Key Points

  • Monocytes that accumulate in patients with a chronic myelomonocytic leukemia have a defective apoptosis through addiction to MCL1

  • Combined MCL1 and MEK inhibition restores monocyte apoptosis and decreases leukemic burden in xenografted animals

Mouse models of chronic myeloid malignancies suggest that targeting mature cells of the malignant clone disrupts feedback loops that promote disease expansion. Here, we show that, in chronic myelomonocytic leukemia (CMML), monocytes that accumulate in the peripheral blood show a decreased propensity to die by apoptosis. BH3 profiling demonstrates their addiction to MCL1 (myeloid cell leukemia-1), which can be targeted with the small molecule inhibitor S63845. RNA sequencing and DNA methylation pattern analysis both point also to the implication of the MAPK (mitogen-activated protein kinase) pathway in the resistance of CMML monocytes to death and reveal an autocrine pathway in which the secreted cytokine CYTL1 (Cytokine-like protein 1) promotes ERK (extracellular signal-regulated kinase) activation through CCR2 (C-C chemokine receptor type 2). Combined MAPK and MCL1 inhibition restores apoptosis of CMML patient monocytes and reduces the expansion of patient-derived xenografts in mice. These results designate the combined inhibition of MCL1 and MAPK as a promising approach to slow down CMML progression by inducing leukemic monocyte apoptosis.

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