Peter Besmer: Mechanisms of the Regulation of Cell-type Specific Expression of Kit

The Kit and PDGFRa receptor tyrosine kinases are encoded in close proximity at the murine white spotting (W) and patch (Ph) loci. While W mutations affect hematopoiesis, melanogenesis, and gametogenesis, the Ph mutation affects melanogenesis and causes early lethality in homozygotes. The Wsh, W57, and Ph mutations diminish Kit expression in certain cell types, such as mast cells, and enhance it in others. The Wsh, W57, and Ph mutations arose from deletions and inversions affecting sequences between the Kit and PDGFRa genes.

We have determined the precise location of the breakpoint of the Wsh inversion and the endpoints of the W57 deletion upstream of the Kit transcription start site, and we examined the effect of these mutations on Kit expression in mast cells and hematopoietic stem cells and progenitors. Our results indicate that positive elements controlling Kit expression in mast cells mapping between -23 and -154 kb from the transcription start site can be dissociated from negative elements controlling Kit misexpression during embryonic development in the vicinity of the PDGFRa gene. In addition, we have identified 2 clusters of hypersensitive sites in mast cells at -23 -28 kb and -147 -154 kb from the Kit gene transcription start site. Analysis of these hypersensitive sites in mutant mast cells indicates a role for HS4-6 in Kit expression in mast cells.

These findings provide a molecular basis for the phenotype of these Kit expression mutations, and they provide insight into the complex mechanisms governing the regulation of Kit expression. Current work is concerned with the elucidation of the role of these upstream hypersensitive sites in the regulation of Kit expression.