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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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