According to the World Health Organization (WHO), over 5% of the world’s population, roughly 360 million people, including 328 million adults and 32 million children, have disabling hearing loss, which refers to hearing loss greater than 40dB in the better hearing ear in adults and greater than 30dB in the better hearing ear in children. WHO tells us that most of these people live in low and middle income countries in South Asia, Asia Pacific and sub-Saharan Africa. In the US, the Centers for Disease Control and Prevention (CDC) have found that approximately 36 million American adults report some degree of hearing loss and that two to three out of 1,000 babies born in the United States each year have a detectable hearing loss, which can affect their speech, language, social, and cognitive development. With these alarming statistics both internationally and locally in the US, the National Institutes of Health (NIH) has embarked upon a program to cure. deafness.

The Problem

Auditory physiology suggests that the ability to hear relies on hair cells, small sensory cells within the cochlea of the inner ear. NIH (2015) presents that these hair cells are named for microscopic hair-like extensions, called stereocilia, projecting from their tops in bundles. These “hair bundles” convert sound vibrations into electrical signals, which travel to the brain by way of the auditory or hearing, nerve. When hair cells are damaged—by disease, injury, or aging—a person experiences hearing loss, and sometimes these hearing losses can be profound. Although fish, amphibians, and birds are able to grow new hair cells to replace damaged ones, mammals cannot regenerate hair cells on their own. Through gene therapy and stem cell research, however, scientists have been able to grow new hair cells in laboratory animals, in some cases restoring some hearing to deafened mammals. Such promising results have led researchers to wonder if we might be able to regenerate hair cells in people one day.

It has been known for some time that:

Scientists have made a number of critical discoveries about hair cell growth and development that are contributing to new ideas about the possibility of regenerating hair cells in people one day.
Knowing that supporting cells have the ability to become hair cells under certain conditions, scientists focused on identifying specific molecules that might be involved in a hair cell developing from a supporting cell.
Studying experimental animals, scientists have identified genes that are necessary for hair cell formation and function. In some cases, they found that these genes were similar even though they were from different animals. For example, a gene in mice, called Atoh1, is necessary to make hair cells and a similar gene is required for hearing in fruit flies.
NIH-supported scientists treated deafened guinea pig ears with a harmless virus carrying the gene Atoh1. The Atoh1 gene caused supporting cells to become hair cells in the deafened guinea pigs and, importantly, the treated animals were able to regain some of their hearing. This was the first demonstration of gene therapy that improved hearing in formerly deaf animals.
NIH-supported scientists have identified a gene in mice, called Rb1, that shuts down the growth of new hair cells early in development. Mice bred to be missing the gene were able to grow more hair cells than mice possessing the gene. In addition, mature hair cells growing in culture dishes were able to regenerate when the Rb1 gene was deleted.
NIH-supported scientists have demonstrated that mouse embryonic stem cells can develop into functional, immature hair cells in the laboratory. The ability to re-grow hair cells will not restore hearing without properly reconnected nerve endings. NIH-supported scientists found that newly formed hair cells and nerve cells successfully reestablish connections in an organized way, although the reconnected nerve endings are simpler than those generated during normal development. This and other research will help reveal how nerve cells form connections with newly generated hair cells.