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

Machine Learning-Directed Conversion of Glioblastoma Cells to Dendritic Cell-Like Antigen-Presenting Cells as Cancer Immunotherapy

Immunotherapy has limited efficacy in glioblastoma (GBM) due to the blood-brain barrier and the immunosuppressed or "cold" tumor microenvironment (TME) of GBM, which is dominated by immune-inhibitory cells and depleted of CTL and dendritic cells (DC). Here, we report the development and application of a machine learning precision method to identify cell fate determinants (CFD) that specifically reprogram GBM cells into induced antigen-presenting cells with DC-like functions (iDC-APC). In murine GBM models, iDC-APCs acquired DC-like morphology, regulatory gene expression profile, and functions comparable to natural DCs. Among these acquired functions were phagocytosis, direct presentation of endogenous antigens, and cross-presentation of exogenous antigens. The latter endowed the iDC-APCs with the ability to prime naïve CD8+ CTLs, a hallmark DC function critical for antitumor immunity. Intratumor iDC-APCs reduced tumor growth and improved survival only in immunocompetent animals, which coincided with extensive infiltration of CD4+ T cells and activated CD8+ CTLs in the TME. The reactivated TME synergized with an intratumor soluble PD1 decoy immunotherapy and a DC-based GBM vaccine, resulting in robust killing of highly resistant GBM cells by tumor-specific CD8+ CTLs and significantly extended survival. Lastly, we defined a unique CFD combination specifically for the human GBM to iDC-APC conversion of both glioma stem-like cells and non-stem-like cell GBM cells, confirming the clinical utility of a computationally directed, tumor-specific conversion immunotherapy for GBM and potentially other solid tumors.

CBFA2T3::GLIS2 pediatric acute megakaryoblastic leukemia is sensitive to BCL-XL inhibition by navitoclax and DT2216

ABSTRACT

Acute megakaryoblastic leukemia (AMKL) is a rare, developmentally restricted, and highly lethal cancer of early childhood. The paucity and hypocellularity (due to myelofibrosis) of primary patient samples hamper the discovery of cell- and genotype-specific treatments. AMKL is driven by mutually exclusive chimeric fusion oncogenes in two-thirds of the cases, with CBFA2T3::GLIS2 (CG2) and NUP98 fusions (NUP98r) representing the highest-fatality subgroups. We established CD34+ cord blood-derived CG2 models (n = 6) that sustain serial transplantation and recapitulate human leukemia regarding immunophenotype, leukemia-initiating cell frequencies, comutational landscape, and gene expression signature, with distinct upregulation of the prosurvival factor B-cell lymphoma 2 (BCL2). Cell membrane proteomic analyses highlighted CG2 surface markers preferentially expressed on leukemic cells compared with CD34+ cells (eg, NCAM1 and CD151). AMKL differentiation block in the mega-erythroid progenitor space was confirmed by single-cell profiling. Although CG2 cells were rather resistant to BCL2 genetic knockdown or selective pharmacological inhibition with venetoclax, they were vulnerable to strategies that target the megakaryocytic prosurvival factor BCL-XL (BCL2L1), including in vitro and in vivo treatment with BCL2/BCL-XL/BCL-W inhibitor navitoclax and DT2216, a selective BCL-XL proteolysis-targeting chimera degrader developed to limit thrombocytopenia in patients. NUP98r AMKL were also sensitive to BCL-XL inhibition but not the NUP98r monocytic leukemia, pointing to a lineage-specific dependency. Navitoclax or DT2216 treatment in combination with low-dose cytarabine further reduced leukemic burden in mice. This work extends the cellular and molecular diversity set of human AMKL models and uncovers BCL-XL as a therapeutic vulnerability in CG2 and NUP98r AMKL.

CBFA2T3-GLIS2-dependent pediatric acute megakaryoblastic leukemia is driven by GLIS2 and sensitive to navitoclax

Pediatric acute megakaryoblastic leukemia (AMKL) is an aggressive blood cancer associated with poor therapeutic response and high mortality. Here we describe the development of CBFA2T3-GLIS2-driven mouse models of AMKL that recapitulate the phenotypic and transcriptional signatures of the human disease. We show that an activating Ras mutation that occurs in human AMKL increases the penetrance and decreases the latency of CBF2AT3-GLIS2-driven AMKL. CBFA2T3-GLIS2 and GLIS2 modulate similar transcriptional networks. We identify the dominant oncogenic properties of GLIS2 that trigger AMKL in cooperation with oncogenic Ras. We find that both CBFA2T3-GLIS2 and GLIS2 alter the expression of a number of BH3-only proteins, causing AMKL cell sensitivity to the BCL2 inhibitor navitoclax both in vitro and in vivo, suggesting a potential therapeutic option for pediatric patients suffering from CBFA2T3-GLIS2-driven AMKL.

The NCOR-HDAC3 co-repressive complex modulates the leukemogenic potential of the transcription factor ERG

The ERG (ETS-related gene) transcription factor is linked to various types of cancer, including leukemia. However, the specific ERG domains and co-factors contributing to leukemogenesis are poorly understood. Drug targeting a transcription factor such as ERG is challenging. Our study reveals the critical role of a conserved amino acid, proline, at position 199, located at the 3' end of the PNT (pointed) domain, in ERG's ability to induce leukemia. P199 is necessary for ERG to promote self-renewal, prevent myeloid differentiation in hematopoietic progenitor cells, and initiate leukemia in mouse models. Here we show that P199 facilitates ERG's interaction with the NCoR-HDAC3 co-repressor complex. Inhibiting HDAC3 reduces the growth of ERG-dependent leukemic and prostate cancer cells, indicating that the interaction between ERG and the NCoR-HDAC3 co-repressor complex is crucial for its oncogenic activity. Thus, targeting this interaction may offer a potential therapeutic intervention.

A neurogenetic mechanism of experience-dependent suppression of aggression

Aggression is an ethologically important social behavior, but excessive aggression can be detrimental to fitness. Social experiences among conspecific individuals reduce aggression in many species, the mechanism of which is largely unknown. We found that loss-of-function mutation of nervy (nvy), a Drosophila homolog of vertebrate myeloid translocation genes (MTGs), increased aggressiveness only in socially experienced flies and that this could be reversed by neuronal expression of human MTGs. A subpopulation of octopaminergic/tyraminergic neurons labeled by nvy was specifically required for such social experience-dependent suppression of aggression, in both males and females. Cell type-specific transcriptomic analysis of these neurons revealed aggression-controlling genes that are likely downstream of nvy. Our results illustrate both genetic and neuronal mechanisms by which the nervous system suppresses aggression in a social experience-dependent manner, a poorly understood process that is considered important for maintaining the fitness of animals.

CTCF: A novel fusion partner of ETO2 in a multiple relapsed acute myeloid leukemia patient

ETO2 is a nuclear co-repressor, which plays a critical role in the regulation of the cell cycle, self-renewal capacity, and differentiation of hematopoietic progenitor cells. We identified novel fusion transcripts involving ETO2 and CTCF by RNA-seq in a multiple relapsed AML case. The CTCF-ETO2 and ETO2-CTCF chimeric genes were validated by RT-PCR and Sanger sequencing. In addition, both transcripts apparently promoted cell proliferation via JAK/STAT3 pathway that is sensitive to STAT3 inhibitors. The novel fusions may have prognostic value and pathogenic mechanisms in acute myeloid leukemia.

The genomics of acute myeloid leukemia in children

Acute myeloid leukemia (AML) is a clinically, morphologically, and genetically heterogeneous disorder. Like many malignancies, the genomic landscape of pediatric AML has been mapped recently through sequencing of large cohorts of patients. Much has been learned about the biology of AML through studies of specific recurrent genetic lesions. Further, genetic lesions have been linked to specific clinical features, response to therapy, and outcome, leading to improvements in risk stratification. Lastly, targeted therapeutic approaches have been developed for the treatment of specific genetic lesions, some of which are already having a positive impact on outcomes. While the advances made based on the discoveries of sequencing studies are significant, much work is left. The biologic, clinical, and prognostic impact of a number of genetic lesions, including several seemingly unique to pediatric patients, remains undefined. While targeted approaches are being explored, for most, the efficacy and tolerability when incorporated into standard therapy is yet to be determined. Furthermore, the challenge of how to study small subpopulations with rare genetic lesions in an already rare disease will have to be considered. In all, while questions and challenges remain, precisely defining the genomic landscape of AML, holds great promise for ultimately leading to improved outcomes for affected patients.

The transcriptional corepressor CBFA2T3 inhibits all-trans-retinoic acid-induced myeloid gene expression and differentiation in acute myeloid leukemia

CBFA2/RUNX1 partner transcriptional co-repressor 3 (CBFA2T3, also known as MTG16 or ETO2) is a myeloid translocation gene family protein that functions as a master transcriptional corepressor in hematopoiesis. Recently, it has been shown that CBFA2T3 maintains leukemia stem cell gene expression and promotes relapse in acute myeloid leukemia (AML). However, a role for CBFA2T3 in myeloid differentiation of AML has not been reported. Here, we show that CBFA2T3 represses retinoic acid receptor (RAR) target gene expression and inhibits all-trans-retinoic acid (ATRA)-induced myeloid differentiation of AML cells. ChIP-Seq revealed that CBFA2T3 targets the RARα/RXRα cistrome in U937 AML cells, predominantly at myeloid-specific enhancers associated with terminal differentiation. CRISPR/Cas9-mediated abrogation of CBFA2T3 resulted in spontaneous and ATRA-induced activation of myeloid-specific genes in a manner correlated with myeloid differentiation. Importantly, these effects were reversed by CBFA2T3 re-expression. Mechanistic studies showed that CBFA2T3 inhibits RAR target gene transcription by acting at an early step to regulate histone acetyltransferase recruitment, histone acetylation, and chromatin accessibility at RARα target sites, independently of the downstream, RAR-mediated steps of transcription. Finally, we validated the inhibitory effect of CBFA2T3 on RAR in multiple AML subtypes and patient samples. To our knowledge, this is the first study to show that CBFA2T3 down-regulation is both necessary and sufficient for enhancing ATRA-induced myeloid gene expression and differentiation of AML cells. Our findings suggest that CBFA2T3 can serve as a potential target for enhancing AML responsiveness to ATRA differentiation therapies.

Emerging Roles of GLI-Similar Krüppel-like Zinc Finger Transcription Factors in Leukemia and Other Cancers

GLI-similar 1-3 (GLIS1-3), a subfamily of Krüppel-like zinc finger transcription factors, function as key regulators of several biological processes important to oncogenesis, including control of cell proliferation, differentiation, self-renewal, and epithelial-mesenchymal transition. This review provides a short overview of the critical roles genetic changes in GLIS1-3 play in the development of several malignancies. This includes intrachromosomal translocations involving GLIS2 and ETO2/CBFA2T3 in the development of pediatric non-Down's syndrome (DS), acute megakaryoblastic leukemia (AMKL), a malignancy with poor prognosis, and an association of interchromosomal translocations between GLIS3, GLIS1, and PAX8, and between GLIS3 and CLPTM1L with hyalinizing trabecular tumors (HTTs) and fibrolamellar hepatocellular carcinoma (FHCC), respectively. Targeting upstream signaling pathways that regulate GLIS signaling may offer new therapeutic strategies in the management of cancer.

The Pluripotency Regulator PRDM14 Requires Hematopoietic Regulator CBFA2T3 to Initiate Leukemia in Mice

PR domain-containing 14 (Prdm14) is a pluripotency regulator central to embryonic stem cell identity and primordial germ cell specification. Genomic regions containing PRDM14 are often amplified leading to misexpression in human cancer. Prdm14 expression in mouse hematopoietic stem cells (HSC) leads to progenitor cell expansion prior to the development of T-cell acute lymphoblastic leukemia (T-ALL), consistent with PRDM14's role in cancer initiation. Here, we demonstrate mechanistic insight into PRDM14-driven leukemias in vivo. Mass spectrometry revealed novel PRDM14-protein interactions including histone H1, RNA-binding proteins, and the master hematopoietic regulator CBFA2T3. In mouse leukemic cells, CBFA2T3 and PRDM14 associate independently of the related ETO family member CBFA2T2, PRDM14's primary protein partner in pluripotent cells. CBFA2T3 plays crucial roles in HSC self-renewal and lineage commitment, and participates in oncogenic translocations in acute myeloid leukemia. These results suggest a model whereby PRDM14 recruits CBFA2T3 to DNA, leading to gene misregulation causing progenitor cell expansion and lineage perturbations preceding T-ALL development. Strikingly, Prdm14-induced T-ALL does not occur in mice deficient for Cbfa2t3, demonstrating that Cbfa2t3 is required for leukemogenesis. Moreover, T-ALL develops in Cbfa2t3 heterozygotes with a significantly longer latency, suggesting that PRDM14-associated T-ALL is sensitive to Cbfa2t3 levels. Our study highlights how an oncogenic protein uses a native protein in progenitor cells to initiate leukemia, providing insight into PRDM14-driven oncogenesis in other cell types. IMPLICATIONS: The pluripotency regulator PRDM14 requires the master hematopoietic regulator CBFA2T3 to initiate leukemia in progenitor cells, demonstrating an oncogenic role for CBFA2T3 and providing an avenue for targeting cancer-initiating cells.

The changing scenario of non-Down syndrome acute megakaryoblastic leukemia in children

Pediatric non-Down-syndrome acute megakaryoblastic leukemia (non-DS-AMKL) is a heterogeneous subtype of leukemia that has historically been associated with poor prognosis. Until the advent of large-scale genomic sequencing, the management of patients with non-DS-AMKL was very difficult due to the absence of reliable biological prognostic markers. The sequencing of large cohort of pediatric non-DS-AMKL samples led to the discovery of novel genetic aberrations, including high-frequency fusions, such as CBFA2T3-GLIS2 and NUP98-KDM5 A, as well as less frequent aberrations, such as HOX rearrangements. These new insights into the genetic landscape of pediatric non-DS-AMKL has allowed refining the risk-group stratification, leading to important changes in the prognostic scenario of these patients. This review summarizes the most important molecular pathogenic mechanisms of pediatric non-DS-AMKL. A critical discussion on how novel genetic abnormalities have refined the risk profile assessment and changed the management of these patients in clinical practice is also provided.

CBFA2T3-GLIS2-positive acute myeloid leukaemia. A peculiar paediatric entity

The scenario of paediatric acute myeloid leukaemia (AML), particularly non‐Down syndrome acute megakaryoblastic leukaemia (non‐DS‐AMKL), has been recently revolutionized by the advent of large‐scale, genomic sequencing technologies. In this changing landscape, a significantly relevant discovery has been represented by the identification of the CBFA2T3‐GLIS2 fusion gene, which is the result of a cryptic inversion of chromosome 16. It is the most frequent chimeric oncogene identified to date in non‐DS‐AMKL, although it seems not to be exclusively restricted to the French‐American‐British M7 subgroup. The CBFA2T3‐GLIS2 fusion gene characterizes a subtype of leukaemia that is specific to paediatrics, having never been identified in adults. It characterizes an extremely aggressive leukaemia, as the presence of this fusion is associated with a grim outcome in almost all of the case series reported, with overall survival rates ranging between 15% and 30%. Although the molecular basis that underlies this leukaemia subtype is still far from being completely elucidated, unique functional properties induced by CBFA2T3‐GLIS2 in the leukaemogenesis driving process have been recently identified. We here review the peculiarities of CBFA2T3‐GLIS2‐positive AML, describing its intriguing clinical and biological behaviour and providing some challenging targeting opportunities.

[Pediatric de novo acute megakaryoblastic leukemia: an affair of complexes]

Pediatric acute megakaryoblastic leukemia (AMKL) are generally associated with poor prognosis and the expression of fusion oncogenes involving transcriptional regulators. Recent results indicate that the ETO2-GLIS2 fusion, associated with 25-30 % of pediatric AMKL, binds and alters the activity of regulatory regions of gene expression, called "enhancers", resulting in the deregulation of GATA and ETS factors essential for the development of hematopoietic stem cells. An imbalance in GATA/ETS factor activity is also found in other AMKL subgroups. This review addresses the transcriptional bases of transformation in pediatric AMKL and therapeutic perspectives.

Splenic hematopoietic stem cells display a pre-activated phenotype

Splenic hematopoiesis is crucial to the pathogenesis of diseases including myocardial infarction and atherosclerosis. The spleen acts as a reservoir of myeloid cells, which are quickly expelled out in response to acute inflammation. In contrast to the well-defined bone marrow hematopoiesis, the cellular and molecular components sustaining splenic hematopoiesis are poorly understood. Surprisingly, we found that, unlike quiescent bone marrow hematopoietic stem cells (HSC), most of splenic HSC are in the G1 phase in C57BL/6 mice. Moreover, splenic HSC were enriched for genes involved in G0-G1 transition and expressed lower levels of genes responsible for G1-S transition. These data indicate that, at steady state, splenic HSC are pre-activated, which may expedite their cell cycle entry in emergency conditions. Consistently, in the acute phase of septic shock induced by LPS injection, splenic HSC entered the S-G2-M phase, whereas bone marrow HSC did not. Mobilization and transplantation experiments displayed that bone marrow HSC, once in the spleen, acquired cell cycle status similar to splenic HSC, strongly suggesting that the splenic microenvironment plays an important role in HSC pre-activation. In addition, we found that myeloid translocation gene 16 (Mtg16) deficiency in C57BL/6 mice resulted in significantly increased S-G2-M entry of splenic but not bone marrow HSC, suggesting that Mtg16 is an intrinsic negative regulator of G1-S transition in splenic HSC. Altogether, this study demonstrates that compared to bone marrow, splenic HSC are in a pre-activated state, which is driven by extracellular signals provided by splenic microenvironment and HSC intrinsic factor Mtg16.

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.

Pediatric Acute Megakaryoblastic Leukemia: Multitasking Fusion Proteins and Oncogenic Cooperations

Pediatric leukemia presents specific clinical and genetic features from adult leukemia but the underpinning mechanisms of transformation are still unclear. Acute megakaryoblastic leukemia (AMKL) is the malignant accumulation of progenitors of the megakaryocyte lineage that normally produce blood platelets. AMKL is diagnosed de novo, in patients showing a poor prognosis, or in Down syndrome (DS) patients with a better prognosis. Recent data show that de novo AMKL is primarily associated with chromosomal alterations leading to the expression of fusions between transcriptional regulators. This review highlights the most recurrent genetic events found in de novo pediatric AMKL patients and, based on recent functional analyses, proposes a mechanism of leukemogenesis common to de novo and DS-AMKL.

MTG16 is a tumor suppressor in colitis-associated carcinoma

MTG16 is a member of the myeloid translocation gene (MTG) family of transcriptional corepressors. While MTGs were originally identified in chromosomal translocations in acute myeloid leukemia, recent studies have uncovered a role in intestinal biology. For example, Mtg16-/- mice have increased intestinal proliferation and are more sensitive to intestinal injury in colitis models. MTG16 is also underexpressed in patients with moderate/severe ulcerative colitis. Based on these findings, we postulated that MTG16 might protect against colitis-associated carcinogenesis. MTG16 was downregulated at the protein and RNA levels in patients with inflammatory bowel disease and in those with colitis-associated carcinoma. Mtg16-/- mice subjected to inflammatory carcinogenesis modeling exhibited worse colitis and increased tumor multiplicity and size. Loss of MTG16 also increased severity of dysplasia, apoptosis, proliferation, DNA damage, and WNT signaling. Moreover, transplantation of WT marrow into Mtg16-/- mice failed to rescue the Mtg16-/- protumorigenic phenotypes, indicating an epithelium-specific role for MTG16. While MTG dysfunction is widely appreciated in hematopoietic malignancies, the role of this gene family in epithelial homeostasis, and in colon cancer, was unrealized. This report identifies MTG16 as an important modulator of colitis and tumor development in inflammatory carcinogenesis.

ETO2-GLIS2 Hijacks Transcriptional Complexes to Drive Cellular Identity and Self-Renewal in Pediatric Acute Megakaryoblastic Leukemia

Chimeric transcription factors are a hallmark of human leukemia, but the molecular mechanisms by which they block differentiation and promote aberrant self-renewal remain unclear. Here, we demonstrate that the ETO2-GLIS2 fusion oncoprotein, which is found in aggressive acute megakaryoblastic leukemia, confers megakaryocytic identity via the GLIS2 moiety while both ETO2 and GLIS2 domains are required to drive increased self-renewal properties. ETO2-GLIS2 directly binds DNA to control transcription of associated genes by upregulation of expression and interaction with the ETS-related ERG protein at enhancer elements. Importantly, specific interference with ETO2-GLIS2 oligomerization reverses the transcriptional activation at enhancers and promotes megakaryocytic differentiation, providing a relevant interface to target in this poor-prognosis pediatric leukemia.

Changing mutational and adaptive landscapes and the genesis of cancer

By the time the process of oncogenesis has produced an advanced cancer, tumor cells have undergone extensive evolution. The cellular phenotypes resulting from this evolution have been well studied, and include accelerated growth rates, apoptosis resistance, immortality, invasiveness, and immune evasion. Yet with all of our current knowledge of tumor biology, the details of early oncogenesis have been difficult to observe and understand. Where different oncogenic mutations may work together to enhance the survival of a tumor cell, in isolation they are often pro-apoptotic, pro-differentiative or pro-senescent, and therefore often, somewhat paradoxically, disadvantageous to a cell. It is also becoming clear that somatic mutations, including those in known oncogenic drivers, are common in tissues starting at a young age. These observations raise the question: how do we largely avoid cancer for most of our lives? Here we propose that evolutionary forces can help explain this paradox. As humans and other organisms age or experience external insults such as radiation or smoking, the structure and function of tissues progressively degrade, resulting in altered stem cell niche microenvironments. As tissue integrity declines, it becomes less capable of supporting and maintaining resident stem cells. These stem cells then find themselves in a microenvironment to which they are poorly adapted, providing a competitive advantage to those cells that can restore their functionality and fitness through mutations or epigenetic changes. The resulting oncogenic clonal expansions then increase the odds of further cancer progression. Understanding how the causes of cancer, such as aging or smoking, affect tissue microenvironments to control the impact of mutations on somatic cell fitness can help reconcile the discrepancy between marked mutation accumulation starting early in life and the somatic evolution that leads to cancer at advanced ages or following carcinogenic insults. This article is part of a Special Issue entitled: Evolutionary principles - heterogeneity in cancer?, edited by Dr. Robert A. Gatenby.

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.

Control of developmentally primed erythroid genes by combinatorial co-repressor actions

How transcription factors (TFs) cooperate within large protein complexes to allow rapid modulation of gene expression during development is still largely unknown. Here we show that the key haematopoietic LIM-domain-binding protein-1 (LDB1) TF complex contains several activator and repressor components that together maintain an erythroid-specific gene expression programme primed for rapid activation until differentiation is induced. A combination of proteomics, functional genomics and in vivo studies presented here identifies known and novel co-repressors, most notably the ETO2 and IRF2BP2 proteins, involved in maintaining this primed state. The ETO2–IRF2BP2 axis, interacting with the NCOR1/SMRT co-repressor complex, suppresses the expression of the vast majority of archetypical erythroid genes and pathways until its decommissioning at the onset of terminal erythroid differentiation. Our experiments demonstrate that multimeric regulatory complexes feature a dynamic interplay between activating and repressing components that determines lineage-specific gene expression and cellular differentiation.

Conserved sets of transcription factors (TFs) regulate hematopoiesis. Here, Stadhouders et al. show that IRF2BP2 is a component of the LDB1 TF complex and together with its co-repressor ETO2, enhances transcriptional repression, which plays a crucial role at the erythroid progenitor stage.

Hhex is Required at Multiple Stages of Adult Hematopoietic Stem and Progenitor Cell Differentiation

Hhex encodes a homeodomain transcription factor that is widely expressed in hematopoietic stem and progenitor cell populations. Its enforced expression induces T-cell leukemia and we have implicated it as an important oncogene in early T-cell precursor leukemias where it is immediately downstream of an LMO2-associated protein complex. Conventional Hhex knockouts cause embryonic lethality precluding analysis of adult hematopoiesis. Thus, we induced highly efficient conditional knockout (cKO) using vav-Cre transgenic mice. Hhex cKO mice were viable and born at normal litter sizes. At steady state, we observed a defect in B-cell development that we localized to the earliest B-cell precursor, the pro-B-cell stage. Most remarkably, bone marrow transplantation using Hhex cKO donor cells revealed a more profound defect in all hematopoietic lineages. In contrast, sublethal irradiation resulted in normal myeloid cell repopulation of the bone marrow but markedly impaired repopulation of T- and B-cell compartments. We noted that Hhex cKO stem and progenitor cell populations were skewed in their distribution and showed enhanced proliferation compared to WT cells. Our results implicate Hhex in the maintenance of LT-HSCs and in lineage allocation from multipotent progenitors especially in stress hematopoiesis.

ANTI-MTG16 ANTIBODIES REVEAL MTG16 SUBCELLULAR DISTRIBUTION AND NUCLEOCYTOPLASMIC TRANSPORT IN ERYTHROLEUKEMIA CELLS

The myeloid translocation gene (MTG) family of transcriptional co-repressors consists of three highly conserved members; MTG8, MTG16 and MTGR1, each evolutionarily related to the Drosophila protein NERVY and with orthologs across the mammalian hierarchy. By coordinating coincident interactions between DNA binding proteins, other co-repressors and epigenetic effectors, MTG proteins occupy a critical nexus in transcriptional control complexes to profoundly impact the specification of cell fate. MTG family members are most conserved within Nervy Homology Regions (NHR) 1-4, with each region fulfilling functions common to the family. Studies of functional differences between MTG proteins require carefully qualified immunologic reagents specific to each family member. We have developed a group of α-MTG16 antibodies and carefully characterized their specificity for MTG16. These tools reveal that MTG16 is concentrated in the cytoplasm of erythroleukemia cell lines from human and mouse. Using the CRM1 antagonist, leptomycin-B, we show that MTG16 levels rise in the nucleus of MEL cells and decline in the cytoplasm. Together, these data indicate bidirectional movement of MTG16 between cytoplasmic and nuclear compartments. Our work reveals an unrecognized feature of MTG16 regulation that may impact cell fate specification and provides reagents to address important questions regarding MTG16 functions in vivo.

ETO family protein Mtg16 regulates the balance of dendritic cell subsets by repressing Id2

Dendritic cells (DCs) comprise two major subsets, the interferon (IFN)-producing plasmacytoid DCs (pDCs) and antigen-presenting classical DCs (cDCs). The development of pDCs is promoted by E protein transcription factor E2-2, whereas E protein antagonist Id2 is specifically absent from pDCs. Conversely, Id2 is prominently expressed in cDCs and promotes CD8(+) cDC development. The mechanisms that control the balance between E and Id proteins during DC subset specification remain unknown. We found that the loss of Mtg16, a transcriptional cofactor of the ETO protein family, profoundly impaired pDC development and pDC-dependent IFN response. The residual Mtg16-deficient pDCs showed aberrant phenotype, including the expression of myeloid marker CD11b. Conversely, the development of cDC progenitors (pre-DCs) and of CD8(+) cDCs was enhanced. Genome-wide expression and DNA-binding analysis identified Id2 as a direct target of Mtg16. Mtg16-deficient cDC progenitors and pDCs showed aberrant induction of Id2, and the deletion of Id2 facilitated the impaired development of Mtg16-deficient pDCs. Thus, Mtg16 promotes pDC differentiation and restricts cDC development in part by repressing Id2, revealing a cell-intrinsic mechanism that controls subset balance during DC development.

Tgif1 regulates quiescence and self-renewal of hematopoietic stem cells

TG-interacting factor 1 (TGIF1) is a transcriptional repressor that can modulate retinoic acid and transforming growth factor β signaling pathways. It is required for myeloid progenitor cell differentiation and survival, and mutations in the TGIF1 gene cause holoprosencephaly. Furthermore, we have previously observed that acute myelogenous leukemia (AML) patients with low TGIF1 levels had worse prognoses. Here, we explored the role of Tgif1 in murine hematopoietic stem cell (HSC) function. CFU assays showed that Tgif1(-/-) bone marrow cells produced more total colonies and had higher serial CFU potential. These effects were also observed in vivo, where Tgif1(-/-) bone marrow cells had higher repopulation potential in short- and long-term competitive repopulation assays than wild-type cells. Serial transplantation and replating studies showed that Tgif1(-/-) HSCs exhibited greater self-renewal and were less proliferative and more quiescent than wild-type cells, suggesting that Tgif1 is required for stem cells to enter the cell cycle. Furthermore, HSCs from Tgif1(+/-) mice had a phenotype similar to that of HSCs from Tgif1(-/-) mice, while bone marrow cells with overexpressing Tgif1 showed increased proliferation and lower survival in long-term transplant studies. Taken together, our data suggest that Tgif1 suppresses stem cell self-renewal and provide clues as to how reduced expression of TGIF1 may contribute to poor long-term survival in patients with AML.

Differential involvement of E2A-corepressor interactions in distinct leukemogenic pathways

E2A is a member of the E-protein family of transcription factors. Previous studies have reported context-dependent regulation of E2A-dependent transcription. For example, whereas the E2A portion of the E2A-Pbx1 leukemia fusion protein mediates robust transcriptional activation in t(1;19) acute lymphoblastic leukemia, the transcriptional activity of wild-type E2A is silenced by high levels of corepressors, such as the AML1-ETO fusion protein in t(8;21) acute myeloid leukemia and ETO-2 in hematopoietic cells. Here, we show that, unlike the HEB E-protein, the activation domain 1 (AD1) of E2A has specifically reduced corepressor interaction due to E2A-specific amino acid changes in the p300/CBP and ETO target motif. Replacing E2A-AD1 with HEB-AD1 abolished the ability of E2A-Pbx1 to activate target genes and to induce cell transformation. On the other hand, the weak E2A-AD1-corepressor interaction imposes a critical importance on another ETO-interacting domain, downstream ETO-interacting sequence (DES), for corepressor-mediated repression. Deletion of DES abrogates silencing of E2A activity by AML1-ETO in t(8;21) leukemia cells or by ETO-2 in normal hematopoietic cells. Our results reveal an E2A-specific mechanism important for its context-dependent activation and repression function, and provide the first evidence for the differential involvement of E2A-corepressor interactions in distinct leukemogenic pathways.

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.

Role of transcriptional corepressor ETO2 in erythroid cells

Transcriptional corepressor ETO2 is a component of a protein complex containing master regulators of hematopoiesis, including GATA-1, SCL/TAL1, LMO2, and LDB1. To elucidate the role of ETO2 during erythroid differentiation, including the effects of ETO2 on GATA-1 targets, we performed gene expression profiling using K562 cells overexpressed with ETO2. The analysis demonstrated that 667 and 598 genes were upregulated and downregulated (more than twofold), respectively, in ETO2-overexpressing cells. ETO2-repressed genes included those encoding prototypical erythroid proteins. To test what percentages of ETO2-repressed genes could be direct target genes of GATA-1 in K562 cells, we merged the microarray results with ChIP-seq profile (n = 5,749), demonstrating that 23.1% of ETO2-repressed genes contained significant GATA-1 in their loci. However, there was no significant enrichment of PU.1 target genes among ETO2-repressed genes. Gene ontology analysis among ETO2-repressed genes revealed significant enrichment of genes related to "oxygen transporter," corresponding to globin genes. Quantitative chromatin immunoprecipitation and ETO2 knockdown analyses confirmed that ETO2 directly regulates globin genes in K562 cells. Next, we evaluated the role of ETO2 in human primary erythroblasts, derived from cord blood CD34-positive cells. In an ex vivo model of erythroid differentiation from CD34-positive cells, ETO2 protein level peaked at day 2-4 and almost diminished at the later stage of differentiation. Furthermore, short hairpin RNA-mediated knockdown and retroviral vector-mediated overexpression of ETO2 in primary erythroblasts suggested that ETO2 significantly represses HBB, HBA, and ALAS2 expression. In summary, ETO2 regulates GATA-1 target genes critical for erythroid differentiation, and the decrease of ETO2 levels during erythroid differentiation would contribute to the activation of these targets.