Motoshi Ichikawa

AML-1 is required for megakaryocytic maturation and lymphocytic differentiation, but not for maintenance of hematopoietic stem cells in adult hematopoiesis

Embryonic development of multilineage hematopoiesis requires the precisely regulated expression of lineage-specific transcription factors, including AML-1 (encoded by Runx1; also known as CBFA-2 or PEBP-2αB). In vitro studies and findings in human diseases, including leukemias, myelodysplastic syndromes and familial platelet disorder with predisposition to acute myeloid leukemia (AML), suggest that AML-1 has a pivotal role in adult hematopoiesis. However, this role has not been fully uncovered in vivo because of the embryonic lethality of Runx1 knockout in mice. Here we assess the requirement of AML-1/Runx1 in adult hematopoiesis using an inducible gene-targeting method. In the absence of AML-1, hematopoietic progenitors were fully maintained with normal myeloid cell development. However, AML-1-deficient bone marrow showed inhibition of megakaryocytic maturation, increased hematopoietic progenitor cells and defective T- and B-lymphocyte development. AML-1 is thus required for maturation of megakaryocytes and differentiation of T and B cells, but not for maintenance of hematopoietic stem cells (HSCs) in adult hematopoiesis.

Notch2 Is Preferentially Expressed in Mature B Cells and Indispensable for Marginal Zone B Lineage Development

The Notch genes play a key role in cellular differentiation. The significance of Notch1 during thymocyte development is well characterized, but the function of Notch2 is poorly understood. Here we demonstrate that Notch2 but no other Notch family member is preferentially expressed in mature B cells and that conditionally targeted deletion of Notch2 results in the defect of marginal zone B (MZB) cells and their presumed precursors, CD1d(hi) fraction of type 2 transitional B cells. Among Notch target genes, the expression level of Deltex1 is prominent in MZB cells and strictly dependent on that of Notch2, suggesting that Deltex1 may play a role in MZB cell differentiation.

Evi-1 Is a Critical Regulator for Hematopoietic Stem Cells and Transformed Leukemic Cells

Evi-1 has been recognized as one of the dominant oncogenes associated with murine and human myeloid leukemia. Here, we show that hematopoietic stem cells (HSCs) in Evi-1-deficient embryos are severely reduced in number with defective proliferative and repopulating capacity. Selective ablation of Evi-1 in Tie2(+) cells mimics Evi-1 deficiency, suggesting that Evi-1 function is required in Tie2(+) hematopoietic stem/progenitors. Conditional deletion of Evi-1 in the adult hematopoietic system revealed that Evi-1-deficient bone marrow HSCs cannot maintain hematopoiesis and lose their repopulating ability. In contrast, Evi-1 is dispensable for blood cell lineage commitment. Evi-1(+/-) mice exhibit the intermediate phenotype for HSC activity, suggesting a gene dosage requirement for Evi-1. We further demonstrate that disruption of Evi-1 in transformed leukemic cells leads to significant loss of their proliferative activity both in vitro and in vivo. Thus, Evi-1 is a common and critical regulator essential for proliferation of embryonic/adult HSCs and transformed leukemic cells.

Risk Factors for Drug-Resistant Pathogens in Community-acquired and Healthcare-associated Pneumonia

RATIONALE Identification of patients with drug-resistant pathogens at initial diagnosis is essential for treatment of pneumonia. OBJECTIVES To elucidate clinical features of community-acquired pneumonia (CAP) and healthcare-associated pneumonia (HCAP), and to clarify risk factors for drug-resistant pathogens in patients with CAP and HCAP. METHODS A prospective observational study was conducted in hospitalized patients with pneumonia at 10 institutions in Japan. Pathogens identified as not susceptible to ceftriaxone, ampicillin-sulbactam, macrolides, and respiratory fluoroquinolones were defined as CAP drug-resistant pathogens (CAP-DRPs). MEASUREMENTS AND MAIN RESULTS In total, 1,413 patients (887 CAP and 526 HCAP) were analyzed. CAP-DRPs were more frequently found in patients with HCAP (26.6%) than in patients with CAP (8.6%). Independent risk factors for CAP-DRPs were almost identical in patients with CAP and HCAP. These included prior hospitalization (adjusted odds ratio [AOR], 2.06; 95% confidence interval [CI], 1.23-3.43), immunosuppression (AOR, 2.31; 95% CI, 1.05-5.11), previous antibiotic use (AOR, 2.45; 95% CI, 1.51-3.98), use of gastric acid-suppressive agents (AOR, 2.22; 95% CI, 1.39-3.57), tube feeding (AOR, 2.43; 95% CI, 1.18-5.00), and nonambulatory status (AOR, 2.45; 95% CI, 1.40-4.30) in the combined patients with CAP and HCAP. The area under the receiver operating characteristic curve for counting the number of risk factors was 0.79 (95% CI, 0.74-0.84). CONCLUSIONS The clinical profile of HCAP was different from that of CAP. However, physicians can predict drug resistance in patients with either CAP or HCAP by taking account of the cumulative number of the risk factors. Clinical trial registered with https://upload.umin.ac.jp/cgi-open-bin/ctr/ctr.cgi?function=brows&action=brows&type=summary&recptno=R000004001&language=E ; number UMIN000003306.

AML1/Runx1 negatively regulates quiescent hematopoietic stem cells in adult hematopoiesis.

Transcription factor AML1/Runx1, initially isolated from the t(8;21) chromosomal translocation in human leukemia, is essential for the development of multilineage hematopoiesis in mouse embryos. AML1 negatively regulates the number of immature hematopoietic cells in adult hematopoiesis, whereas it is required for megakaryocytic maturation and lymphocytic development. However, it remains yet to be determined how AML1 contributes to homeostasis of hematopoietic stem cells (HSCs). To address this issue, we analyzed in detail HSC function in the absence of AML1. Notably, cells in the Hoechst 33342 side population fraction are increased in number in AML1-deficient bone marrow, which suggests enrichment of quiescent HSCs. We also found an increase in HSC number within the AML1-deficient bone marrow using limiting dilution bone marrow transplantation assays. These results indicate that the number of quiescent HSCs is negatively regulated by AML1.

Evi-1 is a transcriptional target of mixed-lineage leukemia oncoproteins in hematopoietic stem cells

Ecotropic viral integration site-1 (Evi-1) is a nuclear transcription factor that plays an essential role in the regulation of hematopoietic stem cells. Aberrant expression of Evi-1 has been reported in up to 10% of patients with acute myeloid leukemia and is a diagnostic marker that predicts a poor outcome. Although chromosomal rearrangement involving the Evi-1 gene is one of the major causes of Evi-1 activation, overexpression of Evi-1 is detected in a subgroup of acute myeloid leukemia patients without any chromosomal abnormalities, which indicates the presence of other mechanisms for Evi-1 activation. In this study, we found that Evi-1 is frequently up-regulated in bone marrow cells transformed by the mixed-lineage leukemia (MLL) chimeric genes MLL-ENL or MLL-AF9. Analysis of the Evi-1 gene promoter region revealed that MLL-ENL activates transcription of Evi-1. MLL-ENL-mediated up-regulation of Evi-1 occurs exclusively in the undifferentiated hematopoietic population, in which Evi-1 particularly contributes to the propagation of MLL-ENL-immortalized cells. Furthermore, gene-expression analysis of human acute myeloid leukemia cases demonstrated the stem cell-like gene-expression signature of MLL-rearranged leukemia with high levels of Evi-1. Our findings indicate that Evi-1 is one of the targets of MLL oncoproteins and is selectively activated in hematopoietic stem cell-derived MLL leukemic cells.

Runx1/AML-1 ranks as a master regulator of adult hematopoiesis.

Runx1 (AML-1) is a critical gene involved in human leukemogenesis, originallyidentified at the 21q22 breakpoint of the leukemic translocations of t(8;21)(q21;q22), and is thoughtto be involved in as much as 25% of human leukemia. It encodes a transcription factor that hasclose homology to a Drosophila protein, runt, and is found to play essential roles in regulation ofhematopoietic systems. Really a gene disruption experiment unequivocally shows that Runx1 isabsolutely required for the establishment of definitive or adult-type hematopoiesis. Moreover,accumulated evidence from a number of in vitro studies and findings in patients with familial plateletdisorder with predisposition to acute myelogenous leukemia (FPD/AML) strongly suggests that italso commits to the control of hematopoietic system in adult life, although the in depth analysis of itsroles in adult hematopoiesis has been largely hampered by premature lethality of Runx1-null animals.Recently we have developed conditional knockout mice in which Runx1 is disrupted specifically inhematopoietic compartments after birth and dissected its roles in adult hematopoiesis. Notably, inthese mice, maturation of megakaryocytes and development of both T and B lymphocytes wereseverely impaired, whereas hematopoietic progenitors were maintained or even expanded withapparently normal myeloid and erythroid differentiation in the periphery and bone marrow. Ourfindings clearly demonstrated differential requirement of Runx1 in stem cell development and in itsmaintenance together with multi-modal functions of this transcription factor that are criticallyrequired for maturation of megakaryocytes and lymphocyte development, also providing a novelinsight into how deeply and meticulously Runx1 is involved in regulation of mammalianhematopoiesis.

Crk-Associated Substrate Lymphocyte Type Is Required for Lymphocyte Trafficking and Marginal Zone B Cell Maintenance

The lymphocyte-specific Cas family protein Cas-L (Crk-associated substrate lymphocyte type) has been implicated to function in lymphocyte movement, mediated mainly by integrin signaling. However, its physiological role is poorly understood. In this study we analyzed the function of Cas-L in lymphocytes using gene-targeted mice. The mutant mice showed a deficit of marginal zone B (MZB) cells and a decrease of cell number in secondary lymphoid organs. An insufficient chemotactic response and perturbed cell adhesion were observed in Cas-L-deficient lymphocytes, suggesting that the aberrant localization was responsible for the deficit of MZB cells. Moreover, we found that lymphocyte trafficking was altered in Cas-L-deficient mice, which gave a potential reason for contraction of secondary lymphoid tissues. Thus, Cas-L affects homeostasis of MZB cells and peripheral lymphoid organs, which is considered to be relevant to impaired lymphocyte migration and adhesion.

A role for RUNX1 in hematopoiesis and myeloid leukemia.

Since its discovery from a translocation in leukemias, the runt-related transcription factor 1/acute myelogenous leukemia-1 (RUNX1/AML1), which is widely expressed in hematopoietic cells, has been extensively studied. Many lines of evidence have shown that RUNX1 plays a critical role in regulating the development and precise maintenance of mammalian hematopoiesis. Studies using knockout mice have shown the importance of RUNX1 in a wide variety of hematopoietic cells, including hematopoietic stem cells and megakaryocytes. Recently, target molecular processes of RUNX1 in normal and malignant hematopoiesis have been revealed. Although RUNX1 is not required for the maintenance of hematopoietic stem cells, it is required for the homeostasis of hematopoietic stem and progenitor cells, and expansion of hematopoietic stem and progenitor cells due to RUNX1 deletion may be an important cause of human leukemias. Molecular abnormalities cooperating with loss of RUNX1 have also been identified. These findings may lead to a further understanding of human leukemias, and suggest novel molecular targeted therapies in the near future.

Pbx1 is a downstream target of Evi-1 in hematopoietic stem/progenitors and leukemic cells.

Ecotropic viral integration site-1 (Evi-1) is a nuclear transcription factor, which is essential for the proliferation/maintenance of hematopoietic stem cells (HSCs). Aberrant expression of Evi-1 has been frequently found in myeloid leukemia, and is associated with a poor patient survival. Recently, we reported candidate target genes of Evi-1 shared in HSCs and leukemic cells using gene expression profiling analysis. In this study, we identified Pbx1, a proto-oncogene in hematopoietic malignancy, as a target gene of Evi-1. Overexpression of Evi-1 increased Pbx1 expression in hematopoietic stem/progenitor cells. An analysis of the Pbx1 promoter region revealed that Evi-1 upregulates Pbx1 transcription. Furthermore, reduction of Pbx1 levels through RNAi-mediated knockdown significantly inhibited Evi-1-induced transformation. In contrast, knockdown of Pbx1 did not impair bone marrow transformation by E2A/HLF or AML1/ETO, suggesting that Pbx1 is specifically required for the maintenance of bone marrow transformation mediated by Evi-1. These results indicate that Pbx1 is a target gene of Evi-1 involved in Evi-1-mediated leukemogenesis.