PRPH2/RDS and ROM-1: Historical context,current views and future considerations

Peripherin 2 (PRPH2), also known as RDS (retinal degeneration slow) is a photoreceptor specific glycoprotein which is essential for normal photoreceptor health and vision. PRPH2/RDS is necessary for the proper formation of both rod and cone photoreceptor outer segments, the organelle specialized for visual transduction. When PRPH2/RDS is defective or absent, outer segments become disorganized or fail to form entirely and the photoreceptors subsequently degenerate. Multiple PRPH2/RDS disease-causing mutations have been found in humans, and they are associated with various blinding diseases of the retina such as macular degeneration and retinitis pigmentosa, the vast majority of which are inherited dominantly, though recessive LCA and digenic RP have also been associated with RDS mutations. Since its initial discovery, the scientific community has dedicated a considerable amount of effort to understanding the molecular function and disease mechanisms of PRPH2/RDS. This work has led to an understanding of how the PRPH2/RDS molecule assembles into complexes and functions as a necessary part of the machinery that forms new outer segment discs, as well as leading to fundamental discoveries about the mechanisms that underlie OS biogenesis. Here we discuss PRPH2/RDS-associated research and how experimental results have driven the understanding of the PRPH2/RDS protein and its role in human disease.     

Recessive pediatric-onset cone-rod dysfunction or dominant maculopathy in a consanguineous family harboring the peripherin mutation p.Arg220Gln

Purpose: Heterozygous peripherin mutation is associated with a wide range of typically adult-onset retinal phenotypes which can include asymptomatic maculopathy. There are few reports of biallelic peripherin mutations, only one of which detailed the ophthalmic phenotype. This report documents the retinal phenotype associated with homozygosity for a known peripherin mutation (c.659G>A; p.Arg220Gln), highlights its similar appearance to what was described in the one previous report, and shows how examination of family members can be useful in genetic diagnosis.

Methods: Retrospective case series.

Results: A 13-year-old Emirati boy was referred for low vision. The parents felt he was blind at birth but noted improvement with time. Retinal examination was significant for central macula horizontal ovoid discoloration as was documented for young adults with homozygous peripherin mutations in the one previous report. Electroretinography revealed cone-rod dysfunction. Both asymptomatic parents were examined and found to have central macular abnormalities. Sanger sequencing of peripherin based on clinical features uncovered the pathogenic variant c.659G>A; p.Arg22Gln (NM_000322.4) in homozygosity in the child and in heterozygosity in each parent. Exome sequencing in the child excluded pathologic variants in other retinal dystrophy genes.

Conclusions: The experience with this family highlights clinical features suggestive for biallelic peripherin mutations, documents cone-rod dysfunction as associated with homozygosity for the p.Arg220Gln peripherin mutation, and is an example of how examination of family members can help to guide genetic testing.     

Genotype–phenotype associations in a large PRPH2‐related retinopathy cohort

Molecular variant interpretation lacks disease gene‐specific cohorts for determining variant enrichment in disease versus healthy populations. To address the molecular etiology of retinal degeneration, specifically the PRPH2‐related retinopathies, we reviewed genotype and phenotype information obtained from 187 eyeGENE® participants from 161 families. Clinical details were provided by referring clinicians participating in the eyeGENE® Network. The cohort was sequenced for variants in PRPH2. Variant complementary DNA clusters and cohort frequency were compared to variants in public databases to help us to determine pathogenicity by current American College of Medical Genetics and Genomics/Association for Molecular Pathology interpretation criteria. The most frequent variant was c.828+3A>T, which affected 28 families (17.4%), and 25 of 79 (31.64%) variants were novel. The majority of missense variants clustered in the D2 intracellular loop of the peripherin‐2 protein, constituting a hotspot. Disease enrichment was noted for 23 (29.1%) of the variants. Hotspot and disease‐enrichment evidence modified variant classification for 16.5% of variants. The missense allele p.Arg172Trp was associated with a younger age of onset. To the best of our knowledge, this is the largest patient cohort review of PRPH2‐related retinopathy. Large disease gene‐specific cohorts permit gene modeling for hotspot and disease‐enrichment analysis, providing novel variant classification evidence, including for novel missense variants.     

Evaluation of the association of single nucleotide polymorphisms in the PRPH2 gene with adult-onset foveomacular vitelliform dystrophy

Objective: A minority of patients with adult-onset foveomacular vitelliform dystrophy (AFVD) carry mutations in the PRPH2 gene. This gene is highly polymorphic and it was suggested that single-nucleotide polymorphisms (SNPs) in PRPH2 may also be associated with AFVD. We aimed to evaluate for such an association.

Methods: A single center cohort from a tertiary referral center including 52 consecutive patients with a clinical diagnosis of AFVD and 91 unaffected individuals was assessed. Sanger sequencing was performed for the PRPH2, BEST1, and IMPG1/2 genes. Investigation as to the frequency of minor alleles for SNPs in PRPH2 was performed and compared to HapMap and Exome Variant Server (EVS) data.

Results: None of the patients carry a mutation in PRPH2, BEST1, or IMPG1/2. Five of 14 known SNPs (rs835, rs361524, rs434102, rs425876, rs390659) in exon 3 of PRPH2 were identified in AFVD patients. A high frequency and percentage of minor alleles of these five SNPs was found in the Israeli AFVD patients and controls compared with European, Chinese, Japanese and African populations identified via HapMap and EVS (p < 0.05). Power calculation suggested that the sample size was sufficient (80%) to rule out an association with an odds ratio above 2.5.

Conclusions: These results suggest that genetic variants in PRPH2 do not compose a major genetic risk factor for AFVD. The Israeli population shows a higher percentage of minor allele frequencies in SNPs in the PRPH2 gene, as compared with other populations. This emphasizes the need for appropriate genetic background when performing SNP association testing.     

Autosomal dominant retinitis pigmentosa with macular involvement associated with a disease haplotype that included a novel PRPH2 variant (p.Cys250Gly)

Background: It is known that PRPH2 variants appear to be rare causes of retinitis pigmentosa (RP) in the Japanese population. The purpose of this study was to describe clinical and genetic features in autosomal dominant RP (adRP) patients with a novel disease-causing variant in the PRHP2 gene.

Materials and methods: A total of 57 unrelated Japanese probands with adRP were investigated in this study. Comprehensive ophthalmic examinations include fundus photography, fundus autofluorescence imaging, spectral-domain optical coherence tomography, and electroretinography. Whole exome sequencing or Sanger sequencing for 25 targeted exons of multiple genes causing adRP was performed to identify disease-causing variants. Co-segregation and haplotype analyses were performed to determine a disease-causing gene variant and its haplotype.

Results: Genetic analysis identified a novel heterozygous PRPH2 variant (c.748T>G, p.Cys250Gly) as disease causing in four probands from four families. The variant co-segregated with the RP phenotype in the eight affected patients in all families. At least three of the four families shared the same haplotype for the variant allele. Clinically, seven of the eight affected patients exhibited typical RP presentation, as well as variable macular involvement including cystoid macular change, vitelliform-like appearance, choroidal neovascularization, and macular atrophy.

Conclusions: The same disease haplotype that included a novel PRPH2 variant (p.Cys250Gly) was identified in three of the four Japanese families with adRP, suggesting a founder effect. Our clinical findings indicate that adRP caused by the p.Cys250Gly variant may accompany macular involvement with high frequency.