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PPM1D encodes a phosphatase that is recurrently activated across cancer, most notably in therapy-related myeloid neoplasms. However, the function of PPM1D in hematopoiesis and its contribution to tumor cell growth remain incompletely understood. Using conditional mouse models, we uncover a central role for Ppm1d in hematopoiesis and validate its potential as a therapeutic target. We find that Ppm1d regulates the competitive fitness and self-renewal of hematopoietic stem cells (HSCs) with and without exogenous genotoxic stresses. We also show that although Ppm1d activation confers cellular resistance to cytotoxic therapy, it does so to a lesser degree than p53 loss, informing the clonal competition phenotypes often observed in human studies. Notably, loss of Ppm1d sensitizes leukemias to cytotoxic therapies in vitro and in vivo, even in the absence of a Ppm1d mutation. Vulnerability to PPM1D inhibition is observed across many cancer types and dependent on p53 activity. Importantly, organism-wide loss of Ppm1d in adult mice is well tolerated, supporting the tolerability of pharmacologically targeting PPM1D. Our data link PPM1D gain-of-function mutations to the clonal expansion of HSCs, inform human genetic observations, and support the therapeutic targeting of PPM1D in cancer., (© 2023 by The American Society of Hematology. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).)

Although the molecular pathways that cause acute myeloid leukemia (AML) are increasingly well understood, the pathogenesis of peripheral blood cytopenia, a major cause of AML mortality, remains obscure. A prevailing assumption states that AML spatially displaces nonleukemic hematopoiesis from the bone marrow. However, examining an initial cohort of 223 AML patients, we found no correlation between bone marrow blast content and cytopenia, questioning the displacement theory. Measuring serum concentration of thrombopoietin (TPO), a key regulator of hematopoietic stem cells and megakaryocytes, revealed loss of physiologic negative correlation with platelet count in AML cases with blasts expressing MPL, the thrombopoietin (scavenging) receptor. Mechanistic studies demonstrated that MPL hi blasts could indeed clear TPO, likely therefore leading to insufficient cytokine levels for nonleukemic hematopoiesis. Microarray analysis in an independent multicenter study cohort of 437 AML cases validated MPL expression as a central predictor of thrombocytopenia and neutropenia in AML. Moreover, t(8;21) AML cases demonstrated the highest average MPL expression and lowest average platelet and absolute neutrophil counts among subgroups. Our work thus explains the pathophysiology of peripheral blood cytopenia in a relevant number of AML cases., (© 2016 by The American Society of Hematology.)

Favorable-risk human acute myeloid leukemia (AML) engrafts poorly in currently used immunodeficient mice, possibly because of insufficient environmental support of these leukemic entities. To address this limitation, we here transplanted primary human AML with isolated nucleophosmin (NPM1) mutation and AML with inv(16) in mice in which human versions of genes encoding cytokines important for myelopoiesis (macrophage colony-stimulating factor [M-CSF], interleukin-3, granulocyte-macrophage colony-stimulating factor, and thrombopoietin) were knocked into their respective mouse loci. NPM1 mut AML engrafted with higher efficacy in cytokine knock-in (KI) mice and showed a trend toward higher bone marrow engraftment levels in comparison with NSG mice. inv(16) AML engrafted with high efficacy and was serially transplantable in cytokine KI mice but, in contrast, exhibited virtually no engraftment in NSG mice. Selected use of cytokine KI mice revealed that human M-CSF was required for inv(16) AML engraftment. Subsequent transcriptome profiling in an independent AML patient study cohort demonstrated high expression of M-CSF receptor and enrichment of M-CSF inducible genes in inv(16) AML cases. This study thus provides a first xenotransplantation mouse model for and informs on the disease biology of inv(16) AML., (© 2016 by The American Society of Hematology.)

Human CD34 + hematopoietic stem and progenitor cells (HSPCs) can reconstitute a human hemato-lymphoid system when transplanted into immunocompromised mice. Although fetal liver-derived and cord blood-derived CD34 + cells lead to high engraftment levels, engraftment of mobilized, adult donor-derived CD34 + cells has remained poor. We generated so-called MSTRG and MISTRG humanized mice on a Rag2 -/- Il2rg -/- background carrying a transgene for human signal regulatory protein α (SIRPα) and human homologs of the cytokine macrophage colony-stimulating factor, thrombopoietin, with or without interleukin-3 and granulocyte-macrophage colony-stimulating factor under murine promoters. Here we transplanted mobilized peripheral blood (PB) CD34 + cells in sublethally irradiated newborn and adult recipients. Human hematopoietic engraftment levels were significantly higher in bone marrow (BM), spleen, and PB in newborn transplanted MSTRG/MISTRG as compared with nonobese diabetic/severe combined immunodeficient Il2rg -/- or human SIRPα-transgenic Rag2 -/- Il2rg -/- recipients. Furthermore, newborn transplanted MSTRG/MISTRG mice supported higher engraftment levels of human phenotypically defined HSPCs in BM, T cells in the thymus, and myeloid cells in nonhematopoietic organs such as liver, lung, colon, and skin, approximating the levels in the human system. Similar results were obtained in adult recipient mice. Thus, human cytokine knock-in mice might open new avenues for personalized studies of human pathophysiology of the hematopoietic and immune system in vivo., (© 2016 by The American Society of Hematology.)