Usher syndrome 1C protein harmonin suppresses canonical Wnt-signaling providing novel insights into the pathophysiology of USH1C

Abstract

Purpose : Human Usher syndrome (USH) is the most common form of hereditary combined deaf-blindness. USH is a complex genetic disorder, and the pathomechanisms underlying the disease are far from being understood, especially in the eye. USH1C encodes the scaffold protein harmonin organizing protein networks including β-catenin, a cell adhesion component but also key effector in canonical Wnt-signaling (cWnt). Besides ocular and inner ear phenotypes and USH1C upregulation in colorectal cancer was reported. We aim to decipher the role of harmonin in the regulation of cWnt-signaling.

Methods : We analyzed harmonin-β-catenin interactions by co-immunoprecipitations and in situ PLAs. cWnt was stimulated by Wnt-medium or β-catenin overexpression and quantified by luciferase assays, Western blotting, and immuncyto-/histochemistry (IC). Differential gene expression was determined in fibroblasts-derived from a USH1C^{R31*/R80Pfs*69} patient and a healthy donor by RNAseq and bioinformatics. USH1C-R31* pig retinas were processed for IC. Ataluren was applied as a translational read-through inducing drug.

Results : Harmonin binds to β-catenin and translocates with acetylated β-catenin into nuclei. Overexpression of harmonin significantly reduced cWnt-signaling, but the USH1C-R31* mutated harmonin did not. Concordantly, we observed an increase in cWnt-signaling in USH1C^{R31*/R80Pfs*69} patient fibroblasts. This was confirmed in Muller glia cells of USH1C^R31* pig retinae. RNAseq analysis revealed that both the expression of genes related to the cWnt-pathway and target genes were significantly altered in USH1C patient-derived fibroblasts. Finally, altered cWnt-signaling was reverted in HEK293T and USH1C patient-derived cells by inducing translational read-through of the USH1C^R31* nonsense mutations by Ataluren.

Conclusions : We provide evidence that harmonin is a potent suppressor of the cWnt-pathway by interacting with b-catenin. USH1C mutations result in an increase of cWnt-signaling in patient-derived fibroblasts and retinal Muller glia cells of USH1C pigs, leading to altered expression of Wnt-related genes that could underlie the pathophysiology of USH1C. The novel role of USH1C/harmonin in the cWnt-signaling pathway opens new paths in research to unravel pathomechanisms, identify new therapeutic targets, and evaluate treatment options.

This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.