ABSTRACT
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 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.
INTRODUCTION
SINCE the discovery of the mouse microphthalmia (Mi) mutation more than 50 years ago (HERTWIG 1942 ), numerous mutant alleles have been identified and genetically characterized. The mutations affect particular cell types, which are derived from neural-crest melanocytes. The size of the mutant eyes is reduced because of the affected retinal pigmented epithelium. The mutants frequently develop deafness owing to the lack of inner ear melanocytes. The mutations detected in the mouse are mainly recessive, but semidominant or dominant phenotypes also have been reported. The wild-type allele encodes a basic-helix-loop-helix leucine zipper (bHLHzip) transcription factor and has been referred to as microphthalmia-associated transcription factor (mitf; STEINGRIMSSON et al. 1994 ; YAJIMA et al. 1999 ; HALLSSON et al. 2000 ; THAUNG et al. 2002 ).
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In the Syrian hamster, one dominant mutation in Mitf (W241X) has been reported and designated as anophthalmic white (Wh). It is predicted that this premature stop codon leads to a truncation of the protein in the loop between helix 1 and helix 2 of the bHLHzip region. It prevents the protein from dimerizing or from binding to its DNA target sites (HODGKINSON et al. 1998 ).
In this article, we describe a novel dominant allele (WhV203) in the Syrian hamster. The phenotype cosegregates with a point mutation in a highly conserved region of intron 6. It leads to skipping of exon 7 of the Mitf gene during the maturation of the transcript.