Specialized Sequencing of the Mutational Hotspot RPGR (isoform C) ORF15 Region

Retinitis Pigmentosa (RP) refers to a group of related eye disorders stemming from the degeneration of photoreceptor cells in the retina (Fahim et al. 1993. PubMed ID: 20301590). RP derives its name from retinal pigmentation changes. In RP patients, pigment granules accumulate in perivascular clusters, called bone-spicules, in the retinal pigment epithelium (RPE). The accumulation of pigment is typically accompanied by death of the photoreceptor cells, first rods and eventually cones. As a result, RP will often present first with night blindness and decreased peripheral vision due to systematic loss of rod cells, followed by deterioration of central vision acuity and day blindness from the loss of cone cells. RP has an incidence of ~1/3500 and can be inherited in autosomal recessive, autosomal dominant, X-linked, oligogenic and even mitochondrial patterns. Currently, pathogenic variants in over 80 genes are known to cause RP (van Soest et al. 1999. PubMed ID: 10025514).

X-linked Retinitis pigmentosa (XLRP) is one of the most severe forms of RP, with early onset and rapid progression in males. Milder symptoms can also present in females, probably due to skewed X inactivation. At least five different genetic loci have been associated with XLRP: Xp11.2 (RP2), Xp21.1 (RPGR), Xp21.2-21.3 (RP6), Xp22 (RP23) and Xp26-27 (RP24) (http://www.sph.uth.tmc.edu/RetNet/disease.htm). To date, only the RPGR, RP2 and OFD1 genes have been identified.

RPGR encodes the retinitis pigmentosa GTPase regulator, a photoreceptor ciliary protein that is essential for photoreceptor survival (Meindl et al. 1996. PubMed ID: 8673101). Pathogenic variants in RPGR account for the vast majority (~80%) of all XLRP cases, (Sharon et al. 2003. PubMed ID: 14564670; Vervoort et al. 2000. PubMed ID: 10932196). The RPGR gene encodes several alternatively spliced isoforms, although Retinitis pigmentosa causative variants are only found in isoform C. Isoform C consists of 15 coding exons, with exon 15 (usually referred to as ORF15; see Bader et al. 2003. PubMed ID: 12657579 for a detailed description of ORF15) coding for nearly 50% of the protein and it is glutamic acid rich region. Pathogenic variants in exons 1-14 account for less than 25% of XLRP cases. Purine rich ORF15 accounts for ~50-60% of XLRP patients (Vervoort et al. 2000. PubMed ID: 10932196; Shu et al. 2007. PubMed ID: 17195164; Sharon et al. 2003. PubMed ID: 14564670). Of note, pathogenic variants in RPGR account for other retinal dystrophies (such as X-linked cone rod dystrophy) and syndromal retinal dystrophies with ciliary dyskinesia and hearing loss (Yang et al. 2002. PubMed ID:11875055). The great majority of the pathogenic variants (63%) are small frameshift deletion/duplications that lead to protein termination, although missense, nonsense, large deletions/duplications and complex genomic rearrangements have been documented (Human Gene Mutation Database).

This test is predicted to detect a causative mutation in ~70-80% of all patients with presumptive X-Linked Retinitis Pigmentosa (Sharon et al. 2003. PubMed ID: 14564670), or ~65% of patients with X-linked cone dystrophy (2 /3 families) (Demirci et al. 2002. PubMed ID: 11857109). The sensitivity for X-linked atrophic macular degeneration is unknown.

This test involves Sanger sequencing of the RPGR ORF15 region.