Motor Protein MYO1C functions in the Pathophysiology of Usher Syndrome

Abstract

Purpose : Usher Syndrome (USH) is an inherited disorder characterized by hearing loss, vestibular dysfunction, and visual impairments. Most of the identified gene loci do not account for all known USH cases. The role of Myosins have been implicated in the pathophysiology of USH. Interestingly, mutations in the motor protein, Myosin 1C (MYO1C) are associated with deafness. We developed a Myo1c-knockout mouse line to investigate a functional role for MYO1C in retinal cell physiology and to confirm its role in hearing.

Methods : Whole-body Myosin 1C knockout (Myo1c-KO) mice were generated and visual function tests using electroretinogram (ERG) and Auditory Brain Response (ABR) tests for hearing were performed on wild-type (WT) and Myo1c-KO animals, at 2-6 months of age. Immunofluorescence and trafficking analysis of key photoreceptor outer segment (OS) proteins were performed on retinal sections from both genotypes. Histology and electron microscopy (TEM) analysis were performed to determine photoreceptor cell structure and to quantify retinal cell layer thickness. Co-immunoprecipitation and reciprocal IP assays were performed to access direct binding of MYO1C to retinal proteins.

Results : Systemic loss of MYO1C in mice specifically affected rhodopsin trafficking, where rhodopsin mislocalized to the rod photoreceptor inner segments and cell bodies, while trafficking of other OS proteins (CNGA1, Arrestin, and Transducin) were largely un-affected. ERG analysis of Myo1c-KO mice showed a progressive loss of visual function. OCT analysis showed thinner photoreceptor cell layer and a reduction of outer nuclear layer thickness, while histological and TEM analysis confirmed shorter photoreceptor OS lengths in Myo1c-KO animals, at the 6-months’ time point. ABR thresholds were significantly decreased in Myo1c-KO vs. WT animals, at 6-months of age, indicating that these mice also suffer from deafness.

Conclusions : We have identified a physiological role for the motor protein, Myosin 1C (MYO1C), in the pathophysiology of Usher Syndrome. Our novel Myo1c knockout mouse model mimicked BOTH the visual dysfunction and hearing loss phenotypes observed in Usher Syndrome patients. The study specifically identified rhodopsin as a cargo of MYO1C. The ongoing goals are to identify the precise molecular mechanisms of visual dysfunction and hearing loss in Myo1c-KO animals and to screen for MYO1C variants in patients with syndromic RP and Usher Syndrome.

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