A MUTATION IN THE RET PROTOONCOGENE IN HIRSCHSPRUNGS-DISEASE AFFECTS THE TYROSINE KINASE-ACTIVITY ASSOCIATED WITH MULTIPLE ENDOCRINE NEOPLASIA TYPE 2A AND 2B

We demonstrate that a Hirschsprung (HSCR) mutation in the tyrosine kin ase domain of the RET proto-oncogene abolishes in cis the tyrosine-pho sphorylation associated with the activating mutation in multiple endoc rine neoplasia type 2A (MEN2A) in transiently transfected Cos cells. Y et the double mutant RET2AHS retains the ability to form stable dimers , thus dissociating the dimerization from the phosphorylation potentia l. Co-transfection experiments with single and double mutants carrying plasmids RET2A and RET2AHS in different ratios drastically reduced th e phosphorylation levels of the RET2A protein, suggesting a dominant-n egative effect of the HSCR mutation. Also, the phosphorylation associa ted with the multiple endocrine neoplasia type 2B (MEN2B) allele was a ffected in experiments with single and double mutants carrying plasmid s co-transfected under the same conditions. Finally, analysis of the e nzymic activity of MEN2A and MEN2B tumours confirmed the relative leve ls of tyrosine phosphorylation observed in Cos cells, indicating that this condition, in vivo, may account for the RET transforming potentia l.