Modeled microgravity affects cell survival and HPRT mutant frequency, but not the expression of DNA repair genes in human lymphocytes irradiated with ionising radiation

We analysed the possibility that a reduced gravitational force impairs the efficiency of DNA repair, increasing the risk of the exposure to conditions occurring during spaceflight: i.e., ionising radiation and microgravity. To obtain information on the effects of the reduced gravity in repairing DNA damage induced by radiation, we compared cell survival and mutant frequency at the hypoxanthine-guanine phosphoribosyl transferase (HPRT) locus in human peripheral blood lymphocytes (PBL) irradiated and subsequently incubated for 24 h in 1 g or modeled microgravity. A weak decrease of the surviving fraction and a significant increase of the HPRT mutant frequency were observed in PBL incubated in modeled microgravity after irradiation compared with those maintained in 1 g. Given the increase of HPRT mutants observed in MMG, we investigated whether modeled microgravity can alter the transcription of 14 genes representative of the main DNA repair pathways: non-homologous end joining (NHEJ), homologous recombination (HR), base excision repair (BER) and nucleotide excision repair (NER). The transcriptional profiles of almost all BER and NER genes were up-regulated in irradiated PBL, whereas the expression of HR and NHEJ genes was only slightly or not affected by radiation. Incubation in modeled microgravity after irradiation did not significantly change the expression of genes involved in DNA repair, suggesting that transcriptional impairment was not responsible for the increase of mutant frequency observed in irradiated cells incubated in microgravity in comparison to the static 1 g condition.