RP1 and autosomal dominant rod-cone dystrophy: novel mutations, a review of published variants, and genotype-phenotype correlation

Rod-cone dystrophies (retinitis pigmentosa [RP]) are a clinically and genetically heterogeneous group of inherited retinal disorders characterized by photoreceptor degeneration. RP1 is a major gene underlying autosomal dominant (ad) RP, though prevalence figures vary depending on the origin of the cases from 0-10% of all adRP. Some mutations in RP1 also lead to autosomal recessive (ar) RP. Herein, we review all previously reported and several novel RP1 mutations in relation to the associated phenotype in RP1 patients from a French adRP cohort. Prevalence studies from this cohort show that 5.3% of the cases have RP1 mutations. This is in accordance with other studies reported from United Kingdom and United States. The majority of mutations represent truncating mutations that are located in a hot spot region of the gene. Similarly, we identified in total four novel deletions and nonsense mutations, of which two may represent recurrent mutations in this population. In addition, a novel missense mutation of uncertain pathogenicity was identified. Including our findings to date, 47 RP1 mutations are known to cause adRP. Variable penetrance of the disease was observed in our and other cohorts. Most patients with RP1 mutations show classical signs of RP with relatively preserved central vision and visual field.     

Mutations P51U and G122E in retinal transcription factor NRL associated with autosomal dominant and sporadic retinitis pigmentosa

Retinitis pigmentosa (RP) is the most frequent form of inherited retinopathy. RP is genetically heterogeneous with autosomal dominant, autosomal recessive and X-linked forms. Autosomal dominant retinitis pigmentosa (adRP) accounts for about 20-25% of all RP cases. At least ten adRP loci have so far been mapped. However, mutations causing adRP have been identified only in four retina-specific genes: RHO (encoding rhodopsin) in approximately 20% of adRP families, peripherin/RDS (3-5% of adRP) and recently RP1 (Pierce et al., 1999, Sulivan et al., 1999) and NRL gene. Only one mutation in the NRL gene causing adRP has so far been reported (Bessant et al., 1999). Here we report a novel mutation Pro51Leu in an adRP Spanish family supporting that mutation in NRL is the cause of adRP. A second missense mutation Gly122Glu has been observed in a simplex RP patient that may represent a sporadic case of retinitis pigmentosa. Hum Mutat 17:520, 2001.     

Polymorphic variation within "conserved" sequences at the 3' end of the human RDS gene which results in amino acid substitutions.

The human RDS gene, previously mapped to chromosome 6p, encodes a protein found in the outer disc membrane of the photoreceptor cells of the retina. The cDNA sequence of the human gene shows 85% identity with the bovine peripherin gene and the rds (retinal degeneration slow) genes from mouse and rat. Mutations in the RDS gene have recently been implicated in autosomal dominant retinitis pigmentosa (adRP) in some families. Here we present evidence that the third exon of this gene is subject to polymorphic variation in humans. The three sequence alterations described in this paper give rise to amino acid substitutions. However, as these missense mutations also occur in the normal population they are not implicated as causing adRP. Interestingly such sequence variation is not found within other species examined including mouse and bovine. These intragenic polymorphisms will be of future potential value in studies to locate further disease causing mutations in adRP patients in the RDS gene.     

Mutation analysis of the ROM1 gene in retinitis pigmentosa

To examine the role of ROM1, a homologue of peripherin/RDS,in autosomal dominant retinitis pigmentosa (adRP), we screened224 adRP and 29 simplex RP probands for ROM1 mutations. FourROM1 alleles were designated as potentially pathogenic becausethey were found only in RP patients but not in 50–100controls nor in 249 other RP probands. The substitutions P60Tand T108M were present in a single alleie in a subject withtypical adRP, and this allele cosegregated with the diseasein the small family. The putative null allele L114 [     

A ninth locus (RP18) for autosomal dominant retinitis pigmentosa maps in the pericentromeric region of chromosome 1

We studied a large Danish family of seven generations in which autosomal dominant retinitis pigmentosa (adRP), a heterogeneous genetic form of retinal dystrophy, was segregating. After linkage had been excluded to all known adRP loci on chromosomes 3q, 6p, 7p, 7q, 8q, 17p, 17q and 19q, a genome screening was performed. Positive lod scores suggestive of linkage with values ranging between Z = 1.58-5.36 at theta = 0.04-0.20 were obtained for eight loci on proximal 1p and 1q. Close linkage without recombination and a maximum lod score of 7.22 at theta = 0.00 was found between the adRP locus (RP18) in this family and D1S498 which is on 1q very near the centromere. Analysis of multiply informative meioses suggests that in this family D1S534 and D1S305 flank RP18 in interval 1p13-q23. No linkage has been found to loci from this chromosomal region in six other medium sized adRP families in which the disease locus has been excluded from all known chromosomal regions harbouring an adRP gene or locus suggesting that there is (at least) one further adRP locus to be mapped in the future.      

Mutations in a protein target of the Pim-1 kinase associated with the RP9 form of autosomal dominant retinitis pigmentosa

The RP9 form of autosomal dominant retinitis pigmentosa (adRP) maps to a locus on human chromosome 7p14. We now report two different disease associated mutations in a previously unidentified human gene, the mouse orthologue of which has been characterised by its interaction with the Pim-1 oncogene. In the original linked family we identified the missense mutation H137L. A second missense mutation, D170G, was found in a single RP patient. The putative RP9 gene appears to be expressed in a wide range of tissues, but its function is unknown and a pathogenic mechanism remains to be determined.     

视网膜色素变性患者中视网膜色素变性1基因点突变的研究

目的 研究中国视网膜色素变性（retinitis pigmentosa，RP)患者RP1基因的突变频率、特征及其在RP发病机理中所起的作用。方法 运用构象敏感凝胶电泳（conformation sensitive gel electrophoresis,CSGE)和DNA直接测序方法对101例香港地区RP患者的RP1基因全编码区进行突变的筛选与检测。结果 101例RP患者中检出1例患者携带常见的RP1致病突变－R677X，另外在3名正常个体及1例Stargardt患者中检出非致病的无义突变－R1933X。RP1基因在所有RP患者中的突变检出率为1／101。突变最终导致RP1蛋白严重截短。此外，在本研究人群中还发现10个错义突变，除M479I的病理意义未确定之外，其余均系RP1基因的多态现象。结论 R1933X无致病意义，提示羧基端224个氨基酸的区域可能为RP1蛋白非功能区，结合最近发现的RP1羧基端的移码突变－Y1053（1bp del)的病理意义，推测RP1蛋白中相应片段（密码子1052－1933）的缺失会导致RP的发生。为证实这种推测，大范围的RP1基因分型工作是有必要的，并且可同时发现更多的RP致病突变以及不同于其他种族人群的RP1基因多态变化。     

Arg120stop nonsense mutation in the RP2 gene: mutational hotspot and germ line mosaicism?

Mutations in the RP2 gene account for up to 20% of X-linked recessive retinitis pigmentosa (RP). Arg120stop is to date the most frequently reported mutation found in RP2. Mutation screening was performed during the course of a large screening program of retinal degenerative disorders (RDDs) in South Africa using exon 1 and 2 of RP2 in 20 unrelated families with an X-linked mode of retinal degenerative inheritance. Direct sequencing analysis revealed a C-->T transition at position 358 in the proband in a family of German origin. Subsequent analysis revealed that this Arg120stop mutation cosegregated with the disease in an additional affected family member. The nonsense mutation, Arg120stop, could not however, be detected in the somatic cells of the obligate carrier female. This, the first report of a germ line mutation for a family with RP, has many implications for genetic counseling of retinal degeneration (RD). To avoid inaccurate risk assessment for RP due to epigenetic events, such as the rare occurrence of germ line mosaicism, genetic counseling in families with XLRP should always be guided by molecular testing.     

Three novel and the common Arg677Ter RP1 protein truncating mutations causing autosomal dominant retinitis pigmentosa in a Spanish population

Background  

Retinitis pigmentosa (RP), a clinically and genetically heterogeneous group of retinal degeneration disorders affecting the photoreceptor cells, is one of the leading causes of genetic blindness. Mutations in the photoreceptor-specific gene RP1 account for 3–10% of cases of autosomal dominant RP (adRP). Most of these mutations are clustered in a 500 bp region of exon 4 of RP1.     

RDS/peripherin gene mutations are frequent causes of central retinal dystrophies.

Patients from 76 independent families with various forms of mostly central retinal dystrophies were screened for mutations in the RDS/peripherin gene by means of SSCP analysis and direct DNA sequencing. Two nonsense mutations (Gln239ter, Tyr285ter), five missense mutations (Arg172Trp, Lys197Glu, Gly208Asp, Trp246Arg, Ser289Leu), and one single base insertion (Gly208insG), heterozygous in all cases, were detected. Only one of these mutations, Arg172Trp, has been reported previously. Cosegregation of the mutation with the disease phenotype could be established in selected families. Other missense mutations were excluded from a panel of 55-75 control subjects. The patients showed remarkable variation in phenotype and disease expression not only between cases with different mutations but also between affected members of the same family. This study indicates that RDS/peripherin mutations are a frequent cause of various types of central retinal dystrophies and that the RDS/peripherin gene exhibits a broad spectrum of allelic mutations. Comparative analysis of known mutations allowed us to hypothesise that the deleterious effect of RDS/peripherin gene mutations is the result of different molecular mechanisms.     

Mutations and polymorphisms in the human peripherin-RDS gene and their involvement in inherited retinal degeneration

The RDS gene codes for the protein peripherin-RDS, which is an integral membrane glycoprotein found in the outer segment of both rod and cone photoreceptor cells. It is thought to function as a structural protein involved in the maintenance of the flattened form of the disc lamellae. The RDS gene has been implicated in the mouse phenotype retinal degeneration slow, and mutations in the human homologue are now known to be associated with both central and peripheral retinal degenerations. In all, 43 sequence variants have been described in the human gene, including 30 missense mutations, two single base substitutions producing termination codons, 7 small in-frame deletions, and 4 insertion/deletion events, which break the reading frame. Of these, 39 are associated with retinal phenotypes, which can be grouped into four broad categories: dominant retinitis pigmentosa, progressive macular degeneration, digenic RP, and pattern dystrophies. The mutations underlying dominant RP and severe macular degeneration are largely missense or small in-frame deletions in a large intradiscal loop between the third and fourth transmembrane domains. In contrast, those associated with the milder pattern phenotypes or with digenic RP are scattered more evenly through the gene and are often nonsense mutations. This observation correlates with the hypothesis that the large loop is an important site of interaction between RDS molecules and other protein components in the disc. © 1996 Wiley-Liss, Inc.     

Novel deletion in the pre-mRNA splicing gene PRPF31 causes autosomal dominant retinitis pigmentosa in a large Chinese family

We report the identification of a novel 12 bp deletion of the pre-mRNA splicing gene PRPF31 in a large Chinese family with autosomal dominant retinitis pigmentosa (adRP). This mutation results in the deletion of four amino acids (DeltaH(111)K(112)F(113)I(114)) including H(111), an amino acid residue that is highly conserved throughout evolution. The 12 bp deletion co-segregates with the disease phenotype in 19 RP patients in the family, but is not present in unaffected relatives or 100 normal individuals. Our data indicate that the novel 12 bp deletion in PRPF31 causes retinitis pigementosa in this Chinese adRP family. In contrast to the incomplete penetrance observed in most adRP families linked to chromosome band 19q13.4 (RP11), the 12 bp PRPF31 deletion identified in this study appears to show high penetrance. These data expand the spectrum of PRPF31 mutations causing adRP, and confirm the role of PRPF31 in the pathogenesis of RP.     

Mutations in the palmitoyl-protein thioesterase gene (PPT; CLN1) causing juvenile neuronal ceroid lipofuscinosis with granular osmiophilic deposits [published erratum appears in Hum Mol Genet 1998 Apr;7(4):765]

A subtype of neuronal ceroid lipofuscinosis (NCL) is well recognized which has a clinical course consistent with juvenile NCL (JNCL) but the ultrastructural characteristics of infantile NCL (INCL): granular osmiophilic deposits (GROD). Evidence supporting linkage of this phenotype, designated vJNCL/GROD, to the INCL region of chromosome 1p32 was demonstrated (pairwise lod score with D1S211 , Z max = 2.63, straight theta = 0.00). The INCL gene, palmitoyl-protein thioesterase (PPT ; CLN1), was therefore screened for mutations in 11 vJNCL/GROD families. Five mutations in the PPT gene were identified: three missense mutations, Thr75Pro, Asp79Gly, Leu219Gln, and two nonsense mutations, Leu10STOP and Arg151STOP. The missense mutation Thr75Pro accounted for nine of the 22 disease chromosomes analysed and the nonsense mutation Arg151STOP for seven. Nine out of 11 patients were shown to combine a missense mutation on one disease chromosome with a nonsense mutation on the other. Mutations previously identified in INCL were not observed in vJNCL/GROD families. Thioesterase activity in peripheral blood lymphoblast cells was found to be markedly reduced in vJNCL/GROD patients compared with controls. These results demonstrate that this subtype of JNCL is allelic to INCL and further emphasize the correlation which exists between genetic basis and ultrastructural changes in the NCLs.      

Genes and mutations causing retinitis pigmentosa

Retinitis pigmentosa (RP) is a heterogeneous set of inherited retinopathies with many disease‐causing genes, many known mutations, and highly varied clinical consequences. Progress in finding treatments is dependent on determining the genes and mutations causing these diseases, which includes both gene discovery and mutation screening in affected individuals and families. Despite the complexity, substantial progress has been made in finding RP genes and mutations. Depending on the type of RP, and the technology used, it is possible to detect mutations in 30–80% of cases. One of the most powerful approaches to genetic testing is high‐throughput ‘deep sequencing’, that is, next‐generation sequencing (NGS). NGS has identified several novel RP genes but a substantial fraction of previously unsolved cases have mutations in genes that are known causes of retinal disease but not necessarily RP. Apparent discrepancy between the molecular defect and clinical findings may warrant reevaluation of patients and families. In this review, we summarize the current approaches to gene discovery and mutation detection for RP, and indicate pitfalls and unsolved problems. Similar considerations apply to other forms of inherited retinal disease.     

A completed screen for mutations of the rhodopsin gene in a panel of patients with autosomal dominant retinitis pigmentosa.

Recently it has been demonstrated that some families with autosomal dominant retinitis pigmentosa (adRP) have mutations in the rhodopsin gene while others do not. Previously we have identified six such mutations in seven adRP families in this laboratory, one of which was previously described in US patients. We now present a completed screen of the rhodopsin gene in a panel of 39 adRP families, by a rapid screening technique which will be of use for routine diagnosis. Nine different mutations were ultimately found, in a total of twelve of the 39 families. These include the six previously identified mutations, in codons 68-71, 190, 211, 255, 296 and 347, two new ones in codons 53 and 106, and another mutation first identified in a single US patient, in codon 58. Thus approximately 30% of adRP families have 'Rhodopsin RP' while the remainder probably have a defect elsewhere in the genome. Of those families in which rhodopsin mutations have been found, four have been classified D type, three as sectorial RP and the remainder are of uncertain classification. All families excluded from chromosome 3q by linkage have been classified R type. These data suggest a correlation between clinical sub-classification and the underlying rhodopsin/non-rhodopsin heterogeneity.     

CRB1 mutation spectrum in inherited retinal dystrophies

Mutations in the Crumbs homologue 1 (CRB1) gene have been reported in patients with a variety of autosomal recessive retinal dystrophies, including retinitis pigmentosa (RP) with preserved paraarteriolar retinal pigment epithelium (PPRPE), RP with Coats-like exudative vasculopathy, early onset RP without PPRPE, and Leber congenital amaurosis (LCA). We extended our investigations of CRB1 in these retinal dystrophies, and identified nine novel CRB1 sequence variants. In addition, we screened patients with "classic" RP and classic Coats disease (without RP), but no pathologic sequence variants were found in the CRB1 gene. In total, 71 different sequence variants have been identified on 184 CRB1 alleles of patients with retinal dystrophies, including amino acid substitutions, frameshift, nonsense, and splice site mutations, in-frame deletions, and large insertions. Recent studies in two animal models, mouse and Drosophila, and in vivo high-resolution microscopy in patients with LCA, have shed light on the role of CRB1 in the pathogenesis of retinal dystrophies and its function in the photoreceptors. In this article, we provide an overview of the currently known CRB1 sequence variants, predict their effect, and propose a genotype-phenotype correlation model for CRB1 mutations.