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It Takes Two: A Year of Twin-formation and the Effects of Spaceflight on Individuals
November 5, 2013

What could twins tell us about the effects of spaceflight on humans? NASA hopes to find out.  In a world as diverse as ours, it can be easy to forget that most humans are approximately 99.5 percent genetically identical. Identical, or “monozygotic,” twins are nearly 100 percent identical. What accounts for the differences we know exist? The remaining 0.5 percent of genetic real estate, which on the surface seems like a small amount, codes nearly all the characteristics that make us unique individual humans.

The complexity of individual differences makes it difficult for scientists to predict responses to disease, treatments or environmental stressors that apply across the entire human population. This is especially relevant to scientists studying the effects of spaceflight on the human body. Spaceflight may magnify the difference between individuals because it exposes crew members to stressors such as radiation, microgravity, isolation, confinement, and altered light/dark cycles. If this might be true for individuals who are genetically different, what does this mean for twins who are nearly identical?[image-51][image-78][image-94]

A study new to NASA, which will take place throughout NASA’s one-year mission beginning in March 2015, should help scientists better understand the impacts of spaceflight on the human body through the study of a pair of identical twins. Astronaut Scott Kelly will spend one year in low-Earth orbit aboard the International Space Station while retired astronaut Mark Kelly, his identical twin, will remain on Earth. The twins’ similarity provides scientists with a reduced number of variables and an ideal control group, both important to scientific investigation. Study investigations will be selected as part of the Human Exploration Research Opportunities solicitation developed and managed by NASA’s Human Research Program and the National Space Biomedical Research Institute.

While the study is a supplemental objective of the one-year mission,  it will support the overarching mission investigations that will likely center around medical concerns such as increased intracranial pressure associated with microgravity, behavioral health and performance issues associated with prolonged isolation, challenges associated with exposure to the space radiation environment, various physiological changes like bone and muscle loss, changes in the sensorimotor system, and changes to nutritional status.

The twin study will also introduce to space physiology a rapidly growing field of research called –omics. Omics is a broad area of biological and molecular studies that, in general, means the study of the entire complement of biomolecules, like proteins (proteomics); metabolites, the end products of metabolism, (metabolomics); or genes (genomics).

“The term –omics means ‘everything’,” says NASA astronaut Kate Rubins, Ph.D. “For example, rather than observing a few genes at a time as researchers did in the early 1990s, scientists in the field of genomics investigate the activity of all of the genes in the genome simultaneously. It’s like taking a snapshot of all of the activity inside a cell.”

NASA’s solicitation for proposals related to –omics studies of the twins was released in July 2013 and closed in September 2013. Selections will take place in January 2014. NASA expects to fund up to ten investigations.

“This study will allow us the opportunity to form a team of experts that collectively will cover all disciplines within the –omics field,” says Graham Scott, Ph.D. and chief scientist at the National Space Biomedical Research Institute. “By conducting an integrated –omics study, we can simultaneously overlay what we observe in the DNA, RNA, proteins and metabolites and form a rich picture of what is going on with Scott [Kelly] as opposed to Mark.”

The twins’ –omics responses will reveal to researchers what impact spaceflight has on humans at the biomolecular level. They should also shed new light on whether individuals are more susceptible to certain adverse conditions which could threaten their health and, ultimately, the mission.

The investigations could also include a study of the length of the twins’ telomeres, the caps at the end of chromosomes that protect its most important parts. Each time a cell replicates, its telomeres slightly shorten, although an enzyme called telomerase can partially repair the telomeres—especially when we are young.

“This shortening is associated with aging and stress,” says Rubins.

Eventually your telomeres shorten to the point that they no longer protect the chromosomes and the cell will either die or potentially malfunction, possibly causing cancer or other diseases. The space environment may also directly damage the telomeres or somehow affect the activity of telomerase, both of which may cause telomeres to shorten prematurely. The difference in the length of the twin’s telomeres after Scott’s year in space may provide valuable data about the effects of spaceflight on humans.

“These –omics- and telomere-focused investigations will allow us to identify specific biological pathways that are being affected by spaceflight,” says Scott. “If we can understand what pathways are being affected and how they correlate to the physiology of the astronaut, we are on the road to designing individualized biological countermeasures for each astronaut.”

This is critical when it comes to long-duration missions like those to asteroids and Mars as long-term exposure to radiation and microgravity will amplify the effects of spaceflight even further.

Ultimately, the fact that the Kelly twins are nearly genetically identical in that small 0.5 percent window that makes most of us different will help NASA understand the specific biological effects of spaceflight.  Their similarities may potentially enhance individual astronaut performance and safety now and years into the future.

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DNA double helix with data
A DNA double helix carries the codes for regulating the activities of cells. Differences in DNA, RNA and proteins are what make individuals unique.
Image Credit: 
Jonathan Bailey, NHGRI
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Human Chromosomes
Human chromosomes contain the DNA found in our cells.
Image Credit: 
Jane Ades, NHGRI
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A telomere is the end of a chromosome that protects the interior of a chromosome from damage during cell division. Each time a cell divides, telomeres become shorter. Eventually, telomeres become so short that the cell dies or becomes cancerous.
Image Credit: 
Darryl Leja, NHGRI
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Page Last Updated: March 4th, 2015
Page Editor: Jeffrey Brief