RNA Toxicity and Missplicing in the Common Eye Disease Fuchs Endothelial Corneal Dystrophy

Abstract Fuchs endothelial corneal dystrophy (FECD) is an inherited, degenerative disease affecting the internal endothelial cell monolayer of the cornea and can result in corneal edema and vision loss in severe cases. FECD affects approximately 5% of middle-aged Caucasians in the United States and accounts for more than 14,000 corneal transplantations annually. Among the several genes and loci associated with FECD, the strongest association is with an intronic (CTG/CAG)n trinucleotide repeat expansion in the transcription factor 4 (TCF4) gene, which is found in a majority of affected patients. Corneal endothelial cells from FECD patients harbor a poly(CUG)n RNA that can be visualized as RNA foci containing this condensed RNA and associated proteins. Similar to myotonic dystrophy type 1 (DM1), the poly (CUG)n RNA co-localizes with and sequesters the mRNA splicing factor MBNL1, leading to missplicing of essential MBNL1-regulated mRNAs. Such foci and missplicing are not observed in similar cells from FECD patients who lack the repeat expansion. RNASeq splicing data from the corneal endothelium of FECD patients and controls reveal hundreds of differential alternative splicing events. These include events previously characterized in the context of DM1 and epithelial-mesenchymal transition (EMT) as well as splicing changes in genes related to proposed mechanisms of FECD pathogenesis. We report the first instance of RNA toxicity and missplicing in a common, non-neurological/neuromuscular disease associated with a repeat expansion. The FECD patient population with this (CTG/CAG)n trinucleotide repeat expansion exceeds that of the combined number of patients in all other microsatellite expansion disorders.