There are many causes of hearing loss. Exposure to noise for an extended length of time is one of the most common, as is a disease, age-related deterioration, and the toxic effects of certain medications. Underlying these causes is another factor that can make us more susceptible to losing our hearing: our genes. Does hearing loss run in the family? Let us find more about genetic hearing loss.
What exactly are genes?
Genes are the microscopic strips of coding that are copied trillions of times over to make up the DNA in every single cell in your body. The DNA in each cell contains about 25,000 genes. Each gene’s code acts as a set of instructions that determine a cell’s structure, what it does, and where it goes in the body. We get all our genes from our parents as a unique combination of both our mother’s and father’s genes. That is, 99.9% of your DNA is exactly the same as your fellow humans; it is that vital 0.01% that makes us unique and interesting. Part of our genetic makeup has the recipe to guide the development of our ears and hearing ability. So far over 100 genes have been identified as being important specifically to hearing. These genes exist in every one of your cells but are only used in the ear. Many other genes have functions in other areas as well as hearing. For example, the eyes and inner ears share a gene that has a particular function in each.
What goes wrong?
When cells are being made and the genetic code copied over, certain mutations or errors can occur that change the gene’s instructions. When this happens, whatever function that gene has will be changed, often in ways that could cause harm to the person. This new code will then be copied over and over as the person develops and ultimately passes it on to their children (and then their grandchildren and great-grandchildren etc). Certain factors will make this inheritance more or less likely to occur. Where the gene is positioned on our DNA, the type of gene, and sometimes even if we are male or female will affect how likely we are to inherit it. For example, most genes are either recessive or dominant. If a hearing-related mutated gene is recessive, you would need to have a copy of that gene from both your mother and your father in order for that gene to cause a genetic hearing loss. If the gene is dominant, you would need only one copy from either your mother or father for it to result in hearing loss.
How often does this happen?
Between 1 and 3 in 1,000 babies are born with hearing loss and approximately 60% of those babies’ hearing loss is inherited. In adults, 30–50% of people over 60 years have significant enough hearing loss that it affects their daily life, increasing to 80% of adults over 85 years old. As we get older, research has found that of our over 100 hearing-specific genes, at least 30 of them contribute to age-related hearing loss. It is estimated that where there is a family history of hearing loss, you may be 25–50% more likely to also develop hearing loss as you get older. However, it is virtually impossible to say exactly to what extent age-related loss is due to your genes and what is due to the environment and natural deterioration. Particular environmental factors that interact with our hearing genes include noise exposure and ototoxic medications.
Genes, drugs, and loud music
Ototoxic medications refer to any drug that causes damage to the ear and hearing. Some examples are antibiotics called aminoglycosides (often used to treat tuberculosis and cystic fibrosis) and a chemotherapy chemical called cisplatin. These drugs behave in such a way in the body that they gain entry into the highly protected inner ear and damage its sensitive cells needed for hearing. The damage can be almost immediate and cause an almost complete loss of hearing, or it can be more gradual, often affecting the high frequencies first. For these people, it is vital that their hearing thresholds are regularly monitored.
Genes play their role in this too. An inherited gene mutation has been found that places people at a significantly greater risk for developing hearing loss as a result of taking medication containing aminoglycosides. One study found that 17–33% of people with ototoxic-related hearing loss also had that particular gene mutation. Similarly, research studies have indicated a link between certain genes and susceptibility to noise-induced hearing loss. Very loud and prolonged sounds can cause structural damage to the inner ear, reduce blood flow to the ears and cause an increase in toxic stress hormones in the inner ear, all of which result in hearing loss.
Other genetic hearing disorders
For the most part, genetic diseases causing hearing loss are non-syndromic, meaning that they only affect the hearing. While most of the disorders discussed here affect the inner ear, one of the most common ear diseases is called otosclerosis which affects the middle ear. It is characterized by abnormal growth of the three tiny bones in the middle ear which restrict their movement, reducing their ability to transmit sound vibrations into the inner ear. A dominant gene has been identified that contributes to the development of the disease, along with environmental factors.
About 30% of genetic hearing loss is caused by a syndrome, which means that the hearing loss occurs along with at least one other disorder such as blindness, kidney, heart, or thyroid problems. Usher’s syndrome is one such example. It is caused by recessive gene mutations that are important for hearing, balance, and vision, leading to hearing loss, balance problems, and blindness. While in most cases the syndrome is present from birth, some variations only become apparent from late childhood into adulthood.
Overall, genes contribute in some way to almost 50% of all hearing loss cases. Some genetic hearing losses will show themselves clearly from the beginning, while others will only become apparent with the right environmental input. In any case, whether hearing loss runs in your family or not, protecting your hearing from noise, being aware of the effects of medications, and understanding that your hearing will likely change with age is vital to ensuring that any future hearing loss does not go unnoticed.