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

Introduction

In mammals, hearing relies on a complex and precisely regulated mechanotransduction process involving the coordinated function of two types of auditory sensory hair cells: outer hair cells (OHCs), which act as the cochlear amplifier for sound signals, and inner hair cells (IHCs), the genuine sensory cells. When environmental sound waves reach the inner ear, they are first amplified by OHCs and then transformed into electrical signals in the auditory nerve. This conversion occurs through an extremely fast and sustained release of neurotransmitters at the primary auditory synapse, the IHC ribbon synapse. In this process, otoferlin, a protein containing six Ca²⁺ binding C2 domain (C2A-C2F) associated with DFNB9 deafness, functions as the major Ca²⁺ sensor for synaptic vesicle exocytosis and replenishment at the active zone of the IHC ribbon synapse^{1, 2–3}. Congenital hearing loss is the most common human sensory deficit in human, affecting ~1 in 700 newborns. Genetic factors account for up to 80% of these deafness cases⁴. To date, 153 genes have been shown to cause hearing loss when mutated⁵. Within the gene encoding otoferlin, about 220 pathogenic and likely pathogenic mutations have been identified, collectively contributing to 2–8% of all cases of genetic congenital deafness⁶. Findings from several preclinical studies indicate that gene therapy shows potential as a treatment for hereditary deafness^{7, 8–9}. Notably, we have proven in a previous work that adeno-associated virus (AAV)-mediated gene replacement therapy can reverse established deafness in a mouse model that faithfully replicates the profound prelingual human DFNB9 deafness caused by frameshift mutations that are predicted to result in truncated forms of otoferlin⁷. This breakthrough led to the initiation of several successful phase II clinical trials for DFNB9 patients^{10, 11–12}. However, these clinical trials target the typical DFNB9 deafness phenotype, characterized by profound prelingual deafness due to mutations that severely disrupt otoferlin-coding gene, leading to a complete loss of otoferlin function¹³. Nevertheless, in about 15% of cases, pathogenic otoferlin mutations lead to DFNB9 deafness with varying levels of severity, such as mild-to-profound, progressive, or temperature-sensitive all of which are associated with difficulties in understanding speech^{14, 15, 16–17}. The mutations involved, primarily in-frame mutations, are expected to...