Endothelial Cell Niche Promotes Leukemic Transformation of Human Cord Blood Stem/Progenitor Cells Expressing CBFA2T3-GLIS2 Oncogenic Fusion

The CBFA2T3-GLIS2 (CBF/GLIS) fusion is a product of a cryptic translocation exclusively seen in refractory infant AML. Lack of relevant model systems that accurately recapitulate this infant AML has limited progress. To overcome this barrier, we developed an endothelial cell (EC) co-culture system to support malignant transformation, self-renewal, and propagation of leukemia-initiating cells (LIC) in CBF/GLIS-transduced human cord blood hematopoietic stem/progenitor cells (CB HSPCs) ex vivo. Lack of recurrent cooperating mutations suggests that CBF/GLIS fusion might be sufficient for malignant transformation. To test this, we expressed the CBF/GLIS fusion or GFP control in CB HSPCs (CBF/GLIS-CB or GFP-CB) by lentiviral transduction and placed transduced cells in either EC co-culture or myeloid-promoting culture (MC). CBF/GLIS-CB cells expanded faster with prolonged lifespan in EC co-culture compared to MC (Figure 1A). Proliferation of CBF/GLIS-CB cells declined after transfer to either an EC trans-well culture or in suspension culture (Figure 1B), suggesting that direct contact as well as secreted factors are required for optimal growth of transduced cells. The CBF/GLIS fusion has been shown to confer enhanced megakaryocytic differentiation. At 6 weeks, CBF/GLIS-CB cells in EC co-culture formed significantly more megakaryocytic colonies than CBF/GLIS-CB cells grown in MC or CBF/GLIS-GFP cells grown in either condition (Figure 1C). At 12 weeks, CBF/GLIS-CB cells cultured in EC co-culture continued to produce numerous megakaryocytic colonies, demonstrating long lived self-renewal and enhance megakaryocytic differentiation of CBF/GLIS-CB cells co-cultured with ECs. To determine whether the EC niche promotes generation and propagation of LICs, we evaluated the murine engraftment of CBF/GLIS-CB cells expanded on ECs or in MC following 3, 6, 9 and 12 weeks of culture. CBF/GLIS-CB cells cultured in EC co-culture at each time point exhibited robust engraftment that progressed to frank leukemia in vivo (Figure 1D), demonstrating that EC co-culture promotes long-term maintenance of functional LICs. CBF/GLIS-CB cells grown in MC also induced leukemia from 3- and 6-week cultures but then became senescent at 9 and 12 weeks, suggesting limited preservation of the LICs. Flow cytometric analysis of CBF/GLIS-CB cells identified a malignant population that is of the RAM immunophenotype (CD56 hi, CD45 dim, and CD38 dim/-) previously reported in infants with CBF/GLIS AML in both culturing conditions. However, CBF/GLIS-CB cells in EC co-culture constituted an almost homogeneous population that expressed the RAM immunophenotype, whereas only a subset was detected in MC at week 6 (Figure 1E). To determine the fidelity of transformation to primary leukemia, we performed RNA-sequencing of CBF/GLIS-CB cells cultured with ECs or in MC. Unsurpervised clustering analysis demonstrated that the CBF/GLIS-CB cells from weeks 6 and 12 in EC co-culture clustered with primary CBF/GLIS-positive patient samples, but not CBF/GLIS-CB cells cultured in MC nor GFP controls (Figure 1F). Further transcriptome analysis revealed CBF/GLIS and HSC signature genes, previously identified to be associated with CBF/GLIS AML, were both significantly enriched in CBF/GLIS-CB cells grown in EC culture relative to MC (Figure 1G). These results suggested that the signaling pathways that are aberrantly dysregulated in primary CBF/GLIS leukemia are faithfully recapitulated in CBF/GLIS-CB cells co-cultured with ECs. Despite concerted efforts, previous attempts to model CBF/GLIS AML in murine hematopoietic cells have failed to generate overt leukemia. In this study, we demonstrate that in an EC co-culture system, the CBF/GLIS oncogenic fusion is sufficient to transform human CB HSPCs that faithfully recapitulates the morphology, transcriptome and immunophenotype of CBF/GLIS AML as well as highly aggressive leukemia in xenograft models. Furthermore, the EC co-culture system provides a tractable model system to further interrogate the mechanisms of leukemogenesis and identify biomarkers for disease diagnosis and targets for therapy in CBF/GLIS AML. Figure 1 Figure 1. Disclosures Hylkema: Quest Diagnostics Inc: Current equity holder in publicly-traded company; Moderna: Current equity holder in publicly-traded company. Pardo: Hematologics, Inc.: Current Employment. Eidenschink Brodersen: Hematologics, Inc.: Current Employment, Other: equity ownership. Loken: Hematologics, Inc.: Current Employment, Other: current equity holder in a privately owned company.

The megakaryocytic transcription factor ARID3A suppresses leukemia pathogenesis

Given the plasticity of hematopoietic stem/progenitor cells, multiple routes of differentiation must be blocked during acute myeloid leukemia pathogenesis - the molecular basis of which is incompletely understood. Here we report that post-transcriptional repression of the transcription factor ARID3A by miR-125b is a key event in megakaryoblastic leukemia (AMKL) pathogenesis. AMKL is frequently associated with trisomy 21 and GATA1 mutations (GATA1s), and children with Down syndrome are at a high risk of developing this disease. We show that chromosome 21-encoded miR-125b synergizes with Gata1s to drive leukemogenesis in this context. Leveraging forward and reverse genetics, we uncover Arid3a as the main miR-125b target behind this synergy. We demonstrate that, during normal hematopoiesis, this transcription factor promotes megakaryocytic differentiation in concert with GATA1 and mediates TGFβ-induced apoptosis and cell cycle arrest in complex with SMAD2/3. While Gata1s mutations perturb erythroid differentiation and induce hyperproliferation of megakaryocytic progenitors, intact ARID3A expression assures their megakaryocytic differentiation and growth restriction. Upon knockdown, these tumor suppressive functions are revoked, causing a dual megakaryocytic/erythroid differentiation blockade and subsequently AMKL. Inversely, restoring ARID3A expression relieves the megakaryocytic differentiation arrest in AMKL patient-derived xenografts. This work illustrates how mutations in lineage-determining transcription factors and perturbation of post-transcriptional gene regulation can interplay to block multiple routes of hematopoietic differentiation and cause leukemia. In AMKL, surmounting this differentiation blockade through restoration of the tumor suppressor ARID3A represents a promising strategy for treating this lethal pediatric disease.

The megakaryocytic transcription factor ARID3A suppresses leukemia pathogenesis

Given the plasticity of hematopoietic stem/progenitor cells, multiple routes of differentiation must be blocked during acute myeloid leukemia pathogenesis - the molecular basis of which is incompletely understood. Here we report that post-transcriptional repression of transcription factor ARID3A by miR-125b is a key event in megakaryoblastic leukemia (AMKL) pathogenesis. AMKL is frequently associated with trisomy 21 and GATA1 mutations (GATA1s), and children with Down syndrome are at a high risk of developing this disease. We show that chromosome 21-encoded miR-125b synergizes with Gata1s to drive leukemogenesis in this context. Leveraging forward and reverse genetics, we uncover Arid3a as the main miR-125b target underlying this synergy. We demonstrate that during normal hematopoiesis this transcription factor promotes megakaryocytic differentiation in concert with GATA1 and mediates TGFbeta-induced apoptosis and cell cycle arrest in complex with SMAD2/3. While Gata1s mutations perturb erythroid differentiation and induce hyperproliferation of megakaryocytic progenitors, intact ARID3A expression assures their megakaryocytic differentiation and growth restriction. Upon knockdown, these tumor suppressive functions are revoked, causing a dual megakaryocytic/erythroid differentiation blockade and subsequently AMKL. Inversely, restoring ARID3A expression relieves the megakaryocytic differentiation arrest in AMKL patient-derived xenografts. This work illustrates how mutations in lineage-determining transcription factors and perturbation of post-transcriptional gene regulation interplay to block multiple routes of hematopoietic differentiation and cause leukemia. Surmounting this differentiation blockade in megakaryoblastic leukemia by restoring the tumor suppressor ARID3A represents a promising strategy for treating this lethal pediatric disease.

Establishment of a novel mesenchymal stem cell-based regimen for chronic myeloid leukemia differentiation therapy

Chronic myeloid leukemia (CML) is characterized by the accumulation of malignant and immature white blood cells which spread to the peripheral blood and other tissues/organs. Despite the fact that current tyrosine kinase inhibitors (TKIs) are capable of achieving the complete remission by reducing the tumor burden, severe adverse effects often occur in CML patients treated with TKIs. The differentiation therapy exhibits therapeutic potential to improve cure rates in leukemia, as evidenced by the striking success of all-trans-retinoic acid in acute promyelocytic leukemia treatment. However, there is still a lack of efficient differentiation therapy strategy in CML. Here we showed that MPL, which encodes the thrombopoietin receptor driving the development of hematopoietic stem/progenitor cells, decreased along with the progression of CML. We first elucidated that MPL signaling blockade impeded the megakaryocytic differentiation and contributed to the progression of CML. While allogeneic human umbilical cord-derived mesenchymal stem cells (UC-MSCs) treatment efficiently promoted megakaryocytic lineage differentiation of CML cells through restoring the MPL expression and activating MPL signaling. UC-MSCs in combination with eltrombopag, a non-peptide MPL agonist, further activated JAK/STAT and MAPK signaling pathways through MPL and exerted a synergetic effect on enhancing CML cell differentiation. The established combinational treatment not only markedly reduced the CML burden but also significantly eliminated CML cells in a xenograft CML model. We provided a new molecular insight of thrombopoietin (TPO) and MPL signaling in MSCs-mediated megakaryocytic differentiation of CML cells. Furthermore, a novel anti-CML treatment regimen that uses the combination of UC-MSCs and eltrombopag shows therapeutic potential to overcome the differentiation blockade in CML.

Acute Coronary Syndrome in Acute Myeloid Leukemia with Maturation Accompanying Megakaryocytic Differentiation

An autopsy case (85-year-old Japanese male) of myeloperoxidase- (MPO-) positive acute myeloid leukemia with maturation (M1) accompanying megakaryocytic differentiation is presented. The patient manifested acute coronary syndrome. Even after emergent percutaneous coronary intervention, his performance status remained poor, so no chemotherapy against leukemia was given. The final white blood cell count reached 291,700/μL, and the platelet count was elevated to 510,000/μL. No cytogenetic studies were performed. He died at the 25th day of hospitalization. Autopsy revealed marked leukemic infiltration to the endocardium and subendocardial myocardium. Subendocardial myonecrosis was surrounded or replaced by the leukemic blasts, and neither granulation tissue reaction nor fibrosis was observed. In the cardiovascular lumen, lard-like blood clots were formed and microscopically consisted of leukemic blasts and platelets (leukemic thrombi). Infiltration of leukemic blasts was seen in the body cavities and systemic organs including the lung. The MPO-positive blasts lacked azurophilic granules and expressed the stem cell markers, CD34 and CD117 (c-kit). No features of myelofibrosis were seen in the 100% cellular marrow. In the endocardium, liver, lymph nodes, and bone marrow, megakaryocytic cells (CD42b/CD61+, MPO-) were distributed, while the small-sized blastic cells in the blood and tissues predominantly expressed MPO. The blasts lacked expression of CD42b/CD61. Megakaryocytic differentiation might be stimulated by certain tissue factors. AML accompanying megakaryocytic differentiation in certain tissues and organs should be distinguished from acute megakaryoblastic leukemia. The mechanisms provoking acute coronary syndrome in acute myeloid leukemia are discussed.