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

Chaperonin TRiC/CCT Modulates the Folding and Activity of Leukemogenic Fusion Oncoprotein AML1-ETO

AML1-ETO is the most common fusion oncoprotein causing acute myeloid leukemia (AML), a disease with a 5-year survival rate of only 24%. AML1-ETO functions as a rogue transcription factor, altering the expression of genes critical for myeloid cell development and differentiation. Currently, there are no specific therapies for AML1-ETO-positive AML. While known for decades to be the translational product of a chimeric gene created by the stable chromosome translocation t(8;21)(q22;q22), it is not known how AML1-ETO achieves its native and functional conformation or whether this process can be targeted for therapeutic benefit. Here, we show that the biosynthesis and folding of the AML1-ETO protein is facilitated by interaction with the essential eukaryotic chaperonin TRiC (or CCT). We demonstrate that a folding intermediate of AML1-ETO binds to TRiC directly, mainly through its β-strand rich, DNA-binding domain (AML-(1-175)), with the assistance of HSP70. Our results suggest that TRiC contributes to AML1-ETO proteostasis through specific interactions between the oncoprotein's DNA-binding domain, which may be targeted for therapeutic benefit.

Genome-wide studies identify a novel interplay between AML1 and AML1/ETO in t(8;21) acute myeloid leukemia

The AML1/ETO fusion protein is essential to the development of t(8;21) acute myeloid leukemia (AML) and is well recognized for its dominant-negative effect on the coexisting wild-type protein AML1. However, the genome-wide interplay between AML1/ETO and wild-type AML1 remains elusive in the leukemogenesis of t(8;21) AML. Through chromatin immunoprecipitation sequencing and computational analysis, followed by a series of experimental validations, we report here that wild-type AML1 is able to orchestrate the expression of AML1/ETO targets regardless of being activated or repressed; this is achieved via forming a complex with AML1/ETO and via recruiting the cofactor AP-1 on chromatin. On chromatin occupancy, AML1/ETO and wild-type AML1 largely overlap and preferentially bind to adjacent and distinct short and long AML1 motifs on the colocalized regions, respectively. On physical interaction, AML1/ETO can form a complex with wild-type AML1 on chromatin, and the runt homology domain of both proteins are responsible for their interactions. More importantly, the relative binding signals of AML1 and AML1/ETO on chromatin determine which genes are repressed or activated by AML1/ETO. Further analysis of coregulators indicates that AML1/ETO transactivates gene expression through recruiting AP-1 to the AML1/ETO-AML1 complex. These findings enrich our knowledge of understanding the significance of the interplay between the wild-type protein and the oncogenic fusion protein in the development of leukemia.

Critical role of retinoid/rexinoid signaling in mediating transformation and therapeutic response of NUP98-RARG leukemia

While the nucleoporin 98-retinoic acid receptor gamma (NUP98-RARG) is the first RARG fusion protein found in acute leukemia, its roles and the molecular basis in oncogenic transformation are currently unknown. Here, we showed that homodimeric NUP98-RARG not only acquired unique nuclear localization pattern and ability of recruiting both RXRA and wild-type NUP98, but also exhibited similar transcriptional properties as RARA fusions found in acute promyelocytic leukemia (APL). Using murine bone marrow retroviral transduction/transformation assay, we further demonstrated that NUP98-RARG fusion protein had gained transformation ability of primary hematopoietic stem/progenitor cells, which was critically dependent on the C-terminal GLFG domain of NUP98 and the DNA binding domain (DBD) of RARG. In contrast to other NUP98 fusions, cells transformed by the NUP98-RARG fusion were extremely sensitive to all-trans retinoic acid (ATRA) treatment. Interestingly, while pan-RXR agonists, SR11237 and LGD1069 could specifically inhibit NUP98-RARG transformed cells, mutation of the RXR interaction domain in NUP98-RARG had little effect on its transformation, revealing that therapeutic functions of rexinoid can be independent of the direct biochemical interaction between RXR and the fusion. Together, these results indicate that deregulation of the retinoid/rexinoid signaling pathway has a major role and may represent a potential therapeutic target for NUP98-RARG-mediated transformation.

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.

AML1/ETO cooperates with HIF1α to promote leukemogenesis through DNMT3a transactivation

The mechanisms by which AML1/ETO (A/E) fusion protein induces leukemogenesis in acute myeloid leukemia (AML) without mutagenic events remain elusive. Here we show that interactions between A/E and hypoxia-inducible factor 1α (HIF1α) are sufficient to prime leukemia cells for subsequent aggressive growth. In agreement with this, HIF1α is highly expressed in A/E-positive AML patients and strongly predicts inferior outcomes, regardless of gene mutations. Co-expression of A/E and HIF1α in leukemia cells causes a higher cell proliferation rate in vitro and more serious leukemic status in mice. Mechanistically, A/E and HIF1α form a positive regulatory circuit and cooperate to transactivate DNMT3a gene leading to DNA hypermethylation. Pharmacological or genetic interventions in the A/E-HIF1α loop results in DNA hypomethylation, a re-expression of hypermethylated tumor-suppressor p15(INK4b) and the blockage of leukemia growth. Thus high HIF1α expression serves as a reliable marker, which identifies patients with a poor prognosis in an otherwise prognostically favorable AML group and represents an innovative therapeutic target in high-risk A/E-driven leukemia.

C-terminal domain of SMYD3 serves as a unique HSP90-regulated motif in oncogenesis

The SMYD3 histone methyl transferase (HMTase) and the nuclear chaperone, HSP90, have been independently implicated as proto-oncogenes in several human malignancies. We show that a degenerate tetratricopeptide repeat (TPR)-like domain encoded in the SMYD3 C-terminal domain (CTD) mediates physical interaction with HSP90. We further demonstrate that the CTD of SMYD3 is essential for its basal HMTase activity and that the TPR-like structure is required for HSP90-enhanced enzyme activity. Loss of SMYD3-HSP90 interaction leads to SMYD3 mislocalization within the nucleus, thereby losing its chromatin association. This results in reduction of SMYD3-mediated cell proliferation and, potentially, impairment of SMYD3's oncogenic activity. These results suggest a novel approach for blocking HSP90-driven malignancy in SMYD3-overexpressing cells with a reduced toxicity profile over current HSP90 inhibitors.

Identification of a dynamic core transcriptional network in t(8;21) AML that regulates differentiation block and self-renewal

Oncogenic transcription factors such as RUNX1/ETO, which is generated by the chromosomal translocation t(8;21), subvert normal blood cell development by impairing differentiation and driving malignant self-renewal. Here, we use digital footprinting and chromatin immunoprecipitation sequencing (ChIP-seq) to identify the core RUNX1/ETO-responsive transcriptional network of t(8;21) cells. We show that the transcriptional program underlying leukemic propagation is regulated by a dynamic equilibrium between RUNX1/ETO and RUNX1 complexes, which bind to identical DNA sites in a mutually exclusive fashion. Perturbation of this equilibrium in t(8;21) cells by RUNX1/ETO depletion leads to a global redistribution of transcription factor complexes within preexisting open chromatin, resulting in the formation of a transcriptional network that drives myeloid differentiation. Our work demonstrates on a genome-wide level that the extent of impaired myeloid differentiation in t(8;21) is controlled by the dynamic balance between RUNX1/ETO and RUNX1 activities through the repression of transcription factors that drive differentiation.

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RUNX1/ETO drives a t(8;21)-specific transcriptional network

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RUNX1/ETO and RUNX1 dynamically compete for the same genomic sites

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RUNX1/ETO targets transcription factor complexes that control differentiation

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RUNX1/ETO depletion activates a transcriptional network dominated by C/EBPα

Histone deacetylases and their inhibitors in cancer, neurological diseases and immune disorders

Epigenetic aberrations, which are recognized as key drivers of several human diseases, are often caused by genetic defects that result in functional deregulation of epigenetic proteins, their altered expression and/or their atypical recruitment to certain gene promoters. Importantly, epigenetic changes are reversible, and epigenetic enzymes and regulatory proteins can be targeted using small molecules. This Review discusses the role of altered expression and/or function of one class of epigenetic regulators--histone deacetylases (HDACs)--and their role in cancer, neurological diseases and immune disorders. We highlight the development of small-molecule HDAC inhibitors and their use in the laboratory, in preclinical models and in the clinic.

From determinants of RUNX1/ETO tetramerization to small-molecule protein-protein interaction inhibitors targeting acute myeloid leukemia

We identified the first small-molecule protein-protein interaction inhibitors of RUNX1/ETO tetramerization applying structure-based virtual screening guided by predicted hot spots and pockets in the interface. A 3D similarity screening revealed specific hot spot mimetics, one of which prevents the proliferation of RUNX1/ETO-dependent SKNO-1 cells at low micromolar concentration. Using solely a protein-protein complex structure to start with, this strategy can be the first step in any comparable structure-based endeavor to identify protein-protein interaction inhibitors.

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.

A stable transcription factor complex nucleated by oligomeric AML1-ETO controls leukaemogenesis

Transcription factors are frequently altered in leukaemia through chromosomal translocation, mutation or aberrant expression¹. AML1-ETO, a fusion protein generated by the t(8;21) translocation in acute myeloid leukaemia (AML), is a transcription factor implicated in both gene repression and activation². AML1-ETO oligomerization, mediated by the NHR2 domain, is critical for leukaemogenesis^3–6, making it important to identify coregulatory factors that “read” the NHR2 oligomerization and contribute to leukaemogenesis⁴. We now show that, in leukaemic cells, AML1-ETO resides in and functions through a stable protein complex (AETFC) that contains several haematopoietic transcription (co)factors. These AETFC components stabilize the complex through multivalent interactions, provide multiple DNA-binding domains for diverse target genes, colocalize genome-wide, cooperatively regulate gene expression, and contribute to leukaemogenesis. Within the AETFC complex, AML1-ETO oligomerization is required for a specific interaction between the oligomerized NHR2 domain and a novel NHR2-binding (N2B) motif in E proteins. Crystallographic analysis of the NHR2-N2B complex reveals a unique interaction pattern in which an N2B peptide makes direct contact with side chains of two NHR2 domains as a dimer, providing a novel model of how dimeric/oligomeric transcription factors create a new protein-binding interface through dimerization/oligomerization. Intriguingly, disruption of this interaction by point mutations abrogates AML1-ETO–induced haematopoietic stem/progenitor cell self-renewal and leukaemogenesis. These results reveal new mechanisms of action of AML1-ETO and a potential therapeutic target in t(8;21)⁺ AML.

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.

Attenuation of AML1-ETO cellular dysregulation correlates with increased leukemogenic potential

AML1-ETO (RUNX1-ETO) fusion proteins are generated by the 8;21 translocation, commonly found in acute myeloid leukemia, which fuses the AML1 (RUNX1) and ETO (MTG8, RUNX1T1) genes. Previous studies have shown that AML1-ETO interferes with AML1 function but requires additional cooperating mutations to induce leukemia development. In mouse models, AML1-ETO forms lacking the C-terminus have been shown to have greatly enhanced leukemogenic potential. Here, we investigate the role of 2 AML1-ETO C-terminal-interacting proteins, N-CoR, a transcriptional corepressor, and SON, a splicing/transcription factor required for cell cycle progression, in AML1-ETO-induced leukemia development. AML1-ETO-W692A loses N-CoR binding at NHR4, displays attenuated transcriptional repression ability and decreased cellular dysregulation, and promotes leukemia in vivo. These results support the importance of the degree of dysregulation by AML1-ETO in cellular transformation and demonstrate that AML1-ETO-W692A can be used as an effective experimental model for determining which factors compromise the leukemogenic potential of AML1-ETO.

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.

RUNX1 mutations in clonal myeloid disorders: from conventional cytogenetics to next generation sequencing, a story 40 years in the making

Translocations and mutations in the core binding factor genes, RUNX1 or CBFB, are found in acute myeloid and lymphocytic leukemia, therapy-related myeloid leukemia, myelodysplastic syndrome, chronic myelomonocytic leukemia, and in familial platelet disorder with predisposition to acute myeloid leukemia. Here we review the biochemical and biological properties of the normal Runx1 protein, discuss the nature of RUNX1 mutations in myeloid leukemia, their prognostic significance, and the mutations that cooperate or co-exist with them in these various diseases.

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.

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.

INPP4B: the new kid on the PI3K block

Dysregulation of phosphatidyl inositol signaling occurs in many cancers and other disorders. The lipid and protein phosphatase, PTEN (Phosphatase and Tensin homology protein on chromosome 10), is a known tumor suppressor whose function is frequently lost in various malignancies due to mutations in the coding region or genomic deletions. Recently, another lipid phosphatase, Inositol Polyphosphate 4-phosphatase type II (INPP4B), has emerged as a potential tumor suppressor in prostate, breast, and ovarian cancers and possibly in leukemia. We will review its structure and function, crosstalk with androgen receptor signaling, and regulation of INPP4B expression, as well as existing data about its role in cancer.

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.

Mtg16/Eto2 contributes to murine T-cell development

Mtg16/Eto2 is a transcriptional corepressor that is disrupted by t(16;21) in acute myeloid leukemia. Using mice lacking Mtg16, we found that Mtg16 is a critical regulator of T-cell development. Deletion of Mtg16 led to reduced thymocyte development in vivo, and after competitive bone marrow transplantation, there was a nearly complete failure of Mtg16(-/-) cells to contribute to thymocyte development. This defect was recapitulated in vitro as Mtg16(-/-) Lineage(-)/Sca1(+)/c-Kit(+) (LSK) cells of the bone marrow or DN1 cells of the thymus failed to produce CD4(+)/CD8(+) cells in response to a Notch signal. Complementation of these defects by reexpressing Mtg16 showed that 3 highly conserved domains were somewhat dispensable for T-cell development but required the capacity of Mtg16 to suppress E2A-dependent transcriptional activation and to bind to the Notch intracellular domain. Thus, Mtg16 integrates the activities of signaling pathways and nuclear factors in the establishment of T-cell fate specification.

Structure of the AML1-ETO NHR3-PKA(RIIα) complex and its contribution to AML1-ETO activity

AML1-ETO is the chimeric protein product of t(8;21) in acute myeloid leukemia. The ETO portion of the fusion protein includes the nervy homology region (NHR) 3 domain, which shares homology with A-kinase anchoring proteins and interacts with the regulatory subunit of type II cAMP-dependent protein kinase A (PKA(RIIα)). We determined the solution structure of a complex between the AML1-ETO NHR3 domain and PKA(RIIα). Based on this structure, a key residue in AML1-ETO for PKA(RIIα) association was mutated. This mutation did not disrupt AML1-ETO's ability to enhance the clonogenic capacity of primary mouse bone marrow cells or its ability to repress proliferation or granulocyte differentiation. Introduction of the mutation into AML1-ETO had minimal impact on in vivo leukemogenesis. Therefore, the NHR3-PKA(RIIα) protein interaction does not appear to significantly contribute to AML1-ETO's ability to induce leukemia.

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.

Myeloid translocation gene 16 (MTG16) interacts with Notch transcription complex components to integrate Notch signaling in hematopoietic cell fate specification

The Notch signaling pathway regulates gene expression programs to influence the specification of cell fate in diverse tissues. In response to ligand binding, the intracellular domain of the Notch receptor is cleaved by the gamma-secretase complex and then translocates to the nucleus. There, it binds the transcriptional repressor CSL, triggering its conversion to an activator of Notch target gene expression. The events that control this conversion are poorly understood. We show that the transcriptional corepressor, MTG16, interacts with both CSL and the intracellular domains of Notch receptors, suggesting a pivotal role in regulation of the Notch transcription complex. The Notch1 intracellular domain disrupts the MTG16-CSL interaction. Ex vivo fate specification in response to Notch signal activation is impaired in Mtg16-/- hematopoietic progenitors, and restored by MTG16 expression. An MTG16 derivative lacking the binding site for the intracellular domain of Notch1 fails to restore Notch-dependent cell fate. These data suggest that MTG16 interfaces with critical components of the Notch transcription complex to affect Notch-dependent lineage allocation in hematopoiesis.

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.

CBFbeta is critical for AML1-ETO and TEL-AML1 activity

AML1-ETO and TEL-AML1 are chimeric proteins resulting from the t(8;21)(q22;q22) in acute myeloid leukemia, and the t(12;21)(p13;q22) in pre-B-cell leukemia, respectively. The Runt domain of AML1 in both proteins mediates DNA binding and heterodimerization with the core binding factor beta (CBFbeta) subunit. To determine whether CBFbeta is required for AML1-ETO and TEL-AML1 activity, we introduced amino acid substitutions into the Runt domain that disrupt heterodimerization with CBFbeta but not DNA binding. We show that CBFbeta 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 is indispensable for its cooperativity with the activated receptor tyrosine kinase TEL-PDGFbetaR in generating acute myeloid leukemia in mice. Similarly, CBFbeta is essential for TEL-AML1's ability to promote self-renewal of B cell precursors in vitro. These studies validate the Runt domain/CBFbeta interaction as a therapeutic target in core binding factor leukemias.

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.

Structure of the AML1-ETO eTAFH domain-HEB peptide complex and its contribution to AML1-ETO activity

AML1-ETO is the chimeric protein product of the t(8;21) in acute myeloid leukemia. The ETO portion of the fusion protein includes the eTAFH domain, which is homologous to several TATA binding protein-associated factors (TAFs) and interacts with E proteins (E2A and HEB). It has been proposed that AML1-ETO-mediated silencing of E protein function might be important for t(8;21) leukemogenesis. Here, we determined the solution structure of a complex between the AML1-ETO eTAFH domain and an interacting peptide from HEB. On the basis of the structure, key residues in AML1-ETO for HEB association were mutated. These mutations do not impair the ability of AML1-ETO to enhance the clonogenic capacity of primary mouse bone marrow cells and do not eliminate its ability to repress proliferation or granulocyte differentiation. Therefore, the eTAFH-E protein interaction appears to contribute relatively little to the activity of AML1-ETO.

Transforming activity of AML1-ETO is independent of CBFbeta and ETO interaction but requires formation of homo-oligomeric complexes

Although both heterodimeric subunits of core binding factors (AML1/RUNX1 and CBFbeta) essential for normal hematopoiesis are frequently mutated to form different chimeric fusion proteins in acute leukemia, the underlying molecular mechanisms and structural domains required for cellular transformation remain largely unknown. Despite the critical role of CBFbeta for wild-type AML1 function and its direct involvement in chromosomal translocation, we demonstrate that both the expression and interaction with CBFbeta are superfluous for AML1-ETO (AE)-mediated transformation of primary hematopoietic cells. Similarly, the hetero-oligomeric interaction with transcriptional repressor ETO family proteins and the highly conserved NHR1 domain in AE fusion are also dispensable for transforming activity. In contrast, AE-mediated transformation is critically dependent on the DNA binding and homo-oligomeric properties of the fusion. Abolishment of homo-oligomerization by a small-molecule inhibitor could specifically suppress AML1 fusion-mediated transformation of primary hematopoietic cells. Together, these results not only identify the essential molecular components but also potential avenues for therapeutic targeting of AE-mediated leukemogenesis.

Proleukemic RUNX1 and CBFbeta mutations in the pathogenesis of acute leukemia

The existence of non-random mutations in critical regulators of cell growth and differentiation is a recurring theme in cancer pathogenesis and provides the basis for our modern, molecular approach to the study and treatment of malignant diseases. Nowhere is this more true than in the study of leukemogenesis, where research has converged upon a critical group of genes involved in hematopoietic stem and progenitor cell self-renewal and fate specification. Prominent among these is the heterodimeric transcriptional regulator, RUNX1/CBFbeta. RUNX1 is a site-specific DNA-binding protein whose consensus response element is found in the promoters of many hematopoietically relevant genes. CBFbeta interacts with RUNX1, stabilizing its interaction with DNA to promote the actions of RUNX1/CBFbeta in transcriptional control. Both the RUNX1 and the CBFbeta genes participate in proleukemic chromosomal alterations. Together they contribute to approximately one-third of acute myelogenous leukemia (AML) and one-quarter of acute lymphoblastic leukemia (ALL) cases, making RUNX1 and CBFbeta the most frequently affected genes known in the pathogenesis of acute leukemia. Investigating the mechanisms by which RUNX1, CBFbeta, and their proleukemic fusion proteins influence leukemogenesis has contributed greatly to our understanding of both normal and malignant hematopoiesis. Here we present an overview of the structural features of RUNX1/CBFbeta and their derivatives, their roles in transcriptional control, and their contributions to normal and malignant hematopoiesis.

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.

Disruption of the NHR4 domain structure in AML1-ETO abrogates SON binding and promotes leukemogenesis

AML1-ETO is generated from t(8;21)(q22;q22), which is a common form of chromosomal translocation associated with development of acute myeloid leukemia (AML). Although full-length AML1-ETO alone fails to promote leukemia because of its detrimental effects on cell proliferation, an alternatively spliced isoform, AML1-ETO9a, without its C-terminal NHR3/NHR4 domains, strongly induces leukemia. However, full-length AML1-ETO is a major form of fusion product in many t(8;21) AML patients, suggesting additional molecular mechanisms of t(8;21)-related leukemogenesis. Here, we report that disruption of the zinc-chelating structure in the NHR4 domain of AML1-ETO by replacing only one critical amino acid leads to rapid onset of leukemia, demonstrating that the NHR4 domain with the intact structure generates inhibitory effects on leukemogenesis. Furthermore, we identified SON, a DNA/RNA-binding domain containing protein, as a novel NHR4-interacting protein. Knock-down of SON by siRNA resulted in significant growth arrest, and disruption of the interaction between AML1-ETO and endogenous SON rescued cells from AML1-ETO-induced growth arrest, suggesting that SON is an indispensable factor for cell growth, and AML1-ETO binding to SON may trigger signals inhibiting leukemogenesis. In t(8;21) AML patient-derived primary leukemic cells and cell lines, abnormal cytoplasmic localization of SON was detected, which may keep cells proliferating in the presence of full-length AML1-ETO. These results uncovered the crucial role of the NHR4 domain in determination of cellular fate during AML1-ETO-associated leukemogenesis.

t(8;21)(q22;q22) Fusion proteins preferentially bind to duplicated AML1/RUNX1 DNA-binding sequences to differentially regulate gene expression

Chromosome abnormalities are frequently associated with cancer development. The 8;21(q22;q22) chromosomal translocation is one of the most common chromosome abnormalities identified in leukemia. It generates fusion proteins between AML1 and ETO. Since AML1 is a well-defined DNA-binding protein, AML1-ETO fusion proteins have been recognized as DNA-binding proteins interacting with the same consensus DNA-binding site as AML1. The alteration of AML1 target gene expression due to the presence of AML1-ETO is related to the development of leukemia. Here, using a 25-bp random double-stranded oligonucleotide library and a polymerase chain reaction (PCR)-based DNA-binding site screen, we show that compared with native AML1, AML1-ETO fusion proteins preferentially bind to DNA sequences with duplicated AML1 consensus sites. This finding is further confirmed by both in vitro and in vivo DNA-protein interaction assays. These results suggest that AML1-ETO fusion proteins have a selective preference for certain AML1 target genes that contain multimerized AML1 consensus sites in their regulatory elements. Such selected regulation provides an important molecular mechanism for the dysregulation of gene expression during cancer development.

Characterizing protein-protein interactions by sedimentation velocity analytical ultracentrifugation

This unit introduces the basic principles and practice of sedimentation velocity analytical ultracentrifugation for the study of reversible protein interactions, such as the characterization of self-association, heterogeneous association, multi-protein complexes, binding stoichiometry, and the determination of association constants. The analytical tools described include sedimentation coefficient and molar mass distributions, multi-signal sedimentation coefficient distributions, Gilbert-Jenkins theory, different forms of isotherms, and global Lamm equation modeling. Concepts for the experimental design are discussed, and a detailed step-by-step protocol guiding the reader through the experiment and the data analysis is available as an Internet resource.

Roles of p15Ink4b and p16Ink4a in myeloid differentiation and RUNX1-ETO-associated acute myeloid leukemia

Inactivation of p15(Ink4b) expression by promoter hypermethylation occurs in up to 80% of acute myeloid leukemia (AML) cases and is particularly common in the FAB-M2 subtype of AML, which is characterized by the presence of the RUNX1-ETO translocation in 40% of cases. To establish whether the loss of p15(Ink4b) contributes to AML progression in association with RUNX1-ETO, we have expressed the RUNX1-ETO fusion protein from a retroviral vector in hematopoietic progenitor cells isolated from wild-type, p15(Ink4b) or p16(Ink4a) knockout bone marrow. Analysis of lethally irradiated recipient mice reconstituted with RUNX1-ETO-expressing cells showed that neither p15(Ink4b) or p16(Ink4a) loss significantly accelerated disease progression over the time period of one year post-transplantation. Loss of p15(Ink4b) alone resulted in increased myeloid progenitor cell frequencies in bone marrow by 10-month post-transplant and a 19-fold increase in the frequency of Lin(-)c-Kit(+)Sca-1(+) (LKS) cells that was not associated with expansion of long-term reconstituting HSC. These results strongly suggest that p15(Ink4b) loss must be accompanied by additional oncogenic changes for RUNX1-ETO-associated AML to develop.

PU.1 is a major downstream target of AML1 (RUNX1) in adult mouse hematopoiesis

Both PU.1 (also called SFPI1), an Ets-family transcription factor, and AML1 (also called RUNX1), a DNA-binding subunit of the CBF transcription factor family, are crucial for the generation of all hematopoietic lineages, and both act as tumor suppressors in leukemia. An upstream regulatory element (URE) of PU.1 has both enhancer and repressor activity and tightly regulates PU.1 expression. Here we show that AML1 binds to functionally important sites within the PU.1 upstream regulatory element and regulates PU.1 expression at both embryonic and adult stages of development. Analysis of mice carrying conditional AML1 knockout alleles and knock-in mice carrying mutations in all three AML1 sites of the URE proximal region demonstrated that AML1 regulates PU.1 both positively and negatively in a lineage dependent manner. Dysregulation of PU.1 expression contributed to each of the phenotypes observed in these mice, and restoration of proper PU.1 expression rescued or partially rescued each phenotype. Thus, our data demonstrate that PU.1 is a major downstream target gene of AML1.

Epigenetic silencing of the myelopoiesis regulator microRNA-223 by the AML1/ETO oncoprotein

Hematopoietic transcription factors are involved in chromosomal translocations, which generate fusion proteins contributing to leukemia pathogenesis. Analysis of patient's primary leukemia blasts revealed that those carrying the t(8;21) generating AML1/ETO, the most common acute myeloid leukemia-associated fusion protein, display low levels of a microRNA-223 (miR-223), a regulator of myelopoiesis. Here, we show that miR-223 is a direct transcriptional target of AML1/ETO. By recruiting chromatin remodeling enzymes at an AML1-binding site on the pre-miR-223 gene, AML1/ETO induces heterochromatic silencing of miR-223. Ectopic miR-223 expression, RNAi against AML1/ETO, or demethylating treatment enhances miR-223 levels and restores cell differentiation. Here, we identify an additional action for a leukemia fusion protein linking the epigenetic silencing of a microRNA locus to the differentiation block of leukemia.

Recruitment of RXR by homotetrameric RARalpha fusion proteins is essential for transformation

While formation of higher-order oncogenic transcriptional complexes is critical for RARalpha fusion proteins in acute promyelocytic leukemia, the essential components and their roles in mediating transformation are still largely unknown. To this end, the present study demonstrates that homodimerization is not sufficient for RARalpha fusion-mediated transformation, which requires higher-order homotetramerization. Surprisingly, intrinsic homo-oligomeric DNA binding by the fusion proteins is also dispensable. Importantly, higher-order RXR/RARalpha fusion hetero-oligomeric complexes that aberrantly recruit transcriptional corepressors to downstream targets are essential for transformation. Intervention of RXR-dependent pathways by panRXR-agonists or RXRalpha shRNAs suppresses RARalpha fusion-mediated transformation. Taken together, these results define the oncogenic threshold for self-association and reveal the pathological significance of higher-order RARalpha fusion/RXR hetero-oligomeric complexes and their potential value as a therapeutic target.

A TAF4-homology domain from the corepressor ETO is a docking platform for positive and negative regulators of transcription

The eight twenty-one protein, ETO, is implicated in 12%-15% of acute human leukemias as part of a gene fusion with RUNX1 (also called AML1). Of the four ETO domains related to Drosophila melanogaster Nervy, only two are required to induce spontaneous myeloid leukemia upon transplantation into the mouse. One of these domains is related in sequence to TAF4, a component of TFIID. The structure of this domain, ETO-TAFH, is similar to yeast Rpb4 and to Escherichia coli sigma(70); it is the first TAF-related protein with structural similarity to the multisubunit RNA polymerases. Overlapping surfaces of ETO-TAFH interact with an autonomous repression domain of the nuclear receptor corepressor N-CoR and with a conserved activation domain from the E-box family of transcription factors. Thus, ETO-TAFH acts as a structural platform that can interchange negative and positive coregulatory proteins to control transcription.

Structural basis for recognition of SMRT/N-CoR by the MYND domain and its contribution to AML1/ETO's activity

AML1/ETO results from the t(8;21) associated with 12%-15% of acute myeloid leukemia. The AML1/ETO MYND domain mediates interactions with the corepressors SMRT and N-CoR and contributes to AML1/ETO's ability to repress proliferation and differentiation of primary bone marrow cells as well as to enhance their self renewal in vitro. We solved the solution structure of the MYND domain and show it to be structurally homologous to the PHD and RING finger families of proteins. We also determined the solution structure of an MYND-SMRT peptide complex. We demonstrated that a single amino acid substitution that disrupts the interaction between the MYND domain and the SMRT peptide attenuated AML1/ETO's effects on proliferation, differentiation, and gene expression.

RUNX1-RUNX1 Homodimerization Modulates RUNX1 Activity and Function

RUNX1 (AML1, CBFalpha2, PEBP2alphaB) is a transcription factor essential for the establishment of the hematopoietic stem cell. It is generally thought that RUNX1 exists as a monomer that regulates hematopoietic differentiation by interacting with tissue-specific factors and its DNA consensus through its N terminus. RUNX1 is frequently altered in human leukemia by gene fusions or point mutations. In general, these alterations do not affect the N terminus of the protein, and it is unclear how they consistently lead to hematopoietic transformation and leukemia. Here we report that RUNX1 homodimerizes through a mechanism involving C terminus-C terminus interaction. This RUNX1-RUNX1 interaction regulates the activity of the protein in reporter gene assays and modulates its ability to induce hematopoietic differentiation of hematopoietic cell lines. The promoters of genes regulated by RUNX1 often contain multiple RUNX1 binding sites. This arrangement suggests that RUNX1 could homodimerize to bring and hold together distant chromatin sites and factors and that if the dimerization region is removed by gene fusions or is altered by point mutations, as observed in leukemia, the ability of RUNX1 to regulate differentiation could be impaired.

Acute myeloid leukemia with the 8q22;21q22 translocation: secondary mutational events and alternative t(8;21) transcripts

Nonrandom and somatically acquired chromosomal translocations can be identified in nearly 50% of human acute myeloid leukemias. One common chromosomal translocation in this disease is the 8q22;21q22 translocation. It involves the AML1 (RUNX1) gene on chromosome 21 and the ETO (MTG8, RUNX1T1) gene on chromosome 8 generating the AML1-ETO fusion proteins. In this review, we survey recent advances made involving secondary mutational events and alternative t(8;21) transcripts in relation to understanding AML1-ETO leukemogenesis.

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.

The p21Waf1 pathway is involved in blocking leukemogenesis by the t(8;21) fusion protein AML1-ETO

The 8;21 translocation is a major contributor to acute myeloid leukemia (AML) of the M2 classification occurring in approximately 40% of these cases. Multiple mouse models using this fusion protein demonstrate that AML1-ETO requires secondary mutagenic events to promote leukemogenesis. Here, we show that the negative cell cycle regulator p21(WAF1) gene is up-regulated by AML1-ETO at the protein, RNA, and promoter levels. Retroviral transduction and hematopoietic cell transplantation experiments with p21(WAF1)-deficient cells show that AML1-ETO is able to promote leukemogenesis in the absence of p21(WAF1). Thus, loss of p21(WAF1) facilitates AML1-ETO-induced leukemogenesis, suggesting that mutagenic events in the p21(WAF1) pathway to bypass the growth inhibitory effect from AML1-ETO-induced p21(WAF1) expression can be a significant factor in AML1-ETO-associated acute myeloid leukemia.

A distinct epigenetic signature at targets of a leukemia protein

Background

Human myelogenous leukemia characterized by either the non random t(8; 21)(q22; q22) or t(16; 21)(q24; q22) chromosome translocations differ for both their biological and clinical features. Some of these features could be consequent to differential epigenetic transcriptional deregulation at AML1 targets imposed by AML1-MTG8 and AML1-MTG16, the fusion proteins deriving from the two translocations. Preliminary findings showing that these fusion proteins lead to transcriptional downregulation of AML1 targets, marked by repressive chromatin changes, would support this hypothesis. Here we show that combining conventional global gene expression arrays with the power of bioinformatic genomic survey of AML1-consensus sequences is an effective strategy to identify AML1 targets whose transcription is epigenetically downregulated by the leukemia-associated AML1-MTG16 protein.

Results

We interrogated mouse gene expression microarrays with probes generated either from 32D cells infected with a retroviral vector carrying AML1-MTG16 and unable of granulocyte differentiation and proliferation in response to the granulocyte colony stimulating factor (G-CSF), or from 32D cells infected with the cognate empty vector. From the analysis of differential gene expression alone (using as criteria a p value < 0.01 and an absolute fold change > 3), we were unable to conclude which of the 37 genes downregulated by AML1-MTG16 were, or not, direct AML1 targets. However, when we applied a bioinformatic approach to search for AML1-consensus sequences in the 10 Kb around the gene transcription start sites, we closed on 17 potential direct AML1 targets. By focusing on the most significantly downregulated genes, we found that both the AML1-consensus and the transcription start site chromatin regions were significantly marked by aberrant repressive histone tail changes. Further, the promoter of one of these genes, containing a CpG island, was aberrantly methylated.

Conclusion

This study shows that a leukemia-associated fusion protein can impose a distinct epigenetic repressive signature at specific sites in the genome. These findings strengthen the conclusion that leukemia-specific oncoproteins can induce non-random epigenetic changes.

Heterochromatic gene repression of the retinoic acid pathway in acute myeloid leukemia

Alteration of lineage-specific transcriptional programs for hematopoiesis causes differentiation block and promotes leukemia development. Here, we show that AML1/ETO, the most common translocation fusion product in acute myeloid leukemia (AML), counteracts the activity of retinoic acid (RA), a transcriptional regulator of myelopoiesis. AML1/ETO participates in a protein complex with the RA receptor alpha (RARalpha) at RA regulatory regions on RARbeta2, which is a key RA target gene mediating RA activity/resistance in cells. At these sites, AML1/ETO recruits histone deacetylase, DNA methyltransferase, and DNA-methyl-CpG binding activities that promote a repressed chromatin conformation. The link among AML1/ETO, heterochromatic RARbeta2 repression, RA resistance, and myeloid differentiation block is indicated by the ability of either siRNA-AML1/ETO or the DNA methylation inhibitor 5-azacytidine to revert these epigenetic alterations and to restore RA differentiation response in AML1/ETO blasts. Finally, RARbeta2 is commonly silenced by hypermethylation in primary AML blasts but not in normal hematopoietic precursors, thus suggesting a role for the epigenetic repression of the RA signaling pathway in myeloid leukemogenesis.