Exonuclease 1 is a Critical Mediator of Survival During DNA Double Strand Break Repair in Nonquiescent Hematopoietic Stem and Progenitor Cells

Authors

  • Amar Desai,

    1. Department of Pharmacology, Case Western Reserve University and University Hospitals Case Medical Center, Cleveland, Ohio, USA
    2. Division of Hematology/Oncology, National Center for Regenerative Medicine, Case Western Reserve University and University Hospitals Case Medical Center, Cleveland, Ohio, USA
    3. Case Comprehensive Cancer Center, Seidman Cancer Center, Case Western Reserve University and University Hospitals Case Medical Center, Cleveland, Ohio, USA
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  • Yulan Qing,

    1. Division of Hematology/Oncology, National Center for Regenerative Medicine, Case Western Reserve University and University Hospitals Case Medical Center, Cleveland, Ohio, USA
    2. Case Comprehensive Cancer Center, Seidman Cancer Center, Case Western Reserve University and University Hospitals Case Medical Center, Cleveland, Ohio, USA
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  • Stanton L. Gerson

    Corresponding author
    1. Department of Pharmacology, Case Western Reserve University and University Hospitals Case Medical Center, Cleveland, Ohio, USA
    2. Division of Hematology/Oncology, National Center for Regenerative Medicine, Case Western Reserve University and University Hospitals Case Medical Center, Cleveland, Ohio, USA
    3. Case Comprehensive Cancer Center, Seidman Cancer Center, Case Western Reserve University and University Hospitals Case Medical Center, Cleveland, Ohio, USA
    • Correspondence: Stanton L. Gerson, M.D., University Hospitals Case Medical Center and Case Western Reserve University, 10900 Euclid Avenue, Wearn 151, Cleveland, Ohio 44106, USA. Telephone: 216-844-8565; Fax: 216-844-4975; e-mail:slg5@case.edu

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Abstract

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 Exo1mut fibroblasts and bone marrow to demonstrate that loss of Exo1 function results in defective HR in cycling cells. Conversely, in Exo1mut mice HR is not required for maintenance of quiescent HSCs at steady state, confirming the steady state HSC reliance on nonhomologous end joining (NHEJ). Exo1mut 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 Exo1mut 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. Stem Cells 2014;32:582–593

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