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A  pioneering treatment has been hailed as a dramatic step toward a cure for children who are born deaf.

Infant deafness has genetic causes more than half the time – and is largely incurable.

But a successful test of a gene editing treatment that reversed hearing loss in mice offers hope for human treatments.

Harvard University researchers used a technique called Crispr CAS-9, a method of modifying DNA in living cells. In mice born with a condition that makes them progressively more deaf the technique turned off a deafness-causing variant of a gene called TMC1.

This is significant because humans who carry the same gene variant also suffer progressive deafness. The Crispr technique alters genetic material by using a virus, enclosed in a blob of liquid, to alter DNA.

The virus was injected into the mice’s cochleas – a part of the inner ear which picks up sounds.

This altered the DNA, allowing hair cells that pick up sounds to grow normally. The researchers, publishing their findings in the scientific journal Nature, found the treatment ‘substantially reduces progressive hearing loss’.

After eight weeks, hair cells in treated ears resembled those in healthy animals – densely packed and tufted with hairlike bundles. The hair cells of untreated mice, in contrast, looked damaged and sparse.

On average, after four weeks, treated ears could hear sounds about 15 decibels lower than untreated ears – the difference the authors said between being able to hear a quiet conversation and a loud radio.

Ralph Holme, director of research at the charity Action on Hearing Loss, said: ‘Inherited deafness is thought to account for more than half of all childhood deafness and can have a massive impact.

‘There are over 100 genes that can cause childhood deafness, but we don’t yet have treatments able to modify any of these genes to prevent hearing loss.

‘That’s why the research showing that it is possible, using a specific protein, to silence one of these genes in mice to reduce hearing loss is such a big deal for those who are seeking treatments.

‘The approach could be used to modify other deafness genes, although it is best suited to progressive forms of deafness that develop after birth. Many types of genetic deafness affect the structures within the ear before birth, which create extra challenges in terms of getting any treatment into the ear at the right time.’

Fyodor Urnov, a leading hearing researcher, said the study was ‘an essential first step toward moving this type of approach nearer the clinic by providing evidence that it is safe and effective in an animal that has a similar genetic mutation and comparable loss to those in humans’.

Commenting in Nature, Dr Urnov said the breakthrough was a ‘reason for optimism’ but further tests would be needed.

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