Medullary thyroid carcinoma (MTC) occurs sporadically or as part of the inherited cancer syndrome multiple endocrine neoplasia (MEN) type 2. In MEN 2A, germline missense mutations are found in one of five cysteine codons within exons 10 and 11 in the extracellular domain of the RET protooncogene. In MEN 2B, germline mutations occur in codon 918 (exon 16) within the catalytic core of the tyrosine kinase domain. To determine if RET mutations similar to those in MEN 2A and 2B play a role in the pathogenesis of sporadic MTC, we analysed 71 sporadic tumours comprising 68 primary tumours and three cell lines, for mutations in RET exons 10, 11, and 16. We found that 23% of sporadic MTC had RET codon 918 mutations, while only 3% had exon 10 mutations, and none had mutations in exon 11. We found no exon 16 mutations in MTC from 14 MEN 2A cases. Thus, exon 10 and 11 mutations, commonly found in familial MTC and MEN 2A, rarely occur in sporadic MTC; somatic mutation of RET codon 918 appears to play a role in the tumourigenesis of a significant minority of sporadic MTC but not MEN 2A tumours. In addition to their biological interest, these findings may have some clinical application in determining whether a patient presenting with isolated MTC is truly sporadic or is part of an inherited cancer syndrome.

Activating germline mutations of the RET receptor tyrosine kinase are found in the majority of cases of inherited cancer syndrome MEN 2, and inactivating mutations in some cases of dominantly inherited Hirschsprung disease. Using RET activated by a MEN 2 mutation, we show that both the SH2 and PTB domains of the adaptor protein Shc interact with RET, and we identify the PTB domain interaction site. Interaction with both the SH2 and PTB domains of Shc contributes to the transcriptional activation of a serum response element. RET alternate splicing aects the strength of interaction with both the Shc SH2 and PTB domains. In addition, a splice isoform-speci®c HSCR missense mutation, which does not inactivate the RET kinase activity, decreases the strength of the PTB domain interaction and the level of RET-dependent Shc phosphorylation.

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