Heterodimerization of AML1/ETO with CBFβ is required for leukemogenesis but not for myeloproliferation

Single-cell RNA sequencing of a new transgenic t(8;21) preleukemia mouse model reveals regulatory networks promoting leukemic transformation

T(8;21)(q22;q22), which generates the AML1-ETO fusion oncoprotein, is a common chromosomal abnormality in acute myeloid leukemia (AML) patients. Despite having favorable prognosis, 40% of patients will relapse, highlighting the need for innovative models and application of the newest technologies to study t(8;21) leukemogenesis. Currently, available AML1-ETO mouse models have limited utility for studying the pre-leukemic stage because AML1-ETO produces mild hematopoietic phenotypes and no leukemic transformation. Conversely, overexpression of a truncated variant, AML1-ETO9a (AE9a), promotes fully penetrant leukemia and is too potent for studying pre-leukemic changes. To overcome these limitations, we devised a germline-transmitted Rosa26 locus AE9a knock-in mouse model that moderately overexpressed AE9a and developed leukemia with long latency and low penetrance. We observed pre-leukemic alterations in AE9a mice, including skewing of progenitors towards granulocyte/monocyte lineages and replating of stem and progenitor cells. Next, we performed single-cell RNA sequencing to identify specific cell populations that contribute to these pre-leukemic phenotypes. We discovered a subset of common myeloid progenitors that have heightened granulocyte/monocyte bias in AE9a mice. We also observed dysregulation of key hematopoietic transcription factor target gene networks, blocking cellular differentiation. Finally, we identified Sox4 activation as a potential contributor to stem cell self-renewal during the pre-leukemic stage.

IL-33-ST2 signaling promotes stemness in subtypes of myeloid leukemia cells through the Wnt and Notch pathways

Cell stemness is characterized by quiescence, pluripotency, and long-term self-renewal capacity. Therapy-resistant leukemic stem cells (LSCs) are the primary cause of relapse in patients with chronic and acute myeloid leukemia (CML and AML). However, the same signaling pathways frequently support stemness in both LSCs and normal hematopoietic stem cells (HSCs), making LSCs difficult to therapeutically target. In cell lines and patient samples, we found that interleukin-33 (IL-33) signaling promoted stemness only in leukemia cells in a subtype-specific manner. The IL-33 receptor ST2 was abundant on the surfaces of CD34+ BCR/ABL1 CML and CD34+ AML cells harboring AML1/ETO and DEK/NUP214 translocations or deletion of chromosome 9q [del(9q)]. The cell surface abundance of ST2, which was lower or absent on other leukemia subtypes and HSCs, correlated with stemness, activated Wnt signaling, and repressed Notch signaling. IL-33-ST2 signaling promoted the maintenance and expansion of AML1/ETO-, DEK/NUP214-, and BCR/ABL1-positive LSCs in culture and in mice by activating Wnt, MAPK, and NF-κB signaling. Wnt signaling and its inhibition of the Notch pathway up-regulated the expression of the gene encoding ST2, thus forming a cell-autonomous loop. IL-33-ST2 signaling promoted the resistance of CML cells to the tyrosine kinase inhibitor (TKI) nilotinib and of AML cells to standard chemotherapy. Thus, inhibiting IL-33-ST2 signaling may target LSCs to overcome resistance to chemotherapy or TKIs in these subtypes of leukemia.

The RUNX Family of Proteins, DNA Repair, and Cancer

The RUNX family of transcription factors, including RUNX1, RUNX2, and RUNX3, are key regulators of development and can function as either tumor suppressors or oncogenes in cancer. Emerging evidence suggests that the dysregulation of RUNX genes can promote genomic instability in both leukemia and solid cancers by impairing DNA repair mechanisms. RUNX proteins control the cellular response to DNA damage by regulating the p53, Fanconi anemia, and oxidative stress repair pathways through transcriptional or non-transcriptional mechanisms. This review highlights the importance of RUNX-dependent DNA repair regulation in human cancers.

Melatonin promotes differentiation and apoptosis of AML1-ETO-positive cells

INTRODUCTION

Acute Myeloid Leukemia 1-Eight-Twenty-One (AML1-ETO) is an oncogenic fusion protein that causes acute myeloid leukemia. We examined the effects of melatonin on AML1-ETO by investigating cell differentiation, apoptosis, and degradation in leukemia cell lines.

METHOD

We evaluated Kasumi-1, U937T, and primary acute myeloid leukemia (AML1-ETO-positive) cell proliferation by Cell Counting Kit-8 assay. Flow cytometry and western blotting were used to evaluate CD11b/CD14 levels (differentiation biomarkers) and the AML1-ETO protein degradation pathway, respectively. CM-Dil-labeled Kasumi-1 cells were also injected into zebrafish embryos to determine the effects of melatonin on vascular proliferation and development and to evaluate the combined effects of melatonin and common chemotherapeutic agents.

RESULTS

AML1-ETO-positive acute myeloid leukemia cells were more sensitive to melatonin than AML1-ETO-negative cells. Melatonin increased apoptosis and CD11b/CD14 expression in AML1-ETO-positive cells and decreased the nuclear/cytoplasmic ratio, together suggesting that melatonin induced cell differentiation. Mechanistically, melatonin degraded AML1-ETO by activating the caspase-3 pathway and regulating the mRNA levels of AML1-ETO downstream genes. Melatonin reduced the number of neovessels in Kasumi-1-injected zebrafish, suggesting that melatonin inhibits cell proliferation in vivo. Finally, combining drugs with melatonin inhibited cell viability.

DISCUSSION

Melatonin is a potential compound for the treatment of AML1-ETO-positive acute myeloid leukemia.

Nanoparticle-mediated targeting of the fusion gene RUNX1/ETO in t(8;21)-positive acute myeloid leukaemia

A hallmark of acute myeloid leukaemias (AMLs) are chromosomal rearrangements that give rise to novel leukaemia-specific fusion genes. Most of these fusion genes are both initiating and driving events in AML and therefore constitute ideal therapeutic targets but are challenging to target by conventional drug development. siRNAs are frequently used for the specific suppression of fusion gene expression but require special formulations for efficient in vivo delivery. Here we describe the use of siRNA-loaded lipid nanoparticles for the specific therapeutic targeting of the leukaemic fusion gene RUNX1/ETO. Transient knockdown of RUNX1/ETO reduces its binding to its target genes and alters the binding of RUNX1 and its co-factor CBFβ. Transcriptomic changes in vivo were associated with substantially increased median survival of a t(8;21)-AML mouse model. Importantly, transient knockdown in vivo causes long-lasting inhibition of leukaemic proliferation and clonogenicity, induction of myeloid differentiation and a markedly impaired re-engraftment potential in vivo. These data strongly suggest that temporary inhibition of RUNX1/ETO results in long-term restriction of leukaemic self-renewal. Our results provide proof for the feasibility of targeting RUNX1/ETO in a pre-clinical setting and support the further development of siRNA-LNPs for the treatment of fusion gene-driven malignancies.

Therapy-related core binding factor acute myeloid leukemia

Therapy-related acute myeloid leukemia (t-AML) usually stems from exposure of the bone marrow to cytotoxic chemotherapy and/or radiation therapy. t-AML is usually associated with poor overall survival, but occasionally t-AML can involve favorable-risk cytogenetics, including core binding factor AML (CBF-AML), which shows a recurrent chromosomal rearrangement with t(8;21) (q22;22) and 'inv(16) (p13.1;q22)/t(16;16)(p13.1;q22)', leading to 'RUNX1::RUNX1T1 and CBFB::MYH11' fusion genes, respectively. Therapy-related CBF-AML (t-CBF-AML) accounts for 5-15% of CBF-AML cases and tends to have better outcomes than t-AML with unfavorable cytogenetics. Although CBF-AML is sensitive to high-dose cytarabine, t-CBF-AML has worse overall survival than de novo CBF- AML. The objective of this review is to discuss the available data on the pathogenesis, mutations, and therapeutic options in patients with t-CBF-AML.

Mechanisms of myeloid leukemogenesis: Current perspectives and therapeutic objectives

Acute myeloid leukemia (AML) is a heterogeneous hematopoietic neoplasm which results in clonal proliferation of abnormally differentiated hematopoietic cells. In this review, mechanisms contributing to myeloid leukemogenesis are summarized, highlighting aberrations of epigenetics, transcription factors, signal transduction, cell cycling, and the bone marrow microenvironment. The mechanisms contributing to AML are detailed to spotlight recent findings that convey clinical impact. The applications of current and prospective therapeutic targets are accentuated in addition to reviews of treatment paradigms stratified for each characteristic molecular lesion - with a focus on exploring novel treatment approaches and combinations to improve outcomes in AML.

Reinforced erythroid differentiation inhibits leukemogenic potential of t(8;21) leukemia

Oncoprotein AML1-ETO (AE) derived from t(8;21)(q22;q22) translocation is typically present in a portion of French-American-British-M2 subtype of acute myeloid leukemia (AML). Although these patients have relatively favorable prognoses, substantial numbers of them would relapse after conventional therapy. Here, we explored whether reinforcing the endogenous differentiation potential of t(8;21) AML cells would diminish the associated malignancy. In doing so, we noticed an expansion of immature erythroid blasts featured in both AML1-ETO9a (AE9a) and AE plus c-KIT (N822K) (AK) murine leukemic models. Interestingly, in the AE9a murine model, a spontaneous step-wise erythroid differentiation path, as characterized by the differential expression of CD43/c-Kit and the upregulation of several key erythroid transcription factors (TFs), accompanied the decline or loss of leukemia-initiating potential. Notably, overexpression of one of the key erythroid TFs, Ldb1, potently disrupted the repopulation of AE9a leukemic cells in vivo, suggesting a new promising intervention strategy of t(8;21) AML through enforcing their erythroid differentiation.

Hydroxylation of the NOTCH1 intracellular domain regulates Notch signaling dynamics

Notch signaling plays a pivotal role in the development and, when dysregulated, it contributes to tumorigenesis. The amplitude and duration of the Notch response depend on the posttranslational modifications (PTMs) of the activated NOTCH receptor - the NOTCH intracellular domain (NICD). In normoxic conditions, the hydroxylase FIH (factor inhibiting HIF) catalyzes the hydroxylation of two asparagine residues of the NICD. Here, we investigate how Notch-dependent gene transcription is regulated by hypoxia in progenitor T cells. We show that the majority of Notch target genes are downregulated upon hypoxia. Using a hydroxyl-specific NOTCH1 antibody we demonstrate that FIH-mediated NICD1 hydroxylation is reduced upon hypoxia or treatment with the hydroxylase inhibitor dimethyloxalylglycine (DMOG). We find that a hydroxylation-resistant NICD1 mutant is functionally impaired and more ubiquitinated. Interestingly, we also observe that the NICD1-deubiquitinating enzyme USP10 is downregulated upon hypoxia. Moreover, the interaction between the hydroxylation-defective NICD1 mutant and USP10 is significantly reduced compared to the NICD1 wild-type counterpart. Together, our data suggest that FIH hydroxylates NICD1 in normoxic conditions, leading to the recruitment of USP10 and subsequent NICD1 deubiquitination and stabilization. In hypoxia, this regulatory loop is disrupted, causing a dampened Notch response.

Roles of Histone Deacetylases in Acute Myeloid Leukemia With Fusion Proteins

Accurate orchestration of gene expression is critical for the process of normal hematopoiesis, and dysregulation is closely associated with leukemogenesis. Epigenetic aberration is one of the major causes contributing to acute myeloid leukemia (AML), where chromosomal rearrangements are frequently found. Increasing evidences have shown the pivotal roles of histone deacetylases (HDACs) in chromatin remodeling, which are involved in stemness maintenance, cell fate determination, proliferation and differentiation, via mastering the transcriptional switch of key genes. In abnormal, these functions can be bloomed to elicit carcinogenesis. Presently, HDAC family members are appealing targets for drug exploration, many of which have been deployed to the AML treatment. As the majority of AML events are associated with chromosomal translocation resulting in oncogenic fusion proteins, it is valuable to comprehensively understand the mutual interactions between HDACs and oncogenic proteins. Therefore, we reviewed the process of leukemogenesis and roles of HDAC members acting in this progress, providing an insight for the target anchoring, investigation of hyperacetylated-agents, and how the current knowledge could be applied in AML treatment.

Chromatin Regulator SPEN/SHARP in X Inactivation and Disease

Simple Summary

Carcinogenesis is a multistep process involving not only the activation of oncogenes and disabling tumor suppressor genes, but also epigenetic modulation of gene expression. X chromosome inactivation (XCI) is a paradigm to study heterochromatin formation and maintenance. The double dosage of X chromosomal genes in female mammals is incompatible with early development. XCI is an excellent model system for understanding the establishment of facultative heterochromatin initiated by the expression of a 17,000 nt long non-coding RNA, known as Xinactivespecifictranscript (Xist), on the X chromosome. This review focuses on the molecular mechanisms of how epigenetic modulators act in a step-wise manner to establish facultative heterochromatin, and we put these in the context of cancer biology and disease. An in depth understanding of XCI will allow a better characterization of particular types of cancer and hopefully facilitate the development of novel epigenetic therapies.

Abstract

Enzymes, such as histone methyltransferases and demethylases, histone acetyltransferases and deacetylases, and DNA methyltransferases are known as epigenetic modifiers that are often implicated in tumorigenesis and disease. One of the best-studied chromatin-based mechanism is X chromosome inactivation (XCI), a process that establishes facultative heterochromatin on only one X chromosome in females and establishes the right dosage of gene expression. The specificity factor for this process is the long non-coding RNA Xinactivespecifictranscript (Xist), which is upregulated from one X chromosome in female cells. Subsequently, Xist is bound by the corepressor SHARP/SPEN, recruiting and/or activating histone deacetylases (HDACs), leading to the loss of active chromatin marks such as H3K27ac. In addition, polycomb complexes PRC1 and PRC2 establish wide-spread accumulation of H3K27me3 and H2AK119ub1 chromatin marks. The lack of active marks and establishment of repressive marks set the stage for DNA methyltransferases (DNMTs) to stably silence the X chromosome. Here, we will review the recent advances in understanding the molecular mechanisms of how heterochromatin formation is established and put this into the context of carcinogenesis and disease.

Transcription Factor RBPJ as a Molecular Switch in Regulating the Notch Response

The Notch signal transduction cascade requires cell-to-cell contact and results in the proteolytic processing of the Notch receptor and subsequent assembly of a transcriptional coactivator complex containing the Notch intracellular domain (NICD) and transcription factor RBPJ. In the absence of a Notch signal, RBPJ remains at Notch target genes and dampens transcriptional output. Like in other signaling pathways, RBPJ is able to switch from activation to repression by associating with corepressor complexes containing several chromatin-modifying enzymes. Here, we focus on the recent advances concerning RBPJ-corepressor functions, especially in regard to chromatin regulation. We put this into the context of one of the best-studied model systems for Notch, blood cell development. Alterations in the RBPJ-corepressor functions can contribute to the development of leukemia, especially in the case of acute myeloid leukemia (AML). The versatile role of transcription factor RBPJ in regulating pivotal target genes like c-MYC and HES1 may contribute to the better understanding of the development of leukemia.

Modelling t(8;21) acute myeloid leukaemia - What have we learned?

Acute myeloid leukaemia (AML) is a heterogeneous haematopoietic malignancy caused by recurrent mutations in haematopoietic stem and progenitor cells that affect both the epigenetic regulatory machinery and signalling molecules. The t(8;21) or RUNX1‐RUNX1T1 translocation generates the RUNX1‐ETO chimeric transcription factor which primes haematopoietic stem cells for further oncogenic mutational events that in their sum cause overt disease. Significant progress has been made in generating both in vitro and in vivo model systems to recapitulate t(8;21) AML which are crucial for the understanding of the biology of the disease and the development of effective treatment. This review provides a comprehensive overview of the in vivo and in vitro model systems that were developed to gain insights into the molecular mechanisms of RUNX1‐ETO oncogenic activity and their contribution to the advancement of knowledge in the t(8;21) AML field. Such models include transgenic mice, patient‐derived xenografts, RUNX1‐ETO transduced human progenitor cells, cell lines and human embryonic stem cell model systems, making the t(8;21) as one of the well‐characterized sub‐type of AML at the molecular level.

Core binding factor acute myeloid leukemia: Advances in the heterogeneity of KIT, FLT3, and RAS mutations (Review)

Core binding factor (CBF) is a heterodimer protein complex involved in the transcriptional regulation of normal hematopoietic process. In addition, CBF molecular aberrations represent approximately 20% of all adult Acute Myeloid Leukemia (AML) patients. Treated with standard therapy, adult CBF AML has higher complete remission (CR) rate, longer CR duration, and better prognosis than that of AML patients with normal karyotype or other chromosomal aberrations. Although the prognosis of CBF AML is better than other subtypes of adult AML, it is still a group of heterogeneous diseases, and the prognosis is often different. Recurrence and relapse-related death are the main challenges to be faced following treatment. Mounting research shows the gene heterogeneity of CBF AML. Therefore, to achieve an improved clinical outcome, the differences in clinical and genotypic characteristics should be taken into account in the evaluation and management of such patients, so as to further improve the risk stratification of prognosis and develop targeted therapy. The present article is a comprehensive review of the differences in some common mutant genes between two subtypes of CBF AML.

The acute myeloid leukemia associated AML1-ETO fusion protein alters the transcriptome and cellular progression in a single-oncogene expressing in vitro induced pluripotent stem cell based granulocyte differentiation model

Acute myeloid leukemia (AML) is characterized by recurrent mutations that affect normal hematopoiesis. The analysis of human AMLs has mostly been performed using end-point materials, such as cell lines and patient derived AMLs that also carry additional contributing mutations. The molecular effects of a single oncogenic hit, such as expression of the AML associated oncoprotein AML1-ETO on hematopoietic development and transformation into a (pre-) leukemic state still needs further investigation. Here we describe the development and characterization of an induced pluripotent stem cell (iPSC) system that allows in vitro differentiation towards different mature myeloid cell types such as monocytes and granulocytes. During in vitro differentiation we expressed the AML1-ETO fusion protein and examined the effects of the oncoprotein on differentiation and the underlying alterations in the gene program at 8 different time points. Our analysis revealed that AML1-ETO as a single oncogenic hit in a non-mutated background blocks granulocytic differentiation, deregulates the gene program via altering the acetylome of the differentiating granulocytic cells, and induces t(8;21) AML associated leukemic characteristics. Together, these results reveal that inducible oncogene expression during in vitro differentiation of iPS cells provides a valuable platform for analysis of aberrant regulation in disease.

Histone variant H2A.Z deposition and acetylation directs the canonical Notch signaling response

A fundamental as yet incompletely understood feature of Notch signal transduction is a transcriptional shift from repression to activation that depends on chromatin regulation mediated by transcription factor RBP-J and associated cofactors. Incorporation of histone variants alter the functional properties of chromatin and are implicated in the regulation of gene expression. Here, we show that depletion of histone variant H2A.Z leads to upregulation of canonical Notch target genes and that the H2A.Z-chaperone TRRAP/p400/Tip60 complex physically associates with RBP-J at Notch-dependent enhancers. When targeted to RBP-J-bound enhancers, the acetyltransferase Tip60 acetylates H2A.Z and upregulates Notch target gene expression. Importantly, the Drosophila homologs of Tip60, p400 and H2A.Z modulate Notch signaling response and growth in vivo. Together, our data reveal that loading and acetylation of H2A.Z are required to assure tight control of canonical Notch activation.

Notch in Leukemia

Notch is commonly activated in lymphoid malignancies through ligand-independent and ligand-dependent mechanisms. In T-cell acute lymphoblastic leukemia/lymphoma (T-ALL), ligand-independent activation predominates. Negative Regulatory Region (NRR) mutations trigger supraphysiological Notch1 activation by exposing the S2 site to proteolytic cleavage in the absence of ligand. Subsequently, cleavage at the S3 site generates the activated form of Notch, intracellular Notch (ICN). In contrast to T-ALL, in mature lymphoid neoplasms such as chronic lymphocytic leukemia (CLL), the S2 cleavage site is exposed through ligand-receptor interactions. Thus, agents that disrupt ligand-receptor interactions might be useful for treating these malignancies. Notch activation can be enhanced by mutations that delete the C-terminal proline (P), glutamic acid (E), serine (S), and threonine (T) (PEST) domain. These mutations do not activate the Notch pathway per se, but rather impair degradation of ICN. In this chapter, we review the mechanisms of Notch activation and the importance of Notch for the genesis and maintenance of lymphoid malignancies. Unfortunately, targeting the Notch pathway with pan-Notch inhibitors in clinical trials has proven challenging. These clinical trials have encountered dose-limiting on-target toxicities and primary resistance. Strategies to overcome these challenges have emerged from the identification and improved understanding of direct oncogenic Notch target genes. Other strategies have arisen from new insights into the "nuclear context" that selectively directs Notch functions in lymphoid cancers. This nuclear context is created by factors that co-bind ICN at cell-type specific transcriptional regulatory elements. Disrupting the functions of these proteins or inhibiting downstream oncogenic pathways might combat cancer without the intolerable side effects of pan-Notch inhibition.