Increased sensitivity of DNA damage response-deficient cells to stimulated microgravity-induced DNA lesions.

Microgravity is a major stress factor that astronauts have to face in space. In the past, the effects of microgravity on genomic DNA damage were studied, and it seems that the effect on genomic DNA depends on cell types and the length of exposure time to microgravity or simulated microgravity (SMG). In this study we used mouse embryonic stem (MES) and mouse embryonic fibroblast (MEF) cells to assess the effects of SMG on DNA lesions. To acquire the insight into potential mechanisms by which cells resist and/or adapt to SMG, we also included Rad9-deleted MES and Mdc1-deleted MEF cells in addition to wild type cells in this study. We observed significant SMG-induced DNA double strand breaks (DSBs) in Rad9-/- MES and Mdc1-/- MEF cells but not in their corresponding wild type cells. A similar pattern of DNA single strand break or modifications was also observed in Rad9-/- MES. As the exposure to SMG was prolonged, Rad9-/- MES cells adapted to the SMG disturbance by reducing the induced DNA lesions. The induced DNA lesions in Rad9-/- MES were due to SMG-induced reactive oxygen species (ROS). Interestingly, Mdc1-/- MEF cells were only partially adapted to the SMG disturbance. That is, the induced DNA lesions were reduced over time, but did not return to the control level while ROS returned to a control level. In addition, ROS was only partially responsible for the induced DNA lesions in Mdc1-/- MEF cells. Taken together, these data suggest that SMG is a weak genomic DNA stress and can aggravate genomic instability in cells with DNA damage response (DDR) defects.