Rhodopsin levels and rod-mediated function in Abyssinian cats with hereditary retinal degeneration

Alternative splicing in CEP290 mutant cats results in a milder phenotype than LCACEP290 patients

PURPOSE

The rdAc cat has an intronic mutation in the centrosomal 290 kDa (CEP290) gene resulting in a frameshift and a premature stop codon (c.6960 + 9 T > G, p.Ile2321AlafsTer3) predicted to truncate the protein by 157 amino acids. CEP290 mutations in human patients cause a range or phenotypes including syndromic conditions and severe childhood loss of vision while the rdAc cat has a milder phenotype. We sought to further characterize the effect of rdAc mutation on CEP290 expression.

METHODS

TaqMan quantitative real-time polymerase chain reaction assays were used to compare wildtype and truncated transcript levels. Relative protein abundance was analyzed by Western blot. Immunohistochemistry (IHC) was performed to detect CEP290 protein.

RESULTS

CEP290 mutant cats show low-level (17.4% of wildtype cats) use of the wildtype splice site and usage of the mutant splice site. Western analysis shows retina from cats homozygous for the mutation has CEP290 protein that likely comprises a combination of both wildtype and truncated protein. IHC detects CEP290 in affected and control retina labeling the region of the interconnecting cilium.

CONCLUSIONS

The comparably milder phenotype of CEP290 mutant cats is likely due to the retained production of some full-length CEP290 protein with possible functional contributions from presence of truncated protein.

Preclinical Models of Retinitis Pigmentosa

Retinitis pigmentosa (RP) is the name for a group of phenotypically-related heritable retinal degenerative disorders. Many genes have been implicated as causing variants of RP, and while the clinical phenotypes are remarkably similar, they may differ in age of onset, progression, and severity. Common inheritance patterns for specific genes connected with the development of the disorder include autosomal dominant, autosomal recessive, and X-linked. Modeling the disease in animals and other preclinical systems offers a cost-conscious, ethical, and time-efficient method for studying the disease subtypes. The history of RP models is briefly examined, and both naturally occurring and transgenic preclinical models of RP in many different organisms are discussed. Syndromic forms of RP and models thereof are reviewed as well.

Central retinal preservation in rdAc cats

OBJECTIVE

Children with Leber congenital amaurosis (LCA) due to CEP290 mutations show characteristic macular preservation. Spectral domain-optical coherence tomography (SD-OCT) is a noninvasive technique to investigate retinal structural changes. Loss of integrity of the ellipsoid zone (EZ) on OCT in people with retinal disease has been associated with loss of visual function and is a useful measure of retinal disease progression. We hypothesized that rdAc felines with Cep290 mutation would have a similar pattern of degeneration, with relative central retinal preservation associated with maintenance of the EZ.

PROCEDURES

Fundus imaging, confocal scanning laser ophthalmoscopy, and SD-OCT cross-sectional imaging was performed on 11 rdAc cats ranging from 6 months to 10 years of age. Images were collected from the area centralis, visual streak, and the mid-superior and mid-inferior retina. Receptor plus (REC+, encompassing the entire length of photoreceptors) thicknesses were measured. Regional rates of degeneration were determined by regression analysis and compared using unpaired t-tests. The EZ was evaluated for the presence, absence, or loss of definition.

RESULTS

RdAc cats showed REC+ thinning over time in all regions. The area centralis and visual streak had a slower rate of thinning than the mid-peripheral retina. There was loss of integrity of the EZ initially in the more peripheral regions, while its integrity was maintained in the area centralis and visual streak at all ages studied.

CONCLUSIONS

rdAc cats show preservation of the central retina with maintenance of EZ integrity, which recapitulates findings in human patients.

Cats: a gold mine for ophthalmology

Over 200 hereditary diseases have been identified and reported in the cat, several of which affect the eye, with homology to human hereditary disease. Compared with traditional murine models, the cat demonstrates more features in common with humans, including many anatomic and physiologic similarities, longer life span, increased size, and a genetically more heterogeneous background. The development of genomic resources in the cat has facilitated mapping and further characterization of feline models. During recent years, the wealth of knowledge in feline ophthalmology and neurophysiology has been extended to include new diseases of significant interest for comparative ophthalmology. This makes the cat an extremely valuable animal species to utilize for further research into disease processes affecting both cats and humans. This is especially true in the advancement and study of new treatment regimens and for extended therapeutic trials. Groups of feline eye diseases reviewed in the following are lysosomal storage disorders, congenital glaucoma, and neuroretinal degenerations. Each has important implications for human ophthalmic research.

Characterization of feline hereditary retinal dystrophies using clinical, functional, structural and molecular genetic studies

Only in recent years have specific mutations been elucidated for feline hereditary retinal dystrophies. Molecular genetic characterization of feline diseases has so far been a slow process but with a full genome sequence for the cat recently completed and the development of a feline single nucleotide polymorphism chip, the characterization of feline monogenic disorders will be significantly simplified. This review summarizes current knowledge with regard to specific hereditary retinal dystrophies in cats and gives an overview of how cats can be used as models in translational research.

The domestic cat as a large animal model for characterization of disease and therapeutic intervention in hereditary retinal blindness

Large mammals, including canids and felids, are affected by spontaneously occurring hereditary retinal diseases with similarities to those of humans. The large mammal models may be used for thorough clinical characterization of disease processes, understanding the effects of specific mutations, elucidation of disease mechanisms, and for development of therapeutic intervention. Two well-characterized feline models are addressed in this paper. The first model is the autosomal recessive, slowly progressive, late-onset, rod-cone degenerative disease caused by a mutation in the CEP290 gene. The second model addressed in this paper is the autosomal dominant early onset rod cone dysplasia, putatively caused by the mutation found in the CRX gene. Therapeutic trials have been performed mainly in the former type including stem cell therapy, retinal transplantation, and development of ocular prosthetics. Domestic cats, having large human-like eyes with comparable spontaneous retinal diseases, are also considered useful for gene replacement therapy, thus functioning as effective model systems for further research.

Cone photoreceptors are the main targets for gene therapy of NPHP5 (IQCB1) or NPHP6 (CEP290) blindness: generation of an all-cone Nphp6 hypomorph mouse that mimics the human retinal ciliopathy

Leber congenital amaurosis (LCA), a severe autosomal recessive childhood blindness, is caused by mutations in at least 15 genes. The most common molecular form is a ciliopathy due to NPHP6 (CEP290) mutations and subjects have profound loss of vision. A similarly severe phenotype occurs in the related ciliopathy NPHP5 (IQCB1)-LCA. Recent success of retinal gene therapy in one form of LCA prompted the question whether we know enough about human NPHP5 and NPHP6 disease to plan such treatment. We determined that there was early-onset rapid degeneration of rod photoreceptors in young subjects with these ciliopathies. Rod outer segment (OS) lamination, when detectable, was disorganized. Retinal pigment epithelium lipofuscin accumulation indicated that rods had existed in the past in most subjects. In contrast to early rod losses, the all-cone human fovea in NPHP5- and NPHP6-LCA of all ages retained cone nuclei, albeit with abnormal inner segments and OS. The rd16 mouse, carrying a hypomorphic Nphp6 allele, was a good model of the rod-dominant human extra-foveal retina. Rd16 mice showed normal genesis of photoreceptors, including the formation of cilia, followed by abnormal elaboration of OS and rapid degeneration. To produce a model of the all-cone human fovea in NPHP6-LCA, we generated rd16;Nrl-/- double-mutant mice. They showed substantially retained cone photoreceptors with disproportionate cone function loss, such as in the human disease. NPHP5- and NPHP6-LCA across a wide age spectrum are thus excellent candidates for cone-directed gene augmentation therapy, and the rd16;Nrl-/- mouse is an appropriate model for pre-clinical proof-of-concept studies.

Leber congenital amaurosis: genes, proteins and disease mechanisms

Leber congenital amaurosis (LCA) is the most severe retinal dystrophy causing blindness or severe visual impairment before the age of 1 year. Linkage analysis, homozygosity mapping and candidate gene analysis facilitated the identification of 14 genes mutated in patients with LCA and juvenile retinal degeneration, which together explain approximately 70% of the cases. Several of these genes have also been implicated in other non-syndromic or syndromic retinal diseases, such as retinitis pigmentosa and Joubert syndrome, respectively. CEP290 (15%), GUCY2D (12%), and CRB1 (10%) are the most frequently mutated LCA genes; one intronic CEP290 mutation (p.Cys998X) is found in approximately 20% of all LCA patients from north-western Europe, although this frequency is lower in other populations. Despite the large degree of genetic and allelic heterogeneity, it is possible to identify the causative mutations in approximately 55% of LCA patients by employing a microarray-based, allele-specific primer extension analysis of all known DNA variants. The LCA genes encode proteins with a wide variety of retinal functions, such as photoreceptor morphogenesis (CRB1, CRX), phototransduction (AIPL1, GUCY2D), vitamin A cycling (LRAT, RDH12, RPE65), guanine synthesis (IMPDH1), and outer segment phagocytosis (MERTK). Recently, several defects were identified that are likely to affect intra-photoreceptor ciliary transport processes (CEP290, LCA5, RPGRIP1, TULP1). As the eye represents an accessible and immune-privileged organ, it appears to be uniquely suitable for human gene replacement therapy. Rodent (Crb1, Lrat, Mertk, Rpe65, Rpgrip1), avian (Gucy2D) and canine (Rpe65) models for LCA and profound visual impairment have been successfully corrected employing adeno-associated virus or lentivirus-based gene therapy. Moreover, phase 1 clinical trials have been carried out in humans with RPE65 deficiencies. Apart from ethical considerations inherently linked to treating children, major obstacles for the treatment of LCA could be the putative developmental deficiencies in the visual cortex in persons blind from birth (amblyopia), the absence of sufficient numbers of viable photoreceptor or RPE cells in LCA patients, and the unknown and possibly toxic effects of overexpression of transduced genes. Future LCA research will focus on the identification of the remaining causal genes, the elucidation of the molecular mechanisms of disease in the retina, and the development of gene therapy approaches for different genetic subtypes of LCA.

Amax to scotopic Imax diagnoses feline hereditary rod cone degeneration more efficiently than any other combination of long protocol electroretinogram parameters

AIM

To evaluate two recent methods for detecting feline hereditary rod cone degeneration with maximum efficiency from a long full-field flash ERG protocol. One combines 12 of these measures in an equation that is derived from iterative principal components factor analysis. The other uses the amplitude of the a-wave to the brightest available flash alone.

METHODS

We tested the original 12-parameter equation, by applying it to 50 new ERG series in 23 backcrossed cats. They were necessarily either heterozygous or homozygous for hereditary rod cone degeneration. A masked observer compared the ERG score and fundus examinations. We reanalyzed the old, new and combined data sets. Data sets with only one session per animal were analysed to avoid problems from non-random sampling. A two factor linear model of the a-wave was evaluated.

RESULTS

The prior equation, applied to the new data, discriminated the groups as well as it had initially. In the reanalysis, group separation continued to increase with even fewer measures compared to the previously reported study. Eventually, one measure, the amplitude of the a-wave (amax) to the brightest scotopic flash (Imax) discriminated the groups better than any other measure or combination of measures in all analyses, including data sets using only one session for each animal and in a two factor linear model of the a-wave.

CONCLUSION

Amax to Imax alone proved to be the best diagnostic criterion in all analyses. No linear model is likely to discriminate affected from unaffected animals more effectively because additional variables increased variance more rapidly than they increased discrimination. Amax to Imax may detect other rod and rod/cone dystrophies equally efficiently.

Stable transgene expression in rod photoreceptors after recombinant adeno-associated virus-mediated gene transfer to monkey retina

Recombinant adeno-associated virus (rAAV) is a promising vector for therapy of retinal degenerative diseases. We evaluated the efficiency, cellular specificity, and safety of retinal cell transduction in nonhuman primates after subretinal delivery of an rAAV carrying a cDNA encoding green fluorescent protein (EGFP), rAAV. CMV.EGFP. The treatment results in efficient and stable EGFP expression lasting >1 year. Transgene expression in the neural retina is limited exclusively to rod photoreceptors. There is neither electroretinographic nor histologic evidence of photoreceptor toxicity. Despite significant serum antibody responses to the vector, subretinal readministration results in additional transduction events. The findings further characterize the retinal cell tropism of rAAV. They also support the development of studies aimed ultimately at treating inherited retinal degeneration by using rAAV-mediated gene therapy.

An early decrease in interphotoreceptor retinoid-binding protein gene expression in Abyssinian cats homozygous for hereditary rod-cone degeneration

Levels of interphotoreceptor retinoid-binding protein (IRBP) protein and message in retinas of Abyssinian cats homozygous for progressive rod-cone degeneration were determined at early ages, well before the onset of clinical retinal degeneration. IRBP gene expression was assessed by immunochemical quantitation of IRBP protein, and by Northern blotting and slot-blotting of total RNA using a human IRBP cDNA probe. Morphology was assessed by electron microscopy and immunocytochemistry. Levels of both IRBP protein and message in affected Abyssinian cat retinas were significantly reduced below normal as early as 4 weeks of age at the earliest stage of retinal disorientation. Opsin mRNA was more abundant in affected Abyssinian cat retinas than in control retinas. This was at least 1 year before the onset of clinical symptoms. The reduction in IRBP gene expression to levels significantly below normal well before the onset of retinal degeneration in affected Abyssinian cat retinas indicates that this represents a primary defect or at least an early problem that could itself cause adverse effects.

Rhodopsin levels in the central retinas of normal miniature poodles and those with progressive rod-cone degeneration

Visual pigment in normal miniature poodles and those with progressive rod-cone degeneration (prcd), a late-onset autosomal recessive photoreceptor degeneration, has been studied using imaging fundus reflectometry (IFR). The stage to which the disease had advanced in the animals with prcd was assessed with electroretinography (ERG). Measurements were carried out on seven affected, two heterozygous and three homozygous normal animals. The IFR measurements showed that the in situ difference spectrum of visual pigment measured in the central retina of the normal poodle is typical of vertebrate rhodopsin, with a maximum at about 510 nm. Rhodopsin regeneration following extensive bleaches continues for up to 70 min. In poodles with prcd, rhodopsin is spectrally normal and regenerates at normal rates. In young affected animals under 1 year of age, the final levels of rhodopsin could already be substantially reduced. Serial measurements of visual pigment in these dogs showed differences in the degree and spatial pattern of pigment loss and rate of progression between animals. The extent of visual pigment loss also differed among the older (greater than 4.5 years) affected animals: while in one animal no pigment could be detected, in another a central band of retina was relatively spared, and significant levels of visual pigment were measured within it. Pigment levels measured within the central 25 degrees of the retinas of poodles heterozygous for prcd were lower than those in normal animals, even though their ERGs were within the normal range.

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.