WASHINGTON — Researchers have for the first time used gene therapy to restore normal hearing to mice that were born deaf. The findings could lead to a successful therapy for people with congenital hearing loss.
Congenital hearing loss or deafness from birth is one of the most common sensory deficits. It is often the result of genetic defects that affect the proper functioning of hair cells inside the inner ear. The hair cells, which move in response to sound vibration, transmit auditory signals to the brain. Experts believe there may be as many as 100 genes whose mutation can cause some form of hearing loss.
Researchers created a mouse model of congenital deafness in humans by breeding rodents that lacked the gene that makes vesicular glutamate transporter-3, or VGLUT3 for short - an essential protein that enables hair cells in the ear to receive and send auditory signals. People with a defective copy of the VGLUT-3 gene suffer diminished hearing over time. Because the experimental mice didn’t have the gene at all, their deafness was profound.
Lawrence Lustig, an otolaryngologist or hearing specialist at the University of California San Francisco, says researchers used a virus that doesn’t cause illness in humans to carry a corrected copy of the VGLUT3 gene into the ear canals of the deaf mice.
Then, they sealed up the tiny hole they had made in a membrane in the rodents’ inner ears to inject the virus, and waited.
“The first time I saw the results, I didn’t believe it. The hearing looked essentially normal in these mice,” Lustig said.
Lustig says that within a week, the mice showed the first signs that they could hear. By the second week, the rodents' hearing was essentially normal, as measured by a stimulation test in which researchers played tones and then monitored the rodents’ brain response to the sounds.
The researchers treated both newborn and adult mice with the therapy. Hearing lasted one-and-a-half years in the grown rodents and at least nine months in the newborns. Mice typically live two years.
Current methods of treating congenital deafness include hearing amplification or hearing aids and cochlear implants, which are surgically embedded in the skull. But the devices often produce audio distortion or sound levels that are too low.
If deafness is caused by a defective gene, Lustig says, gene therapy as demonstrated by the experiment with VGLUT3 has the potential to cure many forms of hearing loss.
“Our next steps that we are actually working on right now are to take a much more common form of genetic hearing loss [caused by a different gene] and then trying that again in mice and see if it works. And if that can work, then the next steps are see how we can implement this in kids who are born with deafnes,” Lustig said.
An article by the University of California’s Lawrence Lustig and colleagues on the use of gene therapy to cure congenital hearing loss is published in the journal Cell Press.