Genetic cause of hearing loss discovered, may lead to cure for deafness

PHILADELPHIA, Pa. — While many people will lose their hearing due to a traumatic injury or other causes, some are unfortunately born without the ability to hear sound. In the United States, between two and three out of every 1,000 children are born with hearing loss in one or both ears. Now, a genetic breakthrough has revealed the cause of this condition. Researchers from University of Pennsylvania say the findings may lead to gene therapies which can reverse genetic deafness.

Scientists identified a mutation that causes hearing loss in babies. The protein, named GAS2, fuels cells that boost sound waves and maintains the “structural stiffness” of the inner ear.

The study finds mice lacking this protein lost their amplifying ability, triggering severe hearing impairment. Researchers add people with GAS2 variants also suffer the same fate. The breakthrough, reported in the journal Developmental Cell, could be a game changer for living with hearing loss.

“Anatomists 150 years ago took pains to draw these support cells with the details of their unique internal structures, but it’s only now, with this discovery about GAS2, that we understand the importance of those structures for normal hearing,” says senior author Professor Douglas Epstein in a university release.

According to the CDC, between 50 and 60 percent of hearing loss cases among infants are genetic in origin. Hearing aids and cochlear implants often help, but the devices seldom restore hearing completely.

Hearing ‘buckles’ under stress without the right genes

Prof. Epstein’s lab focuses on genes that control the development and function of the inner ear; often implicated in congenital deafness. The inner ear contains a snail-shaped structure called the cochlea that “turns up” the volume by increasing vibrations from sound waves. It changes them into nerve signals that are sent towards the auditory cortex of the brain.

In experiments, scientists used genetic engineering to “knock out” GAS2 in mice, leaving them virtually deaf. The deficits at high sound frequencies was up to 50 decibels, equivalent to a loss of 99.999 percent of the normal acoustic energy.

Further analysis revealed GAS2 stabilizes support parts called pillar and Dieters’ cells in the inner ear. If the protein is not functioning properly, these cells weaken, causing dire consequences for the patient’s hearing.

The cells also form the basic structure of the cochlea and serve as the foundation of outer hair cells. These hairs move in response to incoming acoustical vibrations, providing crucial amplification of sound energy. Loss of stiffness in the inner ear, due to the absence of Gas2, dramatically degrades the sound-amplifying properties of the outer hair cells.

“We observed that some of Deiters’ cells in the Gas2-knockout mice even buckled under the tension of the rapid movements of the outer hair cells,” Prof. Epstein explains.

Not all genetic hearing loss will be permanent now

The tests included sophisticated imaging of propagating sound waves in the inner ears of mice without working GAS2 and normal mice. Curiously, the researchers could find no reports of GAS2-associated congenital hearing loss in medical literature. Even when they canvassed colleagues around the world who run deafness clinics, they came up empty-handed.

Then, Prof. Epstein heard from Dutch experts who had been studying four siblings with severe hearing loss from early life. The Somalian family members had no mutations in known hearing-loss genes, but each carried two mutant copies of GAS2. It is the first known case to affect the mechanical properties of inner ear support cells.

“In many genetic hearing loss conditions, the affected cells are permanently damaged or die, but in this one, the affected cells are intact and conceivably could be restored to normal or near-normal by restoring GAS2 function,” the professor of genetics at Penn Medicine says.

Future gene therapies open the door to restoring hearing loss in early childhood. Epstein adds it also offers hope for patients in which inherited mutations have lead to a slower development of deafness in adulthood.

SWNS writer Mark Waghorn contributed to this report.

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