Hair Cell Transduction Efficiency of Single- and Dual-AAV Serotypes in Adult Murine Cochleae

Hearing is an exquisitely complex sense, and while hearing aids and cochlear implants are available to persons with hearing loss, these options do NOT restore normal hearing. As a consequence, there is a major push to develop biotherapies to restore normal auditory function. Among these therapies, gene therapy is the most promising. Within the ear and central to hearing are outer and inner hair cells (OHCs and IHCs). OHCs amplify sound signals while IHCs convert the mechanical information carried by sound waves into electrical signals that are transmitted to the brain. Many forms of genetic hearing loss have their origin in defects in genes in IHCs and OHCs that encode proteins essential to normal hair cell function.

Adeno-associated virus (AAV) is widely used in gene therapy due to its low pathogenicity and sustained expression in transduced cells. Its production is also simple, cheap, and fast. To date, the US Food and Drug Administration has approved AAV-based gene therapies for the treatment of spinal muscular dystrophy and Leber’s congenital amaurosis. AAVs are also being developed for therapeutic applications in the ear. To be successful, the tropism of AAVs for IHCs and OHCs must be defined. Some AAV serotypes, for example AAV1, AAV2, AAV8, AAV9 and Anc80, transduce IHCs and OHCs with different degrees of efficiency. For AAV in the ear to be successful, this variability must be clearly understood. In addition, because of the limited capacity of AAV to carry large genes (AAVs have a small viral loading capacity of no more than 5.0 kb), the ability of a single IHC or OHC to take in two different viruses (so called dual transduction) must be defined. Dual transduction enables delivery of larger transgenes by utilizing the intrinsic property of AAV to undergo concatamerization when separate vectors are co-transduced into a single cell.

These questions were addressed by scientists at University of Iowa – Dr. Ryotaro Omichi, Hidekane Dr. Yoshimura, Dr. Seiji Shibata and Professor Richard J.H. Smith – together with Professor Luk Vandenberghe at Harvard Medical School. Using a surgical approach they developed that involved the combination of round window membrane injection and canal fenestration (RWM+CF), they studied both single and dual AAV transduction in mature mice. The work is published in the Journal Molecular Therapy, Methods & Clinical Development.

In these pre-clinical experiments, the research team selected five AAV serotypes to study, namely AAV1, AAV2, AAV8, AAV9 and Anc80, and injected them into the cochlea. In single AAV experiments, they found that the transduction rate of IHCs and OHCs was highest with AAV2 (IHCs, 97%; OHCs, 84%). In addition, with the exception of AAV8, injection of single AAV serotypes did not alter auditory function. Based on these results, dual vector combinations of AAV2-eGFP and AAV2-mCherry (AAV2-2), AAV2-eGFP and AAV9-mCherry (AAV2-9), and AAV9-eGFP and AAV9-mCherry (AAV9-9) were tested. AAV2-2 dual transduction was found to be as robust and efficient as that of the single AAV2.

Their data indicate that co-transduction is not influenced by cooperation between the two vectors or by preferential selection of the hair cells, which is relevant because dual vector transduction was believed to be inferior to single vector transduction based on ocular and retinal studies. For example, the retinal transduction efficiency of dual AAV8 hybrid vectors has been reported to be 6% and 40% of the transduction rate with a single AAV8 vector in both mice and pigs.  While this study observed a similar trend with dual AAV9-9 co-transduction, for AAV2-2 and AAV2-9, co-transduction rates were similar to single vector transduction rates. These findings suggest that specific combinations of dual vectors are likely to be important, an encouraging and important observation as experiments with AAV continue to explore its suitability as the viral vector of choice in the treatment of human deafness with gene therapy.