Cochlear gene therapy restores hearing and auditory processing in an atypical DFNB9 mouse model.

Background: The autosomal recessive deafness 9 (DFNB9) is caused by mutations in the otoferlin gene that accounts for 2-8% of all inherited deafness cases. In a previous study, we demonstrated that Adeno-associated virus (AAV) gene therapy restored hearing in a preclinical mouse model of profound DFNB9 deafness caused by a frameshift mutation leading to a complete loss of otoferlin expression. However, it remains to be demonstrated that it can address the full spectrum of DFNB9 deafness severity, while also restoring central auditory processing essential for speech understanding.

Methods: Using homologous recombination in mouse embryonic stem cells, we created a knock-in mouse model carrying the E1799del otoferlin mutation, which mirrors the human E1804del variant linked to DFNB9 deafness, characterized by moderate-to-profound deafness during febrile episodes in affected individuals. A mixture of male and female mice was used at P2, P8, and P30. Some were followed for up to 4 months for longevity monitoring and behavioral tests.

Results: The mouse model exhibits abnormal otoferlin distribution, failure of synaptic transmission in inner hair cells, and profound hearing loss, all of which is restored to normal through AAV gene therapy. Notably, we conduct objective behavioral testing to provide the first evidence that gene therapy administered to the cochlea, which is part of the peripheral auditory system, can restore frequency discrimination, indicating the recovery of central auditory processing. This is achieved even when treatment is administered late at the end of the critical period.

Conclusions: These findings indicate that gene therapy can address the entire spectrum of DFNB9 hearing loss, and that profound deafness during critical period may not impede the restoration of central auditory processing.