Cyclic nucleotide metabolism in inherited retinopathy in collies: a biochemical and histochemical study

Large Animal Models of Inherited Retinal Degenerations: A Review

Studies utilizing large animal models of inherited retinal degeneration (IRD) have proven important in not only the development of translational therapeutic approaches, but also in improving our understanding of disease mechanisms. The dog is the predominant species utilized because spontaneous IRD is common in the canine pet population. Cats are also a source of spontaneous IRDs. Other large animal models with spontaneous IRDs include sheep, horses and non-human primates (NHP). The pig has also proven valuable due to the ease in which transgenic animals can be generated and work is ongoing to produce engineered models of other large animal species including NHP. These large animal models offer important advantages over the widely used laboratory rodent models. The globe size and dimensions more closely parallel those of humans and, most importantly, they have a retinal region of high cone density and denser photoreceptor packing for high acuity vision. Laboratory rodents lack such a retinal region and, as macular disease is a critical cause for vision loss in humans, having a comparable retinal region in model species is particularly important. This review will discuss several large animal models which have been used to study disease mechanisms relevant for the equivalent human IRD.

Optical coherence tomography and fundus autofluorescence imaging in an infant with RD3-related leber congenital amaurosis

Background: Leber congenital amaurosis (LCA) is both genetically and phenotypically heterogeneous group of retinal disorder. Mutations in retinal degeneration 3 (RD3) have been reported as an infrequent cause of LCA which account for less than 1% of all known LCA cases. This case report provides Optical Coherence Tomography (OCT) and Fundus Autofluorescence (FAF) findings of an infant with LCA related to a mutation in RD3.Materials and Methods: Single retrospective case report.Results: TruSight One Expanded Sequencing Panel was applied to the patient on the Illumina NextSeq. Homozygous pathogenic variant (c.112 C > T, p.Arg38Ter) was detected in the RD3 gene. Well-demarcated central foveal atrophy was noted in the infrared imaging. FAF imaging showed perifoveal hyperautofluorescent ring and irregular hyperautofluorescence outside the vascular arcade. An arrest in foveal development and loss of outer retinal structure including outer nuclear layer, external limiting membrane, ellipsoid zone and interdigitation zone at the fovea were detected in the OCT imaging.Conclusion: This study indicates that RD3-related LCA has a very severe phenotype with foveal development arrest and very early loss of all photoreceptor layer and external limiting membrane at the fovea.

Assessment of Rod, Cone, and Intrinsically Photosensitive Retinal Ganglion Cell Contributions to the Canine Chromatic Pupillary Response

Purpose

The purpose of this study was to evaluate a chromatic pupillometry protocol for specific functional assessment of rods, cones, and intrinsically photosensitive retinal ganglion cells (ipRGCs) in dogs.

Methods

Chromatic pupillometry was tested and compared in 37 dogs in different stages of primary loss of rod, cone, and combined rod/cone and optic nerve function, and in 5 wild-type (WT) dogs. Eyes were stimulated with 1-s flashes of dim (1 cd/m²) and bright (400 cd/m²) blue light (for scotopic conditions) or bright red (400 cd/m²) light with 25-cd/m² blue background (for photopic conditions). Canine retinal melanopsin/Opn4 was cloned, and its expression was evaluated using real-time quantitative reverse transcription-PCR and immunohistochemistry.

Results

Mean ± SD percentage of pupil constriction amplitudes induced by scotopic dim blue (scDB), scotopic bright blue (scBB), and photopic bright red (phBR) lights in WT dogs were 21.3% ± 10.6%, 50.0% ± 17.5%, and 19.4% ± 7.4%, respectively. Melanopsin-mediated responses to scBB persisted for several minutes (7.7 ± 4.6 min) after stimulus offset. In dogs with inherited retinal degeneration, loss of rod function resulted in absent scDB responses, followed by decreased phBR responses with disease progression and loss of cone function. Primary loss of cone function abolished phBR responses but preserved those responses to blue light (scDB and scBB). Although melanopsin/Opn4 expression was diminished with retinal degeneration, melanopsin-expressing ipRGCs were identified for the first time in both WT and degenerated canine retinas.

Conclusions

Pupil responses elicited by light stimuli of different colors and intensities allowed differential functional assessment of canine rods, cones, and ipRGCs. Chromatic pupillometry offers an effective tool for diagnosing retinal and optic nerve diseases.

Insights into the role of RD3 in guanylate cyclase trafficking, photoreceptor degeneration, and Leber congenital amaurosis

Retinal degeneration 3 (RD3) is an evolutionarily conserved 23 kDa protein expressed in rod and cone photoreceptor cells. Mutations in the gene encoding RD3 resulting in unstable non-functional C-terminal truncated proteins are responsible for early onset photoreceptor degeneration in Leber Congenital Amaurosis 12 patients, the rd3 mice, and the rcd2 collies. Recent studies have shown that RD3 interacts with guanylate cyclases GC1 and GC2 in retinal cell extracts and HEK293 cells co-expressing GC and RD3. This interaction inhibits GC catalytic activity and promotes the exit of GC1 and GC2 from the endoplasmic reticulum and their trafficking to photoreceptor outer segments. Adeno-associated viral vector delivery of the normal RD3 gene to photoreceptors of the rd3 mouse restores GC1 and GC2 expression and outer segment localization and leads to the long-term recovery of visual function and photoreceptor cell survival. This review focuses on the genetic and biochemical studies that have provided insight into the role of RD3 in photoreceptor function and survival.

RD3 gene delivery restores guanylate cyclase localization and rescues photoreceptors in the Rd3 mouse model of Leber congenital amaurosis 12

RD3 is a 23 kDa protein implicated in the stable expression of guanylate cyclase in photoreceptor cells. Truncation mutations are responsible for photoreceptor degeneration and severe early-onset vision loss in Leber congenital amaurosis 12 (LCA12) patients, the rd3 mouse and the rcd2 collie. To further investigate the role of RD3 in photoreceptors and explore gene therapy as a potential treatment for LCA12, we delivered adeno-associated viral vector (AAV8) with a Y733F capsid mutation and containing the mouse Rd3 complementary DNA (cDNA) under the control of the human rhodopsin kinase promoter to photoreceptors of 14-day-old Rb(11.13)4Bnr/J and In (5)30Rk/J strains of rd3 mice by subretinal injections. Strong RD3 transgene expression led to the translocation of guanylate cyclase from the endoplasmic reticulum (ER) to rod and cone outer segments (OSs) as visualized by immunofluorescence microscopy. Guanylate cyclase expression and localization coincided with the survival of rod and cone photoreceptors for at least 7 months. Rod and cone visual function was restored in the In (5)30Rk/J strain of rd3 mice as measured by electroretinography (ERG), but only rod function was recovered in the Rb(11.13)4Bnr/J strain, suggesting that the latter may have another defect in cone phototransduction. These studies indicate that RD3 plays an essential role in the exit of guanylate cyclase from the ER and its trafficking to photoreceptor OSs and provide a 'proof of concept' for AAV-mediated gene therapy as a potential therapeutic treatment for LCA12.

Mutations in RD3 are associated with an extremely rare and severe form of early onset retinal dystrophy

PURPOSE

To identify the underlying mutation and describe the phenotype in a consanguineous Kurdish family with Leber's congenital amaurosis (LCA)/early onset severe retinal dystrophy (EOSRD).

METHODS

Members of the index family were followed up to 22 years by ophthalmological examinations, including best corrected visual acuity (BCVA), Goldmann visual field (GVF), two-color-threshold perimetry (2CTP) and Ganzfeld electroretinogram (ERG), fundus photographs, fundus autofluorescence (FAF), and optical coherence tomography (OCT). After excluding seven of nine known LCA/EOSRD genes in the index patient, linkage analysis was performed in the family using a microarray followed by microsatellite fine mapping and direct sequencing of candidate genes. RD3 was screened by direct sequencing of 85 independent patients with LCA/EOSRD presenting with a BCVA ≥ 1.0 LogMAR before the age of 2 years to assess the prevalence of RD3 mutations in LCA/EOSRD. Since RD3 and RetGC1 have a functional relation, study authors screened for a modifying effect of RD3 mutations in 17 independent patients with mutations in GUCY2D.

RESULTS

BCVA was severely reduced from the earliest examinations (as early as 3 months), never exceeding 1.3 LogMAR. The disease presented as cone-rod dystrophy with dystrophic changes in the macula and bone spicules in the periphery on progression. Linkage analysis narrowed the region of interest towards the LCA12 locus. Direct sequencing of RD3 revealed a homozygous nonsense mutation (c.180C > A) in all affected members tested. Screening of additional unrelated LCA/EOSRD patients revealed only polymorphisms in RD3.

CONCLUSIONS

This is the second family reported so far with mutations in RD3. Mutations in RD3 are a very rare cause of LCA associated with an extremely severe form of retinal dystrophy.

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.

Canine RD3 mutation establishes rod-cone dysplasia type 2 (rcd2) as ortholog of human and murine rd3

Rod-cone dysplasia type 2 (rcd2) is an autosomal recessive disorder that segregates in collie dogs. Linkage disequilibrium and meiotic linkage mapping were combined to take advantage of population structure within this breed and to fine map rcd2 to a 230-kb candidate region that included the gene C1orf36 responsible for human and murine rd3, and within which all affected dogs were homozygous for one haplotype. In one of three identified canine retinal RD3 splice variants, an insertion was found that cosegregates with rcd2 and is predicted to alter the last 61 codons of the normal open reading frame and further extend the open reading frame. Thus, combined meiotic linkage and LD mapping within a single canine breed can yield critical reduction of the disease interval when appropriate advantage is taken of within-breed population structure. This should permit a similar approach to tackle other hereditary traits that segregate in single closed populations.

Excessive activation of cyclic nucleotide-gated channels contributes to neuronal degeneration of photoreceptors

In different animal models, photoreceptor degeneration was correlated to an abnormal increase in cGMP concentration. The cGMP-induced photoreceptor toxicity was demonstrated by applying the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine on retinal explants. To assess the role of cGMP-gated channels in this cGMP toxicity, the Ca(2+) channel blockers verapamil and L- and D-diltiazem, which block cGMP-gated channels with different efficacies, were applied to in vitro animal models of photoreceptor degeneration. These models included: (i) adult rat retinal explants incubated with zaprinast, a more specific inhibitor of the rod phosphodiesterase than 3-isobutyl-1-methylxanthine and (ii) rd mouse retinal explants. Photoreceptor apoptosis was assessed by terminal dUTP nick end labelling and caspase 3 activation. Effects of the blockers on the synaptic rod Ca(2+) channels were measured by patch-clamp recording. In the zaprinast-induced photoreceptor degeneration model, both diltiazem isomers rescued photoreceptors whereas verapamil had no influence. Their neuroprotective efficacy was correlated to their inhibition of cGMP-gated channels (l-diltiazem>d-diltiazem>verapamil=0). In contrast, all three Ca(2+) channel blockers suppressed rod Ca(2+) channel currents similarly. This suppression of the currents by the diltiazem isomers was very weak (16.5%) at the neuroprotective concentration (10 microm). In rd retinal explants, both diltiazem isomers also slowed down rod degeneration in contrast to verapamil. L-diltiazem exhibited this effect at concentrations ranging from 1 to 20 microm. This study further supports the photoreceptor neuroprotection by diltiazem particularly in the rd mouse retina, whereas the absence of neuroprotection by verapamil further suggests the role of cGMP-gated channel activation in the induction of photoreceptor degeneration.

Early retinopathy in the Bernese Mountain Dog in France: preliminary observations

The objective of the study was to describe a form of early retinopathy in the Bernese Mountain Dog in France. Sixty-two Bernese Mountain Dogs (38 males and 24 females), whose ages ranged from 2 months to 9 years, were examined over a period of 3 years. Visual behavior, pupillary light reflexes, menace responses and ocular fundi were evaluated in all animals. Electroretinography (ERG) was performed on six of the affected dogs after dark adaptation. Fluorescein angiography (FA) was performed on one affected dog. Whenever possible, the pedigrees of the affected dogs were evaluated. A histological examination of the retina was performed on one of the affected dogs. Eight dogs (seven males and one female) were diagnosed with retinopathy with an early onset of clinical signs. (Four dogs were aged between 3 months and 1 year, two dogs were aged 2 and 3.5 years, and one dog was 7 years old.) Night vision was impaired in most of the dogs. Retinopathy was characterized ophthalmoscopically by a bilateral, symmetrical horizontal zone of tapetal hyper-reflectivity adjacent to and above the optic disc, and sometimes by peri-papillary hyper-reflectivity. ERG changes included a reduction in b-wave amplitude varying from one case to another. Fluoroscein angiography demonstrated an ischemic-type alteration with epitheliopathy opposite the hyper-reflective zone. Pedigree examinations suggested a familial predisposition. The histological examination indicated photoreceptor degeneration that was more pronounced in the central tapetal zone. In France, retinopathy in the Bernese Mountain Dog involves an early retinal degeneration that produces specific manifestations of the ocular fundus, night visual impairment or blindness, and has familial transmission.

Development and use of a polymerase chain reaction-based diagnostic test for the causal mutation of progressive retinal atrophy in Cardigan Welsh Corgis

OBJECTIVE

To develop an allele-specific polymerase chain reaction (ASPCR)-based diagnostic test for the mutation in the cyclic guanosine monophosphate phosphodiesterase alpha subunit gene (PDE6A) that causes the rcd3 form of progressive retinal atrophy (PRA) in Cardigan Welsh Corgis.

ANIMALS

1 affected homozygote, 1 unaffected carrier, 1 genotypically normal dog, and 500 unknown-PRA status Cardigan Welsh Corgis.

PROCEDURE

Control blood samples were collected from Cardigan Welsh Corgis of known PRA status (ie, affected homozygote, unaffected carrier, and a genotypically normal dog) for test development. Test blood samples were collected from 500 Cardigan Welsh Corgis of unknown PRA status. Genomic DNA was used as a template in ASPCR. One pair of primers was designed to specifically amplify only the mutant allele, and another set to amplify only the wildtype allele. The PCR conditions were adjusted to ensure each reaction was 100% specific.

RESULTS

The PCR conditions were identified so that each ASPCR only amplified the allele it was designed to amplify. Of the 500 Cardigan Welsh Corgis tested using the newly developed ASPCR, 457 were homozygous for the normal allele (genotypically normal), 43 were heterozygous (phenotypically normal carriers), and none were homozygous for the mutant allele.

CONCLUSION AND CLINICAL RELEVANCE

A rapid, ASPCR diagnostic test able to detect the PDE6A gene mutation responsible for the rcd3 form of PRA in Cardigan Welsh Corgis was developed. The test provides a useful service for Cardigan Welsh Corgi breeders and will enable them to prevent the birth of homozygote mutant dogs.

Evaluation of cGMP-phosphodiesterase (PDE) subunits for causal association with rod-cone dysplasia 2 (rcd2), a canine model of abnormal retinal cGMP metabolism

Rod-cone dysplasia types 1 (rcd1; Irish setter) and 2 (rcd2; collie) in dogs are early onset forms of progressive retinal atrophy (PRA) which serve as models of retinitis pigmentosa (RP) in humans. As both rcd1 and rcd2 result from abnormal retinal cGMP metabolism associated with a deficiency in cGMP-phosphodiesterase (PDE) activity, and a nonsense mutation in the PDE6B subunit gene has been shown to cause rcd1, the genes encoding the four subunits of the PDE complex (PDE6A, PDE6B, PDE6G and PDE6D) make compelling candidates for the rcd2 locus. We adopted diverse strategies to evaluate causal association of the four PDE subunit genes with the rcd2 phenotype. Identification in an informative pedigree of obligate recombinations between intragenic polymorphisms within PDE6A and PDE6D and the rcd2 locus unequivocally excludes these two genes. PDE6B was excluded by a breeding strategy demonstrating nonallelism of rcd1 and rcd2. Direct sequencing of PDE6G from an rcd2 -homozygous collie dog revealed no abnormality in the entire genomic sequence. To evaluate cosegregation between PDE6G and rcd2, advantage was taken of prior knowledge that PDE6G and Galactokinase 1 (GALK1) localize to the same canine-rodent somatic hybrid cell line. Linkage analysis using a single nucleotide polymorphism (SNP) in the PDE6G gene, and a (CA)n repeat polymorphism in the GALK1 gene, which were both segregating in an unrelated pedigree, established close linkage of these two genes (theta = 0; Z = 4.21). Identification of obligate recombinations between GALK1 and the rcd2 locus in an informative rcd2 pedigree thus excluded PDE6G as a candidate gene for rcd2; the exclusion distance between GALK1 and rcd2 is at least 0.35 cM. These results therefore exclude the entire set of genes coding for the rod PDE complex as candidates for rcd2.

Molecular characterization and mapping of canine cGMP-phosphodiesterase delta subunit (PDE6D)

cGMP-phosphodiesterase (PDE) is composed of two catalytic (alpha and beta) and two identical inhibitory (gamma) subunits. The human gene (PDE6D) encoding a new subunit (delta) has been characterized and mapped to the long arm of chromosome 2 (HSA2q35-q36) where a new autosomal recessive retinitis pigmentosa (arRP) locus (RP26) has been localized. Characterization of the canine PDE6D shows the gene is about 4.2kb containing four exons interrupted by three introns; the size of the cDNA is 1059bp with an open reading frame (ORF) of 453bp. A single transcript of identical size (1.43kb) was detected in all tissues examined (liver, lung, spleen, kidney, heart, brain and retina), with the highest abundance in the retina. Canine PDE6D has been localized to canine radiation hybrid group 14-a, which extends conserved synteny between the dog, human chromosome 2q and mouse chromosome 1. The characterization of the canine PDE6D gene and its mapping provide important information for testing causal association of the gene with canine retinal degenerations, in particular rod-cone dysplasia 2 (rcd2) in collie dogs. This disease is characterized by abnormal retinal cGMP metabolism due to a deficiency in cGMP-PDE activity, yet the alpha, beta and gamma subunits of PDE have been excluded as candidate gene loci.

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.

Cloning and characterization of the cDNA and gene encoding the gamma-subunit of cGMP-phosphodiesterase in canine retinal rod photoreceptor cells

Rod photoreceptor cyclic GMP-phosphodiesterase (cGMP-PDE) is one of the key enzymes of the visual phototransduction cascade in the vertebrate retina. The enzyme is composed of alpha- and beta-catalytic subunits and two identical inhibitory gamma-subunits. A defect in any of the subunits may potentially alter the activity of the enzyme, leading to aberration in the visual phototransduction. We have cloned and sequenced both the cDNA and gene for the canine gamma-subunit of cGMP-PDE (PDE gamma). The 952-bp cDNA has a coding region of 261 bp which is very similar to those of the PDE gamma cDNAs from human, mouse and bovine retinas. Among the 87 amino acids encoded by the transcribed region, differences in only three residues located within the first 17 amino acids were identified. The carboxyl terminus of PDE gamma, involved in interaction with the catalytic subunits of cGMP-PDE and the alpha-subunit of transducin, is conserved through evolution. The single polyadenylation signal (AATAAA) present in human and bovine PDE gamma cDNAs is replaced by AGTAAA in the canine sequence. The canine gene (2.8 kb) consists of four exons and is much smaller than the human gene (6 kb). The larger size of the human gene is primarily due to the presence of AluI repetitive elements in its first two introns.

Slowly progressive changes of the retina and retinal pigment epithelium in Briard dogs with hereditary retinal dystrophy. A morphological study

Seven eyes from 2 generations of Briard dogs (5 weeks--7 years old) with congenital night blindness and (in the second generation) impairment of day vision to varying degrees, were examined by light and electron microscopy. Specimens from 4 locations were studied: the central area, the midperiphery of the tapetal area, the upper periphery and the lower periphery. Disorientation of rod outer segment disc membranes was seen in the 5-week-old dog. Large electron-lucent inclusions were found in the RPE at 3.5 months of age. These inclusions occurred most frequently in the central and midperipheral-tapetal areas and seemed to increase in numbers and spread towards the periphery with increasing age. The content of these inclusions is not elucidated. Rod photoreceptor degeneration was apparent from 7 months of age and was most prominent in the peripheral areas. The cones were better preserved. The 7-year-old dog showed reduction of photoreceptors in the central and midperipheral-tapetal areas and almost complete photoreceptor degeneration in the periphery. This dog also showed severe changes of the inner retina in the peripheral fundus. It appears that these Briard dogs suffer from a very slowly progressive retinal degeneration, in which the photoreceptor degenerative changes do not correlate anatomically to the changes in the RPE cells. The disease seems to be different from the retinopathy described in the English Briards. It is not clear yet whether the lipid type of retinopathy found in American Briards is identical to the present disease.

Ultrastructural changes of the retina and the retinal pigment epithelium in Briard dogs with hereditary congenital night blindness and partial day blindness

The offspring of two Briard dogs (brother and sister) with congenital, clinically stationary night blindness showed an aggravation of the disease with severe impairment of day vision in addition to night blindness. This ultrastructural study was performed on four such second generation puppies at the age of 4 months. The neuroretina and retinal pigment epithelium (RPE) from four locations were studied: the central area (immediately temporal to the optic disc); the centre of the tapetal area; the upper periphery (border of tapetal area); and the lower periphery (non-tapetal area). The RPE showed large inclusions, seemingly lipid in nature, mainly in the central and tapetal areas of the retina. Small, membrane bound, electron-dense inclusions were scattered in the RPE cytoplasm in all areas examined. The small inclusions were found to be less numerous in normal than in affected dogs and may be lysosomal in nature. Forty to fifty percent of the rod outer segments in the tapetal area showed disorientation of the disc membranes, whereas the corresponding figures were 20-40% in the central and lower peripheral areas and 6-25% in the upper peripheral area. No structural abnormalities were found in the rod inner segments or synaptic bodies. The cones were better preserved. The inner retina appeared normal. These electron microscopic findings seem to correspond to a previously published electrophysiologic evaluation, indicating a defective and delayed rod function (virtually no scotopic a- and b-waves), a better preserved cone function (photopic flicker responses present, although reduced) and impaired RPE activity (a prominent, slow negative potential of long latency at the site of the c-wave). It appears that these Briard dogs, showing structural changes of the rod outer segments in addition to pigment epithelial inclusions, mainly located in the posterior pole, comprise a pigment epitheliopathy and retinopathy morphologically different from other hereditary canine retinopathies that have been described earlier in the literature and different from animal models of congenital night blindness.

Retinal degeneration in the dog and cat

Retinal degenerations in the dog and cat are an important cause of blindness in these species. Particularly in the dog, many retinal degenerations, collectively called progressive retinal atrophy, seen in clinical practice are inherited. The clinical signs, electrophysiological findings, pathology, and underlying biochemical defects in the retina vary from breed to breed. Specific categories of inherited retinal degeneration are now recognized, and classified into early onset photoreceptor dysplasias, late-onset retinal degenerations, or retinal degenerations secondary to primary RPE dystrophy. As new inherited retinal degenerations are reported in different breeds they can generally be assigned to one these categories. Other causes of retinal degeneration include nutritional deficiencies, glaucoma, inflammation, ischemia, and toxins. Idiopathic retinal degeneration occurs in the dog with some frequency.

Isolation and characterization of cDNA encoding the gamma-subunit of cGMP phosphodiesterase in human retina

Cyclic GMP-phosphodiesterase (cGMP-PDE) plays a key role in the normal functioning of retinal rod photoreceptor cells. The enzyme is composed of alpha- and beta-catalytic subunits which are inhibited by two identical gamma-subunits. A cDNA encoding the gamma-subunits (PDE gamma) from human retina has been cloned and sequenced. The 1012-bp cDNA has a coding region of 261 bp which is highly homologous to those of the PDE gamma cDNAs from bovine and mouse retinas. Comparison of the deduced amino acid sequences of the proteins from the three species indicates that PDE gamma has been very well conserved through evolution. The mRNA encoded by the cloned cDNA is 1.0 kb long, is similar in size to the corresponding mRNAs from mouse, dog and bovine retinas and is not detected in ground squirrel retina. The PDEG gene has been assigned to human chromosome 17, probably in the region q21.1.

Molecular characterization of human and bovine rod photoreceptor cGMP phosphodiesterase alpha-subunit and chromosomal localization of the human gene

Defects in proteins that function in photoreceptor signal transduction are prime suspects as causes of some human hereditary retinal degenerations. We have characterized cDNA clones encoding the alpha-subunit of human and bovine rod cell cGMP phosphodiesterase a key phototransduction enzyme. Clones from both species contain an open reading frame capable of coding for an approximately 100-kDa polypeptide of 859 amino acids, 94% of which are identical. Two or more transcripts were detected in both human and bovine retinal poly(A)+ RNA preparations, although the human transcripts ranging from 5.3 to 4.9 kb are significantly larger than the two bovine transcripts of 4.6 and 4.0 kb. The bovine and human genes appear to exist in single copy, with the bovine gene spanning more than 140 kb of genomic DNA. Somatic cell hybrids were used to map the human gene to the long arm of chromosome 5 (5q31.2----q34). Finally, the use of the candidate gene approach in the study of hereditary retinal dystrophies is discussed.

Non-allelism of three genes (rcd1, rcd2 and erd) for early-onset hereditary retinal degeneration

Cross-breeding experiments were utilized to study the genetics of three autosomal recessive, early onset retinal degenerations in dogs. Irish setters affected with rod-cone dysplasia type 1 (rcd1) were bred to Norwegian elkhounds affected with early retinal degeneration (erd). All offspring (15 pups-two litters) surviving to diagnostic age were phenotypically normal, as assessed by electroretinography, retinal morphology and assay of retinal cyclic nucleotide content. One phenotypically normal female Irish setter-Norwegian elkhound crossbred dog (the progeny of the above breeding and thus heterozygous at both the rcd1 and the erd locus) was bred to a collie dog affected with rod-cone dysplasia type 2 (rcd2). All 11 pups from this breeding also proved phenotypically normal by the above methods. These results establish that the genes rcd1, rcd2 and erd are non-allelic. Biochemical data are also presented that establish that erd, unlike rcd1 and rcd2, is not associated with abnormal metabolism of retinal cyclic guanosine monophosphate (cyclic GMP).

Isolation of a candidate cDNA for the gene causing retinal degeneration in the rd mouse

The inherited retinal degeneration of the rd mouse results in the exclusive loss of one cell type, the photoreceptors. We took advantage of this visual-cell loss to devise a strategy for the isolation of photoreceptor-specific cDNAs based on the use of subtractive and differential hybridizations. The resulting pool of photoreceptor-specific cDNAs was screened for a candidate cDNA for the rd gene, and a putative rd cDNA that maps to mouse chromosome 5, the chromosome to which the rd gene has been assigned, was identified. On Northern blots the candidate rd cDNA hybridizes a 3.3-kilobase RNA species from 9- to 11-day-old developing normal retina and, much more faintly, a 3.6-kb RNA species from age-matched rd retina. The 0.3-kilobase difference in the size of the mRNAs hybridized suggests that a structural alteration in the gene corresponding to the candidate rd cDNA has occurred in the rd mouse. This was further supported by the detection of polymorphisms between rd/rd and +/+ mouse genomic DNA after digestion with restriction endonucleases and probing with the candidate rd cDNA. Expression of mRNAs hybridized by the candidate rd cDNA is detected in normal and diseased retinas at postnatal day 1 but the signal intensity is considerably lower in the rd retina. To our knowledge, this is the earliest molecular defect reported in the rd retina that is observed prior to any phenotypic signs of photoreceptor degeneration.

Retinal degenerations in the dog: IV. Early retinal degeneration (erd) in Norwegian elkhounds

A new early onset hereditary retinal degeneration is characterized in Norwegian elkhound dogs. This disease, termed early retinal degeneration (erd), was studied in 10 affected dogs, from 30 days- to 7 years old, clinically, by electroretinography, and by light- and electron-microscopic morphology. Control studies were performed on 49 non-affected dogs. Affected dogs are initially nightblind, and become totally blind between 12- and 18 months of age. The postnatal development of their rod and cone photoreceptors is abnormal both structurally and functionally. Morphologically, rod and cone inner- and outer-segment growth occurs but appears uncoordinated. Adjacent rods become very disparate in the size and proportions of their inner- and outer segments. Prominent villiform processes extend from the inner segments of rods and, to a lesser extent, cones. Synaptic terminals of rods and cones fail to develop properly. The b-wave of the electroretinogram fails to develop and the electroretinogram (ERG) remains a-wave-dominated. Subsequent to these abnormalities of development, the rods and cones degenerate, rapidly at first and later more gradually. In normal dogs, development of the ERG a- and b-waves is shown to follow, respectively, morphologic development of the photoreceptor outer segments and synaptic terminals. Similarly the abnormal development and subsequent degeneration of photoreceptor outer segments and synaptic terminals in affected dogs, correspond in time course to development and degeneration of the ERG a- and b-waves.

Hereditary retinal degeneration in the Rhode Island Red chicken. I. Histology and ERG

Hereditary blindness in Rhode Island Red chickens was analyzed at various post-hatching stages by light microscopy and electrophysiological recordings. At the time of hatching the retina of affected chicks appeared morphologically normal and identical to that of control, non-affected chicks. Whereas the electroretinographic (ERG) response to light stimulus in normal chicks was near the adult level at the time of hatching, no ERG either under light- or dark-adapted conditions was measurable in affected chicks at any stage examined. Photoreceptor cells of affected animals were seen to undergo degenerative changes after about one week post-hatching. Decrease in number of outer segments, spaces between inner segments and large spaces in the outer nuclear layer were apparent by Day 10. By Day 21, most of the photoreceptor inner segments appeared swollen, and the decrease in number of outer segments and photoreceptor nuclei was noteworthy. By the end of the second month no outer segments were seen and the majority of identifiable inner segments were from cones, a larger proportion than normally present being double cones. By six months, very few photoreceptor inner segments and nuclei remained; most inner segments were deformed and diminutive but usually contained a clearstaining oil droplet characteristic of the principal member of the double cone. In all stages after one week of age, pycnotic nuclei and thinning of inner retinal layers accompanied photoreceptor degeneration. In all specimens examined, degeneration of retinal cells was more pronounced in the superior central retina than in the periphery. Pathological changes were frequently also noted in the pigment epithelium overlying degenerating retina. Because the chick retina is well developed at birth, contains a fovea and a significant cone population and because cones (particularly one specific type) survive rods, we believe that this congenitally-blind chicken may be a useful model for studies on human hereditary retinal degenerations.

Spinal cord metabolic changes in murine retrovirus-induced motor neuron disease

Decreased cerebrospinal fluid concentrations of cyclic nucleotides in human motor neuron disease and decreased spinal cord concentrations of cyclic nucleotides in murine (Wobbler) motor neuron disease suggest that an abnormality in cyclic nucleotide metabolism may play a role in motor neuron degeneration. Retroviruses cause decreased cellular levels of cyclic nucleotides in infected cells. We induced a motor neuron degeneration with a neurotropic retrovirus, but not with a non-neurotropic retrovirus. In paralyzed mice, mean cAMP was decreased 21% in posterior horn segments and 34% in anterior horn segments compared to controls. The proportion of spinal cord phosphorylase a decreased 24% in paralyzed mice compared to controls. The content of cGMP decreased 48% in the cerebellum and 25% in both anterior and posterior horn segments of the spinal cords of paralyzed mice compared to controls. White matter content of these chemicals did not decrease in the posterior column of affected animals. Spinal cord content of ATP increased 20-22% in all three compartments, but the spinal cord content of phosphocreatine increased dramatically in white matter (46%), posterior horn gray matter (69%), and anterior horn gray matter (103%) compared to controls. Changes in high-energy phosphate intermediate and cyclic nucleotide metabolites occurred only in topographical regions showing neuronal and astrocyte pathological changes, but did not occur in the cerebral cortex.

Effect of photic injury on the retinal tissues

In order to study the tissue damages in photic injury of the retina, the macula and posterior pole of the rhesus monkey were exposed to the light of an indirect ophthalmoscope. During the acute phase of the injury, disruption of the blood-retinal barrier appeared to be an early and sensitive indicator of retinal pigment epithelial damage and cyclic-GMP-phosphodiesterase of the photoreceptor lamella was inactivated. These lesions were followed for 2 to 5 years. Focal chronic decompensation of the blood-retinal barrier and subretinal pigment epithelial neovascularization were noted in the chronic degenerative phase of this photic maculopathy. A possible protective role of ascorbic acid in mild photic injury was proposed.