Mapping of A gene responsible for cataract formation and its modifier in the UPL rat.

PURPOSE The Upjohn Pharmaceuticals Limited (UPL) rat is a unique model for cataracts, which are inherited as an autosomal semidominant trait and expressed as early-onset (E-type) cataracts in homozygotes and as late-onset (L-type) cataracts in heterozygotes. In this study, a gene and its modifier, which are responsible for formation of cataract, were mapped. METHODS Fifty-five BN x (BN x UPL)F(1) backcross rats and 133 BN x UPL intercross rats were produced. The cataracts present in the rats at eye opening were diagnosed as E-type. Cataracts that developed after eye opening were diagnosed as L-type, and the ages when complete opacity in the lens was observed were used as a quantitative trait to map a gene that modifies the development of mature cataracts. Linkage analysis was performed using 64 arbitrarily primed-representational difference analysis (AP-RDA) markers and 74 microsatellite markers. RESULTS A gene responsible for the formation of cataract was mapped to the vicinity of D2Rat134 on rat chromosome (chr) 2. A candidate gene, connexin 50 (Cx50/Gja8), had a C-to-T transition at codon 340 that is predicted to result in a nonconservative substitution of arginine by tryptophan. Recombination in the Cx50 genotype and formation of cataract was not observed. By quantitative trait loci analysis, a gene that modified the age of the development of mature cataract was mapped on rat chr 5. CONCLUSIONS A candidate gene for formation of cataracts in UPL rats was mapped to rat chr 2, and the Cx50 gene was a strong candidate. In addition, a potential modifier gene was mapped on chr 5. Future cloning of these genes will provide good targets for new therapies that can delay the progression of cataracts.