Functional and structural recovery of the retina after gene therapy in the RPE65 null mutation dog

Improved protocol for histological and histopathological preparation of large eyes

The development of new treatments for ocular diseases often requires investigating eyes similar in size and structure to human eyes. Such studies are challenging because analyzing the histopathology of large, human-sized eyes can be technically difficult. In particular, obtaining high-quality frozen sections is almost impossible due to the formation of ice crystals in the vitreous, which causes crush artifacts during the procedures of section and post sectioning manipulations. Herein, we describe a new method that provides high-quality frozen sections for large eyes and demonstrate its usefulness in the eyes of rabbits, pigs, minipigs, monkeys, and humans. We observed that artifactual separation of the photoreceptors from the retinal pigment epithelium is minimized and photoreceptor morphology is preserved. This method can be highly beneficial for investigators seeking to translate new treatments for ocular disease into the clinic. RESEARCH HIGHLIGHTS: Histopathological analysis of large and human-sized eyes presents significant challenges, particularly in obtaining high-quality frozen sections. A multistep fixation followed by vitreous removal and replacement ensures better cryopreservation and embedding of large eyes, minimizing the morphological and structural retinal loss found in many studies. Our results demonstrate that a multistep fixation and cryopreservation method for large eyes in histopathology consistently minimizes the artifactual separation of photoreceptors from the retinal pigment epithelium, thereby preserving photoreceptor morphology and providing high-quality frozen sections. A new method providing high-quality sections is necessary and will be highly useful for investigators aiming to translate new treatments for ocular diseases into clinical applications.

The landscape of basic gene therapy approaches in inherited retinal dystrophies

The study of gene therapies has been of particular interest in recent decades due to their promising potential to slow or even rescue the degeneration of the retina in inherited retinal dystrophies (IRDs). Here, we review the current approaches to gene therapy trials on IRDs, including the selection of animal models, therapeutic window, vectors and dosages. Mice are typically the first choice of animal models and recombinant adeno-associated virus (rAAV) of serotype 8 is the most common vector for loss-of-function IRDs. Furthermore, the therapeutic window should be considered to ensure efficacy before retinal degeneration occurs if possible, and dosages must be tailored to each approach.

Intravitreal gene therapy preserves retinal function in a canine model of CLN2 neuronal ceroid lipofuscinosis

CLN2 neuronal ceroid lipofuscinosis is a rare hereditary neurodegenerative disorder characterized by deleterious sequence variants in TPP1 that result in reduced or abolished function of the lysosomal enzyme tripeptidyl peptidase 1 (TPP1). Children with this disorder experience progressive neurological decline and vision loss starting around 2-4 years of age. Ocular disease is characterized by progressive retinal degeneration and impaired retinal function culminating in total loss of vision. Similar retinal pathology occurs in a canine model of CLN2 disease with a null variant in TPP1. A study using the dog model was performed to evaluate the efficacy of ocular gene therapy to provide a continuous, long-term source of human TPP1 (hTPP1) to the retina, inhibit retinal degeneration and preserve retinal function. TPP1-/- dogs received an intravitreal injection of 1 x 1012 viral genomes of AAV2.CAG.hTPP1 in one eye and AAV2.CAG.GFP in the contralateral eye at 4 months of age. Ophthalmic exams, in vivo ocular imaging and electroretinography were repeated monthly to assess retinal structure and function. Retinal morphology, hTPP1 and GFP expression in the retina, optic nerve and lateral geniculate nucleus, and hTPP1 concentrations in the vitreous were evaluated after the dogs were euthanized at end stage neurological disease at approximately 10 months of age. Intravitreal administration of AAV2.CAG.hTPP1 resulted in stable, widespread expression of hTPP1 throughout the inner retina, prevented disease-related declines in retinal function and inhibited disease-related cell loss and storage body accumulation in the retina for at least 6 months. Uveitis occurred in eyes treated with the hTPP1 vector, but this did not prevent therapeutic efficacy. The severity of the uveitis was ameliorated with anti-inflammatory treatments. These results indicate that a single intravitreal injection of AAV2.CAG.hTPP1 is an effective treatment to inhibit ocular disease progression in canine CLN2 disease.

Immune Responses to Gene Editing by Viral and Non-Viral Delivery Vectors Used in Retinal Gene Therapy

Inherited retinal diseases (IRDs) are a leading cause of blindness in industrialized countries, and gene therapy is quickly becoming a viable option to treat this group of diseases. Gene replacement using a viral vector has been successfully applied and advanced to commercial use for a rare group of diseases. This, and the advances in gene editing, are paving the way for the emergence of a new generation of therapies that use CRISPR-Cas9 to edit mutated genes in situ. These CRISPR-based agents can be delivered to the retina as transgenes in a viral vector, unpackaged transgenes or as proteins or messenger RNA using non-viral vectors. Although the eye is considered to be an immune-privileged organ, studies in animals, as well as evidence from clinics, have concluded that ocular gene therapies elicit an immune response that can under certain circumstances result in inflammation. In this review, we evaluate studies that have reported on pre-existing immunity, and discuss both innate and adaptive immune responses with a specific focus on immune responses to gene editing, both with non-viral and viral delivery in the ocular space. Lastly, we discuss approaches to prevent and manage the immune responses to ensure safe and efficient gene editing in the retina.

Gene Therapy for Inherited Retinal Disease: Long-Term Durability of Effect

The recent approval of voretigene neparvovec (Luxturna®) for patients with biallelic RPE65 mutation-associated inherited retinal dystrophy with viable retinal cells represents an important step in the development of ocular gene therapies. Herein, we review studies investigating the episomal persistence of different recombinant adeno-associated virus (rAAV) vector genomes and the preclinical and clinical evidence of long-term effects of different RPE65 gene replacement therapies. A targeted review of articles published between 1974 and January 2021 in Medline®, Embase®, and other databases was conducted, followed by a descriptive longitudinal analysis of the clinical trial outcomes of voretigene neparvovec. Following an initial screening, 14 publications examining the episomal persistence of different rAAV genomes and 71 publications evaluating gene therapies in animal models were included. Viral genomes were found to persist for at least 22 months (longest study follow-up) as transcriptionally active episomes. Treatment effects lasting almost a decade were reported in canine disease models, with more pronounced effects the earlier the intervention. The clinical trial outcomes of voretigene neparvovec are consistent with preclinical findings and reveal sustained results for up to 7.5 years for the full-field light sensitivity threshold test and 5 years for the multi-luminance mobility test in the Phase I and Phase III trials, respectively. In conclusion, the therapeutic effect of voretigene neparvovec lasts for at least a decade in animal models and 7.5 years in human subjects. Since retinal cells can retain functionality over their lifetime after transduction, these effects may be expected to last even longer in patients with a sufficient number of outer retinal cells at the time of intervention.

Japan to Global Eye Genetics Consortium: Extending Research Collaboration for Inherited Eye Diseases

Japan Eye Genetics Consortium (JEGC) was established in 2011 to migrate research system to all-Japan structure for collecting phenotype-genotype information for inherited retinal diseases and other retinal diseases including hereditary optic neuropathy and hereditary glaucoma. Diagnostic team was assembled to maintain quality of diagnostic and to collect phenotype information to database in Tokyo Medical Center (TMC). Over the past 10 years, 1538 pedigree [2788 deoxyribonucleic acid (DNA) samples] was collected from 38 ophthalmology departments and eye hospitals. Whole exome analysis has improved diagnostic rate from ~17% in 2011 to 53% in 2021, with 27% of known variants, 18% of novel variants in known gene, 8% of potential novel disease-causing genes, and 47% of pedigree with unknown cause. Approximately 70% of Japanese patients were affected by novel mutation or by unknown cause. In 2014, Asian Eye Genetics Consortium (AEGC) was established by researchers from Hong Kong, India, Japan, and the US, later renamed to Global Eye Genetics Consortium (GEGC) to expand the idea of collaborative research on rare genetic eye diseases in Asia, Middle East, Africa, and South America. GEGC phenotype-genotype database, GenEye, was constructed to collect and catalog genetic eye diseases at global scale. Over 200 members from 30 countries, GEGC now has 200 members from 30 continents, performing scientific programs, young investigator visiting program, and GEGC organized session at the meetings of the Asia-Pacific Academy of Ophthalmology (APAO), The Association for Research in Vision and Ophthalmology (ARVO), All India Ophthalmological Society (AIOS), World Ophthalmology Congress (WOC), and International Society for Eye Research (ISER).

Systemic and local immune responses to intraocular AAV vector administration in non-human primates

Positive clinical outcomes in adeno-associated virus (AAV)-mediated retinal gene therapy have often been attributed to the low immunogenicity of AAVs and immune privilege of the eye. However, several recent studies have shown potential for inflammatory responses. The current understanding of the factors contributing to inflammation, such as the pre-existence of serum antibodies against AAVs and their contribution to increases in antibody levels post-injection, is incomplete. The parameters that regulate the generation of new antibodies in response to the AAV capsid or transgene after intraocular injections are also insufficiently described. This study is a retrospective analysis of the pre-existing serum antibodies in correlation with changes in antibody levels after intraocular injections of AAV in non-human primates (NHPs) of the species Macaca fascicularis. In NHP serums, we analyzed the binding antibody (BAB) levels and a subset of these called neutralizing antibodies (NABs) that impede AAV transduction. We observed significantly higher pre-existing serum BABs against AAV8 compared with other serotypes and a dose-dependent increase in BABs and NABs in the serums collected post-injection, irrespective of the serotype or the mode of injection. Lastly, we were able to demonstrate a correlation between the serum BAB levels with clinical grading of inflammation and levels of transgene expression.

Visual system pathology in a canine model of CLN5 neuronal ceroid lipofuscinosis

CLN5 neuronal ceroid lipofuscinosis is a hereditary neurodegenerative disease characterized by progressive neurological decline, vision loss and seizures. Visual impairment in children with CLN5 disease is attributed to a progressive decline in retinal function accompanied by retinal degeneration as well as impaired central nervous system function associated with global brain atrophy. We studied visual system pathology in five Golden Retriever littermates homozygous for the CLN5 disease allele previously identified in the breed. The dogs exhibited signs of pronounced visual impairment by 21-22 months of age. Electroretinogram recordings showed a progressive decline in retinal function primarily affecting cone neural pathways. Altered visual evoked potential recordings indicated that disease progression affected visual signal processing in the brain. Aside from several small retinal detachment lesions, no gross retinal abnormalities were observed with in vivo ocular imaging and histologically the retinas did not exhibit apparent abnormalities by 23 months of age. However, there was extensive accumulation of autofluorescent membrane-bound lysosomal storage bodies in almost all retinal layers, as well as in the occipital cortex, by 20 months of age. In the retina, storage was particularly pronounced in retinal ganglion cells, the retinal pigment epithelium and in photoreceptor cells just interior to the outer limiting membrane. The visual system pathology of CLN5-affected Golden Retrievers is similar to that seen early in the human disease. It was not possible to follow the dogs to an advanced stage of disease progression due to the severity of behavioral and motor disease signs by 23 months of age. The findings reported here indicate that canine CLN5 disease will be a useful model of visual system disease in CLN5 neuronal ceroid lipofuscinosis. The baseline data obtained in this investigation will be useful in future therapeutic intervention studies. The findings indicate that there is a fairly broad time frame after disease onset within which treatments could be effective in preserving vision.

Animals Models of Inherited Retinal Disease

Inherited retinal diseases (IRDs) are an important cause of blindness worldwide. Over 270 genes have been associated with IRD. Genetic testing can determine the cause of the clinical disease in the majority of patients. However, at least 25-50% of patients with clinical diagnosis of IRD remain unsolved even after whole genome sequencing. Animal models of IRD can be useful for expanding the set of established IRD genes, to gain biological understanding of the function of these genes in the retina, and to test advanced therapeutics prior to human clinical trials. In this chapter some small and large animal models of IRD are discussed including some of the advantages and limitations of each for various forms of retinopathy.

Focused Update on AAV-Based Gene Therapy Clinical Trials for Inherited Retinal Degeneration

Inherited retinal diseases (IRDs) comprise a clinically and genetically heterogeneous group of disorders that can ultimately result in photoreceptor dysfunction/death and vision loss. With over 270 genes known to be involved in IRDs, translation of treatment strategies into clinical applications has been historically difficult. However, in recent years there have been significant advances in basic research findings as well as translational studies, culminating in an increasing number of clinical trials with the ultimate goal of reducing vision loss and associated morbidities. The recent approval of Luxturna^® (voretigene neparvovec-rzyl) for Leber congenital amaurosis type 2 (LCA2) prompts a review of the current clinical trials for IRDs, with a particular focus on the importance of adeno-associated virus (AAV)-based gene therapies. The present article reviews the current state of AAV use in gene therapy clinical trials for IRDs, with a brief background on AAV and the reasons behind its dominance in ocular gene therapy. It will also discuss pre-clinical progress in AAV-based therapies aimed at treating other ocular conditions that can have hereditable links, and what alternative technologies are progressing in the same therapeutic space.

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

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

Voretigene Neparvovec and Gene Therapy for Leber's Congenital Amaurosis: Review of Evidence to Date

Gene therapy has now evolved as the upcoming modality for management of many disorders, both inheritable and non-inheritable. Knowledge of genetics pertaining to a disease has therefore become paramount for physicians across most specialities. Inheritable retinal dystrophies (IRDs) are notorious for progressive and relentless vision loss, frequently culminating in complete blindness in both eyes. Leber’s congenital amaurosis (LCA) is a typical example of an IRD that manifests very early in childhood. Research in gene therapy has led to the development and approval of voretigene neparvovec (VN) for use in patients of LCA with a deficient biallelic RPE65 gene. The procedure involves delivery of a recombinant virus vector that carries the RPE65 gene in the subretinal space. This comprehensive review reports the evidence thus far in support of gene therapy for LCA. We explore and compare the various gene targets including but not limited to RPE65, and discuss the choice of vector and method for ocular delivery. The review details the evolution of gene therapy with VN in a phased manner, concluding with the challenges that lie ahead for its translation for use in communities that differ much both genetically and economically.

Immune responses to retinal gene therapy using adeno-associated viral vectors - Implications for treatment success and safety

Recombinant adeno-associated virus (AAV) is the leading vector for gene therapy in the retina. As non-pathogenic, non-integrating, replication deficient vector, the recombinant virus efficiently transduces all key retinal cell populations. Successful testing of AAV vectors in clinical trials of inherited retinal diseases led to the recent approval of voretigene neparvovec (Luxturna) for the treatment of RPE65 mutation-associated retinal dystrophies. However, studies applying AAV-mediated retinal gene therapy independently reported intraocular inflammation and/or loss of efficacy after initial functional improvements. Both observations might be explained by targeted removal of transduced cells via anti-viral defence mechanisms. AAV has been shown to activate innate pattern recognition receptors (PRRs) such as toll-like receptor (TLR)-2 and TLR-9 resulting in the release of inflammatory cytokines and type I interferons. The vector can also induce capsid-specific and transgene-specific T cell responses and neutralizing anti-AAV antibodies which both limit the therapeutic effect. However, the target organ of retinal gene therapy, the eye, is known as an immune-privileged site. It is characterized by suppression of inflammation and promotion of immune tolerance which might prevent AAV-induced immune responses. This review evaluates AAV-related immune responses, toxicity and inflammation in studies of retinal gene therapy, identifies influencing variables of these responses and discusses potential strategies to modulate immune reactions to AAV vectors to increase the safety and efficacy of ocular gene therapy.

Large Animal Models of Inherited Retinal Degenerations: A Review

Studies utilizing large animal models of inherited retinal degeneration (IRD) have proven important in not only the development of translational therapeutic approaches, but also in improving our understanding of disease mechanisms. The dog is the predominant species utilized because spontaneous IRD is common in the canine pet population. Cats are also a source of spontaneous IRDs. Other large animal models with spontaneous IRDs include sheep, horses and non-human primates (NHP). The pig has also proven valuable due to the ease in which transgenic animals can be generated and work is ongoing to produce engineered models of other large animal species including NHP. These large animal models offer important advantages over the widely used laboratory rodent models. The globe size and dimensions more closely parallel those of humans and, most importantly, they have a retinal region of high cone density and denser photoreceptor packing for high acuity vision. Laboratory rodents lack such a retinal region and, as macular disease is a critical cause for vision loss in humans, having a comparable retinal region in model species is particularly important. This review will discuss several large animal models which have been used to study disease mechanisms relevant for the equivalent human IRD.

Immune Response Mechanisms against AAV Vectors in Animal Models

Early preclinical studies in rodents and other species did not reveal that vector or transgene immunity would present a significant hurdle for sustained gene expression. While there was early evidence of mild immune responses to adeno-associated virus (AAV) in preclinical studies, it was generally believed that these responses were too weak and transient to negatively impact sustained transduction. However, translation of the cumulative success in treating hemophilia B in rodents and dogs with an AAV2-F9 vector to human studies was not as successful. Despite significant progress in recent clinical trials for hemophilia, new immunotoxicities to AAV and transgene are emerging in humans that require better animal models to assess and overcome these responses. The animal models designed to address these immune complications have provided critical information to assess how vector dose, vector capsid processing, vector genome, difference in serotypes, and variations in vector delivery route can impact immunity and to develop approaches for overcoming pre-existing immunity. Additionally, a comprehensive dissection of innate, adaptive, and regulatory responses to AAV vectors in preclinical studies has provided a framework that can be utilized for development of immunomodulatory therapies to overcome or bypass immune responses and for developing strategic approaches toward engineering stealth AAV vectors that can circumvent immunity.

Olfactory Loss and Dysfunction in Ciliopathies: Molecular Mechanisms and Potential Therapies

BACKGROUND

Ciliopathies are a class of inherited pleiotropic genetic disorders in which alterations in cilia assembly, maintenance, and/or function exhibit penetrance in the multiple organ systems. Olfactory dysfunction is one such clinical manifestation that has been shown in both patients and model organisms. Existing therapies for ciliopathies are limited to the treatment or management of symptoms. The last decade has seen an increase in potential curative therapeutic options including small molecules and biologics. Recent work in multiciliated olfactory sensory neurons has demonstrated the capacity of targeted gene therapy to restore ciliation in terminally differentiated cells and rescue olfactory function. This review will discuss the current understanding of the penetrance of ciliopathies in the olfactory system. Importantly, it will highlight both pharmacological and biological approaches, and their potential therapeutic value in the olfactory system and other ciliated tissues.

METHODS

We undertook a structured and comprehensive search of peer-reviewed research literature encompassing in vitro, in vivo, model organism, and clinical studies. From these publications, we describe the olfactory system, and discuss the penetrance of ciliopathies and impact of cilia loss on olfactory function. In addition, we outlined the developing therapies for ciliopathies across different organ and cell culture systems, and discussed their potential therapeutic application to the mammalian olfactory system.

RESULTS

One-hundred sixty-one manuscripts were included in the review, centering on the understanding of olfactory penetrance of ciliopathies, and discussing the potential therapeutic options for ciliopathies in the context of the mammalian olfactory system. Forty-four manuscripts were used to generate a table listing the known congenital causes of olfactory dysfunction, with the first ten listed are linked to ciliopathies. Twenty-three manuscripts were used to outline the potential of small molecules for the olfactory system. Emphasis was placed on HDAC6 inhibitors and lithium, both of which were shown to stabilize microtubule structures, contributing to ciliogenesis and cilia lengthening. Seventy-five manuscripts were used to describe gene therapy and gene therapeutic strategies. Included were the implementation of adenoviral, adeno-associated virus (AAV), and lentiviral vectors to treat ciliopathies across different organ systems and application toward the olfactory system. Thus far, adenoviral and AAVmeditated ciliary restoration demonstrated successful proof-of-principle preclinical studies. In addition, gene editing, ex vivo gene therapy, and transplantation could serve as alternative therapeutic and long-term approaches. But for all approaches, additional assessment of vector immunogenicity, specificity, and efficacy need further investigation. Currently, ciliopathy treatments are limited to symptomatic management with no curative options. However, the accessibility and amenability of the olfactory system to treatment would facilitate development and advancement of a viable therapy.

CONCLUSION

The findings of this review highlight the contribution of ciliopathies to a growing list of congenial olfactory dysfunctions. Promising results from other organ systems imply the feasibility of biologics, with results from gene therapies proving to be a viable therapeutic option for ciliopathies and olfactory dysfunction.

CRISPR-Cas9-mediated therapeutic editing of Rpe65 ameliorates the disease phenotypes in a mouse model of Leber congenital amaurosis

Leber congenital amaurosis (LCA), one of the leading causes of childhood-onset blindness, is caused by autosomal recessive mutations in several genes including RPE65. In this study, we performed CRISPR-Cas9-mediated therapeutic correction of a disease-associated nonsense mutation in Rpe65 in rd12 mice, a model of human LCA. Subretinal injection of adeno-associated virus carrying CRISPR-Cas9 and donor DNA resulted in >1% homology-directed repair and ~1.6% deletion of the pathogenic stop codon in Rpe65 in retinal pigment epithelial tissues of rd12 mice. The a- and b-waves of electroretinograms were recovered to levels up to 21.2 ± 4.1% and 39.8 ± 3.2% of their wild-type mice counterparts upon bright stimuli after dark adaptation 7 months after injection. There was no definite evidence of histologic perturbation or tumorigenesis during 7 months of observation. Collectively, we present the first therapeutic correction of an Rpe65 nonsense mutation using CRISPR-Cas9, providing new insight for developing therapeutics for LCA.

Long-Term Structural Outcomes of Late-Stage RPE65 Gene Therapy

The form of hereditary childhood blindness Leber congenital amaurosis (LCA) caused by biallelic RPE65 mutations is considered treatable with a gene therapy product approved in the US and Europe. The resulting vision improvement is well accepted, but long-term outcomes on the natural history of retinal degeneration are controversial. We treated four RPE65-mutant dogs in mid-life (age = 5-6 years) and followed them long-term (4-5 years). At the time of the intervention at mid-life, there were intra-ocular and inter-animal differences in local photoreceptor layer health ranging from near normal to complete degeneration. Treated locations having more than 63% of normal photoreceptors showed robust treatment-related retention of photoreceptors in the long term. Treated regions with less retained photoreceptors at the time of the intervention showed progressive degeneration similar to untreated regions with matched initial stage of disease. Unexpectedly, both treated and untreated regions in study eyes tended to show less degeneration compared to matched locations in untreated control eyes. These results support the hypothesis that successful long-term arrest of progression with RPE65 gene therapy may only occur in retinal regions with relatively retained photoreceptors at the time of the intervention, and there may be heretofore unknown mechanisms causing long-distance partial treatment effects beyond the region of subretinal injection.

Applying switchable Cas9 variants to in vivo gene editing for therapeutic applications

Progress in targeted gene editing by programmable endonucleases has paved the way for their use in gene therapy. Particularly, Cas9 is an endonuclease with high activity and flexibility, rendering it an attractive option for therapeutic applications in clinical settings. Many disease-causing mutations could potentially be corrected by this versatile new technology. In addition, recently developed switchable Cas9 variants, whose activity can be controlled by an external stimulus, provide an extra level of spatiotemporal control on gene editing and are particularly desirable for certain applications. Here, we discuss the considerations and difficulties for implementing Cas9 to in vivo gene therapy. We put particular emphasis on how switchable Cas9 variants may resolve some of these barriers and advance gene therapy in the clinical setting.

An ABCA4 loss-of-function mutation causes a canine form of Stargardt disease

Autosomal recessive retinal degenerative diseases cause visual impairment and blindness in both humans and dogs. Currently, no standard treatment is available, but pioneering gene therapy-based canine models have been instrumental for clinical trials in humans. To study a novel form of retinal degeneration in Labrador retriever dogs with clinical signs indicating cone and rod degeneration, we used whole-genome sequencing of an affected sib-pair and their unaffected parents. A frameshift insertion in the ATP binding cassette subfamily A member 4 (ABCA4) gene (c.4176insC), leading to a premature stop codon in exon 28 (p.F1393Lfs*1395), was identified. In contrast to unaffected dogs, no full-length ABCA4 protein was detected in the retina of an affected dog. The ABCA4 gene encodes a membrane transporter protein localized in the outer segments of rod and cone photoreceptors. In humans, the ABCA4 gene is associated with Stargardt disease (STGD), an autosomal recessive retinal degeneration leading to central visual impairment. A hallmark of STGD is the accumulation of lipofuscin deposits in the retinal pigment epithelium (RPE). The discovery of a canine homozygous ABCA4 loss-of-function mutation may advance the development of dog as a large animal model for human STGD.

Stargardt disease (STGD) is the most common inherited retinal disease causing visual impairment and blindness in children and young adults, affecting 1 in 8–10 thousand people. For other inherited retinal diseases, the dog has become an established comparative animal model, both for identifying the underlying genetic causes and for developing new treatment methods. To date, there is no standard treatment for STGD and the only available animal model to study the disease is the mouse. As a nocturnal animal, the morphology of the mouse eye differs from humans and therefore the mouse model is not ideal for developing methods for treatment. We have studied a novel form of retinal degeneration in Labrador retriever dogs showing clinical signs similar to human STGD. To investigate the genetic cause of the disease, we used whole-genome sequencing of a family quartet including two affected offspring and their unaffected parents. This led to the identification of a loss-of-function mutation in the ABCA4 gene. The findings of this study may enable the development of a canine model for human STGD.

Gene therapy and the adeno-associated virus in the treatment of genetic and acquired ophthalmic diseases in humans: Trials, future directions and safety considerations

Voretigene neparvovec-rzyl was recently approved for the treatment of Leber congenital amaurosis, and the use of gene therapy for eye disease is attracting even greater interest. The eye has immune privileged status, is easily accessible, requires a reduced dosage of therapy due to its size and is highly compartmentalized, significantly reducing systemic spread. Adeno-associated virus (AAV), with its low pathogenicity, prolonged expression profile and ability to transduce multiple cell types, has become the leading gene therapy vector. Target diseases have moved beyond currently untreatable inherited dystrophies to common, partially treatable acquired conditions such as exudative age-related macular degeneration and glaucoma, but use of the technology in these conditions imposes added obligations for caution in vector design. This review discusses the current status of AAV gene therapy trials in genetic and acquired ocular diseases, and explores new scientific developments, which could help ensure effective and safe use of the therapy in the future.

Dosage Thresholds and Influence of Transgene Cassette in Adeno-Associated Virus-Related Toxicity

Today, there are >500 published studies and 40 clinical trials to treat retinal disorders using gene therapy. The great majority of them rely on the use of adeno-associated virus vectors (AAV) for therapeutic gene delivery. Thus far, AAVs have an excellent safety profile in the clinic. Nevertheless, it is known that AAV-mediated gene delivery leads to toxicity at higher input doses in experimental gene therapy. This study reveals the factors that contribute to retinal toxicity after subretinal administration of AAV vectors in wild-type mice. The study shows that alongside the input dose, the nature of the transgene and the cells mediating the expression determine the extent of toxicity. Importantly, the study shows that AAV vectors encoding green fluorescent protein (GFP) used as controls in experimental gene therapy are toxic at doses as low as 5 × 10⁹ vg, confounding the observed therapeutic effect in gene therapy paradigms. Altogether, the data show the importance of reducing input doses while increasing transgene expression levels via the use of more efficient capsids and promoters in order to avoid side effects in AAV-mediated gene therapy. Furthermore, the toxicity observed with AAV-GFP vectors imply a reinterpretation of previous gene therapy studies where the therapeutic effect was measured in relation to this control.

Phenotypic characterization of complete CSNB in the inbred research beagle: how common is CSNB in research and companion dogs?

PURPOSE

Although congenital stationary night blindness (CSNB) has been described in a Japanese beagle dog research colony, certain clinical correlates with human CSNB have not yet been described, nor has an estimate of frequency of the condition been made in inbred and outbred beagle populations.

METHODS

A beagle with CSNB obtained from a commercial research dog supplier in the USA and matched control dogs (n = 3) underwent examination, refraction, ocular imaging, assessment of visual navigation ability and detailed electroretinography (ERG). Retrospective review of ERGs in two independent groups of inbred (n = 15 and 537, respectively) and one group of outbred dogs (n = 36) was used to estimate CSNB frequency in these populations.

RESULTS

In the affected dog, there were absent dark-adapted b-waves in response to dim-light flashes, severely reduced dark-adapted b-waves in response to bright-light flashes, and normal light-adapted b-waves with a-waves that had broadened troughs. Long-flash ERGs confirmed a markedly reduced b-wave with a preserved d-wave, consistent with cone ON-bipolar cell dysfunction. There was evidence of normal rod photoreceptor a-wave dark adaptation, and rapid light adaptation. In the wider beagle populations, five inbred beagles had a b/a wave ratio of < 1 in dark-adapted bright-flash ERG, whereas no outbred beagles had ERGs consistent with CSNB.

CONCLUSIONS

The identified dog had clinical findings consistent with complete type CSNB, similar to that described in the Japanese colony. CSNB appears to be a rare disorder in the wider beagle population, although its detection could confound studies that use retinal function as an outcome measure in research dogs, necessitating careful baseline studies to be performed prior to experimentation.

A Coding Variant in the Gene Bardet-Biedl Syndrome 4 (BBS4) Is Associated with a Novel Form of Canine Progressive Retinal Atrophy

Progressive retinal atrophy is a common cause of blindness in the dog and affects >100 breeds. It is characterized by gradual vision loss that occurs due to the degeneration of photoreceptor cells in the retina. Similar to the human counterpart retinitis pigmentosa, the canine disorder is clinically and genetically heterogeneous and the underlying cause remains unknown for many cases. We use a positional candidate gene approach to identify putative variants in the Hungarian Puli breed using genotyping data of 14 family-based samples (CanineHD BeadChip array, Illumina) and whole-genome sequencing data of two proband and two parental samples (Illumina HiSeq 2000). A single nonsense SNP in exon 2 of BBS4 (c.58A > T, p.Lys20*) was identified following filtering of high quality variants. This allele is highly associated (P_CHISQ = 3.425e^-14, n = 103) and segregates perfectly with progressive retinal atrophy in the Hungarian Puli. In humans, BBS4 is known to cause Bardet-Biedl syndrome which includes a retinitis pigmentosa phenotype. From the observed coding change we expect that no functional BBS4 can be produced in the affected dogs. We identified canine phenotypes comparable with Bbs4-null mice including obesity and spermatozoa flagella defects. Knockout mice fail to form spermatozoa flagella. In the affected Hungarian Puli spermatozoa flagella are present, however a large proportion of sperm are morphologically abnormal and <5% are motile. This suggests that BBS4 contributes to flagella motility but not formation in the dog. Our results suggest a promising opportunity for studying Bardet-Biedl syndrome in a large animal model.

Outer Plexiform Layer Structures Are Not Altered Following AAV-Mediated Gene Transfer in Healthy Rat Retina

Ocular gene therapy approaches have been developed for a variety of different diseases. In particular, clinical gene therapy trials for RPE65 mutations, X-linked retinoschisis, and choroideremia have been conducted at different centers in recent years, showing that adeno-associated virus (AAV)-mediated gene therapy is safe, but limitations exist as to the therapeutic benefit and long-term duration of the treatment. The technique of vector delivery to retinal cells relies on subretinal injection of the vector solution, causing a transient retinal detachment. Although retinal detachments are known to cause remodeling of retinal neuronal structures as well as significant cell loss, the possible effects of this short-term therapeutic retinal detachment on retinal structure and circuitry have not yet been studied in detail. In this study, retinal morphology and apoptotic status were examined in healthy rat retinas following AAV-mediated gene transfer via subretinal injection with AAV2/5.CMV.d2GFP or sham injection with fluorescein. Outer plexiform layer (OPL) morphology was assessed by immunohistochemical labeling, laser scanning confocal microscopy, and electron microscopy. The number of synaptic contacts in the OPL was quantified after labeling with structural markers. To assess the apoptotic status, inflammatory and pro-apoptotic markers were tested and TUNEL assay for the detection of apoptotic nuclei was performed. Pre- and postsynaptic structures in the OPL, such as synaptic ribbons or horizontal and bipolar cell processes, did not differ in size or shape in injected versus non-injected areas and control retinas. Absolute numbers of synaptic ribbons were not altered. No signs of relevant gliosis were detected. TUNEL labeling of retinal cells did not vary between injected and non-injected areas, and apoptosis-inducing factor was not delocalized to the nucleus in transduced areas. The neuronal circuits in the OPL of healthy rat retinas undergoing AAV-mediated gene transfer were not altered by the temporary retinal detachment caused by subretinal injection, the presence of viral particles, or the expression of green fluorescent protein as a transgene. This observation likely requires further investigations in the dog model for RPE65 deficiency in order to determine the impact of RPE65 transgene expression on diseased retinas in animals and men.

Improved Intravitreal AAV-Mediated Inner Retinal Gene Transduction after Surgical Internal Limiting Membrane Peeling in Cynomolgus Monkeys

The retina is an ideal target for gene therapy because of its easy accessibility and limited immunological response. We previously reported that intravitreally injected adeno-associated virus (AAV) vector transduced the inner retina with high efficiency in a rodent model. In large animals, however, the efficiency of retinal transduction was low, because the vitreous and internal limiting membrane (ILM) acted as barriers to transduction. To overcome these barriers in cynomolgus monkeys, we performed vitrectomy (VIT) and ILM peeling before AAV vector injection. Following intravitreal injection of 50 μL triple-mutated self-complementary AAV serotype 2 vector encoding EGFP, transduction efficiency was analyzed. Little expression of GFP was detected in the control and VIT groups, but in the VIT+ILM group, strong GFP expression was detected within the peeled ILM area. To detect potential adverse effects, we monitored the retinas using color fundus photography, optical coherence tomography, and electroretinography. No serious side effects associated with the pretreatment were observed. These results indicate that surgical ILM peeling before AAV vector administration would be safe and useful for efficient transduction of the nonhuman primate retina and provide therapeutic benefits for the treatment of retinal diseases.

Advances in Gene Therapy for Diseases of the Eye

Over the last few years, huge progress has been made with regard to the understanding of molecular mechanisms underlying the pathogenesis of neurodegenerative diseases of the eye. Such knowledge has led to the development of gene therapy approaches to treat these devastating disorders. Challenges regarding the efficacy and efficiency of therapeutic gene delivery have driven the development of novel therapeutic approaches, which continue to evolve the field of ocular gene therapy. In this review article, we will discuss the evolution of preclinical and clinical strategies that have improved gene therapy in the eye, showing that treatment of vision loss has a bright future.

Investigation of SLA4A3 as a candidate gene for human retinal disease

SLC4A3 has been shown to cause retinal degeneration in a genetically engineered knockout mouse, and in a naturally occurring form of canine progressive retinal atrophy considered to be the equivalent of retinitis pigmentosa in humans (RP). This study was undertaken to investigate if SLC4A3 coding variants were implicated in human retinal degeneration. SLC4A3 exons were amplified and sequenced in 200 patients with autosomal recessive retinal degeneration who had no known molecular diagnosis for their condition, which included 197 unrelated individuals with suspected RP and three individuals with other forms of retinal disease. Three rare variants were identified that were predicted to be potentially pathogenic, however each variant was heterozygous in a single patient and therefore not considered disease-causing in isolation. Of these three variants, SNP-3 was the rarest, with an allele frequency of 7.06 x 10(-5) (>46,000 exomes from the ExAC database). In conclusion, no compound heterozygous or homozygous potentially pathogenic variants were identified that would account for recessive RP or retinal degeneration in this cohort, however the possibility remains that the rare variants identified could be acting with as yet undiscovered mutations in introns or regulatory regions. SLC4A3 remains an excellent candidate gene for human retinal degeneration, and with the advent of whole exome and whole genome sequencing of cohorts of molecularly unsolved patients with syndromic and non-syndromic forms of retinal degeneration, SLC4A3 may yet be implicated in human disease.

Results at 2 Years after Gene Therapy for RPE65-Deficient Leber Congenital Amaurosis and Severe Early-Childhood-Onset Retinal Dystrophy

PURPOSE

To provide an initial assessment of the safety of a recombinant adeno-associated virus vector expressing RPE65 (rAAV2-CB-hRPE65) in adults and children with retinal degeneration caused by RPE65 mutations.

DESIGN

Nonrandomized, multicenter clinical trial.

PARTICIPANTS

Eight adults and 4 children, 6 to 39 years of age, with Leber congenital amaurosis (LCA) or severe early-childhood-onset retinal degeneration (SECORD).

METHODS

Patients received a subretinal injection of rAAV2-CB-hRPE65 in the poorer-seeing eye, at either of 2 dose levels, and were followed up for 2 years after treatment.

MAIN OUTCOME MEASURES

The primary safety measures were ocular and nonocular adverse events. Exploratory efficacy measures included changes in best-corrected visual acuity (BCVA), static perimetry central 30° visual field hill of vision (V30) and total visual field hill of vision (VTOT), kinetic perimetry visual field area, and responses to a quality-of-life questionnaire.

RESULTS

All patients tolerated subretinal injections and there were no treatment-related serious adverse events. Common adverse events were those associated with the surgical procedure and included subconjunctival hemorrhage in 8 patients and ocular hyperemia in 5 patients. In the treated eye, BCVA increased in 5 patients, V30 increased in 6 patients, VTOT increased in 5 patients, and kinetic visual field area improved in 3 patients. One subject showed a decrease in BCVA and 2 patients showed a decrease in kinetic visual field area.

CONCLUSIONS

Treatment with rAAV2-CB-hRPE65 was not associated with serious adverse events, and improvement in 1 or more measures of visual function was observed in 9 of 12 patients. The greatest improvements in visual acuity were observed in younger patients with better baseline visual acuity. Evaluation of more patients and a longer duration of follow-up will be needed to determine the rate of uncommon or rare side effects or safety concerns.

The genetics of inherited retinal disorders in dogs: implications for diagnosis and management

Dogs are affected by many hereditary diseases just as humans are. One group of these diseases comprises of retinal disorders, which are a growing problem in canine breeding. These disorders are heterogeneous, with diverse causative mutations and modes of inheritance. Some affect only one breed, while others may affect many breeds; some breeds are affected by only one disease, while others can be affected by two or more. Dog breeders should take into account the presence of any deleterious alleles when choosing parents for the next generation.

Gene therapy of inherited retinal degenerations: prospects and challenges

Because of its favorable anatomical and immunological characteristics, the eye has been at the forefront of translational gene therapy. Dozens of promising proofs of concept have been obtained in animal models of inherited retinal degenerations (IRDs), and some of them have been relayed to the clinic. The results from the first clinical trials for a congenital form of blindness have generated great interest and have demonstrated the safety and efficacy of intraocular administrations of viral vectors in humans. However, this progress has also generated new questions and posed challenges that need to be addressed to further expand the applicability of gene therapy in the eye, including safe delivery of viral vectors to the outer retina, treatment of dominant IRDs as well as of IRDs caused by mutations in large genes, and, finally, selection of the appropriate IRDs and patients to maximize the efficacy of gene transfer. This review summarizes the strategies that are currently being exploited to overcome these challenges and drive the clinical development of retinal gene therapy.

Non-viral therapeutic approaches to ocular diseases: An overview and future directions

Currently there are no viable treatment options for patients with debilitating inherited retinal degeneration. The vast variability in disease-inducing mutations and resulting phenotypes has hampered the development of therapeutic interventions. Gene therapy is a logical approach, and recent work has focused on ways to optimize vector design and packaging to promote optimized expression and phenotypic rescue after intraocular delivery. In this review, we discuss ongoing ocular clinical trials, which currently use viral gene delivery, but focus primarily on new advancements in optimizing the efficacy of non-viral gene delivery for ocular diseases. Non-viral delivery systems are highly customizable, allowing functionalization to improve cellular and nuclear uptake, bypassing cellular degradative machinery, and improving gene expression in the nucleus. Non-viral vectors often yield transgene expression levels lower than viral counterparts, however their favorable safety/immune profiles and large DNA capacity (critical for the delivery of large ocular disease genes) make their further development a research priority. Recent work on particle coating and vector engineering presents exciting ways to overcome limitations of transient/low gene expression levels, but also highlights the fact that further refinements are needed before use in the clinic.

Shared decision-making, control preferences and psychological well-being in patients with RPE65 deficiency awaiting experimental gene therapy

PURPOSE

Retinal gene therapy trials are currently ongoing in a small number of inherited retinal disorders and this number is expected to rise significantly. The aim of this study was to analyze the psychological aspects of patients with RPE65 deficiency awaiting potential enrollment in gene therapy trials.

METHODS

Five patients with genetically proven RPE65 deficiency took part in this study. They were asked to complete the German versions of (i) the Patient Health Questionnaire (PHQ-D), (ii) the National Eye Institute Visual Function Questionnaire (NEI-VFQ), (iii) the Shared Decision Making Questionnaire (PEF-FB-9), and (iv) the Autonomy Preference Index (API-Dm), and in addition they took part in qualitative interviews.

RESULTS

The evaluations of the questionnaires and the interviews showed that the patients have quite high information needs and wish to take part in medical decisions. The perspective to participate in gene therapy trials does not seem to cause pronounced worries. Only the insecurity about if and when enrollment in a trial takes place may be burdensome.

DISCUSSION

This study generated important data about the psychological situation of patients awaiting potential enrollment in clinical trials, which can be used to improve patient care in the increasing number of future gene therapy trials around the world.

Development of gene and stem cell therapy for ocular neurodegeneration

Retinal degenerative diseases pose a serious threat to eye health, but there is currently no effective treatment available. Recent years have witnessed rapid development of several cutting-edge technologies, such as gene therapy, stem cell therapy, and tissue engineering. Due to the special features of ocular structure, some of these technologies have been translated into ophthalmological clinic practice with fruitful achievements, setting a good example for other fields. This paper reviews the development of the gene and stem cell therapies in ophthalmology.

Dog models for blinding inherited retinal dystrophies

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

The Status of RPE65 Gene Therapy Trials: Safety and Efficacy

Several groups have reported the results of clinical trials of gene augmentation therapy for Leber congenital amaurosis (LCA) because of mutations in the RPE65 gene. These studies have used subretinal injection of adeno-associated virus (AAV) vectors to deliver the human RPE65 cDNA to the retinal pigment epithelial (RPE) cells of the treated eyes. In all of the studies reported to date, this approach has been shown to be both safe and effective. The successful clinical trials of gene augmentation therapy for retinal degeneration caused by mutations in the RPE65 gene sets the stage for broad application of gene therapy to treat retinal degenerative disorders.

Personalized therapeutic strategies for patients with retinitis pigmentosa

INTRODUCTION

Retinitis pigmentosa (RP) encompasses many different hereditary retinal degenerations that are caused by a vast array of different gene mutations and have highly variable disease presentations and severities. This heterogeneity poses a significant therapeutic challenge, although an answer may eventually be found through two recent innovations: induced pluripotent stem cells (iPSCs) and clustered regularly interspaced short palindromic repeats (CRISPR)/Cas genome editing.

AREAS COVERED

This review discusses the wide-ranging applications of iPSCs and CRISPR-including disease modelling, diagnostics and therapeutics - with an ultimate view towards understanding how these two technologies can come together to address disease heterogeneity and orphan genes in a novel personalized medicine platform. An extensive literature search was conducted in PubMed and Google Scholar, with a particular focus on high-impact research published within the last 1 - 2 years and centered broadly on the subjects of retinal gene therapy, iPSC-derived outer retina cells, stem cell transplantation and CRISPR/Cas gene editing.

EXPERT OPINION

For the retinal pigment epithelium, autologous transplantation of gene-corrected grafts derived from iPSCs may well be technically feasible in the near future. Photoreceptor transplantation faces more significant unresolved technical challenges but remains an achievable, if more distant, goal given the rapid pace of advancements in the field.

Gene Therapy of ABCA4-Associated Diseases

The ATP-binding cassette (ABC) transporter gene, ABCA4 (ABCR), was characterized in 1997 as the causal gene for autosomal recessive Stargardt disease (STGD1). Shortly thereafter several other phenotypes were associated with mutations in ABCA4, which now have collectively emerged as the most frequent cause of retinal degeneration phenotypes of Mendelian inheritance. ABCA4 functions as an important transporter (or "flippase") of vitamin A derivatives in the visual cycle. Several ways to alleviate the effects of the defective ABCA4 protein, which cause accumulation of 11-cis and all-trans-retinal in photoreceptors and lipofuscin in the retinal pigment epithelium, have been proposed. Although ABCA4 has proven to be a difficult research target, substantial progress through genetic, functional, and translational studies has allowed major advances in therapeutic applications for ABCA4-associated pathology, which should be available to patients in the (near) future. Here, we summarize the status of the gene therapy-based treatment options of ABCA4-associated diseases.

Gene therapy using stem cells

Viral-mediated gene augmentation therapy has recently shown success in restoring vision to patients with retinal degenerative disorders. Key to this success was the availability of animal models that accurately presented the human phenotype to test preclinical efficacy and safety. These exciting studies support the use of gene therapy in the treatment of devastating retinal degenerative diseases. In some cases, however, in vivo gene therapy for retinal degeneration would not be effective because the cell types targeted are no longer present. The development of somatic cell reprogramming methods provides an attractive source of autologous cells for transplantation and treatment of retinal degenerative disease. This article explores the development of gene therapy and patient-derived stem cells for the purpose of restoring vision to individuals suffering from inherited retinal degenerations.

Patient-specific induced pluripotent stem cells (iPSCs) for the study and treatment of retinal degenerative diseases

Vision is the sense that we use to navigate the world around us. Thus it is not surprising that blindness is one of people's most feared maladies. Heritable diseases of the retina, such as age-related macular degeneration and retinitis pigmentosa, are the leading cause of blindness in the developed world, collectively affecting as many as one-third of all people over the age of 75, to some degree. For decades, scientists have dreamed of preventing vision loss or of restoring the vision of patients affected with retinal degeneration through drug therapy, gene augmentation or a cell-based transplantation approach. In this review we will discuss the use of the induced pluripotent stem cell technology to model and develop various treatment modalities for the treatment of inherited retinal degenerative disease. We will focus on the use of iPSCs for interrogation of disease pathophysiology, analysis of drug and gene therapeutics and as a source of autologous cells for cell transplantation and replacement.

Vector platforms for gene therapy of inherited retinopathies

Inherited retinopathies (IR) are common untreatable blinding conditions. Most of them are inherited as monogenic disorders, due to mutations in genes expressed in retinal photoreceptors (PR) and in retinal pigment epithelium (RPE). The retina's compatibility with gene transfer has made transduction of different retinal cell layers in small and large animal models via viral and non-viral vectors possible. The ongoing identification of novel viruses as well as modifications of existing ones based either on rational design or directed evolution have generated vector variants with improved transduction properties. Dozens of promising proofs of concept have been obtained in IR animal models with both viral and non-viral vectors, and some of them have been relayed to clinical trials. To date, recombinant vectors based on the adeno-associated virus (AAV) represent the most promising tool for retinal gene therapy, given their ability to efficiently deliver therapeutic genes to both PR and RPE and their excellent safety and efficacy profiles in humans. However, AAVs' limited cargo capacity has prevented application of the viral vector to treatments requiring transfer of genes with a coding sequence larger than 5 kb. Vectors with larger capacity, i.e. nanoparticles, adenoviral and lentiviral vectors are being exploited for gene transfer to the retina in animal models and, more recently, in humans. This review focuses on the available platforms for retinal gene therapy to fight inherited blindness, highlights their main strengths and examines the efforts to overcome some of their limitations.

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

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

Lung gene therapy-How to capture illumination from the light already present in the tunnel

Gene therapy has been considered as the most ideal medical intervention for genetic diseases because it is intended to target the cause of diseases instead of disease symptoms. Availability of techniques for identification of genetic mutations and for in vitro manipulation of genes makes it practical and attractive. After the initial hype in 1990s and later disappointments in clinical trials for more than a decade, light has finally come into the tunnel in recent years, especially in the field of eye gene therapy where it has taken big strides. Clinical trials in gene therapy for retinal degenerative diseases such as Leber's congenital amaurosis (LCA) and choroideremia demonstrated clear therapeutic efficacies without apparent side effects. Although these successful examples are still rare and sporadic in the field, they provide the proof of concept for harnessing the power of gene therapy to treat genetic diseases and to modernize our medication. In addition, those success stories illuminate the path for the development of gene therapy treating other genetic diseases. Because of the differences in target organs and cells, distinct barriers to gene delivery exist in gene therapy for each genetic disease. It is not feasible for authors to review the current development in the entire field. Thus, in this article, we will focus on what we can learn from the current success in gene therapy for retinal degenerative diseases to speed up the gene therapy development for lung diseases, such as cystic fibrosis.