Gene-Based Therapeutics for Inherited Retinal Diseases

Abstract

Inherited retinal diseases (IRDs) are a heterogenous group of orphan eye diseases that typically result from monogenic mutations and are considered attractive targets for gene-based therapeutics. Following the approval of an IRD gene replacement therapy for Leber's congenital amaurosis due to RPE65 mutations, there has been an intensive international research effort to identify the optimal gene therapy approaches for a range of IRDs and many are now undergoing clinical trials. In this review we explore therapeutic challenges posed by IRDs and review current and future approaches that may be applicable to different subsets of IRD mutations. Emphasis is placed on five distinct approaches to gene-based therapy that have potential to treat the full spectrum of IRDs: 1) gene replacement using adeno-associated virus (AAV) and nonviral delivery vectors, 2) genome editing via the CRISPR/Cas9 system, 3) RNA editing by endogenous and exogenous ADAR, 4) mRNA targeting with antisense oligonucleotides for gene knockdown and splicing modification, and 5) optogenetic approaches that aim to replace the function of native retinal photoreceptors by engineering other retinal cell types to become capable of phototransduction.

Keywords: CRISPR; RNA editing; adeno-associated virus; antisense oligonucleotides; gene tharapy; inherited retinal diseases (IRDs); optogenetic; retina.

FIGURE 1

Color fundus photographs, autofluorescence, and optical coherence tomography (OCT) of retinitis pigmentosa. The patient was a 42-year-old Chinese male who presented with an incidental finding of peripheral field loss during an inpatient admission for migraine headache. A long-standing history of nyctalopia was elicited but there was no evidence of neurosensory hearing loss. Whole exome sequencing (WES) uncovered the presence of biallelic pathogenic EYS mutations. (A) Fundus imaging shows peripheral bone spicule-like pigmentary retinopathy with outer retinal atrophy and sparing of the macula; (B) fundus autofluorescence highlights the areas of disease, with the hypoautofluorescent (hypoAF) trailing disease front appearing as dark areas and the hyperautofluorescent (hyperAF) leading disease front appearing as brighter areas; (C) horizontal cross-sectional OCT at the level of the fovea shows intact retinal layers at the fovea with loss of the outer retina and photoreceptor layers in the periphery. Selected retinal layers relevant to gene-based therapy are shown: ILM, internal limiting membrane; NFL, nerve fiber layer; RGC, retinal ganglion cell layer; RPE, retinal pigment epithelium; EZ, ellipsoid zone (photoreceptor inner/outer segments); ELM, external limiting membrane.

FIGURE 2

Variation in structural and functional changes due to inherited retinal disease. Case 1 (A–D) was a 51-year-old Indonesian male with retinitis pigmentosa secondary to PRPH2 mutation. Retinal findings include diffuse peripheral RPE atrophy seen on color and autofluorescent imaging (A,B) with sparing of the central macula (B) and a small focus of foveal outer retina intact on OCT imaging (C). Visual acuity was only mildly impaired, but the patient had tunnel vision as demonstrated by Goldmann kinetic perimetry (D). Case 2 (E–H) was a 21-year-old Chinese male with cone dystrophy secondary to PROM1 mutation. Retinal findings are limited to subtle pigmentary changes at the macula (E) which are highlighted on autofluorescent imaging (F), while OCT reveals loss of the outer retina at the foveal region (G). Visual fields are largely preserved but the patient had severely impaired visual acuity (H).

FIGURE 3

Approaches to retinal delivery of gene-based therapies for inherited retinal diseases. Intravitreal injection and the more recently developed suprachoroidal injection may be given as an outpatient clinic-based procedure with multiple repeat injections possible. Subretinal injection is a formal surgical procedure requiring pars plana vitrectomy and gas tamponade and is not easily repeated.

FIGURE 4

Summary diagram of gene-based therapies currently in clinical use or clinical trials for inherited retinal diseases. (1) AAV-mediated gene replacement therapy is currently the predominant modality, and delivers replacement transgenes (e.g., RPE65), or transgene fragments (dual AAV systems) to produce a functional protein in biallelic autosomal recessive IRDs; (2) CRISPR/Cas9 genome editing requires delivery of CRISPR/Cas9 constructs, most commonly with AAV vector systems, and enables site-directed editing or mutagenesis of IRD target genes (e.g., CEP290); (3) ADAR-mediated RNA editing is used to perform sequence-specific RNA nucleotide edits by utilizing guide DNA or RNA and an ADAR recruitment domain, with either endogenous ADAR or transduced and overexpressed exogenous ADAR enzyme; (4) Antisense oligonucleotides (ASOs) induce sequence specific gene silencing via RNAi, RNase H-mediated mRNA knockdown, or targeted exon skipping (e.g., USH2A exon 13); (5) Optogenetics delivers an engineered phototransducing opsin to a specific retinal cell type (e.g., ganglion cells) to render the cell photosensitive and capable of replacing the light-responsive function of degenerating photoreceptor cells in IRDs.