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

Electronic restoration of vision in those with photoreceptor degenerations

Complete loss of vision is one of the most feared sequelae of retinal disease. Currently, there are few if any treatment options available to patients that may slow or prevent blindness in diseases caused by photoreceptor loss, such as retinitis pigmentosa and age-related macular degeneration. Electronic restoration of vision has emerged over recent years as a safe and viable option for those who have lost substantial numbers of photoreceptors and who are severely vision impaired. Indeed, there has been a dramatic increase in our understanding of what is required to restore vision using an electronic retinal prosthesis. Recent reports show that for some patients, restoration of vision to the point of reading large letters is possible. In this review, we examine the types of implants currently under investigation and the results these devices have achieved clinically. We then consider a range of engineering and biological factors that may need to be considered to improve the visual performance of newer-generation devices. With added research, it is hoped that the level of vision achieved with newer generation devices will steadily improve, resulting in enhanced quality of life for those with severe vision impairment.

Pupillometric analysis for assessment of gene therapy in Leber Congenital Amaurosis patients

BACKGROUND

Objective techniques to assess the amelioration of vision in patients with impaired visual function are needed to standardize efficacy assessment in gene therapy trials for ocular diseases. Pupillometry has been investigated in several diseases in order to provide objective information about the visual reflex pathway and has been adopted to quantify visual impairment in patients with Leber Congenital Amaurosis (LCA). In this paper, we describe detailed methods of pupillometric analysis and a case study on three Italian patients affected by Leber Congenital Amaurosis (LCA) involved in a gene therapy clinical trial at two follow-up time-points: 1 year and 3 years after therapy administration.

METHODS

Pupillary light reflexes (PLR) were measured in patients who had received a unilateral subretinal injection in a clinical gene therapy trial. Pupil images were recorded simultaneously in both eyes with a commercial pupillometer and related software. A program was generated with MATLAB software in order to enable enhanced pupil detection with revision of the acquired images (correcting aberrations due to the inability of these severely visually impaired patients to fixate), and computation of the pupillometric parameters for each stimulus. Pupil detection was performed through Hough Transform and a non-parametric paired statistical test was adopted for comparison.

RESULTS

The developed program provided correct pupil detection also for frames in which the pupil is not totally visible. Moreover, it provided an automatic computation of the pupillometric parameters for each stimulus and enabled semi-automatic revision of computerized detection, eliminating the need for the user to manually check frame by frame. With reference to the case study, the amplitude of pupillary constriction and the constriction velocity were increased in the right (treated eye) compared to the left (untreated) eye at both follow-up time-points, showing stability of the improved PLR in the treated eye.

CONCLUSIONS

Our method streamlined the pupillometric analyses and allowed rapid statistical analysis of a range of parameters associated with PLR. The results confirm that pupillometry is a useful objective measure for the assessment of therapeutic effect of gene therapy in patients with LCA.

TRIAL REGISTRATION

ClinicalTrials.gov NCT00516477.

Gene therapy restores vision and delays degeneration in the CNGB1(-/-) mouse model of retinitis pigmentosa

Retinitis pigmentosa (RP) is a group of genetically heterogeneous, severe retinal diseases commonly leading to legal blindness. Mutations in the CNGB1a subunit of the rod cyclic nucleotide-gated (CNG) channel have been found to cause RP in patients. Here, we demonstrate the efficacy of gene therapy as a potential treatment for RP by means of recombinant adeno-associated viral (AAV) vectors in the CNGB1 knockout (CNGB1(-/-)) mouse model. To enable efficient packaging and rod-specific expression of the relatively large CNGB1a cDNA (~4 kb), we used an AAV expression cassette with a short rod-specific promoter and short regulatory elements. After injection of therapeutic AAVs into the subretinal space of 2-week-old CNGB1(-/-) mice, we assessed the restoration of the visual system by analyzing (i) CNG channel expression and localization, (ii) retinal function and morphology and (iii) vision-guided behavior. We found that the treatment not only led to expression of full-length CNGB1a, but also restored normal levels of the previously degraded CNGA1 subunit of the rod CNG channel. Both proteins co-localized in rod outer segments and formed regular CNG channel complexes within the treated area of the CNGB1(-/-) retina, leading to significant morphological preservation and a delay of retinal degeneration. In the electroretinographic analysis, we also observed restoration of rod-driven light responses. Finally, treated CNGB1(-/-) mice performed significantly better than untreated mice in a rod-dependent vision-guided behavior test. In summary, this work provides a proof-of-concept for the treatment of rod channelopathy-associated RP by AAV-mediated gene replacement.

AAV-mediated gene therapy in mouse models of recessive retinal degeneration

In recent years, more and more mutant genes that cause retinal diseases have been detected. At the same time, many naturally occurring mouse models of retinal degeneration have also been found, which show similar changes to human retinal diseases. These, together with improved viral vector quality allow more and more traditionally incurable inherited retinal disorders to become potential candidates for gene therapy. Currently, the most common vehicle to deliver the therapeutic gene into target retinal cells is the adenoassociated viral vector (AAV). Following delivery to the immuno-privileged subretinal space, AAV-vectors can efficiently target both retinal pigment epithelium and photoreceptor cells, the origin of most retinal degenerations. This review focuses on the AAV-based gene therapy in mouse models of recessive retinal degenerations, especially those in which delivery of the correct copy of the wild-type gene has led to significant beneficial effects on visual function, as determined by morphological, biochemical, electroretinographic and behavioral analysis. The past studies in animal models and ongoing successful LCA2 clinical trials, predict a bright future for AAV gene replacement treatment for inherited recessive retinal diseases.

AON-mediated Exon Skipping Restores Ciliation in Fibroblasts Harboring the Common Leber Congenital Amaurosis CEP290 Mutation

Leber congenital amaurosis (LCA) is a severe hereditary retinal dystrophy responsible for congenital or early-onset blindness. The most common disease-causing mutation (>10%) is located deep in intron 26 of the CEP290 gene (c.2991+1655A>G). It creates a strong splice donor site that leads to insertion of a cryptic exon encoding a premature stop codon. In the present study, we show that the use of antisense oligonucleotides (AONs) allow an efficient skipping of the mutant cryptic exon and the restoration of ciliation in fibroblasts of affected patients. These data support the feasibility of an AON-mediated exon skipping strategy to correct the aberrant splicing.

Gene therapy for Leber congenital amaurosis: advances and future directions

BACKGROUND

Leber congenital amaurosis (LCA) is a congenital retinal dystrophy that results in significant and often severe vision loss at an early age. Comprehensive analysis of the genetic mutations and phenotypic correlations in LCA patients has allowed for significant improvements in understanding molecular pathways of photoreceptor degeneration and dysfunction. The purpose of this article is to review the literature on the subject of retinal gene therapy for LCA, including historical descriptions, preclinical animal studies, and human clinical trials.

METHODS

A literature search of peer-reviewed and indexed publications from 1996-2011 using the PubMed search engine was performed. Key terms included "Leber congenital amaurosis", LCA, RPE65, "cone-rod dystrophy", "gene therapy", and "human trials" in various combinations. Seminal articles prior to 1996 were selected from primary sources and reviews from the initial search. Articles were chosen based on pertinence to clinical, genetic, and therapeutic topics reviewed in this manuscript. Fundus photographs from LCA patients were obtained retrospectively from the clinical practice of one of the authors (R.A.S).

RESULTS

Herein, we reviewed the literature on LCA as a genetic disease, the results of human gene therapy trials to date, and possible future directions towards treating inherited retinal diseases at the genetic level. Original descriptions of LCA by Theodor Leber and subsequent research demonstrate the severity of this disease with early-onset blindness. Discoveries of the causative heritable mutations revealed genes and protein products involved in photoreceptor development and visual transduction. Animal models have provided a means to test novel therapeutic strategies, namely gene therapy. Stemming from these experiments, three independent clinical trials tested the safety of subretinal delivery of viral gene therapy to patients with mutations in the RPE65 gene. More recently, efficacy studies have been conducted with encouraging results.

CONCLUSIONS

Initial safety studies indicated promising results of subretinal delivery of viral vector with subclinical immunologic or surgical sequelae. Overall, these initial studies demonstrate that viral vector gene therapy results are very promising, safe, and effective. Future studies measuring potential improvement in photoreceptor function may rely on recent advances in retinal imaging and electrophysiologic testing.

Gucy2f zebrafish knockdown--a model for Gucy2d-related leber congenital amaurosis

Mutations in retinal-specific guanylate cyclase (Gucy2d) are associated with Leber congenital amaurosis-1 (LCA1). Zebrafish offer unique advantages relative to rodents, including their excellent color vision, precocious retinal development, robust visual testing strategies, low cost, relatively easy transgenesis and shortened experimental times. In this study we will demonstrate the feasibility of using gene-targeting in the zebrafish as a model for the photoreceptor-specific GUCY2D-related LCA1, by reporting the visual phenotype and retinal histology resulting from Gucy2f knockdown. Gucy2f zebrafish LCA-orthologous cDNA was identified and isolated by PCR amplification. Its expression pattern was determined by whole-mount in-situ hybridization and its function was studied by gene knockdown using two different morpholino-modified oligos (MO), one that blocks translation of Gucy2f and one that blocks splicing of Gucy2f. Visual function was assessed with an optomotor assay on 6-days-post-fertilization larvae, and by analyzing changes in retinal histology. Gucy2f knockdown resulted in significantly lower vision as measured by the optomotor response compared with uninjected and control MO-injected zebrafish larvae. Histological changes in the Gucy2f-knockdown larvae included loss and shortening of cone and rod outer segments. A zebrafish model of Gucy2f-related LCA1 displays early visual dysfunction and photoreceptor layer dystrophy. This study serves as proof of concept for the use of zebrafish as a simple, inexpensive model with excellent vision on which further study of LCA-related genes is possible.

RPE65 is present in human green/red cones and promotes photopigment regeneration in an in vitro cone cell model

RPE65 is an abundantly expressed protein within the retinal pigment epithelium (RPE) of the eye that is required for retinoid metabolism to support vision. Its genetic mutations are linked to the congenital disease Leber congenital amaurosis Type 2 (LCA2) characterized by the early onset of central vision loss. Current gene therapy trials have targeted restoration of functional RPE65 within the RPE of these patients with some success. Recent data show that RPE65 is also present within mouse cones to promote function. In this study, we evaluated the presence of RPE65 in human cones and investigated its potential mechanism for supporting cone function in the 661W cone cell line. We found that RPE65 was selectively expressed in human green/red cones but absent from blue cones and mediated ester hydrolysis for photopigment synthesis in vitro. These data suggest that cone RPE65 supports human diurnal vision, potentially enhancing our strategies for treating LCA2.

Gene therapy for retinitis pigmentosa caused by MFRP mutations: human phenotype and preliminary proof of concept

Autosomal recessive retinitis pigmentosa (RP), a heterogeneous group of degenerations of the retina, can be due to mutations in the MFRP (membrane-type frizzled-related protein) gene. A patient with RP with MFRP mutations, one of which is novel and the first splice site mutation reported, was characterized by noninvasive retinal and visual studies. The phenotype, albeit complex, suggested that this retinal degeneration may be a candidate for gene-based therapy. Proof-of-concept studies were performed in the rd6 Mfrp mutant mouse model. The fast-acting tyrosine-capsid mutant AAV8 (Y733F) vector containing the small chicken β-actin promoter driving the wild-type mouse Mfrp gene was used. Subretinal vector delivery on postnatal day 14 prevented retinal degeneration. Treatment rescued rod and cone photoreceptors, as assessed by electroretinography and retinal histology at 2 months of age. This AAV-mediated gene delivery also resulted in robust MFRP expression predominantly in its normal location within the retinal pigment epithelium apical membrane and its microvilli. The clinical features of MFRP-RP and our preliminary data indicating a response to gene therapy in the rd6 mouse suggest that this form of RP is a potential target for gene-based therapy.

Gene therapy rescues photoreceptor blindness in dogs and paves the way for treating human X-linked retinitis pigmentosa

Hereditary retinal blindness is caused by mutations in genes expressed in photoreceptors or retinal pigment epithelium. Gene therapy in mouse and dog models of a primary retinal pigment epithelium disease has already been translated to human clinical trials with encouraging results. Treatment for common primary photoreceptor blindness, however, has not yet moved from proof of concept to the clinic. We evaluated gene augmentation therapy in two blinding canine photoreceptor diseases that model the common X-linked form of retinitis pigmentosa caused by mutations in the retinitis pigmentosa GTPase regulator (RPGR) gene, which encodes a photoreceptor ciliary protein, and provide evidence that the therapy is effective. After subretinal injections of adeno-associated virus-2/5-vectored human RPGR with human IRBP or GRK1 promoters, in vivo imaging showed preserved photoreceptor nuclei and inner/outer segments that were limited to treated areas. Both rod and cone photoreceptor function were greater in treated (three of four) than in control eyes. Histopathology indicated normal photoreceptor structure and reversal of opsin mislocalization in treated areas expressing human RPGR protein in rods and cones. Postreceptoral remodeling was also corrected: there was reversal of bipolar cell dendrite retraction evident with bipolar cell markers and preservation of outer plexiform layer thickness. Efficacy of gene therapy in these large animal models of X-linked retinitis pigmentosa provides a path for translation to human treatment.

Gene therapy: light is finally in the tunnel

After two decades of ups and downs, gene therapy has recently achieved a milestone in treating patients with Leber's congenital amaurosis (LCA). LCA is a group of inherited blinding diseases with retinal degeneration and severe vision loss in early infancy. Mutations in several genes, including RPE65, cause the disease. Using adeno-associated virus as a vector, three independent teams of investigators have recently shown that RPE65 can be delivered to retinal pigment epithelial cells of LCA patients by subretinal injections resulting in clinical benefits without side effects. However, considering the whole field of gene therapy, there are still major obstacles to clinical applications for other diseases. These obstacles include innate and immune barriers to vector delivery, toxicity of vectors and the lack of sustained therapeutic gene expression. Therefore, new strategies are needed to overcome these hurdles for achieving safe and effective gene therapy. In this article, we shall review the major advancements over the past two decades and, using lung gene therapy as an example, discuss the current obstacles and possible solutions to provide a roadmap for future gene therapy research.

Tetradecanoylphorbol-13-acetate (TPA) significantly increases AAV2/5 transduction of human neuronal cells in vitro

Recombinant adeno-associated virus type 2 (AAV2) vectors have shown great promise in current ophthalmology clinical trials targeting gene delivery to the retinal pigment epithelium (RPE). To treat the majority of retinal diseases, however, gene delivery would need to be targeted to photoreceptor neurons of the outer retina. AAV2 pseudotyped with the AAV5 capsid (AAV2/5) has shown far greater transduction efficiency in photoreceptors compared to standard AAV2 vectors. For clinical trial applications using gene therapy, it is helpful to generate pre-clinical data in human cells wherever possible. There is however very little data, indeed some controversy, as to whether AAV2/5 can be used effectively in differentiated neurons in culture. In this study we show that transduction of the human neuroblastoma cell line SH-SY5Y with recombinant AAV2/5 expressing GFP is well tolerated. Furthermore, we explore the mechanism whereby exposure to retinoic acid (RA) and the phorbol ester 12-O-Tetradecanoylphorbol-13- acetate (TPA) can induce this cell line to differentiate into a stable population of human neurons, with significantly increased levels of AAV2/5 transduction. These observations may be helpful for assessing AAV2/5 vectors in vitro, particularly where it is necessary to generate pre-clinical data for clinical trials of gene therapy to the human central nervous system.

Downregulation of apoptosis-inducing factor in Harlequin mice induces progressive and severe optic atrophy which is durably prevented by AAV2-AIF1 gene therapy

The Harlequin mutant mouse, characterized by loss of function of apoptosis-inducing factor, represents a reliable genetic model that resembles pathologies caused by human mitochondrial complex I deficiency. Therefore, we extensively characterized the retinal morphology and function of Harlequin mice during the course of neuronal cell death leading to blindness, with the aim of preventing optic atrophy. Retinas and optic nerves from these mice showed an isolated respiratory chain complex I defect correlated with retinal ganglion cell loss, optic atrophy, glial and microglial cell activation. All of these changes led to irreversible vision loss. In control mice, retinas AIF1 messenger RNA was 2.3-fold more abundant than AIF2, both messenger RNAs being sorted to the mitochondrial surface. In Harlequin mouse retinas, there was a 96% decrease of both AIF1 and AIF2 messenger RNA steady-state levels. We attained substantial and long-lasting protection of retinal ganglion cell and optic nerve integrity, the preservation of complex I function in optic nerves, as well as the prevention of glial and microglial responses after intravitreal administration of an AAV2 vector containing the full-length open reading frame and the 3' untranslated region of the AIF1 gene. Therefore, we demonstrate that gene therapy for mitochondrial diseases due to mutations in nuclear DNA can be achieved, so long as the 'therapeutic gene' permits the accurate cellular localization of the corresponding messenger RNA.

Gene therapy for leber congenital amaurosis caused by RPE65 mutations: safety and efficacy in 15 children and adults followed up to 3 years

OBJECTIVE

To determine the safety and efficacy of subretinal gene therapy in the RPE65 form of Leber congenital amaurosis using recombinant adeno-associated virus 2 (rAAV2) carrying the RPE65 gene.

DESIGN

Open-label, dose-escalation phase I study of 15 patients (range, 11-30 years of age) evaluated after subretinal injection of the rAAV2- RPE65 vector into the worse-functioning eye. Five cohorts represented 4 dose levels and 2 different injection strategies.

MAIN OUTCOME MEASURES

Primary outcomes were systemic and ocular safety. Secondary outcomes assayed visual function with dark-adapted full-field sensitivity testing and visual acuity with Early Treatment Diabetic Retinopathy Study charts. Further assays included immune responses to the vector, static visual fields, pupillometry, mobility performance, and optical coherence tomography.

RESULTS

No systemic toxicity was detected; ocular adverse events were related to surgery. Visual function improved in all patients to different degrees; improvements were localized to treated areas. Cone and rod sensitivities increased significantly in the study eyes but not in the control eyes. Minor acuity improvements were recorded in many study and control eyes. Major acuity improvements occurred in study eyes with the lowest entry acuities and parafoveal fixation loci treated with subretinal injections. Other patients with better foveal structure lost retinal thickness and acuity after subfoveal injections.

CONCLUSIONS

Gene therapy for Leber congenital amaurosis caused by RPE65 mutations is sufficiently safe and substantially efficacious in the extrafoveal retina. There is no benefit and some risk in treating the fovea. No evidence of age-dependent effects was found. Our results point to specific treatment strategies for subsequent phases.

APPLICATION TO CLINICAL PRACTICE

Gene therapy for inherited retinal disease has the potential to become a future part of clinical practice.

TRIAL REGISTRATION

clinicaltrials.gov Identifier: NCT00481546.

Functional and anatomic consequences of subretinal dosing in the cynomolgus macaque

OBJECTIVE

To characterize functional and anatomic sequelae of a bleb induced by subretinal injection.

METHODS

Subretinal injections (100 μL) of balanced salt solution were placed in the superotemporal macula of 1 eye in 3 cynomolgus macaques. Fellow eyes received intravitreal injections (100 μL) of balanced salt solution. Fundus photography, ocular coherence tomography, and multifocal electroretinography were performed before and immediately after injection and again at intervals up to 3 months postinjection. Histopathologic analyses included transmission electron microscopy and immunohistochemistry for glial fibrillary acidic protein, rhodopsin, M/L-cone opsin, and S-cone opsin.

RESULTS

Retinas were reattached by 2 days postinjection (seen by ocular coherence tomography). Multifocal electroretinography waveforms were suppressed post-subretinal injection within the subretinal injection bleb and, surprisingly, also in regions far peripheral to this area. Multifocal electroretinography amplitudes were nearly completely recovered by 90 days. The spectral-domain ocular coherence tomography inner segment-outer segment line had decreased reflectivity at 92 days. Glial fibrillary acidic protein and S-cone opsin staining were unaffected. Rhodopsin and M/L-cone opsins were partially displaced into the inner segments. Transmission electron microscopy revealed disorganization of the outer segment rod (but not cone) discs. At all postinjection intervals, eyes with intravitreal injection were similar to baseline.

CONCLUSIONS

Subretinal injection is a promising route for drug delivery to the eye. Three months post-subretinal injection, retinal function was nearly recovered, although reorganization of the outer segment rod disc remained disrupted. Understanding the functional and anatomic effects of subretinal injection is important for interpretation of the effects of compounds delivered to the subretinal space.

CLINICAL RELEVANCE

Subretinal injection is a new potential route for drug delivery to the eye. Separating drug effects from the procedural effects is critical.

Human CRB1-associated retinal degeneration: comparison with the rd8 Crb1-mutant mouse model

PURPOSE

To investigate the human disease due to CRB1 mutations and compare results with the Crb1-mutant rd8 mouse.

METHODS

Twenty-two patients with CRB1 mutations were studied. Function was assessed with perimetry and electroretinography (ERG) and retinal structure with optical coherence tomography (OCT). Cortical structure and function were quantified with magnetic resonance imaging (MRI). Rd8 mice underwent ERG, OCT, and retinal histopathology.

RESULTS

Visual acuities ranged from 20/25 to light perception. Rod ERGs were not detectable; small cone signals were recordable. By perimetry, small central visual islands were separated by midperipheral scotomas from far temporal peripheral islands. The central islands were cone mediated, whereas the peripheral islands retained some rod function. With OCT, there were small foveal islands of thinned outer nuclear layer (ONL) surrounded by thick delaminated retina with intraretinal hyperreflective lesions. MRI showed structurally normal optic nerves and only subtle changes to occipital lobe white and gray matter. Functional MRI indicated that whole-brain responses from patients were of reduced amplitude and spatial extent compared with those of normal controls. Rd8 mice had essentially normal ERGs; OCT and histopathology showed patchy retinal disorganization with pseudorosettes more pronounced in ventral than in dorsal retina. Photoreceptor degeneration was associated with dysplastic regions.

CONCLUSIONS

CRB1 mutations lead to early-onset severe loss of vision with thickened, disorganized, nonseeing retina. Impaired peripheral vision can persist in late disease stages. Rd8 mice also have a disorganized retina, but there is sufficient photoreceptor integrity to produce largely normal retinal function. Differences between human and mouse diseases will complicate proof-of-concept studies intended to advance treatment initiatives.

Clinical gene therapy for the treatment of RPE65-associated Leber congenital amaurosis

INTRODUCTION

The positive results of pioneering clinical trials using gene therapy as treatment for patients with Leber congenital amaurosis (LCA) have ushered in a new era of molecular retinal therapeutics for LCA, other blinding retinal disorders and gene therapy applications.

AREAS COVERED

This review describes the role of retinal pigment epithelium-specific 65 kDa protein (RPE65) in the visual cycle and how RPE65 deficiency results in LCA; the extensive preclinical studies with recombinant adeno-associated virus (rAAV)-RPE65 gene vectors; and the human rAAV-RPE65 and related gene therapy clinical trials and studies. The literature search included a review of primary sources (e.g., journal articles) that reported study data results and key secondary sources such as meta-reviews available through PubMed, as well as reviews of clinical trial descriptions and results as reported in clinicaltrials.gov, conference publications and news releases.

EXPERT OPINION

LCA-RPE65 gene therapy is an example of successful, innovative, translational research. Further research is needed regarding how retinal gene therapy can be improved.

Retinyl ester hydrolases and their roles in vitamin A homeostasis

In mammals, dietary vitamin A intake is essential for the maintenance of adequate retinoid (vitamin A and metabolites) supply of tissues and organs. Retinoids are taken up from animal or plant sources and subsequently stored in form of hydrophobic, biologically inactive retinyl esters (REs). Accessibility of these REs in the intestine, the circulation, and their mobilization from intracellular lipid droplets depends on the hydrolytic action of RE hydrolases (REHs). In particular, the mobilization of hepatic RE stores requires REHs to maintain steady plasma retinol levels thereby assuring constant vitamin A supply in times of food deprivation or inadequate vitamin A intake. In this review, we focus on the roles of extracellular and intracellular REHs in vitamin A metabolism. Furthermore, we will discuss the tissue-specific function of REHs and highlight major gaps in the understanding of RE catabolism. This article is part of a Special Issue entitled Retinoid and Lipid Metabolism.

Key enzymes of the retinoid (visual) cycle in vertebrate retina

A major goal in vision research over the past few decades has been to understand the molecular details of retinoid processing within the retinoid (visual) cycle. This includes the consequences of side reactions that result from delayed all-trans-retinal clearance and condensation with phospholipids that characterize a variety of serious retinal diseases. Knowledge of the basic retinoid biochemistry involved in these diseases is essential for development of effective therapeutics. Photoisomerization of the 11-cis-retinal chromophore of rhodopsin triggers a complex set of metabolic transformations collectively termed phototransduction that ultimately lead to light perception. Continuity of vision depends on continuous conversion of all-trans-retinal back to the 11-cis-retinal isomer. This process takes place in a series of reactions known as the retinoid cycle, which occur in photoreceptor and RPE cells. All-trans-retinal, the initial substrate of this cycle, is a chemically reactive aldehyde that can form toxic conjugates with proteins and lipids. Therefore, much experimental effort has been devoted to elucidate molecular mechanisms of the retinoid cycle and all-trans-retinal-mediated retinal degeneration, resulting in delineation of many key steps involved in regenerating 11-cis-retinal. Three particularly important reactions are catalyzed by enzymes broadly classified as acyltransferases, short-chain dehydrogenases/reductases and carotenoid/retinoid isomerases/oxygenases. This article is part of a Special Issue entitled: Retinoid and Lipid Metabolism.

Gene therapy regenerates protein expression in cone photoreceptors in Rpe65(R91W/R91W) mice

Cone photoreceptors mediate visual acuity under daylight conditions, so loss of cone-mediated central vision of course dramatically affects the quality of life of patients suffering from retinal degeneration. Therefore, promoting cone survival has become the goal of many ocular therapies and defining the stage of degeneration that still allows cell rescue is of prime importance. Using the Rpe65^R91W/R91W mouse, which carries a mutation in the Rpe65 gene leading to progressive photoreceptor degeneration in both patients and mice, we defined stages of retinal degeneration that still allow cone rescue. We evaluated the therapeutic window within which cones can be rescued, using a subretinal injection of a lentiviral vector driving expression of RPE65 in the Rpe65^R91W/R91W mice. Surprisingly, when applied to adult mice (1 month) this treatment not only stalls or slows cone degeneration but, actually, induces cone-specific protein expression that was previously absent. Before the intervention only part of the cones (40% of the number found in wild-type animals) in the Rpe65^R91W/R91W mice expressed cone transducin (GNAT2); this fraction increased to 64% after treatment. Correct S-opsin localization is also recovered in the transduced region. In consequence these results represent an extended therapeutic window compared to the Rpe65^-/- mice, implying that patients suffering from missense mutations might also benefit from a prolonged therapeutic window. Moreover, cones are not only rescued during the course of the degeneration, but can actually recover their initial status, meaning that a proportion of altered cones in chromophore deficiency-related disease can be rehabilitated even though they are severely affected.

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.

Animal models of retinal disease

Diseases of the retina are the leading causes of blindness in the industrialized world. The recognition that animals develop retinal diseases with similar traits to humans has led to not only a dramatic improvement in our understanding of the pathogenesis of retinal disease but also provided a means for testing possible treatment regimes and successful gene therapy trials. With the advent of genetic and molecular biological tools, the association between specific gene mutations and retinal signs has been made. Animals carrying natural mutations usually in one gene now provide well-established models for a host of inherited retinal diseases, including retinitis pigmentosa, Leber congenital amaurosis, inherited macular degeneration, and optic nerve diseases. In addition, the development of transgenic technologies has provided a means by which to study the effects of these and novel induced mutations on retinal structure and function. Despite these advances, there is a paucity of suitable animal models for complex diseases, including age-related macular degeneration (AMD) and diabetic retinopathy, largely because these diseases are not caused by single gene defects, but involve complex genetics and/or exacerbation through environmental factors, epigenetic, or other modes of genetic influence. In this review, we outline in detail the available animal models for inherited retinal diseases and how this information has furthered our understanding of retinal diseases. We also examine how transgenic technologies have helped to develop our understanding of the role of isolated genes or pathways in complex diseases like AMD, diabetes, and glaucoma.

Molecular anthropology meets genetic medicine to treat blindness in the North African Jewish population: human gene therapy initiated in Israel

The history of the North African Jewish community is ancient and complicated with a number of immigration waves and persecutions dramatically affecting its population size. A decade-long process in Israel of clinical-molecular screening of North African Jews with incurable autosomal recessive blindness led to the identification of a homozygous splicing mutation (c.95-2A > T; IVS2-2A > T) in RPE65, the gene encoding the isomerase that catalyzes a key step in the retinoid-visual cycle, in patients from 10 unrelated families. A total of 33 patients (four now deceased) had the severe childhood blindness known as Leber congenital amaurosis (LCA), making it the most common cause of retinal degeneration in this population. Haplotype analysis in seven of the patients revealed a shared homozygous region, indicating a population-specific founder mutation. The age of the RPE65 founder mutation was estimated to have emerged 100-230 (mean, 153) generations ago, suggesting it originated before the establishment of the Jewish community in North Africa. Individuals with this RPE65 mutation were characterized with retinal studies to determine if they were candidates for gene replacement, the recent and only therapy to date for this otherwise incurable blindness. The step from molecular anthropological studies to application of genetic medicine was then taken, and a representative of this patient subgroup was treated with subretinal rAAV2-RPE65 gene therapy. An increase in vision was present in the treated area as early as 15 days after the intervention. This process of genetically analyzing affected isolated populations as a screen for gene-based therapy suggests a new paradigm for disease diagnosis and treatment.

Structural characterization of the dual glycan binding adeno-associated virus serotype 6

The three-dimensional structure of adeno-associated virus (AAV) serotype 6 (AAV6) was determined using cryo-electron microscopy and image reconstruction and using X-ray crystallography to 9.7- and 3.0-Å resolution, respectively. The AAV6 capsid contains a highly conserved, eight-stranded (βB to βI) β-barrel core and large loop regions between the strands which form the capsid surface, as observed in other AAV structures. The loops show conformational variation compared to other AAVs, consistent with previous reports that amino acids in these loop regions are involved in differentiating AAV receptor binding, transduction efficiency, and antigenicity properties. Toward structure-function annotation of AAV6 with respect to its unique dual glycan receptor (heparan sulfate and sialic acid) utilization for cellular recognition, and its enhanced lung epithelial transduction compared to other AAVs, the capsid structure was compared to that of AAV1, which binds sialic acid and differs from AAV6 in only 6 out of 736 amino acids. Five of these residues are located at or close to the icosahedral 3-fold axis of the capsid, thereby identifying this region as imparting important functions, such as receptor attachment and transduction phenotype. Two of the five observed amino acids are located in the capsid interior, suggesting that differential AAV infection properties are also controlled by postentry intracellular events. Density ordered inside the capsid, under the 3-fold axis in a previously reported, conserved AAV DNA binding pocket, was modeled as a nucleotide and a base, further implicating this capsid region in AAV genome recognition and/or stabilization.

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.

Membrane-binding and enzymatic properties of RPE65

Regeneration of visual pigments is essential for sustained visual function. Although the requirement for non-photochemical regeneration of the visual chromophore, 11-cis-retinal, was recognized early on, it was only recently that the trans to cis retinoid isomerase activity required for this process was assigned to a specific protein, a microsomal membrane enzyme called RPE65. In this review, we outline progress that has been made in the functional characterization of RPE65. We then discuss general concepts related to protein-membrane interactions and the mechanism of the retinoid isomerization reaction and describe some of the important biochemical and structural features of RPE65 with respect to its membrane-binding and enzymatic properties.