Deafness can be caused by the loss of sensory hair cells in the ear or by damage to the associated neurons that are responsible for transmitting sound signals to the brain. The study used a model of hearing loss in gerbils that is similar to a human condition known as auditory neuropathy.
In people affected by auditory neuropathy, sound enters the inner ear normally but the transmission of signals from the inner ear to the brain is impaired owing to damage to the auditory neurons. This type of deafness is thought to affect up to 15 per cent of the population across the world with profound hearing loss.
The team developed a method of generating both sensory hair cells and auditory neurons from human embryonic stem cells. They found that when they transplanted the neurons into deaf gerbils, they could obtain a functional recovery in response to sound of around 46 per cent in some animals.
The team used a technique called auditory brainstem evoked responses (ABR), which measures whether the brain can perceive an electrical signal after sound stimulation. They found that an improvement was evident about four weeks after administering the cells.
The responses of the treated animals were substantially better than those untreated, although the range of improvement was broad. Some subjects did very well, whereas recovery was poor in others.
Dr Marcelo Rivolta from the Centre for Stem Cell Biology at the University of Sheffield, who led the project, explains: “We believe this is an important step forward. We have now a method to produce human cochlear sensory cells that we could use to develop new drugs and treatments and to study the function of genes. And, more importantly, we have the proof-of-concept that human stem cells could be used to repair the damaged ear.”
The researchers point out that more research is needed before this therapy could be applied to humans, however. Dr Rivolta adds: “We want to understand the long-term implications of this treatment and its safety.
“Moreover, while in auditory neuropathy patients that retain their hair cells the sole application of stem cells could be beneficial, those with more comprehensive damage may need a cochlear implant to compensate for the hair cell deficit. In these patients, it is possible that stem cells should be administered in combination with a cochlear implant. It is therefore important to explore this interaction.”
Dr Ralph Holme, Head of Biomedical Research for Action on Hearing Loss, said: “The research we have funded at the University of Sheffield is tremendously encouraging and gives us real hope that it will be possible to fix the actual cause of some types of hearing loss in the future. For the millions of people for whom hearing loss is eroding their quality of life, this can’t come soon enough.”
Cochlear implants offer a partial solution for deafness that is caused by loss of hair cells. However, there is currently no treatment available for the loss of sensory neurons that transmit sound information to the brain. This new approach could offer a solution to a wider range of patients if used in combination with cochlear implants, say the researchers.
The study was funded by Action on Hearing Loss, the Wellcome Trust, Medical Research Council and Deafness Research UK.
Image: The hearing organ, or cochlea, of a guinea pig. Credit: Dr David Furness, Wellcome Images.
Chen W et al. Restoration of auditory evoked responses by human ES-cell-derived otic progenitors. Nature 2012 (epub ahead of print).