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June 2, 2003

Gene Therapy Could Be a Hearing Aid

From: Betterhumans, Canada - Jun 2, 2003

Gabe Romain
Betterhumans Staff

Age-related hearing loss and even deafness may be treatable using gene therapy.

Since the discovery in the late 1980s that birds can spontaneously regenerate damaged auditory hair cells, scientists have been trying to find a way to induce the replacement of the cells in mammals.

Normal hearing is made possible with the help of the cochlea. The cochlea is a spiral-shaped, fluid-filled tube in the inner ear that resembles a snail shell.

The cochlea contains nerve endings essential for hearing. When we hear something, sound waves are transferred to cochlear fluid, which stimulates hair cells lining the outside of the cochlea. These stimulated hair cells initiate electrical signals, which are sent to the auditory cortex.

Hearing loss and deafness is often the result of damaged auditory hair cells. Aging, infection, exposure to loud noises and even certain types of medication can cause sensorineural hearing loss , a condition affecting more than 30 million Americans.

Auditory hair cell regrowth

University of Michigan scientists have now used gene therapy to promote new auditory hair cell growth in adult guinea pigs.

"Non-sensory epithelial cells in adult guinea pig cochlea can generate new sensory hair cells following the expression of a gene called Math1, and some of these hair cells can attract the growth of new fibers from auditory neurons," says study director Yehoash Raphael, associate professor of otolaryngology at the University of Michigan.

"During the embryonic stage of an animal's development, hair cells and supporting cells have a common origin. Cells that express Math1 are fated to become hair cells, while Math1 expression is inhibited in the remaining non-sensory cells," Raphael says.

"After embryonic development, hair cell production ceases. Unlike other epithelial cells in the skin or gut, epithelia in the inner ear contain no stem cells, so there is no source for renewal," Raphael explains. "That's the main reason why hair cell loss is permanent."

Over-expressing gene

When Raphael and his team caused the over-expression of the Math1 gene in non-sensory cells of the mature cochlea, they found that the cells switched into auditory hair cells.

Such transdifferentiation is a major source of new auditory hair development in birds, but the scientists were unsure if it would work in mammals.

The next stage of research for the scientists is to determine whether the induced hair cells are functional. They also plan to repeat the procedure using aging and deaf animals.

"This is just the beginning," says Raphael. "With proper therapy, these non-sensory cells have the competence to become hair cells."

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