Multiple regions of ETO cooperate in transcriptional repression

Defined Human Leukemic CD34+ Liquid Cultures to Study HDAC/Transcriptional Repressor Complexes

Defined human primary cell model systems with growth dependence on oncogenes are highly requested to investigate tumor pathogenesis and to validate pharmacological inhibitors that specifically target oncoproteins and their executing protein complex partners. In acute myeloid leukemia (AML), transcription factors such as RUNX1 and MLL1, which are important for normal blood cell development, frequently harbor mutations including chromosomal translocations with other coding genes, resulting in tumor-promoting gain-of-function fusion proteins. These oncoproteins completely modify transcriptional programs, thereby inducing malignant cell phenotypes. A common theme of the chimeric gene products is their physical interaction with a variety of chromatin-modifying effector molecules, including histone acetyltransferases (HATs) and histone deacetylases (HDACs). These aberrant multiprotein machineries disturb gene expression and promote malignant cell growth. In this chapter, we briefly summarize the current understanding regarding AML-associated oncogene-driven human CD34+ blood progenitor cell expansion in ex vivo liquid cultures. We provide a step-by-step protocol to establish oncogene-induced human CD34+ blood progenitor cell cultures suitable to analyze the impact of transcriptional repressor/HDAC activity in these human AML cell models.

AML1/ETO and its function as a regulator of gene transcription via epigenetic mechanisms

The chromosomal translocation t(8;21) and the resulting oncofusion gene AML1/ETO have long served as a prototypical genetic lesion to model and understand leukemogenesis. In this review, we describe the wide-ranging role of AML1/ETO in AML leukemogenesis, with a particular focus on the aberrant epigenetic regulation of gene transcription driven by this AML-defining mutation. We begin by analyzing how structural changes secondary to distinct genomic breakpoints and splice changes, as well as posttranscriptional modifications, influence AML1/ETO protein function. Next, we characterize how AML1/ETO recruits chromatin-modifying enzymes to target genes and how the oncofusion protein alters chromatin marks, transcription factor binding, and gene expression. We explore the specific impact of these global changes in the epigenetic network facilitated by the AML1/ETO oncofusion on cellular processes and leukemic growth. Furthermore, we define the genetic landscape of AML1/ETO-positive AML, presenting the current literature concerning the incidence of cooperating mutations in genes such as KIT, FLT3, and NRAS. Finally, we outline how alterations in transcriptional regulation patterns create potential vulnerabilities that may be exploited by epigenetically active agents and other therapeutics.

Novel t(1;8)(p31.3;q21.3) NFIA-RUNX1T1 Translocation in an Infant Erythroblastic Sarcoma

OBJECTIVES

Pure erythroid leukemia (PEL) is exceptionally rare in the pediatric setting. Four pediatric PEL cases with t(1;16)(p31;q24) NFIA-CBFA2T3 were reported previously. We present a case of an infant with PEL presenting with erythroblastic sarcoma and harboring a novel t(1;8)(p31.3;q21.3) NFIA-RUNX1T1 fusion detected by RNA sequencing and conventional karyotype.

METHODS

Bone marrow (BM) and abdominal mass biopsies from the patient were evaluated with extensive immunohistochemical, flow cytometric, cytogenetic, and molecular studies.

RESULTS

The patient was a female infant who presented between 2 and 5 months of age with cytopenias and an enlarging abdominal mass. Blasts in the BM and abdominal mass expressed CD71 and CD117 with focal expression of CD43, E-cadherin, epithelial membrane antigen, and hemoglobin A. They were negative for additional myeloid, lymphoid, and nonhematolymphoid markers. These findings were most consistent with PEL and erythroblastic sarcoma. RNA sequencing revealed the novel NFIA-RUNX1T1 fusion.

CONCLUSIONS

Along with the previously reported PELs with NFIA-CBFA2T3 fusions, we describe a subset of PELs that occur in children, that frequently display extramedullary disease, and that harbor rearrangements of NFIA with core binding factor genes. We hypothesize that, together, these cases represent a rare but distinct clinicopathologic group of pediatric PELs with recurrent genetic abnormality.

t(8;21) Acute Myeloid Leukemia as a Paradigm for the Understanding of Leukemogenesis at the Level of Gene Regulation and Chromatin Programming

Acute myeloid leukemia (AML) is a heterogenous disease with multiple sub-types which are defined by different somatic mutations that cause blood cell differentiation to go astray. Mutations occur in genes encoding members of the cellular machinery controlling transcription and chromatin structure, including transcription factors, chromatin modifiers, DNA-methyltransferases, but also signaling molecules that activate inducible transcription factors controlling gene expression and cell growth. Mutant cells in AML patients are unable to differentiate and adopt new identities that are shaped by the original driver mutation and by rewiring their gene regulatory networks into regulatory phenotypes with enhanced fitness. One of the best-studied AML-subtypes is the t(8;21) AML which carries a translocation fusing the DNA-binding domain of the hematopoietic master regulator RUNX1 to the ETO gene. The resulting oncoprotein, RUNX1/ETO has been studied for decades, both at the biochemical but also at the systems biology level. It functions as a dominant-negative version of RUNX1 and interferes with multiple cellular processes associated with myeloid differentiation, growth regulation and genome stability. In this review, we summarize our current knowledge of how this protein reprograms normal into malignant cells and how our current knowledge could be harnessed to treat the disease.

Modelling t(8;21) acute myeloid leukaemia - What have we learned?

Acute myeloid leukaemia (AML) is a heterogeneous haematopoietic malignancy caused by recurrent mutations in haematopoietic stem and progenitor cells that affect both the epigenetic regulatory machinery and signalling molecules. The t(8;21) or RUNX1‐RUNX1T1 translocation generates the RUNX1‐ETO chimeric transcription factor which primes haematopoietic stem cells for further oncogenic mutational events that in their sum cause overt disease. Significant progress has been made in generating both in vitro and in vivo model systems to recapitulate t(8;21) AML which are crucial for the understanding of the biology of the disease and the development of effective treatment. This review provides a comprehensive overview of the in vivo and in vitro model systems that were developed to gain insights into the molecular mechanisms of RUNX1‐ETO oncogenic activity and their contribution to the advancement of knowledge in the t(8;21) AML field. Such models include transgenic mice, patient‐derived xenografts, RUNX1‐ETO transduced human progenitor cells, cell lines and human embryonic stem cell model systems, making the t(8;21) as one of the well‐characterized sub‐type of AML at the molecular level.

RUNX1 mutations enhance self-renewal and block granulocytic differentiation in human in vitro models and primary AMLs

To unravel molecular mechanisms by which Runt-related transcription factor 1 (RUNX1) mutations contribute to leukemic transformation, we introduced the RUNX1-S291fs300X mutation in human CD34⁺ stem/progenitor cells and in human induced pluripotent stem cells (iPSCs). In both models, RUNX1mut overexpression strongly impaired myeloid commitment. Instead, self-renewal was enhanced, as shown, by increased long-term culture-initiating cell frequencies and enhanced colony-forming cell replating capacity. Long-term suspension cultures with RUNX1mut-transduced cord blood (CB) CD34⁺ cells continued for more than 100 days, during which the cells displayed an immature granulocyte-macrophage progenitor-like CD34⁺/CD123⁺/CD45RA⁺ phenotype. The CD34⁺/CD38^- hematopoietic stem cell (HSC) population most likely acted as cell of origin, as HSCs provided the best long-term proliferative potential on overexpression of RUNX1mut. CEBPA expression was reduced in RUNX1mut cells, and reexpression of CEBPA partly restored differentiation. RNA-seq analysis on CB/iPSC systems and on primary patient samples confirmed that RUNX1 mutations induce a myeloid differentiation block, and that a common set of RUNX1mut-upregulated target genes was strongly enriched for gene ontology terms associated with nucleosome assembly and chromatin structure. Interestingly, in comparison with AML1-ETO binding in acute myeloid leukemias (AMLs), we found significantly distinct genomic distribution and differential expression for RUNX1mut of genes such as TCF4, MEIS1, and HMGA2 that may potentially contribute to the underlying difference in clinical outcomes between RUNX1mut and AML1-ETO patients. In conclusion, RUNX1mut appears to induce a specific transcriptional program that contributes to leukemic transformation.

Emerging Roles of MTG16 in Cell-Fate Control of Hematopoietic Stem Cells and Cancer

MTG16 (myeloid translocation gene on chromosome 16) and its related proteins, MTG8 and MTGR1, define a small family of transcriptional corepressors. These corepressors share highly conserved domain structures yet have distinct biological functions and tissue specificity. In vivo studies have shown that, of the three MTG corepressors, MTG16 is uniquely important for the regulation of hematopoietic stem/progenitor cell (HSPC) proliferation and differentiation. Apart from this physiological function, MTG16 is also involved in carcinomas and leukemias, acting as the genetic target of loss of heterozygosity (LOH) aberrations in breast cancer and recurrent translocations in leukemia. The frequent involvement of MTG16 in these disease etiologies implies an important developmental role for this transcriptional corepressor. Furthermore, mounting evidence suggests that MTG16 indirectly alters the disease course of several leukemias via its regulatory interactions with a variety of pathologic fusion proteins. For example, a recent study has shown that MTG16 can repress not only wild-type E2A-mediated transcription, but also leukemia fusion protein E2A-Pbx1-mediated transcription, suggesting that MTG16 may serve as a potential therapeutic target in acute lymphoblastic leukemia expressing the E2A-Pbx1 fusion protein. Given that leukemia stem cells share similar regulatory pathways with normal HSPCs, studies to further understand how MTG16 regulates cell proliferation and differentiation could lead to novel therapeutic approaches for leukemia treatment.

RUNX1-ETO Leukemia

AML1-ETO leukemia is the most common cytogenetic subtype of acute myeloid leukemia, defined by the presence of t(8;21). Remarkable progress has been achieved in understanding the molecular pathogenesis of AML1-ETO leukemia. Proteomic surveies have shown that AML-ETO forms a stable complex with several transcription factors, including E proteins. Genome-wide transcriptome and ChIP-seq analyses have revealed the genes directly regulated by AML1-ETO, such as CEBPA. Several lines of evidence suggest that AML1-ETO suppresses endogenous DNA repair in cells to promote mutagenesis, which facilitates acquisition of cooperating secondary events. Furthermore, it has become increasingly apparent that a delicate balance of AML1-ETO and native AML1 is important to sustain the malignant cell phenotype. Translation of these findings into the clinical setting is just beginning.

Role of RUNX1 in hematological malignancies

RUNX1 is a member of the core-binding factor family of transcription factors and is indispensable for the establishment of definitive hematopoiesis in vertebrates. RUNX1 is one of the most frequently mutated genes in a variety of hematological malignancies. Germ line mutations in RUNX1 cause familial platelet disorder with associated myeloid malignancies. Somatic mutations and chromosomal rearrangements involving RUNX1 are frequently observed in myelodysplastic syndrome and leukemias of myeloid and lymphoid lineages, that is, acute myeloid leukemia, acute lymphoblastic leukemia, and chronic myelomonocytic leukemia. More recent studies suggest that the wild-type RUNX1 is required for growth and survival of certain types of leukemia cells. The purpose of this review is to discuss the current status of our understanding about the role of RUNX1 in hematological malignancies.

New insights into transcriptional and leukemogenic mechanisms of AML1-ETO and E2A fusion proteins

BACKGROUND

Nearly 15% of acute myeloid leukemia (AML) cases are caused by aberrant expression of AML1-ETO, a fusion protein generated by the t(8;21) chromosomal translocation. Since its discovery, AML1-ETO has served as a prototype to understand how leukemia fusion proteins deregulate transcription to promote leukemogenesis. Another leukemia fusion protein, E2A-Pbx1, generated by the t(1;19) translocation, is involved in acute lymphoblastic leukemias (ALLs). While AML1-ETO and E2A-Pbx1 are structurally unrelated fusion proteins, we have recently shown that a common axis, the ETO/E-protein interaction, is involved in the regulation of both fusion proteins, underscoring the importance of studying protein-protein interactions in elucidating the mechanisms of leukemia fusion proteins.

OBJECTIVE

In this review, we aim to summarize these new developments while also providing a historic overview of the related early studies.

METHODS

A total of 218 publications were reviewed in this article, a majority of which were published after 2004.We also downloaded 3D structures of AML1-ETO domains from Protein Data Bank and provided a systematic summary of their structures.

RESULTS

By reviewing the literature, we summarized early and recent findings on AML1-ETO, including its protein-protein interactions, transcriptional and leukemogenic mechanisms, as well as the recently reported involvement of ETO family corepressors in regulating the function of E2A-Pbx1.

CONCLUSION

While the recent development in genomic and structural studies has clearly demonstrated that the fusion proteins function by directly regulating transcription, a further understanding of the underlying mechanisms, including crosstalk with other transcription factors and cofactors, and the protein-protein interactions in the context of native proteins, may be necessary for the development of highly targeted drugs for leukemia therapy.

Myeloid translocation gene-16 co-repressor promotes degradation of hypoxia-inducible factor 1

The myeloid translocation gene 16 (MTG16) co-repressor down regulates expression of multiple glycolytic genes, which are targets of the hypoxia-inducible factor 1 (HIF1) heterodimer transcription factor that is composed of oxygen-regulated labile HIF1α and stable HIF1β subunits. For this reason, we investigated whether MTG16 might regulate HIF1 negatively contributing to inhibition of glycolysis and stimulation of mitochondrial respiration. A doxycycline Tet-On system was used to control levels of MTG16 in B-lymphoblastic Raji cells. Results from co-association studies revealed MTG16 to interact with HIF1α. The co-association required intact N-terminal MTG16 residues including Nervy Homology Region 1 (NHR1). Furthermore, electrophoretic mobility shift assays demonstrated an association of MTG16 with hypoxia response elements (HREs) in PFKFB3, PFKFB4 and PDK1 promoters in-vitro. Results from chromatin immunoprecipitation assays revealed co-occupancy of these and other glycolytic gene promoters by HIF1α, HIF1β and MTG16 in agreement with possible involvement of these proteins in regulation of glycolytic target genes. In addition, MTG16 interacted with prolyl hydroxylase D2 and promoted ubiquitination and proteasomal degradation of HIF1α. Our findings broaden the area of MTG co-repressor functions and reveal MTG16 to be part of a protein complex that controls the levels of HIF1α.

RUNX1/ETO blocks selectin-mediated adhesion via epigenetic silencing of PSGL-1

RUNX1/ETO (RE), the t(8;21)-derived leukemic transcription factor associated with acute myeloid leukemia (AML) development, deregulates genes involved in differentiation, self-renewal and proliferation. In addition, these cells show differences in cellular adhesion behavior whose molecular basis is not well understood. Here, we demonstrate that RE epigenetically silences the gene encoding P-Selectin Glycoprotein Ligand-1 (PSGL-1) and downregulates PSGL-1 expression in human CD34+ and murine lin− hematopoietic progenitor cells. Levels of PSGL-1 inversely and dose-dependently correlate with RE oncogene levels. However, a DNA-binding defective mutant fails to downregulate PSGL-1. We show by ChIP experiments that the PSGL-1 promoter is a direct target of RE and binding is accompanied by high levels of the repressive chromatin mark histone H3K27me3. In t(8;21)+ Kasumi-1 cells, PSGL-1 expression is completely restored at both the mRNA and cell surface protein levels following RE downregulation with short hairpin RNA (shRNA) or RE inhibition with tetramerization-blocking peptides, and at the promoter H3K27me3 is replaced by the activating chromatin mark H3K9ac as well as by RNA polymerase II. Upregulation of PSGL-1 restores the binding of cells to P- and E-selectin and re-establishes myeloid-specific cellular adhesion while it fails to bind to lymphocyte-specific L-selectin. Overall, our data suggest that the RE oncoprotein epigenetically represses PSGL-1 via binding to its promoter region and thus affects the adhesive behavior of t(8;21)+ AML cells.

Perspectives of differentiation therapies of acute myeloid leukemia: the search for the molecular basis of patients' variable responses to 1,25-dihydroxyvitamin d and vitamin d analogs

The concept of differentiation therapy of cancer is ~40 years old. Despite many encouraging results obtained in laboratories, both in vitro and in vivo studies, the only really successful clinical application of differentiation therapy was all-trans-retinoic acid (ATRA)-based therapy of acute promyelocytic leukemia (APL). ATRA, which induces granulocytic differentiation of APL leukemic blasts, has revolutionized the therapy of this disease by converting it from a fatal to a curable one. However, ATRA does not work for other acute myeloid leukemias (AMLs). Since 1,25-dihydroxyvitamin D₃ (1,25D) is capable of inducing monocytic differentiation of leukemic cells, the idea of treating other AMLs with vitamin D analogs (VDAs) was widely accepted. Also, some types of solid cancers responded to in vitro applied VDAs, and hence it was postulated that VDAs can be used in many clinical applications. However, early clinical trials in which cancer patients were treated either with 1,25D or with VDAs, did not lead to conclusive results. In order to search for a molecular basis of such unpredictable responses of AML patients toward VDAs, we performed ex vivo experiments using patient’s blast cells. Experiments were also performed using 1,25D-responsive and 1,25D-non-responsive cell lines, to study their mechanisms of resistance toward 1,25D-induced differentiation. We found that one of the possible reasons might be due to a very low expression level of vitamin D receptor (VDR) mRNA in resistant cells, which can be increased by exposing the cells to ATRA. Our considerations concerning the molecular mechanism behind the low VDR expression and its regulation by ATRA are reported in this paper.

The transcriptional co-repressor myeloid translocation gene 16 inhibits glycolysis and stimulates mitochondrial respiration

The myeloid translocation gene 16 product MTG16 is found in multiple transcription factor-containing complexes as a regulator of gene expression implicated in development and tumorigenesis. A stable Tet-On system for doxycycline-dependent expression of MTG16 was established in B-lymphoblastoid Raji cells to unravel its molecular functions in transformed cells. A noticeable finding was that expression of certain genes involved in tumor cell metabolism including 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase 3 and 4 (PFKFB3 and PFKFB4), and pyruvate dehydrogenase kinase isoenzyme 1 (PDK1) was rapidly diminished when MTG16 was expressed. Furthermore, hypoxia-stimulated production of PFKFB3, PFKFB4 and PDK1 was inhibited by MTG16 expression. The genes in question encode key regulators of glycolysis and its coupling to mitochondrial metabolism and are commonly found to be overexpressed in transformed cells. The MTG16 Nervy Homology Region 2 (NHR2) oligomerization domain and the NHR3 protein-protein interaction domain were required intact for inhibition of PFKFB3, PFKFB4 and PDK1 expression to occur. Expression of MTG16 reduced glycolytic metabolism while mitochondrial respiration and formation of reactive oxygen species increased. The metabolic changes were paralleled by increased phosphorylation of mitogen-activated protein kinases, reduced levels of amino acids and inhibition of proliferation with a decreased fraction of cells in S-phase. Overall, our findings show that MTG16 can serve as a brake on glycolysis, a stimulator of mitochondrial respiration and an inhibitor of cell proliferation. Hence, elevation of MTG16 might have anti-tumor effect.

Interference with RUNX1/ETO leukemogenic function by cell-penetrating peptides targeting the NHR2 oligomerization domain

The leukemia-associated fusion protein RUNX1/ETO is generated by the chromosomal translocation t(8;21) which appears in about 12% of all de novo acute myeloid leukemias (AMLs). Essential for the oncogenic potential of RUNX1/ETO is the oligomerization of the chimeric fusion protein through the nervy homology region 2 (NHR2) within ETO. In previous studies, we have shown that the intracellular expression of peptides containing the NHR2 domain inhibits RUNX1/ETO oligomerization, thereby preventing cell proliferation and inducing differentiation of RUNX1/ETO transformed cells. Here, we show that introduction of a recombinant TAT-NHR2 fusion polypeptide into the RUNX1/ETO growth-dependent myeloid cell line Kasumi-1 results in decreased cell proliferation and increased numbers of apoptotic cells. This effect was highly specific and mediated by binding the TAT-NHR2 peptide to ETO sequences, as TAT-polypeptides containing the oligomerization domain of BCR did not affect cell proliferation or apoptosis in Kasumi-1 cells. Thus, the selective interference with NHR2-mediated oligomerization by peptides represents a challenging but promising strategy for the inhibition of the leukemogenic potential of RUNX1/ETO in t(8;21)-positive leukemia.

PRMT1 interacts with AML1-ETO to promote its transcriptional activation and progenitor cell proliferative potential

Fusion protein AML1-ETO, resulting from t(8;21) translocation, is highly related to leukemia development. It has been reported that full-length AML1-ETO blocks AML1 function and requires additional mutagenic events to promote leukemia. We have previously shown that the expression of AE9a, a splice isoform of AML1-ETO, can rapidly cause leukemia in mice. To understand how AML1-ETO is involved in leukemia development, we took advantage of our AE9a leukemia model and sought to identify its interacting proteins from primary leukemic cells. Here, we report the discovery of a novel AE9a binding partner PRMT1 (protein arginine methyltransferase 1). PRMT1 not only interacts with but also weakly methylates arginine 142 of AE9a. Knockdown of PRMT1 affects expression of a specific group of AE9a-activated genes. We also show that AE9a recruits PRMT1 to promoters of AE9a-activated genes, resulting in enrichment of H4 arginine 3 methylation, H3 Lys9/14 acetylation, and transcription activation. More importantly, knockdown of PRMT1 suppresses the self-renewal capability of AE9a, suggesting a potential role of PRMT1 in regulating leukemia development.

The leukemia associated nuclear corepressor ETO homologue genes MTG16 and MTGR1 are regulated differently in hematopoietic cells

Background

MTG16, MTGR1 and ETO are nuclear transcriptional corepressors of the human ETO protein family. MTG16 is implicated in hematopoietic development and in controlling erythropoiesis/megakaryopoiesis. Furthermore, ETO homologue genes are 3'participants in leukemia fusions generated by chromosomal translocations responsible of hematopoietic dysregulation. We tried to identify structural and functional promoter elements of MTG16 and MTGR1 genes in order to find associations between their regulation and hematopoiesis.

Results

5' deletion examinations and luciferase reporter gene studies indicated that a 492 bp sequence upstream of the transcription start site is essential for transcriptional activity by the MTG16 promoter. The TATA- and CCAAT-less promoter with a GC box close to the start site showed strong reporter activity when examined in erythroid/megakaryocytic cells. Mutation of an evolutionary conserved GATA -301 consensus binding site repressed promoter function. Furthermore, results from in vitro antibody-enhanced electrophoretic mobility shift assay and in vivo chromatin immunoprecipitation indicated binding of GATA-1 to the GATA -301 site. A role of GATA-1 was also supported by transfection of small interfering RNA, which diminished MTG16 expression. Furthermore, expression of the transcription factor HERP2, which represses GATA-1, produced strong inhibition of the MTG16 promoter reporter consistent with a role of GATA-1 in transcriptional activation. The TATA-less and CCAAT-less MTGR1 promoter retained most of the transcriptional activity within a -308 to -207 bp region with a GC-box-rich sequence containing multiple SP1 binding sites reminiscent of a housekeeping gene with constitutive expression. However, mutations of individual SP1 binding sites did not repress promoter function; multiple active SP1 binding sites may be required to safeguard constitutive MTGR1 transcriptional activity. The observed repression of MTG16/MTGR1 promoters by the leukemia associated AML1-ETO fusion gene may have a role in hematopoietic dysfunction of leukemia.

Conclusions

An evolutionary conserved GATA binding site is critical in transcriptional regulation of the MTG16 promoter. In contrast, the MTGR1 gene depends on a GC-box-rich sequence for transcriptional regulation and possible ubiquitous expression. Our results demonstrate that the ETO homologue promoters are regulated differently consistent with hematopoietic cell-type- specific expression and function.

Myeloid translocation gene 16 is required for maintenance of haematopoietic stem cell quiescence

The t(8;21) and t(16;21) that are associated with acute myeloid leukaemia disrupt two closely related genes termed Myeloid Translocation Genes 8 (MTG8) and 16 (MTG16), respectively. Many of the transcription factors that recruit Mtg16 regulate haematopoietic stem and progenitor cell functions and are required to maintain stem cell self-renewal potential. Accordingly, we found that Mtg16-null bone marrow (BM) failed in BM transplant assays. Moreover, when removed from the animal, Mtg16-deficient stem cells continued to show defects in stem cell self-renewal assays, suggesting a requirement for Mtg16 in this process. Gene expression analysis indicated that Mtg16 was required to suppress the expression of several key cell-cycle regulators including E2F2, and chromatin immunoprecipitation assays detected Mtg16 near an E2A binding site within the first intron of E2F2. BrdU incorporation assays indicated that in the absence of Mtg16 more long-term stem cells were in the S phase, even after competitive BM transplantation where normal stem and progenitor cells are present, suggesting that Mtg16 plays a role in the maintenance of stem cell quiescence.

Dimer-tetramer transition controls RUNX1/ETO leukemogenic activity

RUNX1/ETO, the fusion protein resulting from the chromosomal translocation t(8;21), is one of the most frequent translocation products in acute myeloid leukemia. Several in vitro and in vivo studies have shown that the homo-tetramerization domain of ETO, the nervy homology region 2 (NHR2), is essential for RUNX1/ETO oncogenic activity. We analyzed the energetic contribution of individual amino acids within the NHR2 to RUNX1/ETO dimer-tetramer transition and found a clustered area of 5 distinct amino acids with strong contribution to the stability of tetramers. Substitution of these amino acids abolishes tetramer formation without affecting dimer formation. Similar to RUNX1/ETO monomers, dimers failed to bind efficiently to DNA and to alter expression of RUNX1-dependent genes. RUNX1/ETO dimers do not block myeloid differentiation, are unable to enhance the self-renewal capacity of hematopoietic progenitors, and fail to induce leukemia in a murine transplantation model. Our data reveal the existence of an essential structural motif (hot spot) at the NHR2 dimer-tetramer interface, suitable for a molecular intervention in t(8;21) leukemias.

The leukemia associated ETO nuclear repressor gene is regulated by the GATA-1 transcription factor in erythroid/megakaryocytic cells

BACKGROUND

The Eight-Twenty-One (ETO) nuclear co-repressor gene belongs to the ETO homologue family also containing Myeloid Translocation Gene on chromosome 16 (MTG16) and myeloid translocation Gene-Related protein 1 (MTGR1). By chromosomal translocations ETO and MTG16 become parts of fusion proteins characteristic of morphological variants of acute myeloid leukemia. Normal functions of ETO homologues have as yet not been examined. The goal of this work was to identify structural and functional promoter elements upstream of the coding sequence of the ETO gene in order to explore lineage-specific hematopoietic expression and get hints to function.

RESULTS

A putative proximal ETO promoter was identified within 411 bp upstream of the transcription start site. Strong ETO promoter activity was specifically observed upon transfection of a promoter reporter construct into erythroid/megakaryocytic cells, which have endogeneous ETO gene activity. An evolutionary conserved region of 228 bp revealed potential cis-elements involved in transcription of ETO. Disruption of the evolutionary conserved GATA -636 consensus binding site repressed transactivation and disruption of the ETS1 -705 consensus binding site enhanced activity of the ETO promoter. The promoter was stimulated by overexpression of GATA-1 into erythroid/megakaryocytic cells. Electrophoretic mobility shift assay with erythroid/megakaryocytic cells showed specific binding of GATA-1 to the GATA -636 site. Furthermore, results from chromatin immunoprecipitation showed GATA-1 binding in vivo to the conserved region of the ETO promoter containing the -636 site. The results suggest that the GATA -636 site may have a role in activation of the ETO gene activity in cells with erythroid/megakaryocytic potential. Leukemia associated AML1-ETO strongly suppressed an ETO promoter reporter in erythroid/megakaryocytic cells.

CONCLUSIONS

We demonstrate that the GATA-1 transcription factor binds and transactivates the ETO proximal promoter in an erythroid/megakaryocytic-specific manner. Thus, trans-acting factors that are essential in erythroid/megakaryocytic differentiation govern ETO expression.

Multivalent binding of the ETO corepressor to E proteins facilitates dual repression controls targeting chromatin and the basal transcription machinery

E proteins are a family of helix-loop-helix transcription factors that play important roles in cell differentiation and homeostasis. They contain at least two activation domains, AD1 and AD2. ETO family proteins and the leukemogenic AML1-ETO fusion protein are corepressors of E proteins. It is thought that ETO represses E-protein activity by interacting with AD1, which competes away p300/CBP histone acetyltransferases. Here we report that E proteins contain another conserved ETO-interacting region, termed DES, and that differential associations with AD1 and DES allow ETO to repress transcription through both chromatin-dependent and chromatin-independent mechanisms. At the chromatin level, AD1 and AD2 cooperatively recruit p300. ETO interacts with AD1 to abolish p300 recruitment and to allow HDAC-dependent silencing. At the post-chromatin-remodeling level, binding to DES enables ETO to directly inhibit activation of the basal transcription machinery. This novel repression mechanism is conserved in ETO family proteins and in the AML1-ETO fusion protein. In addition, the repression capacity exerted by each mechanism is differentially modulated by cross talk among various ETO domains and the AML1 domain of AML1-ETO. In particular, the oligomerization domain of ETO plays a major role in targeting ETO to the DES region and independently potentiates the TAFH domain-mediated AD1 interaction. The ability to exert repression at different levels not only may allow these corepressors to impose robust inhibition of signal-independent transcription but may also allow a rapid response to signals. In addition, our newly defined domain interactions and their interplays have important implications in effectively targeting both E-protein fusion proteins and AML1-ETO found in cancers.

RUNX1/AML1 DNA-binding domain and ETO/MTG8 NHR2-dimerization domain are critical to AML1-ETO9a leukemogenesis

The 8;21 translocation, which involves the gene encoding the RUNX family DNA-binding transcription factor AML1 (RUNX1) on chromosome 21 and the ETO (MTG8) gene on chromosome 8, generates AML1-ETO fusion proteins. Previous analyses have demonstrated that full-length AML1-ETO blocks AML1 function and requires additional mutagenic events to promote leukemia. More recently, we have identified an alternatively spliced form of AML1-ETO, AML1-ETO9a, from t(8;21) acute myeloid leukemia (AML) patient samples. AML1-ETO9a lacks the C-terminal NHR3 and NHR4 domains of AML1-ETO and is highly leukemogenic in the mouse model. Here, we report that the AML1 DNA-binding domain and the ETO NHR2-dimerization domain, but not the ETO NHR1 domain, are critical for the induction of AML by AML1-ETO9a. A region between NHR1 and NHR2 affects latency of leukemogenesis. These results provide valuable insight into further analysis of the molecular mechanism of t(8;21) in leukemogenesis.

Novel RNA-binding properties of the MTG chromatin regulatory proteins

Background

The myeloid translocation gene (MTG) proteins are non-DNA-binding transcriptional regulators capable of interacting with chromatin modifying proteins. As a consequence of leukemia-associated chromosomal translocations, two of the MTG proteins, MTG8 and MTG16, are fused to the DNA-binding domain of AML1, a transcriptional activator crucial for hematopoiesis. The AML1-MTG fusion proteins, as the wild type MTGs, display four conserved homology regions (NHR1-4) related to the Drosophila nervy protein. Structural protein analyses led us to test the hypothesis that specific MTG domains may mediate RNA binding.

Results

By using an RNA-binding assay based on synthetic RNA homopolymers and a panel of MTG deletion mutants, here we show that all the MTG proteins can bind RNA. The RNA-binding properties can be traced to two regions: the Zinc finger domains in the NHR4, which mediate Zinc-dependent RNA binding, and a novel short basic region (SBR) upstream of the NHR2, which mediates Zinc-independent RNA binding. The two AML1-MTG fusion proteins, retaining both the Zinc fingers domains and the SBR, also display RNA-binding properties.

Conclusion

Evidence has been accumulating that RNA plays a role in transcriptional control. Both wild type MTGs and chimeric AML1-MTG proteins display in vitro RNA-binding properties, thus opening new perspectives on the possible involvement of an RNA component in MTG-mediated chromatin regulation.

Deletion of Mtg16, a target of t(16;21), alters hematopoietic progenitor cell proliferation and lineage allocation

While a number of DNA binding transcription factors have been identified that control hematopoietic cell fate decisions, only a limited number of transcriptional corepressors (e.g., the retinoblastoma protein [pRB] and the nuclear hormone corepressor [N-CoR]) have been linked to these functions. Here, we show that the transcriptional corepressor Mtg16 (myeloid translocation gene on chromosome 16), which is targeted by t(16;21) in acute myeloid leukemia, is required for hematopoietic progenitor cell fate decisions and for early progenitor cell proliferation. Inactivation of Mtg16 skewed early myeloid progenitor cells toward the granulocytic/macrophage lineage while reducing the numbers of megakaryocyte-erythroid progenitor cells. In addition, inactivation of Mtg16 impaired the rapid expansion of short-term stem cells, multipotent progenitor cells, and megakaryocyte-erythroid progenitor cells that is required under hematopoietic stress/emergency. This impairment appears to be a failure to proliferate rather than an induction of cell death, as expression of c-Myc, but not Bcl2, complemented the Mtg16(-/-) defect.

The human SIN3B corepressor forms a nucleolar complex with leukemia-associated ETO homologues

Background

SIN3 (SWI-Independent) is part of a transcriptional deacetylase complex, which generally mediates the formation of repressive chromatin. The purpose of this work was to study possible interactions between corepressors human SIN3B (hSIN3B) and the ETO homologues – ETO (eight twenty-one), MTG16 (myeloid-transforming gene 16) and MTGR1 (MTG-related protein 1). In addition, the subnuclear localization of the hSIN3B and the ETO homologues was also examined.

Results

A ubiquitous expression of hSIN3B was observed in adult and fetal tissues. Results with both ectopically expressed proteins in COS-7 cells and endogeneous proteins in the K562 human erytholeukemia cell line demonstrated interactions between hSIN3B and ETO or MTG16 but not MTGR1. Furthermore, nuclear extract of primary placental cells showed complexes between hSIN3B and ETO. The interaction between hSIN3B and ETO required an intact amino-terminus of ETO and the NHR2 domain. A nucleolar localization of hSIN3B and all the ETO homologues was demonstrated upon overexpression in COS-7 cells, and confirmed for the endogeneously expressed proteins in K562 cells. However, hSIN3B did not colocalize or interact with the leukemia-associated AML1 -ETO.

Conclusion

Our data from protein-protein interactions and immunolocalization experiments support that hSIN3B is a potential member of a corepressor complex involving selective ETO homologues.

Mechanism for ubiquitylation of the leukemia fusion proteins AML1-ETO and PML-RARalpha

The chromosomal translocation products AML1-ETO and PML-RARalpha contribute to the pathogenesis of leukemias. Here, we demonstrate that both AML1-ETO and PML-RARalpha are degraded by the ubiquitin-proteasome system and that their turnover critically depends on the E2-conjugase UbcH8 and the E3-ligase SIAH-1. Contrary to its role in HDAC2 degradation, the E3-ligase RLIM does not target AML1-ETO and PML-RARalpha for ubiquitin-dependent elimination. RLIM rather is a substrate of SIAH-1, which indicates that these E3-ligases operate in a hierarchical order. Remarkably, proteasomal degradation of leukemia fusion proteins, in addition to the block of histone deacetylase (HDAC) enzymatic activity is a consequence of HDAC-inhibitor treatment. The former requires the induction of UbcH8 expression and each of these processes might be beneficial for leukemia treatment. Our observations shed light on the mechanism determining the interplay between E2-conjugases, E3-ligases, and their substrates and suggest a strategy for utilizing the ubiquitylation machinery in a therapeutic setting.

Targeting the Oligomerization Domain of ETO Interferes with RUNX1/ETO Oncogenic Activity in t(8;21)-Positive Leukemic Cells

About 12% of all de novo acute myeloid leukemias are characterized by the translocation t(8;21), which generates the oncogenic fusion protein RUNX1/ETO. RUNX1/ETO has a modular structure and contains several docking sites for heterologous proteins, including transcriptional co-repressors like N-CoR, SMART, and mSIN3A. RUNX1/ETO is found in high molecular weight complexes, which are crucial for the block in myeloid differentiation observed in RUNX1/ETO-transformed cells. Essential for high molecular weight complex formation is the nervy homology region 2 (NHR2) within ETO, which serves as interacting surface for oligomerization as well as association with members of the ETO protein family. Here, we show that the expression of a fusion peptide consisting of 128 amino acids (NC128), including the entire NHR2 domain of ETO, disrupts the stability of the RUNX1/ETO high molecular weight complexes, restores transcription of RUNX1/ETO target genes, and reverts the differentiation block induced by RUNX1/ETO in myeloid cells. In the presence of NC128, RUNX1/ETO-transformed cells lose their progenitor cell characteristics, are arrested in cell cycle progression, and undergo cell death. Our results indicate that selective interference with the oligomerization domain of ETO could provide a promising strategy to inhibit the oncogenic properties of the leukemia-associated fusion protein RUNX1/ETO.

A leukemia fusion protein attenuates the spindle checkpoint and promotes aneuploidy

The 8;21 chromosomal translocation occurs in 15% to 40% of patients with the FAB M2 subtype of acute myeloid leukemia (AML). This chromosomal abnormality fuses part of the AML1/RUNX1 gene to the ETO/MTG8 gene and generates the AML1-ETO protein. We previously identified a C-terminal truncated AML1-ETO protein (AEtr) in a mouse leukemia model. AEtr is almost identical to the AML1-ETO exon 9a isoform expressed in leukemia patients. Here, we describe a novel function of AEtr in the development of aneuploidy through spindle checkpoint attenuation. AEtr cells had a reduced mitotic index following nocodazole treatment, suggesting a failure in a subset of cells to arrest in mitosis with a functional spindle checkpoint. Additionally, primary leukemia cells and cell lines expressing AEtr were aneuploid. Moreover, AEtr cells had reduced levels of several spindle checkpoint proteins including BubR1 and securin following treatment with the spindle poison nocodazole. These results suggest that inactivation of the spindle checkpoint may contribute to the development of aneuploidy described in t(8;21) leukemia patients.

Oncogenic pathways of AML1-ETO in acute myeloid leukemia: multifaceted manipulation of marrow maturation

The leukemic fusion protein AML1-ETO occurs frequently in human acute myeloid leukemia (AML) and has received much attention over the past decade. An initial model for its pathogenetic effects emphasized the conversion of a hematopoietic transcriptional activator, RUNX1 (or AML1), into a leukemogenic repressor which blocked myeloid differentiation at the level of target gene regulation. This view has been absorbed into a larger picture of AML1-ETO pathogenesis, encompassing dysregulation of hematopoietic stem cell homeostasis at several mechanistic levels. Recent reports have highlighted a multifaceted capacity of AML1-ETO directly to inhibit key hematopoietic transcription factors that function as tumor suppressors at several nodal points during hematopoietic differentiation. A new model is presented in which AML1-ETO coordinates expansion of the stem cell compartment with diminished lineage commitment and with genome instability.

Histone deacetylase inhibitors induce the degradation of the t(8;21) fusion oncoprotein

The t(8;21) chromosomal translocation that generates the fusion oncoprotein RUNX1-ETO predominates in leukemia patients of the French-American-British (FAB) class M2 subtype. The oncoprotein has the capacity to promote expansion of hematopoietic stem/progenitor cells and induces leukemia in association with other genetic alterations. Here, we show that RUNX1-ETO undergoes degradation in response to treatment with histone deacetylase inhibitors, one of which, depsipeptide (DEP), is currently undergoing phase II clinical testing in a variety of malignancies. These compounds induce turnover of RUNX1-ETO without affecting the stability of RUNX1-ETO partner proteins. In addition, RUNX1-ETO physically interacts with heat shock protein 90 (HSP90). DEP treatment interrupts the association of RUNX1-ETO with HSP90 and induces proteasomal degradation of RUNX1-ETO. DEP and the HSP90 antagonist 17-allylamino-geldanamycin (17-AAG) both triggered RUNX1-ETO degradation, but without any additive or cooperative effects. These findings may stimulate the development of more rational and effective approaches for treating t(8;21) patients using histone deacetylase inhibitors or HSP90 inhibitors.

RUNX1 associates with histone deacetylases and SUV39H1 to repress transcription

RUNX1 (AML1) is a gene that is frequently disrupted by chromosomal translocations in acute leukemia. Like its Drosophila homolog Runt, RUNX1 both activates and represses transcription. Both Runt and RUNX1 are required for gene silencing during development and a central domain of RUNX1, termed repression domain 2 (RD2), was defined as being required for transcriptional repression and for the silencing of CD4 during T-cell maturation in thymic organ cultures. Although transcriptional co-repressors are known to contact other repression domains in RUNX1, the factors that bind to RD2 had not been defined. Therefore, we tested whether RD2 contacts histone-modifying enzymes that may mediate both repression and gene silencing. We found that RD2 contacts SUV39H1, a histone methyltransferase, via two motifs and that endogenous Suv39h1 associates with a Runx1-regulated repression element in murine erythroleukemia cells. In addition, one of these SUV39H1-binding motifs is also sufficient for binding to histone deacetylases 1 and 3, and both of these domains are required for full RUNX1-mediated transcriptional repression. The association between RUNX1, histone deacetylases and SUV39H1 provides a molecular mechanism for repression and possibly gene silencing mediated by RUNX1.

siRNA-mediated AML1/MTG8 depletion affects differentiation and proliferation-associated gene expression in t(8;21)-positive cell lines and primary AML blasts

The chromosomal translocation t(8;21) is associated with 10-15% of all cases of acute myeloid leukaemia (AML). The resultant fusion protein AML1/MTG8 interferes with haematopoietic gene expression and is an important regulator of leukaemogenesis. We studied the effects of small interfering RNA (siRNA)-mediated AML1/MTG8 depletion on global gene expression in t(8;21)-positive leukaemic cell lines and in primary AML blasts using cDNA arrays, oligonucleotide arrays and real-time reverse transcription-polymerase chain reaction (RT-PCR). Suppression of AML1/MTG8 results in the increased expression of genes associated with myeloid differentiation, such as AZU1, BPI, CTSG, LYZ and RNASE2 as well as of antiproliferative genes such as IGFBP7, MS4A3 and SLA both in blasts and in cell lines. Furthermore, expression levels of several genes affiliated with drug resistance or indicative of poor prognosis AML (BAALC, CD34, PRG2, TSPAN7) are affected by AML1/MTG8 depletion. In conclusion, siRNA-mediated suppression of AML1/MTG8 cause very similar changes in gene expression pattern in t(8;21)-positive cell lines and in primary AML blasts. Furthermore, the results suggest that the specific targeting of AML1/MTG8 function may be a promising approach for complementing existing treatment strategies.

ELA2 is regulated by hematopoietic transcription factors, but not repressed by AML1-ETO

A 117 bp fragment of the human ELA2 promoter has been characterized that can act as a minimal promoter for the expression of neutrophil elastase. Chromatin immunoprecipitation and siRNAs revealed that expression of ELA2 is regulated by the acute myeloid human leukemia 1 protein (AML1), C/EBPalpha, PU.1 and c-Myb transcription factors. ELA2 has also been investigated as a possible target of the leukemic fusion protein AML1-ETO resulting from the t(8;21) chromosomal translocation. AML1-ETO, like AML1, binds the ELA2 promoter in the myeloid cell lines Kasumi-1 and U937, but unexpectedly fails to significantly alter expression of ELA2. Although AML1-ETO downregulates the expression of C/EBPalpha, changes in C/EBPalpha expression do not correlate with changes in the expression of ELA2. Our observations indicate that AML1-ETO may not be a constitutive repressor of gene expression in every case in which it can associate with DNA, either on its own or in conjunction with C/EBPalpha. Since neither ETO nor AML1-ETO are typically expressed in hematopoietic progenitors, we hypothesize that it is the interactions between AML1-ETO and regulatory cofactors in disease-state cells that alter gene expression programs during hematopoiesis. These protein-protein interactions may not require simultaneous DNA binding by AML1-ETO for the deleterious effects of the fusion protein to be realized.

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.

Decreased intranuclear mobility of acute myeloid leukemia 1-containing fusion proteins is accompanied by reduced mobility and compartmentalization of core binding factor beta

Acute myeloid leukemia 1 (AML1) gene on chromosome 21 is involved in several chromosomal translocations, including t(8;21) and t(16;21), that produce chimeric fusion proteins AML1-eight twenty-one (ETO) and AML-myeloid transforming gene chromosome 16 (MTG16), which contribute to leukemogenesis. The molecular basis for the leukemogenic effects of these fusion proteins is incompletely understood. Using gel-shift assay, we showed that AML1-ETO and AML1-MTG16 bound to a series of AML1 consensus DNA-binding sites with different affinities. Using fluorescence recovery after photobleaching (FRAP), we demonstrated that a fusion of AML1 with ETO or MTG16 exhibits reduced intranuclear mobility compared with wild-type AML1 or either fusion partner. The dimerization domain (nervy homology region 2) of ETO is responsible for the reduced mobility of AML1-ETO. Dual FRAP studies revealed that CBFbeta colocalized with AML1-ETO within the nucleus, resulting in reduced mobility of CBFbeta. Therefore, AML1 fusion proteins may interfere with normal AML1 function due to aberrant nuclear dynamics, which leads to spatial and temporal sequestration of CBFbeta and perhaps other coregulators critical for myeloid differentiation.

Mtgr1 is a transcriptional corepressor that is required for maintenance of the secretory cell lineage in the small intestine

Two members of the MTG/ETO family of transcriptional corepressors, MTG8 and MTG16, are disrupted by chromosomal translocations in up to 15% of acute myeloid leukemia cases. The third family member, MTGR1, was identified as a factor that associates with the t(8;21) fusion protein RUNX1-MTG8. We demonstrate that Mtgr1 associates with mSin3A, N-CoR, and histone deacetylase 3 and that when tethered to DNA, Mtgr1 represses transcription, suggesting that Mtgr1 also acts as a transcriptional corepressor. To define the biological function of Mtgr1, we created Mtgr1-null mice. These mice are proportionally smaller than their littermates during embryogenesis and throughout their life span but otherwise develop normally. However, these mice display a progressive reduction in the secretory epithelial cell lineage in the small intestine. This is not due to the loss of small intestinal progenitor cells expressing Gfi1, which is required for the formation of goblet and Paneth cells, implying that loss of Mtgr1 impairs the maturation of secretory cells in the small intestine.

Transcriptional repression of the Neurofibromatosis-1 tumor suppressor by the t(8;21) fusion protein

Von Recklinghausen's disease is a relatively common familial genetic disorder characterized by inactivating mutations of the Neurofibromatosis-1 (NF1) gene that predisposes these patients to malignancies, including an increased risk for juvenile myelomonocytic leukemia. However, NF1 mutations are not common in acute myeloid leukemia (AML). Given that the RUNX1 transcription factor is the most common target for chromosomal translocations in acute leukemia, we asked if NF1 might be regulated by RUNX1. In reporter assays, RUNX1 activated the NF1 promoter and cooperated with C/EBPalpha and ETS2 to activate the NF1 promoter over 80-fold. Moreover, the t(8;21) fusion protein RUNX1-MTG8 (R/M), which represses RUNX1-regulated genes, actively repressed the NF1 promoter. R/M associated with the NF1 promoter in vivo and repressed endogenous NF1 gene expression. In addition, similar to loss of NF1, R/M expression enhanced the sensitivity of primary myeloid progenitor cells to granulocyte-macrophage colony-stimulating factor. Our results indicate that the NF1 tumor suppressor gene is a direct transcriptional target of RUNX1 and the t(8;21) fusion protein, suggesting that suppression of NF1 expression contributes to the molecular pathogenesis of AML.

MYND-less splice variants of AML1-MTG8 (RUNX1-CBFA2T1) are expressed in leukemia with t(8;21)

The AML1-MTG8 fusion gene is generated by chromosome translocation t(8;21), which is frequently observed in acute myeloid leukemia. The fusion gene produces a chimeric transcription factor that suppresses the expression of AML1-target genes via the MTG8 part of the chimeric protein, which is thought to be the primary cause of leukemia. The C-terminal region of MTG8 contains the MYND domain, represented by highly conserved zinc-finger-like protein motifs, and is known to interact with corepressor proteins. We found that, instead of the MYND domain, an alternative last exon of MTG8 encoding 27 amino acids in-frame is expressed naturally in human adult testis and in several leukemia cell lines. This type of alternative splicing also occurred in the AML1-MTG8 fusion gene at high levels in leukemia cell lines with t(8;21), as well as in blast cells of leukemia patients with t(8;21). The variant proteins of both MTG8 and AML1-MTG8 reduced transcriptional repressor activity in a mammalian two-hybrid assay. However, mixed expression of these variants with wild-type MTG8 recovered their repressor activity, suggesting that these variants also act as repressors in vivo where wild-type MTG8 and other family members exist in abundance. On the other hand, the MYND-less variants acquired a higher affinity for binding to MTG8 and formed a multimer, whereas the wild-type protein forms a dimer. Thus, expression of the MYND-less variants by the dysregulation of splicing machinery, which stimulates the oligomerization of fusion proteins in leukemia cells, may enhance malignant conversion of hematopoietic cells.

The t(8;21) translocation converts AML1 into a constitutive transcriptional repressor

The human translocation (t8;21) is associated with approximately 12% of the cases of acute myelogenous leukemia. Two genes, AML1 and ETO, are fused together at the translocation breakpoint, resulting in the expression of a chimeric protein called AML1-ETO. AML1-ETO is thought to interfere with normal AML1 function, although the mechanism by which it does so is unclear. Here, we have used Drosophila genetics to investigate two models of AML1-ETO function. In the first model, AML1-ETO is a constitutive transcriptional repressor of AML1 target genes, regardless of whether they are normally activated or repressed by AML1. In the second model, AML1-ETO dominantly interferes with AML1 activity by, for example, competing for a common co-factor. To discriminate between these models, the effects of expressing AML1-ETO were characterized and compared with loss-of-function phenotypes of lozenge (lz), an AML1 homolog expressed during Drosophila eye development. We also present results of genetic interaction experiments with AML1 co-factors that are not consistent with AML1-ETO behaving as a dominant-negative factor. Instead, our data suggest that AML1-ETO acts as a constitutive transcriptional repressor.

The Leukemia-associated ETO homologues are differently expressed during hematopoietic differentiation

The Eight twenty-one (ETO) homologues are nuclear repressor proteins including ETO, myeloid-transforming gene-related protein 1 (MTGR1), and myeloid-transforming gene chromosome 16 (MTG16). ETO and MTG16 are both part of fusion proteins resulting from chromosomal translocations associated with acute myeloid leukemia. Expression of these chimeras results in a differentiation block that contributes to the onset of leukemia. In order to elucidate the relation between the ETO homologues and hematopoietic differentiation, we determined the expression of the homologues during differentiation of leukemic and normal hematopoietic cells. Our results showed MTGR1 and MTG16 to be ubiquitously expressed in leukemic cell lines, whereas expression of ETO was observed only in an erythroleukemic cell line. The MTGR1 and MTG16 proteins decreased during all trans-retinoic acid-, but not vitamin D(3)-induced differentiation of leukemic cells. The reduction seemed to reflect a decrease in transcript levels as well as in protein stability. MTGR1 transcripts were ubiquitously expressed in human bone marrow cells. The MTG16 transcripts of CD34(+) progenitor cells were rapidly downregulated by cytokine-induced differentiation into myeloid or erythroid lineages. ETO transcripts, present at very low abundance in CD34(+) progenitor cells, were transiently upregulated during erythroid differentiation. In conclusion, the differential expression of the ETO homologues suggests that they may have a potential role in hematopoietic differentiation.

The MTG proteins: chromatin repression players with a passion for networking

The human myeloid translocation genes (MTGs) encode a family of proteins with a modular structure that can be traced to the Drosophila protein nervy. The nuclear MTGs can mediate the formation of complex protein networks among nuclear corepressors (Sin3a, N-CoR, SMRT), chromatin-modifying enzymes (histone deacetylases), and DNA-binding transcription factors. Hierarchical modulation of repression at target genes by MTG protein complexes is likely required for fine spatial and temporal gene regulation during development and differentiation. Genomic changes can disrupt these sophisticated protein networks and underlie novel pathogenic causes of cancer and neurodegeneration.

The nuclear receptor co-repressor (N-CoR) utilizes repression domains I and III for interaction and co-repression with ETO

The acute human leukemias are associated with the presence of chimeric gene products that arise from spontaneous chromosomal translocations. The t(8;21) translocation gene product led to the discovery of the Eight Twenty-One (ETO) gene. When fused to RUNX1, ETO is thought to mediate the formation of a repressive complex at RUNX1-dependent genes. ETO has also been found to act as a co-repressor of the promyelocytic zinc finger and Bcl-6 oncoproteins, suggesting that it may play a common role as a transcriptional co-repressor leading to human disease. An analysis of ETO-mediated repression revealed that one of the key binding partners of ETO is the nuclear receptor co-repressor (N-CoR). It is shown that two highly conserved domains of ETO interact with repression domains I and III of N-CoR. One of the ETO domains displays significant homology to Drosophila TAF(II)110, whereas the other is a predicted zinc binding motif that engages a conserved PPLXP motif in repression domain III of N-CoR. Together, these domains of ETO cooperate in repression with N-CoR and the binding sites in N-CoR overlap with those for other repressive factors. Thus, ETO has the potential to participate in a number of repressive complexes, which can be distinguished by their binding partners and target genes.

AML1-ETO Decreases ETO-2 (MTG16) Interactions with Nuclear Receptor Corepressor, an Effect That Impairs Granulocyte Differentiation

The t(8;21) chromosome abnormality in acute myeloid leukemia targets the AML1 and ETO genes to produce the leukemia fusion protein AML1-ETO. Another member of the ETO family, ETO-2/MTG16, is highly expressed in murine and human hematopoietic cells, bears >75% homology to ETO, and like ETO, contains a conserved MYND domain that interacts with the nuclear receptor corepressor (N-CoR). AML1-ETO prevents granulocyte but not macrophage differentiation of murine 32Dcl3 granulocyte/macrophage progenitors. One possible mechanism is recruitment of N-CoR to aberrantly repress AML1 target genes. We wished to examine another mechanism by which AML1-ETO might impair granulocyte differentiation. We demonstrate that AML1-ETO decreases interactions between ETO-2 and N-CoR. Furthermore, overexpression of ETO-2 relieves AML1-ETO-induced granulocyte differentiation arrest. This suggests that decreased interactions between ETO-2 and N-CoR may contribute to granulocyte differentiation impairment. The MYND domain coimmunoprecipitates with N-CoR and inhibits interactions between ETO-2 and N-CoR, presumably by occupying the ETO-2 binding site on N-CoR. This inhibition of ETO-2 interactions with N-CoR is specific because the MYND domain does not inhibit retinoic acid receptor interactions with N-CoR. To examine the effect of decreasing interactions between ETO-2 and N-CoR in hematopoietic cells, without effects of AML1-ETO such as direct repression of AML1 target genes, the MYND domain was expressed in 32Dcl3 and human CD34+ cells. The MYND domain prevented granulocyte but not macrophage differentiation of both 32Dcl3 and human CD34+ cells, recapitulating this effect of AML1-ETO. In conclusion, decreasing interactions between ETO-2 and N-CoR, an effect of AML1-ETO, inhibits granulocyte differentiation.

ETO interacting proteins

The 8;21 translocation produces a fusion between the ETO gene and that encoding the myeloid transcription factor AML1. The AML1-ETO fusion substitutes the majority of the ETO protein for the coregulator recruitment domains of AML1. Biochemical analyses of ETO have led to the identification of numerous interacting proteins including many corepressors. Importantly, the proteins interacting with ETO are different from those of wild-type AML1, suggesting that altered coregulator recruitment underlies the oncogenic properties of AML1-ETO. The list of corepressors capable of binding ETO includes histone deacetylases (HDACs) and components of distinct HDAC core complexes. These investigations have provided mechanistic insight into corepressor recruitment by ETO and clues to the leukemogenic activity of AML1-ETO.

The 8;21 translocation in leukemogenesis

A common chromosomal translocation in acute myeloid leukemia (AML) involves the AML1 (acute myeloid leukemia 1, also called RUNX1, core binding factor protein (CBF alpha), and PEBP2 alpha B) gene on chromosome 21 and the ETO (eight-twenty one, also called MTG8) gene on chromosome 8. This translocation generates an AML1-ETO fusion protein. t(8;21) is associated with 12% of de novo AML cases and up to 40% in the AML subtype M2 of the French-American-British classification. Furthermore, it is also reported in a small portion of M0, M1, and M4 AML samples. Despite numerous studies on the function of AML1-ETO, the precise mechanism by which the fusion protein is involved in leukemia development is still not fully understood. In this review, we will discuss structural aspects of the fusion protein and the accumulated knowledge from in vitro analyses on AML1-ETO functions, and outline putative mechanisms of its leukemogenic potential.