Cytoprotective autophagy maintains leukemia-initiating cells in murine myeloid leukemia.

Despite advances in the treatment of acute myeloid leukemia (AML), relapse and drug resistance frequently occur. Therefore, detailed mechanisms of refractoriness, including leukemia-initiating cell (LIC) biology, should be elucidated to treat AML. The self-degradative property of cytosolic macromolecules is central to autophagy and can contribute to homeostasis and stress response. Recent reports suggest the importance of autophagy in hematopoietic stem cells and various tumors. Thus, this study investigated the functional role of autophagy in AML maintenance and drug resistance using tamoxifen-inducible conditional knockout mice of Atg5 or Atg7, which are essential genes for autophagy, combined with an mixed lineage leukemia-eleven nineteen leukemia-induced murine AML model. Inactivation of autophagy by deletion of Atg5 or Atg7 prolonged survival in leukemic mice and reduced functional LICs. Atg7-deficient LICs displayed enhanced mitochondrial activity and reactive oxygen species production together with increased cell death. In addition, Atg7 deletion markedly decreased peripheral blood leukemia cells, concurrent with increased apoptosis, suggesting a higher dependency on autophagy compared with bone marrow leukemia cells. Finally, cytarabine (AraC) treatment activated autophagy in LICs, and Atg7 deletion potentiated the therapeutic effects of AraC, which included decreased LICs and prolonged survival, suggesting that autophagy contributes to AraC resistance. Our results highlight the intratumoral heterogeneity related to autophagy in AML and the unique role of autophagy in leukemia development and drug resistance.

JAK2V617F+ myeloproliferative neoplasm clones evoke paracrine DNA damage to adjacent normal cells through secretion of lipocalin-2.

Genetic instability is strongly involved in cancer development and progression, and elucidating the mechanism could lead to novel therapeutics for preventing carcinogenesis. Philadelphia-negative myeloproliferative neoplasms (MPNs) are clonal myeloid disorders with a high prevalence of JAK2V617F mutation, and transformation to acute myeloid leukemia through accumulation of additional mutations is a major complication in MPNs. Here, we showed that JAK2V617F(+) cells conferred paracrine DNA damage to neighboring normal cells as well as to themselves through increased reactive oxygen species (ROS). We screened candidate factors responsible for the effect and found that lipocalin-2 (Lcn2) is overexpressed in JAK2V617F(+) cells and that short hairpin RNA-mediated knockdown of Lcn2 significantly alleviated the paracrine DNA damage. Normal hematopoietic cells showed elevated ROS levels through increased intracellular iron levels when treated with lipocalin-2, which led to p53 pathway activation, increased apoptosis, and decreased cellular proliferation. In contrast, JAK2V617F(+) cells did not suffer from lipocalin-2-induced growth suppression resulting from attenuated p53 pathway activation, which conferred a relative growth advantage to JAK2V617F(+) clones. In summary, we demonstrated that JAK2V617F-harboring cells cause paracrine DNA damage accumulation through secretion of lipocalin-2, which gives proliferative advantage to themselves and an increased risk for leukemic transformation to both JAK2V617F(+) and JAK2V617F(-) clones.

Heterozygous Dnmt3a R878C induces expansion of quiescent hematopoietic stem cell pool.

Somatic mutation of DNMT3A (DNA methyltransferase 3 alpha) is implicated in the development of a wide range of hematological disorders, including clonal hematopoiesis of indeterminate potential. To elucidate the functional roles of endogenous levels of a DNMT3A R882 mutant, we generated a novel Dnmt3a R878C conditional knock-in mouse model. In contrast to viable heterozygotes, mice homozygous for the Dnmt3a R878C mutation in the hematopoietic system were not viable (Dnmt3a R878C is homologous to human DNMT3A R882C). Hematopoietic cell-specific heterozygous expression of Dnmt3a R878C led to significant expansion of adult quiescent hematopoietic stem cells (HSCs); however, these mice had no hematological malignancies. The expanding HSC population in heterozygous Dnmt3a R878C knock-in mice had an accumulation of G0-phase cells. In contrast to aberrantly enhanced self-renewal capacity in vitro, heterozygous Dnmt3a R878C knock-in HSCs had no competitive repopulating advantage in vivo over wild-type HSCs. Considering the capacity of the heterozygous Dnmt3a R878C mutant for HSC pool expansion, our Dnmt3a R878C knock-in mouse line is a useful platform on which to dissect the pathophysiology of clonal hematopoiesis. This mouse line can also help to elucidate the biological and molecular actions of DNMT3A mutations in the malignant transformation of normal HSCs.

DNMT3A R882 mutants interact with polycomb proteins to block haematopoietic stem and leukaemic cell differentiation.

Despite the clinical impact of DNMT3A mutation on acute myeloid leukaemia, the molecular mechanisms regarding how this mutation causes leukaemogenesis in vivo are largely unknown. Here we show that, in murine transplantation experiments, recipients transplanted with DNMT3A mutant-transduced cells exhibit aberrant haematopoietic stem cell (HSC) accumulation. Differentiation-associated genes are downregulated without accompanying changes in methylation status of their promoter-associated CpG islands in DNMT3A mutant-transduced stem/progenitor cells, representing a DNA methylation-independent role of mutated DNMT3A. DNMT3A R882H also promotes monoblastic transformation in vitro in combination with HOXA9. Molecularly, the DNMT3A mutant interacts with polycomb repressive complex 1 (PRC1), causing transcriptional silencing, revealing a DNA methylation-independent role of DNMT3A mutation. Suppression of PRC1 impairs aberrant HSC accumulation and monoblastic transformation. From our data, it is shown that DNMT3A mutants can block the differentiation of HSCs and leukaemic cells via PRC1. This interaction could be targetable in DNMT3A-mutated leukaemias.