Mechanistic insight into the repigmentation of piebaldism: functional characterization of a mutant KIT in melanocyte regeneration.

Piebaldism is a pigmentary disorder characterized by white forelock and depigmented patches. Mutations of the KIT gene underlie the autosomal dominant trait of the disease. Piebaldism is generally stable throughout life, however, repigmentation occurs in some patients. We report a Japanese family with piebaldism in which the affected members showed spontaneous repigmentation. A genetic analysis revealed a heterozygous mutation, c.645_650delTGTGTC, in the KIT gene, which resulted in the deletion of Val216 and Ser217 in the extracellular region of KIT. Although melanocytes was absent in the depigmented skin of the proband’s mother who had piebald skin, MITF-positive melanocyte stem cells were observed in the bulge region of the hairs in the mother’s depigmented skin. The KIT mRNA extracted from leg hairs of mother’s normal skin was decreased, however, both wild type (Wt) and mutant KIT mRNA were expressed. Immunofluorescence analyses demonstrated that the mutant KIT had accumulated in the endoplasmic reticulum, and was sparsely expressed on the cell surface when HEK293T cells were transfected with both Wt and mutant KIT. An immunoprecipitation analyses showed that the Wt and mutant KIT formed a heterodimer and was phosphorylated by SCF stimulation. Collectively, these results suggest that melanocyte stem cells were present in the white patches and that the mutant-KIT was expressed on the cell membrane and formed a heterodimer receptor with the Wt KIT that mediated the SCF signals for repigmentation.