Adeno‐associated virus‐based rescue of <i>Myo7a</i> expression restores hair‐cell function and improves hearing thresholds in a <i>USH1B</i> mouse strain

Mutations in MYO7A, the gene encoding the unconventional myosin 7a, cause hereditary deafness in mice and humans. In the cochlea, MYO7A is present in the sensory hair cells from embryonic stages of development, and plays a critical role in the development and maintenance of the mechanosensitive hair bundles composed of actin-rich stereocilia. Shaker-1 mutant mice (Myo7a^Sh1/Sh1), the murine model of Usher 1B syndrome, exhibit a progressive loss of the stereocilia, subsequent degeneration of the sensory epithelium and ultimately profound deafness. In addition to the hair bundle defects, we found that the shaker-1 mutation prevented both inner hair cells (IHCs) and outer hair cells (OHCs) from acquiring their fully mature basolateral current profile. Delivering exogenous Myo7a to newborn Myo7a^Sh1/Sh1 mice using dual-adeno-associated virus 8 (AAV8)-Myo7a or dual-AAV9-PhP.eB-Myo7a, which primarily target IHCs, led to a substantial rescue of their hair bundle structure. The rescued bundles regained their ability to generate mechanoelectrical transducer (MET) currents in response to fluid jet displacement. Although the average MET current was smaller than in control IHCs, the normal resting open probability of the MET channel was fully restored. The IHCs of the treated cochlea also regained a mature basolateral membrane current profile. Functionally, rescue of the IHC structure and function, but not that of OHCs, leads to an average improvement of 20-30 dB in hearing thresholds across most frequencies. These results support dual AAV-induced gene replacement therapy as an effective strategy to recover hair-cell function in Myo7a^Sh1/Sh1 mice. KEY POINTS: Shaker-1 mutant mice (Myo7a^Sh1/Sh1), which carry a mutation in the unconventional myosin MYO7A and are the murine model of Usher 1B syndrome, become profoundly deaf at 1 month of age or soon after. In the mammalian cochlea, MYO7A is expressed in the hair cells, including within their actin-rich stereociliary bundles. We show that hair cells of Myo7a^Sh1/Sh1 mice progressively lose their transducing stereocilia and mechanoelectrical transduction, and fail to acquire their fully mature basolateral current profile. Delivering exogenous Myo7a to newborn Myo7a^Sh1/Sh1 mice using dual-adeno-associated virus (AAVs) led to a substantial rescue of the bundle structure and function of inner hair cells, including mechanoelectrical transduction. This functional rescue led to a 20-30 dB improvement in hearing thresholds across most frequencies. These results support dual AAV-induced gene replacement therapy as an effective strategy to recover the hair-cell function in Myo7a^Sh1/Sh1mice.