Genes play an important role in congenital hearing loss, causing about 60 percent of deafness in infants. One of the most common congenital (present at birth) abnormalities is hearing loss or deafness, affecting as many as three of every 1,000 babies. Exact data is not available, but it is likely that genes also play an important role in hearing loss in the elderly.
Genes are part of your DNA and give you the traits that make you different from everyone else, your eye color and hair, for examples. Genetics is the study of genes and disorders caused by “abnormal” genes. Genes can become mutated, or changed, and this can cause disorders in our bodies. In addition to gene mutation, other causes of hearing loss at any stage of a person’s life include medical problems, environmental exposure, trauma, and medications.
Hearing loss because of gene mutation is often categorized as “syndromic” versus “non-syndromic.” In both cases, the genes can have different ways of being passed down from parents, the most common being “autosomal dominant” (meaning that if you get the abnormal gene from only one parent, you can develop the condition), or “autosomal recessive” (meaning that two copies of the abnormal gene must be present).
Syndromic means that in addition to the hearing loss, there are other disorders in the body. When hearing loss is passed down from parents, 15 to 30 percent are syndromic. Over 400 syndromes are known to include hearing loss. They can be caused by chromosomes that are not normal (having extra or missing chromosomes, or parts of chromosomes), parent passing them to child, or due to many factors working together, including genetic and environmental.
Syndromes can appear in many different ways, and some problems may be mild to severe. An example of when syndromes express themselves in different ways is Waardenburg syndrome. Within the same family, some members may have dystopia canthorum (an unusually wide bridge of the nose due to the inner angles of the eyes moved more sideways than they should be), white forelock (a white patch of hair in front of the head), heterochromia iridis (two different-colored irises or two colors in the same iris), and hearing loss, while others may only have dystopia canthorum.
Non-syndromic hearing loss happens in about 70 percent of genetic hearing loss. This means that the abnormal gene causing the hearing loss does not cause any other associated disorders. Eighty percent of non-syndromic hearing loss cases are due to autosomal recessive genes, and nearly 20 percent are caused by autosomal dominant genes. Less than two percent of cases are caused by X-linked and mitochondrial genetic malfunctions. Mitochondrial genetic disorders happen due to damaged mitochondria—the structures in each cell of the body that are responsible for making energy. X-linked gene problems occur because of disorders in the mother’s X chromosome.
Genes are highly complex and form a roadmap of sorts for the synthesis of proteins, which are the building blocks for everything in the body: hair, eyes, ears, heart, lungs, etc. Every child inherits half of their genes from one parent and half from the other parent. If the inherited genes are abnormal or defective, a health disorder such as hearing loss or deafness can occur. Hearing disorders are commonly inherited in one of four ways:
Advances in molecular biology and genetics over the past 10 years have helped scientists and doctors understand how the inner ear develops, works, and how it can be abnormal. Researchers have identified several genes responsible for deafness or hearing loss, including the “GJB2” gene mutation. As one of the most common genetic causes of hearing loss, GJB2-related hearing loss is an autosomal recessive genetic disorder because the mutations only cause deafness in individuals who inherit two copies of the mutated gene, one from each parent. A person with one mutated copy and one normal copy is a carrier but is not deaf. Screening tests for the GJB2 gene are available for at-risk people to help them determine their risk of having a child with hearing problems. Additionally, panels to screen for genetic hearing loss have been developed if you or your healthcare provider suspect that there may be a genetic cause for the hearing loss.