Expansion of CTG18.1 Trinucleotide Repeat in TCF4 is a Potent Driver of Fuchs Corneal Dystrophy

Abstract

Purpose: To decipher the functional relevance of the expansion of CTG18.1 allele associated with Fuchs Corneal Dystrophy (FCD).

Methods: The (CTG) repeats within the CTG18.1 allele were estimated by STR and TPPCR assays in our cohort of 574 late-onset FCD cases and 354 controls. TCF4 isoforms were identified by 5’RACE of RNA from corneal endothelium. QRT-PCR assays were designed to distinguish the TCF4 isoforms that were subsequently tested in the RNA from corneal endothelium isolated from FCD cases who underwent Endothelial Keratoplasty (EK).

Results: The expanded CTG18.1 for (CTG)_n>40 showed a strong association (P=1.56X10^-82) with FCD. Importantly, we found an expansion of >90 monomers [(CTG)_n>90] in 32% of the FCD cases compared to 0.85% of controls, while regression analyses demonstrated a significant correlation of disease severity with age in individuals that harbor the expanded allele (P=1.54X10^-5). Analysis of the expanded CTG18.1 in sporadic and large multi-generation familial cases suggested a biallelic expansion for (CTG)_n>40 to be sufficient for FCD. Examination of the 11 isoforms of TCF4 identified by 5’RACE from the corneal endothelium in 17 FCD cases detected significant reduction of a specific TCF4 isoform (P = 0.04) in FCD cases who harbor at least one expanded allele.

Conclusions: A monoallelic expansion of CTG18.1 contributes to increased disease severity and is a significant driver of FCD at (CTG)_n>90 whereas, a biallelic expansion for (CTG)_n>40 is causal for FCD likely through its effect on the expression of TCF4 isoforms in the corneal endothelium.