The breakthrough could pave the way for gene-based treatments.
Gene therapy delivered by a benign virus enabled deaf lab mice to hear for the first time, researchers say, offering hope for people with genetic hearing impairments.
The breakthrough could pave the way for gene-based treatments, they reported in two studies, published in Nature Biotechnology.
"With more than 100 genes already known to cause deafness in humans, there are many patients who may eventually benefit from this technology," said Konstantina Stankovic, a professor at Harvard Medical School.
Genetic hearing disorders affect some 125 million people worldwide, according to the World Health Organization.
An expert not involved in the research welcomed the findings as "very encouraging", but cautioned the technique has yet to be proven safe, and that human trials are likely years away.
In the first study, Stankovic and colleagues used a harmless virus to transport -- deep into the mouse ear -- a gene that can fix a specific form of hereditary deafness.
Previous attempts had failed, but this time the viral package was delivered to the right address: the so-called outer hair cells that "tune" the inner ear to sound waves.
"Outer hair cells amplify sound, allowing inner hair cells to send a stronger signal to the brain," explained Gwenaelle Geleoc, a researcher at the F.M. Kirby Neurobiology Center at Boston Children's Hospital.
The technique bestowed hearing and balance "to a level that's never been achieved before," she said in a statement.
"Now you can whisper, and the mice can hear you."
In the second study, a team led by Geleoc used the same viral courier to treat mice with a mutated gene responsible for Usher syndrome, a rare childhood genetic disease that causes deafness, loss of balance, and in some cases blindness.
The virus carried a normal version of the same gene to damaged ear hair cells soon after the mice were born.
Narrow time window
The results far exceeded anything to date: 19 of 25 treated mice heard sounds quieter than 80 decibels. Normal human conversation is about 70 decibels.
A few of the mice could hear sounds as soft as 25 to 30 decibels -- roughly equivalent to whispering.
According to Margaret Kenna, a specialist in genetic hearing loss at Boston Children's Hospital not involved in the studies, "cochlear implants are great, but your own hearing is better."
Electronic implants work by bypassing damaged hair cells in the ear to send sound signals directly to the brain.
"Anything that could stabilise or improve native hearing at an early age would give a huge boost to a child's ability to learn and use spoken language," she said.
The need for early intervention, however, could be a problem in itself, other experts pointed out.
In humans, such an intervention would ideally have to happen before a child is born, said Jonathan Ashmore, a professor at University College London's Ear Institute.
Alan Boyd, president of Britain's Faculty of Pharmaceutical Medicine hailed "a very encouraging result".
"But it is only a mouse model," he cautioned, noting that it is still unknown how the human immune system might react.
Any gene deafness treatment is "at least three years away, if not more," Boyd conjectured.