Enhancer recruitment of transcription repressors RUNX1 and TLE3 by mis-expressed FOXC1 blocks differentiation in acute myeloid leukemia

Despite absent expression in normal hematopoiesis, the Forkhead factor FOXC1, a critical mesenchymal differentiation regulator, is highly expressed in ∼30% of HOXAhigh acute myeloid leukemia (AML) cases to confer blocked monocyte/macrophage differentiation. Through integrated proteomics and bioinformatics, we find that FOXC1 and RUNX1 interact through Forkhead and Runt domains, respectively, and co-occupy primed and active enhancers distributed close to differentiation genes. FOXC1 stabilizes association of RUNX1, HDAC1, and Groucho repressor TLE3 to limit enhancer activity: FOXC1 knockdown induces loss of repressor proteins, gain of CEBPA binding, enhancer acetylation, and upregulation of nearby genes, including KLF2. Furthermore, it triggers genome-wide redistribution of RUNX1, TLE3, and HDAC1 from enhancers to promoters, leading to repression of self-renewal genes, including MYC and MYB. Our studies highlight RUNX1 and CEBPA transcription factor swapping as a feature of leukemia cell differentiation and reveal that FOXC1 prevents this by stabilizing enhancer binding of a RUNX1/HDAC1/TLE3 transcription repressor complex to oncogenic effect.

Stage-specific action of Runx1 and GATA3 controls silencing of PU.1 expression in mouse pro-T cells

PU.1 (encoded by Spi1), an ETS-family transcription factor with many hematopoietic roles, is highly expressed in the earliest intrathymic T cell progenitors but must be down-regulated during T lineage commitment. The transcription factors Runx1 and GATA3 have been implicated in this Spi1 repression, but the basis of the timing was unknown. We show that increasing Runx1 and/or GATA3 down-regulates Spi1 expression in pro-T cells, while deletion of these factors after Spi1 down-regulation reactivates its expression. Leveraging the stage specificities of repression and transcription factor binding revealed an unconventional but functional site in Spi1 intron 2. Acute Cas9-mediated deletion or disruption of the Runx and GATA motifs in this element reactivates silenced Spi1 expression in a pro-T cell line, substantially more than disruption of other candidate elements, and counteracts the repression of Spi1 in primary pro-T cells during commitment. Thus, Runx1 and GATA3 work stage specifically through an intronic silencing element in mouse Spi1 to control strength and maintenance of Spi1 repression during T lineage commitment.

Msx1 cooperates with Runx1 for inhibiting myoblast differentiation

Myogenesis is an important and complicated biological process, especially during the process of embryonic development. The homeoprotein Msx1 is a crucial transcriptional repressor of myogenesis and maintains myogenic precursor cells in an undifferentiated, proliferative state. However, the molecular mechanism through which Msx1 coordinates myogenesis remains to be elucidated. Here, we determine the interacting partner proteins of Msx1 in myoblast cells by a proteomic screening method. Msx1 is found to interact with 55 proteins, among which our data demonstrate that the cooperation of Runt-related transcription factor 1 (Runx1) with Msx1 is required for myoblast cell differentiation. Our findings provide important insights into the mechanistic roles of Msx1 in myoblast cell differentiation, and lays foundation for the myogenic differentiation process.

CHD7 and Runx1 interaction provides a braking mechanism for hematopoietic differentiation

Hematopoietic stem and progenitor cell (HSPC) formation and lineage differentiation involve gene expression programs orchestrated by transcription factors and epigenetic regulators. Genetic disruption of the chromatin remodeler chromodomain-helicase-DNA-binding protein 7 (CHD7) expanded phenotypic HSPCs, erythroid, and myeloid lineages in zebrafish and mouse embryos. CHD7 acts to suppress hematopoietic differentiation. Binding motifs for RUNX and other hematopoietic transcription factors are enriched at sites occupied by CHD7, and decreased RUNX1 occupancy correlated with loss of CHD7 localization. CHD7 physically interacts with RUNX1 and suppresses RUNX1-induced expansion of HSPCs during development through modulation of RUNX1 activity. Consequently, the RUNX1:CHD7 axis provides proper timing and function of HSPCs as they emerge during hematopoietic development or mature in adults, representing a distinct and evolutionarily conserved control mechanism to ensure accurate hematopoietic lineage differentiation.

CBFβ-SMMHC Inhibition Triggers Apoptosis by Disrupting MYC Chromatin Dynamics in Acute Myeloid Leukemia

The fusion oncoprotein CBFβ-SMMHC, expressed in leukemia cases with chromosome 16 inversion, drives leukemia development and maintenance by altering the activity of the transcription factor RUNX1. Here, we demonstrate that CBFβ-SMMHC maintains cell viability by neutralizing RUNX1-mediated repression of MYC expression. Upon pharmacologic inhibition of the CBFβ-SMMHC/RUNX1 interaction, RUNX1 shows increased binding at three MYC distal enhancers, where it represses MYC expression by mediating the replacement of the SWI/SNF complex component BRG1 with the polycomb-repressive complex component RING1B, leading to apoptosis. Combining the CBFβ-SMMHC inhibitor with the BET inhibitor JQ1 eliminates inv(16) leukemia in human cells and a mouse model. Enhancer-interaction analysis indicated that the three enhancers are physically connected with the MYC promoter, and genome-editing analysis demonstrated that they are functionally implicated in deregulation of MYC expression. This study reveals a mechanism whereby CBFβ-SMMHC drives leukemia maintenance and suggests that inhibitors targeting chromatin activity may prove effective in inv(16) leukemia therapy.

RUNX1: A Regulator of NF-kB Signaling in Pulmonary Diseases

Runt-related transcription factor 1 (RUNX1), a member of the RUNX family, is one of the key regulatory proteins in vertebrates. RUNX1 is involved in embryonic development, hematopoiesis, angiogenesis, tumorigenesis and immune response. In the past few decades, studies mainly focused on the effect of RUNX1 on acute leukemia and cancer. Only few studies about the function of RUNX1 in the pathological process of pulmonary diseases have been reported. Recent studies have demonstrated that RUNX1 is highly expressed in both mesenchymal and epithelial compartments of the developing and postnatal lung and that it plays a critical role in the lipopolysaccharide induced lung inflammation by regulating the NF-kB pathway. RUNX1 participates in the regulation of the NF-kB signaling pathway through interaction with IkB kinase complex in the cytoplasm or interaction with the NF-kB subunit P50. NF-kB is well-known signaling pathway necessary for inflammatory response in the lung. This review is to highlight the RUNX1 structure, isoforms and to present the mechanism that RUNX1 regulates NF-kB. This will illustrate the great potential role of RUNX1 in the inflammation signaling pathway in pulmonary diseases.

Crystallization of the Ets1-Runx1-CBFβ-DNA complex formed on the TCRα gene enhancer

Gene transcription is regulated in part through the assembly of multiple transcription factors (TFs) on gene enhancers. To enable examination of the mechanism underlying the formation of these complexes and their response to a phosphorylation signal, two kinds of higher-order TF-DNA assemblies were crystallized composed of an unmodified or phosphorylated Ets1 fragment, a Runx1(L94K) fragment and a CBFβ fragment on the T-cell receptor (TCR) α gene enhancer. Within these complexes, the Ets1 and Runx1 fragments contain intrinsically disordered regulatory regions as well as their DNA-binding domains. Crystals of the complex containing unmodified Ets1 belonged to space group P212121, with unit-cell parameters a = 78.7, b = 102.1, c = 195.0 Å, and diffracted X-rays to a resolution of 2.35 Å, and those containing phosphorylated Ets1 belonged to the same space group, with unit-cell parameters a = 78.6, b = 101.7, c = 194.7 Å, and diffracted X-rays to a similar resolution. To facilitate crystallization, a Runx1 residue involved in a hydrophobic patch that was predicted to be engaged in crystal packing based on the previously reported structures of Runx1-containing crystals was mutated.

CBFβ stabilizes HIV Vif to counteract APOBEC3 at the expense of RUNX1 target gene expression

The HIV-1 accessory protein Vif hijacks a cellular Cullin-RING ubiquitin ligase, CRL5, to promote degradation of the APOBEC3 (A3) family of restriction factors. Recently, the cellular transcription cofactor CBFβ was shown to form a complex with CRL5-Vif and to be essential for A3 degradation and viral infectivity. We now demonstrate that CBFβ is required for assembling a well-ordered CRL5-Vif complex by inhibiting Vif oligomerization and by activating CRL5-Vif via direct interaction. The CRL5-Vif-CBFβ holoenzyme forms a well-defined heterohexamer, indicating that Vif simultaneously hijacks CRL5 and CBFβ. Heterodimers of CBFβ and RUNX transcription factors contribute toward the regulation of genes, including those with immune system functions. We show that binding of Vif to CBFβ is mutually exclusive with RUNX heterodimerization and impacts the expression of genes whose regulatory domains are associated with RUNX1. Our results provide a mechanism by which a pathogen with limited coding capacity uses one factor to hijack multiple host pathways.

The leukemogenicity of AML1-ETO is dependent on site-specific lysine acetylation

The chromosomal translocations found in acute myelogenous leukemia (AML) generate oncogenic fusion transcription factors with aberrant transcriptional regulatory properties. Although therapeutic targeting of most leukemia fusion proteins remains elusive, the posttranslational modifications that control their function could be targetable. We found that AML1-ETO, the fusion protein generated by the t(8;21) translocation, is acetylated by the transcriptional coactivator p300 in leukemia cells isolated from t(8;21) AML patients, and that this acetylation is essential for its self-renewal-promoting effects in human cord blood CD34(+) cells and its leukemogenicity in mouse models. Inhibition of p300 abrogates the acetylation of AML1-ETO and impairs its ability to promote leukemic transformation. Thus, lysine acetyltransferases represent a potential therapeutic target in AML.

Cell cycle and developmental control of hematopoiesis by Runx1

Runx1 binds DNA in cooperation with CBFbeta to activate or repress transcription, dependent upon cellular context and interaction with a variety of co-activators and co-repressors. Runx1 is required for emergence of adult hematopoietic stem cells (HSC) during embryonic development and for lymphoid, myeloid, and megakaryocyte lineage maturation from HSC in adult marrow. Runx1 levels vary during the cell cycle, and Runx1 regulates G1 to S cell cycle progression. Both Cdk and ERK phosphorylate Runx1 to influence its interaction with co-repressors, and the Wnt effector LEF-1/TCF also modulates Runx1 activities. These links likely allow cytokines and signals from adjacent cells to influence HSC proliferation versus quiescence and the rate of progenitor expansion, in response to developmental or environmental demands. J. Cell. Physiol. 219: 520-524, 2009. (c) 2009 Wiley-Liss, Inc.

Post-translational modifications of Runx1 regulate its activity in the cell

In this report we review the current knowledge of the interaction of RUNX1(AML1) with serine/threonine kinases, lysine and arginine methyltransferases, lysine acetyltransferases, and histone deacetylases. We also discuss the effect of RUNX1-ETO fusion gene on DNA methylation. RUNX1 post-transcriptional modification can affect its role in influencing differentiation and self-renewal of hematopoietic cells. The goal of these studies is to develop targets for improved leukemia therapy.

Repression of RUNX1 activity by EVI1: a new role of EVI1 in leukemogenesis

Recurring chromosomal translocations observed in human leukemia often result in the expression of fusion proteins that are DNA-binding transcription factors. These altered proteins acquire new dimerization properties that result in the assembly of inappropriate multimeric transcription complexes that deregulate hematopoietic programs and induce leukemogenesis. Recently, we reported that the fusion protein AML1/MDS1/EVI1 (AME), a product of a t(3;21)(q26;q22) associated with chronic myelogenous leukemia and acute myelogenous leukemia, displays a complex pattern of self-interaction. Here, we show that the 8th zinc finger motif of MDS1/EVI1 is an oligomerization domain involved not only in interaction of AME with itself but also in interactions with the parental proteins, RUNX1 and MDS1/EVI1, from which AME is generated. Because the 8th zinc finger motif is also present in the oncoprotein EVI1, we have evaluated the effects of the interaction between RUNX1 and EVI1 in vitro and in vivo. We found that in vitro, this interaction alters the ability of RUNX1 to bind to DNA and to regulate a reporter gene, whereas in vivo, the expression of the isolated 8th zinc finger motif of EVI1 is sufficient to block the granulocyte colony-stimulating factor-induced differentiation of 32Dcl3 cells, leading to cell death. As EVI1 is not detected in normal bone marrow cells, these data suggest that its inappropriate expression could contribute to hematopoietic transformation in part by a new mechanism that involves EVI1 association with key hematopoietic regulators, leading to their functional impairment.

TEL-AML1 preleukemic activity requires the DNA binding domain of AML1 and the dimerization and corepressor binding domains of TEL

The t(12;21)(p13;q22) translocation generates the TEL-AML1 (TEL, translocation-Ets-leukemia; AML1, acute myeloid leukemia-1) (ETV6-RUNX1) fusion product and is the most common chromosomal abnormality in pediatric leukemia. Our previous studies using a murine fetal liver transplantation model demonstrated that TEL-AML1 promotes the self-renewal of B-cell precursors in vitro and enhances the expansion of hematopoietic stem cells (HSCs) in vivo. This is consistent with the hypothesis that TEL-AML1 induces expansion of a preleukemic clone. Several studies have described domains within TEL-AML1 involved in the transcriptional regulation of specific target genes. However, it is unclear which of these domains is important for the activity of TEL-AML1 in preleukemic hematopoiesis. In order to examine this, we have generated a panel of deletion mutants and expressed them in HSCs. These experiments demonstrate that TEL-AML1 requires multiple domains from both TEL and AML1 to alter hematopoiesis. Furthermore, mutation of a single amino-acid residue within the runt homology domain of AML1, required for DNA binding, was sufficient to abrogate TEL-AML1 activity. These data suggest that TEL-AML1 acts as an aberrant transcription factor to perturb multiple pathways during hematopoiesis.

A previously unidentified alternatively spliced isoform of t(8;21) transcript promotes leukemogenesis

The t(8;21)(q22;q22) translocation is one of the most common genetic abnormalities in acute myeloid leukemia (AML), identified in 15% of all cases of AML, including 40-50% of FAB M2 subtype and rare cases of M0, M1 and M4 subtypes. The most commonly known AML1-ETO fusion protein (full-length AML1-ETO) from this translocation has 752 amino acids and contains the N-terminal portion of RUNX1 (also known as AML1, CBFalpha2 or PEBP2alphaB), including its DNA binding domain, and almost the entire RUNX1T1 (also known as MTG8 or ETO) protein. Although alterations of gene expression and hematopoietic cell proliferation have been reported in the presence of AML1-ETO, its expression does not lead to the development of leukemia. Here, we report the identification of a previously unknown alternatively spliced isoform of the AML1-ETO transcript, AML1-ETO9a, that includes an extra exon, exon 9a, of the ETO gene. AML1-ETO9a encodes a C-terminally truncated AML1-ETO protein of 575 amino acids. Expression of AML1-ETO9a leads to rapid development of leukemia in a mouse retroviral transduction-transplantation model. More importantly, coexpression of AML1-ETO and AML1-ETO9a results in the substantially earlier onset of AML and blocks myeloid cell differentiation at a more immature stage. These results indicate that fusion proteins from alternatively spliced isoforms of a chromosomal translocation may work together to induce cancer development.

The acute myeloid leukemia fusion protein AML1-ETO targets E proteins via a paired amphipathic helix-like TBP-associated factor homology domain

Up to 15% of acute myeloid leukemias (AMLs) are characterized by the abnormal expression of the eight-twenty-one (ETO) transcriptional corepressor within an AML1-ETO fusion protein. The t(8;21) chromosomal translocation serves not only to disrupt WT AML1 function but also to introduce ETO activity during hematopoiesis. AML1-ETO was recently shown to inhibit E protein transactivation by physically displacing WT coactivator proteins in an interaction mediated by ETO. Here, we present the 3D solution structure of the human ETO TAFH (eTAFH) domain implicated in AML1-ETO:E protein interactions and report an unexpected fold similarity to paired amphipathic helix domains from the transcriptional corepressor Sin3. We identify and characterize a conserved surface on eTAFH that is essential for ETO:E protein recognition and show that the mutation of key conserved residues at this site alleviates ETO-based silencing of E protein transactivation. Our results address uncharacterized aspects of the corepression mechanism of ETO and suggest that eTAFH may serve to recruit ETO (or AML1-ETO) to DNA-bound transcription factors. Together, these findings imply that a cofactor exchange mechanism, analogous to that described for E protein inhibition, may represent a common mode of action for ETO.

Mutations of AML1 in non-M0 acute myeloid leukemia: six novel mutations and a high incidence of cooperative events in a South-east Asian population

Point mutations of AML1 are uncommon and predominantly reported in a rare minimally differentiated acute myeloid leukemia (M0 AML). Few data exist regarding the frequency of AML1 mutations in non-M0 cases. We screened 284 consecutive adult Thai patients with de novo AML and found that 3.9% had AML1 mutations. The highest incidence occurred in M6. Six novel mutations were uniquely identified in non-M0 cases. Sixty-four percent of the non-M0 patients with AML1 mutations had coexisting genetic abnormalities including FLT3 mutation in 36%. Our study provides evidence to support the model of multiple co-operating events, which could also be critical in the development of leukemia in non-M0 AML patients with mutated AML1. The prognostic significance of these novel mutations remains to be determined.

Structural basis of multimer-mediated mayhem

Oligomerization of AML1-ETO contributes to leukemogenesis through obscure mechanisms. In this issue of Cancer Cell, Bushweller and colleagues show the crystal structure of the ETO NHR2 domain to be a tetramer. Tetramer formation is important for maturation arrest and self-renewal, and gene expression is altered in the absence of self-association. Loss of oligomer formation disrupts interactions between AML1-ETO and members of the ETO corepressor family, but not other corepressor molecules posited to be important for leukemogenesis. The findings clarify the role of oligomer formation in AML1-ETO function and suggest a possible therapeutic strategy of targeting ETO-corepressor interactions.

The tetramer structure of the Nervy homology two domain, NHR2, is critical for AML1/ETO's activity

AML1/ETO is the chimeric protein resulting from the t(8;21) in acute myeloid leukemia. The Nervy homology 2 (NHR2) domain in ETO mediates oligomerization and AML1/ETO's interactions with ETO, MTGR1, and MTG16, and with the corepressor molecules mSin3A and HDAC1 and HDAC3. We solved the NHR2 domain structure and found it to be an alpha-helical tetramer. We show that oligomerization contributes to AML1/ETO's inhibition of granulocyte differentiation, is essential for its ability to enhance the clonogenic potential of primary mouse bone marrow cells, and affects AML1/ETO's activity on several endogenous genes. Oligomerization is also required for AML1/ETO's interactions with ETO, MTGR1, and MTG16, but not with other corepressor molecules.

Point Mutations in the AML1/RUNX1 Gene Associated with Myelodysplastic Syndrome

Myelodysplastic syndrome (MDS) is a clonal disorder of hematopoietic stem cells characterized by ineffective and inadequate hematopoiesis. Because MDS is a heterogeneous disorder, specific gene abnormalities implicated in the pathogenesis of MDS have been difficult to identify. Cytogenetic abnormalities are seen in half of the MDS patients and generally consist of partial or complete chromosome deletion or addition, whereas balanced translocations are rare. Although point mutations of critical genes had been demonstrated to contribute to the development of MDS, there was no strong correlation between these mutations and clinical features. Recently, we reported the high incidence of somatic mutations in the AML1/RUNX1 gene (which is a critical regulator of definitive hematopoiesis and the most frequent target for translocation of acute myeloid leukemia [AML]) in MDS, especially refractory anemia with excess blasts (RAEB), RAEB in transformation (RAEBt), and AML following MDS (defined here as MDS/AML). The MDS/AML patients with AML1 mutations had a significantly worse prognosis than those without AML1 mutations. Most AML1 mutants lose trans-activation potential, which leads to a loss of AML1 function. These data indicate that AML1 point mutation is one of the major causes of MDS/AML, and "MDS/AML with AML1 mutation" represents a distinct clinicopathologic-genetic entity.

Subnuclear targeting of Runx1 Is required for synergistic activation of the myeloid specific M-CSF receptor promoter by PU.1

Many types of acute myelogenous leukemia involve chromosomal translocations that target the C-terminus of Runx1/AML1 transcription factor, a master regulator of hematopoiesis. The C-terminus of Runx1/AML1 that includes the nuclear matrix targeting signal (NMTS) is essential for embryonic development, hematopoiesis, and target gene regulation. During the onset and normal progression of hematopoiesis, several lineage-specific factors such as C/EBPalpha and PU.1 interact with Runx1 to regulate transcription combinatorially. Here we addressed the functional interplay between subnuclear targeting of Runx1 and gene activation during hematopoiesis. Point mutations were generated in the NMTS of the human Runx1 protein and tested for their effect on transcriptional cooperativity with C/EBPalpha and PU.1 at myeloid-specific promoters. We characterized five mutants that do not alter nuclear import, DNA binding or C/EBPalpha-dependent synergistic activation of the target gene promoters. However a critical tyrosine in the NMTS is required for subnuclear targeting and activation of the granulocyte-macrophage colony stimulating factor (GM-CSF) promoter. Furthermore, this point mutation is defective for transcriptional synergism with PU.1 on the macrophage colony stimulating factor (MCSF) receptor c-FMS promoter. Our results indicate that the NMTS region of Runx1 is required for functional interactions with PU.1. Taken together, our findings establish that subnuclear targeting of Runx1 is a critical component of myeloid-specific transcriptional control.

[Analysis and identification of transcriptional repression domain of ETO]

OBJECTIVE

To further verify the transcriptional repression domains in ETO and their relationship with histone deacetylase (HDAC).

METHODS

Either of the ETO two zinc fingers was mutated respectively by site-directing mutagenesis. The truncation fragments of ETO were amplified by polymerase chain reaction (PCR) and cloned into eukaryotic expression plasmid pFA-CMV. By the means of DNA transfection and analysis of the transcription derived from the promoter of reporter gene, the transcriptional regulation domains of ETO was determined.

RESULTS

The expression plasmids carrying truncated ETO and ETO with point mutation at either zinc finger were successfully constructed. Two repression domains were found within ETO, which were located at two zinc finger motifs and 275 - 487 amino acid residues, respectively.

CONCLUSION

The transcription repression by ETO was mediated by two separated domains and closely associated with HDAC, which may be used as therapeutic target for acute myeloid leukemia M(2b).