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Hearing
impairment has a major impact on affected individuals, greatly influencing
language acquisition, speech and communication, psychosocial development,
and cognitive skills. More than 60% of the cases of profound early-onset
hearing loss are caused by genetic factors, which in most cases are due
to mutations in genes. The term "nonsyndromic" is used to describe
hearing loss that occurs in isolation, in contrast to "syndromic"
hearing loss, in which hearing loss is one of several symptoms of a particular
disorder. There are at least 80 genes associated with nonsyndromic hearing
loss, of which 30 have been identified. Over 400 different syndromes of
which hearing loss is a feature have been defined.
In addition to genetic factors, another significant cause of hearing loss
is congenital infection with cytomegalovirus (CMV), which occurs in up
to 1% of newborns. The hearing loss associated with this infection is
progressive.
Further understanding of the disease
There are three areas
of research related to hearing loss at the Baylor College of Medicine
Mental Retardation Developmental Disabilities Research Center. One area
is an investigation into the genetic causes of hearing loss, another is
a study of the development of inner ear structures, and a third is an
investigation into the hearing loss associated with CMV infection.
Researchers at Baylor College of Medicine are working with families throughout
the world that exhibit an inherited hearing loss. By looking closely at
what DNA (deoxyribonucleic acid) features are associated with the symptoms
of hearing loss in these families (compared to other family members with
normal hearing), investigators can progressively narrow down the region
of DNA that contains the gene responsible for the hearing loss. Once the
region is small enough, then genes that are known to be in the area are
studied further until a direct correlation is found. Good candidates for
hearing loss genes include those that look similar to hearing loss genes
that are already known, and also genes whose function would be involved
in hearing or that express their product in the cochlea.
In addition to searching for new genes that are associated with hearing
loss, researchers are studying genes that have already been identified
and the clinical effect in patients when these genes are defective. Careful
documentation of symptoms, family history, and the effectiveness of different
treatments, in addition to DNA testing, is leading to a correlation between
genetic status and the type and degree of hearing loss. Further development
of this correlation will lead to information that will be helpful in the
assessment of patients regarding their diagnosis, prognosis, and treatment
options.
Tiny cells in the ear, called hair cells, serve to conduct sound waves
into signals that can be processed by the brain and perceived as sound.
Two genes that are found in fruit flies, called atonal and senseless,
have been shown to be involved in the generation and function of hair
cells in the ear. Investigators at Baylor College of Medicine are studying
these genes further in both fruit flies and mice, and also seeking other
genes that interact with or are affected by atonal and senseless, or that
are involved in other aspects of inner ear development.
When congenital infection with CMV occurs, symptoms may or may not appear
at birth. However, even patients who were symptom-free as newborns are
at risk for disabilities later in life, the most common of which is hearing
loss. A clinical study at Baylor College of Medicine is following subjects
who were found to be infected with CMV at birth, and is carefully documenting
the appearance and severity of symptoms that occur later in life. New
subjects are still being enrolled, and others have been in the study for
up to 20 years so far. It has already been found that these children shed
the CMV virus for many years, and that those who shed the virus for a
shorter period of time are at greater risk for hearing loss progression.
Work towards therapy
As more genes associated with nonsyndromic hearing loss are identified,
and the symptoms associated with the disruption of each of these genes
are characterized, then diagnostic and therapeutic tools become more powerful.
For instance, knowledge of a particular hearing loss gene in a family
allows genetic testing of children as early as during pregnancy, and consequently
the opportunity for early treatment intervention. Another advantage is
the opportunity to correlate the most effective treatments with a particular
genetic alteration. This would allow a more efficient use of resources
and improved patient management. Furthermore, as additional hearing loss
genes are identified and studied, a greater understanding of the mechanism
of hearing will develop, which may lead to novel treatment strategies.
The studies of genes that are involved in the production of hair cells
in the inner ear of mice, and a better understanding of this process,
may provide valuable information as to how such hair cells might be generated
or regenerated in cases where they are missing or have been damaged.
Baylor investigators
working on Hearing
loss:
Baylor MRDDRC projects associated with Hearing
loss:
Organizations:
Disease information:
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