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.

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.

Melanoma-associated retinopathy and recurrent exudative retinal detachments in a patient with choroidal melanoma

PURPOSE

To report a patient who presented with photopsias, night blindness, exudative retinal detachments, and melanoma-associated retinopathy in her right eye 23 years after the left eye was enucleated for a choroidal melanoma.

METHODS

Assessment of fundus findings, fluorescein angiograms, and electroretinograms.

RESULTS

The patient had recurrent exudative detachments of the macula in her right eye and electroretinogram responses consistent with the diagnosis of melanoma-associated retinopathy. The abdominal computed tomography (CT) scan was negative, but 13 months later, CT scanning revealed many masses in her liver. Fine-needle biopsy confirmed the diagnosis of metastatic melanoma.

CONCLUSION

To our knowledge, this is the first report of melanoma-associated retinopathy in a patient with a previous choroidal melanoma.

Mutations in ABCR (ABCA4) in patients with Stargardt macular degeneration or cone-rod degeneration

PURPOSE

To determine the spectrum of ABCR mutations associated with Stargardt macular degeneration and cone-rod degeneration (CRD).

METHODS

One hundred eighteen unrelated patients with recessive Stargardt macular degeneration and eight with recessive CRD were screened for mutations in ABCR (ABCA4) by single-strand conformation polymorphism analysis. Variants were characterized by direct genomic sequencing. Segregation analysis was performed on the families of 20 patients in whom at least two or more likely pathogenic sequence changes were identified.

RESULTS

The authors found 77 sequence changes likely to be pathogenic: 21 null mutations (15 novel), 55 missense changes (26 novel), and one deletion of a consensus glycosylation site (also novel). Fifty-two patients with Stargardt macular degeneration (44% of those screened) and five with CRD each had two of these sequence changes or were homozygous for one of them. Segregation analyses in the families of 19 of these patients were informative and revealed that the index cases and all available affected siblings were compound heterozygotes or homozygotes. The authors found one instance of an apparently de novo mutation, Ile824Thr, in a patient. Thirty-seven (31%) of the 118 patients with Stargardt disease and one with CRD had only one likely pathogenic sequence change. Twenty-nine patients with Stargardt disease (25%) and two with CRD had no identified sequence changes.

CONCLUSIONS

This report of 42 novel mutations brings the growing number of identified likely pathogenic sequence changes in ABCR to approximately 250.

Clinical features and mutations in patients with dominant retinitis pigmentosa-1 (RP1)

PURPOSE

To survey patients with dominant retinitis pigmentosa (RP) for mutations in the RP1 gene to determine the spectrum of dominant mutations in this gene, to estimate the proportion of dominant RP caused by this gene, and to determine whether the clinical features of patients with RP1 mutations differ from features of those with rhodopsin mutations.

METHODS

A set of 241 patients who did not have mutations in the rhodopsin gene (based on previous work) formed the basis for the study. Of these patients, 117 had also been previously evaluated and were found not to carry mutations in the RDS gene. The single-strand conformation polymorphism (SSCP) method was used to search for sequence variants, which were then directly sequenced. The relatives of selected patients were recruited for segregation analyses. Clinical evaluations of patients included a measurement of Snellen visual acuity, final dark adaptation thresholds, visual fields, and ERGs. Clinical data were compared with those obtained earlier from a study of 128 patients with dominant rhodopsin mutations.

RESULTS

Of the 241 patients, all were screened for the most common RP1 mutation (Arg677Ter), and 10 patients were found to have this mutation. In addition, an evaluation of a subset of 189 patients in whom the entire coding sequence was evaluated revealed the following mutations: Gln679Ter (1 case), Gly723Ter (2 cases), Glu729(1-bp del) (1 case), Leu762(5-bp del) (2 cases), and Asn763(4-bp del) (1 case). All of these mutations cosegregated with RP in the families of the index patients. Nine missense mutations that were each found in six or fewer patients were encountered. The segregation of eight of these was evaluated in the respective patients' families, and only one segregated with dominant RP. This cosegregating missense change was in cis with the nonsense mutation Gln679Ter. Although patients with RP1 mutations had, on average, slightly better visual acuity than patients with rhodopsin mutations, there was no statistically significant difference in final dark-adaptation thresholds, visual field diameters, or cone electroretinogram (ERG) amplitudes. Comparably aged patients with RP1 mutations had visual function that varied by approximately two orders of magnitude, based on visual fields and ERG amplitudes.

CONCLUSIONS

Dominant RP1 alleles typically have premature nonsense codons occurring in the last exon of the gene and would be expected to encode mutant proteins that are only approximately one third the size of the wild-type protein, suggesting that a dominant negative effect rather than haploinsufficiency is the mechanism leading to RP caused by RP1 mutations. On average, patients with RP1 mutations have slightly better visual acuity than patients with dominant rhodopsin mutations; otherwise, they have similarly severe disease. The wide range in severity among patients with RP1 mutations indicates that other genetic or environmental factors modulate the effect of the primary mutation.

Null RPGRIP1 alleles in patients with Leber congenital amaurosis

We isolated and characterized the entire coding sequence of a human gene encoding a protein that interacts with RPGR, a protein that is absent or mutant in many cases of X-linked retinitis pigmentosa. The newly identified gene, called "RPGRIP1" for RPGR-interacting protein (MIM 605446), is located within 14q11, and it encodes a protein predicted to contain 1,259 amino acids. Previously published work showed that both proteins, RPGR and RPGRIP1, are present in the ciliary structure that connects the inner and outer segments of rod and cone photoreceptors. We surveyed 57 unrelated patients who had Leber congenital amaurosis for mutations in RPGRIP1 and found recessive mutations involving both RPGRIP1 alleles in 3 (6%) patients. The mutations all create premature termination codons and are likely to be null alleles. Patients with RPGRIP1 mutations have a degeneration of both rod and cone photoreceptors, and, early in life, they experience a severe loss of central acuity, which leads to nystagmus.

Nutrition and retinal degenerations

Considerable progress has been made in the understanding and management of degenerative diseases of the retina involving photoreceptors. Nutritional approaches to treatment have proved successful in the case of the common forms of retinitis pigmentosa (supplementation with vitamin A), Bassen-Kornzweig disease (supplementation with vitamins A, E, and K), gyrate atrophy (low-protein, low-arginine diet and/or supplementation with vitamin B6), and Refsum disease (low-phytol, low-phytanic acid diet). The night blindness associated with Sorsby fundus dystrophy can be reversed over the short term with vitamin A. A significant trend for decreased risk for advanced or exudative ARMD has been reported among those whose diets contain a higher content of carotenoids, such as spinach and collard greens. A randomized trial is in progress to determine whether beta-carotene, vitamin C, and vitamin E as well as trace minerals, particularly zinc, will modify the course of ARMD. The difficulties that patients with retinal degenerations face as a result of their diminishing vision, sometimes over decades, cannot be underestimated. Nutritional therapy has proved effective in modifying the course of a number of these conditions; the therapeutic benefit of nutritional modification in diseases that have a genetic basis is of particular interest. Further research is warranted to determine the mechanisms by which these treatments provide their benefit as well as to identify other conditions that may yield to nutritional intervention. Risk-factor analyses of well-defined populations followed over time with food-frequency questionnaires in conjunction with careful assessments of visual function may reveal other dietary constituents that can modify the course of degenerative diseases of the retina.

Treatable forms of retinitis pigmentosa associated with systemic neurological disorders

In this chapter; we have described the role of nutritional supplements or selective dietary restriction (or both) on the maintenance and function of the retina and nervous system in some diseases. Oral vitamin A therapy has proven to be effective in the treatment of the common forms of retinitis pigmentosa. Bassen-Kornzweig disease can be treated with vitamin A and vitamin E and, in some cases, with vitamin K. Vitamin E therapy for Friedreich-like ataxia associated with retinitis pigmentosa has been shown to be effective in the short term. Classic Refsum's disease responds to a low phytol-low phytanic acid diet. Undoubtedly, future research will bring more insight into the biochemical pathways responsible for other diseases and, it is hoped, aid in developing treatments for additional retinal degenerations associated with systemic neurological disease.

Two families from New England with usher syndrome type IC with distinct haplotypes

PURPOSE

To search for patients with Usher syndrome type IC among those with Usher syndrome type I who reside in New England.

METHODS

Genotype analysis of microsatellite markers closely linked to the USH1C locus was done using the polymerase chain reaction. We compared the haplotype of our patients who were homozygous in the USH1C region with the haplotypes found in previously reported USH1C Acadian families who reside in southwestern Louisiana and from a single family residing in Lebanon.

RESULTS

Of 46 unrelated cases of Usher syndrome type I residing in New England, two were homozygous at genetic markers in the USH1C region. Of these, one carried the Acadian USH1C haplotype and had Acadian ancestors (that is, from Nova Scotia) who did not participate in the 1755 migration of Acadians to Louisiana. The second family had a haplotype that proved to be the same as that of a family with USH1C residing in Lebanon. Each of the two families had haplotypes distinct from the other.

CONCLUSION

This is the first report that some patients residing in New England have Usher syndrome type IC. Patients with Usher syndrome type IC can have the Acadian haplotype or the Lebanese haplotype compatible with the idea that at least two independently arising pathogenic mutations have occurred in the yet-to-be identified USH1C gene.

Retinal histopathology of an autopsy eye with advanced retinitis pigmentosa in a family with rhodopsin Glu181Lys

PURPOSE

To compare histologic findings in an autopsy eye of an 84-year-old man with advanced retinitis pigmentosa and rhodopsin, Glu181Lys, with two cases of autosomal dominant retinitis pigmentosa (one with rhodopsin, Pro23His, and one with rhodopsin, Cys110Arg) and with a normal control, all of comparable age.

METHODS

All eyes were prepared for light and electron microscopy within 6 hours after death.

RESULTS

Extensive photoreceptor degeneration was revealed in the eyes with retinitis pigmentosa. Some macular cones showed membranous swirls only in the eye with rhodopsin, Glu181Lys.

CONCLUSION

The retinal degeneration caused by rhodopsin, Glu181Lys, can feature membranous swirls in the inner segments of cones in the macula. These swirls have not been reported in other cases of dominant retinitis pigmentosa studied so far, and their pathogenesis remains to be defined.

Novel rhodopsin mutations Gly114Val and Gln184Pro in dominant retinitis pigmentosa

PURPOSE

To identify mutations in the rhodopsin gene in North American patients with autosomal dominant retinitis pigmentosa (ADRP) and to measure the proportion of cases with rhodopsin mutations.

METHODS

Single-strand conformation polymorphism (SSCP) analysis and direct genomic sequencing were used to evaluate the coding region and intron splice sites of the rhodopsin gene for mutations in 91 unrelated patients.

RESULTS

Nineteen patients heterozygously carried a missense change in the rhodopsin gene (six with Pro23His, two with Pro347Leu, and one each with Thr17Met, Phe45Leu, Gly51Arg, Gly89Asp, Gly114Val, Arg135Trp, Pro171Leu, Gln184Pro, Phe220Leu, Ser297Arg, and Pro347Thr). All these missense changes were previously reported as causes for ADRP except for Gly114Val, Gln184Pro, and Phe220Leu, which were evaluated further by examining the relatives of index patients. The Gly114Val and Gln184Pro alleles cosegregated with ADRP as expected if they were pathogenic. Phe220Leu did not, indicating that it is not a cause of ADRP.

CONCLUSIONS

Summation of the results of cases in this study with those of 272 unrelated cases of ADRP previously evaluated by our group shows that 90 of 363 (25%) of cases were caused by rhodopsin mutations.

Evaluation of the myosin VIIA gene and visual function in patients with Usher syndrome type I

Usher syndrome type I (USH1) is a recessively-inherited disorder consisting of retinitis pigmentosa, profound congenital deafness, and vestibular ataxia. It can be caused by mutations in at least six different loci (USH1A-1F). The gene encoding human myosin VIIA (MYO7A) is the USH1B locus. In this study, 66 unrelated patients with USH1 were evaluated for defects in MYO7A using single-strand conformation polymorphism analysis and direct genomic sequencing. Twenty-nine per cent of cases were found to have likely pathogenic MYO7A mutations. A total of 22 likely pathogenic changes were identified, 18 of which were novel. Cosegregation analysis of mutations in five available families showed that the MYO7A changes segregated with the disease in an autosomal recessive fashion. Average visual function as measured by visual acuity, visual field area, and ERG amplitude was not significantly different between the group of patients with likely pathogenic MYO7A changes and the group in which no likely pathogenic MYO7A changes were detected.

X-linked retinitis pigmentosa: mutation spectrum of the RPGR and RP2 genes and correlation with visual function

PURPOSE

To assess the frequency of RPGR and RP2 mutations in a set of 85 patients with X-linked retinitis pigmentosa (XLRP) and to compare the visual function of patients with mutations in RPGR versus RP2.

METHODS

Eighty-five unrelated patients with XLRP were ascertained, mainly from North America. The single-strand conformation polymorphism (SSCP) and a direct sequencing technique were used to screen their DNA for mutations in the coding region and splice sites of RPGR and RP2. The Snellen visual acuities, visual field areas, and 0.5-Hz and 30-Hz electroretinograms (ERGs) were measured in male patients. The visual function parameters were compared using multiple regression analysis.

RESULTS

A wide spectrum of mutations was found in both genes, including missense, nonsense, splice-site, and frameshift mutations. Twenty putative pathogenic mutations in RPGR, 15 of which were novel, were found in 22 patients (26%), whereas 6 mutations in RP2, 4 of which were novel, were found in 6 patients (7%). A high fraction of the mutations in both genes affected amino acid residues within or adjacent to presumed functional domains. Comparison of visual function between comparably aged patients with mutations in RPGR versus RP2 showed that, on average, patients with RPGR mutations have lower ERG amplitudes and smaller visual field areas.

CONCLUSIONS

Mutations in RPGR and RP2 genes together account for approximately 33% of cases of XLRP in North America. Patients with RPGR mutations have less overall retinal function on average than those with RP2 mutations, on the basis of measurements of visual field areas and full-field ERG amplitudes.

Missense mutation in the USH2A gene: association with recessive retinitis pigmentosa without hearing loss

Microdeletions Glu767(1-bp del), Thr967(1-bp del), and Leu1446(2-bp del) in the human USH2A gene have been reported to cause Usher syndrome type II, a disorder characterized by retinitis pigmentosa (RP) and mild-to-severe hearing loss. Each of these three frameshift mutations is predicted to lead to an unstable mRNA transcript that, if translated, would result in a truncated protein lacking the carboxy terminus. Here, we report Cys759Phe, a novel missense mutation in this gene that changes an amino-acid residue within the fifth laminin-epidermal growth factor-like domain of the USH2A gene and that is associated with recessive RP without hearing loss. This single mutation was found in 4.5% of 224 patients with recessive RP, suggesting that USH2A could cause more cases of nonsyndromic recessive RP than does any other gene identified to date.

A retinitis pigmentosa GTPase regulator (RPGR)- deficient mouse model for X-linked retinitis pigmentosa (RP3)

The X-linked RP3 locus codes for retinitis pigmentosa GTPase regulator (RPGR), a protein of unknown function with sequence homology to the guanine nucleotide exchange factor for Ran GTPase. We created an RPGR-deficient murine model by gene knockout. In the mutant mice, cone photoreceptors exhibit ectopic localization of cone opsins in the cell body and synapses and rod photoreceptors have a reduced level of rhodopsin. Subsequently, both cone and rod photoreceptors degenerate. RPGR was found normally localized to the connecting cilia of rod and cone photoreceptors. These data point to a role for RPGR in maintaining the polarized protein distribution across the connecting cilium by facilitating directional transport or restricting redistribution. The function of RPGR is essential for the long-term maintenance of photoreceptor viability.

Acuity recovery and cone pigment regeneration after a bleach in patients with retinitis pigmentosa and rhodopsin mutations

PURPOSE

To assess visual acuity recovery times and cone photopigment regeneration kinetics after a bleach in the fovea of patients with dominant retinitis pigmentosa due to rhodopsin mutations.

METHODS

The authors measured acuity recovery times by computerized photostress testing in 13 patients with dominant retinitis pigmentosa and one of eight rhodopsin mutations. The authors also measured their time constants of cone photopigment regeneration with a video imaging fundus reflectometer to determine whether acuity recovery time depended on pigment regeneration kinetics. These values were compared with those of normal subjects, by the Mann-Whitney U test. The relationship between acuity recovery time and the time constant of cone photopigment regeneration among the patients was quantified by the Spearman rank correlation.

RESULTS

The visual acuity recovery times, which averaged 22.0 seconds for the patients with retinitis pigmentosa and 11.2 seconds for the normal subjects, were significantly slower for the patient group (P < 0.001). The time constants of cone pigment regeneration, which averaged 172 seconds for the patients with retinitis pigmentosa and 118 seconds for the normal subjects, also were significantly slower for the patient group (P = 0.043). The authors also found a significant, positive correlation between the visual acuity recovery time and the time constant of pigment regeneration for the patients with retinitis pigmentosa (r = 0.65, P = 0.017).

CONCLUSIONS

A slowing of foveal visual acuity recovery and cone pigment regeneration, which are related to each other, can occur in patients with retinitis pigmentosa, due to a rod-specific gene defect.

Frequency of mutations in the gene encoding the alpha subunit of rod cGMP-phosphodiesterase in autosomal recessive retinitis pigmentosa

PURPOSE

To determine the mutation spectrum of the PDE6A gene encoding the alpha subunit of rod cyclic guanosine monophosphate (cGMP)phosphodiesterase and the proportion of patients with recessive retinitis pigmentosa (RP) due to mutations in this gene.

METHODS

The single-strand conformation polymorphism (SSCP) technique and a direct genomic sequencing technique were used to screen all 22 exons of this gene for mutations in 164 unrelated patients with recessive or isolate RP. Variant DNA fragments revealed by SSCP analysis were subsequently sequenced. Selected alleles that altered the coding region or intron splice sites were evaluated further through segregation analysis in the families of the index cases.

RESULTS

Four new families were identified with five novel mutations in this gene that cosegregated with disease. Combining the data presented here with those published earlier by the authors, eight different mutations in six families have been discovered to be pathogenic. Two of the mutations are nonsense, five are missense, and one affects a canonical splice-donor site.

CONCLUSIONS

The PDE6A gene appears to account for roughly 3% to 4% of families with recessive RP in North America. A compilation of the pathogenic mutations in PDE6A and those reported in the homologous gene PDE6B encoding the beta subunit of rod cGMP-phosphodiesterase shows that the cGMP-binding and catalytic domains are frequently affected.

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.

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.

Recessive mutations in the RLBP1 gene encoding cellular retinaldehyde-binding protein in a form of retinitis punctata albescens

PURPOSE

To determine the frequency and spectrum of mutations in the RLBP1 gene encoding cellular retinaldehyde-binding protein (CRALBP) in patients with hereditary retinal degeneration.

METHODS

The single-strand conformation polymorphism (SSCP) technique and a direct genomic sequencing technique were used to screen the coding exons of this gene (exons 2-8) for mutations in 324 unrelated patients with recessive or isolate retinitis pigmentosa, retinitis punctata albescens, Leber congenital amaurosis, or a related disease. Variant DNA fragments revealed by SSCP analysis were subsequently sequenced. Selected alleles that altered the coding region or intron splice sites were evaluated further through segregation analysis in the families of the index cases.

RESULTS

Four novel mutations were identified in this gene among three unrelated patients with recessively inherited retinitis punctata albescens. Two of the mutations were missense: one was a frameshift, and one affected a canonical splice donor site.

CONCLUSIONS

Recessive mutations in the RLBP1 gene are an uncommon cause of retinal degeneration in humans. The phenotype produced by RLBP1 mutations seems to be a form of retinitis punctata albescens.

Safety of <7500 RE (<25000 IU) vitamin A daily in adults with retinitis pigmentosa

BACKGROUND

Vitamin A supplementation is being used successfully to treat some forms of cancer and the degenerative eye disease retinitis pigmentosa. The daily biological need for vitamin A is estimated to be 800 retinol equivalents (RE)/d (2667 IU/d) for adult women and 1000 RE/d (3300 IU/d) for adult men; doses > or = 7500 RE (> or = 25000 IU)/d are considered potentially toxic over the long term.

OBJECTIVE

We assessed the safety in adults of long-term vitamin A supplementation with doses above the daily biological need but <7500 RE (<25000 IU)/d.

DESIGN

Adults aged 18-54 y with retinitis pigmentosa but in generally good health (n = 146) were supplemented with 4500 RE (15000 IU) vitamin A/d for < or = 12 y (group A) and compared with a similar group (n = 149) that received 23 RE (75 IU)/d (trace group). Mean total consumption of vitamin A in group A was 5583 RE (18609 IU)/d (range: 4911-7296 RE/d, or 16369-24318 IU/d) and that in the trace group was 1053 RE (3511 IU)/d (range: 401-3192 RE/d, or 1338-10638 IU/d).

RESULTS

Patients in group A showed an 8% increase in mean serum retinol concentration at 5 y and an 18% increase at 12 y (P < 0.001); no retinol value exceeded the upper normal limit (3.49 micromol/L, or 100 microg/dL). Mean serum retinyl esters were elevated approximately 1.7-fold at 5 y and remained relatively stable thereafter. No clinical symptoms or signs of liver toxicity attributable to vitamin A excess were detected.

CONCLUSIONS

Prolonged daily consumption of <7500 RE (<25000 IU) vitamin A/d can be considered safe in this age group.

Photoreceptor rosettes in autosomal dominant retinitis pigmentosa with reduced penetrance

We performed histopathologic and immunofluorescence studies of autopsy eyes from a 73-year-old woman with autosomal dominant retinitis pigmentosa from a family with reduced penetrance. Light microscopic examination showed extensive photoreceptor loss in most regions. In the temporal midperiphery of the retina, there were patches of remaining photoreceptors, some arranged in rosettes. Electron microscopic examination showed that these rosettes were composed mostly of rods, with a few cone-like inner segments. The malformed photoreceptor elements in the rosette lumens stained positively with anti-rhodopsin, but not with anti-red- and green-cone opsin or anti-blue-cone opsin. To our knowledge, this is the first report of photoreceptor rosettes containing rod photoreceptors in a case of retinitis pigmentosa. Future studies of additional patients will be needed to determine if the rod-abundant rosettes seen in our patient are a characteristic finding of autosomal dominant retinitis pigmentosa with reduced penetrance.

Effect of vitamin A supplementation on rhodopsin mutants threonine-17 --> methionine and proline-347 --> serine in transgenic mice and in cell cultures

A therapeutic effect of vitamin A supplementation on the course of photoreceptor degeneration, previously reported for patients with retinitis pigmentosa, was tested in two transgenic mouse models of this disease, each carrying a dominant rhodopsin mutation. The threonine-17 --> methionine (T17M) mutation is a class II rhodopsin mutation, characterized by a thermal instability/folding defect and minimal regeneration with the chromophore. The proline-347 --> serine (P347S) mutation belongs to class I, comprised of a smaller number of mutations that exhibit no recognized biochemical abnormality in vitro. In the present study, each of the two mouse models was fed a diet containing 2.5 mg of vitamin A palmitate (control) or 102.5 mg of vitamin A palmitate (high vitamin A) per kilogram of diet. Dark-adapted, full-field electroretinograms showed that the high vitamin A diet significantly reduced the rate of decline of a-wave and b-wave amplitudes in the T17M mice but had no significant effect on the decline of electroretinogram amplitude in the P347S mice. Correspondingly, histologic evaluation revealed that the treatment was associated with significantly longer photoreceptor inner and outer segments and a thicker outer nuclear layer in the T17M mice but had no effect on photoreceptor morphology in the P347S mice. In a separate series of experiments, the instability defect of the T17M mutant opsin expressed in vitro was partially alleviated by inclusion of 11-cis-retinal in the culture media. These results show that vitamin A supplementation slows the rate of photoreceptor degeneration caused by a class II rhodopsin mutation. Vitamin A supplementation may confer therapeutic benefit by stabilizing mutant opsins through increased availability of the chromophore.

Histopathologic and immunohistochemical study of dominant cone degeneration

PURPOSE

To report the histopathologic and immunohistochemical findings in autosomal dominant cone degeneration.

METHODS

The autopsy eyes of a 75-year-old man with autosomal dominant cone degeneration were studied with both light and electron microscopy. In addition, immunofluorescent studies using antibodies to cone opsins and cone alpha transducin were performed.

RESULTS

Histopathologic examination of the fovea disclosed loss of photoreceptors and attenuated retinal pigment epithelium. Reduced numbers of cones could be seen in the parafovea, and only occasional cones were visible in the periphery; rods were preserved in the periphery.

CONCLUSION

The histopathologic and immunohistochemical findings in this case of dominantly inherited cone degeneration correlate well with the loss of cone function and preservation of normal or nearly normal rod function upon clinical examination. This condition affects red, green, and blue cones.

Rod and cone function in the Nougaret form of stationary night blindness

BACKGROUND

Recently, a mutation (Gly38Asp) was identified in the alpha subunit of rod transducin in members of the Nougaret pedigree affected with dominantly inherited stationary night blindness.

OBJECTIVE

To evaluate retinal function in patients with the Gly38Asp gene defect.

DESIGN

Ocular examinations, including specialized measures of rod and cone function.

SETTING

A clinical research facility in Boston, Mass.

PATIENTS

A father (aged 48 years) and son (aged 25 years) with the Gly38Asp mutation.

MAIN OUTCOME MEASURES

Psychophysical thresholds to white and narrowband lights and full-field electroretinographic (ERG) responses.

RESULTS

Both patients showed dark-adapted thresholds to white light that were elevated approximately 2 log-units across the retina. Spectral sensitivity testing revealed thresholds that seemed to be governed mostly by rods. Although both patients' dark-adapted ERG responses to a dim blue flash were nondetectable, their dark-adapted ERGs to a white flash showed an a-wave with cone and rod components and a b-wave amplitude larger than what could have been generated by cone function alone. Rod ERGs to bright blue flashes had subnormal, but detectable, amplitudes that seemed to result from a profound reduction in sensitivity. The patients also showed loss of a cone subcomponent in the dark-adapted response to a red flash. The abnormal dark-adapted ERG responses of the patients could be simulated in the ERG responses of normal subjects tested with blue, white, and red flashes presented in the presence of a mesopic background.

CONCLUSIONS

Although the Nougaret form of stationary night blindness has been cited as a prototype of absent rod function with normal cone function, our findings, based on the genealogically and genotypically documented descendants of Jean Nougaret, show that rod function is present, although subnormal, and that there is slight impairment of cone function. The data also suggest that these abnormalities can be simulated by light-adapting the normal retina, compatible with the proposal that the rod transducin encoded by the mutant gene is constitutively active and that the night blindness results from partial desensitization of rods caused by the constitutive activity.

Mutations in the RPE65 gene in patients with autosomal recessive retinitis pigmentosa or leber congenital amaurosis

RPE65 is a protein of unknown function expressed specifically by the retinal pigment epithelium. We examined all 14 exons of this gene in 147 unrelated patients with autosomal recessive retinitis pigmentosa (RP), in 15 patients with isolate RP, and in 45 patients with Leber congenital amaurosis (LCA). Sequence anomalies that were likely to be pathogenic were found in two patients with recessive RP, in one patient with isolate RP recategorized as recessive, and in seven patients with LCA. Cosegregation analysis in each available family showed that all affected individuals were either homozygotes or compound heterozygotes and that all unaffected individuals were either heterozygote carriers or homozygous wild type. In one family, there was one instance of a new mutation not present in either parent of the affected individual. In another family, affected members with recessive RP in three branches (i.e., three distinct pairs of parents) were compound heterozygotes for the same two mutations or homozygous for one of them. Based on our results, mutations in the RPE65 gene appear to account for approximately 2% of cases of recessive RP and approximately 16% of cases of LCA.

Evaluation of the human arrestin gene in patients with retinitis pigmentosa and stationary night blindness

PURPOSE

To establish the DNA sequence of the coding regions of the human arrestin locus and to determine whether defects in this sequence are present among patients with retinitis pigmentosa (RP) or types of stationary night blindness in addition to Oguchi disease.

METHODS

The human genomic locus encoding arrestin was cloned in bacteriophage and P1 vectors. The sequence of the intron DNA flanking each exon was determined from these clones. Single-strand conformation polymorphism analysis and direct genomic sequencing techniques were used to screen 272 unrelated patients, comprising 177 patients with autosomal dominant RP, 85 with recessive RP, and 10 with stationary night blindness.

RESULTS

The arrestin gene is divided into 16 exons ranging in size from 10 bp to 194 bp, with the open reading frame spanning exons 2 through 16. The authors identified several discrepancies between the genomic sequence the authors obtained and the previously published cDNA and genomic sequences. In the set of patients with dominant RP, the authors found one of three heterozygous missense changes (Arg84Cys, Thr125Met, and Val378Ile) in each of four unrelated patients; none of these changes cosegregated with disease in the respective families. In the set of patients with recessive RP, the authors found one of two heterozygous missense changes in each of two unrelated patients with recessive RP (Pro364Leu and Arg384Cys). One of the patients was the offspring of a consanguineous marriage; because the Arg384Cys change in him was heterozygous, it is unlikely to have been the cause of his RP. Cosegregation studies could not be performed on the patient with the Pro364Leu change. The authors confirmed the existence of two previously described polymorphisms (Ile76Val and a multiallelic polymorphism at codon 403), and the authors identified several silent polymorphisms and rare sequence variants. No sequence changes, other than polymorphic changes also found in some patients with RP, were identified in the patients with stationary night blindness.

CONCLUSIONS

We found no evidence that mutations in arrestin are a cause of RP or stationary night blindness other than Oguchi disease. According to the genomic sequence obtained, a region in exon 8 that has been postulated to represent the site of interaction between arrestin and rhodopsin is 100% conserved between humans and all other mammals studied to date.

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.

Histopathologic and immunohistochemical study of an autopsy eye with X-linked cone degeneration

We performed a histopathologic and immunohistochemical study of eyes obtained at autopsy of an 84-year-old man from a family with X-linked cone degeneration in which affected members have a 6.5-kilobase deletion in the red cone pigment gene. At his most recent ocular examination, at age 71 years, this patient had had a visual acuity of 20/200 OU, fundus changes suggestive of macular degeneration, borderline-normal full-field rod electroretinograms, and profoundly reduced full-field cone electroretinograms. Histopathologic examination demonstrated marked loss of cone and rod photoreceptors and the retinal pigment epithelium in the central macula. The peripheral cone population was reduced, while the peripheral rod population was relatively preserved. Immunohistochemical examination with an antibody to both red and green cone opsin and an antibody to blue cone opsin disclosed a prominent loss of the red and green cone population and preservation of the blue cone population. These findings show that a defect in the red cone pigment gene can result in extensive degeneration of the red and green cone population across the retina.

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.

Dominant and digenic mutations in the peripherin/RDS and ROM1 genes in retinitis pigmentosa

PURPOSE

To measure the proportion of cases of retinitis pigmentosa (RP) caused by mutations in the peripherin/RDS (RDS) and ROM1 genes.

METHODS

The single-strand conformation polymorphism (SSCP) method was used to analyze 227 unrelated patients with dominant or recessive RP for mutations in the RDS gene and an overlapping set of 315 unrelated patients for mutations in the ROM1 gene (excluding patients with other known RP genes). Variant bands revealed by SSCP were studied further by polymerase chain reaction-based, direct genomic sequencing and, where possible, by cosegregation analysis in the families of the index cases.

RESULTS

Four index patients were found to have RP as a result of one of four dominant mutations in the RDS gene, two of which are novel. Four other index patients were found to have digenic RP as a result of the combination of heterozygous mutations in both the RDS and the ROM1 gene, with one of the ROM1 mutations being novel. The digenic cases all had the same RDS mutation (the missense change Leu185Pro), but each had one of three different ROM1 mutations. The authors were unable to determine through cosegregation analysis whether three other changes encountered in the RDS gene and five in the ROM1 gene were pathogenic.

CONCLUSIONS

The authors found mutations in the RDS gene as a cause of dominant or digenic RP and mutations in the ROM1 gene as a cause of digenic RP. No cases of RP caused by ROM1 mutations alone have been discovered thus far. Mutations in the RDS and ROM1 genes are infrequent causes of RP, together accounting for only a few percent of patients in the United States and Canada.

Evaluation of the rhodopsin kinase gene in patients with retinitis pigmentosa

We explored the possibility that defects in the rhodopsin kinase gene might cause retinitis pigmentosa (RP) by evaluating 160 unrelated cases with dominant RP and 151 unrelated cases with recessive RP. One of five missense changes was discovered in each of six cases of dominant RP, but none of the missense changes cosegregated with disease among relatives. Heterozygous missense changes were found in two cases of recessive RP, and a heterozygous frameshift mutation was found in one additional case of recessive RP. Although the same DNA sequence alterations could be found heterozygously in the only affected sibling of each index case of recessive RP, no defect could be found in the other allele. Hence, none of the changes found in the cases of dominant or recessive RP was proven to be a cause of RP. The data indicate that defects in the rhodopsin kinase gene causing RP are either rare or nonexistent.

ERG abnormalities in relation to histopathologic findings in vitiligo mutant mice

The vitiligo, mivit, mutation has several prenatal and perinatal effects on development of the retinal pigment epithelium, and later, leads to extensive, progressive degeneration of photoreceptor cells in the neural retina of homozygous affected mice. The aim of the present study was to determine by functional criteria how early can abnormalities be detected in the neural retina. Electroretinograms (ERGs) were correlated with histopathological findings in the same animals. Congenic homozygous mutants, heterozygotes, and homozygous wild-type mice were studied at 2, 3, 6, 24 and 56 weeks of age, the same animals being tested serially at the three older time points. The nontested eye of each animal was embedded in Epon and sectioned at 1 micron for light microscopic study. ERG recordings from vitiligo homozygotes differed from heterozygous and wild-type mice, but the latter two groups did not differ from each other. As early as two weeks of age, homozygous mutants showed a significant reduction of rod dominated maximum ERG a-wave and b-wave amplitude. ERG b-wave sensitivity (sigma) was significantly reduced, and ERG implicit times were delayed for homozygous mutants at 3 (a-wave) and 6 (b-wave) weeks of age. This is the first study to report reduced and delayed ERG a-waves and b-waves in this animal model, like the early functional abnormalities in human retinitis pigmentosa, and also the first to show short and disoriented rod outer segments, beginning retinal separation from the pigment epithelium, and a few macrophage-like cells already present in the subretinal space at 2 weeks of age (in three of four homozygous mutant eyes examined). Given these early functional and structural abnormalities in the neural retina, it remains to be determined whether the mi gene targets the retinal pigment epithelial cell, the photoreceptor cell, or both.

Evaluation of the human gene encoding recoverin in patients with retinitis pigmentosa or an allied disease

PURPOSE

To determine whether defects in the human recoverin gene cause retinitis pigmentosa (RP) or an allied disease such as Usher syndrome, Leber congenital amaurosis, or the Bardet-Biedl syndrome.

METHODS

Single-strand conformation polymorphism analysis and direct genomic sequencing techniques were used to screen 596 unrelated patients, comprising 167 patients with dominant RP, 168 with recessive RP, and 261 with an allied disease.

RESULTS

Four sequence variants were discovered. The first was a missense change (Ala200Thr) found in one family with autosomal dominant RP and in one family with autosomal recessive RP; it did not segregate with disease. The second was a silent, single-base variation affecting codon Ser24 with a minor allele frequency of approximately 0.5%. The third was a silent, single-base variation affecting codon Va1122. The fourth was a single-nucleotide substitution in intron 2, 11 bp upstream of exon 3.

CONCLUSIONS

The authors found no evidence that mutations in the recoverin gene are a cause of RP or another of the hereditary retinal diseases studied. The human phenotype associated with mutations of the recoverin gene remains unknown.

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.

Detection of cone alpha transducin mRNA in human fetal cochlea: negative mutation analysis in Usher syndrome

Cone alpha transducin (GNAT2), known to be expressed in photoreceptors, was found to be transcribed in human fetal cochlea. Due to the unexpected finding of expression of this gene in the inner ear and the success of the candidate gene approach in identifying mutations for a variety of heritable disorders, we investigated the possible role of this gene in Usher syndrome type I and type II. Single-strand conformation polymorphism (SSCP) was used to screen the GNAT2 coding region, as well as splice donor and acceptor sites, for mutations in a total of 140 unrelated patients. Two nucleotide changes leading to two silent amino acid changes and one rare polymorphism were found. In view of these results and those of a previously published Southern blot analysis, it is unlikely that mutations in GNAT2 are a common gene abnormality in Usher syndrome type I or type II.

Clinical and histopathologic findings in clumped pigmentary retinal degeneration

OBJECTIVE

To describe the clinical and histopathologic features of clumped pigmentary retinal degeneration (CPRD).

DESIGN

Retrospective case series.

SETTING

Tertiary referral center.

PATIENTS

Twenty-four patients, aged 7 to 83 years, were identified from the medical record filed of the Berman-Gund Laboratory, Boston, Mass, as having the clinical features of CPRD. The autopsy eye from a 56-year-old man with CPRD was studied with light and electron microscopy.

MAIN OUTCOME MEASURES

Visual acuities, visual fields, dark-adaptation thresholds, and results of electroretinograms; histopathologic study of an autopsy eye.

RESULTS

The functional deficit of patients with CPRD seems to be similar to that of patients with typical retinitis pigmentosa. Different degrees of severity were observed among patients of similar age. The histopathologic data showed that the clinically distinct areas of clumped pigment are due to excessive accumulation of melanin granules in retinal pigment epithelial cells.

CONCLUSION

Based on the distinct clinical and histopathologic appearance, CPRD should be considered as a separate form of retinal degeneration.

The relationship between visual field size and electroretinogram amplitude in retinitis pigmentosa

PURPOSE

To determine to what degree visual field size is correlated with electroretinogram (ERG) amplitude among patients with the common forms of retinitis pigmentosa (RP).

METHODS

Visual field equivalent diameter to the V4e white test light of the Goldmann perimeter was correlated with log ERG amplitude elicited by 0.5 Hz or 30 Hz full-field flashes of white light. Primary analyses were conducted on data from 583 patients with the common forms of RP. Subset analyses were performed on data from patients with ERG responses with different ranges of amplitude to assess to what extent the correlation depends on ERG amplitude, as well as on data from patients of a given genetic type to determine whether the correlation depends on the mode of transmission. Data from patients with the rhodopsin, Pro23His mutation (n = 38) or with the rhodopsin, Pro347Leu mutation (n = 24) were analyzed to determine the correlation between visual field size and ERG amplitude for patients with the same mutation.

RESULTS

Visual field size was significantly correlated with ERG amplitude for every comparison (P < or = 0.0003). Correlations generally were higher for ERGs elicited by 30 Hz flashes (r = 0.62 for the entire sample) than they were for those elicited by 0.5 Hz flashes (r = 0.53 for the entire sample). They were lower for truncated ranges of ERG amplitude, higher for patients with dominant or recessive disease than for patients with x-linked disease or for patients of all genetic types combined, and strong for patients with the same rhodopsin mutation (reaching a value of 0.87).

CONCLUSIONS

Visual field size is significantly correlated with ERG amplitude for patients with RP. Correlation depends on the range of ERG amplitudes, the inheritance type, and, particularly, on whether the analysis is confined to a single gene mutation.

Evaluation of patients with retinitis pigmentosa receiving electric stimulation, ozonated blood, and ocular surgery in Cuba

OBJECTIVE

To evaluate the effect of intervention with electric stimulation, autotransfused ozonated blood, and ocular surgery, performed in Cuba, on the course of the common forms of retinitis pigmentosa.

DESIGN

Ocular evaluations over 6 to 8 months before and after intervention in Cuba.

SETTING

Evaluations performed at a US clinical research facility.

PATIENTS

Ten adult patients aged 25 to 67 years with retinitis pigmentosa.

MAIN OUTCOME MEASURES

Visual acuity, visual field area, and electroretinogram (ERG) amplitude.

RESULTS

No significant change in visual acuity or visual field area was observed on average between preintervention and postintervention values over a 6- to 8-month interval. Mean 30-Hz cone ERG amplitude declined by 15.5% between preintervention and postintervention values (P = .006). When data on change in visual field area from 1 statistically significant outlier were excluded from the analysis, a significant decline of 12.9% in mean visual field area was observed (P = .025).

CONCLUSIONS

These data support the conclusion that the intervention offered in Cuba provides no benefit to patients with retinitis pigmentosa as measured by visual acuity, visual field area, and ERG. The magnitudes of the mean declines observed in ERG amplitude and visual field area over a 6- to 8-month interval, relative to those reported in previous studies, raise the possibility that this intervention may worsen the course of the disease.

Isolation of focal rod electroretinograms from the dark-adapted human eye

PURPOSE

To isolate focal rod electroretinograms (ERGs) from the dark-adapted human eye.

METHODS

In two normal volunteers, dark-adapted focal rod ERGs were recorded from the peripheral retina in response to 30 degree diameter blue flashes of varying retinal illuminance and from different retinal regions in response to 10 degree diameter bright blue flashes. Dark-adapted focal rod ERGs also were recorded from a patient with the multiple evanescent white dot syndrome (MEWDS) and an enlarged blind spot in response to 30 degree diameter blue flashes presented within and outside the scotoma. The slower and larger stray light rod component elicited by these flashes was removed by subtracting the matching rod response to a dimmer, full-field flash, or was ignored when it did not overlap the faster and smaller focal rod component.

RESULTS

The focal rod ERG had a waveform and sensitivity similar to those of the full-field rod ERG, was approximately proportional in amplitude to the density of rods directly illuminated, and was nondetectable within the retinal area corresponding to the enlarged blind spot of the patient with MEWDS.

CONCLUSIONS

Focal rod ERG a- and b-waves, in response to stimuli as small as 10 degrees, can be recorded from different regions of the dark-adapted human retina to evaluate localized rod function.

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 the alpha subunit of the rod cGMP-gated channel in autosomal recessive retinitis pigmentosa

Mutations in the genes encoding two proteins of the retinal rod phototransduction cascade, opsin and the beta subunit of rod cGMP phosphodiesterase, cause retinitis pigmentosa (RP) in some families. Here we report defects in a third member of this biochemical pathway in still other patients with this disease. We screened 94 unrelated patients with autosomal dominant RP and 173 unrelated patients with autosomal recessive RP for mutations in the gene encoding the alpha subunit of the rod cGMP-gated cation channel. Five mutant sequences cosegregated with disease among four unrelated families with autosomal recessive RP. Two of these were nonsense mutations early in the reading frame (Glu76End and Lys139End) and one was a deletion encompassing most if not all of the transcriptional unit; these three alleles would not be expected to encode a functional channel. The remaining two mutations were a missense mutation (Ser316Phe) and a frameshift [Arg654(1-bp del)] mutation truncating the last 32 aa in the C terminus. The latter two mutations were expressed in vitro and found to encode proteins that were predominantly retained inside the cell instead of being targeted to the plasma membrane. We conclude that the absence or paucity of functional cGMP-gated cation channels in the plasma membrane is deleterious to rod photoreceptors and is an uncommon cause of RP.

Low incidence of retinitis pigmentosa among heterozygous carriers of a specific rhodopsin splice site mutation

PURPOSE

To determine whether a rhodopsin splice donor site mutation at the 5' end of intron 4 is a cause of autosomal dominant retinitis pigmentosa.

METHODS

Heterozygous carriers of the same rhodopsin splice site mutation in two pedigrees were identified using single-strand conformation polymorphism analysis. Twelve heterozygous carriers were evaluated by ophthalmoscopy. Goldmann kinetic visual fields, dark adaptation thresholds, and full-field electroretinograms including rod intensity-response functions. Clinical findings from the heterozygous carriers of the splice site mutation were compared with those from heterozygous carriers from a separate family with a known recessive rhodopsin null mutation, Glu249X.

RESULTS

Analysis of DNA from 48 members of two pedigrees revealed 25 heterozygous carriers of the splice site mutation, ranging in age from 14 to 82 years. There were no homozygotes with the rhodopsin splice site mutation. Of the 25 heterozygous carriers, 24 were asymptomatic. Eleven asymptomatic heterozygotes were examined, including four older than 65 years of age. They were found to have normal fundi, full visual fields, and slightly elevated final rod dark adaptation thresholds. Their rod electroretinographic b-wave amplitudes were slightly diminished over the full range of blue light intensities. Rod a-wave implicit times were slightly but significantly prolonged in response to the brightest blue flash of light. These subtle abnormalities in rod function were similar to those found in asymptomatic heterozygous carriers of the recessive Glu249X mutation. Only one of the 25 heterozygous carriers of the splice site mutation had symptoms and signs of retinitis pigmentosa.

CONCLUSIONS

Because 96% of these heterozygous carriers do not have retinitis pigmentosa, it is unlikely that this mutation in intron 4 is a dominant allele. The subtle abnormalities of rod function found in asymptomatic carriers are similar to those found in heterozygous carriers of a recessive rhodopsin allele. The one heterozygous carrier with retinitis pigmentosa probably has a second mutation in the rhodopsin gene or has a defect or defects in another gene that causes his disease.