SOMATIC MUTATIONS OF THE RET PROTOONCOGENE IN SPORADIC MEDULLARY-THYROID CARCINOMA ARE NOT RESTRICTED TO EXON-16 AND ARE ASSOCIATED WITH TUMOR RECURRENCE

Germline point mutations in exons 10, 11, and 16 of the ret protooncog ene have been identified as causative in multiple endocrine neoplasia type 2 and in familial medullary thyroid carcinoma (MTC). Somatic poin t mutations of the same gene, exclusively associated with codon 918 of exon 16, have also been reported in few cases of sporadic medullary t hyroid carcinoma. We analyzed the blood and tumor DNA of 19 patients w ith sporadic MTC and 6 patients with primary parathyroid adenoma for p oint mutations at exons 10, 11, and 16 of the ret protooncogene by res triction analysis of the PCR-amplified product and by sequence analysi s of exons 10 and 11. A Cys(634)-->Tyr mutation was found in both the tumoral and blood DNA of one patient, indicating that he was affected by an hereditary form of MTC, erroneously considered sporadic. In the other 18 patients with MTC, somatic point mutations of ret were found in 8 cases (44.4%). In 5 cases the mutation affected exon 16 (Met(918) -->Thr), and in 3 cases it affected exon 11 (Cys(634)-->Arg in 1 and C ys(634)-->Trp in 2); these 3 mutations were confirmed by sequence anal ysis. The remaining 10 patients had no mutation in exon 10 by either r estriction analysis or sequence analysis. Clinical data showed that 75 % of the patients whose tumor carried ret mutation had tumor recurrenc e and/or increased serum calcitonin concentrations during the postsurg ical follow-up period as opposed to 10% of the patients without mutati ons (P < 0.02, by chi(2) analysis). No ret mutation was found in the t umoral DNA from parathyroid adenomas. Our findings indicate that the s omatic ret point mutation frequently found in sporadic MTC may affect not only exon 16 but also exon 11 and is associated with less favorabl e clinical outcome.