Identification of the mouse and rat orthologs of the gene mutated in Usher syndrome type IIA and the cellular source of USH2A mRNA in retina, a target tissue of the disease.

Usher syndrome type IIA (MIM: 27601) is an autosomal recessive disorder characterized by moderate to severe congenital deafness and progressive retinitis pigmentosa. We recently identified the human Usher syndrome type IIA gene (USH2A) on chromosome 1q41, which encodes a protein possessing 10 laminin epidermal growth factor and four fibronectin type 3 domains, both commonly observed in extracellular matrix proteins. To gain insight into the pathogenesis of Usher syndrome type IIA, we isolated and characterized the murine (Ush2a) and rat (rat Ush2a) orthologs of human USH2A. We mapped mouse Ush2a by fluorescence in situ hybridization to mouse chromosome 1 in the region syntenic to human chromosome 1q41. Rat Ush2a has been localized by radiation hybrid mapping to rat chromosome 13 between d13rat49 and d13rat76. The mouse and rat genes, similar to human USH2A, are expressed primarily in retina and cochlea. Mouse Ush2a encodes a 161-kDa protein that shows 68% identity and 9% similarity to the human USH2A protein. Rat Ush2a encodes a 167-kDa protein with 64% identity and 10% similarity to the human protein and 81% identity and 5% similarity to the mouse USH2A protein. The predicted amino acid sequence of the mouse and rat proteins, like their human counterpart, contains a leader sequence, an amino-terminal globular domain, 10 laminin epidermal growth factor domains, and four carboxy-terminal fibronectin type III motifs. With in situ hybridization, we compared the cellular expression of the USH2A gene in rat, mouse, and human retinas. USH2A mRNA in the adult rat, mouse, and human is expressed in the cells of the outer nuclear layer of the retina, one of the target tissues of the disease. In the developing rat retina, Ush2a mRNA expression appears in the neuroepithelium at embryonic day 17.