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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

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.

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Introduction Development of Acute megakaryoblastic leukemia in Down syndrome children (DS-AMKL) is a multi-step process. Acquired GATA1s mutation during fetal hematopoiesis is responsible of a transient myeloproliferative disorder (TMD) characterized by an accumulation of megakaryoblasts. Although most of TMD regress around birth, some TMD can progress from the initial GATA1s clone to AMKL through the acquisition of additional mutations, including in (i) the cohesin complex (i.e: SMC3), (ii) the JAK/STAT signaling pathway, such as MPL and (iii) the polycomb repressive complex 2 (EZH2). How these mutations cooperate to deregulate megakaryocyte (MK) differentiation and to induce a full-blown AMKL, along with the precise role of trisomy 21 (T21) during this transformation process remain unclear. Because modeling of DS-AMKL is particularly difficult in mice, we performed a step-wise introduction of GATA1s, a gain of function mutation of MPL (MPLW515K) and a heterozygous loss of function mutation in a cohesin (SMC3), separately or in combination, in T21 and isogenic disomic 21 (Dis21) human induced Pluripotent Stem Cells (iPSCs). Methods Trisomy 21 iPSCs were kindly provided by M. Weiss (Memphis, TN). CRISPR/Cas 9 genome editing of GATA1 or SMC3 allowed the generation of GATA1s T21, SMC3+/- T21 and GATA1s SMC3+/- T21 iPSC clones. CRISPR/Cas9-mediated knock-in of MPLW515K was performed in T21 GATA1s iPSCs. The subsequent T21 GATA1sMPLW515K/W515Kclones were selected as well as a revertant Dis21 GATA1sMPLW515K/W515Kclone. Finally, SMC3 insertion/deletion were obtained in isogenic T21 and Dis21 GATA1s MPLW515K/W515K SMC3+/-iPSCs clones. Hematopoietic differentiation was induced in 2D cultures in presence of a matrix and a cocktail of cytokines followed by a MK differentiation with SCF and TPO. MK differentiation was studied by clonogenic assays, flow cytometry, confocal microscopy and ultrastructural studies. Gene expression analyses were performed by RNA-seq on highly purified MK from all genotypes. Results GATA1s alone blocked MK maturation characterized by a persistent CD34 expression, an accumulation of abnormal large granules, a defect in the development of demarcation membranes (DMS), and a marked decrease in proplatelet formation. The typical GATA1s MK were large megakaryoblasts with numerous large granules and rare DMS. However, GATA1s alone had no effect on the clonogenic activity in CFU-MK assays and MK numbers. The introduction of the MPLW515K mutation did not modify this phenotype either in Dis21 or T21 GATA1s MK, but induced a complete TPO independence. SMC3+/- alone enhanced the MK maturation allowing the generation of a higher number of proplatelets-generating MK. Importantly, the combination of GATA1s and SMC3+/- mutations had a marked cooperative effect that worsened the MK maturation defect, led to the generation of abnormal megakaryoblasts with only a pre-DMS and resulted in enhanced proliferation and ploidization both in Dis21 and T21 iPSCs. Interestingly, the proliferation was markedly higher in T21 clones compared to Dis21 counterparts. RNA-seq and GSEA analyses showed that T21 GATA1s SMC3+/- mutant MK exhibited transcriptional signatures consistent with a dramatic decrease in the expression of maturation genes, including GATA1 target genes, while DNA replication gene markers were increased compared with GATA1s alone. T21 GATA1s MPLW515K/W515K SMC3+/- MK were enriched for AMKL signatures as compared to isogenic Dis21 GATA1sMPLW515K/W515K SMC3+/- MK. Ongoing ATAC-seq analyses will define the consequence of the different mutations on chromatin accessibility. Conclusion Using iPSC modeling, we analyzed in a human cell-context the consequences of the different combination of mutations associated with DS-AMKL that would be difficult to model using human primary cells. Our data demonstrate that GATA1s expression cooperates with SMC3+/- to enhance proliferation of megakaryoblasts from T21 iPSCs and isogenic Dis21 iPSCs hence reproducing the abnormalities observed in DS-AMKL. T21 is not directly involved in the MK differentiation defects but rather give a proliferative advantage supporting its role in leukemia development. Disclosures No relevant conflicts of interest to declare.

Deregulated gene expression due to genetic alterations, such as gene fusions affecting transcription and/or epigenetic factors is the hallmark of acute myeloid leukemia and the basis for the differentiation block of hematopoietic progenitors. Acute megakaryoblastic leukemia (AMKL) is a subtype of poor prognosis acute myeloid leukemia (AML) affecting primarily young children. Recently, the ETO2-GLIS2 fusion has been identified in 20-30% of de novo AMKL and associated with the worst prognosis in this subtype of AML. To characterize the transformation induced by ETO2-GLIS2, we first defined the consequences of ETO2-GLIS2 expression on hematopoietic progenitors and the contribution of ETO2 and GLIS2 on differentiation and self-renewal. Using methylcellulose replating assays and phenotype characterization, we show that the GLIS2 moiety drives the megakaryocytic phenotype whereas both the ETO2 and GLIS2 moieties are required for maintaining self-renewal. Global expression profiling and comparison to patients' signature consistently identify ETO2-GLIS2-mediated deregulation of major transcriptional regulators of hematopoiesis and leukemogenesis, including overexpression of the ERG oncogene. ChIP-seq analysis reveals that ETO2-GLIS2 is recruited at normal ETO2 complexes sites and also at GLIS2-specific targets through binding via GLIS2 DNA-binding domain. We demonstrate that ETO2-GLIS2 fusion localize at half of H3K27Ac-dense enhancers, so called super-enhancers, to control transcription of associated genes. We show that interaction of ETO2-GLIS2 with ETO2 complexes is an essential node for the transcriptional control by the fusion at enhancer elements. Indeed, ETO2-GLIS2 dimerizes and interacts with endogenous ETO2 via its NHR2 domains. An NHR2 peptide-interference strategy inhibits oligomerization, reverses the transcriptional activation at enhancers, promotes megakaryocytic differentiation and abrogates human AMKL cells maintenance in vivo. Finally, upregulation of ERG by ETO2-GLIS2 further strengthen enhancers formation as ERG is co-recruited generating a feed forward loop at these elements and its knockdown or genetic inactivation downregulates expression of ETO2-GLIS2 targets required for leukemic cells survival. We propose that the megakaryocytic differentiation arrest and self-renewal controlled by ETO2-GLIS2 results from an imbalance in the expression of master transcription factors imposed by aberrant chromatin structures at enhancers that may be disrupted by targeting the NHR2 interface. Disclosures No relevant conflicts of interest to declare.