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Release: Sept 30, 2002

Fast-paced gene discovery changes clinical practice for care of deaf patients

In 1997, researchers first identified a genetic cause of non-syndromic deafness -- hearing loss that is the patient's only abnormality. Since then, the number of genes known to cause non-syndromic deafness has risen to 29. The rapid pace of discovery has generated great excitement among genetics researchers, and has changed the way physicians diagnose hereditary deafness.

Genetic testing is now becoming a component of clinical care for deaf children and their families. At University of Iowa Health Care, clinicians in the Division of Pediatric Otolaryngology, in conjunction with the Molecular Otolaryngology Research Laboratories, provide genetic testing for 800 families each year. Results of genetic tests can provide helpful diagnostic and prognosis information to these families.

Prior to 1997, in most instances, the only way to diagnose non-syndromic genetic deafness was to exclude all other possible causes of deafness, said Richard Smith, M.D., the Sterba Hearing Research Professor at UI Roy J. and Lucille A. Carver College of Medicine, and director of the Molecular Otolaryngology Research Laboratories.

"Typically, a physician would order a large battery of tests to determine if the mother had been exposed to an illness during pregnancy that caused the baby's deafness," said Smith, who recently was part of a research team that identified a new gene which when mutated can cause deafness. "Now, we don't need to order numerous tests because we know that 50 percent of congenital non-syndromic deafness is due to mutations in a single gene called GJB2. This single finding has changed the practice of clinical medicine."

GJB2-related hearing loss is considered a recessive genetic disorder because the mutations only cause deafness in individuals who inherit two copies of the mutated gene, one from each parent. A person with one mutated copy and one normal copy is a carrier but is not deaf.

In Iowa, 90 percent of newborns are tested for hearing. A probe placed in the baby's ear canals generates sound, and electrodes placed on the baby's head record brain responses to the sound. If this test indicates that a baby is deaf or hearing impaired, the newly available genetic tests may be able to pinpoint an exact genetic cause of the deafness. The Molecular Otolaryngology Research Laboratories at the UI offers mutation screening of three genes known to cause hearing loss, including GJB2.

Many deafness-causing mutations can occur in the GJB2 gene, but one particular mutation is most prevalent in people of Northern European decent, a group that includes most Iowans.

"This mutation arose as a result of a founder effect -- one person has the mutation and passes it on to all of their progeny," Smith said. "This particular founder effect occurred approximately 8,000 years ago, and one in every 40 people in the Midwestern United States is related to that common ancestor."

Just as genetic testing has become part of diagnosing causes of deafness, so genetic counseling also has assumed an increasingly important role in the treatment of deaf patients and their families.

Smith explained that many parents find genetic diagnosis and the accompanying genetic counseling beneficial. They are reassured by the information and the by the knowledge of a specific cause of their child's deafness.

Genetic tests and counseling also can give patients and parents information about the likely progression of a child's hearing impairment

"We know that over the long term, deafness caused by mutations in GJB2 does not generally worsen. A child who has moderate-to-severe deafness caused by GJB2 mutations will have the same level of deafness at ages 10 and 20," Smith said. "That means that if the child is doing well with traditional amplification (hearing aid), they don't necessarily have to worry about further deterioration of hearing."

Current genetic tests have helped improve patient care; however, they do have limitations and subtleties that need to be recognized by health care providers and patients alike. Whereas a positive result establishes with certainty the cause of deafness and also can quantify the likelihood of deafness in future offspring, a negative test does not definitively prove that the deafness is not genetic.

Also, unlike many other medical tests, results of an individual's genetic test have direct implications for other family members. If a child of hearing parents has GJB2-related deafness, it means that both parent are carriers and the child's hearing siblings have a two-thirds chance of being carriers.

Carrier-testing is not recommended for children because carrying one mutated copy of the gene does not cause deafness and therefore has no effect on the child's life. However, the ability to test for susceptibility genes raises the larger question of whether relatives should be told the results of an individual's genetic test or whether those results should remain private as is usually the case with medical information.

Smith also notes that while hearing parents of a deaf child might seek treatment such as hearing aids and cochlear implants, the situation may be different for deaf parents of a deaf child. People who are born deaf and identify themselves with the Deaf community consider deafness an integral part of their culture, and their language (sign language) is simply another language like English or French. The Deaf community does not believe that deafness is something to be treated.

Therefore, it is important that genetic testing and genetic counseling be provided in the context of each patient's wishes and needs, Smith said.

As genetic testing becomes a routine part of clinical practice, physicians will be required to be more familiar with genetic testing and its limitations, and also to understand which patients are good candidates for testing.

"These issues are bigger than hereditary deafness; they apply to physicians irrespective of their specialty," Smith said. "The tremendous advances we are making right now in the field of genetics in all realms of medicine will be a driving force for clinicians to become geneticists, so that they can provide their patients with better care."

University of Iowa Health Care describes the partnership between the UI Roy J. and Lucille A. Carver College of Medicine and UI Hospitals and Clinics and the patient care, medical education and research programs and services they provide. Visit UI Health Care online at