Adeno-associated virus gene replacement for recessive inner ear dysfunction: Progress and challenges

Abstract

Approximately 3 in 1000 children in the US under 4 years of age are affected by hearing loss. Currently, cochlear implants represent the only line of treatment for patients with severe to profound hearing loss, and there are no targeted drug or biological based therapies available. Gene replacement is a promising therapeutic approach for hereditary hearing loss, where viral vectors are used to deliver functional cDNA to "replace" defective genes in dysfunctional cells in the inner ear. Proof-of-concept studies have successfully used this approach to improve auditory function in mouse models of hereditary hearing loss, and human clinical trials are on the immediate horizon. The success of this method is ultimately determined by the underlying biology of the defective gene and design of the treatment strategy, relying on intervention before degeneration of the sensory structures occurs. A challenge will be the delivery of a corrective gene to the proper target within the therapeutic window of opportunity, which may be unique for each specific defective gene. Although rescue of pre-lingual forms of recessive deafness have been explored in animal models thus far, future identification of genes with post-lingual onset that are amenable to gene replacement holds even greater promise for treatment, since the therapeutic window is likely open for a much longer period of time. This review summarizes the current state of adeno-associated virus (AAV) gene replacement therapy for recessive hereditary hearing loss and discusses potential challenges and opportunities for translating inner ear gene replacement therapy for patients with hereditary hearing loss.

Figure 1:. Variants in genes with diverse functions can cause hereditary hearing loss.

This figure illustrates the location of genes that have been successfully targeted for inner ear gene replacement therapy in the cochlea (figure modified from Faridi et al., 2018).

Figure 2:. Most genes that cause hereditary hearing loss affect mechanosensory hair cells in the cochlea.

This figure lists the genes that are associated with hereditary hearing loss by cDNA size. Most of the genes (73%) that cause hereditary hearing loss affect proper mechanosensory hair cell function (shown in black). The size limitations for single AAV and dual AAV gene delivery are also indicated.