A team of US scientists used gene transfer to produce functioning hair cells in mice that the inner ear needs to interpret sounds.

Team leader John Brigande from Oregon Health and Science University used unborn mice to prove gene therapy can be used to encourage cells to become hair cells reports the BBC.

The results are published in Nature.

Normal hearing involves cochlear hair cells converting sound into electrical signals, which are then transmitted to the brain.

If those cells are damaged – or lost – they can’t be replaced.

The Deaf Association of New Zealand says there were over 204,000 adults with some level of hearing impairment in the most recent survey in 1996, although there was no differentiation in the survey between people with progressive hearing loss and those who can’t hear anything.

However, it’s estimated there are between 2500-7000 people who identify as deaf.

Most people lose their hearing gradually with age - partly because of hair cell loss in the cochlea, while prolonged loud noise exposure also damaged hair cells.

To attempt to treat this loss, gene therapy inserted copies of the relevant gene, Atoh1, into cells via a harmless virus, which then replicated.

Cells with Atoh1 functioned the same as original hair cells.

"This capability is a crucial first step in defining translational therapies to ameliorate the effects of inner-ear disease in humans," the BBC reports the researchers saying.

While transferring the technology to humans is not yet imminent, the research suggests a way to repair a cochlea without electrical or mechanical aids, such as the cochlear implants currently used that bypass damaged hair cells to directly stimulate the auditory nerve.