Abstract
Failure to precisely repair DNA damage in self-renewing Hematopoietic Stem and early Progenitor Cells (HSPCs) can disrupt normal hematopoiesis and promote leukemogenesis. Although HSPCs are widely considered a target of ionizing radiation (IR)-induced hematopoietic injury, definitive data regarding cell death, DNA repair, and genomic stability in these rare quiescent cells are scarce. We found that irradiated HSPCs, but not lineage-committed progenitors (CPs), undergo rapid ATM-dependent apoptosis, which is suppressed upon interaction with bone-marrow stroma cells. Using DNA repair reporters to quantify mutagenic Non-Homologous End Joining (NHEJ) processes, we found that HSPCs exhibit reduced NHEJ activities in comparison with CPs. HSPC-stroma interactions did not affect the NHEJ capacity of HSPCs, emphasizing its cell autonomous regulation. We noted diminished expression of multiple double strand break (DSB) repair transcripts along with more persistent 53BP1 foci in irradiated HSPCs in comparison with CPs, which can account for low NHEJ activity and its distinct control in HSPCs. Finally, we documented clonal chromosomal aberrations in 10% of IR-surviving HSPCs. Taken together, our results revealed potential mechanisms contributing to the inherent susceptibility of human HSPC to the cytotoxic and mutagenic effects of DNA damage.
Highlights
Life-long blood production depends on Hematopoietic Stem and early Progenitor Cells (HSPCs) - a subset of primitive hematopoietic cells endowed with high self-renewal potential
We found that 12.9% of irradiated HSPCs became c-PARP positive 6 h post-ionizing radiation (IR), whereas only 1.6% of committed progenitors (CPs) exhibited PARP cleavage at this time point
We investigated clonogenic survival of HSPCs and various CPs sub-types (common cyeloid progenitor (CMP), granulocyte macrophage progenitor (GMP) and megakaryocyte–erythroid progenitor (MEP) exposed to 3 Gy of IR after isolation by flow sorting from cord blood
Summary
Life-long blood production depends on HSPCs - a subset of primitive hematopoietic cells endowed with high self-renewal potential. The initial data sets on DDR in human HSPCs suggested that they have a delayed DSB rejoining capacity and increased IR-sensitivity, relative to CPs isolated from cord blood[17,18] These studies revealed potentially important distinctions in IR-induced DDR in human versus rodent HSPCs as well as between HSPCs and CPs, the underlying mechanism remains poorly understood. Very little is known regarding those factors that affect human HSPC survival after DSB induction as well as the activity and efficiency of the major DNA repair pathways, the importance of intact DDRs in preventing leukemogenesis is well established To bridge this gap, we analyzed in detail the IR-induced cell death and activity of NHEJ repair pathways in human HSPCs and CPs derived from cord blood and bone marrow. Our study emphasizes the inherent vulnerability of human HSPCs to exogenous DNA damage with potential adverse consequences that include bone marrow failure and leukemogenesis
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