Haematopoietic stem cells (HSCs) are responsible for the lifelong production of blood cells. The accumulation of DNA damage in HSCs is a hallmark of ageing and is probably a major contributing factor in age-related tissue degeneration and malignant transformation. A number of accelerated ageing syndromes are associated with defective DNA repair and genomic instability, including the most common inherited bone marrow failure syndrome, Fanconi anaemia. However, the physiological source of DNA damage in HSCs from both normal and diseased individuals remains unclear. Here we show in mice that DNA damage is a direct consequence of inducing HSCs to exit their homeostatic quiescent state in response to conditions that model physiological stress, such as infection or chronic blood loss. Repeated activation of HSCs out of their dormant state provoked the attrition of normal HSCs and, in the case of mice with a non-functional Fanconi anaemia DNA repair pathway, led to a complete collapse of the haematopoietic system, which phenocopied the highly penetrant bone marrow failure seen in Fanconi anaemia patients. Our findings establish a novel link between physiological stress and DNA damage in normal HSCs and provide a mechanistic explanation for the universal accumulation of DNA damage in HSCs during ageing and the accelerated failure of the haematopoietic system in Fanconi anaemia patients.

Abstract/Introduction Aging in the hematopoietic system and the stem cell niche contributes to aging-associated phenotypes of hematopoietic stem cells (HSCs), including leukemia and aging-associated immune remodeling. Among others, the DNA damage theory of aging of HSCs is well established, based on the detection of a significantly higher amount of γH2AX foci and a higher tail moment in the comet assay, both initially thought to be associated with DNA damage in aged HSCs compared to young cells, and bone marrow failure in animals devoid of DNA repair factors. Novel data on the increase and the nature of DNA mutations in the hematopoietic system upon aging, the quality of the DNA damage response in aged HSCs and the nature of γH2AX foci question a direct link between DNA damage and the DNA damage response and aging of HSCs, and rather favor changes in epigenetics, splicing-factors or 3D architecture of the cell as major cell intrinsic factors of HSCs aging. Aging if HSCs is also driven by a strong contribution of aging of the niche. This review discusses the DNA damage theory of HSCs aging in the light of these novel mechanisms of aging of HSCs.

Whether aged hematopoietic stem and progenitor cells (HSPCs) have impaired DNA damage repair is controversial. Using a combination of DNA mutation indicator assays, we observe a 2-3 fold increase in the number of DNA mutations in the hematopoietic system upon aging. Young and aged HSCs and HPCs do not show an increase in mutation upon irradiation-induced DNA damage repair, and young and aged HSPCs respond very similarly to DNA damage with respect to cell cycle checkpoint activation and apoptosis. Both, young and aged HSPCs show impaired activation of the DNA-damage induced G1-S checkpoint. Induction of chronic DNA double strand breaks by zinc-finger nucleases suggest that HSPCs undergo apoptosis rather than faulty repair. These data reveal a protective mechanism in both the young and aged hematopoietic system against accumulation of mutations in response to DNA damage.

SummaryThe spindle assembly checkpoint plays a pivotal role in preventing aneuploidy and transformation. Many studies demonstrate impairment of this checkpoint in cancer cells. While leukemia is frequently driven by transformed hematopoietic stem and progenitor cells (HSPCs), the biology of the spindle assembly checkpoint in such primary cells is not very well understood. Here, we reveal that the checkpoint is fully functional in murine progenitor cells and, to a lesser extent, in hematopoietic stem cells. We show that HSPCs arrest at prometaphase and induce p53-dependent apoptosis upon prolonged treatment with anti-mitotic drugs. Moreover, the checkpoint can be chemically and genetically abrogated, leading to premature exit from mitosis, subsequent enforced G1 arrest, and enhanced levels of chromosomal damage. We finally demonstrate that, upon checkpoint abrogation in HSPCs, hematopoiesis is impaired, manifested by loss of differentiation potential and engraftment ability, indicating a critical role of this checkpoint in HSPCs and hematopoiesis.

Adult hematopoietic stem cells (HSCs) maintain tissue homeostasis and regenerative capacity of the hematopoietic system through self‐renewal and differentiation. Metabolism is recognized as an important regulatory entity controlling stem cells. As purine nucleotides are essential for metabolic functions, we analyzed the role of hypoxanthine guanine phosphoribosyl transferase (HPRT)‐associated purine salvaging in HSCs. Here, we demonstrate that hematopoietic stem and progenitor cells (HSPCs) show a strong dependence on HPRT‐associated purine salvaging. HSPCs with lower HPRT activity had a severely reduced competitive repopulation ability upon transplantation. Strikingly, HPRT deficiency resulted in altered cell‐cycle progression, proliferation kinetics and mitochondrial membrane potential primarily in the HSC compartment, whereas more committed progenitors were less affected. Our data thus imply a unique and important role of HPRT and the purine salvage pathway for HSC function. Stem Cells 2019;37:1606–1614