A team of scientists at the University of New South Wales (UNSW) in Australia has found a way to use the electrodes in cochlear implants to apply targeted gene therapy and regrow damaged auditory nerves in the ear. Their research was published today in Science Translational Medicine.
Hearing loss is the most common type of sensory loss, affecting one in five U.S. adults. For many, a hearing aid is enough to correct their impairment. For more severe cases of hearing loss, a cochlear implant may be necessary.
But the implants don’t restore the full range of hearing. “People with cochlear implants do well with understanding speech, but their perception of pitch can be poor, so they often miss out on the joy of music,” said senior study author Gary Housley, a professor at UNSW, in a press release.
There are many types of hearing loss, depending on where damage occurs on the path between the ear and the brain. In the case of people with cochlear implants, the hearing damage occurs inside the ear itself, in the cochlea. The cochlea is lined with thousands of tiny hairs that vibrate when they detect sound waves, then transmit a signal for nerve cells to carry to the brain. These cells are very sensitive, and can die from any number of causes.
There are also nearby cells that play an important role in hearing, and they too die easily. They make substances called neurotrophins, proteins that support nerve cells and allow them to grow. When these cells die, the nerve cells that send signals to the brain lose their support network. Starved for neurotrophins, the nerve cells also die.
A cochlear implant takes the place of the sound-detecting cells in the ear. It has a microphone to pick up sound and a processor to break the sound up into channels, with an emphasis on wavelengths of sound that correspond to speech. Then, it projects an array of electrodes deep into the cochlea, coming near the nerve cells that transmit the signal to the brain.
However, there’s still a gap, and the nerve cells have still suffered damage from lack of neurotrophins. These problems restrict sound sensitivity for cochlear implant wearers.
The solution for hearing loss seemed straightforward at first: regrow the lost nerve cells. So the team used guinea pigs to find out how.
But getting nerve cells to grow is no easy task. Simply bathing the guinea pigs’ brains in neurotrophins could cause all kinds of nerve cells to grow uncontrollably, which could give the guinea pigs seizures, psychosis, or worse. They needed the neurotrophins to only appear inside the nerve cells that had already been damaged, meaning that the cells had to create the neurotrophins themselves.
This called for gene therapy, which would allow the scientists to deliver a section of DNA to each individual cell giving instructions for how to make neurotrophins. One way that DNA can be persuaded to enter a cell is by zapping the cell’s membrane with an electrical current.
And a guinea pig that’s just received a cochlear implant has dozens of electricity-producing electrodes placed right next to the nerve cells in question.
“No one had tried to use the cochlear implant itself for gene therapy,” said Housley. “With our technique, the cochlear implant can be very effective for this.”
The solution was perfect. The scientists injected their DNA cocktail into the guinea pigs, then used a brief pulse of electricity from the cochlear implant to shock the sound-carrying nerve cells—and only those nerve cells—into accepting the new DNA instructions.
As predicted, the damaged cells began producing their own neurotrophins. With their supply restored, the nerve cells began to regrow and create new connections across the gap to the electrodes in the implant. Deaf guinea pigs that had received the gene therapy had sound-carrying nerve clusters that were 40 percent larger than the guinea pigs that had not had the procedure. The damaged nerve cells even regrew their myelin, a fatty sheath that protects nerve cells and enhances their ability to conduct electrical signals.
Two weeks after treatment, the scientists tested the guinea pigs’ hearing by measuring their brain activity. The results were dramatic. Guinea pigs that had been given gene therapy had hearing that was almost as sensitive as guinea pigs that had never lost their hearing in the first place.
Production of neurotrophins dropped off after a few months as the donated DNA decayed, but with incoming sound signals to keep them active, the nerve cells stayed strong.
This could change everything for people who wear cochlear implants.
“We think it’s possible that in the future this gene delivery would only add a few minutes to the implant procedure,” said the paper’s first author, Jeremy Pinyon, in a press release. “The surgeon who installs the device would inject the DNA solution into the cochlea and then fire electrical impulses to trigger the DNA transfer once the implant is inserted.”
The electrical pulses used to perform the gene therapy procedure are greater than the recommended allowance for cochlear implants, but a burst of electricity for only a few seconds would probably cause few problems in comparison to the potential benefit of restored hearing.
This technique also paves the way for targeted gene therapy to treat other disorders, such as Parkinson’s disease, for which a patient might also receive a bionic implant.
"Our work has implications far beyond hearing disorders,” said co-author Matthias Klugmann, an associate professor at the UNSW School of Medical Sciences, in a press release. “Gene therapy has been suggested as a treatment concept even for devastating neurological conditions, and our technology provides a novel platform for safe and efficient gene transfer into tissues as delicate as the brain.”