Differential in vivo roles of Mpl cytoplasmic tyrosine residues in murine hematopoiesis and myeloproliferative disease.

Thrombopoietin (Tpo), which binds to its specific receptor, the Mpl protein, is the major cytokine regulator of megakaryopoiesis and circulating platelet number. Tpo binding to Mpl triggers activation of Janus kinase 2 (Jak2) and phosphorylation of the receptor, as well as activation of several intracellular signalling cascades that mediate cellular responses. Three tyrosine (Y) residues in the C-terminal region of the Mpl intracellular domain have been implicated as sites of phosphorylation required for regulation of major Tpo-stimulated signalling pathways: Mpl-Y565, Mpl-Y599 and Mpl-Y604.

CD45 limits early Natural Killer cell development.

The clinical development of Natural Killer (NK) cell-mediated immunotherapy marks a milestone in the development of new cancer therapies and has gained traction due to the intrinsic ability of the NK cell to target and kill tumor cells. To fully harness the tumor killing ability of NK cells, we need to improve NK cell persistence and to overcome suppression of NK cell activation in the tumor microenvironment. The trans-membrane, protein tyrosine phosphatase CD45, regulates NK cell homeostasis, with the genetic loss of CD45 in mice resulting in increased numbers of mature NK cells.

The histone lysine acetyltransferase HBO1 (KAT7) regulates hematopoietic stem cell quiescence and self-renewal.

The histone acetyltransferase HBO1 (MYST2, KAT7) is indispensable for postgastrulation development, histone H3 lysine 14 acetylation (H3K14Ac), and the expression of embryonic patterning genes. In this study, we report the role of HBO1 in regulating hematopoietic stem cell function in adult hematopoiesis. We used 2 complementary cre-recombinase transgenes to conditionally delete Hbo1 (Mx1-Cre and Rosa26-CreERT2).

An Erg-driven transcriptional program controls B cell lymphopoiesis.

B lymphoid development is initiated by the differentiation of hematopoietic stem cells into lineage committed progenitors, ultimately generating mature B cells. This highly regulated process generates clonal immunological diversity via recombination of immunoglobulin V, D and J gene segments. While several transcription factors that control B cell development and V(D)J recombination have been defined, how these processes are initiated and coordinated into a precise regulatory network remains poorly understood.

miR17~92 restrains pro-apoptotic BIM to ensure survival of haematopoietic stem and progenitor cells.

The miR17~92 cluster plays important roles in haematopoiesis. However, it is not clear at what stage of differentiation and through which targets miR17~92 exerts this function. Therefore, we generated miR17~92; RosaCreERT2 mice for inducible deletion of miR17~92 in haematopoietic cells.

Altered B-lymphopoiesis in mice with deregulated thrombopoietin signaling.

Thrombopoietin (TPO) is the master cytokine regulator of megakaryopoiesis. In addition to regulation of megakaryocyte and platelet number, TPO is important for maintaining proper hematopoietic stem cell (HSC) function. It was previously shown that a number of lymphoid genes were upregulated in HSCs from Tpo mice.

The combination of reduced MCL-1 and standard chemotherapeutics is tolerable in mice.

A common therapeutic strategy to combat human cancer is the use of combinations of drugs, each targeting different cellular processes or vulnerabilities. Recent studies suggest that addition of an MCL-1 inhibitor to such anticancer drug treatments could be an attractive therapeutic strategy. Thus, it is of great interest to understand whether combinations of conventional anticancer drugs with an MCL-1 inhibitor will be tolerable and efficacious.

Early lineage priming by trisomy of Erg leads to myeloproliferation in a Down syndrome model.

Down syndrome (DS), with trisomy of chromosome 21 (HSA21), is the commonest human aneuploidy. Pre-leukemic myeloproliferative changes in DS foetal livers precede the acquisition of GATA1 mutations, transient myeloproliferative disorder (DS-TMD) and acute megakaryocytic leukemia (DS-AMKL). Trisomy of the Erg gene is required for myeloproliferation in the Ts(1716)65Dn DS mouse model.

Mpl expression on megakaryocytes and platelets is dispensable for thrombopoiesis but essential to prevent myeloproliferation.

Thrombopoietin (TPO) acting via its receptor, the cellular homologue of the myeloproliferative leukemia virus oncogene (Mpl), is the major cytokine regulator of platelet number. To precisely define the role of specific hematopoietic cells in TPO-dependent hematopoiesis, we generated mice that express the Mpl receptor normally on stem/progenitor cells but lack expression on megakaryocytes and platelets (Mpl(PF4cre/PF4cre)). Mpl(PF4cre/PF4cre) mice displayed profound megakaryocytosis and thrombocytosis with a remarkable expansion of megakaryocyte-committed and multipotential progenitor cells, the latter displaying biological responses and a gene expression signature indicative of chronic TPO overstimulation as the underlying causative mechanism, despite a normal circulating TPO level.

Polycomb repressive complex 2 (PRC2) suppresses Eμ-myc lymphoma.

Deregulation of polycomb group complexes polycomb repressive complex 1 (PRC1) and 2 (PRC2) is associated with human cancers. Although inactivating mutations in PRC2-encoding genes EZH2, EED, and SUZ12 are present in T-cell acute lymphoblastic leukemia and in myeloid malignancies, gain-of-function mutations in EZH2 are frequently observed in B-cell lymphoma, implying disease-dependent effects of individual mutations. We show that, in contrast to PRC1, PRC2 is a tumor suppressor in Eµ-myc lymphomagenesis, because disease onset was accelerated by heterozygosity for Suz12 or by short hairpin RNA-mediated knockdown of Suz12 or Ezh2.

Transposon mutagenesis reveals cooperation of ETS family transcription factors with signaling pathways in erythro-megakaryocytic leukemia.

To define genetic lesions driving leukemia, we targeted cre-dependent Sleeping Beauty (SB) transposon mutagenesis to the blood-forming system using a hematopoietic-selective vav 1 oncogene (vav1) promoter. Leukemias of diverse lineages ensued, most commonly lymphoid leukemia and erythroleukemia. The inclusion of a transgenic allele of Janus kinase 2 (JAK2)V617F resulted in acceleration of transposon-driven disease and strong selection for erythroleukemic pathology with transformation of bipotential erythro-megakaryocytic cells.

Characterization of thrombopoietin (TPO)-responsive progenitor cells in adult mouse bone marrow with in vivo megakaryocyte and erythroid potential.

Hematopoietic progenitor cells are the progeny of hematopoietic stem cells that coordinate the production of precise numbers of mature blood cells of diverse functional lineages. Identification of cell-surface antigen expression associated with hematopoietic lineage restriction has allowed prospective isolation of progenitor cells with defined hematopoietic potential. To clarify further the cellular origins of megakaryocyte commitment, we assessed the in vitro and in vivo megakaryocyte and platelet potential of defined progenitor populations in the adult mouse bone marrow.

Thrombocytopenia and erythrocytosis in mice with a mutation in the gene encoding the hemoglobin β minor chain.

Diverse mutations in the genes encoding hemoglobin (Hb) have been characterized in human disease. We describe here a mutation in the mouse Hbb-b2 gene, denoted Plt12, that precisely mimics the human hemoglobin Hotel Dieu variant. The mutation results in increased affinity of Hb for oxygen and Plt12 mutant mice exhibited reduced partial pressure of O(2) in the blood, accompanied by erythrocytosis characterized by elevated erythropoietin levels and splenomegaly with excess erythropoiesis.

Erg is required for self-renewal of hematopoietic stem cells during stress hematopoiesis in mice.

Hematopoietic stem cells (HSCs) are rare residents of the bone marrow responsible for the lifelong production of blood cells. Regulation of the balance between HSC self-renewal and differentiation is central to hematopoiesis, allowing precisely regulated generation of mature blood cells at steady state and expanded production at times of rapid need, as well as maintaining ongoing stem cell capacity. Erg, a member of the Ets family of transcription factors, is deregulated in cancers; and although Erg is known to be required for regulation of adult HSCs, its precise role has not been defined.

Regulation of hematopoietic stem cells by their mature progeny.

Thrombopoietin (TPO), acting through its receptor Mpl, has two major physiological roles: ensuring production of sufficient platelets via stimulation of megakaryocyte production and maintaining hematopoietic stem cell (HSC) quiescence. Mpl also controls circulating TPO concentration via receptor-mediated internalization and degradation. Here, we demonstrate that the megakaryocytosis and increased platelet mass in mice with mutations in the Myb or p300 genes causes reduced circulating TPO concentration and TPO starvation of the stem-cell compartment, which is exacerbated because these cells additionally exhibit impaired responsiveness to TPO.

Deficiency of 5-hydroxyisourate hydrolase causes hepatomegaly and hepatocellular carcinoma in mice.

With the notable exception of humans, uric acid is degraded to (S)-allantoin in a biochemical pathway catalyzed by urate oxidase, 5-hydroxyisourate (HIU) hydrolase, and 2-oxo-4-hydroxy-4-carboxy-5-ureidoimidazoline decarboxylase in most vertebrate species. A point mutation in the gene encoding mouse HIU hydrolase, Urah, that perturbed uric acid metabolism within the liver was discovered during a mutagenesis screen in mice. The predicted substitution of cysteine for tyrosine in a conserved helical region of the mutant-encoded HIU hydrolase resulted in undetectable protein expression.

Trisomy of Erg is required for myeloproliferation in a mouse model of Down syndrome.

Down syndrome is characterized by multiple phenotypic manifestations associated with trisomy of chromosome 21. The transient myeloproliferative disorder and acute megakaryocytic leukemia associated with Down syndrome are uniquely associated with mutations in the transcription factor GATA1; however, the identity of trisomic genes on chromosome 21 that predispose to these hematologic disorders remains unknown. Using a loss-of-function allele, we show that specific reduction to functional disomy of the Erg gene corrects the pathologic and hematologic features of myeloproliferation in the Ts(17(16))65Dn mouse model of Down syndrome, including megakaryocytosis and progenitor cell expansion.

The transcription factor Erg is essential for definitive hematopoiesis and the function of adult hematopoietic stem cells.

Ets-related gene (ERG), which encodes a member of the Ets family of transcription factors, is a potent oncogene. Chromosomal rearrangements involving ERG are found in acute myeloid leukemia, acute lymphoblastic leukemia, Ewing's sarcoma and more than half of all prostate cancers; however, the normal physiological function of Erg is unknown. We did a sensitized genetic screen of the mouse for regulators of hematopoietic stem cell function and report here a germline mutation of Erg.

Polycomb repressive complex 2 (PRC2) restricts hematopoietic stem cell activity.

Polycomb group proteins are transcriptional repressors that play a central role in the establishment and maintenance of gene expression patterns during development. Using mice with an N-ethyl-N-nitrosourea (ENU)-induced mutation in Suppressor of Zeste 12 (Suz12), a core component of Polycomb Repressive Complex 2 (PRC2), we show here that loss of Suz12 function enhances hematopoietic stem cell (HSC) activity. In addition to these effects on a wild-type genetic background, mutations in Suz12 are sufficient to ameliorate the stem cell defect and thrombocytopenia present in mice that lack the thrombopoietin receptor (c-Mpl).

Critical role of thrombopoietin in maintaining adult quiescent hematopoietic stem cells.

The role of cytokines in regulation of hematopoietic stem cells (HSCs) remains poorly understood. Herein we demonstrate that thrombopoietin (THPO) and its receptor, MPL, are critically involved in postnatal steady-state HSC maintenance, reflected in a 150-fold reduction of HSCs in adult Thpo(-/-) mice. Further, whereas THPO and MPL proved not required for fetal HSC expansion, HSC expansion posttransplantation was highly MPL and THPO dependent.

Thrombocytopenia and kidney disease in mice with a mutation in the C1galt1 gene.

An N-ethyl-N-nitrosourea mutagenesis screen in mice was performed to isolate regulators of circulating platelet number. We report here recessive thrombocytopenia and kidney disease in plt1 mice, which is the result of a severe but partial loss-of-function mutation in the gene encoding glycoprotein-N-acetylgalactosamine-3-beta-galactosyltransferase (C1GalT1), an enzyme essential for the synthesis of extended mucin-type O-glycans. Platelet half-life and basic hemostatic parameters were unaffected in plt1/plt1 mice, and the thrombocytopenia and kidney disease were not attenuated on a lymphocyte-deficient rag1-null background.

Synergistic effects on erythropoiesis, thrombopoiesis, and stem cell competitiveness in mice deficient in thrombopoietin and steel factor receptors.

The degree of redundancy between thrombopoietin (Tpo) and steel factor (SF) cytokine pathways in the regulation of hematopoiesis was investigated by generating mice lacking both c-Mpl and fully functional c-Kit receptors. Double-mutant c-Mpl(-/-)Kit(Wv/Wv) mice exhibited reduced viability, making up only 2% of the offspring from c-Mpl(-/-)Kit(Wv/)(+) intercrosses. The thrombocytopenia and megakaryocytopenia characteristic of c-Mpl(-/-) mice was unchanged in c-Mpl(-/-)Kit(Wv/Wv) mice.

Suppressor screen in Mpl-/- mice: c-Myb mutation causes supraphysiological production of platelets in the absence of thrombopoietin signaling.

Genetic screens in lower organisms, particularly those that identify modifiers of preexisting genetic defects, have been used successfully to order components of complex signaling pathways. To date, similar suppressor screens have not been used in vertebrates. To define the molecular pathways regulating platelet production, we have executed a large-scale modifier screen with genetically thrombocytopenic Mpl(-/-) mice by using N-ethyl-N-nitrosourea mutagenesis.