AML1/RUNX1 works as a negative regulator of c-Mpl in hematopoietic stem cells

A newly identified gene Ahed plays essential roles in murine haematopoiesis

The development of haematopoiesis involves the coordinated action of numerous genes, some of which are implicated in haematological malignancies. However, the biological function of many genes remains elusive and unknown functional genes are likely to remain to be uncovered. Here, we report a previously uncharacterised gene in haematopoiesis, identified by screening mutant embryonic stem cells. The gene, 'attenuated haematopoietic development (Ahed)', encodes a nuclear protein. Conditional knockout (cKO) of Ahed results in anaemia from embryonic day 14.5 onward, leading to prenatal demise. Transplantation experiments demonstrate the incapacity of Ahed-deficient haematopoietic cells to reconstitute haematopoiesis in vivo. Employing a tamoxifen-inducible cKO model, we further reveal that Ahed deletion impairs the intrinsic capacity of haematopoietic cells in adult mice. Ahed deletion affects various pathways, and published databases present cancer patients with somatic mutations in Ahed. Collectively, our findings underscore the fundamental roles of Ahed in lifelong haematopoiesis, implicating its association with malignancies.

A RUNX1-FPDMM rhesus macaque model reproduces the human phenotype and predicts challenges to curative gene therapies

Germ line loss-of-function heterozygous mutations in the RUNX1 gene cause familial platelet disorder with associated myeloid malignancies (FPDMM) characterized by thrombocytopenia and a life-long risk of hematological malignancies. Although gene therapies are being considered as promising therapeutic options, current preclinical models do not recapitulate the human phenotype and are unable to elucidate the relative fitness of mutation-corrected and RUNX1-heterozygous mutant hematopoietic stem and progenitor cells (HSPCs) in vivo long term. We generated a rhesus macaque with an FPDMM competitive repopulation model using CRISPR/Cas9 nonhomologous end joining editing in the RUNX1 gene and the AAVS1 safe-harbor control locus. We transplanted mixed populations of edited autologous HSPCs and tracked mutated allele frequencies in blood cells. In both animals, RUNX1-edited cells expanded over time compared with AAVS1-edited cells. Platelet counts remained below the normal range in the long term. Bone marrows developed megakaryocytic dysplasia similar to human FPDMM, and CD34+ HSPCs showed impaired in vitro megakaryocytic differentiation, with a striking defect in polyploidization. In conclusion, the lack of a competitive advantage for wildtype or control-edited HSPCs over RUNX1 heterozygous-mutated HSPCs long term in our preclinical model suggests that gene correction approaches for FPDMM will be challenging, particularly to reverse myelodysplastic syndrome/ acute myeloid leukemia predisposition and thrombopoietic defects.

C11ORF21, a novel RUNX1 target gene, is down-regulated by RUNX1-ETO

The fusion protein RUNX1-ETO is an oncogenic transcription factor generated by t(8;21) chromosome translocation, which is found in FAB-M2-type acute myeloid leukemia (AML). RUNX1-ETO is known to dysregulate the normal RUNX1 transcriptional network, which should involve essential factors for the onset of AML with t(8;21). In this study, we screened for possible transcriptional targets of RUNX1 by reanalysis of public data in silico, and identified C11orf21 as a novel RUNX1 target gene because its expression was down-regulated in the presence of RUNX1-ETO. The expression level of C11orf21 was low in AML patient samples with t(8;21) and in Kasumi-1 cells, which carry RUNX1-ETO. Knockdown of RUNX1-ETO in Kasumi-1 cells restored C11orf21 expression, whereas overexpression of RUNX1 up-regulated C11orf21 expression. In addition, knockdown of RUNX1 in other human leukemia cells without RUNX-ETO, such as K562, led to a decrease in C11orf21 expression. Of note, the C11orf21 promoter sequence contains a consensus sequence for RUNX1 binding and it was activated by exogenously expressed RUNX1 based on our luciferase reporter assay. This luciferase signal was trans-dominantly suppressed by RUNX1-ETO and site-directed mutagenesis of the consensus site abrogated the reporter activity. This study demonstrated that C11orf21 is a novel transcriptional target of RUNX1 and RUNX1-ETO suppressed C11orf21 transcription in t(8;21) AML. Thus, through this in silico approach, we identified a novel transcriptional target of RUNX1, and the depletion of C11orf21, the target gene, may be associated with the onset of t(8;21) AML.

C-terminal RUNX1 mutation in familial platelet disorder with predisposition to myeloid malignancies

Here we report a C-terminal RUNX1 mutation in a family with platelet disorder and predisposition to myeloid malignancies. We identified the mutation c.866delG:p.Gly289Aspfs*22 (NM_001754) (RUNX1 b-isoform NM_001001890; c.785delG:p.Gly262Aspfs*22) using exome sequencing of samples obtained from eight members of a single family. The mutation found in our pedigree is within exon eight and the transactivation domain of RUNX1. One of the affected individuals developed myelodysplastic syndrome (MDS), which progressed to acute myelogenous leukemia (AML). A search for the second hit which led to the development of MDS and later AML in this individual revealed the PHF6 gene variant (exon9:c.872G > A:p.G291E; NM_001015877), BCORL1 (exon3:c.1111A > C:p.T371P; NM_001184772) and BCOR gene variant (exon4:c.2076dupT:p.P693fs; NM_001123383), which appear to be very likely second hits participating in the progression to myeloid malignancy.

Preleukemic and second-hit mutational events in an acute myeloid leukemia patient with a novel germline RUNX1 mutation

BACKGROUND

Germline mutations in the RUNX1 transcription factor give rise to a rare autosomal dominant genetic condition classified under the entity: Familial Platelet Disorders with predisposition to Acute Myeloid Leukaemia (FPD/AML). While several studies have identified a myriad of germline RUNX1 mutations implicated in this disorder, second-hit mutational events are necessary for patients with hereditary thrombocytopenia to develop full-blown AML. The molecular picture behind this process remains unclear. We describe a patient of Malay descent with an unreported 7-bp germline RUNX1 frameshift deletion, who developed second-hit mutations that could have brought about the leukaemic transformation from a pre-leukaemic state. These mutations were charted through the course of the treatment and stem cell transplant, showing a clear correlation between her clinical presentation and the mutations present.

CASE PRESENTATION

The patient was a 27-year-old Malay woman who presented with AML on the background of hereditary thrombocytopenia affecting her father and 3 brothers. Initial molecular testing revealed the same novel RUNX1 mutation in all 5 individuals. The patient received standard induction, consolidation chemotherapy, and a haploidentical stem cell transplant from her mother with normal RUNX1 profile. Comprehensive genomic analyses were performed at diagnosis, post-chemotherapy and post-transplant. A total of 8 mutations (RUNX1, GATA2, DNMT3A, BCORL1, BCOR, 2 PHF6 and CDKN2A) were identified in the pre-induction sample, of which 5 remained (RUNX1, DNMT3A, BCORL1, BCOR and 1 out of 2 PHF6) in the post-treatment sample and none were present post-transplant. In brief, the 3 mutations which were lost along with the leukemic cells at complete morphological remission were most likely acquired leukemic driver mutations that were responsible for the AML transformation from a pre-leukemic germline RUNX1-mutated state. On the contrary, the 5 mutations that persisted post-treatment, including the germline RUNX1 mutation, were likely to be part of the preleukemic clone.

CONCLUSION

Further studies are necessary to assess the prevalence of these preleukemic and secondary mutations in the larger FPD/AML patient cohort and establish their prognostic significance. Given the molecular heterogeneity of FPD/AML and other AML subtypes, a better understanding of mutational classes and their involvement in AML pathogenesis can improve risk stratification of patients for more effective and targeted therapy.

The nonreceptor tyrosine kinase c-Abl phosphorylates Runx1 and regulates Runx1-mediated megakaryocyte maturation

The transcription factor Runx1 is an essential regulator of definitive hematopoiesis, megakaryocyte (MK) maturation, and lymphocyte differentiation. Runx1 mutations that interfere with its transcriptional activity are often present in leukemia patients. Recent work demonstrated that the transcriptional activity of Runx1 is regulated by kinase-mediated phosphorylation. In this study, we showed that c-Abl, but not Arg tyrosine kinase, associated with Runx1 both in cultured cells and in vitro. c-Abl-mediated tyrosine phosphorylation in the Runx1 transcription inhibition domain negatively regulated the transcriptional activity of Runx1 and inhibited Runx1-mediated MK maturation. Consistent with these findings, increased numbers of MKs were detected in the spleens and bone marrow of abl gene conditional knockout mice. Our findings demonstrate an important role of c-Abl kinase in Runx1-mediated MK maturation and platelet formation and provide a potential mechanism of Abl kinase-regulated hematopoiesis.

Human NOTCH4 is a key target of RUNX1 in megakaryocytic differentiation

Megakaryocytes (MKs) in adult marrow produce platelets that play important roles in blood coagulation and hemostasis. Monoallelic mutations of the master transcription factor gene RUNX1 lead to familial platelet disorder (FPD) characterized by defective MK and platelet development. However, the molecular mechanisms of FPD remain unclear. Previously, we generated human induced pluripotent stem cells (iPSCs) from patients with FPD containing a RUNX1 nonsense mutation. Production of MKs from the FPD-iPSCs was reduced, and targeted correction of the RUNX1 mutation restored MK production. In this study, we used isogenic pairs of FPD-iPSCs and the MK differentiation system to identify RUNX1 target genes. Using integrative genomic analysis of hematopoietic progenitor cells generated from FPD-iPSCs, and mutation-corrected isogenic controls, we identified 2 gene sets the transcription of which is either up- or downregulated by RUNX1 in mutation-corrected iPSCs. Notably, NOTCH4 expression was negatively controlled by RUNX1 via a novel regulatory DNA element within the locus, and we examined its involvement in MK generation. Specific inactivation of NOTCH4 by an improved CRISPR-Cas9 system in human iPSCs enhanced megakaryopoiesis. Moreover, small molecules known to inhibit Notch signaling promoted MK generation from both normal human iPSCs and postnatal CD34+ hematopoietic stem and progenitor cells. Our study newly identified NOTCH4 as a RUNX1 target gene and revealed a previously unappreciated role of NOTCH4 signaling in promoting human megakaryopoiesis. Our work suggests that human iPSCs with monogenic mutations have the potential to serve as an invaluable resource for discovery of novel druggable targets.

RUNX1 Mutations in Inherited and Sporadic Leukemia

RUNX1 is a recurrently mutated gene in sporadic myelodysplastic syndrome and leukemia. Inherited mutations in RUNX1 cause familial platelet disorder with predisposition to acute myeloid leukemia (FPD/AML). In sporadic AML, mutations in RUNX1 are usually secondary events, whereas in FPD/AML they are initiating events. Here we will describe mutations in RUNX1 in sporadic AML and in FPD/AML, discuss the mechanisms by which inherited mutations in RUNX1 could elevate the risk of AML in FPD/AML individuals, and speculate on why mutations in RUNX1 are rarely, if ever, the first event in sporadic AML.

Transcription Factor RUNX1 Regulates Platelet PCTP (Phosphatidylcholine Transfer Protein): Implications for Cardiovascular Events: Differential Effects of RUNX1 Variants

BACKGROUND

PCTP (phosphatidylcholine transfer protein) regulates the intermembrane transfer of phosphatidylcholine. Higher platelet PCTP expression is associated with increased platelet responses on activation of protease-activated receptor 4 thrombin receptors noted in black subjects compared with white subjects. Little is known about the regulation of platelet PCTP. Haplodeficiency of RUNX1, a major hematopoietic transcription factor, is associated with thrombocytopenia and impaired platelet responses on activation. Platelet expression profiling of a patient with a RUNX1 loss-of-function mutation revealed a 10-fold downregulation of the PCTP gene compared with healthy controls.

METHODS

We pursued the hypothesis that PCTP is regulated by RUNX1 and that PCTP expression is correlated with cardiovascular events. We studied RUNX1 binding to the PCTP promoter using DNA-protein binding studies and human erythroleukemia cells and promoter activity using luciferase reporter studies. We assessed the relationship between RUNX1 and PCTP in peripheral blood RNA and PCTP and death or myocardial infarction in 2 separate patient cohorts (587 total patients) with cardiovascular disease.

RESULTS

Platelet PCTP protein in the patient was reduced by ≈50%. DNA-protein binding studies showed RUNX1 binding to consensus sites in ≈1 kB of PCTP promoter. PCTP expression was increased with RUNX1 overexpression and reduced with RUNX1 knockdown in human erythroleukemia cells, indicating that PCTP is regulated by RUNX1. Studies in 2 cohorts of patients showed that RUNX1 expression in blood correlated with PCTP gene expression; PCTP expression was higher in black compared with white subjects and was associated with future death/myocardial infarction after adjustment for age, sex, and race (odds ratio, 2.05; 95% confidence interval 1.6-2.7; P<0.0001). RUNX1 expression is known to initiate at 2 alternative promoters, a distal P1 and a proximal P2 promoter. In patient cohorts, there were differential effects of RUNX1 isoforms on PCTP expression with a negative correlation in blood between RUNX1 expressed from the P1 promoter and PCTP expression.

CONCLUSIONS

PCTP is a direct transcriptional target of RUNX1. PCTP expression is associated with death/myocardial infarction in patients with cardiovascular disease. RUNX1 regulation of PCTP may play a role in the pathogenesis of platelet-mediated cardiovascular events.

TWIST-1 promotes cell growth, drug resistance and progenitor clonogenic capacities in myeloid leukemia and is a novel poor prognostic factor in acute myeloid leukemia

Alterations of TWIST-1 expression are often seen in solid tumors and contribute to tumorigenesis and cancer progression. However, studies concerning its pathogenic role in leukemia are scarce. Our study shows that TWIST-1 is overexpressed in bone marrow mononuclear cells of patients with acute myeloid leukemia (AML) and chronic myeloid leukemia (CML). Gain-of-function and loss-of-function analyses demonstrate that TWIST-1 promotes cell growth, colony formation and drug resistance of AML and CML cell lines. Furthermore, TWIST-1 is aberrantly highly expressed in CD34⁺CD38⁻ leukemia stem cell candidates and its expression declines with differentiation. Down-modulation of TWIST-1 in myeloid leukemia CD34⁺ cells impairs their colony-forming capacity. Mechanistically, c-MPL, which is highly expressed in myeloid leukemia cells and associated with poor prognosis, is identified as a TWIST-1 coexpressed gene in myeloid leukemia patients and partially contributes to TWIST-1-mediated leukemogenic effects. Moreover, patients with higher TWIST-1 expression have shorter overall and event-free survival (OS and EFS) in AML. Multivariate analysis further demonstrates that TWIST-1 overexpression is a novel independent unfavourable predictor for both OS and EFS in AML. These data highlight TWIST-1 as a new candidate gene contributing to leukemogenesis of myeloid leukemia, and propose possible new avenues for improving risk and treatment stratification in AML.

Runx1 downregulates stem cell and megakaryocytic transcription programs that support niche interactions

Disrupting mutations of the RUNX1 gene are found in 10% of patients with myelodysplasia (MDS) and 30% of patients with acute myeloid leukemia (AML). Previous studies have revealed an increase in hematopoietic stem cells (HSCs) and multipotent progenitor (MPP) cells in conditional Runx1-knockout (KO) mice, but the molecular mechanism is unresolved. We investigated the myeloid progenitor (MP) compartment in KO mice, arguing that disruptions at the HSC/MPP level may be amplified in downstream cells. We demonstrate that the MP compartment is increased by more than fivefold in Runx1 KO mice, with a prominent skewing toward megakaryocyte (Meg) progenitors. Runx1-deficient granulocyte-macrophage progenitors are characterized by increased cloning capacity, impaired development into mature cells, and HSC and Meg transcription signatures. An HSC/MPP subpopulation expressing Meg markers was also increased in Runx1-deficient mice. Rescue experiments coupled with transcriptome analysis and Runx1 DNA-binding assays demonstrated that granulocytic/monocytic (G/M) commitment is marked by Runx1 suppression of genes encoding adherence and motility proteins (Tek, Jam3, Plxnc1, Pcdh7, and Selp) that support HSC-Meg interactions with the BM niche. In vitro assays confirmed that enforced Tek expression in HSCs/MPPs increases Meg output. Interestingly, besides this key repressor function of Runx1 to control lineage decisions and cell numbers in progenitors, our study also revealed a critical activating function in erythroblast differentiation, in addition to its known importance in Meg and G/M maturation. Thus both repressor and activator functions of Runx1 at multiple hematopoietic stages and lineages likely contribute to the tumor suppressor activity in MDS and AML.

MiR144/451 Expression Is Repressed by RUNX1 During Megakaryopoiesis and Disturbed by RUNX1/ETO

A network of lineage-specific transcription factors and microRNAs tightly regulates differentiation of hematopoietic stem cells along the distinct lineages. Deregulation of this regulatory network contributes to impaired lineage fidelity and leukemogenesis. We found that the hematopoietic master regulator RUNX1 controls the expression of certain microRNAs, of importance during erythroid/megakaryocytic differentiation. In particular, we show that the erythorid miR144/451 cluster is epigenetically repressed by RUNX1 during megakaryopoiesis. Furthermore, the leukemogenic RUNX1/ETO fusion protein transcriptionally represses the miR144/451 pre-microRNA. Thus RUNX1/ETO contributes to increased expression of miR451 target genes and interferes with normal gene expression during differentiation. Furthermore, we observed that inhibition of RUNX1/ETO in Kasumi1 cells and in RUNX1/ETO positive primary acute myeloid leukemia patient samples leads to up-regulation of miR144/451. RUNX1 thus emerges as a key regulator of a microRNA network, driving differentiation at the megakaryocytic/erythroid branching point. The network is disturbed by the leukemogenic RUNX1/ETO fusion product.

Identification of novel biomarkers in chronic immune thrombocytopenia (ITP) by microarray-based serum protein profiling

The pathological mechanisms underlying the development of immune thrombocytopenia (ITP) are unclear and its diagnosis remains a process of exclusion. Currently, there are no known specific biomarkers for ITP to support differential diagnosis and treatment decisions. Profiling of serum proteins may be valuable for identifying such biomarkers. Sera from 46 patients with primary chronic ITP and 34 healthy blood donors were analysed using a microarray of 755 antibodies. We identified 161 differentially expressed proteins. In addition to oncoproteins and tumour-suppressor proteins, including apoptosis regulator BCL2, breast cancer type 1 susceptibility protein (BRCA1), Fanconi anaemia complementation group C (FANCC) and vascular endothelial growth factor A (VEGFA), we detected six anti-nuclear autoantibodies in a subset of ITP patients: anti-PCNA, anti-SmD, anti-Ro/SSA60, anti-Ro/SSA52, anti-La/SSB and anti-RNPC antibodies. This finding may provide a rational explanation for the association of ITP with malignancies and other autoimmune diseases. While RUNX1mRNA expression in the peripheral blood mononuclear cells (PBMC) of patients was significantly downregulated, an accumulation of RUNX1 protein was observed in the platelets of ITP patients. This may indicate dysregulation of RUNX1 expression in PBMC and megakaryocytes and may lead to an imbalanced immune response and impaired thrombopoiesis. In conclusion, we provide novel insights into the pathogenic mechanisms of ITP that warrant further exploration.

Stem cells, megakaryocytes, and platelets

PURPOSE OF REVIEW

Stem cells are an important tool for the study of ex-vivo models of megakaryopoiesis and the production of functional platelets. In this manuscript, we review the optimization of megakaryocyte and platelet differentiation and discuss the mechanistic studies and disease models that have incorporated stem cell technologies.

RECENT FINDINGS

Mechanisms of cytoskeletal regulation and signal transduction have revealed insights into hierarchical dynamics of hematopoiesis, highlighting the close relationship between hematopoietic stem cells and cells of the megakaryocyte lineage. Platelet disorders have been successfully modeled and genetically corrected, and differentiation strategies have been optimized to the extent that utilizing stem cell-derived platelets for cellular therapy is feasible.

SUMMARY

Studies that utilize stem cells for the efficient derivation of megakaryocytes and platelets have played a role in uncovering novel molecular mechanisms of megakaryopoiesis, modeling and correcting relevant diseases, and differentiating platelets that are functional and scalable for translation into the clinic. Efforts to derive megakaryocytes and platelets from pluripotent stem cells foster the opportunity of a revolutionary cellular therapy for the treatment of multiple platelet-associated diseases.

Novel function of PITH domain-containing 1 as an activator of internal ribosomal entry site to enhance RUNX1 expression and promote megakaryocyte differentiation

INTRODUCTION

Altered gene expression coincides with leukemia development and may affect distinct features of leukemic cells. PITHD1 was significantly downregulated in leukemia and upregulated upon PMA induction in K562 cells undergoing megakaryocyte differentiation. We aimed to study the function of PITHD1 in megakaryocyte differentiation.

MATERIALS AND METHODS

K562 cells and fetal liver cells were used for either overexpression or downregulation of PITHD1 by retroviral or lentiviral transduction. FACS was used to detect the expression of CD41 and CD42 to measure megakaryocyte differentiation in these cells. Western blot and quantitative RT-PCR were used to measure gene expression. Dual luciferase assay was used to detect promoter or internal ribosomal entry site (IRES) activity.

RESULTS

Ectopic expression of PITHD1 promoted megakaryocyte differentiation and increased RUNX1 expression while PITHD1 knockdown showed an opposite phenotype. Furthermore, PITHD1 efficiently induced endogenous RUNX1 expression and restored megakaryocyte differentiation suppressed by a dominant negative form of RUNX1. PITHD1 regulated RUNX1 expression at least through two distinct mechanisms: increasing transcription activity of proximal promoter and enhancing translation activity of an IRES element in exon 3. Finally, we confirmed the function of PITHD1 in regulating RUNX1 expression and megakaryopoiesis in mouse fetal liver cells.

CONCLUSION AND SIGNIFICANCE

PITHD1 was a novel activator of IRES and enhanced RUNX1 expression that subsequently promoted megakaryocyte differentiation. Our findings shed light on understanding the mechanisms underlying megakaryopoiesis or leukemogenesis.

MiR-18a increased the permeability of BTB via RUNX1 mediated down-regulation of ZO-1, occludin and claudin-5

The purposes of this study were to investigate the possible molecular mechanisms of miR-18a regulating the permeability of blood-tumor barrier (BTB) via down-regulated expression and distribution of runt-related transcription factor 1 (RUNX1). An in vitro BTB model was established with hCMEC/D3 cells and U87MG cells to obtain glioma vascular endothelial cells (GECs). The endogenous expressions of miR-18a and RUNX1 were converse in GECs. The overexpression of miR-18a significantly impaired the integrity and increased the permeability of BTB, which respectively were detected by TEER and HRP flux assays, accompanied by down-regulated mRNA and protein expressions and distributions of ZO-1, occludin and claudin-5 in GECs. Dual-luciferase reporter assay was carried out and revealed RUNX1 is a target gene of miR-18a. Meanwhile, mRNA and protein expressions and distribution of RUNX1 were downregulated by miR-18a. Most important, miR-18a and RUNX1 could reversely regulate the permeability of BTB as well as the expressions and distributions of ZO-1, occludin and claudin-5. Finally, chromatin immunoprecipitation verified that RUNX1 interacted with "TGGGGT" DNA sequence in promoter region of ZO-1, occludin and claudin-5 respectively. Taken together, our present study indicated that miR-18a increased the permeability of BTB via RUNX1 mediated down-regulation of tight junction related proteins ZO-1, occludin and claudin-5, which would attract more attention to miR-18a and RUNX1 as potential targets of drug delivery across BTB and provide novel strategies for glioma treatment.

Histone lysine-specific demethylase 1 (LSD1) protein is involved in Sal-like protein 4 (SALL4)-mediated transcriptional repression in hematopoietic stem cells

The stem cell protein SALL4 plays a critical role in hematopoiesis by regulating the cell fate. In primitive hematopoietic precursors, it activates or represses important genes via recruitment of various epigenetic factors such as DNA methyltransferases, and histone deacylases. Here, we demonstrate that LSD1, a histone lysine demethylase, also participates in the trans-repressive effects of SALL4. Based on luciferase assays, the amine oxidase domain of LSD1 is important in suppressing SALL4-mediated reporter transcription. In freshly isolated adult mouse bone marrows, both SALL4 and LSD1 proteins are preferentially expressed in undifferentiated progenitor cells and co-localize in the nuclei. Further sequential chromatin immunoprecipitation assay confirmed that these two factors share the same binding sites at the promoter regions of important hematopoietic regulatory genes including EBF1, GATA1, and TNF. In addition, studies from both gain- and loss-of-function models revealed that SALL4 dynamically controls the binding levels of LSD1, which is accompanied by a reversely changed histone 3 dimethylated lysine 4 at the same promoter regions. Finally, shRNA-mediated knockdown of LSD1 in hematopoietic precursor cells resulted in altered SALL4 downstream gene expression and increased cellular activity. Thus, our data revealed that histone demethylase LSD1 may negatively regulate SALL4-mediated transcription, and the dynamic regulation of SALL4-associated epigenetic factors cooperatively modulates early hematopoietic precursor proliferation.

RUNX1, but not its familial platelet disorder mutants, synergistically activates PF4 gene expression in combination with ETS family proteins

BACKGROUND

Familial platelet disorder (FPD) is a rare autosomal dominant disease characterized by thrombocytopenia and abnormal platelet function. Causal mutations have been identified in the gene encoding runt-related transcription factor 1 (RUNX1) of FPD patients.

OBJECTIVES

To elucidate the role of RUNX1 in the regulation of expression of platelet factor 4 (PF4) and to propose a plausible mechanism underlying RUNX1-mediated induction of the FPD phenotype.

METHODS

We assessed whether RUNX1 and its mutants, in combination with E26 transformation-specific-1 (ETS-1), Core-binding factor subunit beta (CBFβ), and Friend leukemia virus integration 1 (FLI-1), cooperatively regulate PF4 expression during megakaryocytic differentiation. In an embryonic stem cell differentiation system, expression levels of endogenous and exogenous RUNX1 and PF4 were determined by real-time RT-PCR. Promoter activation by the transcription factors were evaluated by reporter gene assays with HepG2 cells. DNA binding activity and protein interaction were analyzed by electrophoretic mobility shift assay and immunoprecipitation assay with Cos-7 cells, respectively. Protein localization was analyzed by immunocytochemistry and Western blotting with Cos-7 cells.

RESULTS

We demonstrated that RUNX1 activates endogenous PF4 expression in megakaryocytic differentiation. RUNX1, but not its mutants, in combination with ETS-1 and CBFβ, or FLI-1, synergistically activated the PF4 promoter. Each RUNX1 mutant harbors various functional abnormalities, including loss of DNA-binding activity, abnormal subcellular localization, and/or alterations of binding affinities for ETS-1, CBFβ, and FLI-1.

CONCLUSIONS

RUNX1, but not its mutants, strongly and synergistically activates PF4 expression along with ETS family proteins. Furthermore, loss of the RUNX1 transcriptional activation function is induced by various functional abnormalities.

The Satb1 protein directs hematopoietic stem cell differentiation toward lymphoid lineages

How hematopoietic stem cells (HSCs) produce particular lineages is insufficiently understood. We searched for key factors that direct HSC to lymphopoiesis. Comparing gene expression profiles for HSCs and early lymphoid progenitors revealed that Satb1, a global chromatin regulator, was markedly induced with lymphoid lineage specification. HSCs from Satb1-deficient mice were defective in lymphopoietic activity in culture and failed to reconstitute T lymphopoiesis in wild-type recipients. Furthermore, Satb1 transduction of HSCs and embryonic stem cells robustly promoted their differentiation toward lymphocytes. Whereas genes that encode Ikaros, E2A, and Notch1 were unaffected, many genes involved in lineage decisions were regulated by Satb1. Satb1 expression was reduced in aged HSCs with compromised lymphopoietic potential, but forced Satb1 expression partly restored that potential. Thus, Satb1 governs the initiating process central to the replenishing of lymphoid lineages. Such activity in lymphoid cell generation may be of clinical importance and useful to overcome immunosenescence.

Transcription factors in late megakaryopoiesis and related platelet disorders

Cell type-specific transcription factors regulate the repertoire of genes expressed in a cell and thereby determine its phenotype. The differentiation of megakaryocytes, the platelet progenitors, from hematopoietic stem cells is a well-known process that can be mimicked in culture. However, the efficient formation of platelets in culture remains a challenge. Platelet formation is a complicated process including megakaryocyte maturation, platelet assembly and platelet shedding. We hypothesize that a better understanding of the transcriptional regulation of this process will allow us to influence it such that sufficient numbers of platelets can be produced for clinical applications. After an introduction to gene regulation and platelet formation, this review summarizes the current knowledge of the regulation of platelet formation by the transcription factors EVI1, GATA1, FLI1, NFE2, RUNX1, SRF and its co-factor MKL1, and TAL1. Also covered is how some platelet disorders including myeloproliferative neoplasms, result from disturbances of the transcriptional regulation. These disorders give us invaluable insights into the crucial role these transcription factors play in platelet formation. Finally, there is discussion of how a better understanding of these processes will be needed to allow for efficient production of platelets in vitro.

A novel complex, RUNX1-MYEF2, represses hematopoietic genes in erythroid cells

RUNX1 is known to be an essential transcription factor for generating hematopoietic stem cells (HSC), but much less is known about its role in the downstream process of hematopoietic differentiation. RUNX1 has been shown to be part of a large transcription factor complex, together with LDB1, GATA1, TAL1, and ETO2 (N. Meier et al., Development 133:4913-4923, 2006) in erythroid cells. We used a tagging strategy to show that RUNX1 interacts with two novel protein partners, LSD1 and MYEF2, in erythroid cells. MYEF2 is bound in undifferentiated cells and is lost upon differentiation, whereas LSD1 is bound in differentiated cells. Chromatin immunoprecipitation followed by sequencing (ChIP-seq) and microarray expression analysis were used to show that RUNX1 binds approximately 9,000 target sites in erythroid cells and is primarily active in the undifferentiated state. Functional analysis shows that a subset of the target genes is suppressed by RUNX1 via the newly identified partner MYEF2. Knockdown of Myef2 expression in developing zebrafish results in a reduced number of HSC.

Histone arginine methylation keeps RUNX1 target genes in an intermediate state

The coordinated recruitment of epigenetic regulators of gene expression by transcription factors such as RUNX1 (AML1, acute myeloid leukemia 1) is crucial for hematopoietic differentiation. Here, we identify protein arginine methyltransferase 6 (PRMT6) as a central functional component of a RUNX1 corepressor complex containing Sin3a and HDAC1 in human hematopoietic progenitor cells. PRMT6 is recruited by RUNX1 and mediates asymmetric histone H3 arginine-2 dimethylation (H3R2me2a) at megakaryocytic genes in progenitor cells. H3R2me2a keeps RUNX1 target genes in an intermediate state with concomitant H3K27me3 and H3K4me2 but not H3K4me3. Upon megakaryocytic differentiation PRMT6 binding is lost, the H3R2me2a mark decreases and a coactivator complex containing WDR5/MLL and p300/pCAF is recruited. This leads to an increase of H3K4me3 and H3K9ac, which result in augmented gene expression. Our results provide novel mechanistic insight into how RUNX1 activity in hematopoietic progenitor cells maintains differentiation genes in a suppressed state but poised for rapid transcriptional activation.

Acute myeloid leukemia and transcription factors: role of erythroid Krüppel-like factor (EKLF)

We have investigated the role of erythroid transcription factors mRNA expression in patients with acute myeloid leukemia (AML) in the context of cytogenetic and other prognostic molecular markers, such as FMS-like Tyrosine Kinase 3 (FLT3), Nucleophosmin 1 (NPM1), and CCAAT/enhance-binding protein α (CEBPA) mutations. Further validation of Erythroid Krüppel-like Factor (EKLF) mRNA expression as a prognostic factor was assessed.We evaluated GATA binding protein 1 (GATA1), GATA binding protein 2 (GATA2), EKLF and Myeloproliferative Leukemia virus oncogen homology (cMPL) gene mRNA expression in the bone marrow of 65 AML patients at diagnosis, and assessed any correlation with NPM1, FLT3 and CEBPA mutations. EKLF-positive AML was associated with lower WBC in peripheral blood (P = 0.049), a higher percentage of erythroblasts in bone marrow (p = 0.057), and secondary AMLs (P = 0.036). High expression levels of EKLF showed a trend to association with T-cell antigen expression, such as CD7 (P = 0.057). Patients expressing EKLF had longer Overall Survival (OS) and Event Free Survival (EFS) than those patients not expressing EKLF (median OS was 35.61 months and 19.31 months, respectively, P = 0.0241; median EFS was 19.80 months and 8.03 months, respectively, P = 0.0140). No correlation of GATA1, GATA2, EKLF and cMPL levels was observed with FLT-3 or NPM1 mutation status. Four of four CEBPA mutated AMLs were EKLF positive versus 10 of 29 CEBPA wild-type AMLs; three of the CEBPA mutated, EKLF-positive AMLs were also GATA2 positive. There were no cases of CEBPA mutations in the EKLF-negative AML group. In conclusion, we have validated EKLF mRNA expression as an independent predictor of outcome in AML, and its expression is not associated with FLT3-ITD and NPM1 mutations. EKLF mRNA expression in AML patients may correlate with dysregulated CEBPA.

Thrombopoietin/MPL participates in initiating and maintaining RUNX1-ETO acute myeloid leukemia via PI3K/AKT signaling

Oncogenic mutations in components of cytokine signaling pathways elicit ligand-independent activation of downstream signaling, enhancing proliferation and survival in acute myeloid leukemia (AML). The myeloproliferative leukemia virus oncogene, MPL, a homodimeric receptor activated by thrombopoietin (THPO), is mutated in myeloproliferative disorders but rarely in AML. Here we show that wild-type MPL expression is increased in a fraction of human AML samples expressing RUNX1-ETO, a fusion protein created by chromosome translocation t(8;21), and that up-regulation of Mpl expression in mice induces AML when coexpressed with RUNX1-ETO. The leukemic cells are sensitive to THPO, activating survival and proliferative responses. Mpl expression is not regulated by RUNX1-ETO in mouse hematopoietic progenitors or leukemic cells. Moreover, we find that activation of PI3K/AKT but not ERK/MEK pathway is a critical mediator of the MPL-directed antiapoptotic function in leukemic cells. Hence, this study provides evidence that up-regulation of wild-type MPL levels promotes leukemia development and maintenance through activation of the PI3K/AKT axis, and suggests that inhibitors of this axis could be effective for treatment of MPL-positive AML.

Polycomb group ring finger 1 cooperates with Runx1 in regulating differentiation and self-renewal of hematopoietic cells

The transcription factor runt-related transcription factor 1 (Runx1) is essential for the establishment of definitive hematopoiesis during embryonic development. In adult blood homeostasis, Runx1 plays a pivotal role in the maturation of lymphocytes and megakaryocytes. Furthermore, Runx1 is required for the regulation of hematopoietic stem and progenitor cells. However, how Runx1 orchestrates self-renewal and lineage choices in combination with other factors is not well understood. In the present study, we describe a genome-scale RNA interference screen to detect genes that cooperate with Runx1 in regulating hematopoietic stem and progenitor cells. We identify the polycomb group protein Pcgf1 as an epigenetic regulator involved in hematopoietic cell differentiation and show that simultaneous depletion of Runx1 and Pcgf1 allows sustained self-renewal while blocking differentiation of lineage marker-negative cells in vitro. We found an up-regulation of HoxA cluster genes on Pcgf1 knock-down that possibly accounts for the increase in self-renewal. Moreover, our data suggest that cells lacking both Runx1 and Pcgf1 are blocked at an early progenitor stage, indicating that a concerted action of the transcription factor Runx1, together with the epigenetic repressor Pcgf1, is necessary for terminal differentiation. The results of the present study uncover a link between transcriptional and epigenetic regulation that is required for hematopoietic differentiation.

Megakaryopoiesis and thrombopoiesis: an update on cytokines and lineage surface markers

Megakaryopoiesis is the process by which mature megakaryocytes (MKs) develop from hematopoietic stem cells (HSCs). The biological function of MKs is to produce platelets, which play critical roles in hemostasis and contribute to angiogenesis and wound healing. The generation of platelets from MKs is termed thrombopoiesis. The cytokine thrombopoietin (TPO) is the major regulator of megakaryopoiesis and thrombopoiesis. It binds to its surface receptor, c-Mpl, and acts through multiple downstream signaling pathways, including the PI-3 kinase-Akt, MAPK, and ERK1/ERK2 pathways. However, non-TPO pathways, such as the SDF1/CXCR4 axis, Notch signaling, src family kinases, integrin signaling, and Platelet Factor 4/low-density lipoprotein receptor-related protein 1, have more recently been recognized to influence megakaryopoiesis and thrombopoiesis in vitro and in vivo. In this chapter, we review megakaryopoiesis and thrombopoiesis with emphasis on cell surface marker changes during their differentiation from HSCs, and the classical cytokines that affect these developmental stages. We also discuss non-TPO regulators and their effects on in vitro culture systems.

RUNX1 and RUNX1-ETO: roles in hematopoiesis and leukemogenesis

RUNX1 is a transcription factor that regulates critical processes in many aspects of hematopoiesis. RUNX1 is also integral in defining the definitive hematopoietic stem cell. In addition, many hematological diseases like myelodysplastic syndrome and myeloproliferative neoplasms have been associated with mutations in RUNX1. Located on chromosomal 21, the RUNX1 gene is involved in many forms of chromosomal translocations in leukemia. t(8;21) is one of the most common chromosomal translocations found in acute myeloid leukemia (AML), where it results in a fusion protein between RUNX1 and ETO. The RUNX1-ETO fusion protein is found in approximately 12% of all AML patients. In this review, we detail the structural features, functions, and models used to study both RUNX1 and RUNX1-ETO in hematopoiesis over the past two decades.

The thrombopoietin/MPL pathway in hematopoiesis and leukemogenesis

Hematopoietic stem cells (HSC) comprise a small percentage of total hematopoietic cells. Their ability to self-renewal is key to the continuous replenishment of the hematopoietic system with newly formed functional blood cell types while maintaining their multipotential capacity. Understanding the extrinsic signals that are essential to HSC maintenance will provide insights into the regulation of hematopoiesis at its most primitive stage, and with the knowledge applied, will potentially lead to improved clinical transplantation outcomes. In this review, we will summarize the current understanding of the role of the thrombopoietin/MPL signaling pathway in HSC maintenance during adult and fetal hematopoiesis. We will also speculate on the downstream key players in the pathway based on published data, and summarize the role of this pathway in leukemia.

Chromatin regulation by RUNX1

The transcription factor RUNX1 is essential for definitive hematopoiesis and is required for the expression of a number of important hematopoietic regulator genes. It was recently shown that RUNX1 acts within a narrow developmental window during which it cannot be replaced by other members of the RUNX transcription factor family. Studies of the molecular basis of this phenomenon revealed that RUNX1 is required for the opening of chromatin of important hematopoietic regulator genes and for the formation, but not the maintenance of stable transcription factor complexes on these genes. However, the chromatin opening activity of RUNX1 is context dependent, indicating that it cooperates with alternate transcription factors at different stages of hematopoietic development. This review summarizes recent results on the regulation of chromatin structure by RUNX1 in developing hematopoietic cells.

Cbfb/Runx1 repression-independent blockage of differentiation and accumulation of Csf2rb-expressing cells by Cbfb-MYH11

It is known that CBFB-MYH11, the fusion gene generated by inversion of chromosome 16 in human acute myeloid leukemia, is causative for oncogenic transformation. However, the mechanism by which CBFB-MYH11 initiates leukemogenesis is not clear. Previously published reports showed that CBFB-MYH11 dominantly inhibits RUNX1 and CBFB, and such inhibition has been suggested as the mechanism for leukemogenesis. Here we show that Cbfb-MYH11 caused Cbfb/Runx1 repression-independent defects in both primitive and definitive hematopoiesis. During primitive hematopoiesis, Cbfb-MYH11 delayed differentiation characterized by sustained expression of Gata2, Il1rl1, and Csf2rb, a phenotype not found in Cbfb and Runx1 knockout mice. Expression of Cbfb-MYH11 in the bone marrow induced the accumulation of abnormal progenitor-like cells expressing Csf2rb in preleukemic mice. The expression of all 3 genes was detected in most human and murine CBFB-MYH11(+) leukemia samples. Interestingly, Cbfb-MYH11(+) preleukemic progenitors and leukemia-initiating cells did not express Csf2rb, although the majority of leukemia cells in our Cbfb-MYH11 knockin mice were Csf2rb(+). Therefore Csf2rb can be used as a negative selection marker to enrich preleukemic progenitor cells and leukemia-initiating cells from Cbfb-MYH11 mice. These results suggest that Cbfb/Runx1 repression-independent activities contribute to leukemogenesis by Cbfb-MYH11.

Core binding factor at the crossroads: determining the fate of the HSC

Hematopoietic development requires coordinated actions from a variety of transcription factors. The core binding factor (CBF), consisting of a Runx protein and the CBFbeta protein, is a transcription factor complex that is essential for emergence of the hematopoietic stem cell (HSC) from an endothelial cell stage. The hematopoietic defects observed in either Runx1 or CBFbeta knockout mice underscore the necessity of this complex for definitive hematopoiesis. Despite the requirement for CBF in establishing definitive hematopoiesis, Runx1 loss has minimal impact on maintaining the HSC state postnatally, while CBFbeta may continue to be essential. Lineage commitment, on the other hand, is significantly affected upon CBF loss in the adult, indicating a primary role for this complex in modulating differentiation. Given the impact of normal CBF function in the hematopoietic system, the severe consequences of disrupting CBF activity, either through point mutations or generation of fusion genes, are obvious. The physiologic role of CBF in differentiation is subverted to an active process of self-renewal maintenance by the genetic aberrations, through several possible mechanisms, contributing to the development of hematopoietic malignancies including myelodysplastic syndrome and leukemia. The major impact of CBF on the hematopoietic system in both development and disease highlights the need for understanding the intricate functions of this complex and reiterate the necessity of continued efforts to identify potential points of therapeutic intervention for CBF-related diseases.

Megakaryocytic programming by a transcriptional regulatory loop: A circle connecting RUNX1, GATA-1, and P-TEFb

Transcription factors originally identified as drivers of erythroid differentiation subsequently became linked to megakaryopoiesis, reflecting the shared parentage of red cells and platelets. The divergent development of megakaryocytic and erythroid progenitors relies on signaling pathways that impose lineage-specific transcriptional programs on non-lineage-restricted protein complexes. One such signaling pathway involves RUNX1, a transcription factor upregulated in megakaryocytes and downregulated in erythroid cells. In this pathway, RUNX1 engages the erythro-megakaryocytic master regulator GATA-1 in a megakaryocytic transcriptional complex whose activity is highly dependent on the P-TEFb kinase complex. The implications of this pathway for normal and pathological megakaryopoiesis are discussed.

Runx1 promotes B-cell survival and lymphoma development

Runx1 is essential for the homeostatic control of normal hematopoiesis and is required for lymphoid development. Translocations or point mutations that result in RUNX1 loss or disrupted function predispose to leukemia but data derived from model systems suggests that Runx genes can also be pro-oncogenic. Here we investigate the effects of enforced Runx1 expression in lymphoid lineages both in vivo and in vitro and show that transgene expression enhanced cell survival in the thymus and bone marrow but strongly inhibited the expansion of hematopoietic and B cell progenitors in vitro. Despite this, modestly enhanced levels of Runx1 accelerated Myc-induced lymphomagenesis in both the B cell and T cell lineages. Together these data provide formal proof that wild type Runx1 can promote oncogenesis in lymphoid tissues and that, in addition to loss of function, gain of function may have an aetiological role in leukemia.