Investigation of the role of opsin gene polymorphism in generalized progressive retinal atrophies in dogs

Genetic and phenotypic variations of inherited retinal diseases in dogs: the power of within- and across-breed studies

Considerable clinical and molecular variations have been known in retinal blinding diseases in man and also in dogs. Different forms of retinal diseases occur in specific breed(s) caused by mutations segregating within each isolated breeding population. While molecular studies to find genes and mutations underlying retinal diseases in dogs have benefited largely from the phenotypic and genetic uniformity within a breed, within- and across-breed variations have often played a key role in elucidating the molecular basis. The increasing knowledge of phenotypic, allelic, and genetic heterogeneities in canine retinal degeneration has shown that the overall picture is rather more complicated than initially thought. Over the past 20 years, various approaches have been developed and tested to search for genes and mutations underlying genetic traits in dogs, depending on the availability of genetic tools and sample resources. Candidate gene, linkage analysis, and genome-wide association studies have so far identified 24 mutations in 18 genes underlying retinal diseases in at least 58 dog breeds. Many of these genes have been associated with retinal diseases in humans, thus providing opportunities to study the role in pathogenesis and in normal vision. Application in therapeutic interventions such as gene therapy has proven successful initially in a naturally occurring dog model followed by trials in human patients. Other genes whose human homologs have not been associated with retinal diseases are potential candidates to explain equivalent human diseases and contribute to the understanding of their function in vision.

Analysis of prevalence of presumed inherited eye diseases in Entlebucher Mountain Dogs

We analyzed the prevalence of the presumed inherited eye diseases (PIED) noncongenital cataract and progressive retinal atrophy in the Entlebucher Mountain Dog for systematic environmental influences and the additive genetic variation. Multivariate linear animal models using residual maximum likelihood methods and multivariate threshold animal models using Gibbs sampling in Bayesian analyses were used to estimate variance and covariance components. Data were obtained from the kennel club for Swiss Mountain Dog breeds in Germany. PIED were recorded using the standardized protocols of the Dortmunder Kreis, the German panel of the European Eye Scheme for Diagnosis of Inherited Eye Diseases in Animals (DOK). The material included 515 Entlebucher Mountain Dogs from 344 litters at 77 different kennels. Veterinary diagnoses for PIED were from the years 1981-2001. Pedigree information was available for up to nine generations. The multivariate animal model regarded the fixed effects of sex, birth year, experience of the veterinary ophthalmologist, litter size, percentage of examined dogs per litter, inbreeding coefficient and age at examination. The common environment of the litter and the additive genetic effect of the animal were taken into account as randomly distributed effects. The heritability estimates for PIED in the Entlebucher Mountain Dog were h2=0.15+/-0.06 (noncongenital cataract), and h2=0.34+/-0.08 (progressive retinal atrophy) in the linear model and h2=0.32+/-0.05 (noncongenital cataract) and h2=0.59+/-0.03 (progessive retinal atrophy) in the threshold model. The additive genetic correlation between noncongenital cataract and progressive retinal atrophy was moderately positive (r(g)=0.54+/-0.08) in the threshold model. The number of examinations performed by the veterinary ophthalmologists was associated with slightly higher heritabilities for noncongenital cataract and considerably higher heritabilities for progressive retinal atrophy. The investigated PIED in the Entlebucher Mountain Dog are genetically influenced and the size of the genetic parameters estimated may be sensitive to the accuracy of the diagnosis and how the data were collected.

Genetic analysis of presumed inherited eye diseases in Tibetan Terriers

We analysed the systematic environmental influences and the additive genetic variation for the presumed inherited eye diseases (PIED), membrana pupillaris persistens, distichiasis, primary lens luxation, non-congenital cataract, and progressive retinal atrophy, in Tibetan Terriers. Data were obtained from the International Kennel Club for Tibetan dog breeds in Germany. PIED were recorded in the years 1987 to 2001 by standardised protocols of the Dortmunder Kreis, the association for diagnosis of inherited eye diseases in animals (DOK). The material included 849 Tibetan Terriers from 596 litters in 203 different kennels. The multivariate linear animal model using residual maximum likelihood methods regarded the fixed effects of sex, birth year, experience of the veterinary ophthalmologist, litter size, percentage of examined dogs per litter, inbreeding coefficient, and age at examination. The common environment of the litter and the additive genetic effect of the animal were taken into account as randomly distributed effects. The heritability estimates for PIED in Tibetan Terriers were h2=0.17+/-0.04 (membrana pupillaris persistens), h2=0.04+/-0.03 (distichiasis), h2=0.13+/-0.04 (primary lens luxation), h2=0.13+/-0.04 (non-congenital cataract), and h2=0.49+/-0.08 (progressive retinal atrophy). The additive genetic correlation between non-congenital cataract and progressive retinal atrophy was highly positive rg=0.76+/-0.11, while that between membrana pupillaris persistens and progressive retinal atrophy rg=-0.43+/-0.14 was highly negative. The number of examinations performed by the veterinary ophthalmologists was associated with higher heritabilities for non-congenital cataract and progressive retinal atrophy. We concluded from our analysis that all investigated PIED in Tibetan Terriers are genetically influenced.

Canine inherited retinal degenerations: update on molecular genetic research and its clinical application

Inherited retinal degenerations in the dog include generalised progressive retinal atrophy, retinal pigment epithelial dystrophy, congenital stationary night blindness and day blindness (hemeralopia). The clinical phenotype and pathology of these diseases closely resemble some types of human inherited retinal degeneration, in particular retinitis pigmentosa, one of the most common inherited causes of blindness in man. Molecular genetic investigations aim to identify the genetic mutations underlying the canine inherited retinal degenerations. Two major research strategies, candidate gene analysis and linkage analysis, have been used. To date, candidate gene analysis has definitively identified the genetic mutations underlying nine inherited retinal degenerations, each in a different breed of dog, and linkage studies have identified genetic markers for a further retinal degeneration which is found in at least six different breeds. This review outlines the research strategy behind candidate gene and linkage studies and summarises recent results in the search for genetic causes of canine inherited retinal degenerations. The aim is to increase awareness of this rapidly changing field and to show how the research can be used to develop genetic tests for these diseases and thereby reduce the incidence of inherited eye disease in dogs.

Screening of the arrestin gene in dogs afflicted with generalized progressive retinal atrophy

BACKGROUND

Intronic DNA sequences of the canine arrestin (SAG) gene was screened to identify potential disease causing mutations in dogs with generalized progressive retinal atrophy (gPRA). The intronic sequences flanking each of the 16 exons were obtained from clones of a canine genomic library.

RESULTS

Using polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP) and DNA sequence analyses we screened affected and unaffected dogs of 23 breeds with presumed autosomal recessively (ar) transmitted gPRA. In the coding region of the SAG gene 12 nucleotide exchanges were identified, 5 of which lead to amino acid substitutions (H14C; A111V; A113T; D259T; A379E). 7 other exonic substitutions represent silent polymorphisms (C132C; Q199Q; H225H; V247V; P264P; T288T and L293L). 16 additional sequence variations were observed in intronic regions of different dog breeds.

CONCLUSIONS

In several breeds, these polymorphisms were found in homozygous state in unaffected and in heterozygous state in affected animals. Consequently these informative substitutions provide evidence to exclude mutations in the SAG gene as causing retinal degeneration in 14 of the 23 dog breeds with presumed ar transmitted gPRA.

Naturally occurring rhodopsin mutation in the dog causes retinal dysfunction and degeneration mimicking human dominant retinitis pigmentosa

Rhodopsin is the G protein-coupled receptor that is activated by light and initiates the transduction cascade leading to night (rod) vision. Naturally occurring pathogenic rhodopsin (RHO) mutations have been previously identified only in humans and are a common cause of dominantly inherited blindness from retinal degeneration. We identified English Mastiff dogs with a naturally occurring dominant retinal degeneration and determined the cause to be a point mutation in the RHO gene (Thr4Arg). Dogs with this mutant allele manifest a retinal phenotype that closely mimics that in humans with RHO mutations. The phenotypic features shared by dog and man include a dramatically slowed time course of recovery of rod photoreceptor function after light exposure and a distinctive topographic pattern to the retinal degeneration. The canine disease offers opportunities to explore the basis of prolonged photoreceptor recovery after light in RHO mutations and determine whether there are links between the dysfunction and apoptotic retinal cell death. The RHO mutant dog also becomes the large animal needed for preclinical trials of therapies for a major subset of human retinopathies.

Results of ophthalmologic screening examinations of German Pinschers in Finland--a retrospective study

OBJECTIVE

To retrospectively review ophthalmologic findings in German Pinschers in Finland.

ANIMALS STUDIED

One hundred and twenty-two German Pinschers that had ophthalmologic examination performed according to the Finnish Kennel Club's Eye Scheme before June 15, 1999.

PROCEDURES

A total of 154 eye examination reports of 122 dogs were analysed and all described findings were reported.

RESULTS

Persistent hyperplastic tunica vasculosa lentis (PHTVL) was diagnosed in 8.4% of all cases, and hereditary cataract (HC) in 6.5%. Even higher numbers of similar changes were reported by ophthalmologists; some of the dogs, however, were officially diagnosed as "free of symptoms" of inherited ocular diseases. A relatively high number (4.5%) of dogs had reported changes in Y sutures. A pedigree analysis suggests recessive inheritance for both diseases. Because of missing information about many dogs in the pedigree, an autosomal incomplete inheritance pattern cannot be ruled out in either case.

CONCLUSIONS

Both HC and PHTVL are inherited diseases in German Pinschers. Further studies are needed to determine the importance of the changes found in Y sutures. Discrepancies between the official diagnosis and described changes are probably partially due to the lack of familiarity with the published literature concerning this breed. Further studies are needed to ascertain the inheritance pattern for both diseases. So far breeding with affected animals should be avoided.

Generation and analysis of canine retinal ESTs: isolation and expression of retina-specific gene transcripts

Canine generalized progressive retinal atrophies (gPRA) are a group of degenerative retinal diseases that are a major cause of hereditary blindness in a number of dog breeds. The expressed sequence tag (EST) approach was used to identify and characterize potential candidate genes from canine retinal cDNA libraries. Both conventional and subtractive canine retinal cDNA libraries were constructed and analyzed. Differential hybridization was performed to identify abundantly retinal expressed cDNA clones. Sequences of both random and abundantly expressed clones were analyzed using GCG software and searched against GenEMBL databases. For genes of interest isolated from the libraries, Northern blotting and RT-PCR were performed to determine mRNA expression of the genes. DNA sequences from 85 differentially expressed clones and 100 random cDNAs were obtained and analyzed. A higher percentage of abundantly retina-expressed clones showed homology to database sequences compared with random clones (72 versus 43%). Five retinal genes and 2 anonymous retinal ESTs were selected to analyze mRNA expression. The five known genes, namely HRG4/unc119, cGMP-PDEA, transducin 1A, opsin, and sFRP2 showed retina-specific expression. In anonymous ESTs, clone p81 revealed retina-specific expression, while p3 showed expression in each of 14 canine tissues. Transcripts of the canine secreted frizzled related protein 2 (sFRP2) gene showed surprisingly high abundance in the canine retina. The isolated retinal ESTs here will be useful resources for further investigation of canine retinal function and canine genome mapping.

Isolation and investigation of canine phosducin as a candidate for canine generalized progressive retinal atrophies

A subtractive cDNA cloning strategy was used to isolate canine retina-specific genes. Canine phosducin cDNA was cloned from a canine subtracted retinal cDNA library and was analysed as a candidate for canine generalized progressive retinal atrophies (gPRA). Canine phosducin cDNA is 1230 bp in length encoding 245 amino acids. The nucleotide and amino acid sequences of canine phosducin are highly conserved when compared with those of five other mammalian species, namely human, cat, cow, rat, and mouse. Northern blot analysis demonstrated that the mRNA transcript for phosducin was approximately 1.3 kb in size and was present in canine retina, but showed no visible signals in 13 other canine tissues. The phosducin gene was examined for polymorphisms in a total of 101 pedigree dogs of eight breeds, including normal, obligate gPRA carriers, and gPRA-affected dogs, by single-stranded conformation polymorphisms (SSCP) analysis. Polymorphisms in the phosducin gene were detected only in the 3' untranslated region of the gene in two breeds of dogs: allelic heterozygous polymorphisms in miniature poodles suffering from one form of gPRA (progressive rod-cone degeneration, prcd), and a different polymorphism in a single normal Irish wolfhound. The polymorphisms of phosducin in prcd-affected miniature poodles did not segregate with the autosomal recessive form of gPRA. Heterozygous inheritance of the polymorphisms suggests that phosducin is very unlikely to carry the mutation causing prcd, so phosducin was probably excluded as a candidate for prcd-affected miniature poodles in this study.

The introns of the canine rod opsin gene show higher sequence homology to the human than to the rodent introns

Using genomic DNA from late-onset retinal degenerate and wild type Labrador Retrievers as templates and canine exon-specific oligonucleotides as primers in polymerase chain reaction, all four introns of opsin were cloned and sequenced. Dot-matrix comparisons were made for human, murine and canine introns. Selected sequences containing either intronic or coding sequences were aligned and used for phylogenetic relationship analysis. The opsin gene introns are conserved between the human, the mouse and the dog with regards to number and length. In addition there is an astonishingly high degree of sequence homology between the second and fourth introns. Introns 2(1277 bp in dog) and 4 (863 bp in dog) are 72% and 71% homologous to the human introns, and 57% and 52% homologous to the mouse introns, respectively. The coding sequence (CDS) of the dog shows 93% homology to human CDS and 88% homology to mouse CDS. A phylogenetic analysis of the intronic sequences 2 and 4 confirms the higher relatedness between dog and human than between mouse and human opsin genes. As there are good reasons to believe that the primate and rodent lineages are closer to each other than to the Canis familiaris, there must be some functional constraints on the evolution of human and dog opsins.

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.

Recent advances in understanding the spectrum of canine generalised progressive retinal atrophy

Canine generalised progressive retinal atrophy (gPRA) is a large and ever-increasing collection of naturally occurring, heterogeneous, progressive disorders. Most are inherited in an autosomal recessive manner and new, breed-specific forms continue to be described. The gPRAs cause photoreceptor cell death and subsequent retinal degeneration, culminating in blindness. In humans, similar inherited retinal dystrophies are recognised as retinitis pigmentosa and macular dystrophy. Molecular biological studies have revealed disease-causing mutations in several genes in humans and also in mice with retinal dystrophies. Recently, molecular genetic techniques have identified the cause of one form of gPRA in Irish setters while important candidate genes have been investigated in other breeds. Identification of mutations responsible for different forms of gPRA allows carrier and predegenerate animals to be detected using DNA-based tests. Such genetic tests will greatly facilitate the eradication of these diseases in different breeds.