Long-term cultures under hypoxic conditions have been demonstrated to maintain the phenotype of mesenchymal stromal/stem cells (MSCs) and to prevent the emergence of senescence. According to several studies, hypoxia has frequently been reported to drive genomic instability in cancer cells and in MSCs by hindering the DNA damage response and DNA repair. Thus, we evaluated the occurrence of DNA damage and repair events during the ex vivo expansion of clinical-grade adipose-derived stromal cells (ADSCs) and bone marrow (BM)-derived MSCs cultured with platelet lysate under 21% (normoxia) or 1% (hypoxia) O 2 conditions. Hypoxia did not impair cell survival after DNA damage, regardless of MSC origin. However, ADSCs, unlike BMMSCs, displayed altered cH2AX signaling and increased ubiquitylated cH2AX levels under hypoxic conditions, indicating an impaired resolution of DNA damage-induced foci. Moreover, hypoxia specifically promoted BM-MSC DNA integrity, with increased Ku80, TP53BP1, BRCA1, and RAD51 expression levels and more efficient nonhomologous end joining and homologous recombination repair. We further observed that hypoxia favored mtDNA stability and maintenance of differentiation potential after genotoxic stress. We conclude that long-term cultures under 1% O 2 were more suitable for BM-MSCs as suggested by improved genomic stability compared with ADSCs. STEM CELLS 2015;33:3608-3620
SIGNIFICANCE STATEMENTHuman mesenchymal stem cells (MSCs) feature differentiation capacities, immunomodulatory properties and therefore represent a great potential for medicine. In vitro culture allows their amplification for suitable clinical uses. Hypoxia seems relevant for the prevention of senescence and loss of MSC features. However, hypoxia is shown to promote cancer or DNA damage response (DDR) and repair failures. We demonstrate that long-term culture of MSCs with platelet lysate in hypoxia does not necessarily inhibit DDR and DNA repair. Here, bone marrowMSCs conserved or rather improved their DDR and DNA repair (NHEJ, HR), mtDNA stability in hypoxia compared to adipose-derived stem cells. Moreover, the ubiquitylation status of gH2AX between BM-MSCs and ADSCs might be responsible for changes in their DNA repair activities. Appropriate oxygen tension seems crucial to preserve the MSCs genomic stability according to their origins.