A clinical, psychophysical, and electroretinographic survey of patients with autosomal dominant retinitis pigmentosa

Improved Rod Sensitivity as Assessed by Two-Color Dark-Adapted Perimetry in Patients With RPE65-Related Retinopathy Treated With Voretigene Neparvovec-rzyl

Purpose

The purpose of this study was to evaluate rod-mediated function with two-color dark-adapted perimetry (2cDAP) in patients with RPE65-related retinopathy treated with voretigene neparvovec-rzyl.

Methods

Following dilation and dark adaptation, 2cDAP and FST were performed. The 2cDAP was measured on an Octopus 900 perimeter (Haag-Streit) with cyan (500 nm wavelength) and red (650 nm wavelength) stimuli. Hill of vision (HOV) analysis was performed on 2cDAP perimetry with Visual Field Modeling and Analysis (VFMA). Full field threshold stimulus testing (FST) was also measured as a secondary measure of rod-mediated function, and assessed on a Diagnosys Espion with the ColorDome stimulator (Diagnosys LLC).

Results

Eight eyes from 4 patients who were treated with voretigene bilaterally had rod function assessed by 2cDAP testing at least 1 year after treatment. There was statistically significant improvement in 2cDAP following gene augmentation therapy. HOV VFMA analysis showed widespread improvements that extended beyond the treatment bleb and statistically significant improvement in HOV analysis volumetric measurements post-treatment to cyan and red stimuli. FST testing performed in six eyes from three patients demonstrated statistically significant improvement to all chromatic stimuli following treatment.

Conclusions

These findings demonstrated statistically significant improvement in 2cDAP and FST following treatment with voretigene.

Translational Relevance

These findings provide a sensitive method of assessing rod-mediated function in a topographic manner that may be useful in future clinical trials for inherited retinal dystrophies.

Full-field stimulus testing: Role in the clinic and as an outcome measure in clinical trials of severe childhood retinal disease

Disease mechanisms have become better understood in previously incurable forms of early-onset severe retinal dystrophy, such as Leber congenital amaurosis (LCA). This has led to novel treatments and clinical trials that have shown some success. Standard methods to measure vision were difficult if not impossible to perform in severely affected patients with low vision and nystagmus. To meet the need for visual assays, we devised a psychophysical method, which we named full-field stimulus testing (FST). From early versions based on an automated perimeter, we advanced FST to a more available light-emitting diode platform. The journey from invention to use of such a technique in our inherited retinal degeneration clinic is reviewed and many of the lessons learned over the 15 years of application of FST are explained. Although the original purpose and application of FST was to quantify visual thresholds in LCA, there are rare opportunities for FST also to be used beyond LCA to measure aspects of vision in other inherited retinal degenerations; examples are given. The main goal of the current review, however, remains to enable investigators studying and treating LCA to understand how to best use FST and how to reduce artefact and confounding complexities so the test results become more valuable to the understanding of LCA diseases and results of novel interventions.

Translational Retinal Research and Therapies

The following review summarizes the state of the art in representative aspects of gene therapy/translational medicine and evolves from a symposium held at the School of Veterinary Medicine, University of Pennsylvania on November 16, 2017 honoring Dr. Gustavo Aguirre, recipient of ARVO's 2017 Proctor Medal. Focusing on the retina, speakers highlighted current work on moving therapies for inherited retinal degenerative diseases from the laboratory bench to the clinic.

Dark-Adapted Chromatic Perimetry for Measuring Rod Visual Fields in Patients with Retinitis Pigmentosa

Purpose

Although rod photoreceptors are initially affected in retinitis pigmentosa (RP), the full-field of rod vision is not routinely characterized due to the unavailability of commercial devices detecting rod sensitivity. The purpose of this study was to quantify rod-mediated vision in the peripheral field from patients with RP using a new commercially available perimeter.

Methods

Participants had one eye dilated and dark-adapted for 45 minutes. A dark-adapted chromatic (DAC) perimeter tested 80 loci 144° horizontally and 72° vertically with cyan stimuli. The number of rod-mediated loci (RML) were analyzed based on normal cone sensitivity (method 1) and associated with full-field electroretinography (ERG) responses by Pearson's r correlation and linear regression. In a second cohort of patients with RP, RML were identified by two-color perimetry (cyan and red; method 2). The two methods for ascribing rod function were compared by Bland-Altman analysis.

Results

Method 1 RML were correlated with responses to the 0.01 cd.s/m² flash (P < 0.001), while total sensitivity to the cyan stimulus showed correlation with responses to the 3.0 cd.s/m² flash (P < 0.0001). Method 2 detected a mean of 10 additional RML compared to method 1.

Conclusions

Scotopic fields measured with the DAC detected rod sensitivity across the full visual field, even in some patients who had nondetectable rod ERGs. Two-color perimetry is warranted when sensitivity to the cyan stimulus is reduced to ≤20 dB to get a true estimation of rod function.

Translational Relevance

Many genetic forms of retinitis pigmentosa (RP) are caused by mutations in rod-specific genes. However, treatment trials for patients with RP have relied primarily on photopic (cone-mediated) tests as outcome measures because there are a limited number of available testing methods designed to evaluate rod function. Thus, efficient methods for quantifying rod-mediated vision are needed for the rapidly increasing numbers of clinical trials.

Fundus-controlled two-color dark adaptometry with the Microperimeter MP1

PURPOSE

The aim of this study was to provide fundus-controlled two-color adaptometry with an existing device. A quick and easy approach extends the application possibilities of a commercial fundus-controlled perimeter.

METHODS

An external filter holder was placed in front the objective lens of the MP1 (Nidek, Italy) and fitted with filters to modify background, stimulus intensity, and color. Prior to dark adaptometry, the subject's visual sensitivity profile was measured for red and blue stimuli to determine whether rods or cones or both mediated the absolute threshold. After light adaptation, 20 healthy subjects were investigated with a pattern covering six spots at the posterior pole of the retina up to 45 min of dark adaptation. Thresholds were determined using a 200 ms red Goldmann IV and a blue Goldmann II stimulus.

RESULTS

The pre-test sensitivity showed a typical distribution of values along the meridian, with high peripheral light increment sensitivity (LIS) and low central LIS for rods and the reverse for cones. After bleach, threshold recovery had a classic biphasic shape. The absolute threshold was reached after approximately 10 min for the red and 15 min for the blue stimulus.

CONCLUSIONS

Two-color fundus-controlled adaptometry with a commercial MP1 without internal changes to the device provides a quick and easy examination of rod and cone function during dark adaptation at defined retinal loci of the posterior pole. This innovative method will be helpful to measure rod vs. cone function at known loci of the posterior pole in early stages of retinal degenerations.

Clinical and genetic identification of a large chinese family with autosomal dominant retinitis pigmentosa

BACKGROUND

Retinitis pigmentosa (RP) is a group of inherited retinal dystrophies characterized by night blindness, progressive peripheral visual field loss, and loss of central vision. Fifty-three RP pathogenic genes are responsible for RP. Pre-mRNA processing factor 31(PRPF31) gene is the third most common cause of autosomal dominant retinitis pigmentosa (adRP), and so far more than 40 mutations in PRPF31 have been detected.

PURPOSE

To identify the underlying genetic defect in a five-generation Chinese family affected with adRP and to study the genotype-phenotype relationship of this family.

METHODS

Detailed clinical investigations were undertaken and peripheral blood samples were collected from 25 individuals. Microsatellite (STR) markers tightly linked to genes known to be responsible for adRP were selected for linkage analysis. Exons and adjacent splice junctions of the candidate gene were amplified and sequenced.

RESULTS

This adRP family exhibited an incomplete penetrance of the RP phenotype. In affected individuals, age of disease onset was from infancy to 4 years of age. Typical RP features were associated with this mutation. Linkage analysis identified a maximum two-point LOD score of 3.20 with D19S418, which is close to PRPF31. A mutation PRPF31: (c.358-359 del AA) was identified by linkage analysis.

CONCLUSIONS

A PRPF31 mutation was identified to be responsible for adRP in a large Chinese family. Our findings expand the mutation spectrum of RP in the Chinese population.

Macular thickness and aging in retinitis pigmentosa

PURPOSE

This study was designed to analyze macular tomography in patients of different ages with retinitis pigmentosa (RP) and correlate their visual function with macular thickness, which was measured by optical coherence tomography.

METHODS

In all, 75 RP patients and 75 controls were stratified into three age groups and macular thickness was measured by optical coherence tomography. The tomography was subdivided into three circular zones, four quadrants, and nine areas for analysis. Ophthalmic examinations, which involved ophthalmoscopic examinations, dark adaptation tests, visual acuities, visual field examinations, electrooculography, and color sense discrimination tests, were performed.

RESULTS

Macular thickness of the RP patients decreased in the middle age group (45- to 55-year old), whereas the oldest group showed an increased thickness. The thickness of the outer inferior area remained virtually unchanged, whereas the thickness of the inner temporal area showed the most fluctuation with age. In terms of circular sections, the most dramatic changes in macular thickness were observed in the fovea, and the aging effect decreased outward to the outer ring. Furthermore, the thickness of the fovea was more important than the thickness of the inner ring and the outer ring for electrooculography, visual acuity, and color sense discrimination in RP patients.

CONCLUSIONS

In middle age RP patients, the macular thickness decreased, whereas an increased thickness was observed in patients older than 55 years. In addition, the inner temporal area was the most fragile, and the outer inferior area was the least affected in patients with RP.

Rod sensitivity, cone sensitivity, and photoreceptor layer thickness in retinal degenerative diseases

PURPOSE

To evaluate the effects of selective rod and/or cone loss on frequency-domain optical coherence tomography (fdOCT) measures of photoreceptor structure in patients with retinal degenerative diseases.

METHODS

Six patients with cone dystrophy (CD) and eight patients with retinitis pigmentosa (RP) were recruited from the Southwest Eye Registry on the basis of diagnosis and ERG findings. fdOCT horizontal line scans were segmented to obtain the thicknesses of the outer segments plus RPE (OS+) and the outer nuclear layer (ONL). The normalized product ONL*OS was obtained after dividing by mean ONL*OS values of 23 normal individuals. Visual field sensitivity profiles were obtained with a modified retinal perimeter, from the horizontal midline with short- and long-wave stimuli under dark- and light-adapted conditions.

RESULTS

Patients with CD and normal rod-mediated sensitivity, but decreased cone-mediated sensitivity, showed normal ONL*OS outside the fovea. The total receptor layer was thinned in the fovea, consistent with loss in cone nuclei and Henle's fiber layer. Patients with RP and sensitivity in the dark that was mediated by cones showed ONL*OS thickness that was linearly related to cone sensitivity. ONL*OS thickness was linearly related to rod sensitivity in regions with greater loss of cone than rod sensitivity.

CONCLUSIONS

Both rods and cones can support an intact IS/OS junction and normal photoreceptor thickness measures. The product of ONL and OS thicknesses is proportional to the sensitivity mediated by the less abnormal type of photoreceptor.

The genomic, biochemical, and cellular responses of the retina in inherited photoreceptor degenerations and prospects for the treatment of these disorders

The association of more than 140 genes with human photoreceptor degenerations, together with studies of animal models of these monogenic diseases, has provided great insight into their pathogenesis. Here we review the responses of the retina to photoreceptor mutations, including mechanisms of photoreceptor death. We discuss the roles of oxidative metabolism, mitochondrial reactive oxygen species, metabolic stress, protein misfolding, and defects in ciliary proteins, as well as the responses of Müller glia, microglia, and the retinal vasculature. Finally, we report on potential pharmacologic and biologic therapies, the critical role of histopathology as a prerequisite to treatment, and the exciting promise of gene therapy in animal models and in phase 1 trials in humans.

Retinal laminar architecture in human retinitis pigmentosa caused by Rhodopsin gene mutations

PURPOSE

To determine the underlying retinal micropathology in subclasses of autosomal dominant retinitis pigmentosa (ADRP) caused by rhodopsin (RHO) mutations.

METHODS

Patients with RHO-ADRP (n = 17, ages 6-73 years), representing class A (R135W and P347L) and class B (P23H, T58R, and G106R) functional phenotypes, were studied with optical coherence tomography (OCT), and colocalized visual thresholds were determined by dark- and light-adapted chromatic perimetry. Autofluorescence imaging was performed with near-infrared light. Retinal histology in hT17M-rhodopsin mice was compared with the human results.

RESULTS

Class A patients had only cone-mediated vision. The outer nuclear layer (ONL) thinned with eccentricity and was not detectable within 3 to 4 mm of the fovea. Scotomatous extracentral retina showed loss of ONL, thickening of the inner retina, and demelanization of RPE. Class B patients had superior-inferior asymmetry in function and structure. The superior retina could have normal rod and cone vision, normal lamination (including ONL) and autofluorescence of the RPE melanin; laminopathy was found in the scotomas. With Fourier-domain-OCT, there was apparent inner nuclear layer (INL) thickening in regions with ONL thinning. Retinal regions without ONL had a thick hyporeflective layer that was continuous with the INL from neighboring regions with normal lamination. Transgenic mice had many of the laminar abnormalities found in patients.

CONCLUSIONS

Retinal laminar abnormalities were present in both classes of RHO-ADRP and were related to the severity of colocalized vision loss. The results in human class B and the transgenic mice support the following disease sequence: ONL diminution with INL thickening; amalgamation of residual ONL with the thickened INL; and progressive retinal remodeling with eventual thinning.

Novel rhodopsin mutations and genotype-phenotype correlation in patients with autosomal dominant retinitis pigmentosa

AIM

To identify novel or rare rhodopsin gene mutations in patients with autosomal dominant retinitis pigmentosa and description of their clinical phenotype.

METHODS

The complete rhodopsin gene was screened for mutations by DNA sequencing in index patients. Mutation specific assays were used for segregation analysis and screening for controls. Eight patients from five families and their relatives were diagnosed with autosomal dominant retinitis pigmentosa (adRP) by means of clinical evaluation.

RESULTS

Mutation screening identified five different rhodopsin mutations including three novel mutations: Ser176Phe, Arg314fs16, and Val20Gly and two missense mutations, Pro215Leu and Thr289Pro, that were only reported once in a mutation report. Electrophysiological and psychophysical testings provide evidence of an impaired rod system with additionally affected cone system in subjects from each genotype group. Visual function tended to be less affected in subjects with the Arg314fs16 and Val20Gly mutations than in the Ser176Phe phenotype. In contrast, Pro215Leu and Thr289Pro mutations caused a remarkably severe phenotype.

CONCLUSION

The ophthalmic findings support a correlation between disease expression and structural alteration: (1) extracellular/intradiscal Val20Gly and cytoplasmic Arg314fs16 mutation-mild adRP phenotype; (2) Ser176Phe mutation-"mostly type 1" disease; (3) predicted alteration of transmembrane domains TM V and TM VII induced by Pro215Leu and Thr289Pro-severe phenotype. However, variation of phenotype expression in identical genotypes may still be a typical feature of RHO mutations.

Quantifying rod photoreceptor-mediated vision in retinal degenerations: dark-adapted thresholds as outcome measures

Pre-clinical trials of treatment in retinal degenerations have shown progress toward preventing loss or restoring function of rod photoreceptors. In anticipation of human clinical trials, we assessed two psychophysical methods of quantifying rod photoreceptor-mediated function as potential outcome measures. Modified automated perimeters were used to deliver focal or full-field light stimuli and dark-adapted thresholds were measured. Patients with retinal degeneration were studied in two experimental protocols. Experiment 1 (n = 35 patients) studied dark-adapted focal chromatic stimuli in central retinal locations along the horizontal meridian. Experiment 2 (n = 146 patients) studied dark-adapted responses to a full-field stimulus test (FST) using white and chromatic stimuli. Patients in both experimental groups had testing on two different visits to determine inter-visit variability. In Experiment 1, two subgroups of patients were identified: a group with a majority of test loci detected by rod photoreceptors and a group with only cone-mediated detection. Inter-visit variability (95% confidence interval) was +/-3.1 dB for normals, +/-3.0 dB for patients with rod-mediated function and +/-2.8 dB for patients with only cone-mediated function. In Experiment 2, the dynamic range of the FST using white stimuli was sufficient to quantify sensitivity in all patients studied, including those with severe retinal degenerations. Chromatic stimuli in the FST were detectable by 85% of patients and rod- or cone-mediation could be determined. Regional retinal sources of FST were explored by comparing FST and dark-adapted perimetry in the same patients; there was a strong correlation between FST level and the loci with highest sensitivity by perimetry. Inter-visit variability (95% confidence interval) in the patients was +/-3.9 dB compared to +/-3.5 dB in normals. Dark-adapted focal threshold measurements with an abbreviated protocol in retinal degeneration patients with stable fixation may be useful as an outcome measure for therapies that can affect rod vision. FST measurements were feasible and reproducible in a large spectrum of retinal degenerative diseases and will be most applicable as a psychophysical outcome measure for treatment trials of very severe disorders in which fixation is lost and there is need for a large dynamic range of stimulus intensity.

PROGRESSTOWARDUNDERSTANDING THEGENETIC ANDBIOCHEMICALMECHANISMS OFINHERITEDPHOTORECEPTORDEGENERATIONS

More than 80 genes associated with human photoreceptor degenerations have been identified. Attention must now turn toward defining the mechanisms that lead to photoreceptor death, which occurs years to decades after the birth of the cells. Consequently, this review focuses on topics that offer insights into such mechanisms, including the one-hit or constant risk model of photoreceptor death; topological patterns of photoreceptor degeneration; mutations in ubiquitously expressed splicing factor genes associated only with photoreceptor degeneration; disorders of the retinal pigment epithelium; modifier genes; and global gene expression analysis of the retina, which will greatly increase our understanding of the downstream events that occur in response to a mutation.

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

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

Progression of defects in the central 10-degree visual field of patients with retinitis pigmentosa and choroideremia

PURPOSE

To determine the progression of defects in the central 10-degree visual field in patients with retinitis pigmentosa and choroideremia by means of univariate linear regression with respect to the mean deviation.

METHODS

In a retrospective study, results of automated static perimetry in the central 10-degree visual field were reviewed for patients with retinitis pigmentosa and choroideremia. Univariate linear regression of mean deviation was undertaken for each eye that had field data from at least five tests in addition to the prior static perimetric experience during a period of 3.5 years or more. Data from 30 eyes of 16 patients fulfilled eligibility requirements; the mean number of fields was 5.8 (range, five to 10) and the mean follow-up period was 64 months (range, 42 to 97 months). Eyes were classified as progressive if the regression coefficient was negative and significantly different from zero, with a P value of less than .05.

RESULTS

Among 14 patients in whom both eyes were eligible for univariate linear regression analysis, a statistically significant progression was demonstrated in both eyes in four patients, only in one eye in five patients, and in neither eye in five patients. One of two patients, in whom only one eye was eligible for the study, showed significant progression in the eye. Overall, 14 (47%) of 30 eyes showed statistically significant progression with respect to the mean deviation. No eye showed significant improvement.

CONCLUSIONS

Automated static perimetry of the central 10-degree visual field measured approximately once or twice a year demonstrated the progressive nature of the disease with the use of univariate linear regression of mean deviation in 47% of eyes with retinitis pigmentosa and choroideremia during the follow-up period of 3.5 years or more. These results may be useful in understanding the clinical course of the diseases and counseling patients.

Molecular genetics of human retinal dystrophies

Retinal dystrophies are a heterogeneous group of diseases in which the retina degenerates, leading to either partial or complete blindness. The severe and clearly hereditary forms, retinitis pigmentosa (RP) and various macular degenerations, affect approximately 1 in 3000 people, but many more suffer from aging macular dystrophy in later life. Patients with RP present with narrowing visual fields and night blindness, while those with diseases of the macula lose central vision first. Even before the advent of molecular genetics it was evident that these were heterogeneous disorders, with wide variation in severity, mode of inheritance and phenotype. However, with the widespread application of linkage analysis and mutation detection techniques, a complex underlying pathology has now been revealed. In total, 66 distinct non-overlapping genes or gene loci have been implicated in the various forms of retinal dystrophy, with more being reported regularly in the literature. Within the category of non-syndromic RP alone there are at least 22 genes (and probably many more) involved, with further allelic heterogeneity arising from different mutations in the same gene. This complexity presents a problem for those involved in counselling patients, and also compounds the search for therapies. Nevertheless, several lines of research raise the hope of generic treatments applicable to all such patients, while the greater understanding of normal visual function that arises from genetic studies may open up new avenues for therapy.

Rhodopsin C110Y mutation causes a type 2 autosomal dominant retinitis pigmentosa

PURPOSE

The RHO C110Y mutation has been recently reported to cause a phenotypically unspecified form of autosomal dominant retinitis pigmentosa (adRP). The study of a family affected with this mutation allowed us to hereby describe the genotype/phenotype correlation associated with the RHO C110Y mutation.

METHODS

A six-generation pedigree cosegregating adRP and RHO C110Y in ten accessible individuals was ophthalmologically investigated. All family members affected with RP went through complete eye examination and ERG testing.

RESULTS

The disease first manifested with nyctalopia during adulthood and slowly progressed over the next decades towards tubular visual field defects and relatively preserved central vision. Ophthalmoscopically, the fundus remained almost unaltered until the end of the third decade of life, and then slowly progressed towards typical RP changes with minimal macular involvement by the eighth decade. Color vision remained unaltered. Earliest ERG alteration was limited to the rod system followed by a rod-cone pattern. Scotopic and photopic ERG were recordable until the fourth and sixth decades, respectively.

DISCUSSION

RHO C110Y-associated adRP is characterized by a late onset and a mild progression compatible with type 2 or regional RP with little intrafamilial phenotypic variability and complete penetrance. Characterization of genotype-phenotype correlations plays a role in the improvement of genetic and prognostic counselling.

A new dominant retinitis pigmentosa family mapping to the RP18 locus on chromosome 1q11-21

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Evidence for a major retinitis pigmentosa locus on 19q13.4 (RP11) and association with a unique bimodal expressivity phenotype

Retinitis pigmentosa (RP) is the name given to a heterogeneous group of retinal degenerations mapping to at least 16 loci. The autosomal dominant form (ARP), accounting for approximately 25% of cases, can be caused by mutations in two genes, rhodopsin and peripherin/RDS, and by at least six other loci identified by linkage analysis. The RP11 locus for adRP has previously been mapped to chromosome 19q13.4 in a large English family. This linkage has been independently confirmed in a Japanese family, and we now report three additional unrelated linked U.K. families, suggesting that this is a major locus for RP. Linkage analysis in the U.K. families refines the RP11 interval to 5 cM between markers D19S180 and AFMc001yb1. All linked families exhibit incomplete penetrance; some obligate gene carriers remain asymptomatic throughout their lives, whereas symptomatic individuals experience night blindness and visual field loss in their teens and are generally registered as blind by their 30s. This "bimodal expressivity" contrasts with the variable-expressivity RP mapping to chromosome 7p (RP9) in another family, which has implications for diagnosis and counseling of RP11 families. These results may also imply that a proportion of sporadic RP, previously assumed to be recessive, might result from mutations at this locus.

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

A form of autosomal dominant retinitis pigmentosa (ADRP) mapping to chromosome 17p has been reported in a single large South African family. We now report a new family with severe early onset ADRP which maps to 17p. Linkage and haplotype analysis in this family places the ADRP locus in the 5 cM interval between markers AFMc024za5 and D17S1845, confirming the data obtained in the South African family. The discovery of a second 17p linked family may imply that this is one of the more common loci for dominant RP. In addition, the confirmation of an RP diagnosis at this locus is of interest since loci for a dominant cone dystrophy and Leber's congenital amaurosis (LCA1) have recently been linked to the same markers. While the cone dystrophy locus may be allelic with RP, our data and that of Goliath et al show that distinct genes are responsible for dominant RP and Leber's congenital amaurosis on chromosome 17p.

Recoverin, a calcium-binding protein in photoreceptors

Recoverin is a Ca2+-binding protein found primarily in vertebrate photoreceptors. The proposed physiological function of recoverin is based on the finding that recoverin inhibits light-stimulated phosphorylation of rhodopsin. Recoverin interacts with rod outer segment membranes in a Ca2+-dependent manner. This interaction requires N-terminal acylation of recoverin. Four types of fatty acids have been detected on the N-terminus of recoverin, but the functional significance of this heterogeneous acylation is not yet clear.

Future directions for rhodopsin structure and function studies

NMR (nuclear magnetic resonance) may be useful for determining the structure of retinal and its environment in rhodopsin, but not for determining the complete protein structure. Aggregation and low yield of fragments of rhodopsin may make them difficult to study by NMR. A long-term multidisciplinary attack on rhodopsin structure is required.

More answers about cGMP-gated channels pose more questions

Our understanding of the molecular properties and cellular role of cGMP-gated channels in outer segments of vertebrate photo-receptors has come from over a decade of studies which have continuously altered and refined ideas about these channels. Further examination of this current view may lead to future surprises and further refine the understanding of cGMP-gated channels.

Cyclic nucleotides as regulators of light-adaptation in photoreceptors

Cyclic nucleotides can regulate the sensitivity of retinal rods to light through phosducin. The phosphorylation state of phosducin determines the amount of G available for activation by Rho*. Phosducin phosphorylation is regulated by cyclic nucleotides through their activation of cAMP-dependent protein kinase. The regulation of phosphodiesterase activity by the noncatalytic cGMP binding sites as well as Ca2+/calmodulin dependent regulation of cGMP binding to the cation channel are also discussed.

Long term potentiation and CaM-sensitive adenylyl cyclase: Long-term prospects

The type I CaM-sensitive adenylyl cyclase is in a position to integrate signals from multiple inputs, consistent with the requirements for mediating long term potentiation (LTP). Biochemical and genetic evidence supports the idea that this enzyme plays an important role inc LTP. However, more work is needed before we will be certain of the role that CaM-sensitive adenylyl cyclases play in LTP.

Modulation of the cGMP-gated channel by calcium

Calcium acting through calmodulin has been shown to regulate the affinity of cyclic nucleotide-gated channels expressed in cell lines. But is calmodulin the Ca-sensor that normally regulates these channels?

How many light adaptation mechanisms are there?

The generally positive response to our target article indicates that most of the commentators accept our contention that light adaptation consists of multiple and possibly redundant mechanisms. The commentaries fall into three general categories. The first deals with putative mechanisms that we chose not to emphasize. The second is a more extended discussion of the role of calcium in adaptation. Finally, additional aspects of cGMP involvement in adaptation are considered. We discuss each of these points in turn.

Gene therapy, regulatory mechanisms, and protein function in vision

Hereditary retinal degeneration due to mutations in visual genes may be amenable to therapeutic interventions that modulate, either positively or negatively, the amount of protein product. Some of the proteins involved in phototransduction are rapidly moved by a lightdependent mechanism between the inner segment and the outer segment in rod photoreceptor cells, and this phenomenon is important in phototransduction.

A novel protein family of neuronal modulators

A number of proteins homologous to recoverin have been identified in the brains of the several vertebrate species. The brainderived members originally contain four EF-hand domains, but NH2- terminal domain is aberrant. Many of these proteins inhibited light-induced rhodopsin phosphorylation at high [Ca2+], suggesting that the brain-derived members may act as a Ca2+-sensitive modulator of receptor phosphorylation, as recoverin does.

The structure of rhodopsin and mechanisms of visual adaptation

Rapidly advancing studies on rhodopsin have focused on new strategies for crystallization of this integral membrane protein for x-ray analysis and on alternative methods for structural determination from nuclear magnetic resonance data. Functional studies of the interactions between the apoprotein and its chromophore have clarified the role of the chromophore in deactivation of opsin and in photoactivation of the pigment.

Crucial steps in photoreceptor adaptation: Regulation of phosphodiesterase and guanylate cyclase activities and Ca2+-buffering

This commentary discusses the balance of phosphodiesterase and guanylate cyclase activities in vertebrate photoreceptors at moderate light intensities. The rate of cGMP hydrolysis and synthesis seem to equal each other. Ca2+ as regulator of both enzyme activities is also effectively buffered in photoreceptor cells by cytoplasmic buffer components.

The atomic structure of visual rhodopsin: How and when?

Strong arguments are presented by Hargrave suggesting that the crystallization of visual rhodopsin for high resolution analysis by X-ray crystallography or electron microscopy is feasible. However, the effort needed to achieve this goal will most likely exceed the resources of a single laboratory and a concerted approach to the research is necessary.

Molecular insights gained from covalently tethering cGMP to the ligand-binding sites of retinal rod cGMP-gated channels

A photoaffinity analog of cGMP has been used to biochemically identify a new ligand-binding subunit of the retinal rod cGMP-activated ion channel, as well as amino acids in contact with cGMP in the original subunit. Covalent tethering of this probe to channels in excised menbrane patches has revealed a functional heteogeneity in the ligand-binding sites that may arise from the two biochemically identified subunits.

Genetic and functional complexity of inherited retinal degeneration

Recent findings emphasize the complexity, both genetic and functional, of the manifold genes and mutations causing inherited retinal degeneration in humans. Knowledge of the genetic bases of these diseases can contribute to design of rational therapy, as well as elucidating the function of each gene product in normal visual processes.

Channel structure and divalent cation regulation of phototransduction

The identification of additional subunits of the cGMP-gated cation channel suggests exciting questions about their regulatory roles and about structure/functional relationships. How do the different subunits interact? How is the complex assembled into the plasma membrane? Divalent cations have been implicated in the regulation of adaptation. One often overlooked cation is magnesium. Could this ion play a role in phototransduction?

Structure of the cGMP-gated channel

The subunit structure of the cGMP-gated cation channel of rod photoreceptors is rapidly being defined, and in the process the mode of regulation by Ca2+-calmodulin unraveled. Intriguingly, early results suggest that additional subunits of unknown function are associated with the channel and remain to be identified.

Linking genotypes with phenotypes in human retinal degenerations: Implications for future research and treatment

Although undoubtedly it will be incomplete by the time it is published, the target article by Daiger et al. organizes mutations in genes that produce retinal degenerations in humans into categories of clinically relevant phenotypes. Such classifications should help us understand the link between altered photoreceptor cell proteins and subsequent cell death, and they may yield insight into methods for preventing consequent blindness.

Genetic and clinical heterogeneity in tapetal retinal dystrophies

Large scale DNA-mutation screening in patients with hereditary retinal diseases greatly enhances our knowledge about retinal function and diseases. Scientists, clinicians, patients, and families involved with retinal disorders may directly benefit from these developments. However, certain aspects of this expanding knowledge, such as the correlation between genotype and phenotype, may be much more complicated than we expect at present.

The determination of rhodopsin structure may require alternative approaches

The structure of rhodopsin is a subject of intense interest. Solving the structure by traditional methods has proved exceedingly challenging. It may therefore be useful to confront the problem by a combination of alternate techniques. These include FTIR (Fourier transform infrared spectroscopy) and AFM (atomic force microscopy) on the intact protein. Furthermore, additional insights may be gained through structural investigations of discrete rhodopsin domains.