Evasion of Necroptosis and Inflammasome Activation Promotes Myeloid Leukemogenesis

Acute myeloid leukemia (AML) is a heterogeneous group of hematopoietic neoplasms driven partly by the loss of differentiation and theblockade of cell death. AML is sustained by leukemia-initiating cells (LICs) that arise from pre-leukemic hematopoietic stem and progenitor cells (HSPCs) that carry genetic alterations being selected for during leukemogenesis. The resistance of LICs to standard chemotherapies presents a major clinical challenge as they eventually cause disease relapse and death. Understanding the mechanisms of LIC resistance to undergoing cell death is therefore critical for a curative therapy of AML.

While the regulatory factors that maintain HSPC proliferation and differentiation under normal conditions are well understood, significantly less is known about how LIC fate is regulated. As many hematopoietic disorders are characterized by the overproduction of pro-inflammatory cytokines, we hypothesized that necroptosis controlled cytokine secretion and inflammatory cell death might influence AML development. We therefore addressed the role of MLKL and XIAP in AML and tested whether deletion of Mlkl or Xiap would affect disease progression.

Here we show that MLKL limits oncogene-mediated leukemogenesis by promoting the inflammatory cell death of common myeloid progenitors (CMPs) and short-term hematopoietic stem cells (HSCs) in experimental mice. Upon oncogenic stress MLKL-dependent necroptosis and subsequent inflammasome activation were triggered, promoting the production of IL-1β, a potent stimulator of HSPC differentiation and maturation, thus, suppressing the emergence of LICs and limiting leukemogenesis.

In a murine bone marrow transplantation model of AML the absence of MLKL accelerated AML development significantly. The enhanced disease was due to the expansion of common myeloid progenitors (CMPs) and short-term hematopoietic stem cells (ST-HSCs), being the cellular compartments to contain LICs. The survival advantage of Mlkl^-/- HSPCs became apparent in colony-forming assays and liquid cultures specifically within the CMP and ST-HSC compartments. Sorted ST-HSCs from Mlkl^-/- produced more GEMM colonies than WT, the colony type harboring the multipotential myeloid progenitor cells, and both ST-HSCs and CMPs retained significantly more lineage-negative cells in liquid culture. In addition, Mlkl^-/- colonies showed a reduction in propidium iodide (PI)-positive dead cells compared with WT colonies. Importantly, WT cells showed caspase activation and produced substantial amounts of the inflammatory cytokine IL-1β which was severely blunted by Mlkl deficiency. We also observed reduced expression of MLKL in leukemic cells on both mRNA and protein level, implying that suppression of cell death was beneficial for the survival of LICs.

In contrast, deletion of Xiap did not alter survival or differentiation of leukemic cells when compared with WT cells. Furthermore, XIAP was not differentially expressed on mRNA or protein level compared with WT, indicating that XIAP does not play a critical role in leukemogenesis.

In agreement with the murine data, gene expression analysis from primary leukemia cells from two large patient cohorts newly diagnosed with AML showed significantly lower expression of MLKL, but not XIAP, in a variety of AML subtypes compared to healthy controls.

Overall, our data demonstrate a key role for MLKL-mediated cell death and activation of the inflammasome in AML and represents a novel tumor-suppressive mechanism.