HSC transplantation is routinely used to treat patients with malignant and non-malignant disorders of the blood and immune system, but its therapeutic application is often restricted by difficulties in in vitro maintenance and expansion of HSCs. Studies aimed at understanding the mechanisms governing self-renewal of HSCs within hematopoietic tissues identified a set of growth factors and cytokines that can augment in vitro expansion of HSCs, including stem cell factor (SCF), fms-like tryrosine kinase 3 ligand (FLT3l) and thrombopoietin (TPO) 1. TPO and its receptor, Mpl, are primarily known for their role in megakaryopoiesis, but TPO has also been shown to support HSC quiescence during adult hematopoiesis, with the loss of signaling associated with bone marrow failure and thrombocytopenia 2.
Recently, in Cell Stem Cell, de Lavel et al. identified a novel role of TPO in the regulation of DNA repair in HSCs 3. Exposure to genotoxic agents, such as ionizing radiation (IR), induces DNA damage comprised of double-strand breaks (DSBs). DNA damage is repaired through two main pathways: homologous recombination (HR) and nonhomologous end-joining (NHEJ). DNA repair is essential for cell survival, and studies have shown that NHEJ is necessary for HSC maintenance 4,5. In their study, de Lavel et al. found that γH2AX foci, a marker of DSB formation, were significantly increased in Mpl-deficient HSCs and in their progenitors following IR exposure. Moreover, a TPO injection into mice prior to IR reduced the number of γH2AX foci in HSCs in vivo, while HSCs exposed to IR in the absence of TPO demonstrated an increased number of γH2AX foci. Other experiments showed that TPO modulates the efficiency of the NHEJ pathway by increasing the phosphorylation of the DNA-PK catalytic subunit, a major enzyme involved in NHEJ. Pharmacological or genetic inhibition of DNA-PK abrogated TPO-mediated DNA repair. Interestingly, the other cytokines involved in HSC maintenance and expansion, SCF and FLT3l, did not have the same effects as TPO, suggesting that DNA repair activity is a specific function of TPO.
In short, TPO regulates NHEJ-mediated DNA repair of DSBs by stimulating DNA-PK activity in HSCs. This is the first demonstration that a cytokine involved in HSC maintenance may also regulate DSB repair machinery. Since TPO treatment prior to IR exposure reduces DNA damage, TPO agonists could potentially be given to patients prior to receiving chemotherapy to reduce the risk of developing oncogenic mutations and defects in HSC function. Romiplostim, a TPO peptide mimetic, and eltrombopag, a non-peptide TPO mimetic, have been successfully used for the treatment of immune thrombocytopenic purpura (ITP) and are approved by the FDA 6. de Lavel et al showed that injection of romiplostim prior to IR exposure also completely abolished persistent DNA damage in HSCs, similar to TPO. Thus, these TPO agonists might also be suited for clinical applications involving protection of normal HSC from DNA-damaging agents.
1 Ohmizono, Y. et al. Thrombopoietin augments ex vivo expansion of human cord blood-derived hematopoietic progenitors in combination with stem cell factor and flt3 ligand. Leukemia 11, 524-530 (1997).
2 Ballmaier, M., Germeshausen, M., Krukemeier, S. & Welte, K. Thrombopoietin is essential for the maintenance of normal hematopoiesis in humans: development of aplastic anemia in patients with congenital amegakaryocytic thrombocytopenia. Ann N Y Acad Sci 996, 17-25 (2003).
3 de Laval, B. et al. Thrombopoietin-Increased DNA-PK-Dependent DNA Repair Limits Hematopoietic Stem and Progenitor Cell Mutagenesis in Response to DNA Damage. Cell Stem Cell 12, 37-48, doi:10.1016/j.stem.2012.10.012 (2013).
4 Rossi, D. J. et al. Deficiencies in DNA damage repair limit the function of haematopoietic stem cells with age. Nature 447, 725-729, doi:10.1038/nature05862 (2007).
5 Nijnik, A. et al. DNA repair is limiting for haematopoietic stem cells during ageing. Nature 447, 686-690, doi:10.1038/nature05875 (2007).
6 Kuter, D. J. New thrombopoietic growth factors. Clin Lymphoma Myeloma 9 Suppl 3, S347-356, doi:10.3816/CLM.2009.s.034 (2009).