The ETO2 transcriptional cofactor maintains acute leukemia by driving a MYB/EP300-dependent stemness program.

Transcriptional cofactors of the ETO family are recurrent fusion partners in acute leukemia. We characterized the ETO2 regulome by integrating transcriptomic and chromatin binding analyses in human erythroleukemia xenografts and controlled ETO2 depletion models. We demonstrate that beyond its well-established repressive activity, ETO2 directly activates transcription of MYB, among other genes. The ETO2-activated signature is associated with a poorer prognosis in erythroleukemia but also in other acute myeloid and lymphoid leukemia subtypes. Mechanistically, ETO2 colocalizes with EP300 and MYB at enhancers supporting the existence of an ETO2/MYB feedforward transcription activation loop (e.g., on MYB itself). Both small-molecule and PROTAC-mediated inhibition of EP300 acetyltransferases strongly reduced ETO2 protein, chromatin binding, and ETO2-activated transcripts. Taken together, our data show that ETO2 positively enforces a leukemia maintenance program that is mediated in part by the MYB transcription factor and that relies on acetyltransferase cofactors to stabilize ETO2 scaffolding activity.

In vivo screening characterizes chromatin factor functions during normal and malignant hematopoiesis.

Cellular differentiation requires extensive alterations in chromatin structure and function, which is elicited by the coordinated action of chromatin and transcription factors. By contrast with transcription factors, the roles of chromatin factors in differentiation have not been systematically characterized. Here, we combine bulk ex vivo and single-cell in vivo CRISPR screens to characterize the role of chromatin factor families in hematopoiesis. We uncover marked lineage specificities for 142 chromatin factors, revealing functional diversity among related chromatin factors (i.e. barrier-to-autointegration factor subcomplexes) as well as shared roles for unrelated repressive complexes that restrain excessive myeloid differentiation. Using epigenetic profiling, we identify functional interactions between lineage-determining transcription factors and several chromatin factors that explain their lineage dependencies. Studying chromatin factor functions in leukemia, we show that leukemia cells engage homeostatic chromatin factor functions to block differentiation, generating specific chromatin factor-transcription factor interactions that might be therapeutically targeted. Together, our work elucidates the lineage-determining properties of chromatin factors across normal and malignant hematopoiesis.

High caspase 3 and vulnerability to dual BCL2 family inhibition define ETO2::GLIS2 pediatric leukemia.

Pediatric acute myeloid leukemia expressing the ETO2::GLIS2 fusion oncogene is associated with dismal prognosis. Previous studies have shown that ETO2::GLIS2 can efficiently induce leukemia development associated with strong transcriptional changes but those amenable to pharmacological targeting remained to be identified. By studying an inducible ETO2::GLIS2 cellular model, we uncovered that de novo ETO2::GLIS2 expression in human cells led to increased CASP3 transcription, CASP3 activation, and cell death. Patient-derived ETO2::GLIS2+ leukemic cells expressed both high CASP3 and high BCL2. While BCL2 inhibition partly inhibited ETO2::GLIS2+ leukemic cell proliferation, BH3 profiling revealed that it also sensitized these cells to MCL1 inhibition indicating a functional redundancy between BCL2 and MCL1. We further show that combined inhibition of BCL2 and MCL1 is mandatory to abrogate disease progression using in vivo patient-derived xenograft models. These data reveal that a transcriptional consequence of ETO2::GLIS2 expression includes a positive regulation of the pro-apoptotic CASP3 and associates with a vulnerability to combined targeting of two BCL2 family members providing a novel therapeutic perspective for this aggressive pediatric AML subgroup.

HOXA9 forms a repressive complex with nuclear matrix-associated protein SAFB to maintain acute myeloid leukemia.

HOXA9 is commonly upregulated in acute myeloid leukemia (AML), in which it confers a poor prognosis. Characterizing the protein interactome of endogenous HOXA9 in human AML, we identified a chromatin complex of HOXA9 with the nuclear matrix attachment protein SAFB. SAFB perturbation phenocopied HOXA9 knockout to decrease AML proliferation, increase differentiation and apoptosis in vitro, and prolong survival in vivo. Integrated genomic, transcriptomic, and proteomic analyses further demonstrated that the HOXA9-SAFB (H9SB)-chromatin complex associates with nucleosome remodeling and histone deacetylase (NuRD) and HP1γ to repress the expression of factors associated with differentiation and apoptosis, including NOTCH1, CEBPδ, S100A8, and CDKN1A. Chemical or genetic perturbation of NuRD and HP1γ-associated catalytic activity also triggered differentiation, apoptosis, and the induction of these tumor-suppressive genes. Importantly, this mechanism is operative in other HOXA9-dependent AML genotypes. This mechanistic insight demonstrates the active HOXA9-dependent differentiation block as a potent mechanism of disease maintenance in AML that may be amenable to therapeutic intervention by targeting the H9SB interface and/or NuRD and HP1γ activity.

Screening of ETO2-GLIS2-induced Super Enhancers identifies targetable cooperative dependencies in acute megakaryoblastic leukemia.

Super Enhancers (SEs) are clusters of regulatory elements associated with cell identity and disease. However, whether these elements are induced by oncogenes and can regulate gene modules cooperating for cancer cell transformation or maintenance remains elusive. To address this question, we conducted a genome-wide CRISPRi-based screening of SEs in ETO2-GLIS2+ acute megakaryoblastic leukemia. This approach revealed SEs essential for leukemic cell growth and survival that are induced by ETO2-GLIS2 expression. In particular, we identified a de novo SE specific of this leukemia subtype and regulating expression of tyrosine kinase-associated receptors KIT and PDGFRA. Combined expression of these two receptors was required for leukemic cell growth, and CRISPRi-mediated inhibition of this SE or treatment with tyrosine kinase inhibitors impaired progression of leukemia in vivo in patient-derived xenografts experiments. Our results show that fusion oncogenes, such as ETO2-GLIS2, can induce activation of SEs regulating essential gene modules synergizing for leukemia progression.

Human erythroleukemia genetics and transcriptomes identify master transcription factors as functional disease drivers.

Acute erythroleukemia (AEL or acute myeloid leukemia [AML]-M6) is a rare but aggressive hematologic malignancy. Previous studies showed that AEL leukemic cells often carry complex karyotypes and mutations in known AML-associated oncogenes. To better define the underlying molecular mechanisms driving the erythroid phenotype, we studied a series of 33 AEL samples representing 3 genetic AEL subgroups including TP53-mutated, epigenetic regulator-mutated (eg, DNMT3A, TET2, or IDH2), and undefined cases with low mutational burden. We established an erythroid vs myeloid transcriptome-based space in which, independently of the molecular subgroup, the majority of the AEL samples exhibited a unique mapping different from both non-M6 AML and myelodysplastic syndrome samples. Notably, >25% of AEL patients, including in the genetically undefined subgroup, showed aberrant expression of key transcriptional regulators, including SKI, ERG, and ETO2. Ectopic expression of these factors in murine erythroid progenitors blocked in vitro erythroid differentiation and led to immortalization associated with decreased chromatin accessibility at GATA1-binding sites and functional interference with GATA1 activity. In vivo models showed development of lethal erythroid, mixed erythroid/myeloid, or other malignancies depending on the cell population in which AEL-associated alterations were expressed. Collectively, our data indicate that AEL is a molecularly heterogeneous disease with an erythroid identity that results in part from the aberrant activity of key erythroid transcription factors in hematopoietic stem or progenitor cells.

Ontogenic Changes in Hematopoietic Hierarchy Determine Pediatric Specificity and Disease Phenotype in Fusion Oncogene-Driven Myeloid Leukemia.

Fusion oncogenes are prevalent in several pediatric cancers, yet little is known about the specific associations between age and phenotype. We observed that fusion oncogenes, such as ETO2-GLIS2, are associated with acute megakaryoblastic or other myeloid leukemia subtypes in an age-dependent manner. Analysis of a novel inducible transgenic mouse model showed that ETO2-GLIS2 expression in fetal hematopoietic stem cells induced rapid megakaryoblastic leukemia whereas expression in adult bone marrow hematopoietic stem cells resulted in a shift toward myeloid transformation with a strikingly delayed in vivo leukemogenic potential. Chromatin accessibility and single-cell transcriptome analyses indicate ontogeny-dependent intrinsic and ETO2-GLIS2-induced differences in the activities of key transcription factors, including ERG, SPI1, GATA1, and CEBPA. Importantly, switching off the fusion oncogene restored terminal differentiation of the leukemic blasts. Together, these data show that aggressiveness and phenotypes in pediatric acute myeloid leukemia result from an ontogeny-related differential susceptibility to transformation by fusion oncogenes. SIGNIFICANCE: This work demonstrates that the clinical phenotype of pediatric acute myeloid leukemia is determined by ontogeny-dependent susceptibility for transformation by oncogenic fusion genes. The phenotype is maintained by potentially reversible alteration of key transcription factors, indicating that targeting of the fusions may overcome the differentiation blockage and revert the leukemic state.See related commentary by Cruz Hernandez and Vyas, p. 1653.This article is highlighted in the In This Issue feature, p. 1631.

[Pediatric de novo acute megakaryoblastic leukemia: an affair of complexes]. / Leucémies à mégacaryoblastes de l'enfant - Une affaire de 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.

Partial trisomy 21 contributes to T-cell malignancies induced by JAK3-activating mutations in murine models.

JAK3-activating mutations are commonly seen in chronic or acute hematologic malignancies affecting the myeloid, megakaryocytic, lymphoid, and natural killer (NK) cell compartment. Overexpression models of mutant JAK3 or pharmacologic inhibition of its kinase activity have highlighted the role that these constitutively activated mutants play in the T-cell, NK cell, and megakaryocytic lineages, but to date, the functional impact of JAK3 mutations at an endogenous level remains unknown. Here, we report a JAK3A572V knockin mouse model and demonstrate that activated JAK3 leads to a progressive and dose-dependent expansion of CD8+ T cells in the periphery before colonization of the bone marrow. This phenotype is dependent on the γc chain of cytokine receptors and presents several features of the human leukemic form of cutaneous T-cell lymphoma (L-CTCL), including skin involvements. We also showed that the JAK3A572V-positive malignant cells are transplantable and phenotypically heterogeneous in bone marrow transplantation assays. Interestingly, we revealed that activated JAK3 functionally cooperates with partial trisomy 21 in vivo to enhance the L-CTCL phenotype, ultimately leading to a lethal and fully penetrant disorder. Finally, we assessed the efficacy of JAK3 inhibition and showed that CTCL JAK3A572V-positive T cells are sensitive to tofacitinib, which provides additional preclinical insights into the use of JAK3 inhibitors in these disorders. Altogether, this JAK3A572V knockin model is a relevant new tool for testing the efficacy of JAK inhibitors in JAK3-related hematopoietic malignancies.

B-cell tumor development in Tet2-deficient mice.

The TET2 gene encodes an α-ketoglutarate-dependent dioxygenase able to oxidize 5-methylcytosine into 5-hydroxymethylcytosine, which is a step toward active DNA demethylation. TET2 is frequently mutated in myeloid malignancies but also in B- and T-cell malignancies. TET2 somatic mutations are also identified in healthy elderly individuals with clonal hematopoiesis. Tet2-deficient mouse models showed widespread hematological differentiation abnormalities, including myeloid, T-cell, and B-cell malignancies. We show here that, similar to what is observed with constitutive Tet2-deficient mice, B-cell-specific Tet2 knockout leads to abnormalities in the B1-cell subset and a development of B-cell malignancies after long latency. Aging Tet2-deficient mice accumulate clonal CD19+ B220low immunoglobulin M+ B-cell populations with transplantable ability showing similarities to human chronic lymphocytic leukemia, including CD5 expression and sensitivity to ibrutinib-mediated B-cell receptor (BCR) signaling inhibition. Exome sequencing of Tet2-/- malignant B cells reveals C-to-T and G-to-A mutations that lie within single-stranded DNA-specific activation-induced deaminase (AID)/APOBEC (apolipoprotein B messenger RNA editing enzyme, catalytic polypeptide-like) cytidine deaminases targeted motif, as confirmed by the lack of a B-cell tumor in compound Tet2-Aicda-deficient mice. Finally, we show that Tet2 deficiency accelerates and exacerbates T-cell leukemia/lymphoma 1A-induced leukemogenesis. Together, our data establish that Tet2 deficiency predisposes to mature B-cell malignancies, which development might be attributed in part to AID-mediated accumulating mutations and BCR-mediated signaling.

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.

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.

Characterization of novel genomic alterations and therapeutic approaches using acute megakaryoblastic leukemia xenograft models.

Acute megakaryoblastic leukemia (AMKL) is a heterogeneous disease generally associated with poor prognosis. Gene expression profiles indicate the existence of distinct molecular subgroups, and several genetic alterations have been characterized in the past years, including the t(1;22)(p13;q13) and the trisomy 21 associated with GATA1 mutations. However, the majority of patients do not present with known mutations, and the limited access to primary patient leukemic cells impedes the efficient development of novel therapeutic strategies. In this study, using a xenotransplantation approach, we have modeled human pediatric AMKL in immunodeficient mice. Analysis of high-throughput RNA sequencing identified recurrent fusion genes defining new molecular subgroups. One subgroup of patients presented with MLL or NUP98 fusion genes leading to up-regulation of the HOX A cluster genes. A novel CBFA2T3-GLIS2 fusion gene resulting from a cryptic inversion of chromosome 16 was identified in another subgroup of 31% of non-Down syndrome AMKL and strongly associated with a gene expression signature of Hedgehog pathway activation. These molecular data provide useful markers for the diagnosis and follow up of patients. Finally, we show that AMKL xenograft models constitute a relevant in vivo preclinical screening platform to validate the efficacy of novel therapies such as Aurora A kinase inhibitors.

Identification of regulators of polyploidization presents therapeutic targets for treatment of AMKL.

The mechanism by which cells decide to skip mitosis to become polyploid is largely undefined. Here we used a high-content image-based screen to identify small-molecule probes that induce polyploidization of megakaryocytic leukemia cells and serve as perturbagens to help understand this process. Our study implicates five networks of kinases that regulate the switch to polyploidy. Moreover, we find that dimethylfasudil (diMF, H-1152P) selectively increased polyploidization, mature cell-surface marker expression, and apoptosis of malignant megakaryocytes. An integrated target identification approach employing proteomic and shRNA screening revealed that a major target of diMF is Aurora kinase A (AURKA). We further find that MLN8237 (Alisertib), a selective inhibitor of AURKA, induced polyploidization and expression of mature megakaryocyte markers in acute megakaryocytic leukemia (AMKL) blasts and displayed potent anti-AMKL activity in vivo. Our findings provide a rationale to support clinical trials of MLN8237 and other inducers of polyploidization and differentiation in AMKL.

TET2 inactivation results in pleiotropic hematopoietic abnormalities in mouse and is a recurrent event during human lymphomagenesis.

Loss-of-function mutations affecting one or both copies of the Ten-Eleven-translocation (TET)2 gene have been described in various human myeloid malignancies. We report that inactivation of Tet2 in mouse perturbs both early and late steps of hematopoiesis including myeloid and lymphoid differentiation in a cell-autonomous manner, endows the cells with competitive advantage, and eventually leads to the development of malignancies. We subsequently observed TET2 mutations in human lymphoid disorders. TET2 mutations could be detected in immature progenitors endowed with myeloid colony-forming potential. Our results show that the mutations present in lymphoid tumor cells may occur at both early and later steps of lymphoid development and indicate that impairment of TET2 function or/and expression predisposes to the development of hematological malignancies.

Crosstalk between NOTCH and AKT signaling during murine megakaryocyte lineage specification.

The NOTCH signaling pathway is implicated in a broad range of developmental processes, including cell fate decisions. However, the molecular basis for its role at the different steps of stem cell lineage commitment is unclear. We recently identified the NOTCH signaling pathway as a positive regulator of megakaryocyte lineage specification during hematopoiesis, but the developmental pathways that allow hematopoietic stem cell differentiation into the erythro-megakaryocytic lineages remain controversial. Here, we investigated the role of downstream mediators of NOTCH during megakaryopoiesis and report crosstalk between the NOTCH and PI3K/AKT pathways. We demonstrate the inhibitory role of phosphatase with tensin homolog and Forkhead Box class O factors on megakaryopoiesis in vivo. Finally, our data annotate developmental mechanisms in the hematopoietic system that enable a decision to be made either at the hematopoietic stem cell or the committed progenitor level to commit to the megakaryocyte lineage, supporting the existence of 2 distinct developmental pathways.