Highlights d Patients with severe COVID-19 accumulate HLA-DR Low monocytes and immature neutrophils in blood/lungs d Calprotectin level positively correlates with neutrophil count and disease severity d Loss of non-classical monocytes could identify high risk of severe COVID-19

Key Points Early clonal dominance may distinguish chronic myelomonocytic leukemia from other chronic myeloid neoplasms with similar gene mutations. Early dominance of TET2-mutated cells in the hematopoietic tissue promotes myeloid differentiation skewing toward the granulomonocytic line.

Key Points An increase in the classical monocyte subset to >94% of total monocytes discriminates CMML from other monocytoses with high specificity. This characteristic increase in classical monocytes disappears in CMML patients who respond to hypomethylating agents.

megakaryocytes are unique mammalian cells that undergo polyploidization (endomitosis) during differentiation, leading to an increase in cell size and protein production that precedes platelet production. Recent evidence demonstrates that endomitosis is a consequence of a late failure in cytokinesis associated with a contractile ring defect. Here we show that the non-muscle myosin IIB heavy chain (mYH10) is expressed in immature megakaryocytes and specifically localizes in the contractile ring. mYH10 downmodulation by short hairpin RnA increases polyploidization by inhibiting the return of 4n cells to 2n, but other regulators, such as of the G1/s transition, might regulate further polyploidization of the 4n cells. Conversely, re-expression of mYH10 in the megakaryocytes prevents polyploidization and the transition of 2n to 4n cells. During polyploidization, mYH10 expression is repressed by the major megakaryocyte transcription factor RunX1. Thus, RunX1-mediated silencing of mYH10 is required for the switch from mitosis to endomitosis, linking polyploidization with megakaryocyte differentiation.

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FPD/AML is a familial platelet disorder characterized by platelet defects, predisposition to acute myelogenous leukemia (AML) and germ-line heterozygous RUNX1 alterations. Here we studied the in vitro megakaryopoiesis of 3 FPD/AML pedigrees. A 60% to 80% decrease in the output of megakaryocytes (MKs) from CD34 ؉ was observed. MK ploidy level was low and mature MKs displayed a major defect in proplatelet formation. To explain these defects, we focused on myosin II expression as RUNX1 has been shown to regulate MYL9 and MYH10 in an inverse way. In FPD/AML MKs, expression of MYL9 and MYH9 was decreased, whereas MYH10 expression was increased and the MYH10 protein was still present in the cytoplasm of mature MKs. Myosin II activity inhibition by blebbistatin rescued the ploidy defect of FPD/AML MKs. Finally, we demonstrate that MYH9 is a direct target of RUNX1 by chromatin immunoprecipitation and luciferase assays and we identified new RUNX1 binding sites in the MYL9 promoter region.Together, these results demonstrate that the defects in megakaryopoiesis observed in FPD/AML are, in part, related to a deregulation of myosin IIA and IIB expression leading to both a defect in ploidization and proplatelet formation. IntroductionFamilial platelet disorder with predisposition to acute myeloid leukemia (FPD/AML, OMIM 601399) is an autosomal dominant disorder characterized by dysmegakaryopoiesis, qualitative and quantitative platelet defects, and a propensity to develop myelodysplastic syndromes (MDSs) and/or AML. Several types of heterozygous germ-line mutations or deletions in RUNX1, including missense, frameshift, and nonsense mutations or large intragenic deletion or single nucleotide deletion in the Runt domain have been identified in FPD/AML. The progression to AML is often linked to the somatic alteration of the second RUNX1 allele, 1 supporting the fact that RUNX1 acts as a tumor suppressor gene. RUNX1 (also known as AML1, PEBP2aB, or CBFA2) is 1 of the 3 DNA-binding ␣ subunits of the hematopoietic transcription complex called core binding factor (CBF). RUNX1 contains both a runt homology domain (RHD), which mediates DNA binding and heterodimerization with the core binding factor ␤ (CBF␤) subunit to stabilize the interaction of the complex with DNA and to protect CBF from proteolytic degradation. The C-terminal domain of RUNX1 is responsible for transcriptional activation. RUNX1 can act as a repressor or an activator depending on the cellular context. It regulates positively different hematopoietic genes encoding cytokines and their receptors, such as IL-3, 2 GM-CSF, and M-CSF or negatively the CD4 gene contributing thus to impaired T-cell development. 3 Somatic alterations in RUNX1 are frequently found in AML, MDS, and chronic myelomonocytic leukemia (CMML).In different mouse models, RUNX1 was shown to be essential for establishing definitive hematopoiesis. 4 It is required for the generation of hematopoietic stem cells (HSCs) from the aorta, but not later on. Targeted deletion of RUNX1 in adult HSCs led to their ...

Chimeric transcription factors are a hallmark of human leukemia, but the molecular mechanisms by which they block differentiation and promote aberrant self-renewal remain unclear. Here, we demonstrate that the ETO2-GLIS2 fusion oncoprotein, which is found in aggressive acute megakaryoblastic leukemia, confers megakaryocytic identity via the GLIS2 moiety while both ETO2 and GLIS2 domains are required to drive increased self-renewal properties. ETO2-GLIS2 directly binds DNA to control transcription of associated genes by upregulation of expression and interaction with the ETS-related ERG protein at enhancer elements. Importantly, specific interference with ETO2-GLIS2 oligomerization reverses the transcriptional activation at enhancers and promotes megakaryocytic differentiation, providing a relevant interface to target in this poor-prognosis pediatric leukemia.

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