SomaticRASMutations Occur in a Large Proportion of SporadicRET-Negative Medullary Thyroid Carcinomas and Extend to a Previously Unidentified Exon

Abstract

Medullary thyroid carcinoma (MTC) is characterized by proto-oncogene RET mutations in almost all hereditary cases as well as in more than 40% of sporadic cases. Recently, a high prevalence of RAS mutations was reported in sporadic MTC, suggesting an alternative genetic event in sporadic MTC tumorigenesis. This study aimed to extend this observation by screening somatic mutational status of RET, BRAF, and the three RAS proto-oncogenes in a large series of patients with MTC. Direct sequencing of RET (exons 8, 10, 11, 13, 14, 15, 16), BRAF (exons 11 and 15), and KRAS, HRAS, and NRAS genes (exons 2, 3, and 4) was performed on DNA prepared from 50 MTC samples, including 30 sporadic cases. Activating RET mutations were detected in the 20 hereditary cases (germline mutations) and in 14 sporadic cases (somatic mutations). Among the 16 sporadic MTC without any RET mutation, eight H-RAS mutations and five K-RAS mutations were found. Interestingly, nine RAS mutations correspond to mutation hot spots in exons 2 and 3, but the other four mutations were detected in exon 4. The RET and RAS mutations were mutually exclusive. No RAS gene mutation was found in hereditary MTC, and no BRAF or NRAS mutation was observed in any of the 50 samples. Our study confirms that RAS mutations are frequent events in sporadic MTC. Moreover, we showed that RAS mutation analysis should not be limited to the classical mutational hot spots of RAS genes and should include analysis of exon 4.