Dual AAV-Mediated Gene Replacement Therapy Improves the Vestibular Function in a Mouse Model of USH1B

Abstract

Background: Usher syndrome is the most common cause of deafness-blindness in the world. Usher syndrome type 1B (USH1B) is caused by mutations in MYO7A. Patients with USH1B experience deafness, blindness, and vestibular dysfunction. In this study, we applied dual AAV-mediated gene therapy to a mouse model of USH1B, the shaker-1 (Myo7a4626SB/4626SB) mouse. The shaker-1 mouse has a nonsense mutation in Myo7a. It is profoundly deaf throughout life, and it has significant vestibular dysfunction, similar to patients with USH1B. Methods: Neonatal (P0 to P5) shaker-1 (Myo7a4626SB/4626SB) mutant mice were used in this study. Due to the ~6.7 kb of the Myo7a cDNA, the dual AAV approach was used for gene delivery via the posterior semicircular canal. Auditory brainstem response (ABR) was used to assess auditory function. Vestibular evoked potential (VsEP) and circling behavior recording were used to assess vestibular function. Immunohistochemistry was used to evaluate viral transduction efficiency, sensory hair cell viability, and stereocilia morphology. Results: The shaker-1 mice are profoundly deaf due to significant stereocilia disorganization and rapidly progressive cochlear hair cell loss. Even though vestibular hair cells do not undergo degeneration in shaker-1 mice, they also exhibit significant vestibular dysfunction, due to stereocilia disorganization of the vestibular hair cells as well. When Myo7a cDNA was delivered to shaker-1 inner ears using the dual AAV approach, cochlear hair cell survival was improved. However, stereocilia organization and auditory function were not improved. In the vestibular system, dual AAV-mediated Myo7a delivery caused an improvement in stereocilia organization. In addition, the treated shaker-1 mice also had improved vestibular function, reflected by reduced circling behavior and improved VsEP thresholds. Conclusions: Our results showed that dual AAV gene therapy was able to improve the vestibular function in the shaker-1 mutant mice.