INSTABILITY AT THE W C-KIT LOCUS IN MICE - ANALYSIS OF MELANOCYTE CELL-LINES DERIVED FROM REVERSION SPOTS/

Mutations at the mouse W/c-kit locus have pleiotropic defects includin g impaired development of the melanocyte lineage. We have characterize d the molecular basis of the W-ei mutation. We show here that W-ei is the result of a missense mutation in the ATP binding site domain of c- kit proto-oncogene which affects the tyrosine kinase function of the r eceptor. As a result, few melanoblasts survive during embryogenesis in heterozygous W-ei/+ foetuses. Therefore the adult skin is partly devo id of differentiated pigmented cells giving rise to a mottled coat col our phenotype. However, three per cent of W-ei/+mice exhibit spots of wild-type pigmentation on the coat which is otherwise of mutant phenot ype. Such areas are known as phenotypic reversions. To dissect the mol ecular events responsible for the phenotypic instability of the W-ei m utation, we have isolated pure cultures of continuously proliferating melanocytes from two independent reversion spots. These melanocyte lin es, designated W-ei-R1 and W-ei-R2, were shown to exhibit none of the characteristics associated with transformed melanocytes. We have used a polymorphic restriction site generated by the W-ei mutation to show that both melanocyte lines are still heterozygous at the W focus. Furt hermore, W-ei-R1 and W-ei-R2 melanocytes express both the mutated and the wild-type c-kit RNA. These results indicate that the somatic mutat ion events responsible for reversion spots are not necessarily associa ted with loss of heterozygosity at the W/c-kit locus. Together with pr evious data, this points to the fact that several mechanisms account f or the coat colour reversion phenotype.