An L1 element intronic insertion in the black-eyed white (Mitf(mi-bw)) gene: the loss of a single Mitf isoform responsible for the pigmentary defect and inner ear deafness

Waardenburg syndrome type 2 (WS2) is an autosomal dominant disorder charact erized by a combination of pigmentary and auditory abnormalities. Approxima tely 20% of WS2 cases are associated with mutations in the gene encoding mi crophthalmia-associated transcription factor (MITF), MITF plays a critical role in the development of both neural-crest-derived melanocytes and optic cup-derived retinal pigmented epithelium (RPE); the loss of a functional Mi tf in mice results in complete absence of all pigment cells, which in turn induces microphthalmia and inner ear deafness. The black-eyed white Mitf(ml -bw) homozygous mouse normally has a pigmented RPE but lacks melanocytes es sential for the pigmentation of the body and hearing. We show here that Mit f(ml-bw) is caused by an insertion into intron 3 of a 7.2 kb novel L1 eleme nt, L1(bw), which belongs to an actively retrotransposing T-F subfamily. Th e L1(bw) insertion reduces the amount of mRNAs for two Mitf isoforms, Mitf- A and Mitf-H, by affecting their overall expression levels and pre-mRNA spl icing patterns, while it abolishes mRNA expression of another isoform, Mitf -M, which is specifically expressed in neural-crest-derived melanocytes. Th e consequence of the L1 insertion in the black-eyed white Mitf(ml-bw) mouse is that the developmental programme for RPE cells proceeds normally, most likely because of the presence of residual, full-length Mitf-A and Mitf-H p roteins, whereas the lack of Mitf-M results in loss of the melanocyte popul ation. The results suggest that melanocyte development depends critically o n a single Mitf isoform, Mitf-M, and raise the possibility that specific mu tations affecting MITF-M, the human equivalent of Mitf-M, may be responsibl e for a subset of WS2 conditions.