Why deaf mice can hear

PAPER CUTS on fingers can draw blood, but the skin cells soon regenerate and seal the injury. The hair cells in the inner ear that detect sound, however, do not regenerate when they are damaged, causing hearing loss.

In early January, American researchers reported that for the first time, they had been able to regenerate the hair cells in deaf mice, partially restoring their hearing. The team from Massachusetts Eye and Ear and Harvard Medical School reported their findings in the Jan. 10 issue of journal Neuron.

“We’re excited about these results because they are a step forward in the biology of regeneration and prove that mammalian hair cells have the capacity to regenerate,” said the study’s senior author Albert Edge of Mass. Eye and Ear in a statement. “With more research, we think that regeneration of hair cells opens the door to potential therapeutic applications in deafness.”

Regained partial hearing

The researchers treated the cochleas, the organ in the inner ear that detects and processes hearing, in deaf mice with a drug that turned off certain cell processes. Flipping this biological switch caused some of the cells that normally surround hair cells to turn into hair cells themselves, which allowed them to restore some hearing function in those areas so that the deaf mice regained partial hearing.

Further experiments will be required to see if the preliminary findings in mice can be applied to humans as the results could benefit a lot of people. According to the World Health Organization in 2012, some 5 percent of the global population or an estimated 360 million people have  “disabling” hearing loss, defined as the inability to hear sounds softer than 40 decibels (dB). Conversational volumes generally fall within a range of 55-65 dB, a hair dryer runs at 90 dB and 15 minutes of sustained exposure to noise levels over 100 dB can damage hearing. About a third of all people over the age of 65 have disabling hearing loss, and roughly 15 percent of the world’s adult population has some degree of hearing loss.

A more recent study suggests another incentive from treating hearing loss in older people. In the report published online Jan. 21 in the journal Jama Internal Medicine, researchers analyzed six years worth of results from over 2,000 volunteers between the ages of 70 and 79 who had annual hearing tests and also took several tests that measured memory, concentration, orientation and other cognitive skills.

Hearing loss

“Our results show that hearing loss should not be considered an inconsequential part of aging, because it may come with some serious long-term consequences to healthy brain functioning,” said the study’s senior investigator Frank Lin of Johns Hopkins University School of Medicine in a statement.

Lin and his team found that the cognitive test results for those study volunteers who had hearing loss declined 30 to 40 percent faster—some three years sooner—than the results for the volunteers with normal hearing. “On average, individuals with hearing loss would require 7.7 years to decline by 5 points on … a commonly accepted level of change indicative of cognitive impairment… vs 10.9 years in individuals with normal hearing,” they wrote. They also said the results match those from previous studies that have found associations between hearing loss and brain function.

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