Washington: Space travel may cause changes in gene expression and other biological markers in astronauts, a Nasa study of twins has found.

Scientists studied the genetic differences between astronaut Scott Kelly, who spent nearly a year in space, and his identical twin Mark.

Measurements taken before, during and after Scott Kelly’s mission show changes in gene expression, DNA methylation and other biological markers that are likely to be attributable to his time in orbit.

From the lengths of the twins’ chromosomes to the microbiomes in their guts, “almost everyone is reporting that we see differences," said Christopher Mason, a geneticist at Weill Cornell Medicine in New York City.

The challenge now is to untangle how many of the observed changes are specific to the physical demands of spaceflight - and how many might be simply due to natural variations. Since the Kelly twins are just two people, the results may not be generalisable to others, researchers said.

Still, the work is some of the most detailed molecular profiling ever done, involving some of the most physically demanding environments.

Scott Kelly spent 340 days in space in 2015–16, giving him a lifetime total of 520 days. Mark Kelly, also an astronaut, had previously flown in space for a total of 54 days over four space-shuttle missions between 2001 and 2011.

Since the two men have almost identical genomes and similar life experiences, NASA arranged to have blood and other biological samples taken from them to try and observe biological changes brought about by long-duration spaceflight.

Studies of the twins’ telomeres, the caps on the ends of their chromosomes, showed that during spaceflight Scott’s telomeres grew to be longer than his brother’s. “That is exactly the opposite of what we thought," said Susan Bailey, a radiation biologist at Colorado State University in Fort Collins.

A second lab has studied the same samples and confirmed this puzzling increase in telomere length, she said. Once Scott returned to the ground, the length of his telomeres returned to his pre-flight levels relatively quickly.

The scientists are working to figure out what this means, and are running a separate study of telomere length in ten unrelated astronauts that, when completed in 2018, may shed more light on how spaceflight affects telomeres.

DNA methylation — the reversible addition of a chemical marker to DNA that can affect gene expression — decreased in Scott during flight and increased in Mark over the same period, Feinberg said.

Levels for both men returned close to preflight levels after Scott came back to Earth. What this means is not yet clear, Feinberg said.

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