Retinal gene therapy holds a promise in treating different forms of non-inherited and inherited blindness.
In 2008, three independent research groups reported that patients with the rare genetic retinal disease Leber's congenital amaurosis had been successfully treated using gene therapy with adeno-associated virus (AAV). In all three studies, an AAV vector was used to deliver a functional copy of the RPE65 gene, which restored vision in children suffering from LCA. These results were widely seen as a success in the gene therapy field, and have generated excitement and momentum for AAV-mediated applications in retinal disease.
In retinal gene therapy, the most widely used vectors for ocular gene delivery are based on adeno-associated virus. The great advantage in using adeno-associated virus for the gene therapy is that it poses minimal immune responses and mediates long-term transgene expression in a variety of retinal cell types. For example, tight junctions that form the blood-retina barrier, separate subretinal space from the blood supply, providing protection from microbes and decreasing most immune-mediated damages.
There is still a lot of knowledge missing in regards of retina dystrophies. Detailed characterization is needed in order to improve knowledge. To address this issue, registries are being developed to classify and characterize rare diseases. Registries help to localize, and measure all the phenotypes of these conditions and therefore to provide easy follow-ups and provide a source of information to scientists. Registry designs vary from region to region, however localization and characterization of the phenotype are the gold standard. Examples of registries are: RetMxMap<ARVO 2009>. A Mexican and Latin-American registry created since 2009 by Dr Adda Lízbeth Villanueva Avilés is a work in progress to map inherited retina dystrophies in Mexico and other Latin countries.
Clinical trials
Leber's congenital amaurosis
Preclinical studies in mouse models of Leber's congenital amaurosis (LCA) were published in 1996 and a study in dogs published in 2001. In 2008, three groups reported results of clinical trials using adeno-associated virus for LCA. In these studies, an AAV vector encoding the RPE65 gene was delivered via a "subretinal injection", where a small amount of fluid is injected underneath the retina in a short surgical procedure. Development continued, and in December 2017 the FDA approved Voretigene neparvovec (Luxturna), an adeno-associated virus vector-based gene therapy for children and adults with biallelic RPE65 gene mutations responsible for retinal dystrophy, including Leber congenital amaurosis. People must have viable retinal cells as a prerequisite for the intraocular administration of the drug.
Age-related macular degeneration Following the successful clinical trials in LCA, researchers have been developing similar treatments using adeno-associated virus for age-related macular degeneration (AMD). To date, efforts have focused on long-term delivery of VEGF inhibitors to treat the wet form of macular degeneration. Whereas wet AMD is treated using frequent injections of recombinant protein into the eyeball, the goal of these treatments is long-term disease management following a single administration. One such study is being conducted at the Lions Eye Institute in Australia in collaboration with Avalanche Biotechnologies, a US-based biotechnology start-up. Another early-stage study is sponsored by Genzyme Corporation.
Ixo-vec for Wet AMD Ixoberogene soroparvovec (Ixo-vec) is an investigational intravitreal gene therapy treatment targeting wet age-related macular degeneration (AMD) that aims to reduce the treatment burden by decreasing the frequency of anti-VEGF injections. Delivered as a single intravitreal injection, Ixo-vec enables sustained release of aflibercept, an anti-VEGF protein that helps control abnormal blood vessel growth and fluid leakage, which are key in AMD progression. Results from the OPTIC and LUNA trials demonstrate Ixo-vec's effectiveness in significantly reducing the need for regular injections over extended periods. Patients in these trials experienced a reduction in injection frequency by as much as 90%, with many remaining injection-free for extended periods. Visual acuity remained stable, and anatomical outcomes, like reductions in central subfield thickness (CST), were achieved. Mild intraocular inflammation was the most common side effect, with steroid prophylaxis proving effective in managing this issue. This treatment approach, if proven in further studies, could offer AMD patients a more convenient, long-lasting alternative to frequent anti-VEGF injections, enhancing quality of life and treatment adherence.
Choroideremia In October 2011, the first clinical trial was announced for the treatment of choroideremia. Dr. Robert MacLaren of the University of Oxford, who lead the trial, co-developed the treatment with Dr. Miguel Seabra of the Imperial College, London. This Phase 1/2 trial used subretinal AAV to restore the REP gene in affected patients. Initial results of the trial were reported in January 2014 as promising as all six patients had better vision.
Color blindness
Research has shown that AAV can successfully restore color vision to treat color blindness in adult monkeys. Although this treatment has not yet entered clinical trials for humans, this work was considered a breakthrough for the ability to target cone photoreceptors.
Macular telangiectasia type 2 Revakinagene taroretcel was approved for medical use in the United States in March 2025 for the treatment of macular telangiectasia type 2.
Mechanism
Physiological components in retinal gene therapy The vertebrate neural retina composed of several layers and distinct cell types (see anatomy of the human retina). A number of these cell types are implicated in retinal diseases, including retinal ganglion cells, which degenerate in glaucoma, the rod and cone photoreceptors, which are responsive to light and degenerate in retinitis pigmentosa, macular degeneration, and other retinal diseases, and the retinal pigment epithelium (RPE), which supports the photoreceptors and is also implicated in retinitis pigmentosa and macular degeneration.
In retinal gene therapy, AAV is capable of "transducing" these various cell types by entering the cells and expressing the therapeutic DNA sequence. Since the cells of the retina are non-dividing, AAV continues to persist and provide expression of the therapeutic DNA sequence over a long time period that can last several years.
AAV tropism and routes of administration AAV is capable of transducing multiple cell types within the retina. AAV serotype 2, the most well-studied type of AAV, is commonly administered in one of two routes: intravitreal or subretinal. Using the intravitreal route, AAV is injected in the vitreous humor of the eye. Using the subretinal route, AAV is injected underneath the retina, taking advantage of the potential space between the photoreceptors and RPE layer, in a short surgical procedure. Although this is more invasive than the intravitreal route, the fluid is absorbed by the RPE and the retina flattens in less than 14 hours without complications. Intravitreal AAV targets retinal ganglion cells and a few Muller glial cells. Subretinal AAV efficiently targets photoreceptors and RPE cells.
The reason that different routes of administration lead to different cell types being transfected (e.g., different tropism) is that the inner limiting membrane (ILM) and the various retinal layers act as physical barriers for the delivery of drugs and vectors to the deeper retinal layers. Thus overall, subretinal AAV is 5-10 times more efficient than delivery using the intravitreal route.
Tropism modification and novel AAV vectors One important factor in gene delivery is developing altered cell tropisms to narrow or broaden rAAV-mediated gene delivery and to increase its efficiency in tissues. Specific properties like capsid conformation, cell targeting strategies can determine which cell types are affected and also the efficiency of the gene transfer process. Different kinds of modification can be undertaken. For example, modification by chemical, immunological or genetic changes that enables the AAV2 capsid to interact with specific cell surface molecules.Gene Therapy Net [online]. 2010 [cited 30 March 2010]; Available from: URL:http://www.asgct.org
Initial studies with AAV in the retina have utilized AAV serotype 2. Researchers are now beginning to develop new variants of AAV, based on naturally occurring AAV serotypes and engineered AAV variants.
Several naturally occurring serotypes of AAV have been isolated that can transduce retinal cells. Following intravitreal injection, only AAV serotypes 2 and 8 were capable of transducing retinal ganglion cells. Occasional Muller cells were transduced by AAV serotypes 2, 8, and 9. Following subretinal injection, serotypes 2, 5, 7, and 8 efficiently transduced photoreceptors, and serotypes 1, 2, 5, 7, 8, and 9 efficiently transduce RPE cells.
One example of an engineered variant has been described that efficiently transduces Muller glia following intravitreal injection, and has been used to rescue an animal model of aggressive, autosomal-dominant retinitis pigmentosa.
AAV and immune privilege in the retina Importantly, the retina is immune-privileged, and thus does not experience a significant inflammation or immune-response when AAV is injected. Immune response to gene therapy vectors is what has caused previous attempts at gene therapy to fail, and is considered a key advantage of gene therapy in the eye. Re-administration has been successful in large animals, indicating that no long-lasting immune response is mounted.
Data indicate that the subretinal route may be subject to a greater degree of immune privilege compared to the intravitreal route.
Promoter sequence Expression in various retinal cell types can be determined by the promoter sequence. In order to restrict expression to a specific cell type, a tissue-specific or cell-type specific promoter can be used.
For example, in rats the murine rhodopsin gene drive the expression in AAV2, GFP reporter product was found only in rat photoreceptors, not in any other retinal cell type or in the adjacent RPE after subretinal injection. On the other hand, if ubiquitously expressed immediate-early cytomegalovirus (CMV) enhancer-promoter is expressed in a wide variety of transfected cell types. Other ubiquitous promoters such as the CBA promoter, a fusion of the chicken-actin promoter and CMV immediate-early enhancer, allows stable GFP reporter expression in both RPE and photoreceptor cells after subretinal injections.
Modulation of expression Sometimes modulation of transgene expression may be necessary since strong constitutive expression of a therapeutic gene in retinal tissues could be deleterious for long-term retinal function. Different methods have been utilized for the expression modulation. One way is using exogenously regulatable promoter system in AAV vectors. For example, the tetracycline-inducible expression system uses a silencer/transactivator AAV2 vector and a separate inducible doxycycline-responsive coinjection. When induction occurs by oral doxycycline, this system shows tight regulation of gene expression in both photoreceptor and RPE cells.
Examples and animal models
Targeting RPE