Homozygous deletion related to Alu repeats in RLBP1 causes retinitis punctata albescens

Abstract

PURPOSE. Retinitis punctata albescens (RPA) is an infrequently occurring form of autosomal recessive (and rarely dominant) retinal dystrophy featuring early-onset severe night blindness and tiny, dotlike, white deposits in the fundus. RPA is associated mostly with mutations in RLBP1 and occasionally in RHO, RDS, and RDH5. In this study, mutations were sought in RLBP1, which encodes the retinol binding protein CRALBP in patients with typical RPA. METHODS. Clinical investigation included funduscopy, visual field testing, electroretinogram recording, and adaptometry. The 7 coding exons (3-9) of RLBP1 and the 15th (last) exon of ABDH2 were PCR amplified and sequenced. Long-distance PCR and cloning of genomic DNA were performed to characterize the deletion. RESULTS. The study involved a 24-year-old Moroccan patient with typical RPA, born of first-cousin parents. He carried a 7.36-kb homozygous deletion encompassing the last 3 exons of RLBP1 (7, 8, and 9) and part of the intergenic region between RLBP1 and ABHD2, which lies downstream of RLBP1. This deletion abolishes the retinal binding site of CRALBP. The telomeric breakpoint of the deletion (in RLBP1 intron 6) is embedded in an Alu element, whereas the centromeric breakpoint (in the intergenic region) lies between two Alu elements placed in the opposite orientation. CONCLUSIONS. Because of the high density of Alu elements in RLBP1, a systematic search should be made for deletions in this gene when one or both alleles lack point mutations, in the case of RPA or flecked retinal dystrophy. (Invest Ophthalmol Vis Sci. 2006;47:4719 -4724) DOI:10.1167/iovs.05-1488 T he cellular retinaldehyde-binding protein (CRALBP) belongs to the CRAL-TRIO family whose members bind lipid ligands in a hydrophobic domain. It binds the vitamin A derivatives 11-cis retinol and 11-cis retinal, with more affinity for the aldehyde form. As such, CRALBP is a key actor in the visual cycle, the multistep process that starts with all-trans retinal, the product of the activated rhodopsin, and ends with 11-cis retinal, the chromophore that binds opsins to regenerate rhodopsin and cone photopigments. CRALBP is found in the retina, specifically in the retinal pigment epithelium and the Müller glial cells, where it accelerates the rate of the isomerization to 11-cis retinol. 1 Accordingly, mice lacking CRALBP have considerably delayed dark adaptation. 2 CRALBP is also found in the ciliary epithelium, iris, cornea, pineal gland, and in some oligodendrocytes of the optic nerve and brain, 3,4 where its function remains unclear. In human, mutations in RLBP1, the gene encoding CRALBP, have been found in various types of retinal dystrophiesnamely, retinitis punctata albescens (RPA) in most cases, 14 Although there is an apparent phenotypic heterogeneity, the clinical presentation is in fact quite well characterized and helps in directing the molecular diagnosis that prompts the search for RLBP1 mutations. Clinical features are night blindness from infancy with elevated threshold in adaptometry, progressive loss in visual acuity due to macular degeneration, the presence of tiny white deposits and patches of atrophy in peripheral retina contrasting with the absence or scarcity of pigment deposits, and predominant rod over cone involvement. This condition is in fact appearing as a subtype of autosomal recessive retinitis pigmentosa, leading after several decades to severe visual loss. In this study, we describe a patient with typical RPA who carries a homozygous 7.36-kb deletion that includes the last 3 exons of RLBP1. We show that this deletion occurred in a portion of the genome that is rich in Alu sequences. MATERIALS AND METHODS Clinical Investigations A standard ophthalmic examination (refractometry, visual acuity, slitlamp examination, applanation tonometry, and funduscopy) was performed. Fluorescein angiography was performed, and visual fields were tested with a Goldmann perimeter using targets V 4e , IV 4e , and II 4e . A full-field ERG was performed according to ISCEV (International Society for Clinical Electrophysiology of Vision) recommendations. Dark adaptometry was performed with a Goldmann-Weekers apparatus and a test seen with an angle of 11°, placed at a distance of 30 cm from the eyes and centered on the point of fixation. Patients were light adapted for 5 minutes at 2100 asb before dark adaptation for 30 minutes