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

A Comprehensive Study of the Retinal Phenotype of Rpe65-Deficient Dogs

The Rpe65-deficient dog has been important for development of translational therapies of Leber congenital amaurosis type 2 (LCA2). The purpose of this study was to provide a comprehensive report of the natural history of retinal changes in this dog model. Rpe65-deficient dogs from 2 months to 10 years of age were assessed by fundus imaging, electroretinography (ERG) and vision testing (VT). Changes in retinal layer thickness were assessed by optical coherence tomography and on plastic retinal sections. ERG showed marked loss of retinal sensitivity, with amplitudes declining with age. Retinal thinning initially developed in the area centralis, with a slower thinning of the outer retina in other areas starting with the inferior retina. VT showed that dogs of all ages performed well in bright light, while at lower light levels they were blind. Retinal pigment epithelial (RPE) inclusions developed and in younger dogs and increased in size with age. The loss of photoreceptors was mirrored by a decline in ERG amplitudes. The slow degeneration meant that sufficient photoreceptors, albeit very desensitized, remained to allow for residual bright light vision in older dogs. This study shows the natural history of the Rpe65-deficient dog model of LCA2.

Optogenetic Gene Therapy for the Degenerate Retina: Recent Advances

The degeneration of light-detecting rod and cone photoreceptors in the human retina leads to severe visual impairment and ultimately legal blindness in millions of people worldwide. Multiple therapeutic options at different stages of degeneration are being explored but the majority of ongoing clinical trials involve adeno-associated viral (AAV) vector-based gene supplementation strategies for select forms of inherited retinal disease. Over 300 genes are associated with inherited retinal degenerations and only a small proportion of these will be suitable for gene replacement therapy. However, while the origins of disease may vary, there are considerable similarities in the physiological changes that occur in the retina. When early therapeutic intervention is not possible and patients suffer loss of photoreceptor cells but maintain remaining layers of cells in the neural retina, there is an opportunity for a universal gene therapy approach that can be applied regardless of the genetic origin of disease. Optogenetic therapy offers such a strategy by aiming to restore vision though the provision of light-sensitive molecules to surviving cell types of the retina that enable light perception through the residual neurons. Here we review the recent progress in attempts to restore visual function to the degenerate retina using optogenetic therapy. We focus on multiple pre-clinical models used in optogenetic strategies, discuss their strengths and limitations, and highlight considerations including vector and transgene designs that have advanced the field into two ongoing clinical trials.

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.

Dog models for blinding inherited retinal dystrophies

Spontaneous canine models exist for several inherited retinal dystrophies. This review will summarize the models and indicate where they have been used in translational gene therapy trials. The RPE65 gene therapy trials to treat childhood blindness are a good example of how studies in dogs have contributed to therapy development. Outcomes in human clinical trials are compared and contrasted with the result of the preclinical dog trials.

Gene therapy for eye as regenerative medicine? Lessons from RPE65 gene therapy for Leber's Congenital Amaurosis

Recombinant virus mediated gene therapy of Leber's Congenital Amaurosis has provided a wide range of data on the utility of gene replacement therapy for recessive diseases. Studies to date demonstrate that gene therapy in the eye is safe and can result in long-term recovery of visual function, but they also highlight that further research is required to identify optimum intervention time-points, target populations and the compatibility of associate therapies. This article is part of a directed issue entitled: Regenerative Medicine: the challenge of translation.

Successful gene therapy in older Rpe65-deficient dogs following subretinal injection of an adeno-associated vector expressing RPE65

Young Rpe65-deficient dogs have been used as a model for human RPE65 Leber congenital amaurosis (RPE65-LCA) in proof-of-concept trials of recombinant adeno-associated virus (rAAV) gene therapy. However, there are relatively few reports of the outcome of rAAV gene therapy in Rpe65-deficient dogs older than 2 years of age. The purpose of this study was to investigate the success of this therapy in older Rpe65-deficient dogs. Thirteen eyes were treated in dogs between 2 and 6 years old. An rAAV2 vector expressing the human RPE65 cDNA driven by the human RPE65 promoter was delivered by subretinal injection. Twelve of the 13 eyes had improved retinal function as assessed by electroretinography, and all showed improvement in vision at low lighting intensities. Histologic examination of five of the eyes was performed but found no correlation between electroretinogram (ERG) rescue and numbers of remaining photoreceptors. We conclude that functional rescue is still possible in older dogs and that the use of older Rpe65-deficient dogs, rather than young Rpe65-deficient dogs that have very little loss of photoreceptors, more accurately models the situation when treating human RPE65-LCA patients.

Immuno-histochemical analysis of rod and cone reaction to RPE65 deficiency in the inferior and superior canine retina

Mutations in the RPE65 gene are associated with autosomal recessive early onset severe retinal dystrophy. Morphological and functional studies indicate early and dramatic loss of rod photoreceptors and early loss of S-cone function, while L and M cones remain initially functional. The Swedish Briard dog is a naturally occurring animal model for this disease. Detailed information about rod and cone reaction to RPE65 deficiency in this model with regard to their location within the retina remains limited. The aim of this study was to analyze morphological parameters of cone and rod viability in young adult RPE65 deficient dogs in different parts of the retina in order to shed light on local disparities in this disease. In retinae of affected dogs, sprouting of rod bipolar cell dendrites and horizontal cell processes was dramatically increased in the inferior peripheral part of affected retinae, while central inferior and both superior parts did not display significantly increased sprouting. This observation was correlated with photoreceptor cell layer thickness. Interestingly, while L/M cone opsin expression was uniformly reduced both in the superior and inferior part of the retina, S-cone opsin expression loss was less severe in the inferior part of the retina. In summary, in retinae of young adult RPE65 deficient dogs, the degree of rod bipolar and horizontal cell sprouting as well as of S-cone opsin expression depends on the location. As the human retinal pigment epithelium (RPE) is pigmented similar to the RPE in the inferior part of the canine retina, and the kinetics of photoreceptor degeneration in humans seems to be similar to what has been observed in the inferior peripheral retina in dogs, this area should be studied in future gene therapy experiments in this model.

Human retinal gene therapy for Leber congenital amaurosis shows advancing retinal degeneration despite enduring visual improvement

Leber congenital amaurosis (LCA) associated with retinal pigment epithelium-specific protein 65 kDa (RPE65) mutations is a severe hereditary blindness resulting from both dysfunction and degeneration of photoreceptors. Clinical trials with gene augmentation therapy have shown partial reversal of the dysfunction, but the effects on the degeneration are not known. We evaluated the consequences of gene therapy on retinal degeneration in patients with RPE65-LCA and its canine model. In untreated RPE65-LCA patients, there was dysfunction and degeneration of photoreceptors, even at the earliest ages. Examined serially over years, the outer photoreceptor nuclear layer showed progressive thinning. Treated RPE65-LCA showed substantial visual improvement in the short term and no detectable decline from this new level over the long term. However, retinal degeneration continued to progress unabated. In RPE65-mutant dogs, the first one-quarter of their lifespan showed only dysfunction, and there was normal outer photoreceptor nuclear layer thickness retina-wide. Dogs treated during the earlier dysfunction-only stage showed improved visual function and dramatic protection of treated photoreceptors from degeneration when measured 5-11 y later. Dogs treated later during the combined dysfunction and degeneration stage also showed visual function improvement, but photoreceptor loss continued unabated, the same as in human RPE65-LCA. The results suggest that, in RPE65 disease treatment, protection from visual function deterioration cannot be assumed to imply protection from degeneration. The effects of gene augmentation therapy are complex and suggest a need for a combinatorial strategy in RPE65-LCA to not only improve function in the short term but also slow retinal degeneration in the long term.

Reproducibility of an objective four-choice canine vision testing technique that assesses vision at differing light intensities

OBJECTIVE

The increasing importance of canine retinal dystrophy models means accurate vision testing is needed. This study was performed to evaluate a four-choice vision testing technique for any difference in outcome measures with repeated evaluations of the same dogs.

ANIMALS STUDIED

  Four 11-month-old RPE65-deficient dogs.

PROCEDURES

Vision was evaluated using a previously described four-choice vision testing device. Four evaluations were performed at 2-week intervals. Vision was assessed at six different white light intensities (bright through dim), and each eye was evaluated separately. The ability to select the one of the four exit tunnels that was open at the far end was assessed ('choice of exit') and recorded as correct or incorrect first tunnel choice. 'Time to exit' the device was also recorded. Both outcomes were analyzed for significance using anova. We hypothesized that performance would improve with repeated testing (more correct choices and more rapid time to exit).

RESULTS

'Choice of exit' did not vary significantly between each evaluation (P = 0.12), in contrast 'time to exit' increased significantly (P = 0.012), and showed greater variability in dim light conditions.

CONCLUSIONS

We found no evidence to support the hypothesis that either measure of outcome worsened with repeated testing; in fact, the 'time to exit' outcome worsened rather than improved. The 'choice of exit' gave consistent results between trials. These outcome data indicate the importance of including a choice-based assessment of vision in addition to measurement of device transit time.

RPE65 gene therapy slows cone loss in Rpe65-deficient dogs

Recent clinical trials of retinal pigment epithelium gene (RPE65) supplementation therapy in Leber congenital amaurosis type 2 patients have demonstrated improvements in rod and cone function, but it may be some years before the effects of therapy on photoreceptor survival become apparent. The Rpe65-deficient dog is a very useful pre-clinical model in which to test efficacy of therapies, because the dog has a retina with a high degree of similarity to that of humans. In this study, we evaluated the effect of RPE65 gene therapy on photoreceptor survival in order to predict the potential benefit and limitations of therapy in patients. We examined the retinas of Rpe65-deficient dogs after RPE65 gene therapy to evaluate the preservation of rods and cone photoreceptor subtypes. We found that gene therapy preserves both rods and cones. While the moderate loss of rods in the Rpe65-deficient dog retina is slowed by gene therapy, S-cones are lost extensively and gene therapy can prevent that loss, although only within the treated area. Although LM-cones are not lost extensively, cone opsin mislocalization indicates that they are stressed, and this can be partially reversed by gene therapy. Our results suggest that gene therapy may be able to slow cone degeneration in patients if intervention is sufficiently early and also that it is probably important to treat the macula in order to preserve central function.

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.

Altered expression of retinal molecular markers in the canine RPE65 model of Leber congenital amaurosis

PURPOSE

Leber congenital amaurosis (LCA) is a group of childhood-onset retinal diseases characterized by severe visual impairment or blindness. One form is caused by mutations in the RPE65 gene, which encodes the retinal pigment epithelium (RPE) isomerase. In this study, the retinal structure and expression of molecular markers for different retinal cell types were characterized, and differences between control and RPE65 mutant dogs during the temporal evolution of the disease were analyzed.

METHODS

Retinas from normal and mutant dogs of different ages were examined by immunofluorescence with a panel of 16 different antibodies.

RESULTS

Cones and rods were preserved in the mutant retinas, and the number of cones was normal. However, there was altered expression of cone arrestin and delocalization of rod opsin. The ON bipolar cells showed sprouting of the dendritic arbors toward the outer nuclear layer (ONL) and retraction of their axons in the inner nuclear layer (INL). A decreased expression of GABA, and an increased expression of intermediate filament glial markers was also found in the mutant retinas. These changes were more evident in the adult than the young mutant retinas.

CONCLUSIONS

The structure of the retina is well preserved in the mutant retina, but several molecular changes take place in photoreceptors and in bipolar and amacrine cells. Some of these changes are structural, whereas others reflect a change in localization of the examined proteins. This study provides new information that can be applied to the interpretation of outcomes of retinal gene therapy in animal models and humans.

Leber congenital amaurosis due to RPE65 mutations and its treatment with gene therapy

Leber congenital amaurosis (LCA) is a rare hereditary retinal degeneration caused by mutations in more than a dozen genes. RPE65, one of these mutated genes, is highly expressed in the retinal pigment epithelium where it encodes the retinoid isomerase enzyme essential for the production of chromophore which forms the visual pigment in rod and cone photoreceptors of the retina. Congenital loss of chromophore production due to RPE65-deficiency together with progressive photoreceptor degeneration cause severe and progressive loss of vision. RPE65-associated LCA recently gained recognition outside of specialty ophthalmic circles due to early success achieved by three clinical trials of gene therapy using recombinant adeno-associated virus (AAV) vectors. The trials were built on multitude of basic, pre-clinical and clinical research defining the pathophysiology of the disease in human subjects and animal models, and demonstrating the proof-of-concept of gene (augmentation) therapy. Substantial gains in visual function of clinical trial participants provided evidence for physiologically relevant biological activity resulting from a newly introduced gene. This article reviews the current knowledge on retinal degeneration and visual dysfunction in animal models and human patients with RPE65 disease, and examines the consequences of gene therapy in terms of improvement of vision reported.

Using the NAFX to measure the effectiveness over time of gene therapy in canine LCA

PURPOSE

To use ocular motility recordings to determine the changes over time of infantile nystagmus syndrome (INS) in RPE65-deficient canines with Leber Congenital Amaurosis (LCA) and assess the time course of the recalibration of the ocular motor system (OMS).

METHODS

Nine dogs were treated bilaterally with AAV-RPE65. A second cohort of four dogs was treated with AAV2.RPE65, an optimized vector. Their fixation eye movements were recorded before treatment and at 4-week intervals for 3 months, by using high-speed (500 Hz) digital videography. The dogs were suspended in a sling and encouraged to fixate on distant (57 inches) targets at gaze angles varying between +/-15 degrees horizontally and +/-10 degrees vertically. The records for each eye were examined for qualitative changes in waveform and for quantitative changes in centralisation with the expanded nystagmus acuity function (NAFX) and compared with ERG results for restoration of receptor function.

RESULTS

First group: Before treatment, five of the dogs had clinically apparent INS with jerk, pendular, or both waveforms and with peak-to-peak amplitudes as great as 15 degrees . One dog had intermittent nystagmus. At the 1- and 2-month examinations, no change in nystagmus waveform or NAFX was observed in any of the initial dogs, while at 10 weeks, one dog treated bilaterally with the standard dosage showed reduced nystagmus in only one eye. The other eye did not respond to treatment, as confirmed by ERG. This result was unexpected since it was previously documented that unilateral treatment leads to bilateral reduction of INS. The other dog treated with the standard dosage showed no reduction of its small-amplitude, high-frequency pendular nystagmus despite positive ERG responses. Second group: Only one dog of the four had clinically detectable INS, similar in characteristics to that seen in the affected dogs of the first group. Unlike any previous dog studied, this one showed a damping of the nystagmus within the first 4 weeks after treatment.

CONCLUSIONS

In all but one of the cases in which OMS recalibration occurred, as measured by the clinical appearance of nystagmus and by quantitative measurement using the NAFX, the improvement was apparent no sooner than 10 weeks after treatment. Longer term, dose-related studies are needed to determine the minimum necessary degree of restored receptor functionality, the duration after rescue for recalibration of the OMS, and the conditions under which recalibration information can successfully affect the contralateral eye.

Evaluation of 9-cis-retinyl acetate therapy in Rpe65-/- mice

PURPOSE

Mice lacking retinal pigment epithelium-specific 65-kDa protein (RPE65) develop retinopathy and blindness resembling Leber congenital amaurosis. Effects of 9-cis-retinyl acetate (9-cis-R-Ac) on visual function and retinopathy progression were tested in Rpe65(-/-) mice.

METHODS

Young C57Bl/6 mice were given 9-cis-R-Ac in each of four different oil-based vehicle solutions by gastric gavage to identify the vehicle most suitable for drug delivery by measuring retinoid levels in plasma. Then doses of 9-cis-R-Ac ranging from 1 to 100 mg/kg were administered to 5- to 12-week-old Rpe65(-/-) mice by different treatment regimens, including single doses and either intermittent or daily doses for various periods up to 8 weeks. Retinoid effects on visual function were evaluated by electroretinography, retinoid analyses, histologic methods, and vision-dependent behavioral testing.

RESULTS

Soybean oil vehicle provided the highest 9-cis-R-Ac metabolite levels in plasma. Single doses of 9-cis-R-Ac (6.25-50 mg/kg) provided significant dose-dependent improvement in electroretinographic responses. Well-tolerated daily doses (1-12.5 mg/kg) for 2 weeks induced remarkable improvement of retinal function. Significant dose-dependent improvement of electroretinographic responses was observed 6 days after administration of 9-cis-R-Ac daily for 3 days at 1 to 12.5 mg/kg. Mice given either daily or intermittent 9-cis-R-Ac treatment at 1 and 4 mg/kg and evaluated 8 weeks later displayed dose-dependent improvement of retinal function and morphology, whereas retinal function deteriorated in control animals. Treated mice also performed better than control animals in vision-dependent behavioral tests.

CONCLUSIONS

Treatment with 9-cis-R-Ac improves visual function and preserves retinal morphology in Rpe65(-/-) mice.

Light at the end of the tunnel? Advances in the understanding and treatment of glaucoma and inherited retinal degeneration

Glaucoma and inherited retinal degeneration/dystrophy are leading causes of blindness in veterinary patients. Currently, there is no treatment for the loss of vision that characterizes both groups of diseases. However, this reality may soon change as recent advances in understanding of the disease processes allow researchers to develop new therapies aimed at preventing blindness and restoring vision to blind patients. Elucidating the molecular mechanisms of retinal ganglion cell death in glaucoma patients has led to the development of neuroprotective drugs which protect retinal cells and their function from the disastrous effects of elevated pressure. Identification of the genetic mutation responsible for inherited degenerations and dystrophies of the outer retina has enabled researchers using gene therapy to restore vision to blind dogs. Other patients may benefit from retinal transplantation, stem cell therapy, neuroprotective drugs, nutritional supplementation and even retinal prostheses. It is possible that soon it will be possible to restore sight to some blind patients.

Biological characterization of gene response in Rpe65-/- mouse model of Leber's congenital amaurosis during progression of the disease

RPE65 is the retinal isomerase essential for conversion of all-trans-retinyl ester to 11-cis-retinol in the visual cycle. Leber's congenital amaurosis (LCA), an autosomal recessive form of RP resulting in blindness, is commonly caused by mutations in the Rpe65 gene. Whereas the molecular mechanisms by which these mutations contribute to retinal disease remain largely unresolved, affected patients show marked RPE damage and photoreceptor degeneration. We evaluated gene expression in Rpe65-/- mouse model of LCA before and at the onset of photoreceptor cell death in 2, 4, and 6 month old animals. Microarray analysis demonstrates altered expression of genes involved in phototransduction, apoptosis regulation, cytoskeleton organization, and extracellular matrix (ECM) constituents. Cone-specific phototransduction genes are strongly decreased, reflecting early loss of cones. In addition, remaining rods show modified expression of genes encoding components of the cytoskeleton and ECM. This may affect rod physiology and interaction with the adjacent RPE and lead to loss of survival signals, as reflected by the alteration of apoptosis-related genes Together, these results suggest that RPE65 defect triggers an overall remodeling of the neurosensitive retina that may, in turn, disrupt photoreceptor homeostasis and induce apoptosis signaling cascade toward retinal cell death.

Congenital stationary night blindness in a Thoroughbred and a Paso Fino

This report documents congenital stationary night blindness (CSNB) in two non-Appaloosa horse breeds (Thoroughbred and Paso Fino). History of vision impairment since birth, normal ocular structures on ophthalmic examination, and electroretinographic findings were consistent with CSNB. In one horse (Thoroughbred), a 9-year follow-up was carried out. In the Paso Fino, severe vision impairment from birth to approximately 1 year of age in both dim and bright light situations led to humane euthanasia and histopathologic confirmation of the disorder.

Assessment of Structure and Function Over a 3-year Period after Gene Transfer in RPE65−/− dogs

AIM

To assess retinal structure and function over a 3-year period in a group of five RPE65-/- dogs treated by unilateral rAAV- mediated subretinal gene transfer.

METHODS

Post-operative functional follow-ups were performed using simultaneous, bilateral, full-field ERGs. Structure was evaluated by SLO using FL and ICG angiography and by EM.

RESULTS

Significant improvement of retinal function was observed through ERGs approximately 4 weeks following surgery. Scotopic b-wave amplitudes peaked 3 months after surgery. Then there was a successive reduction, although greater amplitudes than base-line values were observed at all post-operative time points. A-wave amplitudes increased at a later time than b-wave amplitudes and were sustained throughout the follow-up period. The increased cone function was preserved longer than the rod function. Angiography showed structural changes at the site of injection, corroborated by photoreceptor destruction observed ultrastructurally. Immediately adjacent to the subretinal injection area photoreceptor outer segments appeared normal.

CONCLUSION

Despite local structural alterations at the subretinal injection site, subretinal gene transfer in the RPE65 null mutation dog effectively increases retinal function for at least 3 years after surgery.

Large animal models and gene therapy

Over the last two decades, gene transfer experiments for the treatment of inherited or acquired diseases have mainly been performed in mice. While mice provide proof of principle and allow testing of a variety of therapeutic modalities, mouse models have some limitations, as only short-term experiments can be performed, their homogenous genetic background is unlike humans, and the knockout models do not always faithfully represent the human disease. Naturally occurring large animal models of human genetic diseases have become increasingly important despite the costs and the extensive clinical attention they require because of their similarities to human patients. Large animals are reasonably outbred, long lived allowing for longitudinal studies, are more similar in size to a neonate or small child providing an opportunity to address issues related to scaling up therapy, and many physiological parameters including the immune system are more similar to those in humans versus those in mice.

Advances in the molecular understanding of canine retinal diseases

Retinal dystrophies are a common cause of blindness in purebred dogs. Progressive retinal atrophy, the canine equivalent of retinitis pigmentosa in humans, is the most common dystrophy. Molecular studies have led to the identification of the genetic defect underlying some forms of progressive retinal atrophy and the mapping of the chromosomal location of others. Additionally, the gene mutation that causes a severe retinal dystrophy in the briard, which is the equivalent of Leber congenital amaurosis in humans, has been identified. These advances have led to the development of DNA-based diagnostic tests for some retinal dystrophies, thus facilitating their eradication. The study of these dystrophies in dogs has also provided useful information about the equivalent diseases in humans. Recently, gene therapy has been used to restore vision to dogs with a retinal dystrophy due to a mutation in the RPE65 gene. Such studies are important in the quest to develop therapies for similar conditions in humans.

Recombinant adeno-associated virus type 2-mediated gene delivery into the Rpe65-/- knockout mouse eye results in limited rescue

BACKGROUND

Leber's congenital amaurosis (LCA) is a severe form of retinal dystrophy. Mutations in the RPE65 gene, which is abundantly expressed in retinal pigment epithelial (RPE) cells, account for approximately 10-15% of LCA cases. In this study we used the high turnover, and rapid breeding and maturation time of the Rpe65^-/- knockout mice to assess the efficacy of using rAAV-mediated gene therapy to replace the disrupted RPE65 gene. The potential for rAAV-mediated gene treatment of LCA was then analyzed by determining the pattern of RPE65 expression, the physiological and histological effects that it produced, and any improvement in visual function.

METHODS

rAAV.RPE65 was injected into the subretinal space of Rpe65^-/- knockout mice and control mice. Histological and immunohistological analyses were performed to evaluate any rescue of photoreceptors and to determine longevity and pattern of transgene expression. Electron microscopy was used to examine ultrastructural changes, and electroretinography was used to measure changes in visual function following rAAV.RPE65 injection.

RESULTS

rAAV-mediated RPE65 expression was detected for up to 18 months post injection. The delivery of rAAV.RPE65 to Rpe65^-/- mouse retinas resulted in a transient improvement in the maximum b-wave amplitude under both scotopic and photopic conditions (76% and 59% increase above uninjected controls, respectively) but no changes were observed in a-wave amplitude. However, this increase in b-wave amplitude was not accompanied by any slow down in photoreceptor degeneration or apoptotic cell death. Delivery of rAAV.RPE65 also resulted in a decrease in retinyl ester lipid droplets and an increase in short wavelength cone opsin-positive cells, suggesting that the recovery of RPE65 expression has long-term benefits for retinal health.

CONCLUSION

This work demonstrated the potential benefits of using the Rpe65^-/- mice to study the effects and mechanism of rAAV.RPE65-mediated gene delivery into the retina. Although the functional recovery in this model was not as robust as in the dog model, these experiments provided important clues about the long-term physiological benefits of restoration of RPE65 expression in the retina.

Pigment epithelial changes in a strain of pigmented rabbits with low ERG b-wave amplitudes

Retinas from a strain of rabbits with low dark-adapted electroretinographic (ERG) b-wave amplitudes, and declining ERG responses with time, were examined by light and electron microscopy. Seven rabbits from the affected strain (13 months to 5 years old) and six control animals (6-26 months old) were included in the study. Small inclusions with an electron dense border, about 0.2-1.5 microm in diameter, were significantly (p<0.01) more numerous within the retinal pigment epithelial (RPE) cells in the affected rabbits than in the control animals. No morphological evidence of retinal degeneration was found. Further studies are needed to establish the functional defect in this strain of rabbits.

Retinal dystrophy of Swedish briard/briard-beagle dogs is due to a 4-bp deletion in RPE65

The RPE65 gene encodes a 65-kDa microsomal protein expressed exclusively in retinal pigment epithelium (RPE). Mutations in the human RPE65 gene have recently been identified in patients with autosomal recessive, severe, childhood-onset retinal dystrophy. Here we report the characterization of a 2.4-kb canine Rpe65 cDNA. The longest open reading frame predicts a 533-amino-acid protein with a calculated molecular mass of about 61 kDa prior to protein modification. Sequence comparison shows that RPE65 is highly conserved throughout mammalian evolution. We have identified a homozygous 4-bp deletion (485delAAGA) in putative exon 5 of the canine Rpe65 gene in affected animals of a highly inbred kinship of Swedish briard/briard-beagle dogs, in which an autosomal recessive, early-onset, and progressive retinal dystrophy segregates. The deletion results in a frameshift and leads to a premature stop codon after inclusion of 52 canine RPE65-unrelated amino acids from residue 153 onward. More than two-thirds of the wildtype polypeptide chain will be missing, and the mutant protein is most likely nonfunctional (null allele). Clinical features of the canine disease are quite similar to those described in human. Therefore this form of canine retinal dystrophy provides an attractive animal model of the corresponding human disorder with immediate significance for various therapeutic approaches, including RPE transplantation.

Organization of the canine gene encoding the E isoform of retinal guanylate cyclase (cGC-E) and exclusion of its involvement in the inherited retinal dystrophy of the Swedish Briard and Briard-beagle dogs

Intracellular cyclic GMP concentration is known to change in response to a wide variety of agents, including hormones, neurotransmitters or light. In vertebrate photoreceptors, different membrane-bound guanylate cyclase isoforms are responsible for cGMP synthesis and thus directly involved in termination of light signalling via the phototransduction cascade and recovery of the dark state. We have characterized a 4.7 kb long cDNA for the canine retinal guanylate cyclase isoform E (cGC-E) predicting a polypeptide of 1109 amino acids. The genomic structure and the complete sequence of the canine GC-E gene, which consists of 20 exons and spans about 14.5 kb, has also been determined. Northern blot analysis showed that GC-E was expressed in the canine retina as a 4.7 and 6.1 kb large transcript. RT-PCR analysis also detected low expression in cerebrum (occipital lobe). We performed a sequence analysis of the cGC-E gene in animals of a Swedish Briard and Briard-Beagle dog kinship in which an inherited retinal dystrophy is segregating. Several intragenic DNA polymorphisms were identified and used for segregation analysis which excluded cGC-E as a candidate gene for this type of canine retinal dystrophy.

Electrophysiology in some animal and human hereditary diseases involving the retinal pigment epithelium

The present paper surveys slow electrophysiological responses recorded by a d.c. technique in some hereditary eye diseases involving the retinal pigment epithelium (RPE) in animals (English setter dogs and Polish Owczarec Nizinny (PON) dogs with ceroid lipofuscinosis and Briard dogs with a slowly progressive rod-cone dystrophy associated with RPE inclusions) and in humans (Best's disease). The electroretinogram c-wave was typically either decreased in amplitude, lacking or replaced by a negative wave. These c-wave changes could be seen at fairly early stages of disease, when the a- and b-waves of the electroretinogram were still within normal limits.

Cloning and analysis of the cDNA encoding the rod G-protein transduction alpha, beta1 and gamma1 subunits from the canine retina

The canine (Canis familiaris) retinal rod transducin (G(T)) alpha, beta1 and gamma1 subunits were sequenced. Cloning of the cDNAs was accomplished by polymerase chain reaction (PCR) using degenerate and wild type retinal cDNA libraries as templates. The deduced amino acid sequences were highly similar to rod transducins from other species: G(T alpha) differed by 5 amino acids from the corresponding human sequence, whereas beta1 and gamma1 were identical to human sequences. The coding sequence of rod transducin was evaluated as a possible cause for the recessively inherited retinal rod-cone degeneration: there were no nucleotide differences between the wild type and retinal degenerate strains.

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.

Early morphometry of a retinal dystrophy in Labrador retrievers

Retinal morphometry was assessed in 7 dogs from a colony of Labrador Retrievers with dystrophic retinas at 1,2,3,4 and 18 months of age. Rod outer segment length and outer nuclear layer width were measured in the central, midperipheral and peripheral retina at six locations along the vertical meridian. Early striking regional differences in onset and rate of progression were characteristic for this inherited retinal degeneration. Notably, some areas of the retina developed fully and normally before degenerating. The central parts of the vertical meridians showed slightly disorganized rod outer segments already at 1 month of age and they were significantly shorter than those of control animals at 3 and 4 months (p < 0.01 and p < 0.001, respectively). The rod outer segments of the midperipheral and peripheral regions were, however, comparable to control animals as late as at 4 months of age. At 18 months the rod outer segments of dystrophic animals were significantly shorter in all retinal regions (p < 0.0005). At the age the outer nuclear layer of the dystrophic animals had become significantly thinner than that of control animals in all retinal regions (p < 0.001), indicating a clear visual cell loss. It is reasonable to characterize this as a retinal degeneration having a relatively slow progression, which enhances its relevance to conditions of clinical significance.