After 11 years of intensive research, scientists at the University of Michigan Medical School have succeeded in using gene therapy to grow new auditory hair cells and restore hearing in deafened adult guinea pigs, a major step forward in the search for new ways to treat hearing loss in humans.

Results from the study, the first to demonstrate restoration of auditory hair cells at the structural and functional levels in mature living mammals appears in Nature Medicine's advance online publication Web site.
Hair cells are the sensory cells of the auditory and balance organs in the inner ear. Auditory hair cells reside in the organ of Corti, which is part of the cochlea – a spiral-shaped bony organ in the inner ear. They get their name from the numerous microscopic hair-like projections that grow from each cell.

When sound waves reach the inner ear, they cause these projections to move. This triggers electrical signals, which are picked up by auditory nerve fibers and carried to the brain. If hair cells are damaged or missing, the connection between sound waves and the brain's auditory processing center is broken, making it impossible to hear.

Aging, infections, certain medications, autoimmune diseases, and exposure to loud sounds can destroy the delicate hair cells, leading to irreversible sensorineural hearing loss. a condition affecting millions of people worldwide.

For years, scientists have been searching for a way to regenerate functioning hair cells.

Yehoash Raphael, Ph.D., an associate professor of otolaryngology at U-M's Kresge Hearing Research Institute, who directed the U-M study, credits advances made by other scientists worldwide for his team's success. “Progress in gene delivery methods and in understanding of the molecular mechanism that controls hair cell development facilitated the experimental approach used by our group,” Raphael says.

“We inserted a gene called Atoh1, a key regulator of auditory hair cell development, into non-sensory epithelial cells that remain in the deafened inner ears of adult guinea pigs, whose original hair cells were destroyed by exposure to ototoxic drugs,” Raphael explains. “Eight weeks after treatment, we found new auditory hair cells in the Atoh1 -treated ears of the research animals. Auditory tests indicated that the generation of new hair cells coincided with restoration of hearing thresholds.”

In future research, Raphael plans to test Atoh1 treatment in aged animals and animals deafened by noise exposure, rather than drugs. He also wants to determine if the treatment is effective months or years after the original hair cells have degenerated.