NPM1c impedes CTCF functions through cytoplasmic mislocalization in acute myeloid leukemia

Regulation of HOX gene expression in AML

As key developmental regulators, HOX cluster genes have varied and context-specific roles in normal and malignant hematopoiesis. A complex interaction of transcription factors, epigenetic regulators, long non-coding RNAs and chromatin structural changes orchestrate HOX expression in leukemia cells. In this review we summarize molecular mechanisms underlying HOX regulation in clinical subsets of AML, with a focus on NPM1 mutated (NPM1mut) AML comprising a third of all AML patients. While the leukemia initiating function of the NPM1 mutation is clearly dependent on HOX activity, the favorable treatment responses in these patients with upregulation of HOX cluster genes is a poorly understood paradoxical observation. Recent data confirm FOXM1 as a suppressor of HOX activity and a well-known binding partner of NPM suggesting that FOXM1 inactivation may mediate the effect of cytoplasmic NPM on HOX upregulation. Conversely the residual nuclear fraction of mutant NPM has also been recently shown to have chromatin modifying effects permissive to HOX expression. Recent identification of the menin-MLL interaction as a critical vulnerability of HOX-dependent AML has fueled the development of menin inhibitors that are clinically active in NPM1 and MLL rearranged AML despite inconsistent suppression of the HOX locus. Insights into context-specific regulation of HOX in AML may provide a solid foundation for targeting this common vulnerability across several major AML subtypes.

ARID3C Acts as a Regulator of Monocyte-to-Macrophage Differentiation Interacting with NPM1

ARID3C is a protein located on human chromosome 9 and expressed at low levels in various organs, yet its biological function has not been elucidated. In this study, we investigated both the cellular localization and function of ARID3C. Employing a combination of LC-MS/MS and deep learning techniques, we identified NPM1 as a binding partner for ARID3C's nuclear shuttling. ARID3C was found to predominantly localize with the nucleus, where it functioned as a transcription factor for genes STAT3, STAT1, and JUNB, thereby facilitating monocyte-to-macrophage differentiation. The precise binding sites between ARID3C and NPM1 were predicted by AlphaFold2. Mutating this binding site prevented ARID3C from interacting with NPM1, resulting in its retention in the cytoplasm instead of translocation to the nucleus. Consequently, ARID3C lost its ability to bind to the promoters of target genes, leading to a loss of monocyte-to-macrophage differentiation. Collectively, our findings indicate that ARID3C forms a complex with NPM1 to translocate to the nucleus, acting as a transcription factor that promotes the expression of the genes involved in monocyte-to-macrophage differentiation.

NPM1-mutated acute myeloid leukemia: New pathogenetic and therapeutic insights and open questions

The nucleophosmin (NPM1) gene encodes for a multifunctional chaperone protein that is localized in the nucleolus but continuously shuttles between the nucleus and cytoplasm. NPM1 mutations occur in about one-third of AML, are AML-specific, usually involve exon 12 and are frequently associated with FLT3-ITD, DNMT3A, TET2, and IDH1/2 mutations. Because of its unique molecular and clinico-pathological features, NPM1-mutated AML is regarded as a distinct leukemia entity in both the International Consensus Classification (ICC) and the 5th edition of the World Health Organization (WHO) classification of myeloid neoplasms. All NPM1 mutations generate leukemic mutants that are aberrantly exported in the cytoplasm of the leukemic cells and are relevant to the pathogenesis of the disease. Here, we focus on recently identified functions of the NPM1 mutant at chromatin level and its relevance in driving HOX/MEIS gene expression. We also discuss yet controversial issues of the ICC/WHO classifications, including the biological and clinical significance of therapy-related NPM1-mutated AML and the relevance of blasts percentage in defining NPM1-mutated AML. Finally, we address the impact of new targeted therapies in NPM1-mutated AML with focus on CAR T cells directed against NPM1/HLA neoepitopes, as well as XPO1 and menin inhibitors.

NPM1 mutation reprograms leukemic transcription network via reshaping TAD topology

C-terminal mutation of Nucleophosmin 1 (NPM1C+) was thought to be a primary driving event in acute myeloid leukemia (AML) that reprograms leukemic-associated transcription programs to transform hematopoietic stem and progenitor cells (HSPCs). However, molecular mechanisms underlying NPM1C+-driven leukemogenesis remain elusive. Here, we report that NPM1C+ activates signature HOX genes and reprograms cell cycle regulators by altering CTCF-driven topologically associated domains (TADs). Hematopoietic-specific NPM1C+ knock-in alters TAD topology leading to disrupted regulation of the cell cycle as well as aberrant chromatin accessibility and homeotic gene expression, which results in myeloid differentiation block. Restoration of NPM1 within the nucleus re-establishes differentiation programs by reorganizing TADs critical for myeloid TFs and cell cycle regulators that switch the oncogenic MIZ1/MYC regulatory axis in favor of interacting with coactivator NPM1/p300, and prevents NPM1C+-driven leukemogenesis. In sum, our data reveal that NPM1C+ reshapes CTCF-defined TAD topology to reprogram signature leukemic transcription programs required for cell cycle progression and leukemic transformation.

Biomolecular Condensates in Myeloid Leukemia: What Do They Tell Us?

Recent studies have suggested that several oncogenic and tumor-suppressive proteins carry out their functions in the context of specific membrane-less cellular compartments. As these compartments, generally referred to as onco-condensates, are specific to tumor cells and are tightly linked to disease development, the mechanisms of their formation and maintenance have been intensively studied. Here we review the proposed leukemogenic and tumor-suppressive activities of nuclear biomolecular condensates in acute myeloid leukemia (AML). We focus on condensates formed by oncogenic fusion proteins including nucleoporin 98 (NUP98), mixed-lineage leukemia 1 (MLL1, also known as KMT2A), mutated nucleophosmin (NPM1c) and others. We also discuss how altered condensate formation contributes to malignant transformation of hematopoietic cells, as described for promyelocytic leukemia protein (PML) in PML::RARA-driven acute promyelocytic leukemia (APL) and other myeloid malignancies. Finally, we discuss potential strategies for interfering with the molecular mechanisms related to AML-associated biomolecular condensates, as well as current limitations of the field.

An Overview of Targeted Therapies in Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is the most aggressive adult leukemia, characterized by clonal differentiation arrest of progenitor or precursor hematopoietic cells. Intense preclinical and clinical research has led to regulatory approval of several targeted therapeutics, administered either as single agents or as combination therapies. However, the majority of patients still face a poor prognosis and disease relapse frequently occurs due to selection of therapy-resistant clones. Hence, more effective novel therapies, most likely as innovative, rational combination therapies, are urgently needed. Chromosomal aberrations, gene mutations, and epigenetic alterations drive AML pathogenesis but concurrently provide vulnerabilities to specifically target leukemic cells. Other molecules, either aberrantly active and/or overexpressed in leukemic stem cells, may also be leveraged for therapeutic benefit. This concise review of targeted therapies for AML treatment, which are either approved or are being actively investigated in clinical trials or recent preclinical studies, provides a flavor of the direction of travel, but also highlights the current challenges in AML treatment.

NPM1-Mutated Acute Myeloid Leukemia: Recent Developments and Open Questions

Somatic mutations in the nucleophosmin (NPM1) gene occur in approximately 30% of de novo acute myeloid leukemias (AMLs) and are relatively enriched in normal karyotype AMLs. Earlier World Health Organization (WHO) classification schema recognized NPM1-mutated AMLs as a unique subtype of AML, while the latest WHO and International Consensus Classification (ICC) now consider NPM1 mutations as AML-defining, albeit at different blast count thresholds. NPM1 mutational load correlates closely with disease status, particularly in the post-therapy setting, and therefore high sensitivity-based methods for detection of the mutant allele have proven useful for minimal/measurable residual disease (MRD) monitoring. MRD status has been conventionally measured by either multiparameter flow cytometry (MFC) and/or molecular diagnostic techniques, although recent data suggest that MFC data may be potentially more challenging to interpret in this AML subtype. Of note, MRD status does not predict patient outcome in all cases, and therefore a deeper understanding of the biological significance of MRD may be required. Recent studies have confirmed that NPM1-mutated cells rely on overexpression of HOX/MEIS1, which is dependent on the presence of the aberrant cytoplasmic localization of mutant NPM1 protein (NPM1c); this biology may explain the promising response to novel agents, including menin inhibitors and second-generation XPO1 inhibitors. In this review, these and other recent developments around NPM1-mutated AML, in addition to open questions warranting further investigation, will be discussed.

Npm1 haploinsufficiency in collaboration with MEIS1 is sufficient to induce AML in mice

NPM1 is among the most frequently mutated genes in acute myeloid leukemia (AML). Mutations in the NPM1 gene result in the increased export of NPM1 to the cytoplasm (NPM1c) and are associated with multiple transforming events including the aberrant upregulation of MEIS1 that maintains stem cell and cell cycle-associated pathways in NPM1c AML. However, another consequence of the NPM1c mutation is the inadequate levels of NPM1 wild-type in the nucleus and nucleolus, caused by the loss of one wild-type allele in addition to enforced NPM1 nuclear export. The contribution of NPM1 haploinsufficiency independently of the NPM1 mutation to AML development and its relationship with MEIS1 function is poorly understood. Using mouse models, our study shows that NPM1 haploinsufficiency paired with MEIS1 overexpression is sufficient to induce a fully penetrant AML in mice that transcriptionally resembles human NPM1c AML. NPM1 haploinsufficiency alters MEIS1-binding occupancies such that it binds the promoter of the oncogene structural maintenance of chromosome protein 4 (SMC4) in NPM1 haploinsufficient AML cells but not in NPM1 wild-type-harboring Hoxa9/Meis1-transformed cells. SMC4 is higher expressed in haploinsufficient and NPM1c+ AML cells, which are more vulnerable to the disruption of the MEIS1-SMC4 axis compared with AML cells with nonmutated NPM1. Taken together, our study underlines that NPM1 haploinsufficiency on its own is a key factor of myeloid leukemogenesis and characterizes the MEIS1-SMC4 axis as a potential therapeutic target in this AML subtype.

Targeting and Monitoring Acute Myeloid Leukaemia with Nucleophosmin-1 (NPM1) Mutation

Mutations in NPM1, also known as nucleophosmin-1, B23, NO38, or numatrin, are seen in approximately one-third of patients with acute myeloid leukaemia (AML). A plethora of treatment strategies have been studied to determine the best possible approach to curing NPM1-mutated AML. Here, we introduce the structure and function of NPM1 and describe the application of minimal residual disease (MRD) monitoring using molecular methods by means of quantitative polymerase chain reaction (qPCR), droplet digital PCR (ddPCR), next-generation sequencing (NGS), and cytometry by time of flight (CyTOF) to target NPM1-mutated AML. Current drugs, now regarded as the standard of care for AML, as well as potential drugs still under development, will also be explored. This review will focus on the role of targeting aberrant NPM1 pathways such as BCL-2 and SYK; as well as epigenetic regulators (RNA polymerase), DNA intercalators (topoisomerase II), menin inhibitors, and hypomethylating agents. Aside from medication, the effects of stress on AML presentation have been reported, and some possible mechanisms outlined. Moreover, targeted strategies will be briefly discussed, not only for the prevention of abnormal trafficking and localisation of cytoplasmic NPM1 but also for the elimination of mutant NPM1 proteins. Lastly, the advancement of immunotherapy such as targeting CD33, CD123, and PD-1 will be mentioned.

Advances in molecular characterization of myeloid proliferations associated with Down syndrome

Myeloid leukemia associated with Down syndrome (ML-DS) has a unique molecular landscape that differs from other subtypes of acute myeloid leukemia. ML-DS is often preceded by a myeloproliferative neoplastic condition called transient abnormal myelopoiesis (TAM) that disrupts megakaryocytic and erythroid differentiation. Over the last two decades, many genetic and epigenetic changes in TAM and ML-DS have been elucidated. These include overexpression of molecules and micro-RNAs located on chromosome 21, GATA1 mutations, and a range of other somatic mutations and chromosomal alterations. In this review, we summarize molecular changes reported in TAM and ML-DS and provide a comprehensive discussion of these findings. Recent advances in the development of CRISPR/Cas9-modified induced pluripotent stem cell-based disease models are also highlighted. However, despite significant progress in this area, we still do not fully understand the pathogenesis of ML-DS, and there are no targeted therapies. Initial diagnosis of ML-DS has a favorable prognosis, but refractory and relapsed disease can be difficult to treat; therapeutic options are limited in Down syndrome children by their stronger sensitivity to the toxic effects of chemotherapy. Because of the rarity of TAM and ML-DS, large-scale multi-center studies would be helpful to advance molecular characterization of these diseases at different stages of development and progression.

SubCellBarCode: integrated workflow for robust spatial proteomics by mass spectrometry

The molecular functions of a protein are defined by its inherent properties in relation to its environment and interaction network. Within a cell, this environment and network are defined by the subcellular location of the protein. Consequently, it is crucial to know the localization of a protein to fully understand its functions. Recently, we have developed a mass spectrometry- (MS) and bioinformatics-based pipeline to generate a proteome-wide resource for protein subcellular localization across multiple human cancer cell lines ( www.subcellbarcode.org ). Here, we present a detailed wet-lab protocol spanning from subcellular fractionation to MS-sample preparation and analysis. A key feature of this protocol is that it includes all generated cell fractions without discarding any material during the fractionation process. We also describe the subsequent quantitative MS-data analysis, machine learning-based classification, differential localization analysis and visualization of the output. For broad applicability, we evaluated the pipeline by using MS data generated by two different peptide pre-fractionation approaches, namely high-resolution isoelectric focusing and high-pH reverse-phase fractionation, as well as direct analysis without pre-fractionation by using long-gradient liquid chromatography-MS. Moreover, an R package covering the dry-lab part of the method was developed and made available through Bioconductor. The method is straightforward and robust, and the entire protocol, from cell harvest to classification output, can be performed within 1-2 weeks. The protocol enables accurate classification of proteins to 15 compartments and 4 neighborhoods, visualization of the output data and differential localization analysis including treatment-induced protein relocalization, condition-dependent localization or cell type-specific localization. The SubCellBarCode package is freely available at https://bioconductor.org/packages/devel/bioc/html/SubCellBarCode.html .

Nucleophosmin 1: from its pathogenic role to a tantalizing therapeutic target in acute myeloid leukemia

Nucleophosmin 1 (NPM1, also known as B23) is a multifunctional protein involved in a variety of cellular processes, including ribosomal maturation, centrosome replication, maintenance of genomic stability, cell cycle control, and apoptosis. NPM1 is the most commonly mutated gene in adult acute myeloid leukemia (AML) and is present in approximately 40% of all AML cases. The underlying mechanisms of mutant NPM1 (NPM1^mut) in leukemogenesis remain unclear. This review summarizes the structure and physiological function of NPM1, mechanisms underlying the pathogenesis of NPM1-mutated AML, and the potential role of NPM1 as a therapeutic target. It is reported that dysfunctional NPM1 might cause AML pathogenesis via its role as a protein chaperone, inhibiting differentiation of leukemia stem cells and regulation of non-coding RNAs. Besides conventional chemotherapies, NPM1 is a promising therapeutic target against AML that warrants further investigation. NPM1-based therapeutic strategies include inducing nucleolar relocalisation of NPM1 mutants, interfering with NPM1 oligomerization, and NPM1 as an immune response target.

Molecular Classification and Overcoming Therapy Resistance for Acute Myeloid Leukemia with Adverse Genetic Factors

The European LeukemiaNet (ELN) criteria define the adverse genetic factors of acute myeloid leukemia (AML). AML with adverse genetic factors uniformly shows resistance to standard chemotherapy and is associated with poor prognosis. Here, we focus on the biological background and real-world etiology of these adverse genetic factors and then describe a strategy to overcome the clinical disadvantages in terms of targeting pivotal molecular mechanisms. Different adverse genetic factors often rely on common pathways. KMT2A rearrangement, DEK-NUP214 fusion, and NPM1 mutation are associated with the upregulation of HOX genes. The dominant tyrosine kinase activity of the mutant FLT3 or BCR-ABL1 fusion proteins is transduced by the AKT-mTOR, MAPK-ERK, and STAT5 pathways. Concurrent mutations of ASXL1 and RUNX1 are associated with activated AKT. Both TP53 mutation and mis-expressed MECOM are related to impaired apoptosis. Clinical data suggest that adverse genetic factors can be found in at least one in eight AML patients and appear to accumulate in relapsed/refractory cases. TP53 mutation is associated with particularly poor prognosis. Molecular-targeted therapies focusing on specific genomic abnormalities, such as FLT3, KMT2A, and TP53, have been developed and have demonstrated promising results.

Tumour-derived small extracellular vesicles suppress CD8+ T cell immune function by inhibiting SLC6A8-mediated creatine import in NPM1-mutated acute myeloid leukaemia

Acute myeloid leukaemia (AML) carrying nucleophosmin (NPM1) mutations has been defined as a distinct entity of acute leukaemia. Despite remarkable improvements in diagnosis and treatment, the long-term outcomes for this entity remain unsatisfactory. Emerging evidence suggests that leukaemia, similar to other malignant diseases, employs various mechanisms to evade killing by immune cells. However, the mechanism of immune escape in NPM1-mutated AML remains unknown. In this study, both serum and leukemic cells from patients with NPM1-mutated AML impaired the immune function of CD8+ T cells in a co-culture system. Mechanistically, leukemic cells secreted miR-19a-3p into the tumour microenvironment (TME) via small extracellular vesicles (sEVs), which was controlled by the NPM1-mutated protein/CCCTC-binding factor (CTCF)/poly (A)-binding protein cytoplasmic 1 (PABPC1) signalling axis. sEV-related miR-19a-3p was internalized by CD8+ T cells and directly repressed the expression of solute-carrier family 6 member 8 (SLC6A8; a creatine-specific transporter) to inhibit creatine import. Decreased creatine levels can reduce ATP production and impair CD8+ T cell immune function, leading to immune escape by leukemic cells. In summary, leukemic cell-derived sEV-related miR-19a-3p confers immunosuppression to CD8+ T cells by targeting SLC6A8-mediated creatine import, indicating that sEV-related miR-19a-3p might be a promising therapeutic target for NPM1-mutated AML.

NPM1-mutated acute myeloid leukemia: from bench to bedside

The nucleophosmin (NPM1) gene encodes for a multifunctional protein with prominent nucleolar localization that shuttles between nucleus and cytoplasm. NPM1 mutations represent the most common genetic lesion in adult acute myeloid leukemia (AML; about one third of cases), and they act deterministically to cause the aberrant cytoplasmic delocalization of NPM1 mutants. Because of its unique features, NPM1-mutated AML is recognized as a distinct entity in the 2017 World Health Organization (WHO) classification of hematopoietic neoplasms. Here, we focus on recently identified functions of wild-type NPM1 in the nucleolus and address new biological and clinical issues related to NPM1-mutated AML. The relevance of the cooperation between NPM1 and other mutations in driving AML with different outcomes is presented. We also discuss the importance of eradicating NPM1-mutated clones to achieve AML cure and the impact of preleukemic clonal hematopoiesis persistence in predisposing to second AML. The contribution of HOX genes' expression to the development of NPM1-mutated AML is also highlighted. Clinically, yet unsolved diagnostic issues in the 2017 WHO classification of myeloid neoplasms and the importance of NPM1 mutations in defining the framework of European LeukemiaNet genetic-based risk stratification are discussed. Finally, we address the value and limits of NPM1-based measurable residual disease assessment for treatment guidance and present the results of promising preclinical studies with XPO1 and menin-MLL inhibitors.

Cooperation of ATF4 and CTCF promotes adipogenesis through transcriptional regulation

Adipogenesis is a multi-step process orchestrated by activation of numerous TFs, whose cooperation and regulatory network remain elusive. Activating transcription factor 4 (ATF4) is critical for adipogenesis, yet its regulatory network is unclarified. Here, we mapped genome-wide ATF4 binding landscape and its regulatory network by Chip-seq and RNA-seq and found ATF4 directly modulated transcription of genes enriching in fat cell differentiation. Motifs of TFs especially CTCF were found from ATF4 binding sites, suggesting a direct role of ATF4 in regulating adipogenesis associated with CTCF and other TFs. Deletion of CTCF attenuated adipogenesis while overexpression enhanced adipocyte differentiation, indicating CTCF is indispensable for adipogenesis. Intriguingly, combined analysis of Chip-seq data of these two TFs showed that ATF4 co-localized with CTCF in the promoters of key adipogenic genes including Cebpd and PPARg and co-regulated their transactivation. Moreover, ATF4 directly regulated CTCF expression and interacted with CTCF in differentiated 3T3-L1 cells. In vivo, downregulation of ATF4 suppressed the expression of CTCF, Cebpd, and PPARg, leading to reduced adipose tissue expansion in refeeding mice. Consistently, mRNA expression of ATF4 and CTCF was positively correlated with each other in human subcutaneous adipose tissue and inversely associated with BMI, indicating a possible involvement of these two TFs in adipose development. Taken together, our data propose for the first time that ATF4 and CTCF work cooperatively to control adipogenesis and adipose development via orchestrating transcription of adipogenic genes. Our findings reveal novel therapeutic targets in obesity treatment.

A cohesive look at leukemogenesis: The cohesin complex and other driving mutations in AML

Acute myeloid leukemia (AML) affects tens of thousands of patients a year, yet survival rates are as low as 25% in certain populations. This poor survival rate is partially due to the vast genetic diversity of the disease. Rarely do 2 patients with AML have the same mutational profile, which makes the development of targeted therapies particularly challenging. However, a set of recurrent mutations in chromatin modifiers have been identified in many patients, including mutations in the cohesin complex, which have been identified in up to 20% of cases. Interestingly, the canonical function of the cohesin complex in establishing sister chromatid cohesin during mitosis is unlikely to be the affected role in leukemogenesis. Instead, the cohesin complex's role in DNA looping and gene regulation likely facilitates disease. The epigenetic mechanisms by which cohesin complex mutations promote leukemia are not completely elucidated, but alterations of enhancer-promoter interactions and differential histone modifications have been shown to drive oncogenic gene expression changes. Such changes commonly include HoxA upregulation, which may represent a common pathway that could be therapeutically targeted. As cohesin mutations rarely occur alone, examining the impact of common co-occurring mutations, including those in NPM1, the core-binding factor complex, FLT3, and ASXL1, will yield additional insight. While further study of these mutational interactions is required, current research suggests that the use of combinatorial genetics could be the key to uncovering new targets, allowing for the treatment of AML patients based on their individual genetic profiles.

CTCF As an Example of DNA-Binding Transcription Factors Containing Clusters of C2H2-Type Zinc Fingers

In mammals, most of the boundaries of topologically associating domains and all well-studied insulators are rich in binding sites for the CTCF protein. According to existing experimental data, CTCF is a key factor in the organization of the architecture of mammalian chromosomes. A characteristic feature of the CTCF is that the central part of the protein contains a cluster consisting of eleven domains of C2H2-type zinc fingers, five of which specifically bind to a long DNA sequence conserved in most animals. The class of transcription factors that carry a cluster of C2H2-type zinc fingers consisting of five or more domains (C2H2 proteins) is widely represented in all groups of animals. The functions of most C2H2 proteins still remain unknown. This review presents data on the structure and possible functions of these proteins, using the example of the vertebrate CTCF protein and several well- characterized C2H2 proteins in Drosophila and mammals.

NPM1 Biology in Myeloid Neoplasia

PURPOSE OF REVIEW

Nucleophosmin (NPM1) mutations are encountered in myeloid neoplasia and are present in ~ 30% of de novo acute myeloid leukemia cases. This review summarizes features of mutant NPM1-related disease, with a particular emphasis on recent discoveries relevant to disease monitoring, prognostication, and therapeutic intervention.

RECENT FINDINGS

Recent studies have shown that HOX/MEIS gene overexpression is central to the survival of NPM1-mutated cells. Two distinct classes of small molecule drugs, BH3 mimetics and menin-MLL interaction inhibitors, have demonstrated exquisite leukemic cell toxicity in preclinical AML models associated with HOX/MEIS overexpression, and the former of these has shown efficacy in older treatment-naïve NPM1-mutated AML patients. The results of ongoing clinical trials further investigating these compounds will be of particular importance and may alter the clinical management of patients with NPM1-mutated myeloid neoplasms. Significant scientific advancements over the last decade, including improved sequencing and disease monitoring techniques, have fostered a much deeper understanding of mutant NPM1 disease biology, prognostication, and opportunities for therapeutic intervention. These discoveries have led to the development of clinical assays that permit the detection and monitoring of mutant NPM1 and have paved the way for future investigation of targeted therapeutics using emerging cutting-edge techniques.

Determinants and role of chromatin organization in acute leukemia

DNA is compacted into higher order structures that have major implications in gene regulation. These structures allow for long-range interactions of DNA elements, such as the association of promoters with their cognate enhancers. In recent years, mutations in genes that control these structures, including the cohesin-complex and the insulator-binding protein CTCF, have been found in a spectrum of hematologic disorders, and especially in acute leukemias. Cohesin and CTCF are critical for mediating looping and establishing boundaries within chromatin. Cells that harbor mutations in these genes display aberrant chromatin architecture and resulting differences in gene expression that contribute to leukemia initiation and progression. Here, we provide detailed discussion of the nature of 3D interactions and the way that they are disrupted in acute leukemia. Continued research in this area will provide new insights into the mechanisms of leukemogenesis and may shed light on novel treatment strategies.

CTCF-mediated genome organization and leukemogenesis

Recent technological advancements and genome-wide studies provide compelling evidence that dynamic chromatin interaction and three-dimensional genome organization in nuclei play an important role in regulating gene expression. Mammalian genomes consist of many small functional domains termed topologically associated domains (TADs), many of them organized by CCCTC-binding factor (CTCF) and the cohesion complex. Changes in genome TADs might result in inappropriate promoter/enhancer communications leading to activation of oncogenes or suppression of tumor suppressors. During normal hematopoiesis and leukemogenesis, genome structure alters considerably to facilitate normal and malignant hematopoiesis, respectively. Delineating theses normal and abnormal processes will evolve our understanding of disease pathogenesis and development of potential treatment strategies. This review highlights the role of CTCF and its associated protein complexes in three-dimensional genome organization in development and leukemogenesis, as well as the roles of CTCF boundary defined TAD in transcription regulation. We further explore the function of chromatin modulators, such as CTCF, cohesin, and long noncoding RNAs (lncRNAs) in chromosomal interactions and hematopoietic genome organization. Finally, we focus on the implication of 3D genome alteration in the pathogenesis of leukemia and provide a scientific basis for targeted intervention.

Contribution of CTCF binding to transcriptional activity at the HOXA locus in NPM1-mutant AML cells

Transcriptional regulation of the HOXA genes is thought to involve CTCF-mediated chromatin loops and the opposing actions of the COMPASS and Polycomb epigenetic complexes. We investigated the role of these mechanisms at the HOXA cluster in AML cells with the common NPM1c mutation, which express both HOXA and HOXB genes. CTCF binding at the HOXA locus is conserved across primary AML samples, regardless of HOXA gene expression, and defines a continuous chromatin domain marked by COMPASS-associated histone H3 trimethylation in NPM1-mutant primary AML samples. Profiling of the three-dimensional chromatin architecture in primary AML samples with the NPM1c mutation identified chromatin loops between the HOXA cluster and loci in the SNX10 and SKAP2 genes, and an intergenic region located 1.4 Mbp upstream of the HOXA locus. Deletion of CTCF binding sites in the NPM1-mutant OCI-AML3 AML cell line reduced multiple long-range interactions, but resulted in CTCF-independent loops with sequences in SKAP2 that were marked by enhancer-associated histone modifications in primary AML samples. HOXA gene expression was maintained in CTCF binding site mutants, indicating that transcriptional activity at the HOXA locus in NPM1-mutant AML cells may be sustained through persistent interactions with SKAP2 enhancers, or by intrinsic factors within the HOXA gene cluster.