a subset of spor adically occurring medullary thyroid carcinomas. Methods. A nonisotopi c polymerase chain reaction (PCR) based single strand conformation pol ymorphism (SSCP) analysis and heteroduplex gel electrophoresis method was used to screen DNA extracted from 32 formaldehyde fixed and paraff in embedded MTC specimens and normal tissue or blood of the same patie nt for point mutations in RET exons 10, 11, and 16. Point mutations we re identified by nonisotopic cycle sequencing of PCR-products using an automated DNA-sequencer. Results were compared with the disease pheno type, clinical findings, and follow-up. Results. Six different missens e germline mutations were identified at cysteine residues 618, 630, an d 634 of the cysteine-rich extracellular RET domain encoded by exons 1 0 and 11 in all patients with FMTC and MEN 2A. The frequency of mutati ons at codon 634 was higher in patients with MEN 2A than with FMTC and a 634 Cys --> Arg mutation was associated with parathyroid disease in three patients. A germline Met --> Thr point mutation at codon 918 of the RET tyrosine kinase domain was identified in all three patients w ith MEN 2B. Two patients with clinically sporadic MTCs and negative fa mily history exhibited a RET germline mutation at codon 634, indicatin g the presence of an nonpredicted inherited MTC. Furthermore, one pati ent had a 618 Cys --> Ser mutation in the tumor and nontumorous thyroi d DNA but not in blood DNA, indicating a mosaic mutation affecting thy roid tissue but not blood cells. Tumor specific (somatic) Met --> Thr point mutations at codon 918 were identified in 5 of 13 sporadic MTCs. The remaining eight sporadic MTCs lacked mutations in all three RET e xons tested.Conclusions. This study demonstrates that (1) the molecula r methods are not only suitable to identify asymptomatic individuals a t risk for MEN 2A, FMTC, and MEN 2B but also to distinguish heritable from nonheritable MTCs using archival tissue specimens, and (2) that m ore MTCs than clinically expected are heritable, indicating the need f or genetic analysis of all patients with MTC.