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Mark and Scott Kelly
The genetic and health profiles of Mark Kelly and Scott Kelly were compared during a “Twins Study” focusing on the effects of long-term spaceflight. (NASA Photo)

Ten research teams today shared comprehensive scientific results from an unprecedented experiment to gauge the health differences that developed between an astronaut who spent nearly a year in space and his identical twin down on Earth.

The study, published in the journal Science, traces the results of DNA tests and analyses of biological samples from Scott Kelly, who took on the 340-day mission on the International Space Station in 2015-2016; and from his brother Mark Kelly, a former astronaut who underwent parallel tests on Earth.

Many of the findings have been previously reported, but today’s open-access research paper and supporting materials provided broader context for the NASA Twins Study — and pointed to concerns that are likely to be addressed in future space experiments.

Previous reports have noted that Scott Kelly experienced changes in his medical condition during his long stint in space, but that most of those changes were reversed after his return. For example, the makeup of Scott’s gut microbiome shifted, perhaps due to a change in diet, and then shifted back after his flight.

Patterns of gene expression also changed, particularly in genetic regions associated with the immune system and DNA repair. More than 90 percent of those changes reversed themselves, but some of the changes persisted six months after Scott’s landing. (The genes themselves weren’t altered, only the patterns of which genes were switched on or off.)

The detailed findings highlight some concerns about long-lasting effects of long-term spaceflight. Scott’s exposure to radiation in space, for example, led to minor mutations in his chromosomes. “Some of the chromosome rearrangements that we saw, particularly inversions, were persistent,” Susan Bailey, a radiation biologist at Colorado State University, acknowledged during a teleconference.

Another genetic change had to do with the length of Scott’s telomeres — that is, the molecular end caps on his chromosomes. They’ve been compared to the protective ends on shoelaces, and they tend to get shorter as a person ages.

Geneticists were intrigued to find that Scott’s telomeres actually lengthened during his spaceflight, but became shorter when he was back on Earth.

“When we looked at individual telomere length and distributions, he did have many more short telomeres after flight than he did before,” Bailey said. “So in that sense, or from the perspective of aging and health risks, that could be where he might be at increased risk for … cardiovascular disease, for example, or some types of cancer.”

The shape of Scott’s eyeballs changed in weightlessness, leading to the types of vision problems that have been found among male astronauts (but not so much among female astronauts).

Scott also registered a slight loss in cognitive abilities when he returned to Earth, although it’s not clear whether that’s related to long-term spaceflight. The researchers suspected that it’s more likely the result of the added stress he experienced as he readjusted to earthly routines. The now-retired astronaut has acknowledged that it took at least six months for him to readjust fully.

The researchers cautioned against reading too much into their study.

“We’re only studying an n of one — in other words, there’s just one twin pair here — and we’re not corroborating the results in this study by looking at other astronauts,” said Andy Feinberg, director of the Center for Epigenetics at Johns Hopkins University and one of the lead investigators on the Twins Study.

Nevertheless, the findings point to issues that will have to be resolved as NASA plans for trips beyond Earth orbit, to the moon, Mars and beyond.

“We’re looking forward to these results serving as a guide and foundation for future studies and things we need to be aware of and look at in astronauts in upcoming longer-duration missions [going] deeper and deeper in space,” Bailey said.

In a commentary published by Science, the University of Darmstadt’s Markus Löbrich and the University of Sussex’s Penny Jeggo said studying the health impacts of long-term spaceflight, particularly exposure to space radiation, should be a high priority. The newly published study “represents more than one small step for mankind in this endeavor,” they wrote.

Francine Garrett-Bakelman of the University of Virginia School of Medicine is the lead author of the study published in Science, “The NASA Twins Study: A Multidimensional Analysis of a Year-Long Human Spaceflight.” More than 80 other researchers are co-authors.

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