Trinucleotide Repeat-Targeting dCas9 as a Therapeutic Strategy for Fuchs' Endothelial Corneal Dystrophy

Purpose: Fuchs' endothelial corneal dystrophy (FECD) is the leading indication for corneal transplantation. Seventy percent of cases are caused by an intronic CTG triplet repeat expansion in the TCF4 gene that results in accumulation of pathogenic expanded CUG repeat RNA (CUGexp) as nuclear foci in corneal endothelium. A catalytically dead Cas9 (dCas9) can serve as an effective guide to target genomic DNA or RNA transcripts. Here, we examined the utility of the clustered regularly interspaced short palindromic repeats (CRISPR)-dCas9 system to effectively target and reduce CUG(exp). Methods: We delivered dCas9 and repeat-targeting single guide RNA (sgRNA) expression plasmids to patient-derived endothelial cells using lipofection or lentiviral transduction. We used fluorescence in situ hybridization (FISH) and RNA dot-blot hybridization to quantify CUG(exp) foci and repeat RNA levels, respectively. TCF4 expression levels were assessed using quantitative PCR (qPCR). Results: Using FISH, we found that expression of both dCas9 and a (CAG)(n) sgRNA complementary to CUG(exp) are necessary to reduce foci. We observed a reduction in percentage of cells with foci from 59% to 5.6% and number of foci per 100 cells from 73.4 to 7.45 (P < 0.001) in cells stably expressing dCas9-(CAG)(n) sgRNA but saw no decrease in cells expressing dCas9-(CUG)(n) sgRNA or nontargeting control sgRNA. In cells with dCas9-(CAG)(n) sgRNA, we detected a reduction in CUG(exp) RNA by dot-blot without any reduction in TCF4 mRNA levels using qPCR. Conclusions: Using CRISPR-dCas9 to target the trinucleotide repeat is a promising treatment for FECD contingent on effective in vivo delivery. Translational Relevance: This work advances a gene therapy for a commonage-related degenerative disorder.