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The genetics of hearing loss

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There is a wide variation in the causes of deafness. Because of this 9 out of 10 deaf children are born to hearing parents and 1 out of 10 children born to deaf parents are also deaf. With current tests, it is only possible to determine the probable cause of their deafness for about half (40-50%) of children.

In older age, mild hearing loss is much more common and, to date, less is known about how genes contribute to this, though there is research in this area. More is known about childhood deafness and about half is thought to have a genetic cause. Other common causes are premature birth, infections in the mother during pregnancy for example, rubella (german measles) or cytomegalovirus (CMV) or infections after birth for example, mumps or meningitis. Of those cases with a genetic cause, one third has a syndromic link where the deafness is found with other signs or health problems. The other two thirds have deafness alone. This is summarised in the diagram below

A genetic change may have been inherited from further back in the family or may have occurred in that individual for the first time. Many genes are involved in the development and function of the ear and, at the time of writing, 45 recessive, dominant and X-linked genes that cause non-syndromic deafness have been pinpointed and another 128 loci (identified positions in the genome, but where the gene itself has not yet been found) have been determined. This number is growing regularly.

One gene has been found to account for between a third and a half of recessive deafness where there is a further family history e.g. more than one deaf brother or sister. This is the Connexin 26 gene, also known as Cx26, DFNA3, DFNB2 or GJB2. Connexin 26 is a protein that forms channels to regulate the passage of potassium ions in and out of the cells of the cochlea (the inner part of the ear that changes sound waves into nerve signals to send to the brain). Six connexins fit into a hexagonal shape to make one connexon with a hole in the middle. Two connexons join together to make a channel.

Connexin 26 is the only gene causing non-syndromic deafness that can be routinely tested for.

Most of these genetic causes of deafness affect the inner ear and the mechanisms that change sound waves in the air into the electrical signals that are passed to the brain through the hearing nerves. This type of deafness is known as sensorineural hearing loss (SNHL). Sometimes, the cause of the deafness may be the outer or the middle ear not developing as usual. More recently, another, rarer, cause of deafness has been identified. This is called auditory neuropathy and occurs when the nerve from the inner ear to the brain has not developed completely or is not working as usual. This type of deafness is thought to sometimes have a genetic cause, but less is known about it at the time of writing.

The many different causes of deafness show how complex it can be to pinpoint the cause in one family. This is particularly true if both members of a couple are deaf as the cause of their deafness may be the same, but is more likely to be different. This also means that there can be a lot of complicated genetic ideas and terminology to explain during an appointment. If your first or preferred language is Auslan or a spoken language other than English, you should ask before the appointment for an accredited interpreter to be present and preferably one who has experience of interpreting for genetic counselling appointments. Most, if not all, clinical genetics departments will have experience of booking interpreters on the family’s behalf.

Genetic testing is only one part of identifying the cause of hearing loss and it is not yet possible to test for changes in the majority of genes that can cause deafness. A consultation with a genetics professional will involve putting together all the information available through the family history, examination and other tests such as heart and kidney checks as well as any relevant genetic tests.

If it is not possible to identify a specific cause of the hearing loss in the family, the genetics professional may still be able to discuss the chances of another child being born with or developing a hearing loss based on population (or observational) studies:

  • If there is no family history of hearing loss, the chance of a hearing couple who already have one child with hearing loss having another deaf child is 1 in 8
  • If one parent has a hearing loss and the other does not, the chance of them having a deaf child is 1 in 20
  • It is more difficult to give an approximate chance if both parents are deaf themselves and you would need to ask to see a genetics professional.

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Information provided by the National Deaf Children’s Society. Reproduced with permission.

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