Mutation in Intron 6 of the Hamster Mitf Gene Leads to Skipping of the Subsequent Exon and Creates a Novel Animal Model for the Human Waardenburg Syndrome Type II

In the course of analysis of ENU-induced mutations in Syrian hamsters, a novel dominant anophthalmic white mutant (WhV203) with hearing loss was recovered. Because of this phenotype and a close linkage to the Tpi gene, the Mitf gene was considered as a candidate gene. In the Mitf cDNA, a deletion of 76 bp covering the entire exon 7 was detected. Further molecular analysis revealed a T (right arrow) A exchange 16 bp upstream of the end of intron 6, leading to skipping of exon 7. These 16 bp at the end of intron 6 are identical in hamster, rat, mouse, and humans, indicating high conservation during evolution and a functional importance in splicing. Since the loss of exon 7 changes the open reading frame of the MITF transcript, translation will be stopped after 10 new amino acids. The truncated protein is predicted to contain only a part of the basic region and will miss the two helical domains and the leucine zipper. The WhV203 mutation in the Syrian hamster affects the same functional domains of the Mitf transcription factor as the human R124X mutation, causing human Waardenburg syndrome type II. Therefore, the WhV203 hamster mutant provides a novel model for this particular syndrome.