Gene therapy a potential answer for hereditary deafness

14 July 2015

Scientists have partly restored hearing in mice with an inherited form of deafness using gene therapy.

There are more than 70 genes known to cause various forms of inherited deafness, with genetic defects accounting for at least half the cases of childhood hearing loss. The researchers from Boston Children’s Hospital and Harvard Medical School concentrated on the TMC1 gene, which accounts for 4-8 per cent of cases of hereditary deafness. The gene encodes a protein that plays a central role in converting sound waves into electrical signals that travel to the brain.

Published in the journal Science Translations Medicine, the mouse study could pave the way for gene therapy being used as a treatment for people with hearing loss caused by genetic mutations.

Dr Jeffery Holt, one of the researchers from Bostons Children’s Hospital, told the BBC: "We're very excited about it, but we're also cautiously optimistic as we don't want to give false hope. It would be premature to say we've found a cure…But in the not-too-distant future it could become a treatment for genetic deafness so it is an important finding".

Gene therapy was tested on genetically engineered mice that had the TMC1 gene completely deleted. This is comparable to children who inherit two defective copies of the TMC1 gene; these children tend to experience significant hearing loss from as early as two years of age. The team delivered corrected copies of the faulty gene into the inner ear of the mice using a harmless common cold-like virus.

A behavioural trick, known as a startle test, was used to test whether the researchers had cured the mice of their deafness. Undergoing the test after gene therapy the mice reacted with a jump in response to sounds equivalent to the noise inside a moving car (85dB). Tests on the treated mice showed that the hair cells in the inner ear of the mice were able to convert sound waves into electrical signals which could be sent to the brain as well as restoring activity in the portion of the brainstem involved in hearing. However, the mice only received a partial fix, with most of the inner hair cells accepting the viral delivery but very few of the outer hair cells. Inner hair cells allow you to hear sound but the outer hair cells help amplify noises and increase sensitivity.

Currently the gene therapy is not yet ready for clinical trials in humans but the research team hope, if all goes well, to begin trials within 5-10 years. Looking forward, the researchers hope to improve the viral infection rate and test the longevity of the treatment further than the two months already observed.

Despite the current study only being on the TMC1 gene mutation, Holt believes that other forms of genetic deafness may also be responsive to the same gene therapy strategy. He said: “I can envision patients with deafness having their genome sequenced and a tailored, precision medicine treatment injected into their ears to restore hearing".

Commenting on the study, Dr Ralph Holme of Action Hearing Loss said: "These findings are encouraging and open the door for other gene therapies, providing hope for people with certain types of genetic hearing loss that, following diagnosis, gene therapy could be available in the not-too-distant future".

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