A two-hit model for development of multiple endocrine neoplasia type 2B byRET mutations

Multiple endocrine neoplasia (MEN) type 2B mutations have been reported at methionine 918 or alanine 883 in the tyrosine kinase domain of the RET prot ooncogene. Recently, a new combination of two germline missense mutations a t valine 804 and tyrosine 806 was identified in a patient with MEN 2B-like clinical phenotypes including medullary thyroid carcinoma, mucosal neuroma, and marfanoid habitus. In this case, valine 804 and tyrosine 806 were repl aced with methionine and cysteine, respectively. In the present study, biol ogical activities of RET with these new mutations were compared with those with known MEN 2A or MEN 2B mutations. The transforming activity of RET wit h the V804M/Y806C mutation was about 8- to 13-fold higher than that of RET with a single V804M or Y806C mutation. Like RET with the M918T or A883F MEN 2B mutation, the transforming activity of RET with the V804M/YS06C mutatio n was not affected by substitution of phenylalanine for tyrosine 905 that a bolished the activity of RET with the MEN 2A mutation. On the other hand, s ubstitution of phenylalanine for tyrosines 864 and 952 drastically diminish ed the activity of RET with the VS04M/Y806C, M918T or A883F mutation, sugge sting that these three mutant proteins have similar biological properties. (C) 2000 Academic Press.