A new family linked to the RP13 locus for autosomal dominant retinitis pigmentosa on distal 17p

Pre-mRNA Processing Factors and Retinitis Pigmentosa: RNA Splicing and Beyond

Retinitis pigmentosa (RP) is the most common inherited retinal disease characterized by progressive degeneration of photoreceptors and/or retinal pigment epithelium that eventually results in blindness. Mutations in pre-mRNA processing factors (PRPF3, 4, 6, 8, 31, SNRNP200, and RP9) have been linked to 15–20% of autosomal dominant RP (adRP) cases. Current evidence indicates that PRPF mutations cause retinal specific global spliceosome dysregulation, leading to mis-splicing of numerous genes that are involved in a variety of retina-specific functions and/or general biological processes, including phototransduction, retinol metabolism, photoreceptor disk morphogenesis, retinal cell polarity, ciliogenesis, cytoskeleton and tight junction organization, waste disposal, inflammation, and apoptosis. Importantly, additional PRPF functions beyond RNA splicing have been documented recently, suggesting a more complex mechanism underlying PRPF-RPs driven disease pathogenesis. The current review focuses on the key RP-PRPF genes, depicting the current understanding of their roles in RNA splicing, impact of their mutations on retinal cell’s transcriptome and phenome, discussed in the context of model species including yeast, zebrafish, and mice. Importantly, information on PRPF functions beyond RNA splicing are discussed, aiming at a holistic investigation of PRPF-RP pathogenesis. Finally, work performed in human patient-specific lab models and developing gene and cell-based replacement therapies for the treatment of PRPF-RPs are thoroughly discussed to allow the reader to get a deeper understanding of the disease mechanisms, which we believe will facilitate the establishment of novel and better therapeutic strategies for PRPF-RP patients.

Long-term clinical course of 2 Japanese patients with PRPF31-related retinitis pigmentosa

PURPOSE

To assess the long-term clinical course of 2 patients with PRPF31-related retinitis pigmentosa (RP).

PATIENTS AND METHODS

We clinically examined 2 unrelated patients with RP and collected peripheral blood samples from them. Ophthalmic examinations, including best-corrected visual acuity measurements, Goldmann perimetry, full-field electroretinography, fundus autofluorescence imaging, and optical coherence tomography, were also performed. The visual acuity and visual field were continuously monitored. To identify the causative mutations, 74 genes known to cause RP or Leber congenital amaurosis were examined via targeted next-generation sequencing.

RESULTS

The clinical courses of both patients were similar. The onset of nyctalopia occurred in the first decade. Fundus examination showed typical RP. Although the patients' visual acuity was relatively preserved even into the fourth decade, the visual field area exhibited rapid deterioration in the mid-teens, with severe concentric constriction in the third decade. Mutation analysis revealed PRPF31 mutations as the cause for autosomal dominant RP in both patients.

CONCLUSIONS

To the best of our knowledge, few reports of long-term observations pertaining to patients with PRPF31-related RP have been published. The findings reported herein, especially those relating to the progressive degeneration of the visual field, may ultimately play a role in the provision of high-quality counseling for patients with this condition.

Clinical phenotype of an Italian family with a new mutation in the PRPF8 gene

PURPOSE

To report the clinical and functional characteristics of an autosomal dominant retinitis pigmentosa (ADRP) family with a novel point mutation (P2301S) in the PRPF8 gene.

METHODS

PRPF8 gene analysis and complete ophthalmologic examination in an ADRP family.

RESULTS

Clinical examination revealed the typical RP phenotype in all family members. Electroretinography showed preserved ERG photopic responses. Genetic analysis showed that the P2301S missense mutation segregated with the disease in all subjects.

CONCLUSIONS

Unlike previously reported families, the PRPF8 gene mutation in our family is associated with a mild phenotype in which cone function is partially preserved.

[Genetics of retinitis pigmentosa: metabolic classification and phenotype/genotype correlations]

Retinitis pigmentosa (RP, prevalence 1/4000) is a set of hereditary retinal dystrophies characterized by pigment deposits in fundus and progressive death of photoreceptors, always associated with the alteration of retinal pigment epithelium. Genetic heterogeneity of the typical nonsyndromic form (rod cone dystrophy) is extensive: 11 genes and one locus were reported for autosomal dominant RP, 17 genes and five loci for autosomal recessive RP, and two genes and two loci for X-linked RP. A survey of mutation screening reports in large series of patients indicates that the frequency of mutations for all cloned genes varies from 40% to 54% of cases in autosomal dominant RP, from 17% to 24% in autosomal recessive RP (excluding the USH2A gene for which the values remain uncertain) and from 61% to 89% in X-linked RP. Very few studies report on sporadic cases except for the two X-linked genes, RP2 and RPGR, which account for 29% of sporadic cases in males. Altogether, the two most frequently involved genes are RPGR (13% of all RP cases) and RHO (4%), an important consideration for molecular diagnosis. Finally, we roughly estimate that currently known genes do not represent more than 50% of RP cases, suggesting that many genes remain to be discovered. The known genes can be classified into metabolic groups according to the encoded protein: visual transduction, visual cycle, transcription factors, structural proteins, spliceosome complex and cellular traffic, indicating the high level of specialization of photoreceptors and of the retinal pigment epithelium. In parallel with this classification, genotype/phenotype correlations have been established that will help ophthalmologists to suspect particular genes, and thereby mechanisms. This approach will provide better informations to patients and will orient the choice of future therapies.

Clinical characterization, linkage analysis, and PRPC8 mutation analysis of a family with autosomal dominant retinitis pigmentosa type 13 (RP13)

A Dutch family with autosomal dominant retinitis pigmentosa (adRP) displayed a phenotype characterized by an early age of onset, a diffuse loss of rod and cone sensitivity, and constricted visual fields (type I). One male showed a mild progression of the disease. Linkage analysis showed cosegregation of the genetic defect with markers from chromosome 17p13.1-p13.3, a region overlapping the RP13 locus. The critical interval of the RP locus as defined in this family was flanked by D17S926 and D17S786, with a maximal lod score of 4.2 (theta = 0.00) for marker D17S1529. Soon after the mapping of the underlying defect to the 17p13 region, a missense mutation (6970G>A; R2310K) was identified in exon 42 of the splicing factor gene PRPC8 in one patient of this family. Diagnostic restriction enzyme digestion of exon 42 amplified from genomic DNA of all family members revealed that the R2310K mutation segregated fully with the disease. The type I phenotype observed in this family is similar to that described for three other RP13 families with mutations in PRPC8.

Retinitis pigmentosa: defined from a molecular point of view

Retinitis pigmentosa (RP) denotes a group of hereditary retinal dystrophies, characterized by the early onset of night blindness followed by a progressive loss of the visual field. The primary defect underlying RP affects the function of the rod photoreceptor cell, and, subsequently, mostly unknown molecular and cellular mechanisms trigger the apoptotic degeneration of these photoreceptor cells. Retinitis pigmentosa is very heterogeneous, both phenotypically and genetically. In this review we propose a tentative classification of RP based on the functional systems affected by the mutated proteins. This classification connects the variety of phenotypes to the mutations and segregation patterns observed in RP. Current progress in the identification of the molecular defects underlying RP reveals that at least three distinct functional mechanisms may be affected: 1) the daily renewal and shedding of the photoreceptor outer segments, 2) the visual transduction cascade, and 3) the retinol (vitamin A) metabolism. The first group includes the rhodopsin and peripherin/RDS genes, and mutations in these genes often result in a dominant phenotype. The second group is predominantly associated with a recessive phenotype that results, as we argue, from continuous inactivation of the transduction pathway. Disturbances in the retinal metabolism seem to be associated with equal rod and cone involvement and the presence of deposits in the retinal pigment epithelium.

A linkage survey of 20 dominant retinitis pigmentosa families: frequencies of the nine known loci and evidence for further heterogeneity

Autosomal dominant retinitis pigmentosa (ADRP) is caused by mutations in two known genes, rhodopsin and peripherin/Rds, and seven loci identified only by linkage analysis. Rhodopsin and peripherin/Rds have been estimated to account for 20-31% and less than 5% of ADRP, respectively. No estimate of frequency has previously been possible for the remaining loci, since these can only be implicated when families are large enough for linkage analysis. We have carried out such analyses on 20 unrelated pedigrees with 11 or more meioses. Frequency estimates based on such a small sample provide only broad approximations, while the above estimations are based on mutation detection in much larger clinic based patient series. However, when markers are informative, linkage analysis cannot fail to detect disease causation at a locus, whereas mutation detection techniques might miss some mutations. Also diagnosing dominant RP from a family history taken in a genetic clinic may not be reliable. It is therefore interesting that 10 (50%) of the families tested have rhodopsin-RP, suggesting that, in large clearly dominant RP pedigrees, rhodopsin may account for a higher proportion of disease than had previously been suspected. Four (20%) map to chromosome 19q, implying that this is the second most common ADRP locus. One maps to chromosome 7p, one to 17p, and one to 17q, while none maps to 1cen, peripherin/Rds, 8q, or 7q. Three give exclusion of all of these loci, showing that while the majority of dominant RP maps to the known loci, a small proportion derives from loci yet to be identified.

Evidence that the penetrance of mutations at the RP11 locus causing dominant retinitis pigmentosa is influenced by a gene linked to the homologous RP11 allele

A subset of families with autosomal dominant retinitis pigmentosa (RP) display reduced penetrance with some asymptomatic gene carriers showing no retinal abnormalities by ophthalmic examination or by electroretinography. Here we describe a study of three families with reduced-penetrance RP. In all three families the disease gene appears to be linked to chromosome 19q13.4, the region containing the RP11 locus, as defined by previously reported linkage studies based on five other reduced-penetrance families. Meiotic recombinants in one of the newly identified RP11 families and in two of the previously reported families serve to restrict the disease locus to a 6-cM region bounded by markers D19S572 and D19S926. We also compared the disease status of RP11 carriers with the segregation of microsatellite alleles within 19q13.4 from the noncarrier parents in the newly reported and the previously reported families. The results support the hypothesis that wild-type alleles at the RP11 locus or at a closely linked locus inherited from the noncarrier parents are a major factor influencing the penetrance of pathogenic alleles at this locus.