Identical twins are known for having some pretty out of this world similarities. Picking the same clothes, feeling the other’s pain, and often going into the same careers. The latter is true for Mark and Scott Kelly, monozygotic twins who also both happen to have been trained as astronauts. Twins are a favorite model system of many researchers. In this case, the similarity in terms of both genetics and environment (training) of the Kelly twins provides a unique opportunity to study the effects of space-flight on the human body.
The goal of the NASA twin study was to assess the effects of one year of space flight on the human body across multiple metrics. These included telomeric, transcriptomic, epigenetic, proteomic, metabolic, immune, microbiomic, physiological, cardiovascular, vision, and cognitive endpoints. Scott spent 340 days aboard the International Space Station (ISS) while Mark remained on earth. Multiple blood samples were collected from both brothers for 6 months leading up to space-flight, during the year on the ISS, and for 6 months after Scott’s return to earth. Ten teams of researchers joined forces to undertake this ambitious study. The results were varied and quite detailed, so we’re just going to focus on the telomere and epigenomic results here. The authors used real-time PCR and Fluorescence in situ Hybridization (FISH) to assess telomere length in PBMCs, and whole genome bisulfite sequencing (WGBS) to assess DNA methylation in CD4 and CD8 lymphocytes.
Here’s what they found:
- Both subjects had very similar telomere lengths
during the pre-flight period, but during flight Scott’s telomere length
increased about 15% and remained so for the flight duration
- His telomere length rapidly returned to baseline levels after returning to earth
- These results were confirmed by examining the distribution of individual telomere lengths using FISH
- There were limited changes in global DNA
methylation, with mean methylation levels and methylation stochasticity
increasing slightly during spaceflight; however, these changes were within the
range of variation observed in Mark on the ground
- Specific genes involved in platelet-derived growth factor and T cell differentiation pathways were enriched during spaceflight when ranking regions by epigenetic discordance (Jensen-Shannon Distance). These returned to normal after landing and were not enriched in Mark on the ground.
- The majority of the other changes examined also returned to baseline after returning to earth, including changes in gene expression, gut microbiome composition, body weight, carotid artery dimensions, retinal thickness, and serum metabolites
The major conclusion of this work is study is that human health can be “mostly sustained” over one year of space flight. Dramatic changes in health did not arise, and most of the changes that did returned to baseline levels after landing. The observed changes provide specific targets to monitor during spaceflight in the future, perhaps on the longer journey to Mars. So if you, or maybe your identical twin, are planning on heading to space in the future, you may not have to worry about your epigenome.
Go launch yourself into the orbit of the full story in Science, April 2019