Vector platforms for gene therapy of inherited retinopathies

Abstract

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.

Heparan Sulfate Binding Promotes Accumulation of Intravitreally Delivered Adeno-associated Viral Vectors at the Retina for Enhanced Transduction but Weakly Influences Tropism

“…rAAV has shown promise for retinal gene transfer (12)(13)(14)(15)(16). In addition to the influence of the capsid discussed above, the route of administration to the retina determines the transgene expression profile and efficacy.…”

“…Subretinal (SR) delivery deposits the vec-tor between the outer nuclear layer (ONL) and the retinal pigment epithelium (RPE), which causes detachment of these two layers to accommodate the injected solution. Many AAV serotypes display transduction in the ONL and RPE layers, with some serotypes showing restricted tropism (16). Of the natural serotypes, rAAV8 is one of the best for SR delivery based on its rapid transgene expression and transduction of all retinal layers (17,18).…”

Gene Therapy of Inherited Retinal Degenerations: Prospects and Challenges

“…In addition, the availability of various animal models, along with in vivo imaging techniques, allows for noninvasive and consistent monitoring of the effects of gene delivery; outcomes may be compared with disease progression in the contralateral control eye. 6,7 Leber congenital amaurosis type 2 (LCA2) is the first IRD to have been treated with retinal gene therapy in phase I/II clinical trials, the results of which represent the most successful example of ocular gene therapy, to date. LCA2, an autosomal recessive IRD, is caused by mutations in RPE65, an essential gene in the retinal pathway.…”

“…However, when injected intravitreally, most viral vectors, including the majority of AAV serotypes, do not transduce the retina with the exception of AAV2/2, and to some extent AAV2/6 and AAV2/8, whose transduction is, however, mainly restricted to retinal ganglion and Müller cells in the inner retina. 6,40 The failure of vectors delivered intravitreally to transduce PR and RPE in the outer retina appears to be caused by the presence of physical barriers, such as the inner limiting membrane, which is particularly thick in large animals, as well as the relative abundance of AAV receptors that capture vectors after intravitreal administration. 41 Indeed, if retinal architecture is altered by a degenerative process [42][43][44] or by enzymatic digestion, 41 the diffusion of AAV viral particles to the outer retina from the vitreous side increases.…”

“…49 However, Ad vectors efficiently transduce the RPE but less robustly adult mouse PR. 6,50,51 Similarly, LV vectors have been successfully used to transduce PR in newborn rat and mouse retinas and thus, to effectively improve the phenotype of animal models of IRDs. 27,29,[52][53][54][55][56] Although recent reports suggest that LV vectors transduce adult nonhuman primate PR, 28,29 RPE transduction has been primarily observed in adult animals.…”