The Glu632-Leu633 deletion in cysteine rich domain of Bet induces constitutive dimerization and alters the processing of the receptor protein

Mutations of the RET gene, encoding a receptor tyrosine kinase, have been a ssociated with the inherited cancer syndromes MEN 2A and MEN 2B, They have also further been associated with both familial and sporadic medullary thyr oid carcinomas. Missense mutations affecting cysteine residues within the e xtracellular domain of the receptor causes constitutive tyrosine kinase act ivation through the formation of disulfide-bonded homodimers. We have recen tly reported that a somatic 6 bp in-frame deletion, originally coding for G lu632-Leu633, potently activates the RET gene. This activation is increased with respect to the frequent MEN 2A-associated missense mutation Cys634Arg . This finding specifically correlated to the clinic behavior of the corres ponding tumor, which was characterized by an unusually aggressive progressi on with both multiple and recurrent metastases. By examining the possibilit y that this deletion acts in a manner similar to cysteine substitution, we have analysed the molecular mechanism by which this oncogenic activation oc curs. Phosphorylated dimers of the deleted Ret receptor were detected in im munoprecipitates separated under non-reducing conditions. Like other Cys po int mutations, this 6 bp deletion affecting two amino acid residues between two adjacent Cys, is capable of activating the transforming ability of Ret by promoting receptor dimerization, These results suggest that alteration to cysteine residue position or pairing is capable of inducing ligand indep endent dimerization, Furthermore, we present data demonstrating that the pr ocessing and sorting of the Bet membrane receptor to the cell surface is af fected by mutation type.