The response of mouse mesenchymal stromal cells to radiation-induced DNA double-strand breaks

Abstract

Mesenchymal stromal cells (MSCs) are radio-resistant stem cell progenitors that support haematopoiesis in the bone marrow and contribute to the tumour microenvironment. The mechanisms that drive MSC radio-resistance are poorly understood. Ionising radiation (IR) negatively impacts on cell survival largely due to the generation of DNA lesions, particularly of highly genotoxic DNA double-strand breaks (DSBs). The DNA Damage Response (DDR) represents a network of signalling pathways that enable cells to activate biological responses to genotoxic stress, including DNA DSBs. In this study, the role of the DDR in mediating mouse MSC radio-resistance was investigated. Multiple DDR mechanisms synergistically contributed to MSC radio-resistance: robust DDR initiation; DNA damage checkpoint activation and efficient DNA DSB repair. Irradiated mouse MSCs could withstand IR-induced apoptosis; continued to proliferate and could differentiate along mesenchymal-derived lineages. MSCs reside in hypoxic niches within the bone marrow and tumour microenvironments. Herein, hypoxic MSCs exhibited (i) enhanced survival post irradiation; (ii) improved recovery from IR-induced cell cycle arrest and (iii) an increased DNA DSB repair capacity. In addition, HIF-1[alpha] was identified as an important mediator of the increased DNA DSB repair capacity of hypoxic MSCs. Double negative II (DN2) thymocytes are radio-resistant T lymphocyte precursors that reside in the thymus. The mechanisms underlying DN2 radio-resistance are also un-described. Given the important role of the DDR in mediating MSC radio-resistance, the DDR of DN2 thymocytes to IR-induced DNA DSBs was also characterised in this study. Multiple DDR mechanisms were also found to contribute to DN2 radio-resistance including (i) rapid DDR initiation; (ii) induction of a radio-protective G1 checkpoint and (iii) activation of DNA DSB repair. For the first time, this study demonstrates that (i) the DDR is fundamental for mediating mouse MSC resistance to IR-induced DNA DSBs; (ii) hypoxia alters the DDR of irradiated mouse MSCs and (iii) DN2 thymocytes activate the DDR to IR-induced DNA DSBs.