The RPE65 gene encodes a protein of 533 amino acids that is essential for normal vision. It is produced in the retinal pigment epithelium (RPE), a cell layer that supports and nourishes the retina (the light-sensitive tissue that lines the back of the eye). RPE65 plays an important role in the visual cycle. In the cycle, when the light enters the eye, it is converted into electrical signals that are transmitted to the brain. When the light hits photosensitive pigments in the retina, it changes a molecule called 11-cis retinal (a form of vitamin A) to all-trans retinal, which triggers a series of chemical reactions that create electrical signals. RPE65 protein helps in converting all-trans retinal back to 11-cis retinal to start the cycle again.
The RPE65 has two forms; a soluble form called sRPE65, and a palmitoylated, membrane-bound form known as mRPE65. The soluble form binds vitamin A (all-trans-retinol), making it available for LRAT processing to all-trans-retinyl ester. The membrane form, palmitoylated by LRAT, binds all-trans-retinyl esters, making them available for IMH (isomerohydrolase) processing to all-cis-retinol.
Defects in the RPE65 protein are the cause of retinitis pigmentosa 20 (RP) and Leber congenital amaurosis type 2 (LCA 2). These are autosomal recessive disorders characterized by severe childhood blindness.
RPE65 gene has been mapped to chromosome 1p31. It consists of 14 coding exons, spans about 21.1 kb, and encodes a protein of 533 amino acids. More than 20 mutations that cause retinitis pigmentosa have been identified in RPE65 gene. The mutations disrupt the protein function, which result in vision loss by impairing the visual cycle.
At least 30 mutations in the RPE65 gene have also been identified in patients with Leber congenital amaurosis, accounting for 6-16 of all cases of this condition. These mutations result in the accumulation of 11-cis retinal in the retinal pigment epithelium, which leads to severe visual impairment beginning very early in life.
[See: Yemen > Jakobsson et al., 2014].
In 37 consanguineous families from Saudi Arabia with Leber congenital amaurosis, Li et al., (2009) performed direct PCR and sequencing for 13 known LCA genes (GUCY2D, CRX, RPE65, TULP1, AIPL1, CRB1, RPGRIP1, LRAT, RDH12, IMPDH1, CEP290, RD3, and LCA5). Of 417 individuals from these families, 117 were affected. Two different mutations in the RPE65 gene were identified in two families. The first mutation was a p.R91W amino acid substitution, while the second mutation was a novel splice donor site mutation of exon 6 (GGT3->GCT), leading to a frameshift and early termination.
Khan et al. (2014) conducted a retrospective case series analysis for a consecutive cohort of Saudi children diagnosed with LCA between 2012 and 2014. A total of 23 patients from 19 consanguineous or endogamous families were screened for mutations in a panel of 14 known LCA genes. Two of these families were found to have mutations in the RPE65 gene. The two affected male patients in the first family carried homozygous c.271C>T (p.Arg91Trp) mutations. The single affected male patient from the second family carried compound heretrozygous mutations. One of these was a novel c.540C>A (p.His180Gln) missense mutation, while the other was a previously reported c.1067dupA (p.Asn356Lysfs*9) mutation.
El Matri et al. (2006) conducted genome-wide linkage analysis in three consanguineous Tunisian families presenting with early-onset retinal degeneration (EORD). Of 53 examined members, 11 were clinically affected with an EORD. Linkage analysis revealed a maximal lod score of 4.02 (theta=0.1) for the marker D1S207 on 1p31. Sequencing of the RPE65 gene identified a homozygous p.R91W mutation co-segregating with the disease in all affected individuals. Eleven homozygotes had nystagmus and acuities ranging from CF to NLP. Homozygotes had no detectable full-field ERG and an abnormal pupillary light reflex. Eleven heterozygotes had normal visual function.
Al-Gazali et al. (2010) performed mutation analysis for the RPE65 gene in six children from an extended Emirati family affected with retinitis pigmentosa. Homozygous deletion of exons one to seven of the RPE65 gene was identified in the affected patients.
Jacobsson et al. (2014) analyzed a family from Yemen with three members affected with a severe phenotype of Leber Congenital Amaurosis with an early and severe cone dysfunction. Jacobsson et al. (2014) undertook linkage analysis using markers flanking the known LCA genes, which narrowed the locus down to the RPE65 gene. Sequencing of this gene identified a novel homozygous missense mutation (IVS2-3C>G) in this gene in the second intron in all affected patients.