Phenotypic reversions at the W/Kit locus mediated by mitotic recombination in mice.

The mouse W locus encodes Kit, the receptor tyrosine kinase for stem cell factor (SCF). Kit is required for several developmental processes, including the proliferation and survival of melanoblasts. Because of the nearly complete failure of Wrio/+ melanoblasts to colonize the skin, the costs of Wrio/+ mice are characterized by a majority of white hairs interspersed among pigmented hairs, giving a roan effect. However, 3.6% of Wrio/+ mice exhibit phenotypic reversions, i.e., spots of wild-type color on their coats with an otherwise mutant phenotype. Melanocyte cell lines were derived from each of six independent reversion spots on the skin of (C57BL/6 x DBA/2)F1 Wrio/+ mice. All six melanocyte cell lines exhibited the general characteristics common to normal, nonimmortal mouse melanocytes. Of these, three revertant cell lines had lost the dominant-negative Wrio allele following mitotic recombination between the centromere and the W locus. One of the cell lines remained Wrio/+ but showed (i) stimulation in response to SCF and (ii) increased Kit expression, suggesting that the Wrio mutation can be rescued by increased endogenous expression of the c-kit proto-oncogene. Finally, two cell lines showed no detectable genetic change at the W/Kit locus and failed to respond to SCF stimulation in vitro. These results demonstrate that mitotic recombination can create large patches of wild-type hair on the coats of Wrio/+ mutant mice. This shows that mitotic recombination occurs spontaneously in normal healthy tissue in vivo. Moreover, these experiments confirm that other mechanisms, not associated with loss of heterozygosity, may account for the coat color reversion phenotype.