The hair cells, found in the inner ear, are connected to a tuning membrane, which in turn is connected to sensory neurons that conduct sound waves directly to the brain. Although some people are born without these hairs, a condition that causes deafness, many people also suffer a loss of these hairs from infection, aging, exposure to loud noises and certain medications.
"The loss of hair cells is irreversible, along with the deafness caused by it," said Yehoash Raphael, an associate professor of otolaryngology at the UM Medical School in Ann Arbor.
Although research done during the late 1980s proved gene therapy could be used to grow non-sensory hair cells in the ears of chickens, until now, scientists have been unable to grow auditory hair cells in mammals. This demonstration, Raphael said, brings the possibility of achieving auditory hair cell growth in humans.
Raphael and colleagues used adult guinea pigs to implant a gene called Math 1 surgically into the animals' inner ear fluid. The Math 1 gene is an embryonic gene, which determines the type of a premature cell.
After implantation, auditory hair cells began to grow in previously hairless areas of the guinea pigs' inner ears. The researchers also found evidence that sensory neurons began growing toward the hair cells.
The additional development is critical, Raphael explained, because without connection to the neurons the hair cells are useless. However, he added, the research has yet to demonstrate that the neurons and hair cells have achieved normal connection and function.
More than 30 million Americans suffer from hearing impairment and the new approach could be a major step toward restoring their sense of sound, said Doug Cotanche, associate professor of otology and laryngology at Harvard Medical School in Cambridge, Mass.
"Until now gene therapy has not worked that spectacularly in the ear," he added.
Cotanche said the next step in the research will be growing auditory hair cells at specific locations in the ear to stimulate the sense of sound. Although the UM team grew hair successfully in the inner ear, the auditory hair cells need to grow directly on the inner ear organ called the cochlea. This is the snail-shaped tuning membrane of the ear connected to the sensory neurons.
The fluid surrounding the cochlea stimulates movement of tiny projections of the hair cells. This creates electrical signals that are picked up by auditory nerve fibers and carried to the brain.
A surgical process called cochlear implantation already exists that can safely restore a child's or person's ability to hear. From 70 percent to 80 percent of infants who receive the surgical process and finish proper rehabilitation can attend normal schooling, said Dr. Daniel J. Lee, a pediatric cochlear surgeon at the University of Massachusetts Memorial Medical Center in Boston.
However, "it would be great if you could reverse or prevent hearing loss with gene therapy so they would not need a cochlear implant," Lee said.
Susan Fiorillo, of Worcester, Mass., has a 12-year-old son who would benefit from the gene therapy treatments for exactly this reason.
Her son has lived with hearing loss since his birth and it has progressively gotten worse with time. At a young age he received the cochlear transplant, but due to surgical complications he lost his hearing completely in one ear.
Another surgical implantation of a new cochlear was done successfully last April on Fiorillo's son's other ear, but nothing could be done for the surgically damaged one.
"I'm not sure yet about the potential complications of gene therapy, but if it is proven to be safe I would much prefer the growth of healthy hair cells," Fiorillo said. "He lost that one ear forever and it would be wonderful if he would have two good ears instead of one."
Cotanche cautioned, however, that clinical use of gene therapy is still 20 years away. The research is its preliminary stages and before the treatment could go into clinical use it would need testing on humans and approval by the Food and Drug Administration, he said.
In addition, the danger of side effects remains. The gene therapy performed on the guinea pigs resulted in damage to the animals' cochlear organ.
The UM team's research is published in the June 1 issue of the Journal of Neuroscience.
(Reported by Michael Hoffman, UPI Science News, in Washington)