For the first time, scientists have successfully used gene therapy to regenerate hair cells in the inner ear and restore hearing in a deaf mammal.
Dr. Yehoash Raphael and his colleagues at the University of Michigan Medical School used a virus to introduce a corrective gene called Atoh1 into guinea pigs that had acquired hearing loss. The gene induced the regeneration of hair cells that reside in the inner ear and are necessary to trigger the electrical impulses that carry sound waves to the brain.
Damage to these hair cells can result from aging, infection, loud noises and some medications. Once destroyed, hair cells cannot regenerate on their own.
After coaxing the hair cells to regenerate in the organ of Corti -- where they pick up sound vibrations and send them to the brain -- the researchers tested the deafened animals to see how well they could hear.
"In response to sound, [the guinea pigs] were able to generate neural responses indicative of hearing," Raphael says.
Though the evidence suggests at least some hearing was recovered, it is not yet clear how well the animals hear following the treatment. But the results, which appear in the journal Nature Medicine, suggest it may someday be possible to use a similar approach to treat deafness in humans.
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JENNIFER LUDDEN, host:
From NPR News, this is ALL THINGS CONSIDERED. I'm Jennifer Ludden.
For the first time scientists have successfully used gene therapy to restore hearing in a deaf mammal: guinea pigs. It's not yet clear how well the animals hear following the treatment, but it does suggest that using a similar approach to treat deafness in humans is possible. We begin our coverage of this development with a report from NPR's Joe Palca.
JOE PALCA reporting:
Yehoash Raphael has spent more than a decade trying to regenerate a special kind of cell inside the ear called a hair cell.
Dr. YEHOASH RAPHAEL (University of Michigan): Those cells are the ones that are gone, in many cases, of deafness. So if they can be regenerated or replaced once lost, then hearing may also be restored.
PALCA: Two years ago Rafael and his colleagues at the University of Michigan were able to show that by using the techniques of gene therapy to give guinea pigs a gene called Atoh 1, they could get them to make new hair cells. The cells didn't grow in exactly the right place, but the fact that they grew at all was encouraging. Those experiments were done with guinea pigs that could hear and already had the proper number of hair cells. Rafael wondered what effect Atoh 1 would have on animals with no hair cells, so he treated guinea pigs with a drug that destroyed their hair cells and made the animals deaf and then gave them the Atoh 1 gene.
Dr. RAPHAEL: And then as a result of this gene expression, some of these cells became new hair cells right in the organ of Corti where they belong.
PALCA: These organ of Corti is the special structure in the ear where the hair cells pick up sound vibrators and send them to the brain. The fact that they were in the right place was good news, but the bigger question was: Were they working? You can't ask guinea pigs if they hear, so Rafael did the next best thing.
Dr. RAPHAEL: We used a method called ABR; that stands for auditory brainstem responses. And in response to sound, they were actually able to generate neuro responses that are indicative of hearing.
PALCA: In other words, at least some hearing was restored. The new research appears in the journal Nature Medicine. Hair cell researcher Jeffrey Corwin of the University of Virginia is impressed by Raphael's work. He says it's actually a little surprising that animals can't make new hair cells spontaneously when they're needed because a lot of other animals can.
Mr. JEFFREY CORWIN (University of Virginia): Chickens, fish, amphibians, sharks all seem to be able to replace these cells spontaneously. Mammals don't do that spontaneously, but it looks like, through this work of putting in this gene, they've been able to encourage these cells to come back.
PALCA: And Corwin says restoring hearing is remarkable.
Mr. CORWIN: But the other part that's remarkable for somebody who works with this structure is that putting in this gene seems to not only bring hair cells back but to put them back in in the proper pattern.
PALCA: In fact, says Corwin, the pictures of the regenerated hair cells are almost too good to be true, and Corwin wonders whether perhaps the original hair cells weren't all destroyed. He'd like to see the study repeated before he'll be completely convinced. Yehoash Raphael understands Corwin's doubts. He says his junior colleague, Masahiko Izumikawa, carried out the preliminary experiments.
Dr. RAPHAEL: When Masahiko came first time with the images from the scope and showed it to me, I said, 'That's impossible. There must be a mistake.' It looked too good. And then he brought another set, you know, two months later, and I said, 'No, I need to look at that.' And so I followed the whole experiment from the beginning to the end, and I saw it with my own eyes. And it really is amazing.
PALCA: But Rafael emphasizes as promising as these new results are, there's a lot of research needed before the technique will be ready for humans.
Joe Palca, NPR News, Washington.
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