Characterization of USH1C/harmonin in the human retina provides insights into pathophysiology and therapy options for Usher syndrome

Abstract

Human Usher syndrome (USH) is a complex genetic disorder that comprises three clinical subtypes USH1, USH2 and USH3 and the most common form of hereditary combined deaf-blindness. Since rodent USH1 models do not reflect the ocular phenotype observed in human patients to date only little is known about the pathophysiology of USH1 in the human eye. The USH1C gene is heavily alternatively spliced and encodes for numerous harmonin isoforms that function as scaffold proteins and organizer of the USH interactome. RNA-seq analysis of USH1C revealed harmonin a1 as the most abundant transcript of USH1C in the human retina. Bulk mRNA-seq and Western blots confirmed abundant expression of harmonin in Müller glia cells (MGCs) and retinal neurons. We located harmonin in the terminal endfeet and apical microvilli of MGCs and in photoreceptor cells (PRCs), particularly in cone synapses and outer segments of rods as well as at adhesive junctions between both, MGCs and PRCs in the outer limiting membrane (OLM). We evidenced the interaction of harmonin with OLM molecules and rhodopsin in PRCs. We correlate the identified harmonin subcellular expression and colocalization with the clinical phenotype observed in USH1C patients. Furthermore, we demonstrate a phenotype in primary cilia in patient-derived fibroblasts which we were able to revert by gene addition of harmonin a1. Our data provide novel insights for retinal cell biology, USH1C pathophysiology and subsequent gene therapy development.