Recombinant adeno-associated virus targets passenger gene expression to cones in primate retina

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.

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.

Impact of Heparan Sulfate Binding on Transduction of Retina by Recombinant Adeno-Associated Virus Vectors

Adeno-associated viruses (AAVs) currently are being developed to efficiently transduce the retina following noninvasive, intravitreal (Ivt) injection. However, a major barrier encountered by intravitreally delivered AAVs is the inner limiting membrane (ILM), a basement membrane rich in heparan sulfate (HS) proteoglycan. The goal of this study was to determine the impact of HS binding on retinal transduction by Ivt-delivered AAVs. The heparin affinities of AAV2-based tyrosine-to-phenylalanine (Y-F) and threonine-to-valine (T-V) capsid mutants, designed to avoid proteasomal degradation during cellular trafficking, were established. In addition, the impact of grafting HS binding residues onto AAV1, AAV5, and AAV8(Y733F) as well as ablation of HS binding by AAV2-based vectors on retinal transduction was investigated. Finally, the potential relationship between thermal stability of AAV2-based capsids and Ivt-mediated transduction was explored. The results show that the Y-F and T-V AAV2 capsid mutants bind heparin but with slightly reduced affinity relative to that of AAV2. The grafting of HS binding increased Ivt transduction by AAV1 but not by AAV5 or AAV8(Y733F). The substitution of any canonical HS binding residues ablated Ivt-mediated transduction by AAV2-based vectors. However, these same HS variant vectors displayed efficient retinal transduction when delivered subretinally. Notably, a variant devoid of canonical HS binding residues, AAV2(4pMut)ΔHS, was remarkably efficient at transducing photoreceptors. The disparate AAV phenotypes indicate that HS binding, while critical for AAV2-based vectors, is not the sole determinant for transduction via the Ivt route. Finally, Y-F and T-V mutations alter capsid stability, with a potential relationship existing between stability and improvements in retinal transduction by Ivt injection.

IMPORTANCE

AAV has emerged as the vector of choice for gene delivery to the retina, with attention focused on developing vectors that can mediate transduction following noninvasive, intravitreal injection. HS binding has been postulated to play a role in intravitreally mediated transduction of retina. Our evaluation of the HS binding of AAV2-based variants and other AAV serotype vectors and the correlation of this property with transduction points to HS affinity as a factor controlling retinal transduction following Ivt delivery. However, HS binding is not the only requirement for improved Ivt-mediated transduction. We show that AAV2-based vectors lacking heparin binding transduce retina by subretinal injection and display a remarkable ability to transduce photoreceptors, indicating that other receptors are involved in this phenotype.

Retinal Gene Therapy: Current Progress and Future Prospects

Clinical trials treating inherited retinal dystrophy caused by RPE65 mutations had put retinal gene therapy at the forefront of gene therapy. Both successes and limitations in these clinical trials have fueled developments in gene vectors, which continue to further advance the field. These novel gene vectors aim to more safely and efficiently transduce retinal cells, expand the gene packaging capacity of AAV, and utilize new strategies to correct the varying mechanisms of dysfunction found with inherited retinal dystrophies. With recent clinical trials and numerous pre-clinical studies utilizing these novel vectors, the future of ocular gene therapy continues to hold vast potential.

Gene therapy for choroideremia using an adeno-associated viral (AAV) vector

Choroideremia is an outer retinal degeneration with a characteristic clinical appearance that was first described in the nineteenth century. The disorder begins with reduction of night vision and gradually progresses to blindness by middle age. The appearance of the fundus in sufferers is recognizable by the characteristic pale color caused by the loss of the outer retina, retinal-pigmented epithelium, and choroidal vessels, leading to exposure of the underlying sclera. Choroideremia shows X-linked recessive inheritance and the choroideremia gene (CHM) was one of the first to be identified by positional cloning in 1990. Subsequent identification and characterization of the CHM gene, which encodes Rab escort protein 1 (REP1), has led to better comprehension of the disease and enabled advances in genetic diagnosis. Despite several decades of work to understand the exact pathogenesis, no established treatments currently exist to stop or even slow the progression of retinal degeneration in choroideremia. Encouragingly, several specific molecular and clinical features make choroideremia an ideal candidate for treatment with gene therapy. This work describes the considerations and challenges in the development of a new clinical trial using adeno-associated virus (AAV) encoding the CHM gene.

Differential targeting of feline photoreceptors by recombinant adeno-associated viral vectors: implications for preclinical gene therapy trials

The cat is emerging as a promising large animal model for preclinical testing of retinal dystrophy therapies, for example, by gene therapy. However, there is a paucity of studies investigating viral vector gene transfer to the feline retina. We therefore sought to study the tropism of recombinant adeno-associated viral (rAAV) vectors for the feline outer retina. We delivered four rAAV serotypes: rAAV2/2, rAAV2/5, rAAV2/8 and rAAV2/9, each expressing green fluorescent protein (GFP) under the control of a cytomegalovirus promoter, to the subretinal space in cats and, for comparison, mice. Cats were monitored for gene expression by in vivo imaging and cellular tropism was determined using immunohistochemistry. In cats, rAAV2/2, rAAV2/8 and rAAV2/9 vectors induced faster and stronger GFP expression than rAAV2/5 and all vectors transduced the retinal pigment epithelium (RPE) and photoreceptors. Unlike in mice, cone photoreceptors in the cat retina were more efficiently transduced than rod photoreceptors. In mice, rAAV2/2 only transduced the RPE whereas the other vectors also transduced rods and cones. These results highlight species differences in cellular tropism of rAAV vectors in the outer retina. We conclude that rAAV serotypes are suitable for use for retinal gene therapy in feline models, particularly when cone photoreceptors are the target cell.

Advances in gene therapy technologies to treat retinitis pigmentosa

Retinitis pigmentosa (RP) is a class of diseases that leads to progressive degeneration of the retina. Experimental approaches to gene therapy for the treatment of inherited retinal dystrophies have advanced in recent years, inclusive of the safe delivery of genes to the human retina. This review is focused on the development of gene therapy for RP using recombinant adenoassociated viral vectors, which show a positive safety record and have so far been successful in several clinical trials for congenital retinal disease. Gene therapy for RP is under development in a variety of animal models, and the results raise expectations of future clinical application. Nonetheless, the translation of such strategies to the bedside requires further understanding of the mutations and mechanisms that cause visual defects, as well as thorough examination of potential adverse effects.

Gene therapy for retinal disease

Gene therapy strategies for the treatment of inherited retinal diseases have made major advances in recent years. This review focuses on adeno-associated viral (AAV) vector approaches to treat retinal degeneration and, thus, prevent or delay the onset of blindness. Data from human clinical trials of gene therapy for retinal disease show encouraging signs of safety and efficacy from AAV vectors. Recent progress in enhancing cell-specific targeting and transduction efficiency of the various retinal layers plus the use of AAV-delivered growth factors to augment the therapeutic effect and limit cell death suggest even greater success in future human trials is possible.

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

Adeno-associated virus (AAV) has proven an effective gene delivery vehicle for the treatment of retinal disease. Ongoing clinical trials using a serotype 2 AAV vector to express RPE65 in the retinal pigment epithelium have proven safe and effective. While many proof-of-concept studies in animal models of retinal disease have suggested that gene transfer to the neural retina will also be effective, a photoreceptor-targeting AAV vector has yet to be used in the clinic, principally because a vector that efficiently but exclusively targets all primate photoreceptors has yet to be demonstrated. Here, we evaluate a serotype 5 AAV vector containing the human rhodopsin kinase (hGRK1) promoter for its ability to target transgene expression to rod and cone photoreceptors when delivered subretinally in a nonhuman primate (NHP). In vivo fluorescent fundus imaging confirmed that AAV5-hGRK1-mediated green fluorescent protein (GFP) expression was restricted to the injection blebs of treated eyes. Optical coherence tomography (OCT) revealed a lack of gross pathology after injection. Neutralizing antibodies against AAV5 were undetectable in post-injection serum samples from subjects receiving uncomplicated subretinal injections (i.e., no hemorrhage). Immunohistochemistry of retinal sections confirmed hGRK1 was active in, and specific for, both rods and cones of NHP retina. Biodistribution studies revealed minimal spread of vector genomes to peripheral tissues. These results suggest that AAV5-hGRK1 is a safe and effective AAV serotype/promoter combination for targeting therapeutic transgene expression protein to rods and cones in a clinical setting.

A multi-stage color model revisited: implications for a gene therapy cure for red-green colorblindness

In 1993, DeValois and DeValois proposed a 'multi-stage color model' to explain how the cortex is ultimately able to deconfound the responses of neurons receiving input from three cone types in order to produce separate red-green and blue-yellow systems, as well as segregate luminance percepts (black-white) from color. This model extended the biological implementation of Hurvich and Jameson's Opponent-Process Theory of color vision, a two-stage model encompassing the three cone types combined in a later opponent organization, which has been the accepted dogma in color vision. DeValois' model attempts to satisfy the long-remaining question of how the visual system separates luminance information from color, but what are the cellular mechanisms that establish the complicated neural wiring and higher-order operations required by the Multi-stage Model? During the last decade and a half, results from molecular biology have shed new light on the evolution of primate color vision, thus constraining the possibilities for the visual circuits. The evolutionary constraints allow for an extension of DeValois' model that is more explicit about the biology of color vision circuitry, and it predicts that human red-green colorblindness can be cured using a retinal gene therapy approach to add the missing photopigment, without any additional changes to the post-synaptic circuitry.

Dosage thresholds for AAV2 and AAV8 photoreceptor gene therapy in monkey

Gene therapy is emerging as a therapeutic modality for treating disorders of the retina. Photoreceptor cells are the primary cell type affected in many inherited diseases of retinal degeneration. Successfully treating these diseases with gene therapy requires the identification of efficient and safe targeting vectors that can transduce photoreceptor cells. One serotype of adeno-associated virus, AAV2, has been used successfully in clinical trials to treat a form of congenital blindness that requires transduction of the supporting cells of the retina in the retinal pigment epithelium (RPE). Here, we determined the dose required to achieve targeting of AAV2 and AAV8 vectors to photoreceptors in nonhuman primates. Transgene expression in animals injected subretinally with various doses of AAV2 or AAV8 vectors carrying a green fluorescent protein transgene was correlated with surgical, clinical, and immunological observations. Both AAV2 and AAV8 demonstrated efficient transduction of RPE, but AAV8 was markedly better at targeting photoreceptor cells. These preclinical results provide guidance for optimal vector and dose selection in future human gene therapy trials to treat retinal diseases caused by loss of photoreceptors.

Gene therapy for red-green colour blindness in adult primates

Red-green colour blindness, which results from the absence of either the long- (L) or middle- (M) wavelength-sensitive visual photopigments, is the most common single locus genetic disorder. Here, the possibility of curing colour blindness using gene therapy was explored in experiments on adult monkeys that had been colour blind since birth. A third type of cone pigment was added to dichromatic retinas, providing the receptoral basis for trichromatic colour vision. This opened a new avenue to explore the requirements for establishing the neural circuits for a new dimension of colour sensation. Classic visual deprivation experiments1 have led to the expectation that neural connections established during development would not appropriately process an input that was not present from birth. Therefore, it was believed that treatment of congenital vision disorders would be ineffective unless administered to the very young. Here, however, addition of a third opsin in adult red-green colour-deficient primates was sufficient to produce trichromatic colour vision behaviour. Thus, trichromacy can arise from a single addition of a third cone class and it does not require an early developmental process. This provides a positive outlook for the potential of gene therapy to cure adult vision disorders.

The use of canine models of inherited retinal degeneration to test novel therapeutic approaches

Inherited retinal degenerations (RDs) are a common cause of blindness in dogs and in humans. Over the past two decades numerous genes causally associated with these diseases have been identified and several canine models have been used to improve our understanding of the molecular mechanisms of RDs, as well as to test the proof of principle and safety of novel therapies. This review briefly summarizes the drug delivery approaches and therapeutic strategies that have been and are currently tested in dogs, with a particular emphasis on corrective gene therapy, and retinal neuroprotection.

Quantitative analysis of tumor size in a murine model of retinoblastoma

Murine models can provide valuable insight into mechanisms of tumorigenesis. Tumor size is often used to assess the impact of genetic insult or therapeutic treatment, usually using in vivo imaging of advanced tumors. We now describe a highly sensitive method to quantify tumor volume in a mouse model of retinoblastoma, from the earliest stages of tumor initiation to large, advanced tumors. This methodology combines immunohistochemistry, digital slide scanning and computer image analysis, and can be applied to quantitatively assess and characterize early tumor development in other models.

Adapting to altered image statistics using processed video

Perceptual systems can be altered by immersing observers in environments with statistical properties that differ from those naturally encountered. Here we present a novel method for placing observers in naturalistic audio visual environments whose statistics can be manipulated in very targeted ways. We present the results of a case study that used this method. Observers were exposed to an environment where there was a novel statistical relationship between two simple, visual patterns in otherwise natural scenes. Exposure to this altered environment strengthened perceptual interactions between the two patterns.

Longitudinal evaluation of expression of virally delivered transgenes in gerbil cone photoreceptors

Delivery of foreign opsin genes to cone photoreceptors using recombinant adeno-associated virus (rAAV) is a potential tool for studying the basic mechanisms underlying cone based vision and for treating vision disorders. We used an in vivo retinal imaging system to monitor, over time, expression of virally-delivered genes targeted to cone photoreceptors in the Mongolian gerbil (Meriones unguiculatus). Gerbils have a well-developed photopic visual system, with 11-14% of their photoreceptors being cones. We used replication deficient serotype 5 rAAV to deliver a gene for green fluorescent protein (GFP). In an effort to direct expression of the gene specifically to either S or M cones, the transgene was under the control of either the human X-chromosome opsin gene regulatory elements, i.e., an enhancer termed the locus control region (LCR) and L promoter, or the human S-opsin promoter. Longitudinal fluorescence images reveal that gene expression is first detectable about 14 days post-injection, reaches a peak after about 3 months, and is observed more than a year post-injection if the initial viral concentration is sufficiently high. The regulatory elements are able to direct expression to a subpopulation of cones while excluding expression in rods and non-photoreceptor retinal cells. When the same viral constructs are used to deliver a human long-wavelength opsin gene to gerbil cones, stimulation of the introduced human photopigment with long-wavelength light produces robust cone responses.

The role of cis-regulatory elements in the design of gene therapy vectors for inherited blindness

BACKGROUND

Hereditary retinal disease is currently known to involve nearly 200 different genetic loci. There has been remarkable recent progress in the treatment of retinal disease via gene therapy in animal models using virus-based vectors. The majority of retinal diseases affect one of several cell types. In order to target expression of a rescue transgene specifically to the cells in need of therapy, it is necessary to employ a cis-regulatory element (CRE) to drive expression of the transgene specifically in those cells.

OBJECTIVE/METHODS

This review discusses the repertoire of CREs currently available for use in gene therapy vectors for treatment of retinal disease and outlines the issues that must be taken into consideration in the development of novel CREs for the purpose of gene therapy in the retina.

CONCLUSION

There have been a number of important recent advances in the identification and characterization of retinal CREs and their utilization in gene therapy vectors. Nevertheless, future efforts to rationally manipulate existing CREs and design novel synthetic CREs for therapeutic purposes will require a better understanding of the cis-regulatory rules that govern CRE activity in vivo.

Prediction of Cis-Regulatory Elements Controlling Genes Differentially Expressed by Retinal and Choroidal Vascular Endothelial Cells

Cultured endothelial cells of the human retina and choroid demonstrate distinct patterns of gene expression. We hypothesized that differential gene expression reflected differences in the interactions of transcription factors and respective cis-regulatory motifs(s) in these two endothelial cell subpopulations, recognizing that motifs often exist as modules. We tested this hypothesis in silico by using TRANSFAC Professional and CisModule to identify cis-regulatory motifs and modules in genes that were differentially expressed by human retinal versus choroidal endothelial cells, as identified by analysis of a microarray data set. Motifs corresponding to eight transcription factors were significantly (p < 0.05) differentially abundant in genes that were relatively highly expressed in retinal (i.e., glucocorticoid receptor, high mobility group AT-hook 1, heat shock transcription factor 1, p53, vitamin D receptor) or choroidal (i.e., transcription factor E2F, Yin Yang 1, zinc finger 5) endothelial cells. Predicted cis-regulatory modules were quite different for these two groups of genes. Our findings raise the possibility of exploiting specific cis-regulatory motifs to target therapy at the ocular endothelial cells subtypes responsible for neovascular age-related macular degeneration or proliferative diabetic retinopathy.

Inherited diseases of photoreceptors and prospects for gene therapy

The photoreceptor cells of the retina are subject to a wide range of genetic diseases. This review summarizes current knowledge regarding an important group of retinal diseases caused by mutations in photoreceptor-enriched genes. In addition, progress toward treatment of a variety of these diseases in animal models via adeno-associated virus gene therapy is described. Although no human trials have yet been initiated to treat diseases caused by mutations in photoreceptor-enriched genes, there is a great deal of optimism regarding the prospects of treating these diseases using adeno-associated virus gene therapy.

Targeting gene expression to cones with human cone opsin promoters in recombinant AAV

Specific cone-directed therapy is of high priority in the treatment of human hereditary retinal diseases. However, not much information exists about the specific targeting of photoreceptor subclasses. Three versions of the human red cone opsin promoter (PR0.5, 3LCR-PR0.5 and PR2.1), and the human blue cone opsin promoter HB569, were evaluated for their specificity and robustness in targeting green fluorescent protein (GFP) gene expression to subclasses of cones in the canine retina when used in recombinant adeno-associated viral vectors of serotype 5. The vectors were administered by subretinal injection. The promoter PR2.1 led to most effective and specific expression of GFP in the long- and medium-wavelength-absorbing cones (L/M cones) of normal and diseased retinas. The PR0.5 promoter was not effective. Adding three copies of the 35-bp LCR in front of PR0.5 lead to weak GFP expression in L/M cones. The HB569 promoter was not specific, and GFP was expressed in a few L/M cones, some rods and the retinal pigment epithelium. These results suggest that L/M cones, the predominant class of cone photoreceptors in the retinas of dogs and most mammalian species can be successfully targeted using the human red cone opsin promoter.