RESTORING AUDITORY FUNCTION IN MAMMALS USING AAV-BASED GENE THERAPY

Hearing loss is the most common sensory disorder in humans, and it is estimated that approximately 2.5 billion people worldwide will be affected by 2050 (World Health Organisation). Profound deafness and progressive hearing loss have an impact on the quality of life of both children and adults. Despite the prevalence of hearing dysfunction in the population, there are no available treatments for it. In the auditory system, acoustic information is translated into electrical signals by the sensory hair cells in the cochlea, which are the primary target of genetic mutations leading to hearing loss. A key protein involved in the development and function of the hair cells is Myosin VIIa. Shaker-1 mice have a mutation in Myo7a and the same mutation in the orthologous gene in humans leads to Usher syndrome type 1, which is a form of congenital hearing loss. In addition to this, mice that have MYO7A gene conditionally knocked-out (Myosin VIIafl/fl x Myosin15-CRE+/-) exhibit cochlear phenotypes that resemble those seen during progressive or age-related hearing loss. The above two mouse models were used in the current study to examine if the auditory function can be restored by AAV- MYO7A augmentation.
AAV8 and AAV9 vector serotypes were assessed to determine the transduction efficiency of the hair cells both ex vivo and in vivo MYO7Awas delivered to the inner ear using dual-AAV- MYO7Avectors injected through the round window membrane. One-month after surgery, mice were tested for any recovery in their hearing ability using in vivo electrophysiology (auditory brainstem recordings). The potential morphological and functional recovery of the hair cells was tested using immunostaining and scanning electron microscopy. The results from the current project show that dual-AAV-MYO7A gene-based therapy leads to the partial restoration of hearing capability in the congenital deafness mouse model (Shaker-1) and in the progressive hearing loss murine model (Myosin VIIafl/fl x Myosin15-CRE+/-).
In both mouse models, some of the morphological and biophysical characteristics of the dysfunctional hair cells were reinstated after the transduction of the exogenous gene via AAVs. These results highlight that the inner ear retains its ability to be manipulated even in the adult cochlea. However, further pre-clinical research is needed to optimise the delivery and transduction efficiency of the AAVs into the inner ear, which is an essential step before gene-based therapy can be successfully applied to patients.