In a first, scientists have used a gene editing tool to prevent progressive deafness in mice, an advance that may help cure genetic hearing impairments in humans.
The work is among the first to apply a genome editing approach to deafness in animals, said David Liu from Howard Hughes Medical Institute (HHMI) in the US.
A single treatment involving injection of a genome editing cocktail prevented progressive hearing loss in young animals that would have otherwise gone deaf, Liu said.
Since the study was performed in mice, the implications for treating humans are still unclear. "We hope that the work will one day inform the development of a cure for certain forms of genetic deafness in people," Liu said.
Nearly half of all cases of deafness have a genetic root, but treatment options are limited. That is because, until recently, researchers did not have the technology to directly treat the underlying problem: the genetic mutations that sabotage hearing.
One such mutation is in a gene called Tmc1. A single error in this gene causes the loss of the inner ear's hair cells over time.
These delicate bristle-covered cells help detect sound: sound waves bend the bristles, like stalks of wheat in the wind; hair cells then convert that physical information into nerve signals that travel to the brain.
Just one copy of a mutated Tmc1 gene causes progressive hearing loss leading to profound deafness, both in humans and in mice. Researchers surmised that destroying the mutated copy of the gene, called Beethoven in mice, might preserve some hearing.
They tapped the genome editing technology known as CRISPR-Cas9.
They injected the CRISPR-based tool into the inner ears of infant mice with the hearing loss mutation. 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.
Then the researchers measured inner ear function by placing electrodes on mice's heads and monitoring activity of brain regions involved in hearing.
Researchers needed more sound to spark brain activity in untreated mice compared with treated mice.
On average, after four weeks, treated ears could hear sounds about 15 decibels lower than untreated ears.
"That's roughly the difference between a quiet conversation and a garbage disposal," Liu said.
In humans, such a change could make a major difference in hearing-loss patients' quality of life, he said.
Scientists still have a long way to go before trying this approach in humans.
However, if applicable, the treatment could be best during childhood, Liu said. That is because hair cell loss in the inner ear is progressive and irreversible.
"The conventional thinking in the field is that once you've lost your hair cells, it's difficult to get them back," Liu said.Â
