Refinement and Candidate Gene Screening of the Cerulean Cataract Type 1 Locus on 17q24-q25

Abstract

Abstract: : Purpose: Cerulean cataracts (CCA) are a form of autosomal dominant, juvenile-onset hereditary cataracts that are noteworthy for tiny blue or white opacities, predominantly in the lens cortex. There are two reported CCA loci: CCA type 1 on 17q24-q25 (Armitage et al, 1995) and CCA type 2 on 22q11.2-q12.2 (Kramer et al, 1996). CCA2 has been shown to be caused by mutation in the beta-B2-crystallin gene (Litt et al, 1997). The CCA1 locus was originally mapped in a single large family between microsatellite markers D17S802 and D17S836 with 6 recombinations within that interval. Because of new information available since the original study, we have undertaken to remap the disorder in this family and test several candidate genes. Methods: We used Genbank and other databases to establish a BAC contig. We evaluated existing and new microsatellite and SNP markers using a combination of denaturing HPLC (Transgenomic Wave), restriction enzyme digests and sequencing of the PCR products to detect alleles and redefine the critical region. Potential candidate genes were screened for expression using the NEIBank lens cDNA database and by PCR in a human lens cDNA library and evaluated for mutations in exons and splice junctions by sequencing. Results: We evaluated 7 known and 14 previously unreported microsatellite markers, as well as 16 previously reported SNPs and 4 novel SNPs within potential candidate genes spanning our critical region. In addition to the 7 known microsatellite markers, 7 of the 14 new microsatellite markers and 14 of the 20 SNPs were informative in this family. Six possible candidate genes showed expression in a human lens cDNA library. Sequencing of the exons of these genes did not identify any potentially disruptive mutations. Conclusions: With the gradual completion of the Human Genome Project, this region of chromosome 17 is becoming progessively better-defined. In silico methods using Genbank and NEIBank still need to be supplemented by experimental discovery of new markers and testing of candidate genes. We have been able to further localize the CCA1 locus and establish that no known lens-specific genes are within the critical region. Elucidation of the causative gene for CCA1 has the potential of identifying a novel gene that contributes to lens biology.

Keywords: cataract • genetics • gene mapping