The cone dysfunction syndromes

The cone dystrophies comprise a heterogeneous group of disorders characterised by visual loss, abnormalities of colour vision, central scotomata, and a variable degree of nystagmus and photophobia. They may be stationary or progressive. The stationary cone dystrophies are better described as cone dysfunction syndromes since a dystrophy often describes a progressive process. These different syndromes encompass a wide range of clinical and psychophysical findings. The aim is to review current knowledge relating to the cone dysfunction syndromes, with discussion of the various phenotypes, the currently mapped genes, and genotype-phenotype relations. The cone dysfunction syndromes that will be discussed are complete and incomplete achromatopsia, oligocone trichromacy, cone monochromatism, blue cone monochromatism, and Bornholm eye disease. Disorders with a progressive cone dystrophy phenotype will not be discussed.

The rod opsin pigments from two marsupial species, the South American bare-tailed woolly opossum and the Australian fat-tailed dunnart

Rod visual pigment genes have been studied in a wide range of vertebrates including a number of mammalian species. However, no marsupials have yet been examined. To correct this omission, we have studied the rod pigments in two marsupial species, the nocturnal and frugivorous bare-tailed woolly opossum, Caluromys philander, from Central and South America, and the arhythmic and insectivorous fat-tailed dunnart, Sminthopsis crassicaudata, from Australia. Phylogenetic analysis establishes that the cloned opsin sequences are orthologues of rod opsin genes from other vertebrate species. The deduced amino acid sequences show that both possess glutamate at residue 122, a feature of rod opsins, and the corresponding gene follows the typical vertebrate rod opsin pattern of five exons separated by four introns. Compared to other vertebrates, a stretch of five residues near the C-terminus is deleted in the rod opsin of both marsupials and all eutherian mammals. From microspectrophotometric measurements, the pigments in the two species show an 8 nm difference in peak absorbance; the molecular basis for this spectral shift is discussed and two candidate substitutions are identified.

Dominant cone and cone-rod dystrophies: functional analysis of mutations in retGC1 and GCAP1

The regulation of cGMP levels is central to the normal process of phototransduction in both cone and rod photoreceptor cells. Two of the proteins involved in this process are the enzyme, retinal guanylate cyclase (retGC), and its activating protein (GCAP) through which activity is regulated via changes in cellular Ca2+ levels. Dominant cone-rod dystrophies arising from changes in retGC1 are essentially restricted to mutations in codon 838 and result in the replacement of a conserved arginine residue with either cysteine, histidine or serine. In all three cases, the effect of the substitution on the in vitro cyclase activity is a loss of Ca2+ sensitivity arising from an increased stability of the coiled-coil domain of the protein dimer and retention of cyclase activity. In contrast, mutations in the Ca2+-coordinating EF hands of GCAP1 result in dominant cone dystrophy; the consequences of these mutations is a reduced ability of the mutant protein to regulate retGC activity in response to changes in Ca2+ levels. Functionally therefore, the retGC2 and GCAP2 mutations are similar in reducing the feedback inhibition of Ca2+ on cyclase activity and thereby on cGMP levels in the photoreceptors.

Cone dystrophy phenotype associated with a frameshift mutation (M280fsX291) in the alpha-subunit of cone specific transducin (GNAT2)

AIM

To describe the phenotype of a three generation consanguineous Pakistani family containing six individuals with autosomal recessive cone dystrophy caused by mutation in GNAT2.

METHODS

Five of the six affected individuals underwent an ophthalmological examination, electrodiagnostic testing, fundus photography, autofluorescence imaging, and detailed psychophysical testing.

RESULTS

All five examined patients had a history of nystagmus from infancy, photophobia, defective colour vision, and poor visual acuity. The nystagmus in three of the individuals had lessened with time. Fundus examination revealed an abnormal foveal appearance, without frank atrophy or pigmentation. Electroretinography (ERG) revealed absent ISCEV cone flicker ERGs with some preservation of responses to short wavelength stimulation. Rod ERGs showed no definite abnormality, but maximal (mixed rod-cone) response a-wave amplitudes were mildly subnormal. Rudimentary residual colour vision was detected in three individuals. There is clinical evidence of progressive visual acuity reduction in two older individuals.

CONCLUSION

Mutation in the alpha-subunit of cone specific transducin (GNAT2) is characterised by an infantile onset cone dystrophy. Some affected individuals may show deterioration of visual acuity with time.

Divergent mechanisms for the tuning of shortwave sensitive visual pigments in vertebrates

Of the four classes of vertebrate cone visual pigments, the shortwave-sensitive SWS1 class shows the shortest lambda(max) values with peaks in different species in either the violet (390-435 nm) or ultraviolet (around 365 nm) regions of the spectrum. Phylogenetic evidence indicates that the ancestral pigment was probably UV-sensitive (UVS) and that the shifts between violet and UV have occurred many times during evolution. This is supported by the different mechanisms for these shifts in different species. All visual pigments possess a chromophore linked via a Schiff base to a Lys residue in opsin protein. In violet-sensitive (VS) pigments, the Schiff base is protonated whereas in UVS pigments, it is almost certainly unprotonated. The generation of VS from ancestral UVS pigments most likely involved amino acid substitutions in the opsin protein that serve to stabilise protonation. The key residues in the opsin protein for this are at sites 86 and 90 that are adjacent to the Schiff base and the counterion at Glu113. In this review, the different molecular mechanisms for the UV or violet shifts are presented and discussed in the context of the structural model of bovine rhodopsin.

Expression of PRPF31 mRNA in patients with autosomal dominant retinitis pigmentosa: a molecular clue for incomplete penetrance?

PURPOSE

To investigate whether the incomplete penetrance phenotype characteristic of adRP families linked to chromosome 19q13.4 (RP11) with mutations in the PRPF31 gene is due to differentially expressed wild-type alleles in symptomatic and asymptomatic individuals.

METHODS

Real-time quantitative RT-PCR was performed on RNA from lymphoblastoid cell lines derived from a large adRP family (RP856/AD5) that segregates an 11bp deletion in exon 11 of PRPF31. The mRNA levels from only the wild-type allele of PRPF31 were assayed using a probe designed across the deletion. The Mann-Whitney U test was used to compare the median mRNA copy numbers of the symptomatic with the asymptomatic carriers of the mutant PRPF31 allele. The PRPF31 protein levels from symptomatic and asymptomatic individuals were also assayed by Western blot analysis using an antibody specific to the wild-type PRPF31 protein.

RESULTS

The use of cell lines was validated by the observation that cell transformation did not alter PRPF31 expression in the cell lines compared with nucleated blood cells and donor retinas. A significant difference in wild-type PRPF31 mRNA levels was observed between symptomatic and asymptomatic individuals (P < 0.001) and was supported by Western blot analysis of the PRPF31 protein.

CONCLUSIONS

Partial penetrance in RP11 could be due to the coinheritance of a PRPF31 gene defect and a low-expressed wild-type allele. This study revealed a potential avenue for future therapy in that it appears the moderate overexpression of wild-type PRPF31 may prevent clinical manifestation of the disease.

The genetics of inherited macular dystrophies

The inherited macular dystrophies comprise a heterogeneous group of disorders characterised by central visual loss and atrophy of the macula and underlying retinal pigment epithelium (RPE). The different forms of macular degeneration encompass a wide range of clinical, psychophysical and histological findings. The complexity of the molecular basis of monogenic macular disease is now beginning to be elucidated with the identification of many of the disease-causing genes. Age related macular degeneration (ARMD), the leading cause of blind registration in the developed world, may also have a significant genetic component to its aetiology. Genes implicated in monogenic macular dystrophies are good candidate susceptibility genes for ARMD, although to date, with the possible exception of ABCA4, none of these genes have been shown to confer increased risk of ARMD. The aim of this paper is to review current knowledge relating to the monogenic macular dystrophies, with discussion of currently mapped genes, chromosomal loci and genotype-phenotype relationships. Inherited systemic disorders with a macular dystrophy component will not be discussed.

Guanylate cyclase activating proteins, guanylate cyclase and disease

A range of cone and cone-rod dystrophies (CORD) have been observed in man, caused by mutations in retinal guanylate cyclase 1 (RetGC1) and guanylate cyclase activating protein 1 (GCAP 1). The CORD causing mutations in RetGC1 are located at a mutation "hot spot" within the dimerisation domain, where R838 is the key residue. Three disease causing mutations have been found in human GCAP1, resulting in cone or cone-rod degeneration. All three mutations are dominant in their effect although the mechanism by which the P50L mutation exerts its influence remains unclear although it might act due to a haplo-insufficiency, arising from increased susceptibility to protease activity and increased thermal instability. In contrast, loss of Ca2+ sensitivity appears to be the main cause of the diseased state for the Y99C and E155G mutations. The cone and cone-rod dystrophies that are caused by mutations in RetGC1 or GCAP1 arise from a perturbation of the delicate balance of Ca2+ and cGMP within the photoreceptor cells and it is this disruption that is believed to cause cell death. The diseases caused by mutations in RetGC1 and GCAP1 prominently affect cones, consistent with the higher concentrations of these proteins in cone cells.

Genetic linkage analysis of a novel syndrome comprising North Carolina-like macular dystrophy and progressive sensorineural hearing loss

AIM

To characterise the phenotype and identify the underlying genetic defect in a family with deafness segregating with a North Carolina-like macular dystrophy (NCMD).

METHODS

Details of the family were obtained from the Moorfields Eye Hospital genetic clinic database and comprised eight affected, four unaffected members, and two spouses. Pedigree data were collated and leucocyte DNA extracted from venous blood. Positional candidate gene and genetic linkage strategies utilising polymerase chain reaction (PCR) based microsatellite marker genotyping were performed to identify the disease locus.

RESULTS

The non-progressive ocular phenotype shared similarities with North Carolina macular dystrophy. Electro-oculography and full field electroretinography were normal. Progressive sensorineural deafness was also present in all affected individuals over the age of 20 years. Hearing was normal in all unaffected relatives. Haplotype analysis indicated that this family is unrelated to previously reported families with NCMD. Genotyping excluded linkage to the MCDR1 locus and suggested a potential novel disease locus on chromosome 14q (Z=2.92 at theta=0 for marker D14S261).

CONCLUSION

The combination of anomalies segregating in this family represents a novel phenotype. This molecular analysis indicates the disease is genetically distinct from NCMD.

An early-onset autosomal dominant macular dystrophy (MCDR3) resembling North Carolina macular dystrophy maps to chromosome 5

PURPOSE

To characterize the phenotype of an autosomal dominant macular dystrophy and identify the chromosomal locus.

METHODS

Thirteen members of a four-generation, nonconsanguineous British family were examined clinically and also underwent automated perimetry, fundus fluorescein angiography, and fundus autofluorescence imaging. After informed consent was obtained, blood samples were taken for DNA extraction, and genetic linkage analysis was performed.

RESULTS

The retinal changes have an early age of onset and are confined to the macular region. The macular abnormalities vary from mild retinal pigment epithelium (RPE) pigmentary change to atrophy. Drusen-like deposits are present to various degrees and are characteristic of the phenotype. Subretinal neovascular membrane (SRNVM) is an established complication. Genetic linkage analysis established linkage to chromosome 5, region p13.1-p15.33 with a maximum LOD score of 3.61 at a recombination fraction of 0.00 for marker D5S630. The locus for this autosomal dominant macular dystrophy lies between flanking markers D5S1981 and D5S2031.

CONCLUSIONS

A novel locus has been identified for early-onset autosomal dominant macular dystrophy on chromosome 5.

An autosomal dominant bull's-eye macular dystrophy (MCDR2) that maps to the short arm of chromosome 4

PURPOSE

To describe the phenotype of an autosomal dominant macular dystrophy and identify the chromosomal locus.

METHODS

Eleven members of a five-generation, nonconsanguineous British family were examined clinically and also underwent automated perimetry, electrodiagnostic testing, fundus fluorescein angiography, and fundus autofluorescence imaging. Blood samples were taken for DNA extraction and linkage analysis was performed.

RESULTS

The phenotype is characterized by bull's-eye macular dystrophy first evident in the first or second decade of life. There is mild visual impairment, central scotomata, and electrophysiological testing indicates that most affected individuals have disease confined to the central retina but older subjects have more widespread rod and cone abnormalities, demonstrated by flash ERG. Genetic linkage analysis established linkage to chromosome 4 at p15.2-16.3 with a maximum lod score of 3.03 at a recombination fraction of 0.00 for marker D4S391. The locus for this autosomal dominant macular dystrophy lies between flanking markers D4S3023 and D4S3022, and overlaps the Stargardt 4 locus.

CONCLUSIONS

A new locus was identified for a bull's-eye macular dystrophy on the short arm of chromosome 4.

Genomic organisation and alternative splicing of human RIM1, a gene implicated in autosomal dominant cone-rod dystrophy (CORD7)

A mutation has been identified in the Rab3A-interacting molecule (RIM1) gene in CORD7, an autosomal dominant cone-rod dystrophy that localises to chromosome 6q14. The G to A point mutation results in an Arg844His substitution in the C(2)A domain of the protein that segregates with disease. This mutation is absent in over 200 control chromosomes, indicating that it is not a common polymorphism, and the almost complete sequence conservation of the C(2)A domain between human and rat RIM1 is consistent with a disease role for the change. RIM1 is expressed in brain and photoreceptors of the retina where it is localised to the pre-synaptic ribbons in ribbon synapses. The RIM1 gene is composed of at least 35 exons, spans 577 kb of genomic DNA, and encodes a protein of up to 1693 residues. The transcript shows extensive alternative splicing involving exons 17, 21-26 and 28-30.

Disease mechanism for retinitis pigmentosa (RP11) caused by mutations in the splicing factor gene PRPF31

This study investigates the functional consequences of two mutations, A194E and A216P, in the splicing factor gene PRPF31 linked to autosomal dominant retinitis pigmentosa (RP11). Using a yeast complementation assay, we demonstrate that introduction of the human A216P mutation into the yeast orthologue PRP31p results in only partial rescue of growth at the restrictive temperature, indicating that splicing function is not fully restored. An in vivo assay of splicing function in human cells using a bovine rod opsin splicing template did not detect any defect in splicing efficiency or accuracy attributable to either mutation, suggesting that neither has a dominant negative effect on splicing. However, western analysis and immunofluorescence microscopy of mammalian cells transfected with PRPF31 revealed that both mutations substantially hinder translocation of the protein into the nucleus. The overall effect may thus be an insufficiency in splicing function, which is revealed only under conditions of elevated splicing demand. With the need to replenish disc proteins on a daily basis, such conditions will exist in rod photoreceptors and this may underlie the disease pathology.

The molecular mechanism for the spectral shifts between vertebrate ultraviolet- and violet-sensitive cone visual pigments

The short-wave-sensitive (SWS) visual pigments of vertebrate cone photoreceptors are divided into two classes on the basis of molecular identity, SWS1 and SWS2. Only the SWS1 class are present in mammals. The SWS1 pigments can be further subdivided into violet-sensitive (VS), with lambda(max) (the peak of maximal absorbance) values generally between 400 and 430 nm, and ultraviolet-sensitive (UVS), with a lambda(max)<380 nm. Phylogenetic evidence indicates that the ancestral pigment was UVS and that VS pigments have evolved separately from UVS pigments in the different vertebrate lineages. In this study, we have examined the mechanism of evolution of VS pigments in the mammalian lineage leading to present day ungulates (cow and pig). Amino acid sequence comparisons of the UVS pigments of teleost fish, amphibia, reptiles and rodents show that site 86 is invariably occupied by Phe but is replaced in bovine and porcine VS pigments by Tyr. Using site-directed mutagenesis of goldfish UVS opsin, we have shown that a Phe-86-->Tyr substitution is sufficient by itself to shift the lambda(max) of the goldfish pigment from a wild-type value of 360 nm to around 420 nm, and the reverse substitution of Tyr-86-Phe into bovine VS opsin produces a similar shift in the opposite direction. The substitution of this single amino acid is sufficient to account therefore for the evolution of bovine and porcine VS pigments. The replacement of Phe with polar Tyr at site 86 is consistent with the stabilization of Schiff-base protonation in VS pigments and the absence of protonation in UVS pigments.

Mapping of a novel locus for achromatopsia (ACHM4) to 1p and identification of a germline mutation in the alpha subunit of cone transducin (GNAT2)

OBJECTIVE

To determine the molecular basis for achromatopsia using autozygosity mapping and positional candidate gene analysis.

DESIGN AND METHODS

A large consanguineous Pakistani family containing six subjects with autosomal recessive complete achromatopsia was ascertained. After excluding linkage to the two known achromatopsia genes (CNGA3 and CNGB3), a genome wide linkage screen was undertaken.

RESULTS

Significant linkage was detected to a 12 cM autozygous segment between markers D1S485 and D1S2881 on chromosome 1p13. Direct sequence analysis of the candidate gene GNAT2 located within this interval identified a frameshift mutation in exon 7 (c842_843insTCAG; M280fsX291) that segregated with the disease.

CONCLUSIONS

The GNAT2 gene codes for cone alpha-transducin, the G protein that couples the cone pigments to cGMP-phosphodiesterase in phototransduction. Although cone alpha-transducin has a fundamental role in cone phototransduction, mutations in GNAT2 have not been described previously. Since mutations in the CNGA3 gene may cause a variety of retinal dystrophies (complete and incomplete achromatopsia and progressive cone dystrophy), GNAT2 mutations may also prove to be implicated in other forms of retinal dystrophy with cone dysfunction.

Characterisation of two genes for guanylate cyclase activator protein (GCAP1 and GCAP2) in the Japanese pufferfish, Fugu rubripes

cDNA and genomic clones encoding guanylate cyclase activating proteins (GCAP1 and GCAP2) in the Japanese puffer fish (Fugu rubripes) were identified by probing, respectively, a retinal cDNA library and a whole genomic cosmid library with human GCAP1 and GCAP2 cDNA probes. Clones were identified as GCAP1 and GCAP2 on the basis of amino acid identity with the equivalent frog sequences and their placement into GCAP1 and GCAP2 clades within a GCAP phylogenetic tree. The Fugu genes have an identical four exon/three intron structure to GCAP1 and GCAP2 genes from other vertebrates but the introns are smaller, with the result that the four exons spread over approximately 1 kb of DNA in each case. The two genes are separated on to separate cosmids. However, the results of Southern analysis of the cosmids and of genomic DNA are consistent with a tail-to-tail gene arrangement, as in other species, but with a surprisingly large intergenic separation of around 18.7 kb. Recombinant Fugu GCAP1 failed to activate human retinal guanylate cyclase (retGC) in vitro although CD spectroscopy shows that the protein is folded with a similar secondary structure to that of human GCAP1. The failure to activate may be due therefore to a lack of molecular compatibility in this heterologous assay system.

Spectral tuning and evolution of short wave-sensitive cone pigments in cottoid fish from Lake Baikal

The cottoid fishes of Lake Baikal in eastern Siberia provide a unique opportunity to study the evolution of visual pigments in a group of closely related species exposed to different photic environments. Members of this species flock are adapted to different depth habitats down to >1000 m, and both the rod and cone visual pigments display short wave shifts as depth increases. The blue-sensitive cone pigments of the SWS2 class cluster into two species groups with lambda(max) values of 450 and 430 nm, with the pigment in Cottus gobio, a cottoid fish native to Britain, forming a third group with a lambda(max) of 467 nm. The sequences of the SWS2 opsin gene from C. gobio and from two representatives of the 450 and 430 nm Baikal groups are presented. Approximately 6 nm of the spectral difference between C. gobio and the 450 nm Baikal group can be ascribed to the presence of a porphyropsin/rhodopin mixture in C. gobio. Subsequent analysis of amino acid substitutions by site-directed mutagenesis demonstrates that the remainder of the shift from 461 to 450 nm arises from a Thr269Ala substitution and the shift from 450 to 430 nm at least partly from Thr118Ala and Thr118Gly substitutions. The underlying adaptive significance of these substitutions in terms of spectral tuning and signal-to-noise ratio is discussed.

Vitreous-induced modulation of integrins in retinal pigment epithelial cells: effects of fibroblast growth factor-2

Growth in the presence of vitreous results in transformation of human RPE cells from an epithelioid to a fibroblast-like appearance and leads to an elevation of the expression of alpha(5) and alpha(2) integrins, while the level of alpha(3) integrin is reduced. These changes are inhibited by the presence of FGF-2. Vitreous treatment increases mobility, as does antibody neutralization of FGF-2 or antibody blockade of FGF receptors. The vitreous-induced rise in mobility depends on an increase in alpha(5) integrin expression since it is inhibited by anti-alpha(5) integrin antibodies. Expression of alpha(5) integrin as a result of infection of RPE cells with an alpha(5) integrin-encoding adenovirus induced morphological transformation and an increase in mobility similar to that seen with vitreous. It is concluded that a decrease in FGF-2 plays an important role in vitreous-induced alterations of RPE cell morphology, integrin expression and mobility. High FGF-2 levels prevent at least some of the increased mobility of RPE cells induced by vitreous. This is mediated via extracellular FGF-2 binding to FGF receptor(s) since antibodies to FGF-2 or to its receptor(s) mimic the effects of vitreous. Changes in mobility and morphology involve altered alpha(5) integrin expression since mobility is blocked by antibodies against these proteins while elevated alpha(5) integrin expression increases mobility and leads to morphological changes.

Autosomal dominant cone-rod dystrophy with mutations in the guanylate cyclase 2D gene encoding retinal guanylate cyclase-1

OBJECTIVE

To describe the phenotype in 4 families with dominantly inherited cone-rod dystrophy, 1 with an R838C mutation and 1 with an R838H mutation in the guanylate cyclase 2D (GUCY2D) gene encoding retinal guanylate cyclase-1.

METHODS

Psychophysical and electrophysiological evaluation and confocal laser scanning ophthalmoscopic imaging was performed on 10 affected members of 4 British families.

RESULTS

Although subjects had lifelong poor vision in bright light, a major reduction in visual acuity did not occur in most of them until after their late teens. Fundus abnormalities were confined to the central macula, and increasing central atrophy was noted with age. Increased background autofluorescence was observed surrounding the central atrophic area. Electrophysiological testing revealed a marked loss of cone function with only minimal rod involvement, even in older subjects. Photopic and scotopic static perimetry demonstrated central and peripheral cone-mediated threshold elevations with midperipheral sparing.

CONCLUSION

The phenotype associated with autosomal dominant cone-rod dystrophy with either an R838C or R838H mutation in GUCY2D is distinctive, with predominantly cone system involvement. There is some variation in severity within the 3 families with the R838C mutation.

CLINICAL RELEVANCE

Families with the R838C or R838H mutation have a much milder phenotype than the family previously described that had 2 sequence changes, E837D and R838S, in GUCY2D.

Vision in the ultraviolet

Sensitivity to ultraviolet light (UV) is achieved by photoreceptors in the eye that contain a class of visual pigments maximally sensitive to light at wavelengths <400 nm. It is widespread in the animal kingdom where it is used for mate choice, communication and foraging for food. UV sensitivity is not, however, a constant feature of the visual system, and in many vertebrate species, the UV-sensitive (UVS) pigment is replaced by a violet-sensitive (VS) pigment with maximal sensitivity between 410 and 435 nm. The role of protonation of the Schiff base-chromophore linkage and the mechanism for tuning of pigments into the UV is discussed in detail. Amino acid sequence analysis of vertebrate VS/UVS pigments indicates that the ancestral pigment was UVS, with loss of UV sensitivity occurring separately in mammals, amphibia and birds, and subsequently regained by a single amino acid substitution in certain bird species. In contrast, no loss of UV sensitivity has occurred in the UVS pigments of insects.

The molecular basis for spectral tuning of rod visual pigments in deep-sea fish

Most species of deep-sea fish possess of a rod-only retina with a pigment that is generally shortwave shifted in λmax towards the blue region of the spectrum. In addition, the λmax values of different species tend to cluster at particular points in the spectrum. In this study, the rod opsin gene sequences from 28 deep-sea fish species drawn from seven different Orders are compared. The λmax values of the rod pigments vary from approximately 520 nm to &lt;470 nm, with the majority lying between 490 nm and 477 nm. The 520 nm pigment in two species of dragon fish is associated with a Phe261Tyr substitution, whereas the shortwave shifts of the pigments in the other 26 species are accountable by substitutions at a further eight sites (83, 122, 124, 132, 208, 292, 299 and 300). Clustering of λmax values does not, however, involve a common subset of these substitutions in the different species. A phylogenetic analysis predicts that the pigment in the ancestral species would have had a λmax of approximately 480 nm. A total of 27 changes is required to generate the pattern of substitutions seen in the different species, with many sites undergoing multiple changes.