What Twins Are Teaching the Rest of Us

Medically Reviewed by Dan Brennan, MD on October 05, 2016
From the WebMD Archives

Oct. 5, 2016 -- When Scott Kelly blasted off in 2015 to spend more than a year aboard the International Space Station, his twin brother, Mark, had to stay behind.

But the earthbound twin might have taught scientists as much about the effects of space on the human body as his brother. For a year, the identical twins had the same battery of tests, a first-of-its-kind effort to study how a long period in space affects an astronaut, compared with a twin with an identical genetic makeup. The lessons could help scientists understand what happens to the human body on even longer flights; say, for instance, a trip to Mars.

It may be the most dynamic, but the NASA study is only one of many medical, psychological, and behavioral research projects on twins in recent years. Twins offer a way to see how having identical genes affects everything from heart disease and cancer to behavior and longevity in two people with the same DNA but different lives. And scientists are using this to make some surprising discoveries, not just of interest to the 32 out of every 1,000 people in the world who are a twin, but to all of us.

Twins have been studied for years, and previous research involving twins has shown that genetics may play a role in eating disorders, osteoarthritis, cataracts, and even back pain, among other conditions.

Recent twin research has revealed medical surprises, including a very strong link between one twin developing cancer and the other’s cancer risk; that an overweight twin was no more likely than the thinner one to suffer heart problems; and the possibility that the social experience of having a twin is similar to marriage when it comes to a longer lifespan.

Sacrifices for Science

If Scott Kelly felt like a human guinea pig, it was understandable.

NASA hadn’t sent an astronaut into space for such a long period since the 1990s, and nobody had ever had a chance to study the effects of long-term space flight on twins, says John Charles, PhD, chief scientist of NASA’s Human Research Program. So when scientists made twin research a key part of Kelly’s yearlong mission, NASA was flooded with research proposals. Among those selected were studies on how spaceflight affects gene changes in cells in blood, saliva, urine, and stool; how it affects the body’s immune system; gut bacteria; and what happens to different organs.

For the astronaut, it meant poking himself with needles, swabbing his cheek, saving and cataloging his pee and poop, and taking quizzes and many other tests, all while keeping the space station running, warding off scurvy, and spending 2.5 hours a day exercising on the walls and ceiling. For his twin, himself a retired astronaut and pilot, it meant regular visits to NASA labs for similar poking and prodding, though in a much less controlled setting and at fewer intervals, since Mark Kelly is not an employee and was volunteering, says Charles. Scott Kelly returned to Earth in March after 520 days in space.

Charles acknowledged the study’s limitations, and the incredibly small sample size. Still, understanding the role genetics plays in how humans respond to conditions in space is key. While NASA scientists have decades of experience in seeing how unrelated people are affected by spaceflight -- and the differences can vary widely from person to person -- having a genetically identical control group in Mark Kelly is new territory.

Cancer, Heart Disease, Depression, and other Conditions

Meanwhile, for the rest of us who will never go into space or visit Mars, twins-based research is fueling more revelations in medical science.

In the Scandinavian countries of Sweden, Denmark, Finland, and Norway, public health officials have been tracking births of twins for decades, in a database that also contains their medical profiles throughout their lives and how they died.

That unique data has been a treasure trove to medical researchers around the world.

Lorelei Mucci, an associate professor of epidemiology at the Harvard T.H. Chan School of Public Health in Boston, published a study in January based on this data that found a strong risk of cancer between identical twins.

Of 220,000 twins studied between 1943 and 2010, 24,000 people developed cancer, statistics that generally mirror the cancer risk for all people, Mucci says. But when one twin developed cancer, the other was at a much greater risk of developing any kind of cancer: a 37% chance for fraternal twins and a 46% chance for identical twins.

Medical science has long known about the risk of cancer in families. The risk among fraternal twins is similar to that among siblings, but the risk of cancer among identical twins, when the co-twin develops cancer, was eye-opening.

Twins were not statistically more likely to suffer from cancer in the study, which also estimated that 33% of cancer risk is genetic. Some cancers were found to be highly genetic, and others less so. For example, if a twin developed testicular cancer, his fraternal twin was 12 times as likely to have it and his identical twin had 28 times the risk. Other cancers most linked to genetics were melanoma, prostate, and skin cancers.

The data could be of particular interest to twins in evaluating their risk.

Says Mucci, “If your twin is diagnosed with colon cancer, you may want to get screened more frequently.”

For nontwins, the study confirmed what medical science has long known: that genetics account for one-third of a person’s risk of developing cancer, the rest being environmental.

Mucci is not the only scientist poring over this data from Scandinavia.

Researchers at the University of Washington recently determined that twins live longer than their nontwin counterparts. Their social relationship has a similar effect on longevity as being married. Identical twins had higher survival rates than fraternal twins, who in turn had higher survival rates than nontwins. Male twins had a 90% chance of reaching age 45, compared with 84% for the general population. For females, the benefit of having a twin peaks in their 60s. Researchers found that 10% more twins reached their early 60s than non-twins.

“There is benefit to having someone who is socially close to you who is looking out for you,” author David Sharrow said in a news release. “They may provide material or emotional support that lead to better longevity outcomes.”

Swedish scientists have also made some surprising discoveries based on the twins data. Studying data on identical twins -- if one was overweight and the other wasn’t -- researchers at Umea University found the heavier twin was not at greater risk of a heart attack or early death, though they had higher diabetes rates than the skinnier twin. They concluded that while obesity is linked with diabetes, genetic factors leading to high cholesterol and high blood pressure are just as important when it comes to heart disease.

Although medical privacy practices have made such research difficult in the U.S., the Internet age has brought a new focus on medical research on twins.

According to the International Association for Twin Studies, 19 countries have some sort of twin registry. In the U.S., there are nine state and regional twin registries administered by colleges and universities. The registries are voluntary, in some cases offering twins minor prizes for taking part. One registry, the Colorado Twins Registry, has helped researchers at the University of Colorado study thinking and memory, learning ability, substance abuse problems, and antisocial behavior.

The Colorado researchers have found that biology plays strongly in twins’ behavior, even long after they have left home, with similar rates of depression and antisocial behavior. Studies have also shown that twins develop early reading skills similarly but that they are also equally susceptible to attention deficit disorder. And researchers have found no genetic risk among twins for eating disorders or marijuana dependence.

It’s not just universities that have gotten involved in twin research. Nonprofit research center SRI International has compiled a registry of 3,000 twins across the country that is being used for a study on how genetics influences the body’s reaction to the flu vaccine. A smaller-scale version of this study released in early 2015 showed that flu shots produced responses among twins as different as those seen in two nonrelated people, calling into question the relationship between genetics and immunology.

Another study looked at the shingles vaccine and found that while twins showed similar genetic susceptibility to the disease, the vaccine also produced very different antibody responses.

Far-Out Implications

Meanwhile, at NASA, scientists have only just begun analyzing data from Scott Kelly’s spaceflight. Charles, the head scientist, expects preliminary results from at least some of the projects by early next year.

As NASA puts more astronauts in space for longer periods -- Kelly’s record of 520 days was broken just a few months later by another astronaut -- the data will help scientists prepare for the days when astronauts could have space voyages that last for years.

“The mission of NASA’s Human Research Program … is to understand the risk for astronauts of long-duration spaceflight beyond the Earth’s orbit. … That’s just a code phrase for trips to Mars. So we are trying to understand what happens to people on these long flights, what the risks are,” Charles says.

Show Sources


John Charles, PhD, chief scientist of NASA’s Human Research Program.

Lorelei Mucci, associate professor of epidemiology, Harvard T.H. Chan School of Public Health in Boston.

William Jeffs, public relations, NASA.

University of Texas Twin Project.

Colorado Twin Registry.

International Association for Twin Studies.

SRI International.

News release, University of Washington.

NASA Twin Project.

Mucci, L. Journal of the American Medical Association, January 2016.

Sharrow, D. PLOS ONE, May 2016.

Davidson, D. Journal of the American Medical Association Internal Medicine, August 2016.

Wong, A. Human Molecular Genetics, January 2005.

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