Mutations in the ligand-binding domain of the kit receptor: an uncommon site in human piebaldism

Heterozygous mutations in the gene for the Kit transmembrane receptor have been identified recently in human piebaldism and mouse "dominant spotting." Interestingly, all of the 14 known missense mutations that cause depigmentation in these species map to the tyrosine kinase domain of the receptor, whereas none have involved the extracellular ligand-binding domain. In an attempt to detect these uncommon mutations, we screened the nine exons encoding the extracellular portion of Kit for single-strand conformation polymorphisms (SSCP) in eight piebald subjects previously reported to be negative for kinase mutations. Four of these eight kindreds proved to carry novel mutations. The first mutation, found in two apparently unrelated probands with mild piebaldism and English ancestry, substitutes an arginine for a highly conserved cysteine at codon 136. This substitution disrupts a putative disulfide bond required for formation of the second Ig-like (D2) loop of the Kit ligand-binding domain. The second mutation, detected in a piebald kindred characterized by unusually limited depigmentation, substitutes a threonine for an alanine at codon 178, a site just proximal to conserved cysteines at codons 183 and 186. The third mutation, occurring in a kindred with more extensive depigmentation, is a novel four-base insertion in exon 2 that results in a proximal frameshift and premature termination. The data strongly suggest that piebaldism can result from missense mutations in the Kit ligand-binding domain, although the resulting phenotype may be milder than that observed for null or kinase mutations. The apparent clustering of these uncommon mutations at or near the conserved cysteines for the D2 Ig-like loop further suggests a critical role for this region in Kit receptor function.