W-SASH AFFECTS POSITIVE AND NEGATIVE ELEMENTS CONTROLLING C-KIT EXPRESSION - ECTOPIC C-KIT EXPRESSION AT SITES OF KIT-LIGAND EXPRESSION AFFECTS MELANOGENESIS

The receptor tyrosine kinase c-kit and its cognate ligand KL are encod ed at the white spotting (W) and steel (Sl) loci of the mouse, respect ively. Mutations at both the W and the Sl locus cause deficiencies in gametogenesis, melanogenesis and hematopoiesis (erythrocytes and mast cells). The W-sash mutation differs from most W mutations in that it a ffects primarily mast cells and melanogenesis but not other cellular t argets of W and Sl mutations. Thus, W(sh)/W(sh) mice are fertile and n ot anemic, but they lack mast cells in their skin and intestine and ar e devoid of coat pigment. Heterozygotes are black with a broad white s ash/belt in the lumbar region. In order to determine the basis for the phenotypes of W-sash mice, we investigated c-kit RNA and protein expr ession patterns in adult W(sh)/W(sh) mice and during embryonic develop ment. We show that c-kit expression is absent in bone-marrow-derived W (sh)/W(sh) mast cells, the fetal and the adult lung, and the digestive tract at embryonic day 13 1/2 (E13 1/2), tissues that normally expres s c-kit. Unexpectedly, in E10 1/2 and 11 1/2d W(sh)/W(sh) embryos, we found c-kit expression in the dermatome of the somites, the mesenchyme around the otic vesicle and the floorplate of the neural tube, struct ures known to express the c-kit ligand in wild-type embryos. The ectop ic c-kit expression in W(sh) homozygous embryos does not affect c-kit ligand expression. The presumed W(sh)/W(sh) melanoblasts appeared to b e normal and, at E10 1/2, similar numbers were found in normal and hom ozygous mutant embryos. At E13 1/2 +/+ embryos had a graded distributi on of melanoblasts from cranial to caudal with a minimum in the lumbar region. Whereas E13 1/2 homozygous W(sh)/W(sh) embryos essentially la cked c-kit-positive cells in the skin, E13 1/2 heterozygous W(sh)/+ em bryos had reduced numbers of melanoblasts compared to +/+ with few or none in the lumbar region (future sash). It is proposed that ectopic c -kit expression in the somitic dermatome affects early melanogensis in a dominant fashion. Molecular analysis of W(sh) chromosomal DNA revea led a deletion or rearrangement in the vicinity of the c-kit gene. The se results provide an explanation for the W(sh) phenotype and have imp lications for the control of c-kit expression.