Abstract
Abstract: :
Purpose:To characterize a novel retinal gene, RP1L1, which shares sequence similarity with RP1, is expressed in photoreceptors and has an unusual degree of polymorphic variation. Like RP1, RP1L1 appears to be more highly expressed in photoreceptors than other retinal cell types and may be a candidate gene for inherited retinal degeneration. Methods:The complete human sequence of RP1L1 was determined both experimentally and by analysis of the genomic sequence through the RP1L1 region. The RP1L1 transcript was amplified from retinal cDNA libraries and used to confirm the coding sequence. The mouse homologue, Rp1l1, was identified from analysis of the syntenic mouse genomic region. Transcript levels of Rp1l1 in mouse photoreceptors were characterized using quantitative PCR of mRNA from wildtype and rhodopsin knockout mice. Results:The RP1L1 gene has the same structure as RP1, with 3 coding exons, and the protein is similar in length (2,464 amino acids for RP1L1 vs 2,156 for RP1). The sequence similarity between the two proteins is limited to the first 350 amino acids, which have 32% identity and 52% similarity. The region of similarity extends beyond the 280 amino acid doublecortin (DC) domain of RP1, making it the most similar protein to RP1 ever observed. There are two large repetitive regions, one containing a tandemly repeated 16 amino acid sequence that is highly polymorphic. We have identified 6 RP1L1 alleles which contain 0-5 of these repeats. A large number of polymorphisms have also been observed throughout the entire coding sequence. The homologous mouse gene is significantly shorter and does not appear to contain either of the highly repetitive regions. Quantitative PCR of Rp1l1 transcripts in wildtype and rhodopsin knockout mice shows a 16-fold reduction in relative abundance of the transcript in the knockout. Conclusion:Mutations in RP1 cause 6-10% of autosomal dominant retinitis pigmentosa cases but the function of the RP1 protein in the retina is unknown. The presence of a DC domain suggests an interaction with microtubules and this is supported by the localization of the RP1 protein to the connecting cilia of photoreceptors. RP1L1 appears to be RP1's closest relative and also contains a DC domain, possibly signifying a similar cellular location. Because of the similarity of RP1L1 to RP1 we are investigating the possibility that RP1L1 may either cause inherited retinal degeneration itself or be a modifying factor in some forms of disease.
Keywords: 562 retinal degenerations: hereditary • 417 gene/expression • 554 retina