Using a novel form of gene therapy, scientists from Harvard Medical School and the Massachusetts General Hospital have managed to restore partial hearing and balance in mice born with a genetic condition that affects both.
The new model overcomes a long-standing barrier to accessing hair cells, the delicate sensors in the inner ear that capture sound and head movement and convert them to neural signals for hearing and balance. These cells have been notoriously difficult to treat with previous gene delivery techniques.
The team's findings, published in the February issue of Molecular Therapy, show that the treatment leads to notable gains in hearing and allows mice that would normally be completely deaf to hear the equivalent of a loud conversation. The approach also improved the animals' sense of balance.
An accompanying commentary to the study appears in the same issue.
The investigators caution the approach is years away from use in humans, but gene therapy carries the promise of restoring hearing in people with several forms of both genetic and acquired deafness. Some 30 million Americans suffer from hearing loss, and every year about one in 1,000 babies are born with hearing impairment, according to the Centers for Disease Control and Prevention.
On their quest to restore hearing through gene therapy, scientists have long sought ways to improve gene delivery into hair cells. Previous approaches were only marginally effective as they reached one set of hair cells in the inner ear, but another subset—equally critical for hearing—remained largely impenetrable.
"To treat most forms of hearing loss, we need to find a delivery mechanism that works for all types of hair cells," said neurobiologist David Corey, co-senior investigator on the study and the Bertarelli Professor of Translational Medical Science at HMS.
To achieve that, the researchers used the common adeno-associated virus (AAV). The virus has been already used as a gene-delivery vehicle for retinal disorders but thus far has proven far less efficient in penetrating hair cells.