CTG18.1-mediated Fuchs endothelial corneal dystrophy: characterizing biomarkers of pathogenicity in patients with rare intermediate triplet repeat expansions

Abstract

Purpose : Fuchs endothelial corneal dystrophy (FECD) is a degenerative disease commonly associated with a CTG repeat expansion (termed CTG18.1) situated within an intron of the transcription factor encoding gene TCF4. CTG18.1 typically comprises <30 repeats in unaffected or non-CTG18.1-mediated FECD individuals, whereas the majority of FECD patients have a repeat length of ≥50 copies. It is unknown why repeat lengths within the 30-49 range are so rarely detected and if expansions within this range induce defined biomarkers of CTG18.1-expansion mediated FECD (e.g. RNA foci). Here we aim to describe FECD patients within our updated FECD cohort with intermediate repeat lengths (30-49) and, when possible, investigate biomarkers of disease within their corneal endothelial cells (CECs).

Methods : Genomic DNA was extracted from blood samples taken from FECD patients in the UK and the Czech Republic (n=930) and genotyped for CTG18.1-repeat length using short tandem repeat PCR and triplet-primed PCR. Concurrently, primary CECs were cultured following Descemet membrane endothelial keratoplasty (DMEK) in select FECD patients. CECs were probed for repeat containing RNA foci via fluorescence in situ hybridization (FISH) and foci distributions were quantified.

Results : Out of 930 FECD patients, 730 were expansion positive with a CTG18.1 repeat length ≥50 while 186 patients had a repeat length <30 (Figure 1). CTG18.1 repeat length displays a bimodal distribution with only 14 patients with a repeat length between 30-49 (mean age = 65; 9/5 female/male ratio). Primary CECs cultured from a FECD patient within this group expressed RNA foci, but with atypical distribution.

Conclusions : Only 1.5% (14/930) of FECD patients in our cohort have an intermediate CTG18.1 repeat genotype. All cases investigated within this group to date display biomarkers of CTG18.1 expansion-mediated disease within their CECs. However, RNA foci distribution appears abnormal when compared to FECD patients with ≥50 repeats. This suggests a CTG18.1 founder allele length of between 30-49 repeats is sufficient for foci formation but can influence the frequency of RNA foci in CECs and their likely downstream pathogenic effects.

This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.

 

Figure 1: Frequency histogram comparing the longest CTG18.1 allele length (per individual) in FECD (n=930) (grey) and an unaffected aged-matched controls (n=550) (white)