AAV gene therapy rescues hearing in a mouse model of Syne4 deafness

Pathogenic variants in SYNE4 are known to lead to deafness among individuals from Israel and Turkey. SYNE4 codes for nesprin 4, a part of the linker of nucleoskeleton and cytoskeleton (LINC) complex in the nuclear envelope. LINC complexes enable nuclear polarization and transmit mechanical stimuli to the nucleus. We investigated the possibility of rescuing hearing in Syne4^-/- mice using the synthetic AAV Anc80L65. We cloned the coding sequence of Syne4 into the AAV backbone and generated AAV.CMV.Syne4.EGFP viral vectors. Injections of viral preparation were made into the posterior semicircular canals of Syne4^-/-, Syne4^+/- and wild-type mice at day of birth (P0). Transduction efficiency was then evaluated two weeks following the injections based on GFP fluorescence; the majority of inner hair cells and a variable portion of outer hair cells were transduced. Injected mice were evaluated for hearing using open-field ABR at P15. Our results demonstrate that overexpression of Syne4 is safe with respect to auditory function. We also show a modest but statistically significant improvement in hearing. Morphological assessment of injected and control ears demonstrates that while inner hair cells are unchanged, a higher portion of outer hair cells appear to survive in Syne4^-/- inner ears injected with AAV.CMV.Syne4.EGFP, compared with injected ears with AAV.CMV.EGFP. As the Syne4^-/- knock-out mouse phenotype mimics the human one, these mice are a relevant model for development of gene therapy for hearing loss.