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Of Hearing Loss in Mice and Men

Progressive hearing loss, a common but poorly understood condition, is related to a newly discovered gene that affects the sensory hair cells in the inner ear.

Nothing makes Today In Mice happier than to see humans and rodents working on parallel tracks to crack science's toughest challenges.

So we were delighted to see that a pair of recent studies, one on humans and one on mice, has come to the same conclusion: Progressive hearing loss, a common but poorly understood condition that effects one in 500 children by the age of 10, is related to a newly discovered gene that affects the sensory hair cells in the inner ear.

The first research team, comprising scientists from the Hospital Ramón y Cajal in Madrid, Spain, followed families who displayed hearing loss. The second, led by researchers from the Wellcome Trust Sanger Institute in Cambridge, England, focused on mice that showed progressive hearing loss from an early age. They are both published in the journal Nature Genetics; the mice study is here and the human study is here.

"We were able quite quickly to show that if the mice carried one copy of the gene variant, they suffered progressive hearing loss; if they carried two variants, they were profoundly deaf," said professor Karen Steel, principal investigator at the Wellcome Trust Sanger Institute, in a press release announcing the findings. "The important questions were could we determine what the variant is and how does it exert its effect on hearing?"

Both research teams began sequencing all the genes in the equivalent regions of humans and mice that have been implicated in hearing loss; most of the genes in the region, the sequencing showed, were not really responsible for hearing loss. But that's when both groups stumbled across a mutation in a microRNA gene that was associated with the hearing loss.

"We know of a number of genes involved in deafness in humans and mice but, to our great surprise, this was one of a new class of genes called microRNAs," added professor Miguel Angel Moreno-Pelayo, senior author on the human study in Madrid. "No one has seen a disease-causing mutation in the mature sequence of a microRNA. This is the first microRNA gene associated with hearing impairment and, remarkably, it is the first to be associated with an inherited disorder."

The mutation could be further examined in mice; when the rodents had two copies of the mutant gene, their hair cells were malformed from birth and the cells degenerated quickly. In mice with only one copy, the effects were less immediately severe, but grew worse over time. In the human studies, two families showed mutations of the microRNA gene, meaning that deafness in two different organisms were both tied to equivalent genes.

The researchers are hopeful that understanding the genetic mechanism that leads to progressive hearing loss will help in the development of therapies to lessen the effects of deafness, whenever or however it is triggered.

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