RET ACTIVATION BY GERMLINE MEN2A AND MEN2B MUTATIONS

The RET proto-oncogene encodes a receptor tyrosine kinase (TK), It has been shown that distinct germline mutations in the RET proto-oncogene are associated with the dominantly inherited cancer syndromes multipl e endocrine neoplasia type 2A and 2B (MEN 2A and MEN 2B) and familial medullary thyroid carcinoma (FMTC) as well as Hirschsprung disease (HS CR), a congenital disorder characterised by absent enteric innervation , In this study, we have transfected NIH3T3 and PC12 phaeochromocytoma cells with MEN2A (Cys634-->Arg) and MEN2B (Met918-->Thr) RET construc ts, Both caused transformation of the NIH3T3 cells and differentiation of PC12 cells, The Bet (MEN2A) and Bet (MEN2B) proteins were constitu tively phosphorylated on tyrosine, and their in vitro kinase activity was significantly higher than that of the wild type protein, The MTC c ell line TT carries a Cys634-->Trp MEN2A mutation, and we have shown b y immunoelectronmicroscopy that Ret is clustered on the cell surface i n a manner reminiscent of ligand-induced aggregation of cell surface r eceptors, RET is activated, as RET/PTC oncogene, by somatic rearrangem ents which link the TK domain to a constitutive dimerization interface in papillary thyroid carcinomas, We have compared the biological and biochemical activity of the TK domains of the wild type and MEN 2B Ret in the context of the RET/PTC, The results show that the MEN2B mutati on significantly increases the TK domain enzymatic activity suggesting that dimerization may be still necessary for MEN 2B Bet to express it s full activity.