Genetics and Hearing Loss - Did You Know There Is a Connection?
Although genetics and otolaryngology don't often come to mind as related in any way, genetics plays a role in almost every type of human health condition in one form or another. Most patients don't realize it, but even conditions as mundane as hearing loss have a genetic basis - by some estimates, up to half of all patients suffering from hearing loss owe their conditions to genetics rather than to environmental factors. Even more surprising to the average patient, genetic testing is now used to diagnose the causes of hearing loss.
Genetic disease can present itself in three different forms -chromosomal, monogenic, and complex. If large segments of one or more chromosomes occur in only one copy, or in more than the expected two copies, the condition is called a chromosomal disorder. The most well-known example of this is Down syndrome. Single-gene (monogenic) disorders are those that are caused by a mutation in a single gene, and most cases of hearing loss fall into this category. Complex genetic disorders (such as cleft palate), are caused by the interaction of several genes, the environment, and random factors.
It's common for an otolaryngologist to encounter all three types of disorder during the course of his or her practice. Chromosomal disorders are the most severe, and patients usually have both hearing loss as well as head and neck problems The majority of people who go to see an otolaryngologist for treatment are likely to have conditions that are caused by complex disorders, however, meaning a mix of genetic and environmental influences are at play.
Genetic testing for hearing disorders is relatively straightforward, although it does present possible ethical questions (such as whether the patient or his family members wish to receive information about their likely susceptibility to various diseases and illnesses). DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) have two properties that make testing easy. One is that DNA makes copies of itself easily, and the other is that copies stick to each other. For instance, if you start with a double strand of DNA and heat it up, the two strands will come apart into a mixture of single-stranded nucleic acid strands. You can then cool the mixture down and the complementary strands will stick back together again. This allows a type of process called polymearase chain reaction (PCR) to work - it's a method used to amplify targeted sequences of DNA. Although it involves more steps than those described here, the DNA polymearase chain is heated and cooled many times until the DNA fragment has been duplicated enough to use in testing - it's the manufactured DNA that is actually used in testing. Almost every modern genetic test in use today begins with the process of PCR.
To put this technology to work in a clinical setting, an otolaryngologist usually begins by testing the patient's DNA for known mutations. Screening for mutations that are already recognized to occur is usually faster and less expensive than testing to see whether a given patient has some brand-new or never-before-seen mutation. However, if nothing shows up after testing for previously identified mutations in humans, further testing for unknown mutations is also possible. Some mutations are so rare they have only been documented in one family or one patient! At this time, genetic technologies are used almost exclusively as a diagnostic tool - and most of the time the focus remains on disorders caused by major genes. If a patient is discovered to have a mutation responsible for causing hearing loss, head or neck deformities, etc., then there is no further need for clinical testing to determine the cause. In the case of hearing loss, this can save time as well as money.
An understanding of the human genome has opened a vast frontier of new knowledge to scientists, patients, and physicians alike. Without a doubt, new treatments and tests will continue to become available in the future and will continue to influence both otolaryngology as well as other medical specialties.
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