Hearing loss can occur due to several reasons, however the root cause behind each is either damage to sensitive hair cells of inner ear or to auditory nerves. Both play an important role in transmitting sound to the brain, but unfortunately these have no regeneration capability in adults. Consequently, any damage to either of these cells can induce irreversible hearing loss. According to National Institute of Deafness and other Communicative Disorders (NIDCD), one third of the 65-74 years age group and almost half of those above 75 years are affected by age-related hearing loss. Further, National Health and Nutrition Examination Survey (NHANES) showed that noise induces hearing loss to almost 16% of the teenagers (12-19 years age) in US.
Most of the hearing loss cases are incurable by conventional treatment modalities, therefore a wide interest is observed in the research community for establishing gene therapy as a possible cure for irreversible hearing loss. Joan McKechnie BSc Hons Audiology & Speech Pathology, a trained audiologist and a registered hearing aid dispenser at UK based HearingDirect adds “Gene therapy is actually a simple approach to treat certain diseases by introducing a specific gene into the human body”. In this treatment modality, the aim is either to activate certain suppressed genes or to replace the faulty genes with normal ones. With this approach, it has been established by multiple research studies that gene therapy can be helpful in treating as well as preventing the irreversible hearing loss.
At initial stages of research on gene therapy for hearing loss, the primary aim was to identify the genes which can activate regeneration capabilities of hair cells and neurons. Recently, three types of genes are identified whose mutations contribute to about 70% of the hearing loss cases. Atonal homolog 1 (Atoh1, mATH1) is one of such genes that act as a control button for growth of hair cells and is suppressed following the birth of any mammal. It has been shown by research studies that activation of Atoh1 can induce divisions of supporting cells to generate new hair cells, resulting in recovery from alleged incurable hearing loss. Neurotrophic gene therapy is another important mode, with neurotrophins being the key proteins in inducing survival, growth and function of neurons.
As an initial success story, it is reported by few investigators that hearing was restored in mice following various gene therapy modes with no potential side effects, and the same is expected for humans. However, to establish the gene therapy for clinical use, extensive research is required to establish the mechanism for different modalities of gene therapy for humans. Recently a clinical trial has been initiated on a regenerative gene therapy named ‘CGF166’ (GenVec Inc. MD, USA) which is hoped to complete till 2017. In this modality, the gene is attached to a harmless virus, which targets the appropriate cells in ear thus delivering the therapeutic gene. If the results show even little improvement in hearing capability, the modality will be greatly helpful at least in reducing the severity of hearing loss.
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