The development of neural crest-derived melanocytes, as well as haematopoietic and germ cells, is affected by mutations of the Kit and Mgf genes, which lead to dominant spotting (W) or steel (Sl) phenotypes. Mgf codes for the ligand of the receptor tyrosine kinase encoded by the Kit locus. KitW-v, a point mutation exerting a dominant negative effect, causes a substantial reduction in tyrosine kinase activity of the Kit receptor and leads to a characteristic pigmentation phenotype, namely dilute coat colour and a white ventral and head spot with reduced pigmentation of the feet and tail in the heterozygous animal, as well as slight anaemia. Homozygous animals lack coat pigmentation and are severely anaemic and infertile. Dct is a marker for cells of the melanoblast lineage. In order to study these cells in detail we have generated transgenic mouse lines carrying the lacZ reporter under the control of the Dct promoter and have used the embryonic expression of the reporter to identify early melanoblasts before they begin to produce pigment. Our transgenic lines have simplified the study of melanoblasts in the mouse embryo, and by crossing our mice with KitW-v mutants we have been able to identify the midgestation stages at which melanoblasts rely critically on Mgf/Kit interactions. We conclude that the survival of immature melanoblasts depends crucially upon Kit signalling up until E11, and later in development Kit plays a vital role in melanoblast proliferation. Our data do not describe a dependence upon Kit for melanoblast migration or differentiation.

Defects in cilia formation and function result in a range of human skeletal and visceral abnormalities. Mutations in several genes have been identified to cause a proportion of these disorders, some of which display genetic (locus) heterogeneity. Mouse models are valuable for dissecting the function of these genes, as well as for more detailed analysis of the underlying developmental defects. The short-rib polydactyly (SRP) group of disorders are among the most severe human phenotypes caused by cilia dysfunction. We mapped the disease locus from two siblings affected by a severe form of SRP to 2p24, where we identified an in-frame homozygous deletion of exon 5 in WDR35. We subsequently found compound heterozygous missense and nonsense mutations in WDR35 in an independent second case with a similar, severe SRP phenotype. In a mouse mutation screen for developmental phenotypes, we identified a mutation in Wdr35 as the cause of midgestation lethality, with abnormalities characteristic of defects in the Hedgehog signaling pathway. We show that endogenous WDR35 localizes to cilia and centrosomes throughout the developing embryo and that human and mouse fibroblasts lacking the protein fail to produce cilia. Through structural modeling, we show that WDR35 has strong homology to the COPI coatamers involved in vesicular trafficking and that human SRP mutations affect key structural elements in WDR35. Our report expands, and sheds new light on, the pathogenesis of the SRP spectrum of ciliopathies.

Red hair in humans is associated with variant alleles of the alphaMSH receptor gene, MC1R. Loss of MC1R function in other mammals results in red or yellow hair pigmentation. We show that a mouse bacterial artificial chromosome (BAC) which contains Mc1r will efficiently rescue loss of Mc1r in transgenic mice, and that overexpression of the receptor suppresses the effect of the endogenous antagonist, agouti protein. We engineered the BAC to replace the mouse coding region with the human MC1R sequence and used this in the transgenic assay. The human receptor also efficiently rescued Mc1r deficiency, and in addition, appeared to be completely resistant to the effects of agouti, suggesting agouti protein may not play a role in human pigmentary variation. Three human variant alleles account for 60% of all cases of red hair. We engineered each of these in turn into the BAC and find that they have reduced, but not completely absent, function in transgenic mice. Comparison of the phenotypes of alphaMSH-deficient mice and humans in conjunction with this data suggests that red hair may not be the null phenotype of MC1R.