Sigma Institute of Pharmacy, Ajwa Nimeta Road Bakrol, Vadodara- 390019, Gujarat, India
Inherited retinal diseases (IRDs) represent a diverse group of rare, predominantly monogenic disorders characterized by progressive degeneration of photoreceptors and/or the retinal pigment epithelium, ultimately leading to irreversible visual impairment or blindness. Affecting approximately 1 in 3,000–4,000 individuals worldwide, IRDs constitute a major cause of childhood and early-onset blindness. Advances in molecular genetics, next-generation sequencing, and ocular drug delivery systems have transformed the diagnostic and therapeutic landscape of these conditions. Among emerging modalities, gene therapy has demonstrated unprecedented potential to address the underlying molecular defects responsible for retinal degeneration. The approval of voretigene neparvovec-rzyl for RPE65-associated Leber congenital amaurosis established proof-of-concept for retinal gene augmentation and catalyzed rapid progress in the field. This review discusses the genetic and molecular basis of IRDs, principles of ocular gene therapy, vector platforms, delivery routes, clinical applications, genome editing technologies, and future challenges.
Inherited retinal diseases (IRDs) constitute a large and heterogeneous group of rare genetic disorders that progressively impair visual function and often culminate in irreversible blindness. Collectively, IRDs affect an estimated 1 in 3,000–4,000 individuals worldwide, accounting for a major proportion of childhood and adult blindness in developed nations [1, 2]. Unlike age-related macular degeneration or diabetic retinopathy, IRDs are predominantly monogenic in nature, with more than 260 genes identified as causative to date [2]. These genes encode proteins critical for phototransduction, photoreceptor outer segment maintenance, retinoid metabolism, synaptic signaling, and retinal pigment epithelium (RPE) homeostasis. Mutations in these pathways cause progressive degeneration of rods, cones, and/or RPE cells, leading to varied clinical phenotypes such as retinitis pigmentosa (RP), Leber congenital amaurosis (LCA), Stargardt disease, achromatopsia, Usher syndrome, and choroideremia.
2. Historical Perspective of Gene Therapy for IRDs
Until the past decade, IRDs were considered incurable. Management strategies were largely supportive, such as low-vision aids, counseling, and in rare cases, vitamin A supplementation for RP [3]. The paradigm shifted in the early 2000s, when proof-of-concept animal studies demonstrated the feasibility of adeno-associated virus (AAV)-mediated gene delivery to photoreceptors and RPE cells [4, 5]. These preclinical successes laid the foundation for clinical trials, culminating in the 2017 U.S. Food and Drug Administration (FDA) approval of voretigene neparvovec-rzyl (Luxturna) for biallelic RPE65 mutation-associated LCA [6]. Luxturna’s approval not only marked the first gene therapy for an inherited retinal disease, but also established a regulatory framework for genetic therapies in ophthalmology.[7, 8, 9]
3. Advances in Genetics and Diagnostics
The advent of next-generation sequencing (NGS) has revolutionized IRD diagnosis, enabling precise identification of pathogenic mutations at a fraction of the cost of earlier methods [9]. Molecular diagnosis is now considered essential for patient counseling, prognosis, and eligibility for gene therapy clinical trials [10]. Moreover, genetic testing has facilitated the recognition of genotype–phenotype correlations, uncovering significant variability in disease progression even among patients harboring the same mutation. These insights underscore the importance of personalized approaches in therapeutic development.
4. Pathophysiology of Inherited Retinal Diseases
Inherited retinal diseases (IRDs) represent one of the most genetically heterogeneous groups of human disorders. They are caused by pathogenic variants in more than 260 genes, each involved in critical aspects of photoreceptor development, function, and maintenance [11, 12]. Despite their genetic diversity, a unifying theme across IRDs is the progressive loss of rod and cone photoreceptors and/or retinal pigment epithelium (RPE) cells, leading to irreversible visual decline. Understanding the molecular pathophysiology of these disorders is essential for designing targeted therapeutic interventions.
5. Photoreceptor degeneration
Photoreceptors, comprising rods and cones, are highly metabolically active cells that rely on precise protein trafficking, disc renewal, and retinoid cycling. Mutations in genes encoding components of these pathways can trigger cellular stress and apoptosis.
Apoptosis in photoreceptors involves activation of caspase pathways, endoplasmic reticulum (ER) stress, oxidative damage, and mitochondrial dysfunction [8]. Furthermore, toxic accumulation of byproducts such as lipofuscin in the RPE accelerates degeneration [9].
6. Role of the retinal pigment epithelium (RPE)
The RPE is critical for photoreceptor health, mediating phagocytosis of shed outer segments, transport of nutrients, and recycling of retinoids in the visual cycle. Mutations in RPE-specific genes, such as RPE65 or BEST1, lead to dysfunction of the retinoid cycle or impaired ion transport, resulting in retinal degeneration [10]. For instance, RPE65 deficiency blocks conversion of all-trans retinyl esters to 11-cis retinal, leading to defective phototransduction and LCA [11].
RPE degeneration not only disrupts photoreceptor function but also leads to secondary neuroinflammation, with activation of microglia and recruitment of immune mediators contributing to disease progression [12].
7. Classification of major IRDs
Although IRDs are clinically diverse, they can be broadly grouped into several categories:
Figure 1 Retinitis Pigmentosa
Figure 2. Cone–rod dystrophies
Figure 3 Leber Congenital Amaurosis (LCA)
Figure 4 Stargardt disease
Figure 5 Choroideremia
Figure 6 Usher Syndrome
Figure 7 Achromatopsia
Figure 8 IRDs
8. Disease mechanisms
Retinal degeneration arises from multiple interrelated molecular mechanisms that compromise photoreceptor structure and function. Defective phototransduction is a key pathogenic pathway, wherein mutations in phototransduction genes such as PDE6B and CNGA3 disrupt cyclic GMP signaling, leading to dysfunctional rods or cones [13]. In parallel, disruption of the visual cycle due to mutations in RPE65, LRAT, and RDH12 impairs the recycling of 11-cis-retinal, thereby preventing efficient photopigment regeneration [11]. Protein misfolding and intracellular trafficking defects further contribute to disease progression; mutations in RHO or RPGR interfere with outer segment disc assembly and induce endoplasmic reticulum stress [16]. Additionally, impaired clearance of retinoid byproducts caused by ABCA4 mutations results in the accumulation of toxic lipofuscin components such as A2E, which damage the retinal pigment epithelium [9]. Finally, synaptic dysfunction and ciliopathy-related defects arising from mutations in cilia-associated genes, including CEP290 and RPGR, disrupt the transport of essential proteins to photoreceptor outer segments, collectively driving progressive photoreceptor degeneration [17]. Even within the same family, IRD phenotypes can vary widely due to modifier genes and environmental influences. For example, polymorphisms in antioxidant enzymes may influence oxidative stress susceptibility, while lifestyle factors (e.g., light exposure, diet) may modulate progression [18]. Such variability underscores the need for personalized medicine approaches in gene therapy. [19]
9. Principles of Gene Therapy in IRDs
The goal of gene therapy is to introduce, replace, or modify genetic material in affected cells to correct the underlying molecular defect. For inherited retinal diseases (IRDs), gene therapy is particularly promising because the eye is small, compartmentalized, and relatively immune-privileged, allowing efficient and localized delivery of therapeutic constructs [20,21]. Over the past two decades, significant advances have been made in vector design, delivery methods, and therapeutic strategies, culminating in the clinical approval of voretigene neparvovec-rzyl (Luxturna) for RPE65-associated Leber congenital amaurosis (LCA) [22].
12. Vector Platforms for Ocular Gene Therapy
The choice of vector is critical to the success of gene therapy. The ideal vector should efficiently transduce retinal cells, sustain transgene expression, minimize immunogenicity, and accommodate genes of varying sizes.
12.1 Adeno-associated virus (AAV)
Recombinant AAV (rAAV) is the most widely used vector for ocular applications because of its non-pathogenicity, low immunogenicity, and ability to transduce post-mitotic cells [23]. More than 12 AAV serotypes have been engineered, each with unique tissue tropism. In the retina, AAV2 has been the most commonly used, but novel engineered capsids (e.g., AAV8, AAV9, AAV2-7m8, AAV- Anc80L65) demonstrate improved penetration and broader tropism [24].
However, AAV vectors are limited by a 4.7 kb packaging capacity, which excludes large genes such as ABCA4 (Stargardt disease), USH2A (Usher syndrome type 2A), and CEP290 (LCA10) [25]. To overcome this, dual-vector approaches and hybrid delivery systems are under investigation [26].
Figure 9 Adeno-associated virus (AAV)
12.2 Lentiviral vectors
Lentiviruses can accommodate larger transgenes (~8–9 kb), making them suitable for diseases caused by large genes [27]. They integrate into the host genome, enabling stable expression, but integration carries a small risk of insertional mutagenesis. Retinal gene therapy trials with lentiviral vectors (e.g., StarGen for Stargardt disease) have demonstrated safety but limited efficacy so far [28].
12.3 Non-viral vectors
Non-viral approaches, including nanoparticles, liposomes, and electroporation-based plasmid delivery, avoid immune responses associated with viral vectors and allow repeat administration [29]. However, they generally suffer from lower transduction efficiency and transient expression. Current research is focused on DNA nanoparticles and CRISPR ribonucleoprotein complexes for targeted delivery [30].
Figure 10 Non-viral vectors
13. Routes of Ocular Delivery
The route of delivery determines the target cell population and influences both efficacy and safety.
13.1 Subretinal injection
This method involves surgical injection of vector into the subretinal space, creating a temporary retinal detachment to allow direct exposure of photoreceptors and RPE cells [31].
13.2 Intravitreal injection
Vectors are injected into the vitreous cavity, allowing easier, less invasive delivery [34].
13.3 Suprachoroidal delivery
This emerging approach targets the suprachoroidal space between sclera and choroid [37].
14. Therapeutic Strategies
Gene therapy strategies for IRDs can be broadly categorized into gene replacement, gene silencing, and gene editing, with additional emerging approaches such as optogenetics and neuroprotection.
14.1 Gene replacement therapy
This involves delivering a functional copy of a defective gene, restoring protein expression. It is most effective for loss-of-function mutations, such as RPE65-associated LCA [38].
14.2 Gene silencing therapy
For dominant-negative or gain-of-function mutations, gene silencing aims to suppress toxic alleles using:
14.3 Genome editing
Genome editing offers the possibility of precisely correcting pathogenic mutations rather than supplementing or suppressing them.
Figure 11 ZFNs
Sneha Patel*
Vaidehi Pathak