Mutational Screening of the RP 2 and RPGR Genes in Spanish Families with X-Linked Retinitis Pigmentosa

Retinitis pigmentosa (RP; MIM 268000; Mendelian Inheritance in Man; National Center for Biotechnology Information, Bethesda, MD) reflects a heterogeneous group of inherited ocular diseases representing the most recurrent retinal dystrophy. This disease is clinically and genetically heterogeneous, with wide variation in severity, mode of inheritance, age of onset, progression, and phenotype. One hundred twelve causative genes have been identified for RP, and 49 additional genes have been mapped, but not yet identified (www. sph.uth.tmc.edu/RetNet/disease.htm; provided in the public domain by the University of Texas Houston Health Science Center, Houston, TX). The diagnosis criteria for RP were established by Marmor et al. and are characterized by night blindness, progressive constriction of the visual field, and pigmentary retinopathy, that result from the gradual degeneration of photoreceptor cells, which begins in the periphery and progresses to the central region of the retina. The X-linked form of RP accounts for approximately 12% of cases in Spain. It is, on average, the most severe form because of its early onset and rapid progression, typically presenting in the first decade of life and progressing to partial or complete blindness by the beginning of the fourth decade. Five XLRP loci have been mapped: RP23, RP6, RP3, RP2, and RP24. So far, only two XLRP genes have been cloned, RP2 and RPGR (for the RP3 locus), which account for approximately 10% and 70% of XLRP families respectively. The RP2 gene (RP2; MIM 312600) was localized by linkage mapping on Xp11.3-11.23, and it was isolated by positional cloning in Xp11.3. The five exons identified in this gene encode an ubiquitously expressed protein of 350 amino acids. The function of the RP2 protein is not completely known, but it shows homology to human cofactor C (residues 42-192), which is involved in -tubulin folding. Human RP2 has been shown to interact with the Arl3 protein, and the data suggest that both proteins work together in photoreceptors with a linkage function between the cytoskeleton and cell membrane as part of the cell signaling or vesicular transport machinery. Linkage analysis and deletion mapping have been used to localize the RPGR gene (RPGR; MIM 312610) in the interval between the CYBB and OTC genes in Xp21.1. The genomic structure shows a 19-exon gene, spanning 60 kb of genomic DNA. RPGR transcription studies in mouse and human tissues have revealed at least 12 different alternatively spliced isoforms, some of which are tissue specific and contain new exons. In the photoreceptors, RPGR is concentrated in the connecting cilium linked to an RPGR interacting protein (RPGRIP). It has been proposed that it plays a role in the maintenance of the polarized protein distribution across the connecting cilium by regulating directional transport. The RPGR isoform, which contains the new exon open reading frame (ORF) 15, is prominently expressed in the retina. It contains exon 15 and part of intron 15, and it is mutated in most patients with XLRP. No mutation has been identified in exons 16 to 19 of the RPGR gene in XLRP. We screened 30 unrelated XLRP Spanish families to determine the molecular cause of the disease and to give them appropriate genetic counseling.