The human rhodopsin kinase promoter in an AAV5 vector confers rod- and cone-specific expression in the primate retina

Retinal Ciliopathies and Potential Gene Therapies: A Focus on Human iPSC-Derived Organoid Models

The human photoreceptor function is dependent on a highly specialised cilium. Perturbation of cilial function can often lead to death of the photoreceptor and loss of vision. Retinal ciliopathies are a genetically diverse range of inherited retinal disorders affecting aspects of the photoreceptor cilium. Despite advances in the understanding of retinal ciliopathies utilising animal disease models, they can often lack the ability to accurately mimic the observed patient phenotype, possibly due to structural and functional deviations from the human retina. Human-induced pluripotent stem cells (hiPSCs) can be utilised to generate an alternative disease model, the 3D retinal organoid, which contains all major retinal cell types including photoreceptors complete with cilial structures. These retinal organoids facilitate the study of disease mechanisms and potential therapies in a human-derived system. Three-dimensional retinal organoids are still a developing technology, and despite impressive progress, several limitations remain. This review will discuss the state of hiPSC-derived retinal organoid technology for accurately modelling prominent retinal ciliopathies related to genes, including RPGR, CEP290, MYO7A, and USH2A. Additionally, we will discuss the development of novel gene therapy approaches targeting retinal ciliopathies, including the delivery of large genes and gene-editing techniques.

Intravitreal injection of a rationally designed AAV capsid library in non-human primate identifies variants with enhanced retinal transduction and neutralizing antibody evasion

Adeno-associated virus (AAV) continues to be the gold standard vector for therapeutic gene delivery and has proven especially useful for treating ocular disease. Intravitreal injection (IVtI) is a promising delivery route because it increases accessibility of gene therapies to larger patient populations. However, data from clinical and non-human primate (NHP) studies utilizing currently available capsids indicate that anatomical barriers to AAV and pre-existing neutralizing antibodies can restrict gene expression to levels that are "sub-therapeutic" in a substantial proportion of patients. Here, we performed a combination of directed evolution in NHPs of an AAV2-based capsid library with simultaneous mutations across six surface-exposed variable regions and rational design to identify novel capsid variants with improved retinal transduction following IVtI. Following two rounds of screening in NHP, enriched variants were characterized in intravitreally injected mice and NHPs and shown to have increased transduction relative to AAV2. Lead capsid variant, P2-V1, demonstrated an increased ability to evade neutralizing antibodies in human vitreous samples relative to AAV2 and AAV2.7m8. Taken together, this study further contributed to our understanding of the selective pressures associated with retinal transduction via the vitreous and identified promising novel AAV capsid variants for clinical consideration.

Gene augmentation therapy to rescue degenerative photoreceptors in a Cwc27 mutant mouse model

Previous reports have demonstrated that defects in the spliceosome-associated protein CWC27 can lead to the degeneration of retinal cells in Cwc27 mutant mouse models. However, it is unknown whether gene replacement therapy can rescue this phenotype. The purpose of this study was to evaluate whether AAV based gene therapy could rescue the retinal degeneration observed in Cwc27 mutant mice. By 6 months of age, Cwc27 mutant mice show a retinal degenerative phenotype, including morphological and functional abnormalities, primarily driven by the death of photoreceptors. We hypothesize that subretinal injection of AAV8 to drive exogenous CWC27 protein expression will improve the retinal phenotype. We evaluated these improvements after gene therapy with electroretinography (ERG) and histology, either hematoxylin and eosin (H&E) or immunostaining. In this study, we demonstrated that subretinal injection of AAV8-GRK-Cwc27-FLAG in mutant mice can improve the functionality and morphology of the retina. Immunostaining analyses revealed a notable decrease in photoreceptor degeneration, including cone cell degeneration, in the AAV-injected eyes compared to the PBS-injected eyes. Based on these results, gene replacement therapy could be a promising method for treating retinal degeneration caused by mutations in Cwc27.

Preclinical studies in support of phase I/II clinical trials to treat GUCY2D-associated Leber congenital amaurosis

Mutations in GUCY2D are associated with severe early-onset retinal dystrophy, Leber congenital amaurosis type 1 (LCA1), a leading cause of blindness in children. Despite a high degree of visual disturbance stemming from photoreceptor dysfunction, patients with LCA1 largely retain normal photoreceptor structure, suggesting that they are good candidates for gene replacement therapy. The purpose of this study was to conduct the preclinical and IND-enabling experiments required to support clinical application of AAV5-hGRK1-GUCY2D in patients harboring biallelic recessive mutations in GUCY2D. Preclinical studies were conducted in mice to evaluate the effect of vector manufacturing platforms and transgene species on the therapeutic response. Dose-ranging studies were conducted in cynomolgus monkeys to establish the minimum dose required for efficient photoreceptor transduction. Good laboratory practice (GLP) studies evaluated systemic biodistribution in rats and toxicology in non-human primates (NHPs). These results expanded our knowledge of dose response for an AAV5-vectored transgene under control of the human rhodopsin kinase (hGRK1) promoter in NHPs with respect to photoreceptor transduction and safety and, in combination with the rat biodistribution and mouse efficacy studies, informed the design of a first-in-human clinical study in patients with LCA1.

Regulation of the Keratocyte Phenotype and Cell Behavior Derived from Human Induced Pluripotent Stem Cells by Substrate Stiffness

Substrate stiffness has been indicated as an important factor to control stem cell fate, including proliferation and differentiation. To optimize the stiffness for the differentiation process from h-iPSCs (human induced pluripotent stem cells) into h-iCSCs (human corneal stromal cells derived from h-iPSCs) and the phenotypic maintenance of h-iCSCs in vitro, h-iPSCs were cultured on matrigel-coated tissue culture plate (TCP) (10⁶ kPa), matrigel-coated polydimethylsiloxane (PDMS) 184 (1250 kPa), and matrigel-coated PDMS 527 (4 kPa) before they were differentiated to h-iCSCs. Immunofluorescence staining, quantitative real-time polymerase chain reaction (RT-qPCR), and western blot demonstrated that the stiffer substrate TCP promoted the h-iCSCs' differentiation from h-iPSCs. On the contrary, softer PDMS 527 was more effective to maintain the phenotype of h-iCSCs cultured in vitro. Finally, we cultured h-iCSCs on PDMS 527 until P3 and seeded them on a biomimetic collagen membrane to form the single-layer and multiple-layer bioengineered corneal stroma with high transparency properties and cell survival rate. In conclusion, the study is helpful for differentiating h-iPSCs to h-iCSCs and corneal tissue engineering by manipulating stiffness mechanobiology.

Emerging gene therapy products for RPGR-associated X-linked retinitis pigmentosa

INTRODUCTION

Mutations in the RPGR gene are responsible for one of the most prevalent and severe types of retinitis pigmentosa. Gene therapy has shown great promise to treat inherited retinal diseases, and currently, four RPGR gene therapy vectors are being evaluated in clinical trials.

AREAS COVERED

This manuscript reviews the gene therapy products that are in development for X-linked retinitis pigmentosa caused by mutations in RPGR, and the challenges that scientists and clinicians have faced.

EXPERT OPINION

The development of a gene therapy product for RPGR-associated retinal degeneration has been a great challenge due to the incomplete understanding of the underlying genetics and mechanism of action of RPGR, and on the other hand, due to the instability of the RPGR gene. Three of the four gene therapy vectors currently in clinical trials include a codon-optimized version of the human RPGR sequence, and the other vector contains a shortened version of the human RPGR. To date, the only Phase I/II results published in a peer-reviewed journal demonstrate a good safety profile and an improvement in the visual field using a codon optimized version of RPGRORF15.

Cell-specific regulation of gene expression using splicing-dependent frameshifting

Precise and reliable cell-specific gene delivery remains technically challenging. Here we report a splicing-based approach for controlling gene expression whereby separate translational reading frames are coupled to the inclusion or exclusion of mutated, frameshifting cell-specific alternative exons. Candidate exons are identified by analyzing thousands of publicly available RNA sequencing datasets and filtering by cell specificity, conservation, and local intron length. This method, which we denote splicing-linked expression design (SLED), can be combined in a Boolean manner with existing techniques such as minipromoters and viral capsids. SLED can use strong constitutive promoters, without sacrificing precision, by decoupling the tradeoff between promoter strength and selectivity. AAV-packaged SLED vectors can selectively deliver fluorescent reporters and calcium indicators to various neuronal subtypes in vivo. We also demonstrate gene therapy utility by creating SLED vectors that can target PRPH2 and SF3B1 mutations. The flexibility of SLED technology enables creative avenues for basic and translational research.

Advancing precision medicines for ocular disorders: Diagnostic genomics to tailored therapies

Successful sequencing of the human genome and evolving functional knowledge of gene products has taken genomic medicine to the forefront, soon combining broadly with traditional diagnostics, therapeutics, and prognostics in patients. Recent years have witnessed an extraordinary leap in our understanding of ocular diseases and their respective genetic underpinnings. As we are entering the age of genomic medicine, rapid advances in genome sequencing, gene delivery, genome surgery, and computational genomics enable an ever-increasing capacity to provide a precise and robust diagnosis of diseases and the development of targeted treatment strategies. Inherited retinal diseases are a major source of blindness around the world where a large number of causative genes have been identified, paving the way for personalized diagnostics in the clinic. Developments in functional genetics and gene transfer techniques has also led to the first FDA approval of gene therapy for LCA, a childhood blindness. Many such retinal diseases are the focus of various clinical trials, making clinical diagnoses of retinal diseases, their underlying genetics and the studies of natural history important. Here, we review methodologies for identifying new genes and variants associated with various ocular disorders and the complexities associated with them. Thereafter we discuss briefly, various retinal diseases and the application of genomic technologies in their diagnosis. We also discuss the strategies, challenges, and potential of gene therapy for the treatment of inherited and acquired retinal diseases. Additionally, we discuss the translational aspects of gene therapy, the important vector types and considerations for human trials that may help advance personalized therapeutics in ophthalmology. Retinal disease research has led the application of precision diagnostics and precision therapies; therefore, this review provides a general understanding of the current status of precision medicine in ophthalmology.

Engineered Capsids for Efficient Gene Delivery to the Retina and Cornea

Adeno-associated viral (AAV) vectors represent an ideal vehicle for human gene transfer. One advantage to the AAV vector system is the availability of multiple naturally occurring serotypes that provide selective tropisms for various target cells. Strategies to enhance the properties of the natural AAV isolates have been developed and can be divided into two approaches, rational design or directed evolution. The rational design approach utilizes knowledge of AAV capsids to make targeted changes to the capsid to alter transduction efficiency or specificity, while the directed evolution approach does not require a priori knowledge of capsid structure and includes random mutagenesis, capsid shuffling, or random peptide insertion. In this study, we describe the generation of novel variants for both AAV2 and AAV5 using a rational design approach and knowledge of AAV receptor binding, surface charge, and AAV capsid protein posttranslational modifications. The novel AAV2 and AAV5 variants demonstrate improved transduction properties in both the mouse retina and cornea. The translational fidelity of the novel AAV2 variant was confirmed in the context of the nonhuman primate (NHP) retina, whereas a NHP tissue explant model was established to allow the rapid assessment of translational fidelity between species for the AAV5 variants. The capsid-modified AAV2 and AAV5 variants described in this study have novel attributes that will add to the efficacy and specificity of their potential use in gene therapy for a range of human ocular diseases.

Effects of Altering HSPG Binding and Capsid Hydrophilicity on Retinal Transduction by AAV

Adeno-associated viruses (AAVs) have recently emerged as the leading vector for retinal gene therapy. However, AAV vectors which are capable of achieving clinically relevant levels of transgene expression and widespread retinal transduction are still an unmet need. Using rationally designed AAV2-based capsid variants, we investigate the role of capsid hydrophilicity and hydrophobicity as it relates to retinal transduction. We show that hydrophilic, single amino acid (aa) mutations (V387R, W502H, E530K, L583R) in AAV2 negatively impact retinal transduction when heparan sulfate proteoglycan (HSPG) binding remains intact. Conversely, addition of hydrophobic point mutations to an HSPG binding deficient capsid (AAV2ΔHS) lead to increased retinal transduction in both mouse and macaque. Our top performing vector, AAV2(4pMut)ΔHS, achieved robust rod and cone photoreceptor (PR) transduction in macaque, especially in the fovea, and demonstrates the ability to spread laterally beyond the borders of the subretinal injection (SRI) bleb. This study both evaluates biophysical properties of AAV capsids that influence retinal transduction, and assesses the transduction and tropism of a novel capsid variant in a clinically relevant animal model.ImportanceRationally guided engineering of AAV capsids aims to create new generations of vectors with enhanced potential for human gene therapy. By applying rational design principles to AAV2-based capsids, we evaluated the influence of hydrophilic and hydrophobic amino acid (aa) mutations on retinal transduction as it relates to vector administration route. Through this approach we identified a largely deleterious relationship between hydrophilic aa mutations and canonical HSPG binding by AAV2-based capsids. Conversely, the inclusion of hydrophobic aa substitutions on a HSPG binding deficient capsid (AAV2ΔHS), generated a vector capable of robust rod and cone photoreceptor (PR) transduction. This vector AAV2(4pMut)ΔHS also demonstrates a remarkable ability to spread laterally beyond the initial subretinal injection (SRI) bleb, making it an ideal candidate for the treatment of retinal diseases which require a large area of transduction.

Delivering AAV to the Central Nervous and Sensory Systems

As gene therapy enters mainstream medicine, it is more important than ever to have a grasp of exactly how to leverage it for maximum benefit. The development of new targeting strategies and tools makes treating patients with genetic diseases possible. Many Mendelian disorders are amenable to gene replacement or correction. These often affect post-mitotic tissues, meaning that a single stably expressing therapy can be applied. Recent years have seen the development of a large number of novel viral vectors for delivering specific therapies. These new vectors - predominately recombinant adeno-associated virus (AAV) variants - target nervous tissues with differing efficiencies. This review gives an overview of current gene therapies in the brain, ear, and eye, and describes the optimal approaches, depending on cell type and transgene. Overall, this work aims to serve as a primer for gene therapy in the central nervous and sensory systems.

Gene editing technology: Towards precision medicine in inherited retinal diseases

Purpose: To review preclinical and clinical advances in gene therapy, with a focus on gene editing technologies, and application to inherited retinal disease.Methods: A narrative overview of the literature, summarizing the state-of-the-art in clinical gene therapy for inherited retinal disease, as well as the science and application of new gene editing technology.Results: The last three years has seen the first FDA approval of an in vivo gene replacement therapy for a hereditary blinding eye disease and, recently, the first clinical application of an in vivo gene editing technique. Limitations and challenges in this evolving field are highlighted, as well as new technologies developed to address the multitude of molecular mechanisms of disease.Conclusion: Genetic therapy for the treatment of inherited retinal disease is a rapidly expanding area of ophthalmology. New technologies have revolutionized the field of genome engineering and rekindled an interest in precision medicines for these conditions.

Organoids and organ chips in ophthalmology

Recent advances have driven the development of stem cell-derived, self-organizing, three-dimensional miniature organs, termed organoids, which mimic different eye tissues including the retina, cornea, and lens. Organoids and engineered microfluidic organ-on-chips (organ chips) are transformative technologies that show promise in simulating the architectural and functional complexity of native organs. Accordingly, they enable exploration of facets of human disease and development not accurately recapitulated by animal models. Together, these technologies will increase our understanding of the basic physiology of different eye structures, enable us to interrogate unknown aspects of ophthalmic disease pathogenesis, and serve as clinically-relevant surrogates for the evaluation of ocular therapeutics. Both the burden and prevalence of monogenic and multifactorial ophthalmic diseases, which can cause visual impairment or blindness, in the human population warrants a paradigm shift towards organoids and organ chips that can provide sensitive, quantitative, and scalable phenotypic assays. In this article, we review the current situation of organoids and organ chips in ophthalmology and discuss how they can be leveraged for translational applications.

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.

The new landscape of retinal gene therapy

Novel therapeutics for inherited retinal dystrophies (IRDs) have rapidly evolved since groundbreaking clinical trials for LCA due to RPE65 mutations led to the first FDA-approved in vivo gene therapy. Since then, advancements in viral vectors have led to more efficient AAV transduction and developed other viral vectors for gene augmentation therapy of large gene targets. Furthermore, significant developments in gene editing and RNA modulation technologies have introduced novel capabilities for treatment of autosomal dominant diseases, intronic mutations, and/or large genes otherwise unable to be treated with current viral vectors. We highlight strategies currently being evaluated in gene therapy clinical trials and promising preclinical developments for IRDs.

Translating CRISPR-Cas Therapeutics: Approaches and Challenges

CRISPR-Cas clinical trials have begun, offering a first glimpse at how DNA and RNA targeting could enable therapies for many genetic and epigenetic human diseases. The speedy progress of CRISPR-Cas from discovery and adoption to clinical use is built on decades of traditional gene therapy research and belies the multiple challenges that could derail the successful translation of these new modalities. Here, we review how CRISPR-Cas therapeutics are translated from technological systems to therapeutic modalities, paying particular attention to the therapeutic cascade from cargo to delivery vector, manufacturing, administration, pipelines, safety, and therapeutic target profiles. We also explore potential solutions to some of the obstacles facing successful CRISPR-Cas translation. We hope to illuminate how CRISPR-Cas is brought from the academic bench toward use in the clinic.

Modeling and Rescue of RP2 Retinitis Pigmentosa Using iPSC-Derived Retinal Organoids

RP2 mutations cause a severe form of X-linked retinitis pigmentosa (XLRP). The mechanism of RP2-associated retinal degeneration in humans is unclear, and animal models of RP2 XLRP do not recapitulate this severe phenotype. Here, we developed gene-edited isogenic RP2 knockout (RP2 KO) induced pluripotent stem cells (iPSCs) and RP2 patient-derived iPSC to produce 3D retinal organoids as a human retinal disease model. Strikingly, the RP2 KO and RP2 patient-derived organoids showed a peak in rod photoreceptor cell death at day 150 (D150) with subsequent thinning of the organoid outer nuclear layer (ONL) by D180 of culture. Adeno-associated virus-mediated gene augmentation with human RP2 rescued the degeneration phenotype of the RP2 KO organoids, to prevent ONL thinning and restore rhodopsin expression. Notably, these data show that 3D retinal organoids can be used to model photoreceptor degeneration and test potential therapies to prevent photoreceptor cell death.

AAV-Mediated Gene Delivery to Foveal Cones

Adeno-associated virus (AAV) has emerged as the vector of choice for delivering genes to the mammalian retina. From the first gene therapy to receive FDA approval for the inherited retinal disease (Luxturna™) to more recent clinical trials using microbial opsins to regain light sensitivity, therapeutic transgenes rely on AAV vectors for safe and efficient gene delivery to retinal cells. Such vectors are administered to the retina via subretinal (SR) injection or intravitreal (IVT) injection routes depending on the targeted retinal cell type. An attractive target for gene therapy is the fovea, bearing the highest concentration of cone cells responsible for our high acuity daylight vision. However, previous clinical trials and large animal studies reported that SR administration of vector under the cone-exclusive fovea disrupts its fine structure and might impair visual acuity. Due to its technical difficulty and potential risks, alternatives to vector injection under this delicate region have been investigated by using novel AAV capsid variants identified via rational design or directed evolution. We recently established new vector-promoter combinations to overcome the limitations associated with AAV-mediated cone transduction in the fovea. Our methods provide efficient foveal cone transduction without detaching this delicate region and rely on the use of engineered AAVs and optimal promoters compatible with optogenetic vision restoration. Here we describe in detail our AAV vectors, methods for intravitreal and subretinal injections as well as pre- and postoperative procedures as performed in cynomolgus macaques.

Recombinant Adeno-Associated Viral Vectors (rAAV)-Vector Elements in Ocular Gene Therapy Clinical Trials and Transgene Expression and Bioactivity Assays

Inherited retinal dystrophies and optic neuropathies cause chronic disabling loss of visual function. The development of recombinant adeno-associated viral vectors (rAAV) gene therapies in all disease fields have been promising, but the translation to the clinic has been slow. The safety and efficacy profiles of rAAV are linked to the dose of applied vectors. DNA changes in the rAAV gene cassette affect potency, the expression pattern (cell-specificity), and the production yield. Here, we present a library of rAAV vectors and elements that provide a workflow to design novel vectors. We first performed a meta-analysis on recombinant rAAV elements in clinical trials (2007-2020) for ocular gene therapies. We analyzed 33 unique rAAV gene cassettes used in 57 ocular clinical trials. The rAAV gene therapy vectors used six unique capsid variants, 16 different promoters, and six unique polyadenylation sequences. Further, we compiled a list of promoters, enhancers, and other sequences used in current rAAV gene cassettes in preclinical studies. Then, we give an update on pro-viral plasmid backbones used to produce the gene therapy vectors, inverted terminal repeats, production yield, and rAAV safety considerations. Finally, we assess rAAV transgene and bioactivity assays applied to cells or organoids in vitro, explants ex vivo, and clinical studies.

Novel AAV44.9-Based Vectors Display Exceptional Characteristics for Retinal Gene Therapy

The majority of inherited retinal diseases (IRDs) are caused by mutations in genes expressed in photoreceptors (PRs). The ideal vector to address these conditions is one that transduces PRs in large areas of retina with the smallest volume/lowest titer possible, and efficiently transduces foveal cones, the cells responsible for acute, daylight vision that are often the only remaining area of functional retina in IRDs. The purpose of our study was to evaluate the retinal tropism and potency of a novel capsid, AAV44.9, and rationally designed derivatives thereof. We found that AAV44.9 and AAV44.9(E531D) transduced retinas of subretinally injected (SRI) mice with higher efficiency than did benchmark AAV5- and AAV8-based vectors. In macaques, highly efficient cone and rod transduction was observed following submacular and peripheral SRI. AAV44.9- and AAV44.9(E531D)-mediated GFP fluorescence extended laterally well beyond SRI bleb margins. Notably, extrafoveal injection (i.e., fovea not detached during surgery) led to transduction of up to 98% of foveal cones. AAV44.9(E531D) efficiently transduced parafoveal and perifoveal cones, whereas AAV44.9 did not. AAV44.9(E531D) was also capable of restoring retinal function to a mouse model of IRD. These novel capsids will be useful for addressing IRDs that would benefit from an expansive treatment area.

Metabolic and Redox Signaling of the Nucleoredoxin-Like-1 Gene for the Treatment of Genetic Retinal Diseases

The loss of cone photoreceptor function in retinitis pigmentosa (RP) severely impacts the central and daily vision and quality of life of patients affected by this disease. The loss of cones follows the degeneration of rods, in a manner independent of the causing mutations in numerous genes associated with RP. We have explored this phenomenon and proposed that the loss of rods triggers a reduction in the expression of rod-derived cone viability factor (RdCVF) encoded by the nucleoredoxin-like 1 (NXNL1) gene which interrupts the metabolic and redox signaling between rods and cones. After providing scientific evidence supporting this mechanism, we propose a way to restore this lost signaling and prevent the cone vision loss in animal models of RP. We also explain how we could restore this signaling to prevent cone vision loss in animal models of the disease and how we plan to apply this therapeutic strategy by the administration of both products of NXNL1 encoding the trophic factor RdCVF and the thioredoxin enzyme RdCVFL using an adeno-associated viral vector. We describe in detail all the steps of this translational program, from the design of the drug, its production, biological validation, and analytical and preclinical qualification required for a future clinical trial that would, if successful, provide a treatment for this incurable disease.

GUCY2D-Associated Leber Congenital Amaurosis: A Retrospective Natural History Study in Preparation for Trials of Novel Therapies

PURPOSE

To describe the natural history of Leber congenital amaurosis (LCA) associated with GUCY2D variants (GUCY2D-LCA) in a cohort of children and adults, in preparation for trials of novel therapies.

DESIGN

Retrospective case series.

METHODS

Participants: Patients with GUCY2D-LCA at a single referral center.

PROCEDURES

Review of clinical notes, retinal imaging including fundus autofluorescence (FAF) and optical coherence tomography (OCT), electroretinography (ERG), and molecular genetic testing.

MAIN OUTCOME MEASURES

Demographic data, symptoms at presentation, visual acuity, evidence of progression, OCT and FAF findings, ERG assessment, and molecular genetics.

RESULTS

Twenty-one subjects with GUCY2D-LCA were included, with a mean follow-up ± standard deviation (SD) of 10 ± 11.85 years. Marked reduction in visual acuity (VA) and nystagmus was documented in all patients within the first 3 years of life. Fifty-seven percent (n = 12) exhibited photophobia and 38% (n = 8) had nyctalopia. VA was worse than hand motion in 71% of the patients (n = 15). Longitudinal assessment of VA showed stability in all patients, except 1 patient who experienced deterioration over a follow-up of 44 years. Hyperopia was reported in 13 of the 17 subjects (71%) with available refraction data. Eighteen subjects had either normal fundus appearance (n = 14) or a blond fundus (n = 3), while only 4 of the eldest subjects had mild retinal pigment epithelium (RPE) atrophy (mean, 49 years; range 40-54 years). OCT data were available for 11 subjects and 4 different grades of ellipsoid zone (EZ) integrity were identified: (1) continuous/intact EZ (n = 6), (2) focally disrupted EZ (n = 2), (3) focally disrupted with RPE changes (n = 2), and (4) diffuse EZ disruption with RPE changes (n = 1). All examined subjects had stable OCT findings over the long follow-up period. Full-field ERGs showed evidence of a severe cone-rod dystrophy in 5 of 6 patients and undetectable ERGs in 1 subject. Novel genotype-phenotype correlations are also reported.

CONCLUSION

GUCY2D-LCA is a severe early-onset retinal dystrophy associated with very poor VA from birth. Despite the severely affected photoreceptor function, the relatively preserved photoreceptor structure based on EZ integrity until late in the disease in the majority of subjects suggests a wide therapeutic window for gene therapy trials.

RPGR gene therapy presents challenges in cloning the coding sequence

Introduction: Currently, there are three Phase I/II clinical trials based on gene therapy ongoing to test different AAV.RPGR or deleted RPGR vectors on patients affected by X-linked retinitis pigmentosa. These three vectors differ in the adeno-associated viral (AAV) vector capsid used, and the coding sequences: two contain codon optimized versions of RPGR which give the full-length protein, whilst the third uses a wild-type sequence that contains a large deletion encoding part of the functional domain of the RPGR protein.Areas covered: This review approaches the different studies that have led to the initiation of three different clinical trials for RPGR related X-linked retinitis pigmentosa.Expert opinion: The development of a gene therapy vector to deliver a normal copy of the RPGR gene into the photoreceptors has presented a challenge for the scientific community. The instability of its sequence and the fact that its function is not well understood can lead to the production of a nonfunctional or deleterious protein for the human retina. Since the RPGR protein undergoes post-translational glutamylation in the protein domain that may be particularly affected by gene instability, a functional assay of glutamylation is essential to verify the correct coding sequence.

The primate model for understanding and restoring vision

Retinal degenerative diseases caused by photoreceptor cell death are major causes of irreversible vision loss. As only primates have a macula, the nonhuman primate (NHP) models have a crucial role not only in revealing biological mechanisms underlying high-acuity vision but also in the development of therapies. Successful translation of basic research findings into clinical trials and, moreover, approval of the first therapies for blinding inherited and age-related retinal dystrophies has been reported in recent years. This article explores the value of the NHP models in understanding human vision and reviews their contribution to the development of innovative therapeutic strategies to save and restore vision.

Loss of CRB2 in Müller glial cells modifies a CRB1-associated retinitis pigmentosa phenotype into a Leber congenital amaurosis phenotype

Variations in the human Crumbs homolog-1 (CRB1) gene lead to an array of retinal dystrophies including early onset of retinitis pigmentosa (RP) and Leber congenital amaurosis (LCA) in children. To investigate the physiological roles of CRB1 and CRB2 in retinal Müller glial cells (MGCs), we analysed mouse retinas lacking both proteins in MGC. The peripheral retina showed a faster progression of dystrophy than the central retina. The central retina showed retinal folds, disruptions at the outer limiting membrane, protrusion of photoreceptor nuclei into the inner and outer segment layers and ingression of photoreceptor nuclei into the photoreceptor synaptic layer. The peripheral retina showed a complete loss of the photoreceptor synapse layer, intermingling of photoreceptor nuclei within the inner nuclear layer and ectopic photoreceptor cells in the ganglion cell layer. Electroretinography showed severe attenuation of the scotopic a-wave at 1 month of age with responses below detection levels at 3 months of age. The double knockout mouse retinas mimicked a phenotype equivalent to a clinical LCA phenotype due to loss of CRB1. Localization of CRB1 and CRB2 in non-human primate (NHP) retinas was analyzed at the ultrastructural level. We found that NHP CRB1 and CRB2 proteins localized to the subapical region adjacent to adherens junctions at the outer limiting membrane in MGC and photoreceptors. Our data suggest that loss of CRB2 in MGC aggravates the CRB1-associated RP-like phenotype towards an LCA-like phenotype.

Somatic Gene Editing of GUCY2D by AAV-CRISPR/Cas9 Alters Retinal Structure and Function in Mouse and Macaque

Mutations in GUCY2D, the gene encoding retinal guanylate cyclase-1 (retGC1), are the leading cause of autosomal dominant cone-rod dystrophy (CORD6). Significant progress toward clinical application of gene replacement therapy for Leber congenital amaurosis (LCA) due to recessive mutations in GUCY2D (LCA1) has been made, but a different approach is needed to treat CORD6 where gain of function mutations cause dysfunction and dystrophy. The CRISPR/Cas9 gene editing system efficiently disrupts genes at desired loci, enabling complete gene knockout or homology directed repair. Here, adeno-associated virus (AAV)-delivered CRISPR/Cas9 was used specifically to edit/disrupt this gene's early coding sequence in mouse and macaque photoreceptors in vivo, thereby knocking out retGC1 expression and demonstrably altering retinal function and structure. Neither preexisting nor induced Cas9-specific T-cell responses resulted in ocular inflammation in macaques, nor did it limit GUCY2D editing. The results show, for the first time, the ability to perform somatic gene editing in primates using AAV-CRISPR/Cas9 and demonstrate the viability this approach for treating inherited retinal diseases in general and CORD6 in particular.

Human iPSC-Derived Retinas Recapitulate the Fetal CRB1 CRB2 Complex Formation and Demonstrate that Photoreceptors and Müller Glia Are Targets of AAV5

Human retinal organoids from induced pluripotent stem cells (hiPSCs) can be used to confirm the localization of proteins in retinal cell types and to test transduction and expression patterns of gene therapy vectors. Here, we compared the onset of CRB protein expression in human fetal retina with human iPSC-derived retinal organoids. We show that CRB2 protein precedes the expression of CRB1 in the developing human retina. Our data suggest the presence of CRB1 and CRB2 in human photoreceptors and Müller glial cells. Thus the fetal CRB complex formation is replicated in hiPSC-derived retina. CRB1 patient iPSC retinal organoids showed disruptions at the outer limiting membrane as found in Crb1 mutant mice. Furthermore, AAV serotype 5 (AAV5) is potent in infecting human Müller glial cells and photoreceptors in hiPSC-derived retinas and retinal explants. Our data suggest that human photoreceptors can be efficiently transduced by AAVs in the presence of photoreceptor segments.

Co-Delivery of a Short-Hairpin RNA and a shRNA-Resistant Replacement Gene with Adeno-Associated Virus: An Allele-Independent Strategy for Autosomal-Dominant Retinal Disorders

Recombinant adeno-associated virus (rAAV) has become an important gene delivery vector for the treatment of inherited retinal degenerative diseases. Many of the mutations leading to retinal degeneration are inherited in an autosomal-dominant pattern and can produce toxic gain-of-function and/or dominant-negative effects. Here we describe an allele-independent gene therapy strategy with rAAV to treat autosomal-dominant retinal degenerative diseases. In this methodology, we co-deliver a short-hairpin RNA (shRNA) to inhibit expression of both the toxic and (WT) copies of the gene as well as an shRNA-resistant cDNA for functional gene replacement with a rAAV.

SubILM Injection of AAV for Gene Delivery to the Retina

Adeno-associated virus (AAV) has emerged as the vector of choice for delivering genes to the retina. Indeed, the first gene therapy to receive FDA approval in the United States is an AAV-based treatment for the inherited retinal disease, Leber congenital amaurosis-2. Voretigene neparvovec (Luxturna™) is delivered to patients via subretinal (SR) injection, an invasive surgical procedure that requires detachment of photoreceptors (PRs) from the retinal pigment epithelium (RPE). It has been reported that subretinal administration of vector under the cone-exclusive fovea leads to a loss of central retinal structure and visual acuity in some patients. Due to its technical difficulty and potential risks, alternatives to SR injection have been explored in primates. Intravitreally (Ivt) delivered AAV transduces inner retina and foveal cones, but with low efficiency. Novel AAV capsid variants identified via rational design or directed evolution have offered only incremental improvements, and have failed to promote pan-inner retinal transduction or significant outer retinal transduction beyond the fovea. Problems with retinal transduction by Ivt-delivered AAV include dilution in the vitreous, potential antibody-mediated neutralization of capsid in this nonimmune privileged space, and the presence of the inner limiting membrane (ILM), a basement membrane separating the vitreous from the neural retina. We have developed an alternative "subILM" injection method that overcomes all three hurdles. Specifically, vector is placed in a surgically induced, hydrodissected space between the ILM and neural retina. We have shown that subILM injection promotes more efficient retinal transduction by AAV than Ivt injection, and results in uniform and extensive transduction of retinal ganglion cells (RGCs) beneath the subILM bleb. We have also demonstrated transduction of Muller glia, ON bipolar cells, and photoreceptors by subILM injection. Our results confirm that the ILM is a major barrier to transduction by AAV in primate retina and that, when it is circumvented, the efficiency and depth to which AAV2 promotes transduction of multiple retinal cell classes is greatly enhanced. Here we describe in detail methods for vector preparation, vector dilution, and subILM injection as performed in macaque (Macaca sp.).

Six Years and Counting: Restoration of Photopic Retinal Function and Visual Behavior Following Gene Augmentation Therapy in a Sheep Model of CNGA3 Achromatopsia

Achromatopsia causes severely reduced visual acuity, photoaversion, and inability to discern colors due to cone photoreceptor dysfunction. In 2010, we reported on day-blindness in sheep caused by a stop-codon mutation of the ovine CNGA3 gene and began gene augmentation therapy trials in this naturally occurring large animal model of CNGA3 achromatopsia. The purpose of this study was to evaluate long-term efficacy and safety results of treatment, findings that hold great relevance for clinical trials that started recently in CNGA3 achromatopsia patients. Nine day-blind sheep were available for long-term follow up. The right eye of each sheep was treated with a single subretinal injection of an Adeno-Associated Virus Type 5 (AAV5) vector carrying either a mouse (n = 4) or a human (n = 5) CNGA3 transgene under control of the 2.1-Kb red/green opsin promoter. The efficacy of treatment was assessed periodically with photopic maze tests and electroretinographic (ERG) recordings for as long as 74 months postoperatively. Safety was assessed by repeated ophthalmic examinations and scotopic ERG recordings. The retinas of three animals that died of unrelated causes >5 years post-treatment were studied histologically and immunohistochemically using anti-hCNGA3 and anti-red/green cone opsin antibodies. Passage time and number of collisions of treated sheep in the photopic maze test were significantly lower at all follow-up examinations as compared with pretreatment values (p = 0.0025 and p < 0.001, respectively). ERG Critical Flicker Fusion Frequency and flicker amplitudes at 30 and 40 Hz showed significant improvement following treatment (p < 0.0001) throughout the study. Ophthalmic examinations and rod ERG recordings showed no abnormalities in the treated eyes. Immunohistochemistry revealed the presence of CNGA3 protein in red/green opsin-positive cells (cones) of the treated eyes. Our results show significant, long-term improvement in cone function, demonstrating a robust rescue effect up to six years following a single treatment with a viral vector that provides episomal delivery of the transgene. This unique follow-up duration confirms the safe and stable nature of AAV5 gene therapy in the ovine achromatopsia model.

Nonhuman Primate Studies to Advance Vision Science and Prevent Blindness

Most primate behavior is dependent on high acuity vision. Optimal visual performance in primates depends heavily upon frontally placed eyes, retinal specializations, and binocular vision. To see an object clearly its image must be placed on or near the fovea of each eye. The oculomotor system is responsible for maintaining precise eye alignment during fixation and generating eye movements to track moving targets. The visual system of nonhuman primates has a similar anatomical organization and functional capability to that of humans. This allows results obtained in nonhuman primates to be applied to humans. The visual and oculomotor systems of primates are immature at birth and sensitive to the quality of binocular visual and eye movement experience during the first months of life. Disruption of postnatal experience can lead to problems in eye alignment (strabismus), amblyopia, unsteady gaze (nystagmus), and defective eye movements. Recent studies in nonhuman primates have begun to discover the neural mechanisms associated with these conditions. In addition, genetic defects that target the retina can lead to blindness. A variety of approaches including gene therapy, stem cell treatment, neuroprosthetics, and optogenetics are currently being used to restore function associated with retinal diseases. Nonhuman primates often provide the best animal model for advancing fundamental knowledge and developing new treatments and cures for blinding diseases.

Can an in vivo imaging system be used to determine localization and biodistribution of AAV5-mediated gene expression following subretinal and intravitreal delivery in mice?

Recombinant adeno associated viruses (AAV) are the most commonly used vectors in animal model studies of gene therapy for retinal diseases. The ability of a vector to localize and remain in the target tissue, and in this manner to avoid off-target effects beyond the site of delivery, is critical to the efficacy and safety of the treatment. The in vivo imaging system (IVIS) is a non-invasive imaging tool used for detection and quantification of bioluminescence activity in rodents. Our aim was to investigate whether IVIS can detect localization and biodistribution of AAV5 vector in mice following subretinal (SR) and intravitreal (IVT) injections. AAV5 carrying firefly luciferase DNA under control of the ubiquitous cytomegalovirus (CMV) promoter was injected unilaterally IVT or SR (in the central or peripheral retina) of forty-one mice. Luciferase activity was tracked for up to 60 weeks in the longest surviving animals, using repeated (up to 12 times) IVIS bioluminescence imaging. Luciferase presence was also confirmed immunohistochemically (IHC) and by PCR in representative animals. In the SR group, IVIS readings demonstrated luciferase activity in all (32/32) eyes, and luciferase presence was confirmed by IHC (4/4 eyes) and PCR (12/12 eyes). In the IVT group, IVIS readings demonstrated luciferase activity in 7/9 eyes, and luciferase presence was confirmed by PCR in 5/5 eyes and by IHC (2/2 eyes). In two SR-injected animals (one each from the central and peripheral injection sites), PCR detected luciferase presence in the ipsilateral optic nerves, a finding that was not detected by IVIS or IHC. Our results show that when evaluating SR delivery, IVIS has a sensitivity and specificity of 100% compared with the gold standard PCR. When evaluating IVT delivery, IVIS has a sensitivity of 78% and specificity of 100%. These finding confirm the ability of IVIS to detect in-vivo localized expression of AAV following SR delivery in the retina up to 60 weeks post-treatment, using repeated imaging for longitudinal evaluation, without fading of the biological signal, thereby replacing the need for post mortem processing in order to confirm vector expression. However, IVIS is probably not sensitive enough, compared with genome detection, to demonstrate biodistribution to the optic nerve, as it could not detect luciferase activity in ipsilateral optic nerves following SR delivery in mice.

High-Throughput Screening Identifies Kinase Inhibitors That Increase Dual Adeno-Associated Viral Vector Transduction In Vitro and in Mouse Retina

Retinal gene therapy based on adeno-associated viral (AAV) vectors is safe and efficient in humans. The low intrinsic DNA transfer capacity of AAV has been expanded by dual vectors where a large expression cassette is split in two halves independently packaged in two AAV vectors. Dual AAV transduction efficiency, however, is greatly reduced compared to that obtained with a single vector. As AAV intracellular trafficking and processing are negatively affected by phosphorylation, this study set to identify kinase inhibitors that can increase dual AAV vector transduction. By high-throughput screening of a kinase inhibitors library, three compounds were identified that increase AAV transduction in vitro, one of which has a higher effect on dual than on single AAV vectors. Importantly, the transduction enhancement is exerted on various AAV serotypes and is not transgene dependent. As kinase inhibitors are promiscuous, siRNA-mediated silencing of targeted kinases was performed, and AURKA and B, PLK1, and PTK2 were among those involved in the increase of AAV transduction levels. The study shows that kinase inhibitor administration reduces AAV serotype 2 (AAV2) capsid phosphorylation and increases the activity of DNA-repair pathways involved in AAV DNA processing. Importantly, the kinase inhibitor PF-00562271 improves dual AAV8 transduction in photoreceptors following sub-retinal delivery in mice. The study identifies kinase inhibitors that increase dual and single AAV transduction by modulating AAV entry and post-entry steps.

Screening for Neutralizing Antibodies Against Natural and Engineered AAV Capsids in Nonhuman Primate Retinas

Adeno-associated virus (AAV) has shown promise as a therapeutic gene delivery vector for inherited retinal degenerations in both preclinical disease models and human clinical trials. The retinas of nonhuman primates (NHPs) share many anatomical similarities to humans and are an important model for evaluating AAV gene delivery. Recent evidence has shown that preexisting immunity in the form of neutralizing antibodies (NABs) in NHPs strongly correlates with weak or lack of AAV transduction in the retina when administered intravitreally, work with translational implications. This necessitates prescreening of NHPs before intravitreal delivery of AAV. In this chapter, we describe a method for screening NHP serum for preexisting NABs.

Assessment of AAV Vector Tropisms for Mouse and Human Pluripotent Stem Cell-Derived RPE and Photoreceptor Cells

Adeno-associated viral vectors are showing great promise as gene therapy vectors for a wide range of retinal disorders. To date, evaluation of therapeutic approaches has depended almost exclusively on the use of animal models. With recent advances in human stem cell technology, stem cell-derived retina now offers the possibility to assess efficacy in human organoids in vitro. Here we test six adeno-associated virus (AAV) serotypes [AAV2/2, AAV2/9, AAV2/8, AAV2/8T(Y733F), AAV2/5, and ShH10] to determine their efficiency in transducing mouse and human pluripotent stem cell-derived retinal pigment epithelium (RPE) and photoreceptor cells in vitro. All the serotypes tested were capable of transducing RPE and photoreceptor cells in vitro. AAV ShH10 and AAV2/5 are the most efficient vectors at transducing both mouse and human RPE, while AAV2/8 and ShH10 achieved similarly robust transduction of human embryonic stem cell-derived cone photoreceptors. Furthermore, we show that human embryonic stem cell-derived photoreceptors can be used to establish promoter specificity in human cells in vitro. The results of this study will aid capsid selection and vector design for preclinical evaluation of gene therapy approaches, such as gene editing, that require the use of human cells and tissues.

Synthetic Adeno-Associated Viral Vector Efficiently Targets Mouse and Nonhuman Primate Retina In Vivo

Gene therapy is a promising approach in the treatment of inherited and common complex disorders of the retina. Preclinical and clinical studies have validated the use of adeno-associated viral vectors (AAV) as a safe and efficient delivery vehicle for gene transfer. Retinal pigment epithelium and rods-and to a lesser extent, cone photoreceptors-can be efficiently targeted with AAV. Other retinal cell types however are more challenging targets. The aim of this study was to characterize the transduction profile and efficiency of in silico designed, synthetic Anc80 AAVs for retinal gene transfer. Three Anc80 variants were evaluated for retinal targeting in mice and primates following subretinal delivery. In the murine retina Anc80L65 demonstrated high level of retinal pigment epithelium and photoreceptor targeting with comparable cone photoreceptor affinity compared to other AAVs. Remarkably, Anc80L65 enhanced transduction kinetics with visible expression as early as day 1 and steady state mRNA levels at day 3. Inner retinal tropism of Anc80 variants demonstrated distinct transduction patterns of Müller glia, retinal ganglion cells and inner nuclear layer neurons. Finally, murine findings with Anc80L65 qualitatively translated to the Rhesus macaque in terms of cell targets, levels and onset of expression. Our findings support the use of Anc80L65 for therapeutic subretinal gene delivery.

Rod Outer Segment Development Influences AAV-Mediated Photoreceptor Transduction After Subretinal Injection

Vectors based on the adeno-associated virus (AAV) are currently the preferred tools for delivering genes to photoreceptors (PR) in small and large animals. AAVs have been applied successfully in various models of PR dystrophies. However, unknown barriers still limit AAV's efficient application in several forms of severe PR degenerations due to insufficient transgene expression and/or treated cells at the time of injection. Optimizations of PR gene therapy strategies will likely benefit from the identification of the cellular factors that influence PR transduction. Interestingly, recent studies have shown that the AAV transduction profile of PRs differs significantly between neonatal and adult mouse retinas after subretinal injection. This phenomenon may provide clues to identify host factors that influence the efficiency of AAV-mediated PR transduction. This study demonstrates that rod outer segments are critical modulators of efficient AAV-mediated rod transduction. During retinal development, rod transduction correlated temporally and spatially with the differentiation order of PRs when vectors were introduced subretinally but not when introduced intravitreally. All subretinally injected vectors had an initial preference to transduce cones in the absence of formed rod outer segments and then displayed a preference for rods as the cells matured, independently of the expression cassette or AAV serotype. Consistent with this observation, altered development of rod outer segments was associated with a strong reduction of rod transduction and an increase in the percentage of transduced cones by 2- to 2.8-fold. A similar increase of cone transduction was observed in the adult retinal degeneration 1 (rd1) retina compared to wild-type mice. These results suggest that the loss of rod outer segments in diseased retinas could markedly affect gene transfer efficiency of AAV vectors by limiting the ability of AAVs to infect dying rods efficiently. This information could be exploited for the development of more efficient AAV-based PR gene delivery procedures.

Assessment of Adeno-Associated Virus Serotype Tropism in Human Retinal Explants

Advances in the discovery of the causes of monogenic retinal disorders, combined with technologies for the delivery of DNA to the retina, offer enormous opportunities for the treatment of previously untreatable blinding diseases. However, for gene augmentation to be most effective, vectors that have the correct cell-type specificity are needed. While animal models are very useful, they often exhibit differences in retinal cell surface receptors compared to the human retina. This study evaluated the use of an ex vivo organotypic explant system to test the transduction efficiency and tropism of seven different adeno-associated virus type 2 (AAV2) serotypes in the human retina and retinal pigment epithelium-choroid-AAV2/1, AAV2/2, AAV2/4, AAV2/5, AAV2/6, AAV2/8, and AAV2/9-all driving expression of GFP under control of the cytomegalovirus promoter. After 7 days in culture, it was found that AAV2/4 and AAV2/5 were particularly efficient at transducing photoreceptor cells and that AAV2/5 was highly specific to the outer nuclear layer, whereas AAV2/8 displayed consistently low transduction of photoreceptors. To validate the authenticity of the organotypic culture system, the transduction of the same set of AAVs was also compared in a pig model, in which sub-retinal injections in vivo were compared to cultured and transduced organotypic cultures ex vivo. This study shows how different AAV serotypes behave in the human retina and provides insight for further investigation of each of these serotypes for gene augmentation-based treatment of inherited retinal degeneration.

Neutralizing Antibodies Against Adeno-Associated Virus (AAV): Measurement and Influence on Retinal Gene Delivery

Adeno-associated viral vectors have become widely used in the clinic for retinal gene therapy. Thanks to AAVs impeccable safety profile and positive functional outcomes in its clinical application, interest in retinal gene therapy has increased exponentially over the past decade. Although early clinical trials have shown there is little influence of neutralizing antibodies on the performance of AAV when vector is administered into the subretinal space, recent findings suggest neutralizing antibodies may play a role when AAV is delivered via the intravitreal route. These findings highlight the importance of microenvironment on gene delivery and stress the need for a versatile assay to screen subjects for the presence of AAV-neutralizing antibodies. Measuring NAb titers against AAV prior and after gene therapy will help us better understand the impact of preexisting immunity on gene transfer, especially when the vector is administered intravitreally.

Triple Vectors Expand AAV Transfer Capacity in the Retina

Retinal gene transfer with adeno-associated viral (AAV) vectors holds great promise for the treatment of inherited retinal degenerations (IRDs). One limit of AAV is its transfer capacity of about 5 kb, which can be expanded to about 9 kb, using dual AAV vectors. This strategy would still not suffice for treatment of IRDs such as Usher syndrome type 1D or Alström syndrome type I (ALMS) due to mutations in CDH23 or ALMS1, respectively. To overcome this limitation, we generated triple AAV vectors, with a maximal transfer capacity of about 14 kb. Transcriptomic analysis following triple AAV transduction showed the expected full-length products along a number of aberrant transcripts. However, only the full-length transcripts are efficiently translated in vivo. We additionally showed that approximately 4% of mouse photoreceptors are transduced by triple AAV vectors and showed correct localization of recombinant ALMS1. The low-photoreceptor transduction levels might justify the modest and transient improvement we observe in the retina of a mouse model of ALMS. However, the levels of transduction mediated by triple AAV vectors in pig retina reached 40% of those observed with single vectors, and this bodes well for further improving the efficiency of triple AAV vectors in the retina.

Highly Efficient Delivery of Adeno-Associated Viral Vectors to the Primate Retina

Adeno-associated virus (AAV) has emerged as the preferred vector for targeting gene expression to the retina. Subretinally injected AAV can efficiently transduce retinal pigment epithelium and photoreceptors in primate retina. Inner and middle primate retina can be transduced by intravitreally delivered AAV, but with low efficiency. This is due to dilution of vector, potential neutralization of capsid because it is not confined to the immune-privileged retinal compartment, and the presence of the inner limiting membrane (ILM), a barrier separating the vitreous from the neural retina. We here describe a novel "subILM" injection method that addresses all three issues. Specifically, vector is placed in a surgically induced, hydrodissected space between the ILM and neural retina. In an initial experiment, we injected viscoelastic (Healon(®)), a substance we confirmed was biocompatible with AAV, to create a subILM bleb and subsequently injected AAV2-GFP into the bleb after irrigation with basic salt solution. For later experiments, we used a Healon-AAV mixture to place single, subILM injections. In all cases, subILM delivery of AAV was well tolerated-no inflammation or gross structural changes were observed by ophthalmological examination or optical coherence tomography. In-life fluorescence imaging revealed profound transgene expression within the area of the subILM injection bleb that persisted for the study duration. Uniform and extensive transduction of retinal ganglion cells (RGCs) was achieved in the areas beneath the subILM bleb. Transduction of Müller glia, ON bipolar cells, and photoreceptors was also observed. Robust central labeling from green fluorescent protein-expressing RGCs confirmed their continued survival, and was observed in the lateral geniculate nucleus, the superior colliculus, and the pretectum. Our results confirm that the ILM is a major barrier to transduction by AAV in primate retina and that, when it is circumvented, the efficiency and depth to which AAV2 promotes transduction of multiple retinal cell classes are greatly enhanced.

Optimization of Retinal Gene Therapy for X-Linked Retinitis Pigmentosa Due to RPGR Mutations

X-linked retinitis pigmentosa (XLRP) caused by mutations in the RPGR gene is an early onset and severe cause of blindness. Successful proof-of-concept studies in a canine model have recently shown that development of a corrective gene therapy for RPGR-XLRP may now be an attainable goal. In preparation for a future clinical trial, we have here optimized the therapeutic AAV vector construct by showing that GRK1 (rather than IRBP) is a more efficient promoter for targeting gene expression to both rods and cones in non-human primates. Two transgenes were used in RPGR mutant (XLPRA2) dogs under the control of the GRK1 promoter. First was the previously developed stabilized human RPGR (hRPGRstb). Second was a new full-length stabilized and codon-optimized human RPGR (hRPGRco). Long-term (>2 years) studies with an AAV2/5 vector carrying hRPGRstb under control of the GRK1 promoter showed rescue of rods and cones from degeneration and retention of vision. Shorter term (3 months) studies demonstrated comparable preservation of photoreceptors in canine eyes treated with an AAV2/5 vector carrying either transgene under the control of the GRK1 promoter. These results provide the critical molecular components (GRK1 promoter, hRPGRco transgene) to now construct a therapeutic viral vector optimized for RPGR-XLRP patients.