Missense mutation in the gene encoding the alpha subunit of rod transducin in the Nougaret form of congenital stationary night blindness.

Patients with congenital stationary night blindness enjoy normal daytime vision, which is mediated by cone photoreceptors, but are blind when ambient light is so dim that a normal individual would utilize only rod photoreceptors to see without colour discrimination. The disease is genetically heterogeneous. One form of dominantly inherited congenital night blindness is eponymously named "Nougaret' because pedigree analysis reveals that the disease originated in Jean Nougaret (1637-1719), a butcher who lived in Vendémian in southern France. Here we report that his affected descendants carry a missense mutation in the gene encoding the alpha subunit of rod transducin the G-protein that couples rhodopsin to cGMP-phosphodiesterase in the phototransduction cascade. Based on these results, rod transducin joins rhodopsin and the beta subunit of rod cGMP-phosphodiesterase to become the third component of the rod phototransduction cascade where a defect is implicated as a cause of stationary night blindness. Interestingly, the amino acid residue in transducin affected by the Nougaret mutation is in the position homologous to that affected by the oncogenic mutation originally reported in p21ras, a distant relative in the G-protein superfamily.

Defects in the rhodopsin kinase gene in the Oguchi form of stationary night blindness.

Oguchi disease is a recessively inherited form of stationary night blindness due to malfunction of the rod photoreceptor mechanism. Patients with this disease show a distinctive golden-brown colour of the fundus that occurs as the retina adapts to light, called the Mizuo phenomenon. Recently a defect in arrestin, a member of the rod phototransduction pathway, was found to cause this disease in some Japanese patients. As rhodopsin kinase works with arrestin in shutting off rhodopsin after it has been activated by a photon of light, it is reasonable to propose that some cases of Oguchi disease might be caused by defects in rhodopsin kinase. This report describes an analysis of the arrestin and rhodopsin kinase genes in three unrelated cases of Oguchi disease. No defects in arrestin were detected, but all three cases had mutations in the rhodopsin kinase gene. Two cases were found to be homozygous for a deletion encompassing exon 5, predicted to lead to a nonfunctional protein. The third case was a compound heterozygote with two allelic mutations, a missense mutation (Val380Asp) affecting a residue in the catalytic domain, and a frameshift mutation (Ser536(4-bp del)) resulting in truncation of the carboxy terminus. Our results indicate that null mutations in the rhodopsin kinase gene are a cause of Oguchi disease and extend the known genetic heterogeneity in congenital stationary night blindness.

Recessive mutations in the gene encoding the beta-subunit of rod phosphodiesterase in patients with retinitis pigmentosa.

We have found four mutations in the human gene encoding the beta-subunit of rod cGMP phosphodiesterase (PDE beta) that cosegregate with autosomal recessive retinitis pigmentosa, a degenerative disease of photoreceptors. In one family two affected siblings both carry allelic nonsense mutations at codons 298 and 531. Affected individuals have abnormal rod and cone electroretinograms. PDE beta is the second member of the phototransduction cascade besides rhodopsin that is absent or altered as a cause of retinitis pigmentosa, suggesting that other members of this pathway may be defective in other forms of this disease.

Heterozygous missense mutation in the rhodopsin gene as a cause of congenital stationary night blindness.

A number of mutations in the rhodopsin gene have been shown to cause both dominant and recessive retinitis pigmentosa. Here we describe another phenotype associated with a defect in this gene. We discovered a patient with congenital stationary night blindness who carries the missense mutation Ala292Glu. When coupled with 11-cis-retinal in vitro, Ala292Glu rhodopsin is able to activate transducin in a light-dependent manner like wild-type rhodopsin. However, without a chromophore, Ala292Glu opsin anomalously activates transducin. We speculate that the rod dysfunction in this patient is due to an abnormal, continuous activation of transducin by mutant opsin molecules in photoreceptor outer segments.

A null mutation in the human peripherin/RDS gene in a family with autosomal dominant retinitis punctata albescens.

The murine rds (retinal degeneration slow) allele is a semidominant null allele that causes photoreceptor degeneration. The wild-type sequence at the rds locus encodes a photoreceptor disc membrane protein named peripherin/RDS. Mutations in the homologous human peripherin/RDS gene can cause autosomal dominant retinitis pigmentosa, but these are missense mutations or deletions of single codons. No obvious null alleles have been reported in humans, so that the human phenotype corresponding to rds is as yet unknown. Here we report a 2-basepair deletion in codon 25 of the human gene in a family with autosomal dominant retinitis punctata albescens, suggesting that this disease, rather than retinitis pigmentosa, is the comparable human phenotype.

Autosomal recessive retinitis pigmentosa caused by mutations in the alpha subunit of rod cGMP phosphodiesterase.

Retinitis pigmentosa (RP) constitutes a group of genetically heterogeneous progressive photoreceptor degenerations leading to blindness and affecting 50,000-100,000 people in the U.S. alone. Over 20 different RP loci have been mapped, of which six have been identified. Three of these encode members of the rod photoreceptor visual transduction cascade: rhodopsin, the rod cGMP-gated cation channel alpha subunit, and the beta subunit of cGMP-phosphodiesterase (PDEB). As null mutations in PDEB cause some cases of RP and since both alpha and beta subunits are required for full phosphodiesterase activity, we examined the gene encoding the alpha subunit of cGMP phosphodiesterase (PDEA) in 340 unrelated patients with RP. We found three point mutations in PDEA in affected members of two pedigrees with recessive RP. Each mutation alters an essential functional domain of the encoded protein and likely disrupts its catalytic function. PDEA is the seventh RP gene identified, highlighting the extensive genetic heterogeneity of the disorder and encouraging further investigation into the role of other members of the phototransduction cascade in RP.

Mutations in the gene encoding 11-cis retinol dehydrogenase cause delayed dark adaptation and fundus albipunctatus.

The metabolic pathways that produce 11-cis retinal are important for vision because this retinoid is the chromophore residing in rhodopsin and the cone opsins. The all-trans retinal that is generated after cone and rod photopigments absorb photons of light is recycled back to 11-cis retinal by the retinal pigment epithelium and Müller cells of the retina. Several of the enzymes involved have recently been purified and molecularly cloned; here we focus on 11-cis retinol dehydrogenase (encoded by the gene RDH5; chromosome 12q13-14; ref. 4), the first cloned enzyme in this pathway. This microsomal enzyme is abundant in the retinal pigment epithelium, where it has been proposed to catalyse the conversion of 11-cis retinol to 11-cis retinal. We evaluated patients with hereditary retinal diseases featuring subretinal spots (retinitis punctata albescens and fundus albipunctatus) and patients with typical dominant or recessive retinitis pigmentosa for mutations in RDH5. Mutations were found only in two unrelated patients, both with fundus albipunctatus; they segregated with disease in the respective families. Recombinant mutant 11-cis retinol dehydrogenases had reduced activity compared with recombinant enzyme with wild-type sequence. Our results suggest that mutant alleles in RDH5 are a cause of fundus albipunctatus, a rare form of stationary night blindness characterized by a delay in the regeneration of cone and rod photopigments.

Mutations in a gene encoding a new oxygen-regulated photoreceptor protein cause dominant retinitis pigmentosa.

The autosomal dominant retinitis pigmentosa (RP) locus, designated RP1, has been mapped through linkage studies to a 4-cM interval at 8q11-13. Here we describe a new photoreceptor-specific gene that maps in this interval and whose expression is modulated by retinal oxygen levels in vivo. This gene consists of at least 4 exons that encode a predicted protein of 2,156 amino acids. A nonsense mutation at codon 677 of this gene is present in approximately 3% of cases of dominant RP in North America. We also detected two deletion mutations that cause frameshifts and introduce premature termination codons in three other families with dominant RP. Our data suggest that mutations in this gene cause dominant RP, and that the encoded protein has an important but unknown role in photoreceptor biology.

Recessive mutations in the gene encoding the tubby-like protein TULP1 in patients with retinitis pigmentosa.

A recessive mutation in the tub gene causes obesity, deafness and retinal degeneration in tubby mice. The tub gene is a member of a family of tubby-like genes (TULPs) that encode proteins of unknown function. Members of this family have been identified in plants, vertebrates and invertebrates. The TULP proteins share a conserved carboxy-terminal region of approximately 200 amino-acid residues. Here we report the analysis of the human gene TULP1, which is expressed specifically in the retina. Upon analysing 162 patients with nonsyndromic recessive retinitis pigmentosa (RP) and 374 simplex cases of RP, we found two who were compound heterozygotes for mutations that cosegregated with disease in the respective families. Three of the mutations are missense changes affecting the conserved C-terminal region; the fourth mutation affects a splice donor site upstream of this region. Our data suggest that mutations in TULP1 are a rare cause of recessive RP and indicate that TULP1 has an essential role in the physiology of photoreceptors.

A null mutation in the rhodopsin gene causes rod photoreceptor dysfunction and autosomal recessive retinitis pigmentosa.

Mutations within the rhodopsin gene are known to give rise to autosomal dominant retinitis pigmentosa (RP), a common hereditary form of retinal degeneration. We now describe a patient with autosomal recessive RP who is homozygous for a nonsense mutation at codon 249 within exon 4 of the rhodopsin gene. This null mutation, the first gene defect identified in autosomal recessive retinitis pigmentosa, should result in a functionally inactive rhodopsin protein that is missing the sixth and seventh transmembrane domains including the 11-cis-retinal attachment site. We also found a different null mutation carried heterozygously by an unrelated unaffected individual. Heterozygous carriers of either mutation had normal ophthalmologic examinations but their electroretinograms revealed an abnormality in rod photoreceptor function.

Digenic retinitis pigmentosa due to mutations at the unlinked peripherin/RDS and ROM1 loci.

In spite of recent advances in identifying genes causing monogenic human disease, very little is known about the genes involved in polygenic disease. Three families were identified with mutations in the unlinked photoreceptor-specific genes ROM1 and peripherin/RDS, in which only double heterozygotes develop retinitis pigmentosa (RP). These findings indicate that the allelic and nonallelic heterogeneity known to be a feature of monogenic RP is complicated further by interactions between unlinked mutations causing digenic RP. Recognition of the inheritance pattern exemplified by these three families might facilitate the identification of other examples of digenic inheritance in human disease.

Point mutational inactivation of the retinoblastoma antioncogene.

The retinoblastoma (Rb) antioncogene encodes a nuclear phosphoprotein, p105-Rb, that forms protein complexes with the adenovirus E1A and SV40 large T oncoproteins. A novel, aberrant Rb protein detected in J82 bladder carcinoma cells was not able to form a complex with E1A and was less stable than p105-Rb. By means of a rapid method for the detection of mutations in Rb mRNA, this defective Rb protein was observed to result from a single point mutation within a splice acceptor sequence in J82 genomic DNA. This mutation eliminates a single exon and 35 amino acids from its encoded protein product.

Transgenic mice with a rhodopsin mutation (Pro23His): a mouse model of autosomal dominant retinitis pigmentosa.

We inserted into the germline of mice either a mutant or wild-type allele from a patient with retinitis pigmentosa and a missense mutation (P23H) in the rhodopsin gene. All three lines of transgenic mice with the mutant allele developed photoreceptor degeneration; the one with the least severe retinal photoreceptor degeneration had the lowest transgene expression, which was one-sixth the level of endogenous murine rod opsin. Of two lines of mice with the wild-type allele, one expressed approximately equal amounts of transgenic and murine opsin and maintained normal retinal function and structure. The other expressed approximately 5 times more transgenic than murine opsin and developed a retinal degeneration similar to that found in mice carrying a mutant allele, presumably due to the overexpression of this protein. Our findings help to establish the pathogenicity of mutant human P23H rod opsin and suggest that overexpression of wild-type human rod opsin leads to a remarkably similar photoreceptor degeneration.

Recessive mutations in the CYP4V2 gene in East Asian and Middle Eastern patients with Bietti crystalline corneoretinal dystrophy.

BACKGROUND: Bietti crystalline corneoretinal dystrophy (BCD) is an autosomal recessively inherited disorder characterised by tiny yellowish glittering retinal crystals, choroidal sclerosis, and crystals in the peripheral cornea, associated with progressive night blindness. CYP4V2, encoding a member of cytochrome p450 (CYP450) protein family, was recently identified as the causative gene. METHODS: We recruited 11 unrelated patients with BCD and characteristic clinical features; eight of Japanese, two of Middle Eastern, and one of Chinese ancestry. Genomic DNA was extracted from peripheral blood leucocytes, and all 11 exons and the flanking intron splice sites of the CYP4V2 gene were amplified and sequenced. A complete ophthalmological examination was performed. RESULTS: We found recessive mutations in the CYP4V2 gene in all of the 11 patients. Two novel mutations, L173W and Q450X, were identified in a Japanese patient and two unrelated patients from Middle Eastern countries, respectively. Each patient was a homozygote. A previously reported mutation IVS6-8_810delinsGC was identified in seven unrelated Japanese patients and the Chinese patient with BCD. All patients with BCD shared a characteristic fundus appearance with numerous intraretinal crystal deposits and atrophy of the retinal pigment epithelium. However, the clinical findings, including elecroretinograph recordings, indicated that there was considerable variation in the degree of visual dysfunction even among patients of similar ages carrying the same mutation. CONCLUSIONS: Defects in CYP4V2 are the main cause of BCD. The IVS6-8_810delinsGC mutant allele may be especially prevalent among patients with BCD in East Asian countries, resulting from a single founder. Variation of disease severity suggests that environmental or additional genetic factors influence the course of the retinal disease.

A silencer element in the retinoblastoma tumor-suppressor gene.

We have previously identified two positive regulatory elements in the retinoblastoma gene (RB) promoter. One is an ATF site and the other we have called a retinoblastoma binding factor 1 site. In addition, a consensus E2F site is located directly 3' to the ATF site. Here we demonstrate with gel shift assays that E2F and E2F-dependent complexes can bind in vitro to the E2F site of the RB promoter (RB-E2F site). Moreover, we demonstrate that deletion of the RB-E2F site results in stimulation of RB promoter activity in transient transfection assays in several cell lines. Specific point mutations in the E2F site that inhibit the binding of E2F and its complexes also stimulate the RB promoter activity, suggesting that a factor(s) that can bind to the RB-E2F site could be a silencer. RB promoter activity was also stimulated two- to four-fold by mutation of the E2F site in the RB-negative cell lines, J82 and HTB9. Taken together, our results show that the E2F site in the promoter of the RB tumor-suppressor gene acts as a silencer element and that the silencer effect of the E2F site is not necessarily dependent on the presence of the RB protein.

Dominant Leber congenital amaurosis, cone-rod degeneration, and retinitis pigmentosa caused by mutant versions of the transcription factor CRX.

We summarize 18 mutations in the human CRX gene that have been associated with Leber congenital amaurosis (congenital retinal blindness), cone-rod degeneration, or retinitis pigmentosa. Except for one obviously null allele not definitely associated with a phenotype (a frameshift in codon 9), all CRX mutations appear to be completely penetrant and cause disease in heterozygotes. These dominant alleles fall into two categories. In one group are missense mutations and short, in-frame deletions; in the second group are frameshift mutations, all of which are in the last exon. All of these dominant mutations are likely to produce stable mRNA that is translated. Mutations in the missense group preferentially affect the conserved homeobox (codons 39-98), and all frameshift mutations leave the homeodomain intact but alter the OTX motif encoded by codons 284-295 at the carboxy terminus. We could not uncover any correlation between type of disease (congenital amaurosis vs. cone-rod degeneration or retinitis pigmentosa) and the type of mutation (missense vs. frameshift). Four of the 18 mutations (approximately 20%) were de novo mutations, and all of these were found in isolate cases of Leber congenital amaurosis. Dominant CRX mutations have not been associated with mental retardation or developmental delay that has sometimes been found in Leber congenital amaurosis caused by other genes. Implications regarding potential future therapies are discussed.

Novel frameshift mutations in CRX associated with Leber congenital amaurosis.

Mutations in CRX, a photoreceptor-specific transcription factor, can cause Leber congenital amaurosis (LCA), cone-rod dystrophy (CORD), and retinitis pigmentosa (RP), all of which feature severe visual impairment. Upon screening 55 patients with Leber congenital amaurosis, 75 patients with cone-rod dystrophy, 13 with cone dystrophy, and 36 with recessive or isolate RP for changes in the CRX sequence, we found two patients with Leber congenital amaurosis who carried heterozygously one of two novel frameshift mutations. The first mutation, Tyr191(1-bp del), was a de novo change and the second change, Pro263(1-bp del) was inherited from the proband's affected father. Both mutations are predicted to encode mutant versions of CRX with altered carboxy termini. We also found a previously reported missense mutation, Arg41Gln, heterozygously in a 47-year-old patient with a form of RP. The missense change Val242Met was found in an isolate case of CORD and no controls; however, its pathogenicity remains uncertain because only limited segregation analysis was possible. A nonpathogenic missense change, Ala158Thr, was found to be a variant present at relatively high frequency among African-Americans.

Structure and partial genomic sequence of the human retinoblastoma susceptibility gene.

This report describes the genomic organization of the human retinoblastoma susceptibility locus. This gene spans approximately 200 kb of DNA within human chromosome 13, band q14. The previously determined cDNA sequence comprises 27 exons, ranging in size from 31 bp to 1873 bp, and 26 introns, ranging in size from 80 bp to 70,500 bp. We have mapped the positions of the exons and the positions of the recognition sites for six restriction endonucleases. We also present the sequence of 9.2% of the locus (18,335 bp), including approximately 200 bp of intron sequence immediately flanking each exon. This map of a wild-type allele will form the foundation for future studies of mutant, oncogenic alleles at this locus.

Retrieval of human DNA from rodent-human genomic libraries by a recombination process.

Human Alu repeat ("BLUR") sequences have been cloned into the mini-plasmid vector piVX. The resulting piBLUR clones have been used to rescue selectively, by recombination, bacteriophage carrying human DNA sequences from genomic libraries constructed using DNA from rodent-human somatic cell hybrids. piBLUR clones are able to retrieve human clones from such libraries because at least one Alu family repeat is present on most 15 to 20 kb fragments of human DNA and because of the relative species-specificity of the sequences comprising the Alu family. The rapid, selective plaque purification achieved results in the construction of a collection of recombinant phage carrying diverse human DNA inserts from a specific subset of the human karyotype. Subfragments of two recombinants rescued from a mouse-human somatic cell hybrid containing human chromosomes X, 10, 13, and 22 were mapped to human chromosomes X and 13, respectively, demonstrating the utility of this protocol for the isolation of human chromosome-specific DNA sequences from appropriate somatic cell hybrids.