You might remember the NASA twins study in which 10 different teams of researchers studied astronaut Scott Kelly, who spent a year aboard the International Space Station in 2015 and 2016, and compared him to his identical twin, fellow astronaut Mark Kelly, who had remained behind on Earth.
Part of that research involved studying and comparing the twins' DNA, and as this Atlantic article details, it created a stir when numerous news outlets misinterpreted the results and reported erroneously that Scott Kelly's DNA had been altered significantly by his time in space. In reality, as this NASA media release explains, Scott Kelly's DNA didn't fundamentally change. But researchers did observe changes in gene expression — that is, how genes react to the environment.
Most of Scott's gene expression returned to normal after he landed back on Earth, but 7 percent of his gene expression didn't revert. As the NASA release notes, that 7 percent difference points to possible longer-term changes in genes related to Kelly's immune system, DNA repair and bone formation networks. The changes might also affect how Kelly's genes respond to hypoxia, or oxygen deprivation, and hypercapnia, the condition of having too much carbon dioxide in the bloodstream. (The latter is a potential problem on the ISS, where, as this 2012 study notes, ambient CO2 levels rise above normal atmospheric conditions on Earth, and crews routinely report symptoms such as headaches and lethargy.)
Space or Stress?
But were the changes in Scott Kelly's gene expression triggered by being in space, or simply the result of an extremely stressful environment? One way to answer that question would be to study mountain climbers at a high elevation, where the rarified atmosphere and low temperatures would contribute to stress, and compare them to twins who remained at a lower elevation.
Christopher Mason, an associate professor at Weill Cornell Medicine in New York City who led the NASA gene expression study, has designed a comparable experiment, involving two climbers, Willie Benegas and Matt Moniz, who have planned an ascent to the summit of Mount Everest in May. According to an account in Science magazine, both climbers have twins who remain at lower elevations — an identical twin for Benegas and a fraternal twin for Moniz — for comparison. (Identical twins share 100 percent of the same genes, while fraternal twins share 50 percent, according to the Michigan State University Twin Registry.)
That's if all takes place as planned. Outside magazine's website reported on May 11 that a regulatory issue in Nepal might interfere with the climb. However, Benegas said in a May 14 email that the ascent was still on, with the two climbers targeting May 20 as the day to reach the summit.
But there is already scientific evidence that venturing into high altitudes on Earth can alter gene expression. Zac Cheviron, an assistant professor at the University of Montana, has been involved in research on the effect of altitude on the deer mouse. That tiny creature has the distinction of having the most extreme altitude range of any North American mammal, from below sea level in Death Valley to mountainsides more than 14,000 feet (4,300 meters) up.
Cheviron — who isn't involved with the Everest study — says that lowlander deer mice, when subjected to simulated high altitude, experience changes in gene expression that affect the structure of their muscles. The changes in expression will cause them to develop more slow-twitch, oxidative muscle fibers, and to grow more blood vessels. Those changes are an acclimation response that enables the mice to cope with the mountain environment, where the thinner air makes it more challenging to supply needed oxygen to muscle tissue.
Changes Mimic Evolution — To a Point
Gene expression is an extension of physiology, turning genes up and turning them down, because it enhances survival in those conditions, Cheviron explains. "For certain traits, plasticity that's driven by gene expression does seem to mimic adaptation as driven by evolution," he says.
But that's true only to a point, Cheviron emphasizes. Deer mice born on mountains, the descendants of previous generations of mice who've evolved at high altitudes, have a mutation that allows them to express those same genes affecting the muscle fibers and blood vessels at much higher levels than the lowland mice ever could.
"If you take a lowlander and expose them to high altitudes they'll get gene expression to develop more blood vessels," Cheviron says. "But they won't have as many as the highlanders."
Not all gene-expression adaptions that lowlanders make at high altitudes are necessarily good ones, either. As this 2014 Science magazine article details, many Tibetans who live at high altitudes have inherited genes that enable their bodies to use oxygen more efficiently, without having a high number of hemoglobin-laden red blood cells. When a lowlander ventures into the same high places, his or her body will try to cope by making more red blood cells — a change that thickens the blood, making a person more vulnerable to blood clots and strokes.