From Scientific American:

From the lengths of the twins’ chromosomes to the microbiomes in their guts, “almost everyone is reporting that we see differences”, says Christopher Mason, a geneticist at Weill Cornell Medical College in New York City. He and other project scientists reported the early results on 26 January in Galveston, Texas, at a meeting of scientists working in NASA’s Human Research Program. “The data are so fresh that some of them are still coming off the sequencing machines,” Mason says.

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. And because the Kelly twins are just two people, the results may not be generalizable to others.

Still, the work is some of the most detailed molecular profiling ever done, involving some of the most physically demanding environments. “The greatest importance of the study is to show that we can do it,” says team member Andrew Feinberg, a geneticist at Johns Hopkins University in Baltimore, Maryland. “I don’t think people realized it would be so easy to do genomics on astronauts in space.”

Scott Kelly spent 340 days in space in 2015-2016, 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.

Because they have the same genome and similar life experiences, NASA arranged to have blood and other biological samples taken from the men to try and observe biological changes brought about by long-duration spaceflight.

DNA methylation — the reversible addition of a chemical marker that can affect gene expression — decreased in Scott during flight and increased in Mark over the same period, Feinberg says. Levels for both men …

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