More than 70 different genes are known to cause deafness when mutated. Jeffrey Holt, PhD, envisions a day when patients with hearing loss have their genome sequenced and their hearing restored by gene therapy. A proof-of-principle study published today by the journal Science Translational Medicine takes a clear step in that direction, restoring hearing in deaf mice.
“Our gene therapy protocol is not yet ready for clinical trials – we need to tweak it a bit more – but in the not-too-distant future we think it could be developed for therapeutic use in humans,” says Holt, a scientist in the F.M. Kirby Neurobiology Center at Boston Children’s Hospital and an associate professor of Otolaryngology at Harvard Medical School.
Holt, with first author Charles Askew and colleagues at École Polytechnique Fédérale de Lausanne in Switzerland, focused on deafness caused by a gene called TMC1. Not only does TMC1 account for 4 to 8 percent of genetic deafness, it also encodes a protein that’s critical for hearing – helping to convert sound into electrical signals that travel to the brain.
To deliver the functioning TMC1 gene into the ear, the team inserted into an engineered virus called adeno-associated virus 1, or AAV1. For good measure, they added a promoter – a genetic sequence that turns the gene on only in certain sensory cells in the cochlea, known as hair cells.
They then injected the engineered AAV1 into the inner ears of mutant, deaf mice modeling the more common recessive form of TMC1 deafness, which causes profound hearing loss in children from a very young age, usually by around 2 years. After the injection, the animals’ sensory hair cells began responding to sound – producing a measurable electrical current – and activity began showing up in the auditory portion of their brainstems.
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