Lab Notes: The Biology of Deafness
Gregoret, Lydia M., Volta Voices
When it comes to the biology of hearing loss, sensory hair cell regeneration and the connexin 26 gene often share the limelight. The former holds promise for restoring hearing, and the latter is the most common inherited form of deafness.
This research update focuses on other causes of deafness, which, though less common, still affect tens of thousands each year. Ototoxicity - poisoning of the hearing system by medicines and other chemicals - is the fourth leading cause of hearing loss. It will likely be the first to benefit from new preventive medicines and treatments, as the number of individuals affected presents a potentially lucrative market for the pharmaceutical industry. This is evidenced by the fact that a recent issue of the trade journal Drug Discovery Today was dedicated to hearing loss (Volume 10, number 19, October 2005).
Knowledge of the cause of their child's hearing loss has brought closure and understanding for many families whose children are deaf due to mutations in the connexin 26 gene. Now, some families whose children have been diagnosed with auditory neuropathy or with CHARGE syndrome may also gain that sense of understanding. Described here are the discoveries of two new genes associated with these disorders.
Ototoxicity: More Common Than You Think
Hearing loss caused by ototoxicity is the most common acquired form of hearing loss among children and young adults between the ages of 3 and 25. Every year over 80,000 Americans develop hearing loss due to ototoxic drug exposure, which permanently damages the sensory hair cells in the inner ear. Even greater numbers are affected in countries where ototoxic drug use is more widespread, particularly in the Asian Pacific Rim countries.
Awareness of the dangers that certain ototoxic medicines pose to our hearing is very good within the medical community, and patients given these drugs are carefully monitored. However, prescribing ototoxic drugs is sometimes unavoidable because for some life-threatening diseases like cancer or meningitis, there is currently no other treatment to save patients' lives.
The drugs most often responsible for hearing loss are the class of antibiotics called aminoglycosides. These include gentamicin, streptomycin and other related compounds. Repeated exposure to these drugs has a cumulative effect; unfortunately, as highly drug-resistant strains of tuberculosis become more common, aminoglycoside use will rise along with its side effects. The anti-cancer drugs in the cisplatin family are also ototoxic and cause about as many new cases of hearing loss in the United States each year as the aminoglycoside antibiotics.
Fortunately, there has been a great deal of progress in understanding how aminoglycoside antibiotics and other chemicals poison our hearing at the cellular and molecular levels. Many of these exciting developments are described in AG Bell's soon-to-be-published monograph issue of The Volta Review. The monograph, guest edited by Peter Steyger, Ph.D., of Oregon Health Sciences University, features research on ototoxicity. "There is a great deal of excitement in the area of ototoxicity research. We've had a number of breakthroughs in the last few years and expect some major ones within the next year or two," said Steyger. He and his fellow researchers hope that a higher level of understanding will result in the design of new treatment protocols that minimize or completely avoid hearing damage.
Aminoglycoside antibiotics damage the sensory hair cells and spiral ganglion neurons of the cochlea. It appears that they are selectively absorbed into the cochlear fluids and then into the sensory cells themselves. Once in the cells, the aminoglycosides enter and attack the mitochondria the parts of the cell where nutrients and oxygen are converted to a form of energy the cell can use. If the mitochondria stop being able to produce energy, they can burst and release many oxidative enzymes and reactive oxygen-containing molecules into the cell interior. …