Epigenetic modifications of DNA methylation in lymphocytes exposed to galactic cosmic radiation: a twin study within the NASA twins study program

The NASA Twins Study is a singular experiment: one identical twin spent a year aboard the International Space Station while his genetically identical brother remained on the ground, and the comparison between them isolated the biology of spaceflight from the confound of ge- netic difference. Among the changes the study documented were alterations in DNA methylation in the astronaut's immune cells, near genes governing immune function. Those changes have since been cited as evidence of how galactic cosmic radiation rewrites the epigenome. This article argues that the citation outruns the evidence. The Twins Study has a sample of one twin pair, and the spaceflight environment it sampled bundles galactic cosmic radiation together with micro- gravity, isolation, sleep disruption, elevated carbon dioxide, and altered diet — so a methylation change observed in the flown twin cannot, on the study's design, be assigned to radiation rather than to any other component of the bundle. Compounding this, the International Space Station orbits within the protection of Earth's magnetosphere, where the galactic cosmic radiation dose is a fraction of what a crew bound for Mars would receive. The original contribution of this article is the Spaceflight–Radiation Methylation Corpus, the first cross-study catalogue of differentially methylated immune-cell loci and functional pathways assembled from eighteen Scopus-indexed studies and cross-classified by exposure source — the spaceflight bundle, isolated ionizing radi- ation, and galactic-cosmic-radiation analog irradiation — and by post-exposure reversibility. The corpus permits, for the first time, a side-by-side reading: a radiation-specific methylation signa- ture, concentrated in DNA-damage-response loci, oxidative-stress genes, and repetitive-element hypomethylation, is partially separable from a spaceflight-general signature concentrated in im- mune-activation and inflammatory pathways. Read against the corpus, the Twins Study lympho- cyte methylation changes overlap predominantly with the spaceflight-general signature, and most reverted after return to Earth. The corpus reframes the interpretive task from detecting a meth- ylation change to locating it within an exposure-resolved and reversibility-resolved reference frame.