Cell type-specific gene therapy approach for TOMT-linked congenital hearing loss

PROJECT SUMMARY/ABSTRACT Hearing loss is one of the most prevalent sensory deficits, yet there are no FDA-approved biological therapies. There are over 100 distinct forms of nonsyndromic genetic hearing loss, each caused by mutations in different genes. Advances in adeno-associated viral (AAV)-based gene therapies for Otoferlin-linked genetic hearing loss have shown great promise for translating therapeutics developed in mouse models to humans. However, many challenges exist, and most gene therapies lead to incomplete hearing restoration and may cause side effects. Namely, the cochlear expression patterns and levels, and the structural and functional impacts of deafness- linked mutations are gene- and cell-type-specific. Thus, a one-size-fits-all approach is not optimal for hearing loss gene therapies, and the development of precision, cell-type-specific therapies is an essential unmet need. To establish a cell-type-specific, precision medicine approach for maximal therapeutic benefit and minimal adverse effects, we formed Echogenesis Therapeutics. We used a computational approach with in vivo screening of candidates to generate candidate cell-type-specific genomic promoter fragments for precise therapeutic expression. Using this platform approach, we discovered a novel promoter that is highly specific for cochlear hair cells. Here, we present proof-of-concept of our platform for the development of our first target cell-type-specific therapy for congenital genetic hearing loss. Mutations in TOMT/LRTOMT/COMT2 (TOMT) cause profound hearing loss in humans—0.5% to 8% of all cases of genetic hearing loss, depending on the population sampled. Mouse models with Tomt mutations faithfully recapitulate the human hearing loss phenotype. Consistent with our hypothesis for the need for cell-type-specific gene therapies, we found that inner- ear delivery of TOMT driven by gene therapies utilizing promoters currently used for hearing loss gene therapy led to significant hearing loss, as evidenced by elevated auditory brainstem response (ABR) thresholds in normal hearing mice. Using our novel hair-cell-type-specific promoter, we found that our cell-type specific approach: 1) is much more specific for hair cells than other promoters; 2) significantly restores ABR thresholds in congenitally deaf Tomt mutant mice; and 3) does not cause hearing deficits in normal hearing mice, in contrast to other therapies. This STTR Phase I project will address the efficacy of our AAV-based cell-type-specific gene therapy expressing human TOMT in congenitally deaf Tomt mutant mice for hearing restoration. Specifically, we will 1) determine effective gene therapy dosages for hearing restoration; 2) determine the timing window for therapeutic intervention; and 3) determine the duration of the therapeutic benefit. Overall, our findings will have significant implications for patients with hereditary hearing loss, including those with TOMT mutations, and broader impacts toward improving the understanding of cell-type-specific delivery in the cochlea. Importantly, our proposed cell- type-specific approach will be also relevant for other forms of hearing loss with a gene-related etiology or genetic predisposition, such as noise-induced and age-related hearing loss, and for vestibular disorders.