Ectopic expression of a c-kit^W42 minigene in transgenic mice: Recapitulation of W phenotypes and evidence for c-kit function in melanoblast progenitors

The proto-oncogene c-kit encodes a transmembrane tyrosine kinase receptor that is allelic with the murine white-spotting locus (W). W mutations affect melanogenesis, gametogenesis, and hematopoiesis during development and adult life, and they result from the partial or complete loss of c-kit function. The W⁴² allele is a W mutation with severe effects in both the homozygous and the heterozygous states. Previous analysis of the W⁴² allele identified a missense mutation in an essential amino acid of the c-kit^W42 kinase domain that abolishes the in vitro kinase activity of the c-kit^W42 protein but does not affect its normal expression. These results suggested that the c-kit^W42 allele was a dominant negative mutation within the context of c-kit-mediated signal transduction. To further explore the dominant negative characteristics of the W⁴² mutation, we have generated transgenic mice in which ectopic expression is driven by the human β-actin promotor (hAP). Two mouse lines carrying the hAP-c-kit^W42 transgene show an effect on pigmentation and the number of tissue mast cells. The patchy coat color pattern of the line 695 mice may reflect variable expression of the transgene in melanoblast progenitors and their descendents and, consequently, is indicative of a function for c-kit in early melanoblasts. Germ cell development and erythropoiesis, however, do not appear to be affected by the transgene. Mice expressing the c-kit^W42 transgene therefore recapitulate some of the phenotypes of mice with W mutations. These results are therefore in agreement with the molecular basis of the W⁴² mutation and the dominant-negative characteristics of the c-kit^W42 protein product.