Apoptosis is important in controlling hematopoietic stem cell (HSC) numbers. However, the specific BCL-2 family member(s) that regulate HSC homeostasis are not precisely defined. We tested myeloid leukemia-1 (MCL-1) as an attractive candidate that is highly expressed in HSCs and regulated by growth factor signals. Inducible deletion of Mcl-1 in mice resulted in ablation of bone marrow. This resulted in the loss of early bone marrow progenitor populations, including HSCs. Moreover, growth factors including stem cell factor increased transcription of the Mcl-1 gene and required MCL-1 to augment survival of purified bone marrow progenitors. Deletion of Mcl-1 in other tissues, including liver, did not impair survival. Thus, MCL-1 is a critical and specific regulator essential for ensuring the homeostasis of early hematopoietic progenitors.

During 'emergency' situations such as infections, host defense requires rapid mobilization of bone marrow granulocyte progenitors. 'Steady-state' granulopoiesis is absolutely dependent on the C/EBPalpha transcription factor, but the transcriptional mechanisms underlying emergency granulopoiesis remain unclear. Here we show that large numbers of granulocytes were generated from C/EBPalpha-deficient progenitors after cytokine stimulation in vivo. Cytokine treatment or fungal infection induced upregulation of C/EBPbeta but not C/EBPalpha or C/EBPepsilon transcripts in granulocyte progenitors, and C/EBPbeta-deficient progenitors showed decreased emergency-induced granulopoiesis in vitro and in vivo. C/EBPbeta inhibited proliferation less severely than did C/EBPalpha. These data suggest a critical function for C/EBPbeta in emergency granulopoiesis, which demands both differentiation and proliferation of granulocyte precursors.

Basophils and mast cells, which are selectively endowed with the high-affinity IgE receptor and mediate a range of adaptive and innate immune responses, have an unknown developmental relationship. Here, by evaluating the expression of the ␤7 integrin, a molecule that is required for selective homing of mast cell progenitors (MCPs) to the periphery, we identified bipotent progenitors that are capable of differentiating into either cell type in the mouse spleen. These basophil͞mast cell progenitors (BMCPs) gave rise to basophils and mast cells at the single-cell level and reconstituted both mucosal and connective tissue mast cells. We also identified the basophil progenitor (BaP) and the MCP in the bone marrow and the gastrointestinal mucosa, respectively. We further show that the granulocyte-related transcription factor CCAAT͞ enhancer-binding protein ␣ (C͞EBP␣) plays a primary role in the fate decision of BMCPs, being expressed in BaPs but not in MCPs. Thus, circulating basophils and tissue mast cells share a common developmental stage at which their fate decision might be controlled principally by C͞EBP␣.integrin ͉ progenitor ͉ CCAAT͞enhancer-binding protein

No abstract

Aberrant hypermethylation of tumor suppressor genes plays an important role in the development of many tumors. Recently identified new DNA methyltransferase (DNMT) genes, DNMT3A and DNMT3B, code for de novo methyltransferases. To determine the roles of DNMT3A, DNMT3B, as well as DNMT1, in the development of leukemia, competitive polymerase chain reaction (PCR) assays were performed and the expression levels of DNMTs were measured in normal hematopoiesis, 33 cases of acute myelogenous leukemia (AML), and 17 cases of chronic myelogenous leukemia (CML). All genes were constitutively expressed, although at different levels, in T lymphocytes, monocytes, neutrophils, and normal bone marrow cells. Interestingly, DNMT3B was expressed at high levels in CD34 ؉ bone marrow cells but down-regulated in differentiated cells. In AML, 5.3-, 4.4-, and 11.7-fold mean increases were seen in the levels of DNMT1, 3A, and 3B, respectively, compared with the control bone marrow cells. Although CML cells in the chronic phase did not show significant changes, cells in the acute phase showed 3.2-, 4.5-, and 3.4-fold mean increases in the levels of DNMT1, 3A, and 3B, respectively. Using methylation-specific PCR, it was observed that the p15 INAK4B IntroductionDNA methylation plays an important role in tissue-and stagespecific gene regulation, 1,2 genomic imprinting, 3,4 and X-chromosome inactivation, 5 and has been shown to be essential for normal mammalian development. 6 Recent studies have revealed that both global DNA hypomethylation and regional hypermethylation occur in tumorigenesis. [7][8][9] Such aberrant DNA methylation is observed in a nonrandom, tumor type-specific manner. 10 In particular, certain types of tumors show regional hypermethylation of CpG islands associated with the promoter regions of tumor suppressor genes, such as RB, 11 VHL, 12 p16 INAK4A , 13 and hMLH1. 14 Furthermore, the regional hypermethylation is often associated with the inactivation of the tumor suppressor genes. 15 These data suggest that this epigenetic process has a pathogenetic role in the clonal evolution of cancer. 9 In hematologic malignancies, aberrant DNA hypermethylation is thought to have relevance to leukemogenesis. 16 For example, during the progression of chronic myelogenous leukemia (CML), the ABL1 promoter of the BCR-ABL fusion gene becomes significantly hypermethylated. 17,18 Also, aberrant hypermethylation of the p15 INAK4B tumor suppressor gene is associated with its inactivation in at least half of the patients with acute lymphoblastic leukemia (ALL) and acute myelogenous leukemia (AML). 19,20 Furthermore, hypermethylation of p15 INAK4B is observed concomitant with the disease progression in myelodysplastic syndrome (MDS). 21 In addition to these tumor-related genes, a number of other genes are concurrently hypermethylated in AML, 22 suggesting that there might be a dysregulation in the normal DNA methylation mechanism, by which the leukemic cells become predisposed to hypermethylation.Until recently, only one mammalian DNA methyl...

The mechanism of lineage specification in multipotent stem cells has not been fully understood. We recently isolated progenitors with the eosinophil, basophil, or mast cell lineage potential, all of which originate from granulocyte/monocyte progenitors (GMPs). By using these prospectively purified progenitors, we show here that the expression timing of GATA-2 and CCAAT enhancer-binding protein ␣ (C/EBP␣) can differentially control their lineage commitment. The expression of GATA-2 instructed C/EBP␣-expressing GMPs to commit exclusively into the eosinophil lineage, while it induced basophil and/or mast cell lineage commitment if C/EBP␣ was suppressed at the GMP stage. Furthermore, simply by switching the order of C/EBP␣ and GATA-2 transduction, even lymphoid-committed progenitors recaptured these developmental processes to be reprogrammed into each of these lineages. We propose that the order of expression of key transcription factors is critical for their interplay to selectively drive lineage specification programs, by which stem cells could generate multiple lineage cells in a hierarchical manner.[Keywords: Hematopoiesis; hematopoietic progenitors; transcription factor] Supplemental material is available at http://www.genesdev.org.

A hierarchical hematopoietic development with myeloid versus lymphoid bifurcation has been proposed downstream of the multipotent progenitor (MPP) stage, based on prospective isolation of progenitors capable of generating only myeloerythroid cells (common myeloid progenitor, CMP) or only lymphocytes (common lymphoid progenitor, CLP). By utilizing GATA-1 and PU.1 transcription factor reporters, here we identified progenitor populations that are precursors for either CMPs or CLPs. Two independent populations expressing either GATA-1 or PU.1 resided within the CD34(+)Sca-1(+)c-Kit(+) MPP fraction. The GATA-1(+) MPP displayed potent myeloerythroid potential without giving rise to lymphocytes, whereas the PU.1(+) MPP showed granulocyte/monocyte/lymphoid-restricted progenitor activity without megakaryocyte/erythroid differentiation. Furthermore, GATA-1(+) and PU.1(+) MPPs possessed huge expansion potential and differentiated into the original CMPs and CLPs, respectively. Thus, the reciprocal activation of GATA-1 and PU.1 primarily organizes the hematopoietic lineage fate decision to form the earliest hematopoietic branchpoint that comprises isolatable myeloerythroid and myelolymphoid progenitor populations.

The developmental origin of type I interferon (IFN)-producing plasmacytoid dendritic cells (PDCs) is controversial. In particular, the rearrangement of immunoglobulin heavy chain (IgH) genes in murine PDCs and the expression of pre-T cell receptor alpha (pTalpha) gene by human PDCs were proposed as evidence for their "lymphoid" origin. Here we demonstrate that PDCs capable of IFN production develop efficiently from both myeloid- and lymphoid-committed progenitors. Rearranged IgH genes as well as RAG transcripts were found in both myeloid- and lymphoid-derived PDCs. The human pTalpha transgenic reporter was activated in both myeloid- and lymphoid-derived PDCs at a level comparable to pre-T cells. PDCs were the only cell population that activated murine RAG1 knockin and human pTalpha transgenic reporters outside the lymphoid lineage. These results highlight a unique developmental program of PDCs that distinguishes them from other cell types including conventional dendritic cells.