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Genetics of Hearing Loss

Posted on January 12, 2014 at 9:50 PM
Genetics of Hearing Loss 


Though inherited genetic defects are just one factor that can lead to hearing loss and deafness, which may occur at any stage of a person’s lifespan, this is often overlooked as one does not consider probability in this. 

50%-60% or 3 in every 1,000 infants born who suffer from hearing loss have obtained this through DNA. Although exact data is not available for the aging it is likely that genetics play an important role in hearing loss for the elderly as well. Other factors may include: medical problems, environmental exposure, trauma, and medications. 

Inherited genetic defects play an important role in congenital hearing loss. The majority of hearing loss is caused by mutations ("misspellings") in the DNA (deoxyribonucleic acid) sequence of genes. There are a number of other factors in the environment that can cause hearing loss too. 

25% or more of hearing loss in babies is due to "environmental” causes such as maternal infections during pregnancy and complications after birth.  Sometimes, both genes and environment work together as a team to cause hearing loss. For example, there are some medicines that can cause hearing loss, but only in people who have certain mutations in their genes. 

There are four compounds known as "bases" in a strand of DNA: adenine (A), guanine (G), thymine (T), and cytosine (C). Each DNA molecule consists of two strands that form the shape of a double helix, through the pairing of A with T, and C with G. Thus, knowing the sequence of bases on one DNA strand automatically gives the sequence on the other strand of the double helix. 

Genes contain the instructions that tell the cells of people’s bodies how to grow and work. For example, the instructions in genes control what color a person’s eyes will be. There are many genes that are involved in hearing. Sometimes, a gene does not form in the expected manner. This is called a mutation. Some mutations run in families and others do not.  If more than one person in a family has hearing loss, it is said to be "familial". That is, it runs in the family. If only one person in the family has hearing loss, it is called "sporadic". Meaning, it does not run in the family. About 70% of all mutations causing hearing loss are non-syndromic. This means that the person does not have any other symptoms. 

About 30% of the mutations causing hearing loss are syndromic. This means that the person has other symptoms besides hearing loss.  For example, some people with hearing loss are also vision impaired or blind. 

The cochlea (the part of the ear that changes sounds in the air into nerve signals to the brain) is a very complex and specialized part of the body that needs many instructions to guide its development and function. These instructions come from genes. Changes in any one of these genes can result in hearing loss. The GJB2 gene is one of the genes that contains the instructions for a protein called connexin 26; this protein plays an important role in the functioning of the cochlea. In some populations about 40% of newborns with a genetic hearing loss who do not have a syndrome, have a mutation in the GJB2 gene. 

Diagnosis/testing:
Genetic forms of hearing loss must be distinguished from acquired (non-genetic) causes of hearing loss. The genetic forms of hearing loss are diagnosed by otologic, audiologic, and physical examination, family history, ancillary testing (e.g., CT examination of the temporal bone), and molecular genetic testing. Molecular genetic testing, available in clinical laboratories for many types of syndromic and nonsyndromic deafness, plays a prominent role in diagnosis and genetic counseling. 

Genetic  counseling: 
Hereditary hearing loss can be inherited in an autosomal dominant, autosomal recessive, or X-linked recessive manner, as well as by mitochondrial inheritance. Genetic counseling and risk assessment depend on accurate determination of the specific genetic diagnosis. In the absence of a specific diagnosis, empiric recurrence risk figures, coupled with GJB2 and GJB6 molecular genetic testing results, can be used for genetic counseling

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