Restoration of Vision and Retinal Responses After Adeno-Associated Virus-Mediated Optogenetic Therapy in Blind Dogs

Cone-Driven, Geniculocortical Responses in Canine Models of Outer Retinal Disease

Purpose

Canine models of inherited retinal degeneration are used for proof of concept of emerging gene and cell-based therapies that aim to produce functional restoration of cone-mediated vision. We examined functional magnetic resonance imaging (MRI) measures of the postretinal response to cone-directed stimulation in wild-type (WT) dogs, and in three different retinal disease models.

Methods

Temporal spectral modulation of a uniform field of light around a photopic background was used to target the canine L/M (hereafter "L") and S cones and rods. Stimuli were designed to separately target the postreceptoral luminance (L+S) and chrominance (L-S) pathways, the rods, and all photoreceptors jointly (light flux). These stimuli were presented to WT, and mutant PDE6B-RCD1, RPGR-XLPRA2, and NPHP5-CRD2 dogs during pupillometry and functional MRI (fMRI).

Results

Pupil responses in WT dogs to light flux, L+S, and rod-directed stimuli were consistent with responses being driven by cone signals alone. For WT animals, both luminance and chromatic (L-S) stimuli evoked fMRI responses in the lateral geniculate nucleus or visual cortex; RCD1 animals with predominant rod loss had similar responses. Responses to cone-directed stimulation were reduced in XLPRA2 and absent in CRD2. NPHP5 gene augmentation restored the cortical response to luminance stimulation in a CRD2 animal.

Conclusions

Cone-directed stimulation during fMRI can be used to measure the integrity of luminance and chrominance responses in the dog visual system. The NPHP5-CRD2 model is appealing for studies of recovered cone function.

Translational Relevance

fMRI assessment of cone-driven cortical response provides a tool to translate cell/gene therapies for vision restoration.

Cone-driven, geniculo-cortical responses in canine models of outer retinal disease

Purpose

Canine models of inherited retinal degeneration are used for proof-of-concept of emerging gene and cell-based therapies that aim to produce functional restoration of cone-mediated vision. We examined functional MRI measures of the post-retinal response to cone-directed stimulation in wild type (WT) dogs, and in three different retinal disease models.

Methods

Temporal spectral modulation of a uniform field of light around a photopic background was used to target the canine L/M (hereafter "L") and S cones and rods. Stimuli were designed to separately target the post-receptoral luminance (L+S) and chrominance (L-S) pathways, the rods, and all photoreceptors jointly (light flux). These stimuli were presented to WT, and mutant PDE6B-RCD1, RPGR-XLPRA2, and NPHP5-CRD2 dogs during pupillometry and fMRI.

Results

Pupil responses in WT dogs to light flux, L+S, and rod-directed stimuli were consistent with responses being driven by cone signals alone. For WT animals, both luminance and chromatic (L-S) stimuli evoked fMRI responses in the lateral geniculate nucleus (LGN) or visual cortex; RCD1 animals with predominant rod loss had similar responses. Responses to cone-directed stimulation were reduced in XLPRA2 and absent in CRD2. NPHP5 gene augmentation restored the cortical response to luminance stimulation in a CRD2 animal.

Conclusions

Cone-directed stimulation during fMRI can be used to measure the integrity of luminance and chrominance responses in the dog visual system. The NPHP5-CRD2 model is appealing for studies of recovered cone function.

Translational Relevance

fMRI assessment of cone driven cortical response provides a tool to translate cell/gene therapies for vision restoration.

Retinitis Pigmentosa: Novel Therapeutic Targets and Drug Development

Retinitis pigmentosa (RP) is a heterogeneous group of hereditary diseases characterized by progressive degeneration of retinal photoreceptors leading to progressive visual decline. It is the most common type of inherited retinal dystrophy and has a high burden on both patients and society. This condition causes gradual loss of vision, with its typical manifestations including nyctalopia, concentric visual field loss, and ultimately bilateral central vision loss. It is one of the leading causes of visual disability and blindness in people under 60 years old and affects over 1.5 million people worldwide. There is currently no curative treatment for people with RP, and only a small group of patients with confirmed RPE65 mutations are eligible to receive the only gene therapy on the market: voretigene neparvovec. The current therapeutic armamentarium is limited to retinoids, vitamin A supplements, protection from sunlight, visual aids, and medical and surgical interventions to treat ophthalmic comorbidities, which only aim to slow down the progression of the disease. Considering such a limited therapeutic landscape, there is an urgent need for developing new and individualized therapeutic modalities targeting retinal degeneration. Although the heterogeneity of gene mutations involved in RP makes its target treatment development difficult, recent fundamental studies showed promising progress in elucidation of the photoreceptor degeneration mechanism. The discovery of novel molecule therapeutics that can selectively target specific receptors or specific pathways will serve as a solid foundation for advanced drug development. This article is a review of recent progress in novel treatment of RP focusing on preclinical stage fundamental research on molecular targets, which will serve as a starting point for advanced drug development. We will review the alterations in the molecular pathways involved in the development of RP, mainly those regarding endoplasmic reticulum (ER) stress and apoptotic pathways, maintenance of the redox balance, and genomic stability. We will then discuss the therapeutic approaches under development, such as gene and cell therapy, as well as the recent literature identifying novel potential drug targets for RP.

Therapeutic landscape for inherited ocular diseases: Current and emerging therapies

Inherited ocular diseases comprise a heterogeneous group of rare and complex diseases, including inherited retinal diseases (IRDs) and inherited optic neuropathies. Recent success in adeno-associated virus-based gene therapy, voretigene neparvovec (Luxturna^®) for RPE65-related IRDs, has heralded rapid evolution in gene therapy platform technologies and strategies, from gene augmentation to RNA editing, as well as gene agnostic approaches such as optogenetics. This review discusses the fundamentals underlying the mode of inheritance, natural history studies and clinical trial outcomes, as well as current and emerging therapies covering gene therapy strategies, cell-based therapies and bionic vision.

Optogenetic Therapy for Visual Restoration

Optogenetics is a recent breakthrough in neuroscience, and one of the most promising applications is the treatment of retinal degenerative diseases. Multiple clinical trials are currently ongoing, less than a decade after the first attempt at visual restoration using optogenetics. Optogenetic therapy has great value in providing hope for visual restoration in late-stage retinal degeneration, regardless of the genotype. This alternative gene therapy consists of multiple elements including the choice of target retinal cells, optogenetic tools, and gene delivery systems. Currently, there are various options for each element, all of which have been developed as a product of technological success. In particular, the performance of optogenetic tools in terms of light and wavelength sensitivity have been improved by engineering microbial opsins and applying human opsins. To provide better post-treatment vision, the optimal choice of optogenetic tools and effective gene delivery to retinal cells is necessary. In this review, we provide an overview of the advancements in optogenetic therapy for visual restoration, focusing on available options for optogenetic tools and gene delivery methods.