Quiescent (5-fluorouracil-resistant) aplastic anemia hematopoietic cells in vitro

Insufficiency of FZR1 disturbs HSC quiescence by inhibiting ubiquitin-dependent degradation of RUNX1 in aplastic anemia

FZR1 has been implicated as a master regulator of the cell cycle and quiescence, but its roles and molecular mechanisms in the pathogenesis of severe aplastic anemia (SAA) are unclear. Here, we report that FZR1 is downregulated in SAA HSCs compared with healthy control and is associated with decreased quiescence of HSC. Haploinsufficiency of Fzr1 shows impaired quiescence and self-renewal ability of HSC in two Fzr1 heterozygous knockout mouse models. Mechanistically, FZR1 insufficiency inhibits the ubiquitination of RUNX1 protein at lysine 125, leading to the accumulation of RUNX1 protein, which disturbs the quiescence of HSCs in SAA patients. Moreover, downregulation of Runx1 reversed the loss of quiescence and impaired long-term self-renew ability in Fzr1^+/- HSCs in vivo and impaired repopulation capacity in BM from SAA patients in vitro. Our findings, therefore, raise the possibility of a decisive role of the FZR1-RUNX1 pathway in the pathogenesis of SAA via deregulation of HSC quiescence.

Exonuclease 1 is a critical mediator of survival during DNA double strand break repair in nonquiescent hematopoietic stem and progenitor cells

Hematopoietic stem cell (HSC) populations require DNA repair pathways to maintain their long-term survival and reconstitution capabilities, but mediators of these processes are still being elucidated. Exonuclease 1 (Exo1) participates in homologous recombination (HR) and Exo1 loss results in impaired 5' HR end resection. We use cultured Exo1(mut) fibroblasts and bone marrow to demonstrate that loss of Exo1 function results in defective HR in cycling cells. Conversely, in Exo1(mut) mice HR is not required for maintenance of quiescent HSCs at steady state, confirming the steady state HSC reliance on nonhomologous end joining (NHEJ). Exo1(mut) mice sustained serial repopulation, displayed no defect in competitive repopulation or niche occupancy, and exhibited no increased sensitivity to whole body ionizing radiation. However, when Exo1(mut) HSCs were pushed into cell cycle in vivo with 5-fluorouracil or poly IC, the hematopoietic population became hypersensitive to IR, resulting in HSC defects and animal death. We propose Exo1-mediated HR is dispensable for stem cell function in quiescent HSC, whereas it is essential to HSC response to DNA damage processing after cell cycle entry, and its loss is not compensated by intact NHEJ. In HSCs, the maintenance of stem cell function after DNA damage is dependent on the DNA repair capacity, segregated by active versus quiescent points in cell cycle.

Abnormal immunity and stem/progenitor cells in acquired aplastic anemia

Acquired aplastic anemia (AA) is considered as an immune-mediated bone marrow failure syndrome, characterized by hypoplasia and pancytopenia with fatty bone marrow. Abnormal immunity is the major factor mediating the pathogenesis of acquired AA. Activated DCs might promote the polarization to Th1 cells, and activate CD8(+) T cells. A variety of immune molecules including IFN-gamma, TNF-alpha, MIP-1alpha and IL-2, 8, 12, 15, 17, 23, produced by them and stromal cells, compose a cytokine network to destruct stem/progenitor cells as well as hematopoietic stem/progenitor cells, mesenchymal stem cells (MSCs) and angioblasts/endothelial progenitor cells. Inversely, deficient MSCs, CD4(+)CD25(+) T cells, NK cells, NKT cells and early hematopoietic growth factors diminish the capacity of immune regulation and the support of hematopoiesis. As a result, stem/progenitor cells are significantly impaired to be disabled cells with markedly deficient proliferation, differentiation, induced apoptosis and dysfunctional response to growth factor stimuli, together with rare normal ones. Although some patients can be ameliorated by stem-cell transplantation or immunosuppressive therapy, more effective and convenient therapies such as patient-specific pluripotent iPS cells based on definite pathogenesis are expected.