Background
Mutations in the OTOF gene, which cause autosomal recessive non-syndromic hearing loss, DFNB9, are a leading cause of auditory neuropathy. Gene augmentation and RNA and DNA editing have been used to restore the auditory function of mouse models, and gene augmentation has even been used in patients with DFNB9. However, the therapeutic time window for DFNB9 treatment is largely unknown.

Methods
We screened multiple adenine base editors to identify an optimal editor for correcting the pathogenic Otof c.2815C>T (p.Gln939?) mutation. The selected editor was delivered to the cochlea of Otof-deficient mice at different postnatal ages using an adeno-associated-virus-mediated strategy. Auditory function was assessed in the treated Otof-deficient mice, while safety was evaluated in wild-type mice.

Findings
Nme2ABE8e exhibited superior on-target editing efficiency compared with other base editors. Adeno-associated-virus-mediated delivery of Nme2ABE8e to 1-month-old adult Otof-deficient mice restored auditory function to near-normal levels, with sustained effects for at least 6 months. Importantly, treatment at 2, 3, and 4 months of age also resulted in hearing improvement, indicating an extended therapeutic time window. Comprehensive safety assessments revealed no detectable ototoxicity or neurological deficits.
Conclusions
These results underscore the safety and efficacy of Nme2ABE8e for DFNB9 and suggest that patients with DFNB9 may have a relatively long therapeutic time window for gene therapy.
Funding
This work was funded by the National Key R&D Program of China, the National Natural Science Foundation of China, and other governmental and institutional funding sources.