An extreme form of inherited deafness has been cured by gene therapy.

The technique, demonstrated in mice, could pave the way to life-changing treatments for humans born with gene defects that affect hearing and balance, scientists believe.

In a groundbreaking experiment, scientists in the US used a laboratory-made virus to deliver corrective DNA into the inner ear.

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The treated newborn mice were affected by Usher syndrome, a rare genetic disorder that causes deafness, disturbed balance and blindness in humans.

After the therapy, a number of the profoundly deaf animals could hear well enough to respond to whispered sounds. Their sense of balance was also greatly improved.

"Now, you can whisper, and they can hear you," said Dr Gwenaelle Geleoc, from Boston Children's Hospital, who led the research.

Nineteen out of 25 mice heard sounds quieter than 80 decibels, about twice the normal volume of a human conversation. A few picked up sounds as soft as 25-30 decibels, demonstrating a normal level of hearing.

The treatment targeted the ush1c gene, whose mutated form is the cause of Usher syndrome.

Defective ush1c disrupts the function of hair cells in the cochlea, the snail-like inner ear structure where sound vibrations are converted to nerve signals.

Sound waves transferred from the ear drum cause the microscopic hairs to move, triggering nerve impulses to the brain.

The gene therapy virus, known as Anc80, was able to "infect" a hard-to-reach subset of the cells called "outer hair cells" that effectively act as a pre-amplifier.

Dr Geleoc said: "This strategy is the most effective one we've tested. Outer hair cells amplify sound, allowing inner hair cells to send a stronger signal to the brain. We now have a system that works well and rescues auditory and vestibular function to a level that's never been achieved before."

The research, reported in the journal Nature Biotechnology, also showed that the treatment restored balance, which is controlled by a different type of hair cell.

Mice with dysfunctional balance were able to stay longer on a rotating rod without falling off and stopped moving erratically.

Co-author Dr Jeffrey Holt, also from Boston Children's Hospital, said: "This is a landmark study. Here we show, for the first time, that by delivering the correct gene sequence to a large number of sensory cells in the ear, we can restore both hearing and balance to near-normal levels."

Further work is needed before the technology can be brought to patients, the researchers said. They pointed out that the mice were treated straight after birth and when the therapy was delayed by 10 to 12 days, it failed to work.

Usher syndrome leads to blindness over time by causing the light-sensing cells in the retina to deteriorate.

Although the research did not test for vision restoration, gene therapy for eye disorders is already at an advanced state of development.

Dr Luk Vandenberghe, another member of the team from Harvard Medical School, who helped create the Anc80 virus, said: "Progress in gene therapy for blindness is much further along than for hearing and I believe our studies take an important step toward unlocking a future of hearing gene therapy."

British expert Professor Alan Boyd, President of the Faculty of Pharmaceutical Medicine, said: "This is a very encouraging result, particularly as this treatment has been shown to improve both hearing and balance aspects of the disease, but it is only in a mouse model.

"One of the biggest risks is that the new synthetic viral vector has not been given to humans yet. it is not known how humans may react to this new vector from an immunological standpoint and this needs to be investigated fully.

"Much more work needs to be done before it could be given to a patient and that's at least three years away, if not more."