4 – Genetics of Hearing Loss

Geneticists and other researchers have discovered that hearing loss can be caused by a multitude of underlying genetic changes. Significant  hearing loss at birth occurs in about  3 per 1,000 newborns, and among babies in neonatal intensive care the rate is about 1 to 2 per 200 — a much higher incidence. About 60% of all congenital hearing loss is genetic. Within that group, about 75% are due to autosomal recessive changes, 20% to autosomal dominant  differences, 4% are X-linked, while the remaining 1% or so are “other.” Another way to look at congenital and genetic hearing loss is that about 30% is due to syndromic causes and the other 70% is non-syndromic. A syndrome is characterized as having multiple symptoms co-occur while non-syndromic means that only the auditory system is affected. Within the non-syndromic class 70% are recessively inherited and 30% are dominantly transmitted.

Congenital hearing loss can also be caused by non-genetic events/factors such  as reduced oxygen flow, meningitis, ototoxic drugs, cerebral hemorrhage and blood type incompatibility, among others. Many of these non-genetic conditions can cause hearing loss in older children and adults as well.

Even before the explosion of knowledge about genetics with the Human Genome Project, it was known that hearing loss could be, though wasn’t always, a symptom in over 400 syndromes. A few of the syndromes in which hearing loss appears most frequently through dominant genetic transmission are: Waardenburg Syndrome, Branchio-Oto-Renal, Neurofibromatosis Type II (NFII), Stickler Syndrome and Treacher-Collins Syndrome.

The most common recessively inherited syndromes in which hearing loss is more common are: Usher Syndrome, Alport Syndrome, Jervell and Lange-Nielson Syndrome and Pendred Syndrome. A few syndromes that less often have hearing loss are: CHARGE Syndrome (CHARGE is an acronym and each letter stands for a different piece of the syndrome; the ‘E’ is for the ears), Stapes Fixation (which is X-linked) and several conditions resulting from genetic changes in the mitochondria, the energy producers of cells, which actually have a 37-gene DNA structure separate from each cell itself.

See more information on the genetics of hearing loss and syndromes.

In addition to particular mutations or differences in DNA that result in a syndrome, there are a variety of changes in DNA that result in hearing loss being the sole consequence. The one change in DNA that accounts for approximately 50% of all congenital hearing loss is in genes that code for proteins within the cochlea called Connexins.  Individuals with hearing loss from Connexin changes may have anything degree of hearing loss from mild to profound.  In most cases the changed Connexin occurs as a recessive trait, that is, both parents must pass the changed DNA on to a child in order for that change to be expressed as hearing loss. However, there are now known cases in which Connexin-based hearing loss is known to be passed to the next generation as a dominant trait, which means that only one parent need contribute the altered DNA in order for the child to have a hearing loss. As of summer 2011, more than 100 different mutations to the Connexin genes, which result in hearing loss, have been identified.

The amount of research which has been performed on the genetics of hearing loss is astounding. Although earlier studies exist, the earliest study listed in the government’s database Pubmed, is from 1946 entitled: Studies on the inheritance of deafness in the pupils of the Clarke School for the Deaf. One example of a paper from 2011 is: Hereditary hearing loss: From human mutation to mechanism. During this 65-year period, but especially during the last 20 years, advances in technology and knowledge about DNA and genetics has afforded geneticists and genetic counselors the opportunity to offer a much greater amount of information to families. That is not to say that every family is interested in pursuing the specific etiology of the hearing loss. However, because of this new amount of information families can make much better informed choices for what is right in their particular situation.

See new findings on the genetics of hearing loss.

One interesting aspect of genetic hearing loss lies in the fact that there are certain conditions in which hearing loss does not appear until the teenage years, young adulthood, or middle-age. This fact might be quite important in your role as a vocational rehabilitation counselor in knowing that an individual can develop hearing loss seemingly overnight as an adult. In such a situation, most of these individuals will be quite anxious to obtain personal amplification such as hearing aids and perhaps other types of assistive devices, because they are used to operating in “the hearing world.”

Source: [Schaefer BG. (2011). Genetics of Hearing Loss. Presentation to Genetics of Hearing Loss Class, University of Arkansas for Medical Sciences, June, 2011.]