Scientists hope it may be possible to treat age-related hearing loss by stimulating the growth of new hair cells in the inner ear.
The ear gradually loses hair cells
A US team led by Harvard Medical School has discovered that deletion of a specific gene can lead to the proliferation of new hair cells.
The loss of hair cells is the most common cause of hearing loss in older people.
The research is published in the magazine Science.
Hair cells in the cochlea detect sound by vibrating in response to sound waves, triggering nerve impulses that travel to the auditory region of the brain.
Normally, humans are born with a complement of about 50,000 hair cells.
But since the cells do not regenerate, the steady rate of hair-cell loss that can accompany aging produces significant hearing loss in about a third of the population by the time they reach 70-years-old.
Hair cell loss can be caused by disease, certain drugs and the general cacophony of modern life.
The researchers examined gene activity during the embryonic development of the inner ear and concluded that there might be a gene that produces a protein which acts as a permanent brake on hair cell regeneration.
One gene in particular - the retinoblastoma (pRb) gene - seemed to be particularly active.
Mice bred to lack this gene showed signs of problems with the working of their inner ear because they had too much hair.
Further examination showed that they had more hair cells than normal mice - and the cells were actively proliferating.
Tests showed that these cells functioned normally.
The researchers then knocked out the retinoblatoma gene in mature inner ear cells from mice in the lab - and found that this triggered the cells to begin proliferating.
Researcher Dr David Corey said: "Deletion of this gene can allow functioning hair cells to continue to divide.
"They are no longer limited by whatever growth controls existed before.
"This work gives us an invaluable window into the control mechanism, which could lead to eventual clinical application in regenerating lost hair cells."
Further work needed
However, Dr Corey warned that more research was needed to refine the technique.
Simply deactivating the key gene might lead to the uncontrolled cell growth that can lead to cancer.
The key will be to switch off the gene just long enough to generate enough hair cells to restore hearing, and then to turn it back on again to avoid possible complications.
Ralph Holme, biomedical research manager at the charity RNID, said the findings presented an exciting prospect for millions of people affected by age-related hearing loss.
"The discovery that deleting the pRb gene 'switches on' the ear's ability to continue generating hair cells is a significant step towards the goal of restoring hearing.
"But there are still many problems to solve before this type of treatment reaches the clinic.
"To prevent excessive proliferation, reversible ways of controlling pRb activity need to be found.
"Further research is also needed to show that these regenerated hair cells actually improve hearing following damage to the cochlea.
"The hair cells need to be integrated into the rest of inner ear and connected to the auditory nerve before they are useful."
It is estimated that over 50 million people in Western Europe and the USA have age-related hearing loss.