DNA damage response and DNA repair during hematopoietic differentiation and development

DNA damage is deleterious to all cells, but especially to stem cells because they maintain tissue integrity by constantly self-renewing (to replicate themselves) and differentiating (to produce all mature cells of the tissue). Hence, damage occurring in stem cells can be transmitted both horizontally (to other stem cells) and vertically (to differentiating cells). To study the DNA damage response (DDR) and DNA repair mechanisms employed by stem cells, we used hematopoietic stem cells (HSCs) isolated from young adult mice. We found that quiescent HSCs have cell-intrinsic mechanisms ensuring their survival in response to ionizing radiation (IR), including enhanced pro-survival gene expression and activation of p53-mediated DDR. We show how nonhomologous end joining (NHEJ) DNA repair in quiescent HSCs is associated with acquisition of genomic rearrangements, which can persist in vivo and contribute to hematopoietic abnormalities. Hence, we demonstrate that quiescence, long believed to be strictly protective, actually has a dark side and renders HSCs intrinsically vulnerable to mutagenesis. It has been suggested that the decline of tissue homeostasis seen with age may be caused by impaired stem cell activity. To understand how aging affects DNA repair capability of stem cells we studied the DDR and repair of HSCs isolated from old mice. We confirmed the previously documented numerical expansion, lineage skewing, and high basal levels of genomic stress in old HSCs. We found that old HSCs have a G1 arrest and exhibit a senescence phenotype compared to young HSCs. We show that the intrinsic genomic stress in old HSCs happens at the nucleolus and telomeres, two known fragile sites of the genome, and is potentially caused by replication stress. We demonstrate that old HSCs use NHEJ to repair IR induced DNA damage as efficiently as young HSCs. Taken together, these data indicate that old HSCs have decreased protective mechanisms and increased levels of intrinsic stress that may account for their decreased functionality. Moreover, they suggest that the life-long use of error-prone NHEJ repair in HSCs may be the driving force for the increased cancer incidence occurring with age in the hematopoietic system