Gene therapy and genome surgery in the retina

Abstract

Precision medicine seeks to treat disease with molecular specificity. Advances in genome sequence analysis, gene delivery, and genome surgery have allowed clinician-scientists to treat genetic conditions at the level of their pathology. As a result, progress in treating retinal disease using genetic tools has advanced tremendously over the past several decades. Breakthroughs in gene delivery vectors, both viral and nonviral, have allowed the delivery of genetic payloads in preclinical models of retinal disorders and have paved the way for numerous successful clinical trials. Moreover, the adaptation of CRISPR-Cas systems for genome engineering have enabled the correction of both recessive and dominant pathogenic alleles, expanding the disease-modifying power of gene therapies. Here, we highlight the translational progress of gene therapy and genome editing of several retinal disorders, including RPE65-, CEP290-, and GUY2D-associated Leber congenital amaurosis, as well as choroideremia, achromatopsia, Mer tyrosine kinase- (MERTK-) and RPGR X-linked retinitis pigmentosa, Usher syndrome, neovascular age-related macular degeneration, X-linked retinoschisis, Stargardt disease, and Leber hereditary optic neuropathy.

Keywords: Genetics; Ophthalmology; Retinopathy.

Figure 1. Examples of gene supplementation versus genome surgery in the retina.

Conventional gene supplementation works well for mutations that are inherited in an autosomal recessive manner; however, dominant-negative conditions require elimination or repression of the mutant allele to correct the disease phenotype and are unlikely to be ameliorated by supplementation. (A) Schematic of gene supplementation as well as vectors that have been used to treat retinal diseases in current or planned clinical trials. (B) Schematic of genome surgery. For dominant-negative conditions, scientists have focused on genetic tools to modulate gene expression, such as RNAi, or tools that modify the patient’s genome to mutate the pathogenic allele, such as site-specific nucleases like CRISPR-Cas. (C) Description of different approaches used to affect gene expression.

Figure 2. RPE65-associated LCA2.

Mutations in the gene encoding RPE65 isomerase results in autosomal recessive LCA. This gene is largely active in the RPE and is responsible for the isomerization of all-trans-retinyl esters to 11-cis-retinyl esters, the rate-limiting step in the retinal visual cycle, and mutations in RPE65 result in RPE degeneration and photoreceptor death. Salt and pepper retinopathy and arteriole attenuation seen in a 28-year-old man with compound heterozygote mutations (c.11 + 5G > A) and (c.715T > G) in RPE65. Fundoscopy revealed optic disc pallor and a relatively spared macula. Note the absence of bone spicule–like pigmentation in the periphery.

Figure 3. Intravitreal versus subretinal delivery.

Intravitreal delivery is less technical and has fewer risks associated with structural damage to ocular tissues, however, concentrated delivery of vector to the disease tissue can be problematic because of the diffusional volume of the vitreous. Subretinal delivery is more technically challenging and causes transient retinal detachment, which poses a risk for permanent retinal damage, however, the gene therapy concentration is much higher and the transduction to nearby tissue is often significantly greater.