Genetic changes induced in human cells in Space Shuttle experiment (STS-95)

Background: Results of past space experiments suggest that the biological effect of space radiation could be enhanced under microgravity. To assess the radiation risk for humans during long-term spaceflight, it is very important to clarify whether human cells exhibit a synergistic effect of radiation and microgravity. Hypothesis: if significant synergism occurs in human cells, genetic changes induced during spaceflight may be detected by using human tumor HCT-116 cells which are hypermutable due to a defect in the DNA mismatch repair system. Methods: Cultured HCT-116 cells were loaded on the Space Shuttle Discovery (STS-95) and grown during the 9-d mission. After landing, many single-cell clones were isolated, microsatellite repetitive sequences in each clone were amplified by PCR, and mutations in the microsatellite loci were detected as changes in the length of PCR fragments. Mutation frequencies of ouabain-resistant phenotype were also analyzed. Results: The frequencies of microsatellite mutations as well as ouabain-resistant mutations in the flight sample were similar to those of the ground control samples. Some cells were treated in space with bleomycin which mimics the action of radiation, but the frequencies of microsatellite mutations were not significantly different between the flight and the ground control samples. Conclusion: Under the present flight conditions, neither space radiation (about 20 mSv during this mission) nor microgravity caused excess mutations in human cells.