US and Japanese researchers have used gene therapy to restore the hearing of deaf guinea pigs, an achievement they say is 'the first time anyone has biologically repaired the hearing of an animal'. The team, based at the University of Michigan in Ann Arbour, delivered a gene to the inner ear of the guinea pigs, which triggered the growth of crucial 'hair cells'. The findings, published in the journal Nature Medicine, are a step towards new treatments for people with acquired hearing loss.
Humans have around 16,000 hair cells in the cochlea of each inner ear. The 'hairs', tiny projections on the outside of the cells, detect sound waves and convert them to electrical nerve impulses to send to the brain. Hair cells are easily damaged by loud noises, aging, infections and some antibiotics, and cannot regenerate themselves.
To find out if they could use gene therapy to replenish hair cells, the team first gave the guinea pigs high doses of antibiotics, which destroyed the delicate inner ear cells and left the animals profoundly deaf. They then used a virus to deliver a gene called Atoh1, which is known to control hair cell growth in developing embryos. The gene was injected into the left inner ear of the guinea pigs, while the right was left untreated. After two months, the animals were still deaf in their right ears, but their left ears showed signs that some hearing had been restored.
The technique worked better than anyone had expected: 'The recovery of hair cells brought the treated ears to between 50 and 80 per cent of their original hearing thresholds', said team leader Yehoash Raphael. When they examined the inner ear, the researchers saw regenerated hair cells. However, the outer hair cells, responsible for filtering different sounds, did not grow back. 'What they hear is probably pretty distorted', said Raphael.
The team are also studying animals that have been deafened by other means, older animals, and animals that have been deaf for a long time. This is because if the technique is eventually used to treat deafness in people, then it is likely that most of the potential patients would have been deaf for a long time, says Raphael. He also cautions that before it can be used in humans, the technique will have to be refined, since the human inner ear is not nearly as accessible as it is in rodents.
Another US team, based at the Massachusetts Eye and Ear Infirmary in Boston, are attempting to use chick embryonic stem cells to grow hair cells in the laboratory. Team leader Stefan Heller welcomed the latest study: 'There are now at least two possibilities for the development of a cure for deafness. It is highly likely that both approaches or a combination of those will find their way into the clinic within the next decade', he told New Scientist.