An international team of researchers has used a virus to correct genetic defects and partially restore hearing in deaf mice. It’s an important proof-of-concept that could eventually lead to therapies in humans.
Above: Those upside-down V’s are sensory hair bundles in the ear, each containing 50 to 100 microvilli tipped with TMC proteins. The new gene therapy restores hearing by providing working copies of those proteins.
Genetic faults are responsible for approximately half of all cases of hearing loss in early life, and there are at least 70 genes that, when mutated, cause deafness. It’s no wonder, then, that scientists would like to use gene therapies to treat—and even possibly cur —certain forms of hearing impairments. Now, a research team led by Jeffrey Holt from the F. M. Kirby Neurobiology Center at Boston Children’s Hospital, along with Charles Askew and colleagues at École Polytechnique Fédérale de Lausanne in Switzerland, has shown that this may indeed be the right approach.
By focusing on a gene called TMC1—a gene that accounts for 4-to-8% of genetic deafness—the researchers have demonstrated that it’s possible to use a virus, in this case the adeno-associated virus 1 (AAV1), to deliver a functional version of TMC1 and restore partial hearing in mice. TMC1 is critical in that it encodes a protein that converts sound into electrical signals that travel to the brain. The results of their work now appears at Science Translational Medicine.
Sound-sensing cells are shown in green. (Askew et al./Science Translational Medicine 2015)
Writing in Science News, Sarah Schwartz explains more:
The scientists tried this therapy on two different deafness-causing mutations. Within a month, around half the mice with one mutation showed brainwave activity consistent with hearing and jumped when exposed to loud noises. Treated mice with the other mutation didn’t respond to noises, but the gene therapy helped their hair cells — which normally die off quickly due to the mutation — survive. All of the untreated mice remained deaf.
The mice that recovered hearing received a partial fix. Most of their inner hair cells, which allow basic hearing, used the new genes. But few outer hair cells, which amplify noises, accepted the viral delivery. It’s hard to get outer hair cells to respond to gene therapy...Still, inner hair cells control most sound transmission...
Eventually, the scientists would like to use viruses to treat all hair cells and achieve complete recovery of hearing. The next step is to improve the viral infection rate and to see if they can get these treatments to last longer.
“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,” noted Holt in a statement.
A trio of hair bundles from the cochlea. (Boston Children’s Hospital)
Excitingly, AAV1 is considered safe as a viral vector and has already been used in human gene therapy trials to treat blindness, heart disease, muscular dystrophy, and other conditions.
“This is a great example of how the basic science can lead to clinical therapies,” says Holt.
Read more at Science News. And check out the scientific study at Science Translational Medicine: “Tmc gene therapy restores auditory function in deaf mice”.