This gene therapy only works on very young mice and more steps are needed to make it useful for reversing hearing loss in the aged.
Researchers at Emory University School of Medicine have shown that introducing a gene called Atoh1 into the cochleae of young mice can induce the formation of extra sensory hair cells.
I want this for a very important non-aged purpose: I want to turn up the volume on music much higher than I let myself listen to now. The ability to repair damage will enable us to get damaged in ways we (or at least the more prudent among us) avoid getting damaged today. We might even be able to identify genetic variants that make more robust hair cells. Then we could use gene therapy or cell therapy to grow cilia hair cells that can handle higher volume.
Their results show the potential of a gene therapy approach, but also demonstrate its current limitations. The extra hair cells produce electrical signals like normal hair cells and connect with neurons. However, after the mice are two weeks old, which is before puberty, inducing Atoh1 has little effect. This suggests that an analogous treatment in adult humans would also not be effective by itself.
Likely at some point in development changes happen to the genome (e.g. DNA methylation) that block gene activity so that Atoh1 can no longer initiate hearing hair cell growth. To grow replacement hearing hair will require turning on more genes than Atoh1 by itself can activate. Once scientists figure out how to turn on all the genes needed to grow replacement they'll still need techniques to safely deliver gene therapy. Or perhaps the repair will get done using cell therapy where cells are primed to do repair and then injected into the inner ear.