Developments in Gene Therapies for Hearing Loss

Approximately ten per cent of the world’s population is affected by impaired hearing resulting from the degeneration of mechanosensory cells in the inner ear [1]. Until twenty years ago, an external hearing aid was the only therapeutic option for the hearing-impaired. This device, which filters and amplifies sound, is useful for in- dividuals with some peripheral sensory loss, but it cannot help individuals suffering from major damage to hair cells. The first innovation in direct transmission of electrical sig- nals to the auditory neurons came about in the 1980s with the introduction of cochlear implants [2].  These surgical implants provide a mechanical alternative to the hearing aid by bypassing damaged or nonexistent hair cells and directly stimulating auditory nerve fibers. However, due to their invasive and irreversible nature, cochlear implants are far from the best possible solution to hearing impairment. Current lines of research, such as cellular replacement and gene therapy, are bringing us closer to the goal of improving the functionality of the damaged inner ear without utilizing mechanical implants. 

 Damage to the inner ear can occur as a result of over- stimulation, head trauma, disease, ototoxic drugs, noise exposure, or aging. While defects of the outer or middle ear are often treatable, destruction of hair cells is gener- ally permanent. The inner ear facilitates the transduction of mechanical to electrical signals, which are then sent to the brain via auditory neurons. This process is carried out by the organ of Corti, which contains four rows of mecha- nosensory hair cells. Human hair cells can repair following minor injury but undergo facilitated cell death when severely damaged [3]. Research into the auditory systems of species that naturally regenerate inner ear hair cells may help sci- entists develop technologies for making similar processes available to humans. 

 The areas of research investigating therapeutic ap- proaches to defects in the inner ear can be sorted into four main categories: prevention, intervention, substitution, and regeneration. Prevention refers to the use of protec- tive treatments that are administered before the onset of an anticipated trauma, such as a cancer regimen, that has known damaging effects to the auditory system [4]. While this line of research is important, the majority of auditory defects are unforeseen and therefore unaided by this type of therapy. Intervention therapy, which involves the modifica- tion of biological processes following trauma, is similarly limited in its therapeutic value since it may only be used to immediately treat some cases of accidental exposure to damagingly loud noises [4]. Neither prevention nor inter- vention can be used in unexpected or congenital cases or in cases of complete destruction of hair cells. 

 Currently, the two remaining strategies for addressing hearing impairments are entirely experimental, but both are intended to address acquired damage to the inner ear. In substitution therapy, the missing or dysfunctional hair cells are replaced with stem cells. These undifferentiated or partially differentiated stem cells are implanted into the cochlea, where they are guided towards differentiation into new hair cells. Regeneration therapy involves inducing the differentiation of progenitor or supporting cells already pres- ent in the inner ear into functional hair cells. Most of the recent and therapeutically promising advancements in the field of hair cell regeneration have been a combination of substitution and regeneration therapies [4]. 

 Although it has long been known that there are many pathways, genes, and transcription factors involved in the development and differentiation of hair cells, it wasn’t until 1999 that researchers determined Math1 to be a singularly necessary gene in these processes [5]. Since then, both in vivo and in vitro studies have demonstrated that it is possible to trigger the transdifferentiation of non-sensory supporting cells into sensory hair cells simply by increasing the expres- sion levels of this gene [2,6,7]. 

 In 2003, two important studies measured the effect of the overexpression of Math1 (or homolog Atoh1) on mature mammalian cochleae. Both used viral-mediated gene transfer (replicating or utilizing existing mechanisms of induced gene expression belonging to viruses) to insert multiple copies of the gene into the epithelial tissue. However, where Shou et al. performed their experimentation on rat inner ear organs in vitro [8], Kawamoto and colleagues inoculated mature guinea pigs in vivo [9]. Kawamoto’s experiment resulted in hair cell regeneration in regions of the inner ear that normally contain hair cells as well as those that do not (ectopic regions). 

 While the ectopic hair cells were stunted and immature, those that grew in the organ of Corti appeared morphologically normal [9]. This demonstrated that not only does ATOH1 trigger hair cell growth in embryonic cells, but also that mature mammalian cochleae retain the ability to generate new hair cells. Furthermore, data from this study showed axonal extension towards new ectopic hair cells, suggest- ing that new hair cells can produce signals that effectively attract axons [9]. The study performed by Kawamoto et al. was the first to succeed in inducing hair cell regrowth in a mammalian cochlea in vivo and generated hope that gene therapy would soon be able to produce functional as well as morphologically normal hair cells. 

 Two years later, Izumikawa et al. further demonstrated repair of mature mammalian hair cells in the organ of Corti [6]. Following eight weeks of Atoh1 expression, guinea pigs showed large numbers of hair cells in the organ of Corti as well as in ectopic cells. Since no new progenitor cells were implanted with the adenovirus, it is clear that existing cells changed differentiation – or transdifferentiated – following overexpression of Atoh1. The morphology and orientation of the cells in the organ of Corti were normal, suggesting that the cues for positioning and organization remain in the supporting cells rather than the hair cells themselves [6]. 

 These studies not only prove the importance of the transcription factor ATOH1 during differentiation, but they also make it clear that it is possible to induce the cells of fully matured, mammalian cochlea to change from one cell type to another. Multiple independent laboratories [6,8,9] have shown that the overexpression of a single gene could cause non-sensory cells to sprout stereocilia and attract auditory neurons – characteristics normally distinct to hair cells.  This is a major feat; until now, modern medicine has been un- able to reproduce the complex mechanosensory cells that nature has created. 

 Additionally, a recent study conducted in utero, induced hair cells displayed stereociliary bundles that attracted nerve fibers, transduced signals as normal inner and outer hair cells do, and even expressed proteins unique to hair cells [10]. When added to the studies conducted in vivo, these findings provide compelling evidence that it will soon be possible to regenerate functional hair cells in the human organ of Corti – or, at the very least, to prevent hereditary hearing impairments prior to birth. 

 Though we are still far from understanding the secrets of the nervous system, scientists may soon be able to use the existing neural machinery to develop reliable methods of converting supporting cells into functional sensory cells. If and when this happens, it will completely change the rules of therapeutic neuroscience. 

 Despite the many obstacles still present in the field of hair cell regeneration and repair, the last ten years have generated rapid developments and increasingly promis- ing results; it seems only a matter of time before cochlear implants become a thing of the past.   

Alissa Groisser is an undergraduate at Brown University