Tuesday November 10 2009
Hearing impairment is common as people get older
Researchers have identified a gene that causes deafness in the elderly, The Daily Telegraph reported.
It said that about four in 10 people experience a reduction in their hearing ability as they age as a result of gradually losing hair cells and nerve cells in the ear that are essential for hearing.
The researchers removed a gene, called Bak, in mice and found that these mice had better hearing as they aged than mice that had the gene. The Bak gene causes the hair cells in the ear to ‘self-destruct’ as people age.
Further studies will be needed to confirm the findings in humans. There may be other genes involved in age-related hearing loss, and it is likely that environmental factors, such as noise exposure, also contribute.
Where did the story come from?
The research was carried out by Dr Shinichi Someya and colleagues from the University of Wisconsin and other universities in the US and Tokyo. The study was funded by the National Institutes of Health in the US, the Ministry of Education, Culture, Sports, Science and Technologies in Japan and the Marine Bio Foundation. Two of the researchers have filed a patent for any future treatments that use Bak inhibition for age-related hearing loss.
The study was published in the peer-reviewed scientific journal Proceedings of the National Academy of Sciences of the United States of America.
The Daily Mail and Daily Telegraph gave generally accurate and balanced reports of the study, and both said that a drug treatment making use of this genetic discovery is a long way off.
What kind of research was this?
This laboratory research examined whether a gene called Bak is involved in age-related hearing loss.
To investigate this, mice were genetically engineered so that they were missing this gene, and their hearing was then tested as they aged to see what effect this had.
This type of experiment can be informative as similar genes tend to perform similar roles in different species. However, there are differences, and this means that what is observed in mice may not be the same as what will occur in humans.
What did the research involve?
As mammals age, they gradually lose hair cells and nerve cells in their ears. These cells are essential for hearing and, as they are not replaced, their loss leads to a reduction in hearing ability. This is called age-related hearing loss (AHL). The researchers suggested that over 40% of people aged over 65 in the US have AHL.
One gene that the researchers suspected might be involved in hearing loss was Bak. This gene causes cells to self-destruct and has been found to be less active in mice with lower levels of AHL. In order to investigate the role of Bak, the researchers genetically engineered mice that were lacking the gene, and looked at the effects on the mice’s hearing and the cells in their ears. The type of mice they used would normally show AHL by 12 to 15 months of age if they were not genetically engineered.
One theory for how ageing affects cells is that reactive chemicals produced inside the energy-generating part of a cell (the mitochondria) damage DNA and proteins inside the mitochondria. This is called oxidative stress. The accumulation of this damage over time is thought to lead to the ageing of the cell, and to contribute to AHL.
Believing that AHL is at least partly caused by oxidative stress, the researchers looked at the effect of chemicals that cause oxidative stress on the Bak gene in cells taken from the cochlea, a part of the inner ear. They also looked at whether supplementing the diets of normal mice with any of 17 different antioxidants from the age of four months to 15 months reduced AHL.
What were the basic results?
The researchers found that 15-month-old mice lacking the Bak gene had better hearing than normal mice of this age. Mice lacking Bak had less loss of nerve cells and hair cells in the ear than normal mice. The researchers showed that in normal mice, more nerve cells and hair cells were self-destructing than in the mice lacking Bak.
It was also found that, in cells taken from the cochleas of normal mice, exposure to a chemical that causes oxidative stress leads to Bak being ‘switched on’, which causes the cells to die. Cells from the cochleas of mice lacking Bak were more resistant to this happening.
Feeding the mice certain antioxidant chemicals (α-lipoic acid or coenzyme Q10) reduced the activity of the Bak gene in the ear cells and slowed the development of AHL.
How did the researchers interpret the results?
The researchers conclude that their findings support the theory that mitochondria-related oxidative stress triggers Bak-induced cell death in the ear, which leads to AHL.
This study has identified the role of the Bak gene in age-related hearing loss (AHL) in mice. The gene may play a similar role in humans and further studies in human ear cells will help confirm this.
However, the Bak gene may not be the only gene involved in AHL, and it is likely that environmental factors, such as noise exposure, also contribute to hearing loss.
Due to the differences between mice and humans, it is not clear whether taking supplements containing α-lipoic acid or coenzyme Q10 (which reduced the activity of the Bak gene in mice) would help reduce AHL in humans. Studies in humans would be required to determine if this is the case.