Nuclear import and subnuclear localization of the proto-oncoprotein ETO (MTG8)

RUNX1T1, a potential prognostic marker in breast cancer, is co-ordinately expressed with ERα, and regulated by estrogen receptor signalling in breast cancer cells

BACKGROUND

RUNX1T1 is extensively studied in the context of AML1-RUNX1T1 fusion protein in acute myeloid leukemia. Little is known about the function of RUNX1T1 itself, although data on its function and regulation have begun to emerge from clinical, and in vitro studies. It is a putative tumor suppressor, whose expression is altered in a variety of solid tumors. Recently, reduced expression of RUNX1T1 in triple-negative breast tumors, and its influence on prognosis was reported.

METHODS AND RESULTS

The Kaplan-Meier Plotter online tool was used to study the relationship between RUNX1T1 expression and survival of breast cancer patients. High RUNX1T1 expression was associated with longer overall survival (OS), relapse-free survival (RFS) and distant metastasis free survival (DMFS). RUNX1T1 expression positively and negatively influenced OS of patients with ERα-positive and ERα-negative breast tumors, respectively. It was also associated with prolonged RFS, and DMFS in tamoxifen-treated patients. Expression of RUNX1T1 and ERα mRNA was analyzed in 40 breast tumor samples, and breast cancer cell lines using RT-PCR. TCGA-BRCA data was mined to study the relationship between RUNX1T1 and ERα mRNA expression. ERα-positive breast tumors showed significantly higher RUNX1T1 mRNA expression compared to ERα-negative tumors. RUNX1T1 mRNA expression was analyzed by qRT-PCR in MCF-7 or T47D cells, which were treated with 17β-estradiol, or the ERα agonist PPT, alone or in combination with 4-hydroxytamoxifen. Effect of ERα knockdown was also investigated. Results indicate that estrogen downmodulated RUNX1T1 mRNA expression via ERα.

CONCLUSION

Higher expression of RUNX1T1 in breast tumors is associated with favourable prognosis. RUNX1T1 and ERα show co-ordinated expression in breast tumors, and breast cancer cell lines. Estrogen-ERα signalling downmodulates the expression of RUNX1T1 mRNA in ERα-positive breast cancer cells. In-depth investigations on the interaction between RUNX1T1 and ERα are warranted to unravel the role and relevance of RUNX1T1 in breast cancer.

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.

Targeting nuclear import and export in hematological malignancies

The transport of proteins across the nuclear membrane is a highly regulated process, essential for the cell function. This transport is actively mediated by members of the karyopherin family, termed importins, or exportins, depending on the direction of transport. These proteins play an active part in tumorigenesis, through aberrant localization of their cargoes, which include oncogenes, tumor-suppressor genes and mediators of key signal transduction pathways. Overexpression of importins and exportins is reported in many malignancies, with implications in cell growth and viability, differentiation, drug resistance, and tumor microenvironment. Given their broad significance across tumors and pathways, much effort is being put to develop specific inhibitors as a novel anticancer therapeutics. Already, selinexor, a specific inhibitor of exportin-1 (XPO1), is approved for clinical use. This review will focus on the role of importins and exportins in hematological malignancies. We will discuss current preclinical and clinical data on importins and exportins, and demonstrate how our growing understanding of their functions has identified new therapeutic targets.

RUNX1-ETO: Attacking the Epigenome for Genomic Instable Leukemia

Oncogenic fusion protein RUNX1-ETO is the product of the t(8;21) translocation, responsible for the most common cytogenetic subtype of acute myeloid leukemia. RUNX1, a critical transcription factor in hematopoietic development, is fused with almost the entire ETO sequence with the ability to recruit a wide range of repressors. Past efforts in providing a comprehensive picture of the genome-wide localization and the target genes of RUNX1-ETO have been inconclusive in understanding the underlying mechanism by which it deregulates native RUNX1. In this review; we dissect the current data on the epigenetic impact of RUNX1 and RUNX1-ETO. Both share similarities however, in recent years, research focused on epigenetic factors to explain their differences. RUNX1-ETO impairs DNA repair mechanisms which compromises genomic stability and favors a mutator phenotype. Among an increasing pool of mutated factors, regulators of DNA methylation are frequently found in t(8;21) AML. Together with the alteration of both, histone markers and distal enhancer regulation, RUNX1-ETO might specifically disrupt normal chromatin structure. Epigenetic studies on the fusion protein uncovered new mechanisms contributing to leukemogenesis and hopefully will translate into clinical applications.

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.

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.

Physical and functional interaction of Runt-related protein 1 with hypoxia-inducible factor-1alpha

Angiogenesis and hematopoiesis are closely linked and interactive with each other, but few studies were given to identify possible links between angiogenesis-promoting proteins and hematopoiesis-related transcription factors. Here we investigated the potential relationship of oxygen-sensitive alpha-subunit of angiogenesis-related hypoxia-inducible factor-1alpha (HIF-1alpha) with Runt-related protein 1 (Runx1, also known as acute myeloid leukemia-1, AML-1), an important hematopoietic transcription factor. The results demonstrated that Runx1 and HIF-1alpha proteins directly interacted with each other to a degree, in which Runt homology domain of Runx1 was mainly involved. Leukemia-related abnormal Runx1 fusion protein AML1-ETO, which fuses the N-terminal 177 amino acid residues of the Runx1 protein in frame to ETO (eight-twenty-one) protein, also interacted with HIF-1alpha protein with greater ability than Runx1 itself. More intriguingly, Runx1 overexpression inhibited DNA-binding and transcriptional activity of HIF-1 protein with reduced expression of HIF-1-targeted genes such as vascular endothelial growth factor, while silence of Runx1 expression by specific small interfering RNA significantly increased transcriptional activity of HIF-1 protein, suggesting that Runx1 inhibited transcription-dependent function of HIF-1. Vice versa, HIF-1alpha increased DNA-binding ability and transcriptional activity of Runx1 protein. All these data would shed new insight to understanding Runx1 and HIF-1alpha-related hematopoietic cell differentiation and angiogenesis.

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.

ETO/MTG8 is an inhibitor of C/EBPbeta activity and a regulator of early adipogenesis

The putative transcriptional corepressor ETO/MTG8 has been extensively studied due to its involvement in a chromosomal translocation causing the t(8;21) form of acute myeloid leukemia. Despite this, the role of ETO in normal physiology has remained obscure. Here we show that ETO is highly expressed in preadipocytes and acts as an inhibitor of C/EBPbeta during early adipogenesis, contributing to its characteristically delayed activation. ETO prevents both the transcriptional activation of the C/EBPalpha promoter by C/EBPbeta and its concurrent accumulation in centromeric sites during early adipogenesis. ETO expression rapidly reduces after the initiation of adipogenesis, and this is essential to the normal induction of adipogenic gene expression. These findings define, for the first time, a molecular role for ETO in normal physiology as an inhibitor of C/EBPbeta and a novel regulator of early adipogenesis.

A small-scale survey identifies selective and quantitative nucleo-cytoplasmic shuttling of a subset of CREM transcription factors

Elucidating dynamic aspects of intracellular localization of proteins is essential to decipher their functional interaction networks. Although transcription factors lacking a detectable cytoplasmic fraction have been generally considered compartmentalized in the nucleus, some were found to shuttle into the cytoplasm, suggesting functional interactions therein. To further investigate how common, specific and quantitative is this traffic, we have employed the heterokaryon assay for a small-scale survey of nuclear factors not previously tested for their nucleo-cytoplasmic motion. We show that a subset of cAMP response element (CRE) binding proteins of the CREM type shuttles within a biologically meaningful time frame, revealing a continuous flow into the cytoplasm that persists during signaling. Their dynamic behavior, not involving the classical Exportin-1 pathway, could be ascribed to C-terminal sequences, containing, in addition to the bZIP domain and the NLS, a nuclear export activity and an inhibitory activity at an adjacent site. Other proteins examined in this study either did not shuttle significantly or, like CREB and distinct CREM isoforms, shuttled with markedly delayed kinetics, denoting considerable selectivity of this traffic. These findings raise the possibility that events associated with bi-directional transport and periodic transit through the cytoplasm may modulate activities of select nuclear transcription factors.

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.

Gfi-1 attaches to the nuclear matrix, associates with ETO (MTG8) and histone deacetylase proteins, and represses transcription using a TSA-sensitive mechanism

Gfi-1 and Gfi-1B can repress transcription and play important roles in hematopoietic cell survival and differentiation. Although these proteins are known to bind DNA through a C-terminal zinc-finger domain and may require an N-terminal SNAG domain (SNAIL/Gfi-1) to repress transcription, the mechanism by which Gfi-1 and Gfi-1B act is unknown. A first step towards understanding the mechanism by which these proteins repress transcription is to identify interacting proteins that could contribute to transcriptional repression. ETO (also termed MTG8), was first identified through its involvement in the (8;21) translocation associated with acute myelogenous leukemia. It attaches to the nuclear matrix and associates with histone deacetylases and the co-repressors N-CoR, SMRT, and mSin3A, and may act as a co-repressor for site-specific transcriptions factors. In this report we demonstrate that Gfi-1 interacts with ETO and related proteins both in vitro and in vivo and with histone deacetylase proteins in vivo. We observed that a portion of Gfi-1 and Gfi-1B associated with the nuclear matrix, as is the case with ETO. Moreover, Gfi-1 and ETO co-localize to punctate subnuclear structures. When co-expressed in mammalian cells, Gfi-1 associates with histone deacetylse-1 (HDAC-1), HDAC-2, and HDAC-3. These data identify ETO as a partner for Gfi-1 and Gfi-1B, and suggest that Gfi-1 proteins repress transcription through recruitment of histone deacetylase-containing complexes.

The ETO (MTG8) gene family

Cloning and characterization of the 8;21 chromosomal breakpoint identified AML1 on chromosome 21 and ETO (MTG8) on chromosome 8, and the resultant chimeric gene product, AML-1/ETO. The ETO gene family now includes three human members encoding proteins composed of four evolutionarily conserved domains termed nervy homology regions (NHR) 1-4. ETO associates with N-CoR/Sin3a/HDAC complexes in vivo and acts as a corepressor for the promyelocytic zinc finger protein. Moreover, ETO is nuclear matrix attached at sites coincident with histone deacetylase enzymes and mSin3a. These data suggest that ETO proteins function as transcriptional corepressors. This review focuses on the ETO gene family in terms of expression and function. Specifically, the role of ETO as a co-repressor will be detailed. Additionally, the impact of this recent discovery on treatment of t(8;21)-containing leukemia will be discussed.

Multiple subnuclear targeting signals of the leukemia-related AML1/ETO and ETO repressor proteins

Leukemic disease can be linked to aberrant gene expression. This often is the result of molecular alterations in transcription factors that lead to their misrouting within the nucleus. The acute myelogenous leukemia-related fusion protein AML1ETO is a striking example. It originates from a gene rearrangement t(8;21) that fuses the N-terminal part of the key hematopoietic regulatory factor AML1 (RUNX1) to the ETO (MTG8) repressor protein. AML1ETO lacks the intranuclear targeting signal of the wild-type AML1 and is directed by the ETO component to alternate nuclear matrix-associated sites. To understand this aberrant subnuclear trafficking of AML1ETO, we created a series of mutations in the ETO protein. These were characterized biochemically by immunoblotting and in situ by immunofluorescence microscopy. We identified two independent subnuclear targeting signals in the N- and C-terminal regions of ETO that together direct ETO to the same binding sites occupied by AML1ETO. However, each segment alone is targeted to a different intranuclear location. The N-terminal segment contains a nuclear localization signal and the conserved hydrophobic heptad repeat domain responsible for protein dimerization and interaction with the mSin3A transcriptional repressor. The C-terminal segment spans the nervy domain and the zinc finger region, which together support interactions with the corepressors N-CoR and HDACs. Our findings provide a molecular basis for aberrant subnuclear targeting of the AML1ETO protein, which is a principal defect in t(8;21)-related acute myelogenous leukemia.

Transcription factor fusions in acute leukemia: variations on a theme

The leukemia-associated fusion proteins share several structural or functional similarities, suggesting that they may impart a leukemic phenotype through common modes of transcriptional dysregulation. The fusion proteins generated by these translocations usually contain a DNA-binding domain, domains responsible for homo- or hetero-dimerization, and domains that interact with proteins involved in chromatin remodeling (e.g., co-repressor molecules or co-activator molecules). It is these shared features that constitute the 'variations on the theme' that underling the aberrant growth and differentiation that is the hallmark of acute leukemia cells.

Analysis of the nuclear distribution of the translocation t(8;21)-derived fusion protein AML1/ETO by confocal laser scanning microscopy

The AML1/ETO protein derived from the t(8;21) translocation retains the DNA binding domain of AML1, the runt homology domain (RHD), and nearly the complete ETO protein with its four nervy homology regions (NHR1-4). To analyze which domains of AML1/ETO are responsible for its intranuclear transport and its subnuclear distribution, AML1/ETO deletion constructs tagged with green fluorescence protein were expressed transiently in 293 cells. The subcellular distribution was analyzed by confocal laser scanning microscopy. The nuclear localization signal (NLS) of AML1/ETO was mapped to a region encoded by the carboxy-terminal part of NHR1 and the sequences following up to NHR2 corresponding to the amino acids 304-489 of the AML1/ETO protein. A speckled subnuclear distribution was found with those constructs containing the NHR2 and/or the NHR3 and NHR4 domains. Co-localization with AML1/ETO was complete with constructs containing the NHR2 domain, indicating that NHR2 has a crucial role in the subnuclear distribution of AML1/ETO. Co-localization with AML1 seems to be supported by RHD, whereas the NHR3 and NHR4 regions possibly counterbalance this effect. Finally, AML1/ETO could not be co-localized with PML and SUMO-1, indicating that AML1/ETO is not part of the nuclear bodies and probably not SUMOylated.

Alterations in subnuclear trafficking of nuclear regulatory factors in acute leukemia

The nuclear matrix plays an important role in the functional organization of the nucleus in part by locally concentrating regulatory factors involved in nucleic acid metabolism. A number of nuclear regulatory proteins initially identified due to their involvement in human cancer are localized to discrete nuclear matrix-attached foci and correct nuclear partitioning likely plays a role in their function. Two such examples are promyelocytic leukemia (PML) and acute myelogenous leukemia-1 (AML-1; Runx1). PML, the target of the t(15;17) in acute PML, is localized to PML nuclear bodies (also termed Nuclear Domain 10 and PML oncogenic domains), a nuclear matrix-associated body whose function appears to be quite complex, with probable roles in cancer, apoptosis, and in acute viral infections. In t(15;17)-containing leukemic cells, the PML nuclear bodies are disrupted, but reform when the leukemic cells are induced to differentiate in the presence of all-trans retinoic acid. AML1 (RUNX1) is a key regulator of hematopoietic differentiation and AML1 proteins are found in nuclear compartments that reflect their roles in transcriptional activation and repression. The t(8;21), associated with AML, results in a chimeric transcription factor, AML-1/ETO (eight twenty one), that remains attached to the nuclear matrix through targeting signals contained in the ETO protein. When co-expressed, ETO and AML-1/ETO co-localize to a nuclear compartment distinct from that of AML1 or PML nuclear bodies. Interestingly, enforced expression of ETO or AML-1/ETO changes the average number of PML nuclear bodies per cell. Thus, chromosomal translocations involving AML1 result in altered nuclear trafficking of the transcription factor as well as other changes to the nuclear architecture. J. Cell. Biochem. Suppl. 35:93-98, 2000.