Autosomal dominant retinitis pigmentosa (ADRP): a rhodopsin mutation in a Scottish family

A review of research to elucidate the causes of the generalized progressive retinal atrophies

Progressive retinal atrophy (PRA) is a leading hereditary cause of blindness in pedigree dogs as is its counterpart retinitis pigmentosa (RP) in humans. PRA shows genetic heterogeneity, as does RP, with several distinct forms already recognized and several more remaining to be investigated. Progress in molecular genetics has allowed the identification of the gene mutation responsible for an early onset form of PRA in the Irish setter, classified as rod-cone dysplasia type 1. The gene involved is the beta-subunit of cyclic guanosine monophosphate phosphodiesterase which encodes a protein of the visual transduction cascade. Investigation of this gene in other breeds of dog with PRA has failed to find further breeds with the same mutation. Other genes that have been investigated include those encoding other proteins in the visual transduction cascade and for photoreceptor specific structural proteins. Further disease causing mutations have not yet been identified. Recently, developments in the mapping of the canine genome have produced sufficient markers to allow preliminary mapping of PRA genes. Already linkage to the most common form of PRA, progressive rod-cone degeneration (prcd), has been established. prcd occurs in poodles, cocker spaniels and Labrador retrievers and possibly other breeds. The prcd-linked marker should enable development of a DNA-based test for the disease locus and facilitate identification of the actual disease causing gene mutation. Over the next few years we can look forward to the identification of several more PRA-causing gene mutations. This article will review research that seeks to characterize PRA in the dog, identify the responsible gene mutations, and elucidate the disease processes involved.

Linkage of a medium sized Scottish autosomal dominant retinitis pigmentosa family to chromosome 7q

Retinitis pigmentosa is a group of hereditary retinopathies which is both clinically and genetically heterogeneous. Autosomal dominant (ADRP), autosomal recessive (ARRP), and X linked recessive (XLRP), as well as digenic forms of inheritance have been reported. ADRP has been linked to 3q, 6p, 7p, 7q, 8cen, 17p, 17q, and 19q. Three unrelated ADRP families have been reported to show linkage to 7q. We tested a Scottish ADRP family with microsatellite markers mapping within the 7q31-q35 region, and found three markers (D7S487, D7S514, D7S530) showing statistically significant evidence of linkage. A maximum two point lod score of 3.311 at 0% recombination was obtained for D7S514.

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.

Na-Ca + K exchanger and Ca2+ homeostasis in retinal rod outer segments: Inactivation of the Ca2+ efflux mode and possible involvement of intracellular Ca2+ stores in Ca2+ homeostasis

Inactivation of the Ca2+ extrusion mode of the retinal rod Na- Ca + K exchanger is suggested to be the mechanism that prevents lowering of cytosolic free Ca2+ to < 1 nM when rod cells are saturated for a prolonged time under bright light conditions. Under these conditions, Ca2+ fluxes across disk membranes can contribute significantly to Ca2+ homeostasis in rods.

Nuclear magnetic resonance studies on the structure and function of rhodopsin

Magic angle spinning (MAS) NMR methods provide a means of obtaining high resolution structural data on rhodopsin and its photoin termediates. Current work has focused on the structure of the retinal chromophore and its interactions with surrounding protein charges. The recent development of MAS NMR methods for measuring internuclear distances with a resolution of ∼0.2 will complement diffraction methods for addressing key mechanistic questions.

Glutamate accumulation in the photoreceptor-presumed final common path of photoreceptor cell death

Genetic abnormalities of three factors related to the photoreceptor mechanism have been reported in both animal models and humans. Apoptotic mechanism has also been suggested as a final common pathway of photoreceptor cell death. Our findings of increased level of glutamate in photoreceptor cells in rds mice suggest that amino acid might mediate between these two pathological mechanisms.

Unique lipids and unique properties of retinal proteins

Amino-terminal heteroacylation has been identified in retinal proteins including recoverin and α subunit of G-protein, transducin. The tissue-specific modification seems to mediate not only a proteinmembrane interaction but also a specific protein-protein interaction. The mechanism generating the heterogeneity and its physiological role are still unclear, but an interesting idea for the latter postulates a fine regulation of the signal transduction pathway by distinct N-acyl groups.

Further insight into the structural and regulatory properties of the cGMP-gated channel

Recent studies from several different laboratories have provided further insight into structure-function relationships of cyclic nucleotide-gated channel and in particular the cCMPgated channel of rod photoreceptors. Site-directed mutagenesis and rod-olfactory chimeria constructs have defined important amino acids and peptide segments of the channel that are important in ion blockage, ligand specificity, and gating properties. Molecular cloning studies have indicated that cyclic nucleotide-gated channels consist of two subunits that are required to reproduce the properties of the native channels. Biochemical analysis of the cGMP-gated channel of rodcells have indicated that the 240 kDa protein that co-purifies with the 63 kDa channel subunit contains both the previously cloned second subunit of the channel and a glutamic acid-rich protein. The regulatory properties of the cGMP-gated channel from rod cells has also been studied in more detail. Studies indicate that the beta subunit of the cGMP-gated channel of rod cells contains the binding site for calmodulin. Interaction of calmodulin with the channel alters the apparent affinity of the channel for cGMP in all in vitro systems that have been studied. The significance of these recent studies are discussed in relation to the commentaries on the target article.

Unsolved issues in S-modulin/recoverin study

S-Modulin is a frog homolog of recoverin. The function and the underlying mechanism of the action of these proteins are now understood in general. However, there remain some unsolved issues including; two distinct effects of S-modulin; Ca2+-dependent binding of S-modulin to membranes and a possible target protein; S-modulin-like proteins in other neurons. These issues are considered in this commentary.

Mechanisms of photoreceptor degenerations

The candidate gene approach has identified many causes of photoreceptor rod cell death in retinitis pigmentosa. Some mutations lead to increased cyclicGMP concentrations in rods. Rod photoreceptors are also particularly susceptible to some mutations in housekeeping genes. Although many more cases of macular degeneration than retinitis pigmentosa occur each year, there is much less known about both genetic and sporadic forms of this disease.

Reduced cytoplasmic calcium concentration may be both necessary and sufficient for photoreceptor light adaptation

Light adaptation is modulated almost exclusively by changes in intracellular Ca2+ concentration, and other Ca2+-independent mechanisms are likely to play only a minor role. Changes in Ca2+i may be not only necessary for light adaptation to take place but sufficient to cause it.

The genetic kaleidoscope of vision

Site-specific phenotypic effects of the 73 known alleles in the rhodopsin gene that cause retinal degeneration are difficult to interpret because most alleles are documented in only one case or one family, which means variation in effects could actually arise from interactions with other loci. However, sample sizes necessary to detect epistatic interaction may place an answer to this question beyond our grasp.

Evidence that the type I adenylyl cyclase may be important for neuroplasticity: Mutant mice deficient in the gene for type I adenylyl cyclase show altered behavior and LTP

The regulatory properties of the neurospecific, type I adenylyl cyclase and its distribution within brain have suggested that this enzyme may be important for neuroplasticity. To address this issue, the murine, Ca2+ -stimulated adenylyl cyclase (type I), was inactivated by targeted mutagenesis. Ca2+ -stimulated adenylyl cyclase activity was reduced 40% to 60% in the hippocampus, neocortex, and cerebellum. Long term potentiation in the CA1 region of the hippocampus from mutants was perturbed relative to controls. Both the initial slope and maxim um extent of changes in synaptic response were reduced. Although mutant mice learned to find a hidden platform normally in the Morris water task, they did not display a preference for the region where the platform had been when it was removed. The behavioral phenotype of these mice is very similar to that exhibited by mice which have been surgically lesioned in the hippocampus. These results indicate that disruption of the gene for the type I adenylyl cyclase produces changes in spatial memory and indicate that the cAMP signal transduction pathway may play an important role for synaptic plasticity.

Calcium/calmodulin-sensitive adenylyl cyclase as an example of a molecular associative integrator

Evidence suggests that the Ca2+/calmodulin-sensitive adenylyl cyclase may play a key role in neural plasticity and learning in Aplysia, Drosophila, and mammals. This dually-regulated enzyme has been proposed as a possible site of stimulus convergence during associative learning. This commentary discusses the evidence that is required to demonstrate that a protein in a second messenger cascade actually functions as a molecular site of associative integration. It also addresses the issue of how a dually-regulated protein could contribute to the temporal pairing requirements of classical conditioning: that relationship between stimuli display both temporal contiguity and predictability.

The key to rhodopsin function lies in the structure of its interface with transducin

Light activated rhodopsin functions by catalyzing the exchange of GTP for GDP on numerous copies of transducin. Peptide mapping has shown that at least six regions, three on rhodopsin and three on the transducin alpha subunit, are involved in the active interface between the two proteins. The most informative structural studies of rhodopsin should include focus on the transducin interaction.

Retinal photoreceptor dystrophies LI. Edward Jackson Memorial Lecture

PURPOSE

To assess the state of knowledge of photoreceptor dystrophies.

METHODS

The current literature concerning photoreceptor dystrophies is reviewed, and their potential impact on concepts of pathogenesis of disease and clinical practice is assessed.

RESULTS

As a result of cooperative investigative work between researchers in various disciplines, major advances in the classification of retinal photoreceptor dystrophies have been made. Until recently, classification of retinal dystrophies was based on clinical observation alone, and it was evident that this method was imprecise and of limited value. Largely through the work of molecular biologists, it has been shown that diseases clinically indistinguishable from one another may be a result of mutations on a variety of genes; conversely, different mutations on a single gene may give rise to a variety of phenotypes. It is reassuring that it is possible to generate concepts as to potential pathogenetic mechanisms that exist in retinal dystrophies in light of this new knowledge. More important for the clinician is the potential impact on clinical practice. There is as yet no therapy by which the course of most of these disorders can be modified. However, there is a considerable body of work in which therapeutic intervention is being explored, and many researchers now see treatment as a justifiable objective of their work.

CONCLUSIONS

Knowledge of the causative mutation is of value to the clinician in that it provides a precise diagnosis and allows the distribution of the abnormal gene to be documented fully within a family. To take full advantage of the opportunities provided by current research, clinical practice will have to be modified, particularly if therapy can be justified.

Rhodopsin mutations in a Scottish retinitis pigmentosa population, including a novel splice site mutation in intron four

Retinitis pigmentosa (RP) is the name given to a group of disorders, both clinically and genetically heterogeneous, that primarily affect the photoreceptor function of the eye. Mutations in the genes encoding for rhodopsin, RDS-peripherin, or the beta subunit of the cGMP phosphodiesterase enzyme can be responsible for the phenotype. In this study the rhodopsin gene has been screened for mutations in a panel of RP individuals and five different sequence changes have been detected to date in three dominantly inherited and two unclassified families. One of these, a base substitution in the 3'UTR, has not yet been confirmed as disease specific, while three missense substitutions have previously been reported and are likely to be responsible for the phenotype. The fifth change, a base substitution at the intron 4 acceptor splice site, represents a novel mutation and is assumed to be the causative mutation.

Identification of novel rhodopsin mutations responsible for retinitis pigmentosa: implications for the structure and function of rhodopsin

Ten rhodopsin mutations have been found in a screen of 282 subjects with retinitis pigmentosa (RP), 76 subjects with Leber congenital amaurosis, and 3 subjects with congenital stationary night blindness. Eight of these mutations (gly51-to-ala, val104-to-ile, gly106-to-arg, arg135-to-gly, cys140-to-ser, gly188-to-glu, val209-to-met, and his211-to-arg) produce amino acid substitutions, one (gln64-to-ter) introduces a stop codon, and one changes a guanosine in the intron 4 consensus splice donor sequence to thymidine. Cosegregation of RP with gln64-to-ter, gly106-to-arg, arg135-to-gly, cys140-to-ser, gly188-to-glu, his211-to-arg, and the splice site guanosine-to-thymidine indicates that these mutations are likely to cause retinal disease. Val104-to-ile does not cosegregate and is therefore unlikely to be related to retinal disease. The relevance of gly51-to-ala and val209-to-met remains to be determined. The finding of gln64-to-ter in a family with autosomal dominant RP is in contrast to a recent report of a recessive disease phenotype associated with the rhodopsin mutation glu249-to-ter. In the present screen, all of the mutations that cosegregate with retinal disease were found among patients with RP. The mutations described here bring to 35 the total number of amino acid substitutions identified thus far in rhodopsin that are associated with RP. The distribution of the substitutions along the polypeptide chain is significantly nonrandom: 63% of the substitutions involve those 19% of amino acids that are identical among vertebrate visual pigments sequenced to date.

Autosomal dominant retinitis pigmentosa: no evidence for nonallelic genetic heterogeneity on 3q

Since the initial report of linkage of autosomal dominant retinitis pigmentosa (adRP) to the long arm of chromosome 3, several mutations in the gene encoding rhodopsin, which also maps to 3q, have been reported in adRP pedigrees. However, there has been some discussion as to the possibility of a second adRP locus on 3q. This suggestion has important diagnostic and research implications and must raise doubts about the usefulness of linked markers for reliable diagnosis of RP patients. In order to address this issue we have performed an admixture test (A-test) on 10 D3S47-linked adRP pedigrees and have found a likelihood ratio of heterogeneity versus homogeneity of 4.90. We performed a second A-test, combining the data from all families with known rhodopsin mutations. In this test we obtained a reduced likelihood ratio of heterogeneity versus homogeneity, of 1.0. On the basis of these statistical analyses we have found no significant support for two adRP loci on chromosome 3q. Furthermore, using 40 CEPH families, we have localized the rhodopsin gene to the D3S47-D3S20 interval, with a maximum lod score (Zm) of 20 and have found that the order qter-D3S47-rhodopsin-D3S20-cen is significantly more likely than any other order. In addition, we have mapped (Zm = 30) the microsatellite marker D3S621 relative to other loci in this region of the genome.