Retinitis pigmentosa (RP) refers to a group of inherited retinal dystrophy that affects the photoreceptors and retinal pigment epithelium; resulting in blindness usually after several decades. Prevalence of RP is approximately 1/3,000 to 1/5,000. There is broad variability in age of onset, rate of progression and secondary clinical manifestations. The first sign of retinitis pigmentosa is usually a loss of night vision (nyctalopia) due to loss of rod function, which becomes apparent in adolescence or earlier. Later, patients develop peripheral visual field impairment, and overtime loss of central vision, usually at late stages, often around adulthood. Clinical diagnosis of RP is based on the presence of night blindness and peripheral visual field defects, lesions in the fundus, hypovolted electroretinogram traces, and decreased or abolished responses as measured by electroretinography (ERG). To date, there are no proven or effective cures for RP. Treatment is primarily aimed at slowing progression of the disease by sunlight protection and vitamin therapy, treating the complications (cataract and macular edema), and helping patients to cope with the social and psychological impact of blindness.
There are various inheritance patterns for RP, including autosomal dominant (30-40%), autosomal recessive (50-60%) and X-linked (5-15%). To date, more than 3,000 different mutations in over 60 different genes or loci are currently known to cause non-syndromic RP. Mutations in the RP1 gene (8q11.2-q12.1), which encodes rhodopsin, have been identified in patients with retinitis pigmentosa 1.
Al-Rashed et al. (2012) described 20 patients from six Arab families, with autosomal recessive retinitis pigmentosa. All families were consanguineous, and five of the families were multiplex. Using linkage analysis, homozygosity mapping, and direct sequencing of the RP1 gene, four novel mutations, all recessive, were identified in the affected patients. These mutations consisted of a homozygous nonsense mutation (c.4552A>T) in one family, a homozygous single base deletion (c.3428delA) in three families, a homozygous nonsense mutation (c.33396 G>A) in a sporadic case, and a homozygous frameshift mutation (c.3677_3678dupA) in one family. All these mutations were in exon 4, and they resulted in a truncated RP1 protein.