Identification of Eys: The Power of Homozygosity Mapping in Outbred Patients With RP

Abstract

Purpose: : To unravel the causative gene defects underlying autosomal recessive retinitis pigmentosa (arRP) in a large group of patients from outbred populations.

Methods: : 226 RP patients (including 31 families with 2-to-5 affected siblings and 154 isolated cases) were genotyped using high-density SNP microarrays, and analyzed for homozygous regions that may harbor the causative gene defect.

Results: : In a few patients, large homozygous stretches of DNA ranging from 30-90 Mb were identified, implying consanguinity between their parents. In the majority of cases however, patients carried three to six homozygous regions varying from 1 to 30 Mb in size. A large number of patients were not homozygous for the known arRP genes, suggesting that many new RP genes remain to be identified. Interestingly, twelve patients were homozygous for SNPs at the RP25 locus. By combining their SNP data, the critical region could be diminished to 5.0 Mb. In this 5.0 Mb region, we identified a novel gene named EYS, the human orthologue of Drosophila eyes shut/spacemaker which is essential for photoreceptor morphology in the insect eye. Mutations in EYS have thus far been identified in ten patients from seven families, indicating that the gene is a frequent cause of arRP. In many other families, microsatellite analysis is currently ongoing to confirm regions carrying novel arRP genes.

Conclusions: : Using homozygosity mapping in outbred RP-patients, it is possible to pinpoint relatively small regions in the genome carrying genes causative for arRP, as proven by the identification of the EYS gene. Further analysis of our SNP data will reveal the genetic defects underlying RP in many patients, both in novel genes as well as in previously identified genes.

Keywords: retinal degenerations: hereditary • gene mapping • candidate gene analysis