A novel gene therapy technique has partially restored hearing and balance in deaf mice.
Mice that were completely deaf because they lacked a critical hair cell gene were able to hear the equivalent of a loud conversation after the treatment.
This is the first time that a gene therapy has been able to access hair cells in the inner ear – delicate sensors detect sound and head movement and turn them into signals that travel to the brain.
'To treat most forms of hearing loss, we need to find a delivery mechanism that works for all types of hair cells,' said Professor David Corey, a senior investigator on the study from Harvard Medical School.
Nine out of 12 mice treated showed signs of improved hearing, and four mice could hear the equivalent of conversation in a loud restaurant, approximately 70 to 80 decibels. The mice also had improved balance and reduced instability or disorientation, a common symptom in mice with this form of deafness.
As they describe in the journal Molecular Therapy, the research team improved on a commonly used gene therapy that uses an adeno-associated virus (AAV), by packaging the virus into small protective bubbles of cell membrane, called exosomes. As with the standard gene therapy, the virus contains a copy of the missing or faulty gene responsible for the mice's deafness.
This new method – termed exo-AAV – was much more effective at delivering the virus to hair cells because the exosomes seem to bind more easily to receptors on the surface of hair cells. The exosomes then fused with the hair cells and released their cargo inside them.
The researchers injected these exo-AAVs into the inner ear of mouse pups that were born completely deaf because they lacked a hair-cell gene. The treatment resulted in a 30–50 percent higher penetration rate of the virus, when compared with AAV delivery. Previous studies using this standard treatment resulted in no therapeutic benefit.
'Unlike current approaches in the field, we didn't change or directly modify the virus. Instead, we gave it a vehicle to travel in, making it better capable of navigating the terrain inside the inner ear and accessing previously resistant cells,' said Dr Casey Maguire of Massachusetts General Hospital, a co-author on the study.
The research team acknowledge that gene therapy for deafness in humans is many years away, but they suggest this study is 'an important proof of principle'. One in 1000 babies each year are born with a hearing impairment, and 60 percent of these have a genetic cause.