TIF1β activates leukemic transcriptional program in HSCs and promotes BCR::ABL1-induced myeloid leukemia

TIF1β/KAP1/TRIM28, a chromatin modulator, both represses and activates the transcription of genes in normal and malignant cells. Analyses of datasets on leukemia patients revealed that the expression level of TIF1β was increased in patients with chronic myeloid leukemia at the blast crisis and acute myeloid leukemia. We generated a BCR::ABL1 conditional knock-in (KI) mouse model, which developed aggressive myeloid leukemia, and demonstrated that the deletion of the Tif1β gene inhibited the progression of myeloid leukemia and showed longer survival than that in BCR::ABL1 KI mice, suggesting that Tif1β drove the progression of BCR::ABL1-induced leukemia. In addition, the deletion of Tif1β sensitized BCR::ABL1 KI leukemic cells to dasatinib. The deletion of Tif1β decreased the expression levels of TIF1β-target genes and chromatin accessibility peaks enriched with the Fosl1-binding motif in BCR::ABL1 KI stem cells. TIF1β directly bound to the promoters of proliferation genes, such as FOSL1, in human BCR::ABL1 cells, in which TIF1β and FOSL1 bound to adjacent regions of chromatin. Since the expression of Fosl1 was critical for the enhanced growth of BCR::ABL1 KI cells, Tif1β and Fosl1 interacted to activate the leukemic transcriptional program in and cellular function of BCR::ABL1 KI stem cells and drove the progression of myeloid leukemia.

Exposure to microbial products followed by loss of Tet2 promotes myelodysplastic syndrome via remodeling HSCs

Aberrant innate immune signaling in myelodysplastic syndrome (MDS) hematopoietic stem/progenitor cells (HSPCs) has been implicated as a driver of the development of MDS. We herein demonstrated that a prior stimulation with bacterial and viral products followed by loss of the Tet2 gene facilitated the development of MDS via up-regulating the target genes of the Elf1 transcription factor and remodeling the epigenome in hematopoietic stem cells (HSCs) in a manner that was dependent on Polo-like kinases (Plk) downstream of Tlr3/4-Trif signaling but did not increase genomic mutations. The pharmacological inhibition of Plk function or the knockdown of Elf1 expression was sufficient to prevent the epigenetic remodeling in HSCs and diminish the enhanced clonogenicity and the impaired erythropoiesis. Moreover, this Elf1-target signature was significantly enriched in MDS HSPCs in humans. Therefore, prior infection stress and the acquisition of a driver mutation remodeled the transcriptional and epigenetic landscapes and cellular functions in HSCs via the Trif-Plk-Elf1 axis, which promoted the development of MDS.

Stem cell regulation and dynamics in myeloid malignancies

In this issue of PIH, we asked four researchers to write about basic research on the molecular mechanisms of the development of myeloid malignancies, in particular two epigenetic regulation and two space- and time-dependent factors. Regarding epigenomic regulation, Dr. Yang reviewed ASXL1, a polycomb modifier gene that is often mutated in myeloid malignancies, but also in clonal hematopoiesis in healthy elderly people, and Dr. Vu reviewed RNA modifications, which are critical for development and tissue homeostasis, and are now recognized as an important driver for cancer development. Regarding spatiotemporal factors, Dr. Inoue reviewed the role of extracellular vesicles in leukemic stem cell niches. As some cancers develop preferentially in infancy or old age, Dr. Osato discussed the time-specific development of leukemia involving the RUNX1-ETO mutation, which is often found in leukemia in adolescents and young adults. Recent studies on hematopoietic development have shown that hematopoietic stem cells do not generate multipotent progenitor cells, but that these cells develop in parallel. We hope that reconsideration of the definition of leukemic stem cells and their origin will help us understand the regulatory mechanisms of these cells, but also enable us to develop future therapies by targeting factors that regulate the leukemic stem cell and the niche.

A gain-of-function mutation in micro-RNA-142 is sufficient to cause the development of T-cell leukemia in mice

MicroRNAs (miRNAs) play a crucial role in regulating gene expression. miRNA expression levels fluctuate, and point mutations and methylation occur in cancer cells; however, to date, there are no reports of carcinogenic point mutations in miRNAs. MicroRNA-142 (miR-142) is frequently mutated in patients with follicular lymphoma, diffuse large B-cell lymphoma, chronic lymphocytic leukemia (CLL), and acute myeloid leukemia/myelodysplastic syndrome (AML/MDS). To understand the role of miR-142 mutation in blood cancers, the CRISPR-Cas9 system was utilized to successfully generate miR-142-55A>G mutant knock-in (Ki) mice, simulating the most frequent mutation in patients with miR-142 mutated AML/MDS. Thereafter, bone marrow cells from miR-142 mutant heterozygous Ki mice were transplanted, and we found that the miR-142 mutant/wild-type cells were sufficient for the development of CD8⁺ T-cell leukemia in mice post-transplantation. RNA-sequencing analysis in hematopoietic stem/progenitor cells and CD8⁺ T-cells revealed that miR-142-Ki/+ cells had increased expression of the mTORC1 activator, a potential target of wild-type miR-142-3p. Notably, the expression of genes involved in apoptosis, differentiation, and the inhibition of the Akt-mTOR pathway was suppressed in miR-142-55A>G heterozygous cells, indicating that these genes are repressed by the mutant miR-142-3p. Thus, in addition to the loss of function due to the halving of wild-type miR-142-3p alleles, mutated miR-142-3p gained the function to suppress the expression of distinct target genes, sufficient to cause leukemogenesis in mice.

CARD11 mutation and HBZ expression induce lymphoproliferative disease and adult T-cell leukemia/lymphoma

Adult T-cell leukemia/lymphoma (ATL) is caused by human T-cell leukemia virus type 1 (HTLV-1). In addition to HTLV-1 bZIP factor (HBZ), a leukemogenic antisense transcript of HTLV-1, abnormalities of genes involved in TCR-NF-κB signaling, such as CARD11, are detected in about 90% of patients. Utilizing mice expressing CD4⁺ T cell-specific CARD11(E626K) and/or CD4⁺ T cell-specific HBZ, namely CARD11(E626K)^CD4-Cre mice, HBZ transgenic (Tg) mice, and CARD11(E626K)^CD4-Cre;HBZ Tg double transgenic mice, we clarify these genes' pathogenetic effects. CARD11(E626K)^CD4-Cre and HBZ Tg mice exhibit lymphocytic invasion to many organs, including the lungs, and double transgenic mice develop lymphoproliferative disease and increase CD4⁺ T cells in vivo. CARD11(E626K) and HBZ cooperatively activate the non-canonical NF-κB pathway, IRF4 targets, BATF3/IRF4/HBZ transcriptional network, MYC targets, and E2F targets. Most KEGG and HALLMARK gene sets enriched in acute-type ATL are also enriched in double transgenic mice, indicating that these genes cooperatively contribute to ATL development.

ATP citrate lyase controls hematopoietic stem cell fate and supports bone marrow regeneration

In order to support bone marrow regeneration after myeloablation, hematopoietic stem cells (HSCs) actively divide to provide both stem and progenitor cells. However, the mechanisms regulating HSC function and cell fate choice during hematopoietic recovery remain unclear. We herein provide novel insights into HSC regulation during regeneration by focusing on mitochondrial metabolism and ATP citrate lyase (ACLY). After 5-fluorouracil-induced myeloablation, HSCs highly expressing endothelial protein C receptor (EPCR^high ) were enriched within the stem cell fraction at the expense of more proliferative EPCR^Low HSCs. These EPCR^High HSCs were initially more primitive than EPCR^Low HSCs and enabled stem cell expansion by enhancing histone acetylation, due to increased activity of ACLY in the early phase of hematopoietic regeneration. In the late phase of recovery, HSCs enhanced differentiation potential by increasing the accessibility of cis-regulatory elements in progenitor cell-related genes, such as CD48. In conditions of reduced mitochondrial metabolism and ACLY activity, these HSCs maintained stem cell phenotypes, while ACLY-dependent histone acetylation promoted differentiation into CD48⁺ progenitor cells. Collectively, these results indicate that the dynamic control of ACLY-dependent metabolism and epigenetic alterations is essential for HSC regulation during hematopoietic regeneration.

Inflammation-driven senescence-associated secretory phenotype in cancer-associated fibroblasts enhances peritoneal dissemination

In the tumor microenvironment, senescent non-malignant cells, including cancer-associated fibroblasts (CAFs), exhibit a secretory profile under stress conditions; this senescence-associated secretory phenotype (SASP) leads to cancer progression and chemoresistance. However, the role of senescent CAFs in metastatic lesions and the molecular mechanism of inflammation-related SASP induction are not well understood. We show that pro-inflammatory cytokine-driven EZH2 downregulation maintains the SASP by demethylating H3K27me3 marks in CAFs and enhances peritoneal tumor formation of gastric cancer (GC) through JAK/STAT3 signaling in a mouse model. A JAK/STAT3 inhibitor blocks the increase in GC cell viability induced by senescent CAFs and peritoneal tumor formation. Single-cell mass cytometry revealed that fibroblasts exist in the ascites of GC patients with peritoneal dissemination, and the fibroblast population shows p16 expression and SASP factors at high levels. These findings provide insights into the inflammation-related SASP maintenance by histone modification and the role of senescent CAFs in GC peritoneal dissemination.

Two faces of RUNX3 in myeloid transformation

RUNX3, a transcription factor, has been implicated as a tumor suppressor in various cancers, including hematological malignancies; however, recent studies revealed an oncogenic function of RUNX3 in the pathogenesis of myeloid malignancies, such as myelodysplastic syndrome and acute myeloid leukemia. In contrast to the high frequency of mutations in the RUNX1 gene, deletion of and loss-of-function mutations in RUNX3 are rarely detected in patients with hematopoietic malignancies. Although RUNX3 is expressed in normal hematopoietic stem and progenitor cells, its expression decreases with aging in humans. The loss of Runx3 did not result in the development of lethal hematological diseases in mice despite the expansion of myeloid cells. Therefore, RUNX3 does not appear to initiate the transformation of normal hematopoietic stem cells. However, the overexpression of RUNX3 inhibits the expression and transcriptional function of the RUNX1 gene, but activates the expression of key oncogenic pathways, such as MYC, resulting in the transformation of premalignant stem cells harboring a driver genetic mutation. We herein discuss the mechanisms by which RUNX3 is activated and how RUNX3 exerts oncogenic effects on the cellular function of and transcriptional program in premalignant stem cells to drive myeloid transformation.

Epithelial-Mesenchymal Transition in Liver Fluke-Induced Cholangiocarcinoma

Simple Summary

Parasitic infection remains a health threat in many countries. Liver flukes, parasitic flatworms endemic to southeast and east Asia, cause bile duct inflammation and are major risk factors of bile duct cancer (cholangiocarcinoma). As the only group of eukaryotic organisms listed as carcinogens, liver flukes can increase cholangiocarcinoma incidence by 100-fold in some parts of Thailand. How they interact with bile duct epithelial cells during tumor initiation and progression is unknown. In this review, we summarize molecular and cellular evidence linking liver fluke-associated cholangiocarcinoma with mis-regulation of epithelial–mesenchymal transition (EMT), a multicellular morphogenetic process known to be involved in many normal and pathological settings, including cancer. EMT markers and regulators can potentially be used to facilitate cholangiocarcinoma diagnosis and treatment.

Abstract

Cholangiocarcinoma (CCA) is the second most common type of hepatic cancer. In east and southeast Asia, intrahepatic CCA is caused predominantly by infection of Opisthorchis viverrini and Clonorchis sinensis, two species of parasitic liver flukes. In this review, we present molecular evidence that liver fluke-associated CCAs have enhanced features of epithelial–mesenchymal transition (EMT) in bile duct epithelial cells (cholangiocytes) and that some of those features are associated with mis-regulation at the epigenetic level. We hypothesize that both direct and indirect mechanisms underlie parasitic infection-induced EMT in CCA.

Diphtheria toxin-mediated transposon-driven poly (A)-trapping efficiently disrupts transcriptionally silent genes in embryonic stem cells

Random gene trapping is the application of insertional mutagenesis techniques that are conventionally used to inactivate protein-coding genes in mouse embryonic stem (ES) cells. Transcriptionally silent genes are not effectively targeted by conventional random gene trapping techniques, thus we herein developed an unbiased poly (A) trap (UPATrap) method using a Tol2 transposon, which preferentially integrated into active genes rather than silent genes in ES cells. To achieve efficient trapping at transcriptionally silent genes using random insertional mutagenesis in ES cells, we generated a new diphtheria toxin (DT)-mediated trapping vector, DTrap that removed cells, through the expression of DT that was induced by the promoter activity of the trapped genes, and selected trapped clones using the neomycin-resistance gene of the vector. We found that a double-DT, the dDT vector, dominantly induced the disruption of silent genes, but not active genes, and showed more stable integration in ES cells than the UPATrap vector. The dDT vector disrupted differentiated cell lineage genes, which were silent in ES cells, and labeled trapped clone cells by the expression of EGFP upon differentiation. Thus, the dDT vector provides a systematic approach to disrupt silent genes and examine the cellular functions of trapped genes in the differentiation of target cells and development.

Overexpression of RUNX3 Represses RUNX1 to Drive Transformation of Myelodysplastic Syndrome

RUNX3, a RUNX family transcription factor, regulates normal hematopoiesis and functions as a tumor suppressor in various tumors in humans and mice. However, emerging studies have documented increased expression of RUNX3 in hematopoietic stem/progenitor cells (HSPC) of a subset of patients with myelodysplastic syndrome (MDS) showing a worse outcome, suggesting an oncogenic function for RUNX3 in the pathogenesis of hematologic malignancies. To elucidate the oncogenic function of RUNX3 in the pathogenesis of MDS in vivo, we generated a RUNX3-expressing, Tet2-deficient mouse model with the pancytopenia and dysplastic blood cells characteristic of MDS in patients. RUNX3-expressing cells markedly suppressed the expression levels of Runx1, a critical regulator of hemaotpoiesis in normal and malignant cells, as well as its target genes, which included crucial tumor suppressors such as Cebpa and Csf1r. RUNX3 bound these genes and remodeled their Runx1-binding regions in Tet2-deficient cells. Overexpression of RUNX3 inhibited the transcriptional function of Runx1 and compromised hematopoiesis to facilitate the development of MDS in the absence of Tet2, indicating that RUNX3 is an oncogene. Furthermore, overexpression of RUNX3 activated the transcription of Myc target genes and rendered cells sensitive to inhibition of Myc-Max heterodimerization. Collectively, these results reveal the mechanism by which RUNX3 overexpression exerts oncogenic effects on the cellular function of and transcriptional program in Tet2-deficient stem cells to drive the transformation of MDS. SIGNIFICANCE: This study defines the oncogenic effects of transcription factor RUNX3 in driving the transformation of myelodysplastic syndrome, highlighting RUNX3 as a potential target for therapeutic intervention.

RUNX3 levels in human hematopoietic progenitors are regulated by aging and dictate erythroid-myeloid balance

Healthy bone marrow progenitors yield a co-ordinated balance of hematopoietic lineages. This balance shifts with aging toward enhanced granulopoiesis with diminished erythropoiesis and lymphopoiesis, changes which likely contribute to the development of bone marrow disorders in the elderly. In this study, RUNX3 was identified as a hematopoietic stem and progenitor cell factor whose levels decline with aging in humans and mice. This decline is exaggerated in hematopoietic stem and progenitor cells from subjects diagnosed with unexplained anemia of the elderly. Hematopoietic stem cells from elderly unexplained anemia patients had diminished erythroid but unaffected granulocytic colony forming potential. Knockdown studies revealed human hematopoietic stem and progenitor cells to be strongly influenced by RUNX3 levels, with modest deficiencies abrogating erythroid differentiation at multiple steps while retaining capacity for granulopoiesis. Transcriptome profiling indicated control by RUNX3 of key erythroid transcription factors, including KLF1 and GATA1 These findings thus implicate RUNX3 as a participant in hematopoietic stem and progenitor cell aging, and a key determinant of erythroid-myeloid lineage balance.

Ascl1-induced Wnt11 regulates neuroendocrine differentiation, cell proliferation, and E-cadherin expression in small-cell lung cancer and Wnt11 regulates small-cell lung cancer biology

The involvement of Wnt signaling in human lung cancer remains unclear. This study investigated the role of Wnt11 in neuroendocrine (NE) differentiation, cell proliferation, and epithelial-to-mesenchymal transition (EMT) in human small-cell lung cancer (SCLC). Immunohistochemical staining of resected specimens showed that Wnt11 was expressed at higher levels in SCLCs than in non-SCLCs; 58.8% of SCLC, 5.2% of adenocarcinoma (ADC), and 23.5% of squamous cell carcinoma tissues stained positive for Wnt11. A positive relationship was observed between Achaete-scute complex homolog 1 (Ascl1) and Wnt11 expression in SCLC cell lines, and this was supported by transcriptome data from SCLC tissue. The expression of Wnt11 and some NE markers increased after the transfection of ASCL1 into the A549 ADC cell line. Knockdown of Ascl1 downregulated Wnt11 expression in SCLC cell lines. Ascl1 regulated Wnt11 expression via lysine H3K27 acetylation at the enhancer region of the WNT11 gene. Wnt11 controlled NE differentiation, cell proliferation, and E-cadherin expression under the regulation of Ascl1 in SCLC cell lines. The phosphorylation of AKT and p38 mitogen-activated protein kinase markedly increased after transfection of WNT11 into the SBC3 SCLC cell line, which suggests that Wnt11 promotes cell proliferation in SCLC cell lines. Ascl1 plays an important role in regulating the Wnt signaling pathway and is one of the driver molecules of Wnt11 in human SCLC. Ascl1 and Wnt11 may employ a cooperative mechanism to control the biology of SCLC. The present results indicate the therapeutic potential of targeting the Ascl1-Wnt11 signaling axis and support the clinical utility of Wnt11 as a biological marker in SCLC.

Antitumor immunity augments the therapeutic effects of p53 activation on acute myeloid leukemia

The negative regulator of p53, MDM2, is frequently overexpressed in acute myeloid leukemia (AML) that retains wild-type TP53 alleles. Targeting of p53-MDM2 interaction to reactivate p53 function is therefore an attractive therapeutic approach for AML. Here we show that an orally active inhibitor of p53-MDM2 interaction, DS-5272, causes dramatic tumor regressions of MLL-AF9-driven AML in vivo with a tolerable toxicity. However, the antileukemia effect of DS-5272 is markedly attenuated in immunodeficient mice, indicating the critical impact of systemic immune responses that drive p53-mediated leukemia suppression. In relation to this, DS-5272 triggers immune-inflammatory responses in MLL-AF9 cells including upregulation of Hif1α and PD-L1, and inhibition of the Hif1α-PD-L1 axis sensitizes AML cells to p53 activation. We also found that NK cells are important mediators of antileukemia immunity. Our study showed the potent activity of a p53-activating drug against AML, which is further augmented by antitumor immunity.

Systematic Review of Normal Subjects Harbouring BCR-ABL1 Fusion Gene

The treatment of chronic myeloid leukaemia (CML) requires quantitative polymerase chain reaction (qPCR) to monitor BCR-ABL1 in International Scale (IS). Some normal subjects were found to harbour BCR-ABL1. We performed a systematic review on normal subjects harbouring BCR-ABL1. A literature search was done on July 16, 2017 using EBSCOhost Research Databases interface and Western Pacific Region Index Medicus. Two authors selected the studies, extracted the data, and evaluated the quality of studies using the modified Appraisal Tool for Cross-Sectional Studies independently. The outcomes were prevalence, level of BCR-ABL1IS, proportion, and time of progression to CML. The initial search returned 4,770 studies. Eleven studies, all having used convenient sampling, were included, with total of 1,360 subjects. Ten studies used qualitative PCR and one used qPCR (not IS). The mean prevalence of M-BCR was 5.9, 15.5, and 15.9% in cord blood/newborns/infants (CB/NB/I) (n = 170), children (n = 90), and adults (n = 454), respectively, while m-BCR was 15, 26.9, and 23.1% in CB/NB/I (n = 786), children (n = 67), and adults (n = 208), respectively. No study reported the proportion and time of progression to CML. Nine studies were graded as moderate quality, one study as poor quality, and one study as unacceptable. The result of the studies could neither be inferred to the general normal population nor compared. Follow-up data were scarce.

Lineage-specific RUNX2 super-enhancer activates MYC and promotes the development of blastic plasmacytoid dendritic cell neoplasm

Blastic plasmacytoid dendritic cell neoplasm (BPDCN) is an aggressive subtype of acute leukemia, the cell of origin of which is considered to be precursors of plasmacytoid dendritic cells (pDCs). Since translocation (6;8)(p21;q24) is a recurrent anomaly for BPDCN, we demonstrate that a pDC-specific super-enhancer of RUNX2 is associated with the MYC promoter due to t(6;8). RUNX2 ensures the expression of pDC-signature genes in leukemic cells, but also confers survival and proliferative properties in BPDCN cells. Furthermore, the pDC-specific RUNX2 super-enhancer is hijacked to activate MYC in addition to RUNX2 expression, thereby promoting the proliferation of BPDCN. We also demonstrate that the transduction of MYC and RUNX2 is sufficient to initiate the transformation of BPDCN in mice lacking Tet2 and Tp53, providing a model that accurately recapitulates the aggressive human disease and gives an insight into the molecular mechanisms underlying the pathogenesis of BPDCN.

Deregulated Polycomb functions in myeloproliferative neoplasms

Polycomb proteins function in the maintenance of gene silencing via post-translational modifications of histones and chromatin compaction. Genetic and biochemical studies have revealed that the repressive function of Polycomb repressive complexes (PRCs) in transcription is counteracted by the activating function of Trithorax-group complexes; this balance fine-tunes the expression of genes critical for development and tissue homeostasis. The function of PRCs is frequently dysregulated in various cancer cells due to altered expression or recurrent somatic mutations in PRC genes. The tumor suppressive functions of EZH2-containing PRC2 and a PRC2-related protein ASXL1 have been investigated extensively in the pathogenesis of hematological malignancies, including myeloproliferative neoplasms (MPN). BCOR, a component of non-canonical PRC1, suppresses various hematological malignancies including MPN. In this review, we focus on recent findings on the role of PRCs in the pathogenesis of MPN and the therapeutic impact of targeting the pathological functions of PRCs in MPN.

Correction to: Systematic review of pre-clinical chronic myeloid leukaemia

The author would like to correct the error in the publication of the original article. The corrected detail is given below for your reading.

Systematic review of pre-clinical chronic myeloid leukaemia

BACKGROUND

Studies of a provisional entity pre-clinical chronic myeloid leukaemia (CML), which precedes chronic phase (CP) without leucocytosis or blood/marrow feature of CML CP, has been increasing.

OBJECTIVE

To perform a systematic review of pre-clinical CML and analysis the data relevant to disease progression to CML CP.

METHOD

We performed a literature search on 16 July 2017 using EBSCOhost Research Databases interface and Western Pacific Region Index Medicus. Two authors selected the studies, extracted the data and evaluated the quality of studies using an 8-item tool, independently. The outcomes were percentage of Philadelphia chromosome in the number of metaphases examined (Ph%), correlation between Ph% and blood count and time progress to CML.

RESULT

Our initial search returned 4770 studies. A total of 10 studies with a total 17 subjects were included. The lowest Ph%, which eventually progresses to CML, was 10%. Absolute basophil count seemed to correlate better with Ph% compared to total white cell and absolute eosinophil count. The time from the first documented pre-clinical CML to CML ranged from 12 to 48 months. The overall quality of the included studies was average.

CONCLUSION

This is the first systematic review on pre-clinical CML. This entity requires additional large-scale studies.

Ezh2 loss propagates hypermethylation at T cell differentiation-regulating genes to promote leukemic transformation

Early T cell precursor acute lymphoblastic leukemia (ETP-ALL) is a new pathological entity with poor outcomes in T cell ALL (T-ALL) that is characterized by a high incidence of loss-of-function mutations in polycomb repressive complex 2 (PRC2) genes. We generated a mouse model of ETP-ALL by deleting Ezh2, one of the PRC2 genes, in p53-null hematopoietic cells. The loss of Ezh2 in p53-null hematopoietic cells impeded the differentiation of ETPs and eventually induced ETP-ALL-like disease in mice, indicating that PRC2 functions as a bona fide tumor suppressor in ETPs. A large portion of PRC2 target genes acquired DNA hypermethylation of their promoters following reductions in H3K27me3 levels upon the loss of Ezh2, which included pivotal T cell differentiation-regulating genes. The reactivation of a set of regulators by a DNA-demethylating agent, but not the transduction of single regulator genes, effectively induced the differentiation of ETP-ALL cells. Thus, PRC2 protects key T cell developmental regulators from DNA hypermethylation in order to keep them primed for activation upon subsequent differentiation phases, while its insufficiency predisposes ETPs to leukemic transformation. These results revealed a previously unrecognized epigenetic switch in response to PRC2 dysfunction and provide the basis for specific rational epigenetic therapy for ETP-ALL with PRC2 insufficiency.

Pathobiological Pseudohypoxia as a Putative Mechanism Underlying Myelodysplastic Syndromes

Myelodysplastic syndromes (MDS) are heterogeneous hematopoietic disorders that are incurable with conventional therapy. Their incidence is increasing with global population aging. Although many genetic, epigenetic, splicing, and metabolic aberrations have been identified in patients with MDS, their clinical features are quite similar. Here, we show that hypoxia-independent activation of hypoxia-inducible factor 1α (HIF1A) signaling is both necessary and sufficient to induce dysplastic and cytopenic MDS phenotypes. The HIF1A transcriptional signature is generally activated in MDS patient bone marrow stem/progenitors. Major MDS-associated mutations (Dnmt3a, Tet2, Asxl1, Runx1, and Mll1) activate the HIF1A signature. Although inducible activation of HIF1A signaling in hematopoietic cells is sufficient to induce MDS phenotypes, both genetic and chemical inhibition of HIF1A signaling rescues MDS phenotypes in a mouse model of MDS. These findings reveal HIF1A as a central pathobiologic mediator of MDS and as an effective therapeutic target for a broad spectrum of patients with MDS.Significance: We showed that dysregulation of HIF1A signaling could generate the clinically relevant diversity of MDS phenotypes by functioning as a signaling funnel for MDS driver mutations. This could resolve the disconnection between genotypes and phenotypes and provide a new clue as to how a variety of driver mutations cause common MDS phenotypes. Cancer Discov; 8(11); 1438-57. ©2018 AACR. See related commentary by Chen and Steidl, p. 1355 This article is highlighted in the In This Issue feature, p. 1333.

Epigenetic dysregulation in myelodysplastic syndromes

Myelodysplastic syndrome (MDS) is a clonal hematopoietic stem cell disease characterized by impaired hematopoiesis and an increased risk of transformation to acute myeloid leukemia (AML). Epigenetic regulators, including TET2, DNMT3A and EZH2, are often mutated in patients with MDS. Recently, exome sequencing of blood cells from aged people without hematological malignancies have demonstrated the presence of clonal hematopoiesis with myeloid malignancies-associated mutations, such as TET2 and DNMT3A. Here, I will discuss the molecular mechanisms underlying the accumulation of epigenetic alterations and genetic mutations, including TET2, DNMT3A and EZH2, and how these promote the development of MDS and hematological malignancies in aged people with clonal hematopoiesis.

Multifaceted role of the polycomb-group gene EZH2 in hematological malignancies

Polycomb repressive complex (PRC) is a critical regulator of normal tissue homeostasis as well as tumorigenesis. EZH2, an enzymatic subunit of PRC2, is a histone H3K27 methyltransferase that functions in the regulation of gene silencing. EZH2 overexpression was first identified in prostate and breast cancers and is associated with poor clinical outcome. Subsequently, gain- and loss-of-function mutations of EZH2 have been identified in various tumors, including hematological malignancies, implicating EZH2 as either an oncogene or a tumor suppressor gene, depending on the cancer type. Molecular mechanisms underlying the multifaceted function of EZH2 have been analyzed extensively. However, because EZH2 dysregulation is functionally integrated with multiple other epigenetic events in a context-dependent manner, the precise manner in which EZH2 dysregulation impacts the pathogenesis of hematological malignancies remains to be clarified. In this perspective, we describe recent findings in pathogenic role of EZH2 in hematological malignancies, which may provide insights into the treatment of with cancers with EZH2 dysregulation and the development of novel therapies targeting epigenetic regulators.

The loss of Ezh2 drives the pathogenesis of myelofibrosis and sensitizes tumor-initiating cells to bromodomain inhibition

Loss of Ezh2 in the presence of activating mutation in JAK2 (JAK2^V617F) cooperatively alters transcriptional programs of hematopoiesis, activates specific oncogenes, and promotes the development of myelofibrosis.

EZH2 is a component of polycomb repressive complex 2 (PRC2) and functions as an H3K27 methyltransferase. Loss-of-function mutations in EZH2 are associated with poorer outcomes in patients with myeloproliferative neoplasms (MPNs), particularly those with primary myelofibrosis (MF [PMF]). To determine how EZH2 insufficiency is involved in the pathogenesis of PMF, we generated mice compound for an Ezh2 conditional deletion and activating mutation in JAK2 (JAK2V617F) present in patients with PMF. The deletion of Ezh2 in JAK2^V617F mice markedly promoted the development of MF, indicating a tumor suppressor function for EZH2 in PMF. The loss of Ezh2 in JAK2^V617F hematopoietic cells caused significant reductions in H3K27 trimethylation (H3K27me3) levels, resulting in an epigenetic switch to H3K27 acetylation (H3K27ac). These epigenetic switches were closely associated with the activation of PRC2 target genes including Hmga2, an oncogene implicated in the pathogenesis of PMF. The treatment of JAK2^V617F/Ezh2-null mice with a bromodomain inhibitor significantly attenuated H3K27ac levels at the promoter regions of PRC2 targets and down-regulated their expression, leading to the abrogation of MF-initiating cells. Therefore, an EZH2 insufficiency not only cooperated with active JAK2 to induce MF, but also conferred an oncogenic addiction to the H3K27ac modification in MF-initiating cells that was capable of being restored by bromodomain inhibition.

[Mutations of epigenetic regulator genes and myeloid malignancies]

Recent genome studies have identified recurrent somatic mutations in various myeloid malignancies, including acute myeloid leukemia, myelodysplastic syndrome and myeloproliferative neoplasm. These mutations frequently occur in epigenetic regulator genes, and functions of the proteins encoded by these genes in hematopoietic cells have been extensively analyzed, as reported recently. It is noteworthy that several epigenetic regulator genes, such as DNMT3A, TET2 and ASXL1, have also been identified in pre-leukemic stem cells. As targeting pre-leukemic stem cells would be a promising therapeutic approach, further investigations of epigenetic abnormalities in hematopoietic cells are anticipated to lead to the development of novel epigenetic therapies. In this review, we discuss recent genetic and functional data regarding epigenetic regulator genes and the future landscape of this new research field.

Leukemia Inhibitory Factor Enhances Endogenous Cardiomyocyte Regeneration after Myocardial Infarction

Cardiac stem cells or precursor cells regenerate cardiomyocytes; however, the mechanism underlying this effect remains unclear. We generated CreLacZ mice in which more than 99.9% of the cardiomyocytes in the left ventricular field were positive for 5-bromo-4-chloro-3-indolyl-β-d-galactoside (X-gal) staining immediately after tamoxifen injection. Three months after myocardial infarction (MI), the MI mice had more X-gal-negative (newly generated) cells than the control mice (3.04 ± 0.38/mm², MI; 0.47 ± 0.16/mm², sham; p < 0.05). The cardiac side population (CSP) cell fraction contained label-retaining cells, which differentiated into X-gal-negative cardiomyocytes after MI. We injected a leukemia inhibitory factor (LIF)-expression construct at the time of MI and identified a significant functional improvement in the LIF-treated group. At 1 month after MI, in the MI border and scar area, the LIF-injected mice had 31.41 ± 5.83 X-gal-negative cardiomyocytes/mm², whereas the control mice had 12.34 ± 2.56 X-gal-negative cardiomyocytes/mm² (p < 0.05). Using 5-ethynyl-2'-deoxyurinide (EdU) administration after MI, the percentages of EdU-positive CSP cells in the LIF-treated and control mice were 29.4 ± 2.7% and 10.6 ± 3.7%, respectively, which suggests that LIF influenced CSP proliferation. Moreover, LIF activated the Janus kinase (JAK)signal transducer and activator of transcription (STAT), mitogen-activated protein kinase/extracellular signal-regulated (MEK)extracellular signal-regulated kinase (ERK), and phosphatidylinositol 3-kinase (PI3K)–AKT pathways in CSPs in vivo and in vitro. The enhanced green fluorescent protein (EGFP)-bone marrow-chimeric CreLacZ mouse results indicated that LIF did not stimulate cardiogenesis via circulating bone marrow-derived cells during the 4 weeks following MI. Thus, LIF stimulates, in part, stem cell-derived cardiomyocyte regeneration by activating cardiac stem or precursor cells. This approach may represent a novel therapeutic strategy for cardiogenesis.

Non-Lethal Ionizing Radiation Promotes Aging-Like Phenotypic Changes of Human Hematopoietic Stem and Progenitor Cells in Humanized Mice

Precise understanding of radiation effects is critical to develop new modalities for the prevention and treatment of radiation-induced damage. We previously reported that non-lethal doses of X-ray irradiation induce DNA damage in human hematopoietic stem and progenitor cells (HSPCs) reconstituted in NOD/Shi-scid IL2rγnull (NOG) immunodeficient mice and severely compromise their repopulating capacity. In this study, we analyzed in detail the functional changes in human HSPCs in NOG mice following non-lethal radiation. We transplanted cord blood CD34+ HSPCs into NOG mice. At 12 weeks post-transplantation, the recipients were irradiated with 0, 0.5, or 1.0 Gy. At 2 weeks post-irradiation, human CD34+ HSPCs recovered from the primary recipient mice were transplanted into secondary recipients. CD34+ HSPCs from irradiated mice showed severely impaired reconstitution capacity in the secondary recipient mice. Of interest, non-lethal radiation compromised contribution of HSPCs to the peripheral blood cells, particularly to CD19+ B lymphocytes, which resulted in myeloid-biased repopulation. Co-culture of limiting numbers of CD34+ HSPCs with stromal cells revealed that the frequency of B cell-producing CD34+ HSPCs at 2 weeks post-irradiation was reduced more than 10-fold. Furthermore, the key B-cell regulator genes such as IL-7R and EBF1 were downregulated in HSPCs upon 0.5 Gy irradiation. Given that compromised repopulating capacity and myeloid-biased differentiation are representative phenotypes of aged HSCs, our findings indicate that non-lethal ionizing radiation is one of the critical external stresses that promote aging of human HSPCs in the bone marrow niche.

Gene expression profiling of loss of TET2 and/or JAK2V617F mutant hematopoietic stem cells from mouse models of myeloproliferative neoplasms

Myeloproliferative neoplasms (MPNs) are clinically characterized by the chronic overproduction of differentiated peripheral blood cells and the gradual expansion of malignant intramedullary/extramedullary hematopoiesis. In MPNs mutations in JAK2 MPL or CALR are detected mutually exclusive in more than 90% of cases [1], [2]. Mutations in them lead to the abnormal activation of JAK/STAT signaling and the autonomous growth of differentiated cells therefore they are considered as “driver” gene mutations. In addition to the above driver gene mutations mutations in epigenetic regulators such as TET2 DNMT3A ASXL1 EZH2 or IDH1/2 are detected in about 5%–30% of cases respectively [3]. Mutations in TET2 DNMT3A EZH2 or IDH1/2 commonly confer the increased self-renewal capacity on normal hematopoietic stem cells (HSCs) but they do not lead to the autonomous growth of differentiated cells and only exhibit subtle clinical phenotypes [[4], [6], [7], [8],5]. It was unclear how mutations in such epigenetic regulators influenced abnormal HSCs with driver gene mutations how they influenced the disease phenotype or whether a single driver gene mutation was sufficient for the initiation of human MPNs. Therefore we focused on JAK2V617F and loss of TET2—the former as a representative of driver gene mutations and the latter as a representative of mutations in epigenetic regulators—and examined the influence of single or double mutations on HSCs (Lineage⁻Sca-1⁺c-Kit⁺ cells (LSKs)) by functional analyses and microarray whole-genome expression analyses [9]. Gene expression profiling showed that the HSC fingerprint genes [10] was statistically equally enriched in TET2-knockdown-LSKs but negatively enriched in JAK2V617F–LSKs compared to that in wild-type-LSKs. Double-mutant-LSKs showed the same tendency as JAK2V617F–LSKs in terms of their HSC fingerprint genes but the expression of individual genes differed between the two groups. Among 245 HSC fingerprint genes 100 were more highly expressed in double-mutant-LSKs than in JAK2V617F–LSKs. These altered gene expressions might partly explain the mechanisms of initiation and progression of MPNs which was observed in the functional analyses [9]. Here we describe gene expression profiles deposited at the Gene Expression Omnibus (GEO) under the accession number GSE62302 including experimental methods and quality control analyses.

[Epigenetic dysregulation in myelodysplastic syndrome]

Myelodysplastic syndrome (MDS) is a clonal hematopoietic stem cell disease characterized by impaired hematopoiesis and an increased risk of transformation to acute myeloid leukemia. Various epigenetic regulators are mutated in MDS patients, indicating that accumulation of epigenetic alterations together with genetic alterations plays a crucial role in the development of MDS.

Ezh2 loss promotes development of myelodysplastic syndrome but attenuates its predisposition to leukaemic transformation

Loss-of-function mutations of EZH2, a catalytic component of polycomb repressive complex 2 (PRC2), are observed in ~\n10% of patients with myelodysplastic syndrome (MDS), but are rare in acute myeloid leukaemia (AML). Recent studies have shown that EZH2 mutations are often associated with RUNX1 mutations in MDS patients, although its pathological function remains to be addressed. Here we establish an MDS mouse model by transducing a RUNX1S291fs mutant into hematopoietic stem cells and subsequently deleting Ezh2. Ezh2 loss significantly promotes RUNX1S291fs-induced MDS. Despite their compromised proliferative capacity of RUNX1S291fs/Ezh2-null MDS cells, MDS bone marrow impairs normal hematopoietic cells via selectively activating inflammatory cytokine responses, thereby allowing propagation of MDS clones. In contrast, loss of Ezh2 prevents the transformation of AML via PRC1-mediated repression of Hoxa9. These findings provide a comprehensive picture of how Ezh2 loss collaborates with RUNX1 mutants in the pathogenesis of MDS in both cell autonomous and non-autonomous manners.

Concurrent loss of Ezh2 and Tet2 cooperates in the pathogenesis of myelodysplastic disorders

Deletion of Ezh2 results in transcriptional repression of developmental regulator genes, derepression of oncogenic polycomb targets, and induction of MDS/MPN-like disease in mice that is exacerbated by concurrent deletion of Tet2.

Polycomb group (PcG) proteins are essential regulators of hematopoietic stem cells. Recent extensive mutation analyses of the myeloid malignancies have revealed that inactivating somatic mutations in PcG genes such as EZH2 and ASXL1 occur frequently in patients with myelodysplastic disorders including myelodysplastic syndromes (MDSs) and MDS/myeloproliferative neoplasm (MPN) overlap disorders (MDS/MPN). In our patient cohort, EZH2 mutations were also found and often coincided with tet methylcytosine dioxygenase 2 (TET2) mutations. Consistent with these findings, deletion of Ezh2 alone was enough to induce MDS/MPN-like diseases in mice. Furthermore, concurrent depletion of Ezh2 and Tet2 established more advanced myelodysplasia and markedly accelerated the development of myelodysplastic disorders including both MDS and MDS/MPN. Comprehensive genome-wide analyses in hematopoietic progenitor cells revealed that upon deletion of Ezh2, key developmental regulator genes were kept transcriptionally repressed, suggesting compensation by Ezh1, whereas a cohort of oncogenic direct and indirect polycomb targets became derepressed. Our findings provide the first evidence of the tumor suppressor function of EZH2 in myeloid malignancies and highlight the cooperative effect of concurrent gene mutations in the pathogenesis of myelodysplastic disorders.

Abstract 3855: Modeling MLL-PTD related Myelodysplastic Syndromes (MDS) in mouse

MLL partial tandem duplication (MLL-PTD) is found in 5-8% of human MDS, secondary acute myeloid leukemia (s-AML) and de novo AML. Mll-PTD knock-in mouse model shows that Mll-PTD HSPCs exhibited a proliferative advantage over wild-type HSPCs, and The ST-HSCs/MPPs and even GMPs have self-renewal capabilities. However, Mll-PTD HSPCs never develop MDS or s-AML in primary or transplanted recipients, suggesting that additional genetic and/or epigenetic defects are necessary for transformation. Recently, high frequent co-existences of both MLL-PTD and RUNX1 mutations have been reported in MDS/AML patients, which suggest a potential cooperation for transformation between these two mutations. Thus, we hypothesize that reducing RUNX1 dosage may facilitate the MLL-PTD mediated transformation toward MDS and/or s-AML.

We first generated the mice containing one allele of Mll-PTD in a Runx1+/- background and assessed HSPCs of Mll-PTD/Runx1+/− double heterozygous (DH) mice. The DH newborns are runty; they frequently die in early postnatal stage and barely survive to adulthood, compared to the normal life span of wild type (WT) or single heterozygous (Mllwt/wt/Runx1+/− and Mll-PTD/Runx1+/+) mice. We studied DH embryos fetal liver hematopoiesis and found reduced LK and LSK cells, partly because of increased apoptosis. Enhanced proliferation was found in DH fetal liver cells (FLCs) in vitro CFU replating assays over WT controls. FLCs of DH also showed dominant expansion in both serial competitive and non-competitive BMT assays compared to WT controls. The DH derived ST-HSCs/MPPs and GMPs also have enhanced self-renewal capabilities, rescuing hematopoiesis by giving rise to long-term repopulating cells in recipient mice better than cells derived from Mll-PTD mice. However, DH HSPCs do not develop MDS or s-AML in primary or in BMT recipient mice. We further generated Mll-PTD/Runx1Δ/Δ mice using Mx1-Cre mediated deletion. These mice showed thrombocytopenia one month after pI-pC injection, and developed pancytopenia 2-4 months later. The complete blood count exhibited increased MCV, RDW and severe anemia. All these Mll-PTD/Runx1Δ/Δ mice died of MDS induced complications within 7-8 months, and tri-lineages dysplasias (TLD) were found in bone marrow aspirate. However, there are no spontaneous s-AML found in Mll-PTD/Runx1Δ/Δ mice, which suggests that RUNX1 mutants found in MLL-PTD patients may not be simply loss-of-function mutations and present gain-of-function activities which cooperate with MLL-PTD in human diseases onsets. In conclusion, our study demonstrates that: 1) RUNX1 gene dosage reverse-correlates with HSPCs self-renewal activity; 2) Runx1 complete deletion causes MDS in Mll-PTD background. Future studies are needed to fully understand the collaboration between MLL-PTD and RUNX1 mutations for MDS development and leukemic transformation, which should facilitate improved therapies and patient outcomes.

Citation Format: Yue Zhang, Xiao-mei Yan, Goro Sashida, Ai-Li Chen, Xing-Hui Zhao, Susan P. Whitman, Michael A. Caligiuri, Zhi-Jian Xiao, Harry Leighton Grimes, Gang Huang. Modeling MLL-PTD related Myelodysplastic Syndromes (MDS) in mouse. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3855. doi:10.1158/1538-7445.AM2013-3855

Akt phosphorylates the transcriptional repressor bmi1 to block its effects on the tumor-suppressing ink4a-arf locus

The Polycomb group protein Bmi1 is a transcriptional silencer of the Ink4a-Arf locus, which encodes the cell cycle regulator p16(Ink4a) and the tumor suppressor p19(Arf). Bmi1 plays a key role in oncogenesis and stem cell self-renewal. We report that phosphorylation of human Bmi1 at Ser³¹⁶ by Akt impaired its function by triggering its dissociation from the Ink4a-Arf locus, which resulted in decreased ubiquitylation of histone H2A and the inability of Bmi1 to promote cellular proliferation and tumor growth. Moreover, Akt-mediated phosphorylation of Bmi1 also inhibited its ability to promote self-renewal of hematopoietic stem and progenitor cells. Our study provides a mechanism for the increased abundance of p16(Ink4a) and p19(Arf) seen in cancer cells with an activated phosphoinositide 3-kinase to Akt signaling pathway and identifies crosstalk between phosphorylation events and chromatin structure.

Epigenetic regulation of hematopoiesis

Epigenetic regulation is required not only for development, but also for tissue homeostasis, which is maintained via the self-renewal and differentiation of somatic stem cells. Accumulating evidence suggests that epigenetic regulators play critical roles in the maintenance of both self-renewing hematopoietic stem cells and leukemic stem cells. Recent genome-wide comprehensive analyses have identified mutations in epigenetic regulator genes, including genes whose products modify DNA and histones in hematological malignancies. Among these epigenetic regulators, repressive histone modifications by Polycomb-group complexes have been most fully characterized in hematopoietic stem cells, and are recognized as general regulators of stem cells. Hematopoietic stem cells are controlled by both cell-intrinsic and -extrinsic regulators, including transcription factors, signal transduction pathways, and niche factors. However, there is little insight into the mechanism of how epigenetic regulators act in concert with these factors to ensure blood homeostasis. In this review, we highlight recent findings in epigenetic regulation of hematopoiesis with emphasis on the role of Polycomb-group proteins and DNA-methylation modulators in hematopoietic stem cells and their progeny.

The mef/elf4 transcription factor fine tunes the DNA damage response

The ATM kinase plays a critical role in initiating the DNA damage response that is triggered by genotoxic stresses capable of inducing DNA double-strand breaks. Here, we show that ELF4/MEF, a member of the ETS family of transcription factors, contributes to the persistence of γH2AX DNA damage foci and promotes the DNA damage response leading to the induction of apoptosis. Conversely, the absence of ELF4 promotes the faster repair of damaged DNA and more rapid disappearance of γH2AX foci in response to γ-irradiation, leading to a radio-resistant phenotype despite normal ATM phosphorylation. Following γ-irradiation, ATM phosphorylates ELF4, leading to its degradation; a mutant form of ELF4 that cannot be phosphorylated by ATM persists following γ-irradiation, delaying the resolution of γH2AX foci and triggering an excessive DNA damage response. Thus, although ELF4 promotes the phosphorylation of H2AX by ATM, its activity must be dampened by ATM-dependent phosphorylation and degradation to avoid an excessive DNA damage response.

The oncogenic role of the ETS transcription factors MEF and ERG

Several ETS transcription factors, including MEF/ELF4 and ERG, can function as oncogenes and are overexpressed in human cancer. MEF cooperates in tumorigenesis in retroviral insertional mutagenesis-based mouse models of cancer and MEF is overexpressed in human lymphoma and ovarian cancer tissues via unknown mechanisms. ERG (Ets related gene) overexpression or increased activity has been found in various human cancers, including sarcomas, acute myeloid leukemia and prostate cancer, where the ERG gene is rearranged due to chromosomal translocations. We have been examining how MEF functions as an oncogene and recently showed that MEF can cooperate with H-Ras(G12V) and can inhibit both p53 and p16 expression thereby promoting transformation. In fact, in cells lacking p53, the absence of Mef abrogates H-Ras(G12V)-induced transformation of mouse embryonic fibroblasts, at least in part due to increased p16 expression. We discuss the known mechanisms by which the ETS transcription factors MEF and ERG contribute to the malignant transformation of cells.

Cyclin C regulates human hematopoietic stem/progenitor cell quiescence

Hematopoietic stem cells (HSCs) can remain quiescent or they can enter the cell cycle, and either self-renew or differentiate. Although cyclin C and cyclin dependent kinase (cdk3) are essential for the transition from the G(0) to the G(1) phase of the cell cycle in human fibroblasts, the role of cyclin C in hematopoietic stem/progenitor cells (HSPCs) is not clear. We have identified an important role of cyclin C (CCNC) in regulating human HSPC quiescence, as knocking down CCNC expression in human cord blood CD34(+) cells resulted in a significant increase in quiescent cells that maintain CD34 expression. CCNC knockdown also promotes in vitro HSPC expansion and enhances their engraftment potential in sublethally irradiated immunodeficient mice. Our studies establish cyclin C as a critical regulator of the G(0)/G(1) transition of human HSPCs and suggest that modulating cyclin C levels may be useful for HSC expansion and more efficient engraftment.

The p53 tumor suppressor protein is a critical regulator of hematopoietic stem cell behavior

In response to diverse stresses, the tumor suppressor p53 differentially regulates its target genes, variably inducing cell-cycle arrest, apoptosis or senescence. Emerging evidence indicates that p53 plays an important role in regulating hematopoietic stem cell (HSC) quiescence, self-renewal, apoptosis and aging. The p53 pathway is activated by DNA damage, defects in ribosome biogenesis, oxidative stress and oncogene induced p19 ARF upregulation. We present an overview of the current state of knowledge about p53 (and its target genes) in regulating HSC behavior, with the hope that understanding the molecular mechanisms that control p53 activity in HSCs and how p53 mutations affect its role in these events may facilitate the development of therapeutic strategies for eliminating leukemia (and cancer) propagating cells.

p53 regulates hematopoietic stem cell quiescence

The importance of the p53 protein in the cellular response to DNA damage is well known, but its function during steady-state hematopoiesis has not been established. We have defined a critical role of p53 in regulating hematopoietic stem cell quiescence, especially in promoting the enhanced quiescence seen in HSCs that lack the MEF/ELF4 transcription factor. Transcription profiling of HSCs isolated from wild-type and p53 null mice identified Gfi-1 and Necdin as p53 target genes, and using lentiviral vectors to upregulate or knockdown the expression of these genes, we show their importance in regulating HSC quiescence. Establishing the role of p53 (and its target genes) in controlling the cell-cycle entry of HSCs may lead to therapeutic strategies capable of eliminating quiescent cancer (stem) cells.