Gene therapy and stem cell transplantation: strategies for hearing restoration

Strategies to restore sensorineural hearing loss focus on the replacement of lost hair cells, the specialized mechanoreceptors in the organ of Corti that convert the mechanical energy of sound into electrical energy. Hair cells in mammalian systems do not have the capacity to regenerate, but two exciting lines of research hold promise in restoring inner ear function. Here we review basic principles of gene therapy and discuss its application in the inner ear. We survey the various viral vectors and routes of delivery into the inner ear. Applications of gene therapy in the inner include hair cell protection in the face of chemical or noise-induced ototoxicity, spiral ganglion cell survival following hair cell death or injury, and hair cell regeneration. More recently, the viability of gene therapy in human inner ear tissue has been reported. Transplantation of progenitor cells that can differentiate into functioning hair cells with the appropriate connections to their corresponding spiral ganglion cells is yet another strategy to restore sensorineural hearing loss. Neonatal or embryonic stem cells, adult mouse inner ear stem cells, and stem cells from the central nervous system have been shown to differentiate into cells containing hair cell markers and proteins. Prospects for stem cell therapy in the inner ear, and its limitations, will also be examined.