A novel type of mutation in the cysteine rich domain of the RET receptor causes ligand independent activation

Multiple endocrine neoplasia type 2A (MEN 2A) is a dominantly inherited can ter syndrome, which involves the triad of MTC, pheochromocytoma, and hypcrp arathyridism, Missense mutations in one of sis cysteine codons in the extra cellular cysteine-rich domain of the RET proto-oncogene predispose to this disease. These mutations cause ligand-independent constitutive activation o f the tyrosine kinase receptor by the formation of disulfide-bonded homodim ers. We examined a different type of mutation, which results in an addition al cysteine in the cysteine rich domain. A duplication of 9 bp in the first case resulted in an insertion of three amino acids between codon 633 and 6 34, In the second case a 12 bp duplication in exon 11 results in four addit ional amino acids between codon 634 and 635. Here we demonstrate that an ad ditional cysteine causes a ligand independent dimerization of the RET recep tor in transfected NIH3T3 cells, which results in an activation of the intr acellular tyrosine kinase.