Genome-Editing Strategies for Treating Human Retinal Degenerations.

Joel Quinn,Kanmin Xue,Michelle E Mcclements,Ariel Kantor,Jasmina Cehajic-Kapetanovic,Ayesha Musa,Robert E Maclaren

doi:10.1089/hum.2020.231

Abstract

Inherited retinal degenerations (IRDs) are a leading cause of blindness. Although gene-supplementation therapies have been developed, they are only available for a small proportion of recessive IRD mutations. In contrast, genome editing using clustered-regularly interspaced short palindromic repeats (CRISPR) CRISPR-associated (Cas) systems could provide alternative therapeutic avenues for treating a wide range of genetic retinal diseases through targeted knockdown or correction of mutant alleles. Progress in this rapidly evolving field has been highlighted by recent Food and Drug Administration clinical trial approval for EDIT-101 (Editas Medicine, Inc., Cambridge, MA), which has demonstrated efficacious genome editing in a mouse model of CEP290-associated Leber congenital amaurosis and safety in nonhuman primates. Nonetheless, there remains a significant number of challenges to developing clinically viable retinal genome-editing therapies. In particular, IRD-causing mutations occur in more than 200 known genes, with considerable heterogeneity in mutation type and position within each gene. Additionally, there are remaining safety concerns over long-term expression of Cas9 in vivo. This review highlights (i) the technological advances in gene-editing technology, (ii) major safety concerns associated with retinal genome editing, and (iii) potential strategies for overcoming these challenges to develop clinical therapies.

Highlights

INHERITED RETINAL DEGENERATIONS (IRDs) comprise a heterogeneous group of disorders associated with mutations in more than 250 genes, and they are characterized by degeneration of photoreceptors and/or the underlying retinal pigment epithelium (RPE), which lead to irreversible sight loss.[1]
Despite remarkable progress made in the development of therapies for these diseases over the past decade, which have culminated in approved gene therapy for the inherited retinal dystrophy, Leber congenital amaurosis (LCA) associated with biallelic mutations in RPE65 (Luxturna, Hoffmann-La Roche, Basel, Switzerland),[3] and implantable retinal prostheses for end-stage retinal degeneration (Argus II; Second Sight Medical Products, Inc., Sylmar, CA; and Retinal Implant AG, Reutlingen, Germany), the majority of IRDs remain untreatable
Nine-fold increase in photoreceptor nuclei 53% Improvement in the optokinetic response 25% Increase in cone photoreceptor preservation and electroretinogram B waves amplitude by *60% Increase in wild-type mRNA by *20% compared with untreated control Delayed outer nuclear layer degeneration Preserved electroretinogram B-waves and outer nuclear layer thickness in Cas9-treated mice compared with mice only given gene supplementation Electroretinogram showing improved rod and cone responses compared with untreated and homology-directed repair (HDR)-treated controls Increased survival of rod photoreceptors five-fold compared with nontreated controls Editing rate of 27.9 – 20.7% at 1 · 1012 vg/mL vector dose in non-human primates (NHPs) Efficacy and tolerance shown in NHPs 58 – 12% efficiency for indel Reduced choroidal neovascularization area by 20 – 4%

Summary

Introduction

INHERITED RETINAL DEGENERATIONS (IRDs) comprise a heterogeneous group of disorders associated with mutations in more than 250 genes, and they are characterized by degeneration of photoreceptors and/or the underlying retinal pigment epithelium (RPE), which lead to irreversible sight loss (https://sph.uth.edu/RETNET/).[1]. In the Royal College of Surgeons mouse model of autosomal recessive retinitis pigmentosa caused by a 1.9 kB deletion in the Mertk gene, HITI-based CRISPR-Cas[9] delivered via dual AAV vectors showed effective editing and improved rod-cone response compared with HDR-treated controls[20] (Table 1).

Results

Conclusion