Washington (ISJ) ? In what could be a precursor to therapeutically restoring hearing and balance for people born with genetic conditions, scientists at the Harvard Medical School and Massachusetts General Hospital have managed to restore partial hearing and balance in mice born with such impairments.
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 show 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.
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.� In the USA alone, some 30 million people 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.
Scientists have long sought ways to improve gene delivery into hair cells in their quest to restore hearing through gene therapy. But 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- investigator of the study and Professor of Translational Medical Science at Harvard Medical School.
The researchers worked with mice born without a gene critical for hair cell function to test the approach in living animals. Such animals normally cannot hear even the loudest sounds and exhibit poor balance.
Post-treatment tests revealed that the gene entered between 30 and 70 percent of hair cells, reaching both inner and outer hair cells.
A month after treatment, nine of 12 mice had some level of hearing restored and could be startled by a loud clap, a standard behavioural test for hearing. Four could hear sounds of 70 to 80 decibel intensity, the rough equivalent of conversation in a loud restaurant.
Because hair cells are also critical for the sense of balance, mice with damaged or missing hair cells show balance abnormalities. Treated mice had notably improved balance, compared with their untreated counterparts, showing far less head tossing or running in circles, both markers of instability or disorientation.
The team now plans to improve their gene-delivery technique in an attempt to reach an even greater proportion of hair cells. The scientists will test the approach in other forms of deafness, including conditions that cause both deafness and blindness.
