Deafness
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Deafness or “hearing loss” is the total or partial inability to hear or to clearly understand sound. It is estimated that nine million people in the UK, 1 in 7 of the population, suffer from deafness or significant hearing difficulty1. In 2005, about 278 million people had moderate to profound hearing impairment, according to the World Health Organization2. The origin of hearing loss in the majority of this population is thought to be impaired function of the inner ear3, usually associated with damage of hair cells.
>How do researchers know if animals can hear or not?
>Future: treatment and research
There are two main types of hearing loss: sensorineural and conductive. Conductive or mechanical deafness results from a blockage in the outer or middle ear that stops the sound passing to the inner ear, and can be caused by excess ear wax, ear infections or as a result of abnormalities in the structure of the ear. Conductive hearing loss can often be corrected with medical management or minor surgery.
Sensorineural hearing, the most common cause of deafness, is often the result of damage or loss of cells within the inner ear. These sensory cells or nerve endings, known as hair cells, are responsible for detecting the sound vibrations that pass from the outside via the eardrum and the bones of the middle ear, to the cochlea where the hair cells are situated. These cells function as a microphone, converting the sound waves into electrical signals that can be understood and processed by the brain.
One of the most common causes for this deafness is the ageing process (more than half those over the age of 60 have hearing loss4), but there are other factors such as exposure to loud sounds, toxic drugs (including some antibiotics), infectious diseases, birth complications or injuries. Genetic predisposition is also important5: about one in every 1,000 babies born in the UK is moderately to profoundly deaf and genetic factors are thought to be the cause of at least half of these cases6.
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Sensorineural deafness can be treated through cochlear implant, which can bypass the damaged hair cells and stimulate the nerves. But this is only suitable for people with severe hearing loss and requires a high-risk operation. Researchers have worked on a promising alternative: the repair or regeneration of hair cells within the inner ear.
Because hair cells are so important in deafness – up to 90% of the cases of sensorineural deafness involve the loss of hair cells – and since damage to these cells is irreversible, researchers have tried to find the factors that may allow their regeneration. Unlike mammals, birds, fish and amphibians possess the ability to regenerate hair cells. To develop more effective treatments, scientists must understand first what makes the difference between mammals and other species in their ability to regenerate and repair hair cells7.
Through gene therapy and stem cell research, it has been possible to grow new hair cells in laboratory animals, in some cases restoring some hearing to deaf mammals.
In 2003, for the first time researchers regenerated hair cells in guinea pigs using gene therapy8. This experiment proved that the gene Math1 (believed to help young cells conversion into auditory cells) was able to “teach” old non-sensory cells to behave like hair cells. Yet another study, reported in February 2010, demonstrated that this gene could repair damaged hair cells for life, if applied within the first 10 days of damage.
A different gene, Atho1, has proven to be a key factor in the production of hair cells. A paper published in Nature in 20089 showed that injecting the gene into newborn mice resulted in new functioning hair cells in the right location within the ear. Although the scientists are not expecting to use it in human newborns, this technique could lead to the production of better treatments.
Recent work led by Nicholas Daudet showed that the protein Notch plays an important role in regulation of the number of hair cells that are produced following damage in the regenerating chick ear. For hair cells to work properly, they must be present in an ordered pattern within the inner ear. Signalling by Notch from one cell to another stops cells becoming hair cells, thus preventing too many forming10.
Another important area of research is the development of hair cells using embryonic stem cells. In 2009 the first human hair cells were developed in the University of Sheffield11. The researchers cultivated stem cells lines obtained from stem cells in the cochlea of discarded human foetuses. After expansion in vitro, the cells retained their capacity to become sensory hair cells and auditory neurons.
In another study with stem cells, researchers at Stanford University, California, found a way to develop mouse cells that look and act like the inner hair cells in the animal12. Using embryonic stem cells from mice and reprogrammed mouse fibroblasts, the researchers found a way to convert these cells into hair cells.
Both these studies suggest that artificial hair cells can be useful to study in greater detail how damage occurs in deafness, and also to find better ways to develop medicines to reverse it. The new research even raises the possibility of transplanting the hair cells into the ear to restore hearing.
How do researchers know if animals can hear or not?
When carrying out deafness research on animals, scientists need know how much the animals can actually hear when under test. Training the animals with a reward system, researchers can interpret the responses to hunger or thirst following exposure to sounds at different frequencies. The responses are plottedon an audiogram to produce a graph of the softest intensity at which the subject was able to detect a specific sound13.
Another way to know how much the animals can hear is by using the test known as the brainstem auditory evoked response or brainstem auditory evoked potential which detects electrical activity in the cochlea and in auditory pathways in the brain.
Future: treatment and research
Although there is not yet a definitive cure for deafness, research has demonstrated that is possible to significantly improve the quality of life on people with hearing loss, if treated in time. Treatments vary from case to case, but as science advances, researches have gained a better understanding of the hearing system and the different genetic and environmental factors that can affect the ability to hear.
One of the most promising areas of research is the regeneration of hair cells for transplantation into the inner ear, but scientists are also working on genetic tests to predict hearing loss and finding news ways to prevent hair cells damaged by understanding the molecular mechanisms that the body uses to protect and regenerate those cells in other animals. It is believed that 100 different genes are involved in deafness. Mistakes in any one of these can then result in deafness. Already more than 20 of these genes have been identified and studied in mice. For instance, Karen Steel at Wellcome Trust Sanger Institute in the UK works with genetically modified mice to screen genetic mutations and their effects on deafness14.
The improvement of cochlear implants also continues, with research groups trying to find better ways to enhance hearing and reduce the risk of the surgery involved.
June 2010
References
1. Davis, A. (1995), Hearing in Adults. Whurr: London
2. WHO, Deafness and hearing impairment, in http://www.who.int/mediacentre/factsheets/fs300/en/ (Accessed 17 May 2010)
3. Deafness Research UK, The science of sound and the mechanics of listening, in http://www.deafnessresearch.org.uk/The%2 0science%20of%20sound%20and%20the%20mechanics%20of%20listening+4357.twl (Accessed on 16 May 2010)
4. National Institute on Deafness and Other Communication Disorders, Hearing Loss, in http://nihseniorhealth.gov/hearingloss/hearinglossdefined/06.html (Accessed on 17 May 2010)
5. Deafness Research UK, Deafness fact sheet http://www.deafnessresearch.org.uk/factsheets/deafness-the-facts.pdf (Accessed on 17 May 2010)
6. The Royal National Institute for Deaf People in http://www.rnid.org.uk/information_resources/aboutdeafness/causes/genetics/< /a> (Accessed on 16 May 2010)
7. Deafness Research UK, Deafness in later life, in http://www.deafnessresearch.org.uk/factsheets/deafness-in-later-life.pdf (Accessed on 17 May 2010)
8. Kawamoto et al (2003) Math1 Regenerates Cochlear Hair Cells In Vivo J Neurosci 23, 4395
9. Gubbels, Samuel et al (2008) Functional auditory hair cells produced in the mammalian cochlea by in utero gene transfer Nature 455 DOI:10.1038/nature07265
10.Daudet et al (2009), Notch regulation of progenitor cell behaviour in quiescent and regenerating auditory epithelium of mature birds Dev Biol 326 86
11. Chen W, Johnson SL, Marcotti W, Andrews PW, Moore HD, Rivolta MN, (2009) Human fetal auditory stem cells can be expanded in vitro and differentiate into functional auditory neurons and hair cell-like cells Stem Cells 27 1196
12. Oshima K, Shin K, Diensthuber M, Peng AW, Ricci AJ, Heller S (2010) Mechanosensitive Hair Cell-like Cells from Embryonic and Induced Pluripotent Stem Cells Cell 141 704
13. Louisiana State University, How Well Do Dogs and Other Animals Hear? in http://www.lsu.edu/deafness/HearingRange.html (Accessed on 17 May 2010)
14. http://www.sanger.ac.uk/research/projects/geneticsofdeafness/ (Accessed on 10 June 2010)
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Research Fields: Anatomy & development, Disease characteristics, Brain & nervous system(yes - 3 items)Animals Used: Mouse, Guinea pig, Dog, Cat, Chicken, Ferret(required - 6 items)
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