Deafness is an unusual disability, in that some deaf people would dispute that they have a disability at all.
Those individuals maintain that there is a deaf culture which is equal to hearing culture, and the problems of deaf individuals stem solely from the refusal of hearing culture to accept the equality.
Deaf individuals do have a rich subculture that includes a language, complete with dialects, and a top-notch university, Gallaudet.
The vast majority of the deaf and hearing impaired population consists of either deaf children born to hearing parents, or those who lose their hearing as they age. Those age-related individuals do not live in a deaf culture, and suffer deeply from social isolation, as well as from often being mistaken for less intelligent than they are due to communication problems.
In society, the deaf population as a whole is at a disadvantage compared to the hearing. Gallaudet notwithstanding, literacy rates are low among the deaf, with the average deaf high school graduate reading at about a 4th grade level. That leads to a cascade of other disadvantages, including low average education and income levels.
The most common cause of hearing loss is the loss of hair cells, the cells that turn sound waves into neural impulses in the inner ear. Such hearing loss can have many causes, ranging from the hereditary to exposure to loud noises or infections. Both those with born and acquired hearing loss might one day benefit from new research published in the March 2005 issue of Nature Medicine, though senior author Yehoash Raphael, associate professor at the University of Michigan, cautions that clinical applications are years away.
In the paper, scientists from the University of Michigan and Gaithersburg, Md.-based biotech company GenVec Inc., along with Japanese colleagues from Kansai Medical University in Osaka and the Kumamoto University School of Medicine, report on a new gene therapy that is able to regenerate hair cells. Raphael also pointed out that the work builds on a lot of existing technology and knowledge in the fields of molecular biology, gene therapy and hearing research.
"It's not like we just stood up one morning and did this," he told BioWorld Today.
The scientists treated deafened guinea pigs with Atoh1. Expression of Atoh1, which used to be called Math1, during development leads to the formation of hair cells, and previous research had shown that in vitro and in normal inner ears, it is possible to induce the formation of new hair cells by overexpressing Atoh1. The new research showed it was possible in damaged inner ears, as well, and that such overexpression can restore hearing thresholds.
The scientists first deafened guinea pigs in both ears and confirmed the destruction of hair cells by staining, as well as hearing loss by electrophysiological techniques. After a few days, the newly deaf animals were treated with Atoh1 gene delivery, via an adenoviral vector, to one ear. The scientists then checked for Atoh1 gene expression, hair cells in the treated inner ear, and brainstem responses to sound at varying times after the treatment. They found that Atoh1 gene expression was widespread by a few days after gene delivery; eight weeks after the treatment, new hair cells had formed.
In electrophysiological experiments, the treated ear was once again responsive to sound. Untreated ears showed no hair cell regeneration, and brainstem on the untreated side remained largely unresponsive to sound, showing that neither the cells nor hearing regenerated spontaneously via endogenous repair mechanisms.
By far the most likely source of the new hair cells is the existing supporting cells in the inner ear. After the treatment, there are new hair cells and fewer supporting cells in the inner ear, suggesting that the supporting cells have changed their phenotype. Stem cells are a theoretical possibility, but a very unlikely one. "Stem cells have never been demonstrated in this part of the auditory system; there is no evidence for them," Raphael said. "They may be present in remote areas and migrate, but that is a speculation at present."
Raphael envisions treating acquired hearing loss first, though both adult-onset and congenital hearing loss can ultimately be targeted with the technology. Genetic deafness will be more complicated to treat.
"In hereditary deafness, you also need to address genetic issues; many times, hair cells die due to a gene mutation, so if you induce a new hair cell to grow, it will just die again." Potentially, both Atoh1 and the normal version of the gene that causes the deafness could be delivered, but that is more complicated.
Raphael also pointed out that the research might actually be applied to the balance system first. Loss of balance is also caused by hair cell problems, and while many people with hearing loss benefit from hearing aids or cochlear implants, "for disturbance of the balance system, right now, there no treatment whatsoever," he said. "And it is at least as devastating, if not more devastating, than hearing loss."