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Vol. LVIII, No. 19
September 22, 2006

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It Takes Two
Hearing Loss Study Has NIDCD Scientists Doing Double Duty

On the front page...

The line-up of volunteers looked like a Hollywood casting call for colorful character actors. There were bead-bedecked cowgirls, whiskered men in sailor hats and 84-year-old Southern belles toting lavender lace parasols. And everyone — everyone — showed up with his or her own double

But this was no casting call. For a fifth consecutive year, the National Institute on Deafness and Other Communication Disorders took part in the world's largest annual gathering of twins, this time to learn more about the genetics behind age-related hearing loss, or presbycusis. Nearly 2,000 sets of twins, triplets and perhaps higher multiples attended the legendary Twinsburg, Ohio, festival-Twins Days-which celebrated its 31st anniversary this past August. The study is the first to definitively address an observation that most hearing health professionals and researchers have made but have yet to prove: that people tend to lose their hearing as they age and that this type of hearing loss seems to run in families.


Twins of all ages show up each year at Twins Days.

"Hearing can decline over the years from a variety of factors, including noise, chemotherapy, ear infections, head injuries and overall health," said Dr. Carmen Brewer, NIDCD's chief of audiology and lead investigator on the study. "And hearing loss can also occur in families. We want to obtain a more precise picture of the role heredity plays in hearing loss as a person ages."

Brewer and her research team are looking to twins to help sort out this question because they can offer something the rest of us can't: a clone. Monozygotic, or "identical," twins result when a fertilized egg divides in half to form two embryos, which means that they possess identical DNA. Dizygotic, or fraternal, twins result when two eggs are fertilized by two sperm. They are no more alike genetically than any other sibling pair — sharing roughly 50 percent of their DNA. If a type of hearing loss (or any other disorder) is strictly genetic, one would expect monozygotic twins to be alike 100 percent of the time and dizygotic twins to be alike approximately half the time.

In the study, the research team is administering hearing tests to identical and fraternal twins ages 50 and up, and then comparing results between twins to see if their hearing abilities are highly correlated. If twins tend to have the same hearing status, statistical analyses will determine the extent to which their shared genes account for the similarities. Other NIDCD scientists contributing to the study are Drs. Andrew Griffith, Thomas Friedman, Robert Morell and Dennis Drayna.

Dr. Carmen Brewer
Dr. Carmen Brewer, NIDCD's chief of audiology

"We know that there's an environmental component to presbycusis and we know there's also a genetic component. What we're trying to figure out is how much each of these factors contributes to a person's hearing loss," said Brewer. The findings might one day help people who know that they carry a gene for age-related hearing loss to take early steps to help limit or even prevent its effects.

Of course, testing a person's hearing at the world's largest twins event can be rather difficult, what with the bagpipe music playing in the nearby talent tent and the P.A. system booming the names of siblings who are most alike, oldest and farthest traveled, or who are wearing the best costume. In addition, because the research team was testing the hearing of older individuals, who have a higher incidence of hearing loss in comparison to young people, it was more important than ever to filter out competing outside noises.

To address these concerns, Brewer and the NIDCD scientists, along with doctoral students from the University of Maryland and Gallaudet University, set up a high-tech testing facility not unlike what one would encounter on a typical trip to the audiologist. In contrast to previous years, where testing was conducted in a rented trailer, the team was permitted to install a temporary clinic in a brick municipal building on the festival grounds — complete with two soundproof booths, audiometers and other instruments and areas for counseling.

Twins sigining up for testing
Twin volunteers fill out their medical histories.

"We basically set up a base camp 350 miles away from home that functioned at the same high quality as our facilities on campus," said Chris Zalewski, an NIDCD audiologist who served as both heavy-equipment hauler and hearing tester that weekend.

When a pair of twins showed up for their scheduled appointment, the researchers would take a medical history of each, then would conduct the hearing test and provide follow-up counseling. They also took cheek swab samples to determine whether the pair was monozygotic or dizygotic. (This isn't always obvious, incidentally. Last year, two 19-year-olds who had considered themselves fraternal twins were elated to discover they are identical.) By the end of the weekend, 26 sets of twins had been tested. The researchers plan to participate in Twins Days for at least 3 more years to test a grand total of 100 sets of twins. Brewer is also considering adding two more soundproof booths to improve efficiency.

When asked what made the effort such a success, Zalewski remarked that the outcome was facilitated in large part by the extremely gracious and accommodating people on the Twins Days staff. “Because of them, we were able to operate like a well-oiled machine,” he said.

Brewer agrees. “That,” she said, “and the way the volunteers walked away — audiogram in hand — after receiving their hearing exam and counseling. They forgot that they were participating in a study. They saw this as something that was beneficial to them.”

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