Retinal photoreceptor dystrophies LI. Edward Jackson Memorial Lecture

A mutation in ZNF513, a putative regulator of photoreceptor development, causes autosomal-recessive retinitis pigmentosa

Retinitis pigmentosa (RP) is a phenotypically and genetically heterogeneous group of inherited retinal degenerations characterized clinically by night blindness, progressive constriction of the visual fields, and loss of vision, and pathologically by progressive loss of rod and then cone photoreceptors. Autosomal-recessive RP (arRP) in a consanguineous Pakistani family previously linked to chromosome 2p22.3-p24.1 is shown to result from a homozygous missense mutation (c.1015T>C [p.C339R]) in ZNF513, encoding a presumptive transcription factor. znf513 is expressed in the retina, especially in the outer nuclear layer, inner nuclear layer, and photoreceptors. Knockdown of znf513 in zebrafish reduces eye size, retinal thickness, and expression of rod and cone opsins and causes specific loss of photoreceptors. These effects are rescued by coinjection with wild-type (WT) but not p.C339R-znf513 mRNA. Both normal and p.C339R mutant ZNF513 proteins expressed in COS-7 cells localize to the nucleus. ChIP analysis shows that only the wild-type but not the mutant ZNF513 binds to the Pax6, Sp4, Arr3, Irbp, and photoreceptor opsin promoters. These results suggest that the ZNF513 p.C339R mutation is responsible for RP in this family and that ZNF513 plays a key role in the regulation of photoreceptor-specific genes in retinal development and photoreceptor maintenance.

Blindness and visual impairment in retinitis pigmentosa: a Cameroonian hospital-based study

Aim:

We performed a retrospective, analytical study in February 2010 on all retinitis pigmentosa cases seen during ophthalmologic consultation at the Gyneco-Obstetrics and Pediatric Hospital of Yaounde between March 2002 and December 2009 (82 months). The aim of this research was to determine the significance of blindness and visual impairment associated with retinitis pigmentosa in Cameroon.

Results:

Forty cases were reported, corresponding to a hospital prevalence of 1.6/1000 (21 men and 19 women). The average age of the patients was 43.3 ± 18 years, ranging between 6 and 74 years. Bilateral blindness and low vision was noted in 30% and 27.5% of patients, respectively. The average age of patients with low vision was 40.38 ± 16.27 years and the average age of those with bilateral blindness was 51.08 ± 15.79 years. Retinitis pigmentosa was bilateral in all cases and isolated (without any eye or general additional disease) in 67.5% of cases.

Conclusion:

Visual impairment is common and becomes even more severe with aging. Patients should be screened to enable them to benefit from management focusing on both appropriate treatment and genetic counseling.

Nonsense mutation in MERTK causes autosomal recessive retinitis pigmentosa in a consanguineous Pakistani family

BACKGROUND

Retinitis pigmentosa (RP) is one of the most common ophthalmic disorders affecting one in approximately 5000 people worldwide. A nuclear family was recruited from the Punjab province of Pakistan to study the genetic basis of autosomal recessive RP.

METHODS

All affected individuals underwent a thorough ophthalmic examination and the disease was characterised based upon results for fundus photographs and electroretinogram recordings. Genomic DNA was extracted from peripheral leucocytes. Exclusion studies were performed with short tandem repeat (STR) markers flanking reported autosomal recessive RP loci. Haplotypes were constructed and results were statistically evaluated.

RESULTS

The results of exclusion analyses suggested that family PKRP173 was linked to chromosome 2q harbouring mer tyrosine kinase protooncogene (MERTK), a gene previously associated with autosomal recessive RP. Additional STR markers refined the critical interval and placed it in a 13.4 cM (17 Mb) region flanked by D2S293 proximally and D2S347 distally. Significant logarithm of odds (LOD) scores of 3.2, 3.25 and 3.18 at theta=0 were obtained with markers D2S1896, D2S2269 and D2S160. Sequencing of the coding exons of MERTK identified a mutation, c.718G-->T in exon 4, which results in a premature termination of p.E240X that segregates with the disease phenotype in the family.

CONCLUSION

Our results strongly suggest that the nonsense mutation in MERTK, leading to premature termination of the protein, is responsible for RP phenotype in the affected individuals of the Pakistani family.

A novel locus for autosomal recessive retinitis pigmentosa in a consanguineous Pakistani family maps to chromosome 2p

OBJECTIVE

To identify a disease locus for autosomal recessive retinitis pigmentosa in a consanguineous Pakistani family.

DESIGN

Prospective linkage study.

METHODS

Blood samples were collected and genomic DNA was extracted. A genome-wide scan was performed using 382 polymorphic microsatellite markers on genomic DNA from 4 affected and 5 unaffected family members, and logarithm of odds scores were calculated.

RESULTS

A maximum 2-point logarithm of odds score of 3.14 at theta = 0 was obtained for marker D2S165 during the genome-wide scan. Fine mapping markers identified a 20.92-cM (19.98-Mb) interval flanked by D2S149 and D2S367 that cosegregates with the disease phenotype. Haplotype analyses further refined the critical interval, distal to D2S220 in a 12.31-cM (13.35-Mb) region that does not harbor any genes that previously have been associated with retinitis pigmentosa.

CONCLUSIONS

Linkage analysis identified a new locus for autosomal recessive retinitis pigmentosa that maps to chromosome 2p22.3-p24.1 in a consanguineous Pakistani family.

Morphological alterations in retinal neurons in the S334ter-line3 transgenic rat

The S334ter-line-3 rat is a transgenic model of retinal degeneration developed to express a rhodopsin mutation similar to that found in human retinitis pigmentosa (RP) patients. Previous studies have focused on physiological changes in retinal cells and higher centers of the visual system with this model of retinal degeneration. However, little is known about the morphological changes in retinal cells during the development of the S334ter-line-3 rat. In order to understand and aid vision-rescue strategies, our aim has been to describe the retinal degeneration pattern in this model. We focus on changes in the morphologies of horizontal, bipolar, and amacrine cells in developing S334ter-line-3 rat retinas. Degeneration of photoreceptors begins in the central retina and progresses toward the periphery. In retinas at post-natal day 15 (P15), horizontal and rod bipolar cells show normal morphology. However, at P21, horizontal and rod bipolar cells exhibit abnormal processes at the outer plexiform layer, whereas the outer nuclear layer is significantly thinner. A glial reaction occurs concomitantly. In contrast, modifications in cone-bipolar and amacrine cells are much slower and do not occur until P90 and P180, respectively. The density of horizontal and rod-bipolar cells significantly drops after P60. Overall, the S334ter-line-3 model exhibits the hallmarks of cellular remodeling caused by photoreceptor degeneration. Its moderately fast time course makes the S334ter-line-3 a good model for studying vision-rescue strategies.

Morphological changes of short-wavelength cones in the developing S334ter-3 transgenic rat

The S334ter-3 rat is a transgenic model of retinal degeneration (RD) developed to express a rhodopsin mutation similar to that found in human retinitis pigmentosa. Due to this advantage over other models of RD, a few retina transplant studies have been reported on this animal model. Currently, no information is available on cone photoreceptor changes that occur in the S334ter RD model. In this study, we investigated the effect of RD on the morphology, distribution, and synaptic connectivity of short-wavelength cones (S-cones) during development of S334ter-3 rat retinas. At P21 RD retinas, the outer-nuclear layer was significantly narrower, while S-cones showed shortening of their segments and axons compared to control retinas. From P90 onward, S-opsin-immunoreactive cells appeared at the outer margin of the inner-nuclear layer of RD retinas. Double-labelling experiments showed these cells contained recoverin and cone arrestin. Furthermore, ultra-structure study showed that synaptic ribbons are conserved in the S-cone at P180 RD retinas. Although cell density of S-cones significantly dropped after P90, survival rates depended on the retinal region. Overall, the S334ter-3 RD model shows hallmarks of cone remodelling due to photoreceptor degeneration.

Autosomal-dominant retinitis pigmentosa caused by a mutation in SNRNP200, a gene required for unwinding of U4/U6 snRNAs

Mutations in genes associated with the U4/U6-U5 small nuclear ribonucleoprotein (snRNP) complex of the spliceosome are implicated in autosomal-dominant retinitis pigmentosa (adRP), a group of progressive retinal degenerative disorders leading to visual impairment, loss of visual field, and even blindness. We recently assigned a locus (RP33) for adRP to 2cen-q12.1, a region that harbors the SNRNP200 gene encoding hBrr2, another U4/U6-U5 snRNP component that is required for unwinding of U4/U6 snRNAs during spliceosome activation and for disassembly of the spliceosome. Here, we report the identification of a missense mutation, c.3260C>T (p.S1087L), in exon 25 of the SNRNP200 gene in an RP33-linked family. The c.3260C>T substitution showed complete cosegregation with the retinitis pigmentosa (RP) phenotype over four generations, but was absent in a panel of 400 controls. The p.S1087L mutation and p.R1090L, another adRP-associated allele, reside in the "ratchet" helix of the first of two Sec63 domains implicated in the directionality and processivity of nucleic acid unwinding. Indeed, marked defects in U4/U6 unwinding, but not U4/U6-U5 snRNP assembly, were observed in budding yeast for the analogous mutations (N1104L and R1107L) of the corresponding Brr2p residues. The linkage of hBrr2 to adRP suggests that the mechanism of pathogenesis for splicing-factor-related RP may fundamentally derive from a defect in hBrr2-dependent RNA unwinding and a consequent defect in spliceosome activation.

Assessment of functional vision score and vision-specific quality of life in individuals with retinitis pigmentosa

PURPOSE

To determine the relationship between the American Medical Association's (AMA) functional vision score (FVS) and vision-specific quality of life in retinitis pigmentosa (RP) patients using the National Eye Institute's Visual Functioning Questionnaire (NEI-VFQ 25).

METHODS

One hundred eight patients with RP participated in the study. We measured best-corrected visual acuity, conducted Goldmann perimetry, and collected the self-reported NEI-VFQ 25. The FVS was calculated using the functional field score (FFS) and the functional acuity score (FAS). The correlations of the VFQ composite scores to the FVS, FFS, and FAS were determined using correlation and regression analyses.

RESULTS

FVS was highly correlated to the BCVA (r=0.69, p<0.001), the FFS (r=0.86, p<0.001) and the FAS (r=0.73, p<0.001). Significant correlations of the VFQ composite score to the BCVA (r=0.60, p<0.001), FFS (r=0.44, p<0.001), FAS (r=0.60, p<0.001), FVS (r=0.58, p<0.001) were also found. However, the correlation strengths of BCVA, FVS, FAS, and FFS to NEI-FVQ were not different.

CONCLUSIONS

In RP patients, the vision-specific quality of life was correlated with the AMA guidelines' FVS, FFS, and FAS. Their correlation degrees to NEI-FVQ were not different. This result suggests that vision-specific quality of life can be explained by both visual acuity and visual field in RP patients.

Mutations in ASCC3L1 on 2q11.2 are associated with autosomal dominant retinitis pigmentosa in a Chinese family

PURPOSE

To localize and identify the gene and mutations causing autosomal dominant retinitis pigmentosa in a Chinese Family.

METHODS

Families were ascertained and patients underwent complete ophthalmic examinations. Blood samples were collected and DNA was extracted. A linkage scan of genomic regions containing known candidate genes was performed by using 34 polymorphic microsatellite markers on genomic DNA from affected and unaffected family members, and lod scores were calculated. Candidate genes were sequenced and mutations analyzed.

RESULTS

A genome-wide scan yielded a lod score of 3.5 at theta = 0 for D2S2333 and 3.46 at theta = 0 for D2S2216. This region harbors the ASCC3L1 gene. Sequencing of ASCC3L1 in an affected family member showed a heterozygous single-base-pair change; c.3269G-->T, predicted to result in an Arg1090Leu amino acid change.

CONCLUSIONS

The results provide strong evidence that mutations in ASCC3L1 have resulted in autosomal dominant retinitis pigmentosa in this Chinese family.

Retinal dystrophies and genetic counselling

Increased patient demand is leading to a corresponding increase in the need for informed genetic counselling in ophthalmic practice which requires refined diagnosis, and a detailed knowledge of molecular genetics. Accurate assessment of risk and visual potential in prospective children is becoming available for a range of retinal dystrophies allowing for more educated decisions to be made by parents.

Preconditioning-induced protection from oxidative injury is mediated by leukemia inhibitory factor receptor (LIFR) and its ligands in the retina

Preconditioning with moderate oxidative stress (e.g., moderate bright light or mild hypoxia) can induce changes in retinal tissue that protect photoreceptors from a subsequent dose of lethal oxidative stress. The mechanism underlying this induced protection is likely a general mechanism of endogenous protection which has been demonstrated in heart and brain using ischemia and reperfusion. While multiple factors like bFGF, CNTF, LIF and BDNF have been hypothesized to play a role in preconditioning-induced endogenous neuroprotection, it has not yet been demonstrated which factors or receptors are playing an essential role. Using quantitative PCR techniques we provide evidence that in the retina, LIFR activating cytokines leukemia inhibitory factor (LIF), cardiotrophin-1 (CT-1) and cardiotrophin like cytokine (CLC) are strongly upregulated in response to preconditioning with bright cyclic light leading to robust activation of signal transducer and activator of transcription-3 (STAT3) in a time-dependent manner. Further, we found that blocking LIFR activation during preconditioning using a LIFR antagonist (LIF05) attenuated the induced STAT3 activation and also resulted in reduced preconditioning-induced protection of the retinal photoreceptors. These data demonstrate that LIFR and its ligands play an essential role in endogenous neuroprotective mechanisms triggered by preconditioning-induced stress.

A novel mutation and phenotypes in phosphodiesterase 6 deficiency

PURPOSE

To develop a systematic approach for the molecular diagnosis of retinitis pigmentosa (RP) and to report new genotype-phenotype correlations for phosphodiesterase 6 (PDE6)-based RP mutations.

DESIGN

Clinical and molecular studies on a retrospective case series.

METHODS

We screened 40 unrelated RP patients with an autosomal recessive RP microarray. Individuals with RP caused by PDE6 deficiency underwent genetic segregation and phenotype analysis.

RESULTS

A disease-associated allele was identified in 32% of patients. Two probands (5%) had PDE6 mutations. The first proband was a compound heterozygote for known R102C and N216S alleles in PDE6A (MIM#180071). Pedigree analysis determined that the N216S variant was benign and direct sequencing discovered a novel, S303C allele. The second proband had a homozygous D600N mutation in the PDE6B gene (MIM#180072). Visual acuities of PDE6-deficient patients ranged from 20/40 to 20/200. Clinical studies showed unusual vitreomacular traction, cystoid macular edema, macular atrophy, and ring hyperfluorescence in PDE6-deficient patients. Such extensive vitreoretinal degeneration is not characteristic of photoreceptor-specific enzyme deficiencies.

CONCLUSION

High-throughput deoxyribonucleic acid microarray chips can be used in combination with clinical imaging to precisely characterize patients with RP. Identifying the precise mutation in RP may become the standard of care as gene therapy emerges.

Identification of two new mutations in the GPR98 and the PDE6B genes segregating in a Tunisian family

Autosomal recessive retinitis pigmentosa (ARRP) is a genetically heterogeneous disorder. ARRP could be associated with extraocular manifestations that define specific syndromes such as Usher syndrome (USH) characterized by retinal degeneration and congenital hearing loss (HL). The USH type II (USH2) associates RP and mild-to-moderate HL with preserved vestibular function. At least three genes USH2A, the very large G-protein-coupled receptor, GPR98, and DFNB31 are responsible for USH2 syndrome. Here, we report on the segregation of non-syndromic ARRP and USH2 syndrome in a consanguineous Tunisian family, which was previously used to define USH2B locus. With regard to the co-occurrence of these two different pathologies, clinical and genetic reanalysis of the extended family showed (i) phenotypic heterogeneity within USH2 patients and (ii) excluded linkage to USH2B locus. Indeed, linkage analysis disclosed the cosegregation of the USH2 phenotype with the USH2C locus markers, D5S428 and D5S618, whereas the ARRP perfectly segregates with PDE6B flanking markers D4S3360 and D4S2930. Molecular analysis revealed two new missense mutations, p.Y6044C and p.W807R, occurring in GPR98 and PDE6B genes, respectively. In conclusion, our results show that the USH2B locus at chromosome 3p23-24.2 does not exist, and we therefore withdraw this locus designation. The combination of molecular findings for GPR98 and PDE6B genes enable us to explain the phenotypic heterogeneity and particularly the severe ocular affection first observed in one USH2 patient. This report presents an illustration of how consanguinity could increase familial clustering of multiple hereditary diseases within the same family.

The spectrum of retinal dystrophies caused by mutations in the peripherin/RDS gene

Peripherin/rds is an integral membrane glycoprotein, mainly located in the rod and cone outer segments. The relevance of this protein to photoreceptor outer segment morphology was first demonstrated in retinal degeneration slow (rds) mice. Thus far, over 90 human peripherin/RDS gene mutations have been identified. These mutations have been associated with a variety of retinal dystrophies, in which there is a remarkable inter- and intrafamilial variation of the retinal phenotype. In this paper, we discuss the characteristics of the peripherin/RDS gene and its protein product. An overview is presented of the broad spectrum of clinical phenotypes caused by human peripherin/RDS gene mutations, ranging from various macular dystrophies to widespread forms of retinal dystrophy such as retinitis pigmentosa. Finally, we review the proposed genotype-phenotype correlation and the pathophysiologic mechanisms underlying this group of retinal dystrophies.

Recurrent mutation in the first zinc finger of the orphan nuclear receptor NR2E3 causes autosomal dominant retinitis pigmentosa

"Autosomal dominant retinitis pigmentosa" (adRP) refers to a genetically heterogeneous group of retinal dystrophies, in which 54% of all cases can be attributed to 17 disease loci. Here, we describe the localization and identification of the photoreceptor cell-specific nuclear receptor gene NR2E3 as a novel disease locus and gene for adRP. A heterozygous mutation c.166G-->A (p.Gly56Arg) was identified in the first zinc finger of NR2E3 in a large Belgian family affected with adRP. Overall, this missense mutation was found in 3 families affected with adRP among 87 unrelated families with potentially dominant retinal dystrophies (3.4%), of which 47 were affected with RP (6.4%). Interestingly, affected members of these families display a novel recognizable NR2E3-related clinical subtype of adRP. Other mutations of NR2E3 have previously been shown to cause autosomal recessive enhanced S-cone syndrome, a specific retinal phenotype. We propose a different pathogenetic mechanism for these distinct dominant and recessive phenotypes, which may be attributed to the dual key role of NR2E3 in the regulation of photoreceptor-specific genes during rod development and maintenance.

Large-scale molecular analysis of a 34 Mb interval on chromosome 6q: major refinement of the RP25 interval

A large scale bioinformatics and molecular analysis of a 34 Mb interval on chromosome 6q12 was undertaken as part of our ongoing study to identify the gene responsible for an autosomal recessive retinitis pigmentosa (arRP) locus, RP25. Extensive bioinformatics analysis indicated in excess of 110 genes within the region and we also noted unfinished sequence on chromosome 6q in the Human Genome Database, between 58 and 61.2 Mb. Forty three genes within the RP25 interval were considered as good candidates for mutation screening. Direct sequence analysis of the selected genes in 7 Spanish families with arRP revealed a total of 244 sequence variants, of which 67 were novel but none were pathogenic. This, together with previous reports, excludes 60 genes within the interval ( approximately 55%) as disease causing for RP. To investigate if copy number variation (CNV) exists within RP25, a comparative genomic hybridization (CGH) analysis was performed on a consanguineous family. A clone from the tiling path array, chr6tp-19C7, spanning approximately 100-Kb was found to be deleted in all affected members of the family, leading to a major refinement of the interval. This will eventually have a significant impact on cloning of the RP25 gene.

Delay of photoreceptor degeneration intubbymouse by sulforaphane

In this study, the homozygous tubby (tub/tub) mutant mouse, with an early progressive hearing loss and photoreceptor degeneration, was used as a model system to examine the effects of systemic administration of a naturally occurring isothiocyanate, sulforaphane (SF), on photoreceptor degeneration. Several novel observations have been made: (i) the mRNA and protein expression of thioredoxin (Trx), thioredoxin reductase (TrxR) and NF-E2-related factor-2 (Nrf2) were significantly reduced even prior to photoreceptor cell degeneration in the retinas of tub/tub mice, suggesting that retinal expression of the Trx system is impaired and that Trx regulation is involved in the pathogenesis of retinal degeneration in this model, (ii) intraperitoneal injection with SF significantly up-regulated retinal levels of Trx, TrxR, and Nrf2, and effectively protected photoreceptor cells in tub/tub mice as evaluated functionally by electroretinography and morphologically by quantitative histology, and (iii) treatment with PD98059, an inhibitor of extracellular signal-regulated kinases (ERKs), blocked SF-mediated ERKs activation and up-regulation of Trx/TrxR/Nrf2 in the retinas of tub/tub mice. This suggests that ERKs and Nrf2 are involved in the mechanism of SF-mediated up-regulation of the Trx system to protect photoreceptor cells in this model. These novel findings are significant and could provide important information for the development of a unique strategy to prevent sensorineural deafness/retinal dystrophic syndromes and also other forms of inherited neurological disorders.

Retinal remodelling

Retinal degenerative diseases that progress through loss of photoreceptors initiate a sequence of events that culminates in negative remodelling of the retina. Initially, photoreceptor loss ablates glutamatergic signalling to the neural retina and eliminates coordinate Ca++-coupled homeostatic signalling. Retinal neurons react to this loss of glutamatergic input through retinal rewiring and migration of neurons throughout the axis of the retina. All diseases that kill photoreceptors trigger retinal remodelling as the final common pathway and cell death is a common feature. Retinal remodelling resembles CNS pathologic remodelling and constitutes a major challenge to all rescue strategies.

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.

Higher-order wavefront aberrations in retinitis pigmentosa

PURPOSE

The purpose of this study was to characterize higher-order wavefront aberrations associated with posterior subcapsular (PSC) cataracts in patients with retinitis pigmentosa (RP).

METHODS

Wavefront aberrations were measured by Shack-Hartmann (SH) aberrometry in eight patients with RP who had PSC cataracts, 10 patients with RP who had minimal or no PSC cataracts, and 16 age-equivalent visually normal control subjects. Higher-order wavefront aberrations for 3-mm and 6-mm pupil diameters were defined as the root mean square (RMS) of the wavefront aberration functions.

RESULTS

For a 6-mm pupil diameter, the mean RMS for total higher-order wavefront aberrations was significantly larger for the patients with RP than for the control subjects, both for patients with PSC cataract (F = 17.30, p < 0.001) and for those with minimal or no PSC cataract (F = 4.80, p < 0.05). The mean RMS for third-order aberrations was not significantly different for patients with RP than for the control subjects. However, the mean RMS for fourth-order aberrations was significantly larger for the patients with RP than for the control subjects, both for patients with PSC cataract (F = 8.85, p < 0.01) and those with minimal or no PSC cataract (F = 5.07, p < 0.05). There were no significant differences in higher-order aberrations between the patients with RP and the control subjects for a 3-mm pupil diameter.

CONCLUSIONS

Increased higher-order wavefront aberrations were present in patients with RP with and without clinically observable PSC cataracts. The measurement of wavefront aberrations in patients with RP provides an objective and quantitative method for detecting and monitoring disease-related changes in the optics of the eye.

Retinal remodeling during retinal degeneration

Retinal degenerations, regardless of the initiating event or gene defect, often result in a loss of photoreceptors. This formal deafferentation of the neural retina eliminates the intrinsic glutamatergic drive of the sensory retina and, perhaps more importantly, removes coordinated Ca++-coupled signaling to the neural retina. As in other central nervous system degenerations, deafferentation activates remodeling. Neuronal remodeling is the common fate of all photoreceptor degenerations.

Glare sensitivity and professional drivers' safety: a case of rod-cone dystrophy with negative electroretinogram

BACKGROUND

To obtain a driver's licence certain requirements for visual acuity and visual field have to be fulfilled. Mesopic contrast and glare sensitivity are not regularly tested and are not crucial to passing the driving test.

CASE REPORT

We report a case of a 39-year-old professional bus driver whose only complaint was increased glare sensitivity. After he had been involved in four traffic accidents, ophthalmological investigations revealed binocular annular scotomata and night blindness, leading to the diagnosis of rod-cone dystrophy.

DISCUSSION

Enhanced glare sensitivity is a common complaint in elderly people or people with the beginnings of cataract but may also represent an initial symptom of a retinal disorder. It is therefore advisable for traffic safety if drivers with such complaints undergo a complete ophthalmological investigation including visual field testing.

Molecular mechanisms of light-induced photoreceptor apoptosis and neuroprotection for retinal degeneration

Human retinal dystrophies and degenerations and light-induced retinal degenerations in animal models are sharing an important feature: visual cell death by apoptosis. Studying apoptosis may thus provide an important handle to understand mechanisms of cell death and to develop potential rescue strategies for blinding retinal diseases. Apoptosis is the regulated elimination of individual cells and constitutes an almost universal principle in developmental histogenesis and organogenesis and in the maintenance of tissue homeostasis in mature organs. Here we present an overview on molecular and cellular mechanisms of apoptosis and summarize recent developments. The classical concept of apoptosis being initiated and executed by endopeptidases that cleave proteins at aspartate residues (Caspases) can no longer be held in its strict sense. There is an increasing number of caspase-independent pathways, involving apoptosis inducing factor, endonuclease G, poly-(ADP-ribose) polymerase-1, proteasomes, lysosomes and others. Similarly, a considerable number and diversity of pro-apoptotic stimuli is being explored. We focus on apoptosis pathways in our model: light-damage induced by short exposures to bright white light and highlight those essential conditions known so far in the apoptotic death cascade. In our model, the visual pigment rhodopsin is the essential mediator of the initial death signal. The rate of rhodopsin regeneration defines damage threshold in different strains of mice. This rate depends on the level of the pigment epithelial protein RPE65, which in turn depends on the amino acid (leucine or methionine) encoded at position 450. Activation of the pro-apoptotic transcription factor AP-1 constitutes an essential death signal. Inhibition of rhodopsin regeneration as well as suppression of AP-1 confers complete protection in our system. Furthermore, we describe observations in other light-damage systems as well as characteristics of animal models for RP with particular emphasis on rescue strategies. There is a vast array of different neuroprotective cytokines that are applied in light-damage and RP animal models and show diverging efficacy. Some cytokines protect against light damage as well as against RP in animal models. At present, the mechanisms of neuroprotective/anti-apoptotic action represent a "black box" which needs to be explored. Even though acute light damage and RP animal models show different characteristics in many respects, we hope to gain insights into apoptotic mechanisms for both conditions by studying light damage and comparing results with those obtained in animal models. In our view, future directions may include the investigation of different apoptotic pathways in light damage (and inherited animal models). Emphasis should also be placed on mechanisms of removal of dead cells in apoptosis, which appears to be more important than initially recognized. In this context, a stimulating concept concerns age-related macular degeneration, where an insufficiency of macrophages removing debris that results from cell death and photoreceptor turnover might be an important pathogenetic event. In acute light damage, the appearance of macrophages as well as phagocytosis by the retinal pigment epithelium are a consistent and conspicuous feature, which lends itself to the study of removal of cellular debris in apoptosis. We are aware of the many excellent reviews and the earlier work paving the way to our current knowledge and understanding of retinal degeneration, photoreceptor apoptosis and neuroprotection. However, we limited this review mainly to work published in the last 7-8 years and we apologize to all the researchers which have contributed to the field but are not cited here.

Ectopic synaptogenesis during retinal degeneration in the royal college of surgeons rat

Rod photoreceptor-specific mutations cause ectopic synapses to form between cone photoreceptor terminals and rod bipolar cell dendrites in degenerating retinas of rhodopsin transgenic (P347L) pigs and retinal degeneration mice. Since the mutations occur in rod photoreceptor-specific genes in these two models, it is not known if ectopic synaptogenesis occurs specifically due to some rod photoreceptor cell-autonomous properties of a mutation or as a general consequence of photoreceptor degeneration. In the Royal College of Surgeons (RCS) rat, a mutation in the receptor tyrosine kinase gene, Mertk, causes failure of the retinal pigment epithelial (RPE) cells to phagocytose shed photoreceptor outer segments; subsequently, both rod and cone photoreceptors die. The non-phagocytic phenotype of the RCS rat is RPE cell-autonomous and the photoreceptors degenerate secondarily. Here we show that in 35-day-old RCS rats, where a majority of rod and cone photoreceptors remained, rod bipolar cell dendrites had abnormal (flat-contact type) synaptic contacts with rod and cone terminals. Demonstration of ectopic synapses in the RCS rat suggested that ectopic synaptogenesis could occur as a result of photoreceptor degeneration, even when the rods and cones were developmentally normal. This further supported the hypothesis that ectopic synaptogenesis may be a common step in the disease progression of different forms of retinal degeneration that include photoreceptor death as a feature, such as retinitis pigmentosa.

Activated microglia in human retinitis pigmentosa, late-onset retinal degeneration, and age-related macular degeneration

Many gaps exist in our knowledge of human retinal microglia in health and disease. We address the hypothesis that primary death of rod photoreceptors leads to activation of resident microglia in human retinas with retinitis pigmentosa (RP), late-onset retinal degeneration (L-ORD), or age-related macular degeneration (AMD). Regions of ongoing photoreceptor cell death were studied by immunocytochemistry with microglia- and other retinal cell-specific markers. In normal human retinas, quiescent microglia were small, stellate cells associated with inner retinal blood vessels. In retinas with RP, L-ORD, or AMD, numerous activated microglia were present in the outer nuclear layer in regions of ongoing rod cell death. These microglia were enlarged, amoeboid cells that contained rhodopsin-positive cytoplasmic inclusions. We conclude that activated microglia migrate to the outer nuclear layer and remove rod cell debris. In other central nervous system diseases such as stroke, activated microglia phagocytose debris from the primary injury and also secrete molecules that kill nearby normal neurons. By analogy with these diseases, we suggest that microglia activated by primary rod cell death may kill adjacent photoreceptors. Activated microglia may be a missing link in understanding why initial rod cell death in the human diseases RP, L-ORD, and AMD leads to death of the cones that are critical for high acuity daytime vision.

Transplantation of EGF-responsive neurospheres from GFP transgenic mice into the eyes of rd mice

The isolation of stem cells from various regions of the central nervous system has raised the possibility of using them as a donor cell source for cell transplantation, where they offer great promise for repair of the diseased brain, spinal cord, and retina. Here, we have studied the migration, integration, and differentiation of EGF-responsive neurospheres isolated from the brains of green fluorescent protein transgenic mice and transplanted into the eyes of mature rd mice, a model of retinitis pigmentosa. While grafts of freshly isolated postnatal day 8 retina expressed many markers characteristic of mature retina (e.g. rhodopsin, protein kinase C), very few of the grafted cells migrated into host retina. EGF-responsive neurospheres, conversely, readily migrated into and integrated with the remaining host retina, but showed a very limited ability to differentiate into mature retinal neurons. While the progenitor cells used here show remarkable ability to integrate with host retina and develop some attributes of retinal cells, the failure to fully differentiate into retinal cells suggests that they already express some level of terminal commitment that precludes using them to replace lost photoreceptors.

Distribution and density of medium- and short-wavelength selective cones in the domestic pig retina

The topography of medium (M)- and short (S)-wavelength sensitive cone photoreceptors was studied in the domestic pig retina. Antisera specific for M or S opsin as well as cone photoreceptor proteins arrestin and alpha-transducin were used to label cone types. Retinal wholemounts and their blood vessel patterns were drawn and specific regions removed. The wholemounts were immunocytochemically labelled to detect both M and S cones, and the specific regions labelled to detect S cones. Cones were counted in a 1 mm grid pattern, using the drawings as a guide. Pig retina has a high cone density retinal streak extending across the retina covering the optic disc (OD) and horizontal meridian. Densities in the streak are 20,000-35,000 mm(-2). Two higher peaks occur in the streak, one in temporal retina near the OD (39,000 mm(-2)) and the other in nasal retina 5-7 mm from the OD (40,500 mm(-2)). The lowest cone density is in far peripheral inferior retina (7000 mm(-2)). The total number of cones in pig retina is 17-20 million. Both types of cones are found throughout the retina, with S cone percentages ranging from 7.4 to 17.5% in no consistent topographical pattern. S cones have an irregular local distribution which can vary from a regular hexagonal pattern to small clusters of adjacent S cones to small areas lacking S cones. Double-label immunocytochemistry found that virtually all S cone outer segments (OS) contain some M opsin. M cone OS do not label at detectible levels for S opsin. Domestic pig retina is widely available, large, has a high cone density and has two types of cones. This tissue should be an excellent source for biochemical analysis of cone proteins, and for in vitro approaches to understanding cone survival factors.

A novel mutation of the RP1 gene (Lys778ter) associated with autosomal dominant retinitis pigmentosa

BACKGROUND

Besides the three known genes (RHO, RDS/Peripherin, NRL) involved in autosomal dominant retinitis pigmentosa (adRP), a fourth gene, RP1, has been recently identified. Initial reports suggest that mutations in the RP1 gene are the second most frequent cause of adRP. The clinical findings were described in a family with adRP and a novel mutation in the RP1 gene.

METHOD

Index patients from 15 independent families with adRP in which RHO mutations had been excluded in previous examinations were screened for mutations in the RP1 gene by means of direct DNA sequencing. Evaluation of the RP1 phenotype in patients included funduscopy, kinetic perimetry, dark adapted final threshold test, standard electroretinography and, in one case, multifocal electroretinography.

RESULTS

One novel nonsense mutation (Lys778ter) in one of these 15 patients was detected. Cosegregation of the mutation with the disease phenotype could be established in the index patient's family. The phenotype comprises variable expression of clinical disease probably including one case of incomplete penetrance, a onset of symptoms beginning in adulthood, and evidence of regionally varying retinal function loss.

CONCLUSION

The Lys778ter mutation localises inside the critical region harbouring all mutations described so far. The ophthalmic findings support previous observations that variation of disease expression appears as a typical feature of the RP1 phenotype.

The complexities of ocular genetics

The cliché 'a picture is worth a thousand words' is a testament to the power of the visual system in helping us deal with our physical environment. Rarely do perturbations to the visual system, even minor ones, go unnoticed. Major defects in eye development may occur in the absence of systemic problems which threaten health. Ocular anomalies offer a window into many developmental events which would otherwise be difficult to study.

Protection of photoreceptor cells in adult rats from light-induced degeneration by adaptation to bright cyclic light

Light history has been shown to affect the susceptibility of the albino rat retina to the damaging effects of constant light exposure. Retinas of animals raised in relatively bright cyclic light are protected against light-induced degeneration compared with dim-reared animals. These effects were observed in animals raised from birth in bright cyclic light and are part of an adaptive response that protects photoreceptors from stress-induced degeneration. To determine if retinas of adult animals are capable of such adaptive changes or flexibility by switching between different light environments which do not pathologically damage photoreceptor cells, albino rats were maintained in less than 250 lux cyclic light for more than 3 weeks. At 12-13 weeks of age, they were placed into 800 lux cyclic light for 1 week, after which they were exposed to constant illumination of 1500-lux for 1, 3 or 7 days. Retinal function was evaluated by electroretinography and photoreceptor cell death was quantified by measuring outer nuclear layer thickness. After 1 week in bright cyclic light, the retinas were completely protected against 1 day constant light exposure that significantly damaged retinas of animals without 800 lux cyclic light adaptation. Significant protection was also observed in 3 day constant light exposed animals; limited protection occurred after 7 days exposure. These results indicate that the retinas of adult rats adapted to bright cyclic light within certain ranges that did not significantly damage photoreceptor cells are protected from constant light challenge. This phenomenon is a post-developmental response that demonstrates a remarkable plasticity of the retina. The mechanism(s) underlying the ability of this adaptation/flexibility in protecting photoreceptors could involve endogenous molecules that encompass many aspects of retinal cell and molecular biology and physiology. Identification of these molecules may provide insight into the development of therapeutic approaches to treat retinal degeneration.

Pattern electroretinography (PERG) and an integrated approach to visual pathway diagnosis

The pattern electroretinogram (PERG) provides an objective measure of central retinal function, and has become an important element of the author's clinical visual electrophysiological practice. The PERG contains two main components, a positivity at approximately 50ms (P50) and a larger negativity at approximately 95ms (N95). The P50 component is affected by macular dysfunction with concomitant reduction in N95. The PERG therefore complements the Ganzfeld ERG in the assessment of patients with retinal disease. In contrast, the ganglion cell origins of the N95 component allow electrophysiological evaluation of ganglion cell function both in primary disease and in dysfunction secondary to optic nerve disease, where selective loss of N95 can be observed. Both macular dysfunction and optic nerve disease can give abnormalities in the visual evoked cortical potential (VEP), and the PERG thus facilitates more meaningful VEP interpretation. This review addresses the origins and recording of the PERG, and then draws on extensive clinical data from patients with genetically determined retinal and macular dystrophies, other retinal diseases and a variety of optic nerve disorders, to present an integrated approach to diagnosis.

Contrast discrimination deficits in retinitis pigmentosa are greater for stimuli that favor the magnocellular pathway

Luminance contrast discrimination was measured in 14 patients with retinitis pigmentosa (RP) and 14 control observers with normal vision, using steady-pedestal and pulsed-pedestal paradigms [Pokorny, J., & Smith, V. C. (1997). Psychophysical signatures associated with magnocellular and parvocellular pathway contrast gain. Journal of the Optical Society of America A, 14, 2477-2486] to bias performance toward the magnocellular (MC) or parvocellular (PC) pathway, respectively. The aim was to determine the relative effects of retinal degeneration on MC- and PC-pathway function in RP. For five of the RP patients, contrast discrimination thresholds were within normal limits for both the steady-pedestal and pulsed-pedestal paradigms. The other nine RP patients showed threshold elevations for the steady-pedestal paradigm (presumed magnocellular mediation), whereas their thresholds for the pulsed-pedestal paradigm (presumed parvocellular mediation) were within normal limits for all but the two patients who had the most extreme threshold elevations using the steady-pedestal paradigm. A control experiment on four of the RP patients, using a greater number of pedestal contrasts, verified that the patients' thresholds for the pulsed-pedestal paradigm showed the pattern expected for contrast discrimination mediated by the PC pathway. The higher threshold elevations for the steady-pedestal paradigm than for the pulsed-pedestal paradigm indicate that the retinal degeneration that occurs in RP predominantly disrupts contrast discrimination under stimulus conditions that favor the MC pathway.

An immune response after intraocular administration of an adenoviral vector containing a beta galactosidase reporter gene slows retinal degeneration in the rd mouse

BACKGROUND/AIMS

Retinal degenerations are a leading cause of blindness for which there are currently no effective treatments. This has stimulated interest in the investigation of gene therapy strategies for these diseases in a variety of animal models. A number of attempts have been made to prevent photoreceptor loss in the rd mouse model of retinal degeneration using adenoviral vectors containing either a copy of the missing functional gene or a gene encoding either a neurotrophic factor or an antiapoptotic factor. The authors have previously demonstrated that intraocular administration of an adenoviral vector containing a beta galactosidase gene (AV.LacZ) results in an immune response to viral gene products and beta galactosidase. Here the effect of the immune response on retinal degeneration is examined.

METHODS

Juvenile rd mice were injected intravitreally with AV.LacZ and a proportion were depleted of either CD4+ or CD8+ T cells or both. Control animals were injected with PBS. The mice were sacrificed 10 and 20 days post-injection and their eyes embedded in paraffin wax and sectioned.

RESULTS

10 days after intravitreal injection of AV.LacZ, the outer nuclear layer contains an average of 2.5 rows compared with 1.5 in PBS injected animals (p<0.005). The protective effect of AV.LacZ is negated by immune suppression and does not extend beyond 20 days.

CONCLUSION

An immune response to vector and transgene products is able to slow degeneration in the rd mouse. This phenomenon should be taken into account when analysing the degeneration in the rd mouse following gene transfer.

Harnessing the power of forward genetics--analysis of neuronal diversity and patterning in the zebrafish retina

The seven major cell classes of the vertebrate retina are organized with remarkable precision into distinct layers. The appearance of this architecture during embryogenesis raises two questions of general importance. How do individual cell classes acquire their specialized structures and functions if they all originate from a morphologically uniform cell population? What mechanisms are responsible for the formation of such a complex and exact pattern? Recent advances present an opportunity to apply the tools of forward genetic analysis to identify mutations that affect these mechanisms in zebrafish. Molecular characterization will follow, providing insight into the basis of neuronal patterning in the vertebrate CNS.

Ectopic synaptogenesis in the mammalian retina caused by rod photoreceptor-specific mutations

In addition to rod photoreceptor loss, many mutations in rod photoreceptor-specific genes cause degeneration of other neuronal types. Identifying mechanisms of cell-cell interactions initiated by rod-specific mutations and affecting other retinal cells is important for understanding the pathogenesis and progression of retinal degeneration. Here we show in animals with rod and cone degeneration due to mutations in the genes encoding rhodopsin and cGMP phosphodiesterase beta-subunit (PDE-beta) respectively, that rod bipolar cells received ectopic synapses from cones in the absence of rods. Thus, synaptic plasticity links certain rod-specific mutations to retina-wide structural alterations that involve different types of neurons.

Molecular genetics of human retinal disease

The past decade has witnessed extraordinary progress in retinal disease gene identification, the analysis of animal and tissue culture models of disease processes, and the integration of this information with clinical observations and with retinal biochemistry and physiology. During this period over twenty retinal disease genes were identified and for many of these genes there are now significant insights into their role in disease. This review presents an overview of the basic and clinical biology of the retina, summarizes recent progress in understanding the molecular mechanisms of inherited retinal diseases, and offers an assessment of the role that genetics will play in the next phase of research in this area.

Difference between RP2 and RP3 phenotypes in X linked retinitis pigmentosa

AIM

X linked retinitis pigmentosa (XLRP) has two genetic loci known as "RP2" and "RP3". Clinical features reported to differentiate RP2 from RP3 include a higher prevalence of myopia and primary cone dysfunction in RP2, and late onset night blindness and tapetal reflex in RP3. Members from 14 XLRP families were examined in an attempt to verify these differences.

METHODS

16 affected males and 37 females from 14 XLRP families assigned as either RP2 or RP3 by haplotype analysis and/or by heterogeneity analysis were examined. Members of all 14 families who were willing to participate but unavailable for examination were contacted and detailed interviews carried out.

RESULTS

No clear phenotypic differences were found that could be used to reliably differentiate RP2 from RP3 with respect to myopia and onset of night blindness. The tapetal reflex was also found to be present in carriers of both RP2 and RP3.

CONCLUSIONS

XLRP is a heterogeneous class of rod degenerative disorders with no clear phenotypic differentiation between the two genetic loci RP2 and RP3. There is a continuum of clinical presentations which can be seen in both RP2 and RP3, but the features within a given family tend to be consistent. However, interfamilial variability is prevalent leading to a wide range of clinical presentations and more than one abnormal allele at each gene locus cannot be excluded.

Maintenance of retinoid metabolism in human retinal pigment epithelium cell culture

If transplantation of cultured retinal pigment epithelium (RPE) or iris pigment epithelium (IPE) is to be successful in the treatment of ocular disease, it is imperative to demonstrate that these cells can perform all of their necessary metabolic functions. Unfortunately, a critical function of the RPE, retinoid metabolism, is often lost rapidly in culture. We have examined whether or not nonspecific proteolytic enzymes commonly used in cell isolation and serial passaging may be responsible for this loss of function, and we have investigated novel isolation and passaging techniques which can alleviate this loss of retinoid metabolism.RPE cells were obtained from human donor eyes by enzymatic and nonenzymatic methods. Cells were cultured either on control tissue culture inserts or on inserts coated with a layer of thermally responsive poly(N -isopropylacrylamide-co-cinnamoylcarbamidemethylstyrene). Upon confluence, cells were detached either by trypsinization or by lowering dish temperature. Retinoid metabolism of cells was assessed after isolation and culture by incubating membrane fractions with3H-all- trans -retinol. Retinoid metabolism was also measured in freshly isolated IPE, corneal endothelium (CE), an RPE cell line (D407), and two hepatocyte cell lines (Hepa 6 and HepG2). Membrane fractions from cells isolated nonenzymatically or using collagenase/hyaluronidase formed 11- cis -retinol, retinal isomers and retinyl esters. Retinoid metabolism of RPE cells freshly isolated by trypsinization showed no 11- cis -retinal and little 11- cis -retinol formation. Nondamaged cells cultured on thermally responsive surfaces detached in sheets upon temperature change. They showed metabolism similar to that of cells freshly isolated by nonenzymatic means. After trypsinization, confluent cultures dissociated into individual cells, but these cells showed poor retinoid metabolism, including no detectable retinyl esters or 11- cis -retinoid isomers. IPE, CE and Hepa 6 did not show any retinoid metabolism. D407 and HepG2 produced retinals, but not the 11- cis isomer.RPE cells isolated using trypsin lose the ability to form critical intermediates in the visual cycle. Collagenase/hyaluronidase or nonenzymatic cell isolation techniques enable these functions to be maintained. After cell culture, thermally responsive surfaces allow nonenzymatic cell detachment and excellent maintenance of retinoid metabolism.

Retinal rod photoreceptor-specific gene mutation perturbs cone pathway development

Rod-specific photoreceptor dystrophies are complicated by the delayed death of genetically normal neighboring cones. In transgenic (Tg) swine with a rod-specific (rhodopsin) gene mutation, cone photoreceptor physiology was normal for months but later declined, consistent with delayed cone cell death. Surprisingly, cone postreceptoral function was markedly abnormal when cone photoreceptor physiology was still normal. The defect was localized to hyperpolarizing cells postsynaptic to the middle wavelength-sensitive cones. Recordings throughout postnatal development indicated a failure of cone circuitry maturation, a novel mechanism of secondary cone abnormality in rod dystrophy. The results have implications for therapy for human retinal dystrophies and raise the possibility that rod afferent activity plays a role in the postnatal maturation of cone retinal circuitry.

Coherence and the judgment of spatial displacements in retinitis pigmentosa

We used a motion coherence paradigm to test the hypothesis that patients with retinitis pigmentosa (RP) have difficulty discriminating the direction of spatial displacements because of a random loss of motion-sensitive units owing to cone photoreceptor dropout. Minimum (Dmin) and maximum (Dmax) displacement thresholds of patients with typical RP or Usher syndrome were compared with those of age-similar, visually normal subjects. Two-frame random dot cinematograms were used, in which a group of target dots, which comprised 40-100% of the dot array in steps of 20%, were displaced in one of four directions, whereas the non-target dots were randomly repositioned between frames. Reducing the dot coherence in this way increased Dmin and reduced Dmax for both the RP patients and control subjects. Furthermore, the displacement thresholds of the RP patients were displaced laterally from normal along a log coherence axis, consistent with the hypothesis that the patients had a reduced effective (intrinsic) coherence. However, the displacement thresholds of control subjects, when measured at a reduced coherence, did not mimic those of RP patients at full coherence when both groups were tested with a range of dot contrasts and dot areas. These apparently discrepant findings can be reconciled if it is assumed that the patients' effective coherence varies with stimulus visibility.

Retinal degeneration in retinitis pigmentosa and neuronal ceroid lipofuscinosis: An overview

Retinal degeneration is an early consequence of the group of lysosomal storage diseases collectively referred to as the neuronal ceroid lipofuscinoses (NCLs). This review details specialized techniques that have evolved for retinal assessment in patients with hereditary retinal degeneration. A standard ERG protocol is described for assessing rod- and cone-mediated function. Standardization will be crucial for planning and implementing multicenter trials as rational therapeutic intervention becomes available. In recent years, there has been a dramatic increase in knowledge of the molecular biological bases of retinitis pigmentosa and allied retinal degenerations. Rather than attempting a comprehensive summary, this review stresses the concepts of genetic, allelic, and clinical heterogeneity, which have obvious parallels in the NCLs. Many of the mutations that cause retinal degeneration are in genes that encode photoreceptor cascade proteins; others are in genes that encode photoreceptor structural proteins. Recent advances in linking the retinal degeneration slow (RDS) and ATP-binding cassette transporter retina (ABCR) genes to a variety of disease phenotypes will be summarized. Clinical heterogeneity even among family members with the same mutation raises the possibility that modifying factors, either genetic or environmental, could influence the severity of the disease. Here, we focus on vitamin A and docosahexaenoic acid, two potential nutritional modifiers that have received considerable attention in recent years.

Retinitis pigmentosa: defined from a molecular point of view

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

Changes in Bruch's membrane and related structures with age

Age-related macular disease is a major and growing public health burden in developed Caucasian societies, accounting for about 50% of blind registration. Evidence exists that this is an emerging problem in Eastern Asia, although the phenotype appears to differ from that seen in Western society. It is likely that several genes are involved, and that the genes or allelic variants conferring are common. Environment plays a major role in its pathogenesis, and it is believed that genetic susceptibility becomes apparent only if there are sufficient environmental pressures. There is no therapy currently available that will have an impact on the prevalence of blindness from age-related macular disease. It has been shown that visual loss occurs as a reaction to ageing changes in Bruch's membrane, which is interposed between the choriocapillaris and the retinal pigment epithelium. The age changes in all three structures have been partly characterised, and as a consequence, multiple putative pathogenic mechanisms have been proposed. Cross-sectional studies of populations with different genetic background and life styles would serve to prove the importance of inheritance and environment. Molecular genetic analysis of blood from affected sibling pairs from these sources may indicate the relevant genes, the prevalence of which may differ in different communities. Enquiries as to life styles may determine important environmental influences. Examination of donor eyes from these communities may reveal distinctive features that may reflect the variation in genetic predisposition and environmental pressures. It is hoped that the findings from such studies will lead to novel and potentially successful management strategies.