MLL fusions: pathways to leukemia

Discovery, Structure-Activity Relationship and In Vitro Anticancer Activity of Small-Molecule Inhibitors of the Protein-Protein Interactions between AF9/ENL and AF4 or DOT1L

Chromosomal translocations involving the mixed lineage leukemia (MLL) gene cause 5-10% acute leukemias with poor clinical outcomes. Protein-protein interactions (PPI) between the most frequent MLL fusion partner proteins AF9/ENL and AF4 or histone methyltransferase DOT1L are drug targets for MLL-rearranged (MLL-r) leukemia. Several benzothiophene-carboxamide compounds were identified as novel inhibitors of these PPIs with IC50 values as low as 1.6 μM. Structure-activity relationship studies of 77 benzothiophene and related indole and benzofuran compounds show that a 4-piperidin-1-ylphenyl or 4-pyrrolidin-1-ylphenyl substituent is essential for the activity. The inhibitors suppressed expression of MLL target genes HoxA9, Meis1 and Myc, and selectively inhibited proliferation of MLL-r and other acute myeloid leukemia cells with EC50 values as low as 4.7 μM. These inhibitors are useful chemical probes for biological studies of AF9/ENL, as well as pharmacological leads for further drug development against MLL-r and other leukemias.

Mechanisms of Secondary Leukemia Development Caused by Treatment with DNA Topoisomerase Inhibitors

Leukemia is a blood cancer originating in the blood and bone marrow. Therapy-related leukemia is associated with prior chemotherapy. Although cancer therapy with DNA topoisomerase II inhibitors is one of the most effective cancer treatments, its side effects include development of secondary leukemia characterized by the chromosomal rearrangements affecting AML1 or MLL genes. Recurrent chromosomal translocations in the therapy-related leukemia differ from chromosomal rearrangements associated with other neoplasias. Here, we reviewed the factors that drive chromosomal translocations induced by cancer treatment with DNA topoisomerase II inhibitors, such as mobility of ends of double-strand DNA breaks formed before the translocation and gain of function of fusion proteins generated as a result of translocation.

RNA binding induces an allosteric switch in Cyp33 to repress MLL1-mediated transcription

Mixed-lineage leukemia 1 (MLL1) is a transcription activator of the HOX family, which binds to specific epigenetic marks on histone H3 through its third plant homeodomain (PHD3) domain. Through an unknown mechanism, MLL1 activity is repressed by cyclophilin 33 (Cyp33), which binds to MLL1 PHD3. We determined solution structures of Cyp33 RNA recognition motif (RRM) free, bound to RNA, to MLL1 PHD3, and to both MLL1 and the histone H3 lysine N6-trimethylated. We found that a conserved α helix, amino-terminal to the RRM domain, adopts three different positions facilitating a cascade of binding events. These conformational changes are triggered by Cyp33 RNA binding and ultimately lead to MLL1 release from the histone mark. Together, our mechanistic findings rationalize how Cyp33 binding to MLL1 can switch chromatin to a transcriptional repressive state triggered by RNA binding as a negative feedback loop.

Defined Human Leukemic CD34+ Liquid Cultures to Study HDAC/Transcriptional Repressor Complexes

Defined human primary cell model systems with growth dependence on oncogenes are highly requested to investigate tumor pathogenesis and to validate pharmacological inhibitors that specifically target oncoproteins and their executing protein complex partners. In acute myeloid leukemia (AML), transcription factors such as RUNX1 and MLL1, which are important for normal blood cell development, frequently harbor mutations including chromosomal translocations with other coding genes, resulting in tumor-promoting gain-of-function fusion proteins. These oncoproteins completely modify transcriptional programs, thereby inducing malignant cell phenotypes. A common theme of the chimeric gene products is their physical interaction with a variety of chromatin-modifying effector molecules, including histone acetyltransferases (HATs) and histone deacetylases (HDACs). These aberrant multiprotein machineries disturb gene expression and promote malignant cell growth. In this chapter, we briefly summarize the current understanding regarding AML-associated oncogene-driven human CD34+ blood progenitor cell expansion in ex vivo liquid cultures. We provide a step-by-step protocol to establish oncogene-induced human CD34+ blood progenitor cell cultures suitable to analyze the impact of transcriptional repressor/HDAC activity in these human AML cell models.

Histone Readers and Their Roles in Cancer

Histone proteins in eukaryotic cells are subjected to a wide variety of post-translational modifications, which are known to play an important role in the partitioning of the genome into distinctive compartments and domains. One of the major functions of histone modifications is to recruit reader proteins, which recognize the epigenetic marks and transduce the molecular signals in chromatin to downstream effects. Histone readers are defined protein domains with well-organized three-dimensional structures. In this Chapter, we will outline major histone readers, delineate their biochemical and structural features in histone recognition, and describe how dysregulation of histone readout leads to human cancer.

Molecular regulators of HOXA9 in acute myeloid leukemia

Dysregulation of the oncogenic transcription factor HOXA9 is a prominent feature for most aggressive acute myeloid leukemia cases and a strong indicator of poor prognosis in patients. Leukemia subtypes with hallmark overexpression of HOXA9 include those carrying MLL gene rearrangements, NPM1c mutations, and other genetic alternations. A growing body of evidence indicates that HOXA9 dysregulation is both sufficient and necessary for leukemic transformation. The HOXA9 mRNA and protein regulation includes multilayered controls by transcription factors (such as CDX2/4 and USF2/1), epigenetic factors (such as MLL-menin-LEDGF, DOT1L, ENL, HBO1, NPM1c-XPO1, and polycomb proteins), microRNAs (such as miR-126 and miR-196b), long noncoding RNAs (such as HOTTIP), three-dimensional chromatin interactions, and post-translational protein modifications. Recently, insights into the dynamic regulation of HOXA9 have led to an advanced understanding of the HOXA9 regulome and provided new cancer therapeutic opportunities, including developing inhibitors targeting DOT1L, menin, and ENL proteins. This review summarizes recent advances in understanding the molecular mechanisms controlling HOXA9 regulation and the pharmacological approaches that target HOXA9 regulators to treat HOXA9-driven acute myeloid leukemia.

The phosphorylation to acetylation/methylation cascade in transcriptional regulation: how kinases regulate transcriptional activities of DNA/histone-modifying enzymes

Transcription factors directly regulate gene expression by recognizing and binding to specific DNA sequences, involving the dynamic alterations of chromatin structure and the formation of a complex with different kinds of cofactors, like DNA/histone modifying-enzymes, chromatin remodeling factors, and cell cycle factors. Despite the significance of transcription factors, it remains unclear to determine how these cofactors are regulated to cooperate with transcription factors, especially DNA/histone modifying-enzymes. It has been known that DNA/histone modifying-enzymes are regulated by post-translational modifications. And the most common and important modification is phosphorylation. Even though various DNA/histone modifying-enzymes have been classified and partly explained how phosphorylated sites of these enzymes function characteristically in recent studies. It still needs to find out the relationship between phosphorylation of these enzymes and the diseases-associated transcriptional regulation. Here this review describes how phosphorylation affects the transcription activity of these enzymes and other functions, including protein stability, subcellular localization, binding to chromatin, and interaction with other proteins.

Histone-lysine N-methyltransferase 2 (KMT2) complexes - a new perspective

Histone H3 Lys4 (H3K4) methylation is catalyzed by the Histone-Lysine N-Methyltransferase 2 (KMT2) protein family, and its members are required for gene expression control. In vertebrates, the KMT2s function in large multisubunit complexes known as COMPASS or COMPASS-like complexes (COMplex of Proteins ASsociated with Set1). The activity of these complexes is critical for proper development, and mutation-induced defects in their functioning have frequently been found in human cancers. Moreover, inherited or de novo mutations in KMT2 genes are among the etiological factors in neurodevelopmental disorders such as Kabuki and Kleefstra syndromes. The canonical role of KMT2s is to catalyze H3K4 methylation, which results in a permissive chromatin environment that drives gene expression. However, current findings described in this review demonstrate that these enzymes can regulate processes that are not dependent on methylation: noncatalytic functions of KMT2s include DNA damage response, cell division, and metabolic activities. Moreover, these enzymes may also methylate non-histone substrates and play a methylation-dependent function in the DNA damage response. In this review, we present an overview of the new, noncanonical activities of KMT2 complexes in a variety of cellular processes. These discoveries may have crucial implications for understanding the functions of these methyltransferases in developmental processes, disease, and epigenome-targeting therapeutic strategies in the future.

Design, Synthesis, and Biological Evaluations of DOT1L Peptide Mimetics Targeting the Protein-Protein Interactions between DOT1L and MLL-AF9/MLL-ENL

On the basis of a previously identified DOT1L peptide mimetic (compound 3), a series of novel peptide mimetics were designed and synthesized. These compounds can potently bind to AF9 and ENL either in cell-free binding assays or in leukemia cells, and selectively inhibit the growth of leukemia cells containing mixed lineage leukemia (MLL) fusion proteins. The most potent compound 12 exhibited comparable anticancer cellular activities to those of EPZ5676, a clinical stage enzymatic inhibitor of DOT1L in several leukemia cell lines containing MLL fusion proteins. Mechanism studies for compound 12 indicated that it did not affect the global methylation of H3K79 catalyzed by DOT1L but could effectively suppress the methylation of H3K79 at MLL fusion proteins targeted genes and inhibit the expressions of these genes. Our studies thus demonstrated that inhibiting the protein-protein interactions between DOT1L and MLL fusion proteins is a potentially effective strategy for the treatment of MLL rearranged leukemias.

Targeting matrix metallopeptidase 2 by hydroxyurea selectively kills acute myeloid mixed-lineage leukemia

Oncogene-induced tumorigenesis results in the variation of epigenetic modifications, and in addition to promoting cell immortalization, cancer cells undergo more intense cellular stress than normal cells and depend on other support genes for survival. Chromosomal translocations of mixed-lineage leukemia (MLL) induce aggressive leukemias with an inferior prognosis. Unfortunately, most MLL-rearranged (MLL-r) leukemias are resistant to conventional chemotherapies. Here, we showed that hydroxyurea (HU) could kill MLL-r acute myeloid leukemia (AML) cells through the necroptosis process. HU target these cells by matrix metallopeptidase 2 (MMP2) deficiency rather than subordinate ribonucleotide reductase regulatory subunit M2 (RRM2) inhibition, where MLL directly regulates MMP2 expression and is decreased in most MLL-r AMLs. Moreover, iron chelation of HU is also indispensable for inducing cell stress, and MMP2 is the support factor to protect cells from death. Our preliminary study indicates that MMP2 might play a role in the nonsense-mediated mRNA decay pathway that prevents activation of unfolding protein response under innocuous endoplasmic reticulum stress. Hence, these results reveal a possible strategy of HU application in MLL-r AML treatment and shed new light upon HU repurposing.

Gene Transcription as a Therapeutic Target in Leukemia

Blood malignancies often arise from undifferentiated hematopoietic stem cells or partially differentiated stem-like cells. A tight balance of multipotency and differentiation, cell division, and quiescence underlying normal hematopoiesis requires a special program governed by the transcriptional machinery. Acquisition of drug resistance by tumor cells also involves reprogramming of their transcriptional landscape. Limiting tumor cell plasticity by disabling reprogramming of the gene transcription is a promising strategy for improvement of treatment outcomes. Herein, we review the molecular mechanisms of action of transcription-targeted drugs in hematological malignancies (largely in leukemia) with particular respect to the results of clinical trials.

A Fibrinogen Alpha Fragment Mitigates Chemotherapy-Induced MLL Rearrangements

Rearrangements in the Mixed Lineage Leukemia breakpoint cluster region (MLLbcr) are frequently involved in therapy-induced leukemia, a severe side effect of anti-cancer therapies. Previous work unraveled Endonuclease G as the critical nuclease causing initial breakage in the MLLbcr in response to different types of chemotherapeutic treatment. To identify peptides protecting against therapy-induced leukemia, we screened a hemofiltrate-derived peptide library by use of an enhanced green fluorescent protein (EGFP)-based chromosomal reporter of MLLbcr rearrangements. Chromatographic purification of one active fraction and subsequent mass spectrometry allowed to isolate a C-terminal 27-mer of fibrinogen α encompassing amino acids 603 to 629. The chemically synthesized peptide, termed Fα27, inhibited MLLbcr rearrangements in immortalized hematopoietic cells following treatment with the cytostatics etoposide or doxorubicin. We also provide evidence for protection of primary human hematopoietic stem and progenitor cells from therapy-induced MLLbcr breakage. Of note, fibrinogen has been described to activate toll-like receptor 4 (TLR4). Dissecting the Fα27 mode-of action revealed association of the peptide with TLR4 in an antagonistic fashion affecting downstream NFκB signaling and pro-inflammatory cytokine production. In conclusion, we identified a hemofiltrate-derived peptide inhibitor of the genome destabilizing events causing secondary leukemia in patients undergoing chemotherapy.

Aberrant Activity of Histone-Lysine N-Methyltransferase 2 (KMT2) Complexes in Oncogenesis

KMT2 (histone-lysine N-methyltransferase subclass 2) complexes methylate lysine 4 on the histone H3 tail at gene promoters and gene enhancers and, thus, control the process of gene transcription. These complexes not only play an essential role in normal development but have also been described as involved in the aberrant growth of tissues. KMT2 mutations resulting from the rearrangements of the KMT2A (MLL1) gene at 11q23 are associated with pediatric mixed-lineage leukemias, and recent studies demonstrate that KMT2 genes are frequently mutated in many types of human cancers. Moreover, other components of the KMT2 complexes have been reported to contribute to oncogenesis. This review summarizes the recent advances in our knowledge of the role of KMT2 complexes in cell transformation. In addition, it discusses the therapeutic targeting of different components of the KMT2 complexes.

Novel therapeutic strategies for MLL-rearranged leukemias

MLL rearrangement is one of the key drivers and generally regarded as an independent poor prognostic marker in acute leukemias. The standard of care for MLL-rearranged (MLL-r) leukemias has remained largely unchanged for the past 50 years despite unsatisfying clinical outcomes, so there is an urgent need for novel therapeutic strategies. An increasing body of evidence demonstrates that a vast number of epigenetic regulators are directly or indirectly involved in MLL-r leukemia, and they are responsible for supporting the aberrant gene expression program mediated by MLL-fusions. Unlike genetic mutations, epigenetic modifications can be reversed by pharmacologic targeting of the responsible epigenetic regulators. This leads to significant interest in developing epigenetic therapies for MLL-r leukemia. Intriguingly, many of the epigenetic enzymes also involve in DNA damage response (DDR), which can be potential targets for synthetic lethality-induced therapies. In this review, we will summarize some of the recent advances in the development of epigenetic and DDR therapeutics by targeting epigenetic regulators or protein complexes that mediate MLL-r leukemia gene expression program and key players in DDR that safeguard essential genome integrity. The rationale and molecular mechanisms underpinning the therapeutic effects will also be discussed with a focus on how these treatments can disrupt MLL-fusion mediated transcriptional programs and impair DDR, which may help overcome treatment resistance.

Breakage-Fusion-Bridge Events Trigger Complex Genome Rearrangements and Amplifications in Developmentally Arrested T Cell Lymphomas

To reveal the relative contribution of the recombination activating gene (RAG)1/2 nuclease to lymphomagenesis, we conducted a genome-wide analysis of T cell lymphomas from p53-deficient mice expressing or lacking RAG2. We found that while p53^−/− lymphoblastic T cells harbor primarily ectopic DNA deletions, Rag2^−/−p53^−/− T cell lymphomas display complex genomic rearrangements associated with amplification of the chromosomal location 9qA4-5.3. We show that this amplicon is generated by breakage-fusion-bridge during mitosis and arises distinctly in T cell lymphomas originating from an early progenitor stage. Notably, we report amplification of the corresponding syntenic region (11q23) in a subset of human leukemia leading to the overexpression of several cancer genes, including MLL/KMT2A. Our findings provide direct evidence that lymphocytes undergo malignant transformation through distinct genome architectural routes that are determined by both RAG-dependent and RAG-independent DNA damage and a block in cell development.

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Lymphomas from RAG2/p53- and p53-deficient mice bear distinct genome architectures

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Block in T cell development leads to 9qA4-5.3 rearrangements and amplifications

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Breakage-fusion-bridge events trigger 9qA4-5.3 aberrations in early T cell lymphomas

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The syntenic region 11q23 is amplified in some human hematological cancers

Targeting epigenetic protein-protein interactions with small-molecule inhibitors

Epigenetic protein-protein interactions (PPIs) play essential roles in regulating gene expression, and their dysregulations have been implicated in many diseases. These PPIs are comprised of reader domains recognizing post-translational modifications on histone proteins, and of scaffolding proteins that maintain integrities of epigenetic complexes. Targeting PPIs have become focuses for development of small-molecule inhibitors and anticancer therapeutics. Here we summarize efforts to develop small-molecule inhibitors targeting common epigenetic PPI domains. Potent small molecules have been reported for many domains, yet small domains that recognize methylated lysine side chains on histones are challenging in inhibitor development. We posit that the development of potent inhibitors for difficult-to-prosecute epigenetic PPIs may be achieved by interdisciplinary approaches and extensive explorations of chemical space.

Characterization of myelodysplastic syndromes progressing to acute lymphoblastic leukemia

Myelodysplastic syndromes (MDS) are a heterogeneous group of diseases, with a variable probability of transformation into acute leukemia, which is, in the vast majority of cases, of myeloid lineage. Nevertheless, rare cases of acute lymphoblastic leukemia in patients with previously diagnosed MDS have been reported. We describe a series of 3 cases of MDS/CMML marked with evolution to acute lymphoblastic leukemia (ALL) and provide a comprehensive review of the 49 cases documented in the literature so far. These sporadic events have only been published as single-case reports or small series to date. Such atypical cases emphasize the possibility of major phenotypic switches arising at the leukemic stem cell (LSC) and/or early progenitor levels, as a consequence of epigenetic and genomic events driving these changes in the bone marrow niche.

PARP goes the weasel! Emerging role of PARP inhibitors in acute leukemias

Poly (ADP-ribose) polymerase (PARP) inhibitors, which induce synthetic lethality of BRCA mutant breast and ovarian cancers, are now under active exploration for treatment of acute leukemias, specifically acute myeloid leukemia (AML). Experimental data has revealed that DNA repair deficiencies similar to those found in BRCA mutant solid tumors function in malignant hematopoietic cells to enhance cell survival and promote therapy resistance. Preclinical studies have demonstrated that inhibition of PARP with a variety of agents can dramatically enhance the efficacy of other therapeutic approaches including cytotoxic and epigenetic chemotherapy, small molecule inhibitors (IDH and FLT3 inhibitors) and antibody drug conjugates. This has led to early stage clinical trials of multiple PARP inhibitors (PARPi) for AML patients. Despite small patient numbers, evidence of modest clinical efficacy and tolerability in combinatorial regimens support the further development of PARP inhibition as a novel therapeutic strategy for AML, particularly in select molecular subsets (MLL rearranged, FLT3 and IDH1 mutant disease.

Keeping RNA polymerase II on the run: Functions of MLL fusion partners in transcriptional regulation

Since the identification of key MLL fusion partners as transcription elongation factors regulating expression of HOX cluster genes during hematopoiesis, extensive work from the last decade has resulted in significant progress in our overall mechanistic understanding of role of MLL fusion partner proteins in transcriptional regulation of diverse set of genes beyond just the HOX cluster. In this review, we are going to detail overall understanding of role of MLL fusion partner proteins in transcriptional regulation and thus provide mechanistic insights into possible MLL fusion protein-mediated transcriptional misregulation leading to aberrant hematopoiesis and leukemogenesis.

t(11;16)(q23;p13)/KMT2A-CREBBP in hematologic malignancies: presumptive evidence of myelodysplasia or therapy-related neoplasm?

Fusion partners of KMT2A affect disease phenotype and influence the current World Health Organization classification of hematologic neoplasms. The t(11;16)(q23;p13)/KMT2A-CREBBP is considered presumptive evidence of a myelodysplastic syndrome (MDS) and a MDS-related cytogenetic abnormality in the classification of acute myeloid leukemia (AML). Here, we report 18 cases of hematologic neoplasms with t(11;16). There were 8 males and 10 females with a median age of 51.9 years at time of detection of t(11;16). Of 17 patients with enough clinical information and pathological materials for review, 16 had a history of cytotoxic therapies for various malignancies including 12/15 patients who received topoisomerase II inhibitors, and 15 were classified as having therapy-related neoplasms. The median interval from the diagnosis of primary malignancy to the detection of t(11;16) was 23.2 months. Dysplasia, usually mild, was observed in 7/17 patients. Blasts demonstrated monocytic differentiation in 8/8 patients who developed AML at the time or following detection of t(11;16). t(11;16) was observed as the sole chromosomal abnormality in 10/18 patients. KMT2A rearrangement was confirmed in 11/11 patients. The median survival from the detection of t(11;16) was 15.4 months. In summary, t(11;16)(q23;p13) is rare and overwhelmingly associated with prior exposure of cytotoxic therapy. Instead of being considered presumptive evidence of myelodysplasia, we suggest that the detection of t(11;16) should automatically prompt a search for a history of malignancy and cytotoxic therapy so that proper risk stratification and clinical management are made accordingly. The dismal outcome of patients with t(11;16) is in keeping with that of therapy-related neoplasms.

Facile target validation in an animal model with intracellularly expressed monobodies

Rapidly determining the biological effect of perturbing a site within a potential drug target could guide drug discovery efforts but it remains challenging. Here, we describe a facile target validation approach that exploits monobodies, small synthetic binding proteins that can be fully functionally expressed in cells. We developed a potent and selective monobody to WDR5, a core component of the Mixed Lineage Leukemia (MLL) methyltransferase complex. The monobody bound to the MLL-interaction site of WDR5, the same site for small molecule inhibitors whose efficacy has been demonstrated in cells but not in animals. As a genetically encoded reagent, the monobody inhibited proliferation of an MLL-AF9 cell line in vitro, suppressed its leukemogenesis and conferred survival benefit in an in vivo mouse leukemia model. The capacity of this approach to readily bridge biochemical, structural, cellular characterization and tests in animal models may accelerate discovery and validation of druggable sites.

ChREBP promotes the differentiation of leukemia-initiating cells to inhibit leukemogenesis through the TXNIP/RUNX1 pathways

Targeting leukemia-initiating cells (LICs) is the key to eradicating leukemia and preventing its relapse. Recent studies have indicated that metabolic regulation may play a critical role in the maintenance of stemness in LICs, although the detailed mechanisms are poorly understood. Herein, we provide intriguing evidence showing that a glucose-responsive transcription factor, carbohydrate responsive element binding protein (ChREBP), served as a tumor suppressor rather than an oncogene, as previously described, to inhibit the development of acute myeloid leukemia by promoting the differentiation of LICs. Using an MLL-AF9-induced murine leukemia model, we demonstrated that the deletion of ChREBP resulted in the blockage of the differentiation of LICs and significantly reduced survival in ChREBP-null leukemic mice. However, ChREBP was not required for the normal repopulation abilities of hematopoietic stem cells. ChREBP promoted leukemia cell differentiation through the direct inhibition of RUNX1 or the transactivation of TXNIP to downregulate the RUNX1 level and ROS generation. Moreover, knockdown of ChREBP in human leukemia THP1 cells led to markedly enhanced proliferation and decreased differentiation upon PMA treatment. Collectively, we unraveled an unexpected role of ChREBP in leukemogenesis, which may provide valuable clues for developing novel metabolic strategies for leukemia treatment.

Acute myeloid leukaemia at an early age: Reviewing the interaction between pesticide exposure and KMT2A-rearrangement

Acute myeloid leukaemia (AML) in early childhood is characterised by a high frequency of recurrent genomic aberrations associated with distinct myeloid subtypes, clinical outcomes and pathogenesis. Genomic instability is the first step of pathogenic mechanism in early childhood AML. A sum of adverse events is necessary to the development of infant AML (i-AML), which includes latency of biochemical-molecular and cellular effects. Inherited genetic susceptibility associated with exposures to biotransformation substances can modulate the risk of DNA damage and it is a very important piece in the pathogenic puzzle. In this review, we have aimed to explore the chain of events in the time-points of the natural history of i-AML, which includes maternal exposures during pregnancy, the speculations about the formation of somatic mutations during foetal life and the secondary genomic aberrations associated with i-AML. The modulation of risk conferred by xenobiotic metabolism´s genes variants is the bottom line of the pathogenic process. Since we have conducted observational and molecular investigations in early childhood leukaemia, the data focused here is based on Brazilian findings with summarised results of our experience with epidemiological and molecular studies in early-age leukaemia.

MLL-AF9 leukemias are sensitive to PARP1 inhibitors combined with cytotoxic drugs

PARP1 is required for the maintenance of MLL-AF9 leukemias.PARP1 inhibitors enhance the therapeutic effect of cytotoxic drugs against MLL-AF9 leukemias.

KMT2A promotes melanoma cell growth by targeting hTERT signaling pathway

Melanoma is an aggressive cutaneous malignancy, illuminating the exact mechanisms and finding novel therapeutic targets are urgently needed. In this study, we identified KMT2A as a potential target, which promoted the growth of human melanoma cells. KMT2A knockdown significantly inhibited cell viability and cell migration and induced apoptosis, whereas KMT2A overexpression effectively promoted cell proliferation in various melanoma cell lines. Further study showed that KMT2A regulated melanoma cell growth by targeting the hTERT-dependent signal pathway. Knockdown of KMT2A markedly inhibited the promoter activity and expression of hTERT, and hTERT overexpression rescued the viability inhibition caused by KMT2A knockdown. Moreover, KMT2A knockdown suppressed tumorsphere formation and the expression of cancer stem cell markers, which was also reversed by hTERT overexpression. In addition, the results from a xenograft mouse model confirmed that KMT2A promoted melanoma growth via hTERT signaling. Finally, analyses of clinical samples demonstrated that the expression of KMT2A and hTERT were positively correlated in melanoma tumor tissues, and KMT2A high expression predicted poor prognosis in melanoma patients. Collectively, our results indicate that KMT2A promotes melanoma growth by activating the hTERT signaling, suggesting that the KMT2A/hTERT signaling pathway may be a potential therapeutic target for melanoma.

Regulation of IL-20 Expression by Estradiol through KMT2B-Mediated Epigenetic Modification

Cytokines are low molecular weight regulatory proteins, or glycoproteins, with both tumor-promoting and inhibitory effects on breast cancer growth. Different cytokines play important roles in breast cancer initiation and progression. Here, we show that of the 39 interleukin (IL) genes, IL-20 is the only gene over-expressed in MCF-7 cells treated with estradiol (E2) and that induction of IL-20 expression by estrogen was epigenetically regulated. Methylation of histone H3K4 in the IL-20 promoter was shown to occur via the specific recruitment of KMT2B by estrogen receptor alpha (ERα), but not by other members of the mixed-lineage leukemia (MLL) family of histone methyltransferases. Depletion of KMT2B, or IL-20, disrupts estrogen signaling, attenuates cell proliferation, reduces colony formation, and results in cell cycle arrest. Furthermore, we demonstrated that KMT2B-mediated epigenetic modification also affected the expression of several ERα target genes. IL-20 and KMT2B expression were also associated with ERα-positive breast cancer tissues. We have revealed an important role for KMT2B in the epigenetic transcriptional regulation of cytokine IL-20, and other ERα-responsive genes, in breast cancer cells. Inhibition of IL-20 and KMT2B may have therapeutic benefits in ERα-positive breast cancer.

High-affinity small molecular blockers of mixed lineage leukemia 1 (MLL1)-WDR5 interaction inhibit MLL1 complex H3K4 methyltransferase activity

MLL1-WDR5 protein-protein interaction is essential for MLL1 H3K4 methyltransferase activity. Targeting MLL1 enzymatic activity to regulate expression level of MLL-dependent genes represents a therapeutic strategy for acute leukemia harboring MLL fusion proteins. Herein we reported a series of biphenyl compounds disturbed MLL1-WDR5 interaction. These compounds effectively inhibited MLL1 histone methyltransferase (HMT) activity in vitro and in MV4-11 cell line. The representative compound 30 (DDO-2084) inhibited proliferation and induced apoptosis of MV4-11 cells through deregulating expression level of Hoxa9 and Meis-1 genes, which emphasized our compounds were on-target. Optimization of compound 30 led to high-affinity inhibitors. Especially, compound 42 (DDO-2117, IC₅₀ = 7.6 nM) bearing an amino and a 4-aminobutanamido group was the most potent inhibitor reported to-date, and showed the most potent inhibitory activity (IC₅₀ = 0.19 μM) in HMT assay.

Design and synthesis of benzylpiperidine inhibitors targeting the menin-MLL1 interface

Menin is an essential oncogenic cofactor for mixed lineage leukemia (MLL)-mediated leukemogenesis, functioning through its direct interaction with MLL1 protein. Therefore, targeting the menin-MLL1 protein-protein interface represents a promising strategy to block MLL-mediated leukemogenesis. On the basis of co-crystal structure analysis, starting from thienopyrimidine chemotype, we have investigated the detailed structure-activity relationship of the piperazinyl-dihydrothiazole moiety. Several compounds were found with potent inhibitory activity against menin and better activities in cell-based experiments than MI-2-2. Molecular docking analysis revealed a less explored subpocket, which could be used for the design of new menin-MLL1 inhibitors.

Lessons Learned: HIV Points the Way Towards Precision Treatment of Mixed-Lineage Leukemia

Protein-protein interactions are involved in most if not all pathogenic and pathophysiological processes and represent attractive therapeutic targets. Extensive biological and clinical research efforts have led to the identification and validation of several cellular hubs that are crucially involved in disease pathogenesis. An interesting example of such a hub is the lens epithelium-derived growth factor (LEDGF/p75), a protein that tethers multiple unrelated proteins and protein complexes to the chromatin. Its chromatin-tethering ability is linked to at least two unrelated diseases-HIV infection and MLL-rearranged acute leukemia. In this review we discuss recent progress in our understanding of the interaction of LEDGF/p75 with its binding partners and focus on the first steps towards therapies targeting protein-protein interactions of LEDGF/p75.

Discovering and understanding oncogenic gene fusions through data intensive computational approaches

Although gene fusions have been recognized as important drivers of cancer for decades, our understanding of the prevalence and function of gene fusions has been revolutionized by the rise of next-generation sequencing, advances in bioinformatics theory and an increasing capacity for large-scale computational biology. The computational work on gene fusions has been vastly diverse, and the present state of the literature is fragmented. It will be fruitful to merge three camps of gene fusion bioinformatics that appear to rarely cross over: (i) data-intensive computational work characterizing the molecular biology of gene fusions; (ii) development research on fusion detection tools, candidate fusion prioritization algorithms and dedicated fusion databases and (iii) clinical research that seeks to either therapeutically target fusion transcripts and proteins or leverages advances in detection tools to perform large-scale surveys of gene fusion landscapes in specific cancer types. In this review, we unify these different-yet highly complementary and symbiotic-approaches with the view that increased synergy will catalyze advancements in gene fusion identification, characterization and significance evaluation.

Identification of novel small-molecule inhibitors targeting menin–MLL interaction, repurposing the antidiarrheal loperamide

Scaffold hopping combines with biochemical studies and medicinal chemistry optimizations, leading to potent inhibitors of the menin–MLL interaction.

Taspase1: a 'misunderstood' protease with translational cancer relevance

Proteolysis is not only a critical requirement for life, but the executing enzymes also play important roles in numerous pathological conditions, including cancer. Therefore, targeting proteases is clearly relevant for improving cancer patient care. However, to effectively control proteases, a profound knowledge of their mechanistic function as well as their regulation and downstream signalling in health and disease is required. The highly conserved protease Threonine Aspartase1 (Taspase1) is overexpressed in numerous liquid and solid malignancies and was characterized as a 'non-oncogene addiction' protease. Although Taspase1 was shown to cleave various regulatory proteins in humans as well as leukaemia provoking mixed lineage leukaemia fusions, our knowledge on its detailed functions and the underlying mechanisms contributing to cancer is still incomplete. Despite superficial similarity to type 2 asparaginases as well as Ntn proteases, such as the proteasome, Taspase1-related research so far gives us the picture of a unique protease exhibiting special features. Moreover, neither effective genetic nor chemical inhibitors for this enzyme are available so far, thus hampering not only to further dissect Taspase1's pathobiological functions but also precluding the assessment of its clinical impact. Based on recent insights, we here critically review the current knowledge of Taspase1's structure-function relationship and its mechanistic relevance for tumorigenesis obtained from in vitro and in vivo cancer models. We provide a comprehensive overview of tumour entities for which Taspase1 might be of predictive and therapeutic value, and present the respective experimental evidence. To stimulate progress in the field, a comprehensive overview of Taspase1 targeting approaches is presented, including coverage of Taspase1-related patents. We conclude by discussing future inhibition strategies and relevant challenges, which need to be resolved by the field.

Cleaving for growth: threonine aspartase 1--a protease relevant for development and disease

From the beginning of life, proteases are key to organismal development comprising morphogenesis, cellular differentiation, and cell growth. Regulated proteolytic activity is essential for the orchestration of multiple developmental pathways, and defects in protease activity can account for multiple disease patterns. The highly conserved protease threonine aspartase 1 is a member of such developmental proteases and critically involved in the regulation of complex processes, including segmental identity, head morphogenesis, spermatogenesis, and proliferation. Additionally, threonine aspartase 1 is overexpressed in numerous liquid as well as in solid malignancies. Although threonine aspartase 1 is able to cleave the master regulator mixed lineage leukemia protein as well as other regulatory proteins in humans, our knowledge of its detailed pathobiological function and the underlying molecular mechanisms contributing to development and disease is still incomplete. Moreover, neither effective genetic nor chemical inhibitors for this enzyme are available so far precluding the detailed dissection of the pathobiological functions of threonine aspartase 1. Here, we review the current knowledge of the structure-function relationship of threonine aspartase 1 and its mechanistic impact on substrate-mediated coordination of the cell cycle and development. We discuss threonine aspartase 1-mediated effects on cellular transformation and conclude by presenting a short overview of recent interference strategies.

Use of Genome Engineering to Create Patient Specific MLL Translocations in Primary Human Hematopoietic Stem and Progenitor Cells

One of the challenging questions in cancer biology is how a normal cell transforms into a cancer cell. There is strong evidence that specific chromosomal translocations are a key element in this transformation process. Our studies focus on understanding the developmental mechanism by which a normal stem or progenitor cell transforms into leukemia. Here we used engineered nucleases to induce simultaneous specific double strand breaks in the MLL gene and two different known translocation partners (AF4 and AF9), which resulted in specific chromosomal translocations in K562 cells as well as primary hematopoietic stem and progenitor cells (HSPCs). The initiation of a specific MLL translocation in a small number of HSPCs likely mimics the leukemia-initiating event that occurs in patients. In our studies, the creation of specific MLL translocations in CD34+ cells was not sufficient to transform cells in vitro. Rather, a variety of fates was observed for translocation positive cells including cell loss over time, a transient proliferative advantage followed by loss of the clone, or a persistent proliferative advantage. These studies highlight the application of genome engineering tools in primary human HSPCs to induce and prospectively study the consequences of initiating translocation events in leukemia pathogenesis.

Direct and Propagated Effects of Small Molecules on Protein-Protein Interaction Networks

Networks of protein–protein interactions (PPIs) link all aspects of cellular biology. Dysfunction in the assembly or dynamics of PPI networks is a hallmark of human disease, and as such, there is growing interest in the discovery of small molecules that either promote or inhibit PPIs. PPIs were once considered undruggable because of their relatively large buried surface areas and difficult topologies. Despite these challenges, recent advances in chemical screening methodologies, combined with improvements in structural and computational biology have made some of these targets more tractable. In this review, we highlight developments that have opened the door to potent chemical modulators. We focus on how allostery is being used to produce surprisingly robust changes in PPIs, even for the most challenging targets. We also discuss how interfering with one PPI can propagate changes through the broader web of interactions. Through this analysis, it is becoming clear that a combination of direct and propagated effects on PPI networks is ultimately how small molecules re-shape biology.

Identification of CD34+ and CD34- leukemia-initiating cells in MLL-rearranged human acute lymphoblastic leukemia

Translocation of the mixed-lineage leukemia (MLL) gene with AF4, AF9, or ENL results in acute leukemia with both lymphoid and myeloid involvement. We characterized leukemia-initiating cells (LICs) in primary infant MLL-rearranged leukemia using a xenotransplantation model. In MLL-AF4 patients, CD34(+)CD38(+)CD19(+) and CD34(-)CD19(+) cells initiated leukemia, and in MLL-AF9 patients, CD34(-)CD19(+) cells were LICs. In MLL-ENL patients, either CD34(+) or CD34(-) cells were LICs, depending on the pattern of CD34 expression. In contrast, in patients with these MLL translocations, CD34(+)CD38(-)CD19(-)CD33(-) cells were enriched for normal hematopoietic stem cells (HSCs) with in vivo long-term multilineage hematopoietic repopulation capacity. Although LICs developed leukemic cells with clonal immunoglobulin heavy-chain (IGH) rearrangement in vivo, CD34(+)CD38(-)CD19(-)CD33(-) cells repopulated recipient bone marrow and spleen with B cells, showing broad polyclonal IGH rearrangement and recipient thymus with CD4(+) single positive (SP), CD8(+) SP, and CD4(+)CD8(+) double-positive (DP) T cells. Global gene expression profiling revealed that CD9, CD32, and CD24 were over-represented in MLL-AF4, MLL-AF9, and MLL-ENL LICs compared with normal HSCs. In patient samples, these molecules were expressed in CD34(+)CD38(+) and CD34(-) LICs but not in CD34(+)CD38(-)CD19(-)CD33(-) HSCs. Identification of LICs and LIC-specific molecules in primary human MLL-rearranged acute lymphoblastic leukemia may lead to improved therapeutic strategies for MLL-rearranged leukemia.

Epigenetic control of immunity

Immunity relies on the heterogeneity of immune cells and their ability to respond to pathogen challenges. In the adaptive immune system, lymphocytes display a highly diverse antigen receptor repertoire that matches the vast diversity of pathogens. In the innate immune system, the cell's heterogeneity and phenotypic plasticity enable flexible responses to changes in tissue homeostasis caused by infection or damage. The immune responses are calibrated by the graded activity of immune cells that can vary from yeast-like proliferation to lifetime dormancy. This article describes key epigenetic processes that contribute to the function of immune cells during health and disease.

Nucleoporin Gene Fusions and Hematopoietic Malignancies

Nuclear pore complexes (NPCs) are the sole gateways between the nucleus and the cytoplasm of eukaryotic cells and they mediate all macromolecular trafficking between these cellular compartments. Nucleocytoplasmic transport is highly selective and precisely regulated and as such an important aspect of normal cellular function. Defects in this process or in its machinery have been linked to various human diseases, including cancer. Nucleoporins, which are about 30 proteins that built up NPCs, are critical players in nucleocytoplasmic transport and have also been shown to be key players in numerous other cellular processes, such as cell cycle control and gene expression regulation. This review will focus on the three nucleoporins Nup98, Nup214, and Nup358. Common to them is their significance in nucleocytoplasmic transport, their multiple other functions, and being targets for chromosomal translocations that lead to haematopoietic malignancies, in particular acute myeloid leukaemia. The underlying molecular mechanisms of nucleoporin-associated leukaemias are only poorly understood but share some characteristics and are distinguished by their poor prognosis and therapy outcome.

Classifying leukemia types with chromatin conformation data

Background

Although genetic or epigenetic alterations have been shown to affect the three-dimensional organization of genomes, the utility of chromatin conformation in the classification of human disease has never been addressed.

Results

Here, we explore whether chromatin conformation can be used to classify human leukemia. We map the conformation of the HOXA gene cluster in a panel of cell lines with 5C chromosome conformation capture technology, and use the data to train and test a support vector machine classifier named 3D-SP. We show that 3D-SP is able to accurately distinguish leukemias expressing MLL-fusion proteins from those expressing only wild-type MLL, and that it can also classify leukemia subtypes according to MLL fusion partner, based solely on 5C data.

Conclusions

Our study provides the first proof-of-principle demonstration that chromatin conformation contains the information value necessary for classification of leukemia subtypes.

Proteasome inhibitors evoke latent tumor suppression programs in pro-B MLL leukemias through MLL-AF4

Chromosomal translocations disrupting MLL generate MLL-fusion proteins that induce aggressive leukemias. Unexpectedly, MLL-fusion proteins are rarely observed at high levels, suggesting excessive MLL-fusions may be incompatible with a malignant phenotype. Here, we used clinical proteasome inhibitors, bortezomib and carfilzomib, to reduce the turnover of endogenous MLL-fusions and discovered that accumulated MLL-fusions induce latent, context-dependent tumor suppression programs. Specifically, in MLL pro-B lymphoid, but not myeloid, leukemias, proteasome inhibition triggers apoptosis and cell cycle arrest involving activation cleavage of BID by caspase-8 and upregulation of p27, respectively. Furthermore, proteasome inhibition conferred preliminary benefit to patients with MLL-AF4 leukemia. Hence, feasible strategies to treat cancer-type and oncogene-specific cancers can be improvised through harnessing inherent tumor suppression properties of individual oncogenic fusions.

The epigenetic landscape of acute myeloid leukemia

Acute myeloid leukemia (AML) is a genetically heterogeneous disease. Certain cytogenetic and molecular genetic mutations are recognized to have an impact on prognosis, leading to their inclusion in some prognostic stratification systems. Recently, the advent of high-throughput whole genome or exome sequencing has led to the identification of several novel recurrent mutations in AML, a number of which have been found to involve genes concerned with epigenetic regulation. These genes include in particular DNMT3A, TET2, and IDH1/2, involved with regulation of DNA methylation, and EZH2 and ASXL-1, which are implicated in regulation of histones. However, the precise mechanisms linking these genes to AML pathogenesis have yet to be fully elucidated as has their respective prognostic relevance. As massively parallel DNA sequencing becomes increasingly accessible for patients, there is a need for clarification of the clinical implications of these mutations. This review examines the literature surrounding the biology of these epigenetic modifying genes with regard to leukemogenesis and their clinical and prognostic relevance in AML when mutated.

Challenges and opportunities in targeting the menin-MLL interaction

Menin is an essential co-factor of oncogenic MLL fusion proteins and the menin-MLL interaction is critical for development of acute leukemia in vivo. Targeting the menin-MLL interaction with small molecules represents an attractive strategy to develop new anticancer agents. Recent developments, including determination of menin crystal structure and development of potent small molecule and peptidomimetic inhibitors, demonstrate the feasibility of targeting the menin-MLL interaction. On the other hand, biochemical and structural studies revealed that MLL binds to menin in a complex bivalent mode engaging two MLL motifs, and therefore inhibition of this protein-protein interaction represents a challenge. This review summarizes the most recent achievements in targeting the menin-MLL interaction as well as discusses potential benefits of blocking menin in cancer.

A chromatin modifier genetic screen identifies SIRT2 as a modulator of response to targeted therapies through the regulation of MEK kinase activity

Resistance to targeted therapies is a major problem in cancer treatment. The epidermal growth factor receptor (EGFR) antibody drugs are effective in a subset of colorectal cancers, but the molecular mechanisms of resistance are understood poorly. Genes involved in epigenetic regulation are frequently deregulated in cancer, raising the possibility that such genes also contribute to drug resistance. Using a focused RNA interference library for genes involved in epigenetic regulation, we identify sirtuin2 (SIRT2), an NAD(+)-dependent deacetylase, as a modulator of the response to EGFR inhibitors in colon and lung cancer. SIRT2 loss also conferred resistance to BRAF and MEK inhibitors in BRAF mutant melanoma and KRAS mutant colon cancers, respectively. These results warrant further investigation into the potential role of SIRT2 in resistance to drugs that act in the receptor tyrosine kinase-RAS-RAF-MEK-ERK signaling pathway.

Downregulation of RUNX1/CBFβ by MLL fusion proteins enhances hematopoietic stem cell self-renewal

RUNX1/CBFβ (core binding factor [CBF]) is a heterodimeric transcription factor complex that is frequently involved in chromosomal translocations, point mutations, or deletions in acute leukemia. The mixed lineage leukemia (MLL) gene is also frequently involved in chromosomal translocations or partial tandem duplication in acute leukemia. The MLL protein interacts with RUNX1 and prevents RUNX1 from ubiquitin-mediated degradation. RUNX1/CBFβ recruits MLL to regulate downstream target genes. However, the functional consequence of MLL fusions on RUNX1/CBFβ activity has not been fully understood. In this report, we show that MLL fusion proteins and the N-terminal MLL portion of MLL fusions downregulate RUNX1 and CBFβ protein expression via the MLL CXXC domain and flanking regions. We confirmed this finding in Mll-Af9 knock-in mice and human M4/M5 acute myeloid leukemia (AML) cell lines, with or without MLL translocations, showing that MLL translocations cause a hypomorph phenotype of RUNX1/CBFβ. Overexpression of RUNX1 inhibits the development of AML in Mll-Af9 knock-in mice; conversely, further reducing Runx1/Cbfβ levels accelerates MLL-AF9-mediated AML in bone marrow transplantation assays. These data reveal a newly defined negative regulation of RUNX1/CBFβ by MLL fusion proteins and suggest that targeting RUNX1/CBFβ levels may be a potential therapy for MLLs.

Complex patterns of chromosome 11 aberrations in myeloid malignancies target CBL, MLL, DDB1 and LMO2

Exome sequencing of primary tumors identifies complex somatic mutation patterns. Assignment of relevance of individual somatic mutations is difficult and poses the next challenge for interpretation of next generation sequencing data. Here we present an approach how exome sequencing in combination with SNP microarray data may identify targets of chromosomal aberrations in myeloid malignancies. The rationale of this approach is that hotspots of chromosomal aberrations might also harbor point mutations in the target genes of deletions, gains or uniparental disomies (UPDs). Chromosome 11 is a frequent target of lesions in myeloid malignancies. Therefore, we studied chromosome 11 in a total of 813 samples from 773 individual patients with different myeloid malignancies by SNP microarrays and complemented the data with exome sequencing in selected cases exhibiting chromosome 11 defects. We found gains, losses and UPDs of chromosome 11 in 52 of the 813 samples (6.4%). Chromosome 11q UPDs frequently associated with mutations of CBL. In one patient the 11qUPD amplified somatic mutations in both CBL and the DNA repair gene DDB1. A duplication within MLL exon 3 was detected in another patient with 11qUPD. We identified several common deleted regions (CDR) on chromosome 11. One of the CDRs associated with de novo acute myeloid leukemia (P=0.013). One patient with a deletion at the LMO2 locus harbored an additional point mutation on the other allele indicating that LMO2 might be a tumor suppressor frequently targeted by 11p deletions. Our chromosome-centered analysis indicates that chromosome 11 contains a number of tumor suppressor genes and that the role of this chromosome in myeloid malignancies is more complex than previously recognized.

The broken MLL gene is frequently located outside the inherent chromosome territory in human lymphoid cells treated with DNA topoisomerase II poison etoposide

The mixed lineage leukaemia (MLL) gene is frequently rearranged in secondary leukaemias, in which it could fuse to a variety of different partners. Breakage in the MLL gene preferentially occurs within a ~8 kb region that possesses a strong DNA topoisomerase II cleavage site. It has been proposed that DNA topoisomerase II-mediated DNA cleavage within this and other regions triggers translocations that occur due to incorrect joining of broken DNA ends. To further clarify a possible mechanism for MLL rearrangements, we analysed the frequency of MLL cleavage in cells exposed to etoposide, a DNA topoisomerase II poison commonly used as an anticancer drug, and positioning of the broken 3'-end of the MLL gene in respect to inherent chromosomal territories. It was demonstrated that exposure of human Jurkat cells to etoposide resulted in frequent cleavage of MLL genes. Using MLL-specific break-apart probes we visualised cleaved MLL genes in ~17% of nuclei. Using confocal microscopy and 3D modelling, we demonstrated that in cells treated with etoposide and cultivated for 1 h under normal conditions, ~9% of the broken MLL alleles were present outside the chromosome 11 territory, whereas in both control cells and cells inspected immediately after etoposide treatment, virtually all MLL alleles were present within the chromosomal territory. The data are discussed in the framework of the "breakage first" model of juxtaposing translocation partners. We propose that in the course of repairing DNA topoisomerase II-mediated DNA lesions (removal of stalled DNA topoisomerase II complexes and non-homologous end joining), DNA ends acquire additional mobility, which allows the meeting and incorrect joining of translocation partners.

p27kip1 maintains a subset of leukemia stem cells in the quiescent state in murine MLL-leukemia

MLL (mixed-lineage leukemia)-fusion genes induce the development of leukemia through deregulation of normal MLL target genes, such as HOXA9 and MEIS1. Both HOXA9 and MEIS1 are required for MLL-fusion gene-induced leukemogenesis. Co-expression of HOXA9 and MEIS1 induces acute myeloid leukemia (AML) similar to that seen in mice in which MLL-fusion genes are over-expressed. p27(kip1) (p27 hereafter), a negative regulator of the cell cycle, has also been defined as an MLL target, the expression of which is up-regulated in MLL leukemic cells (LCs). To investigate whether p27 plays a role in the pathogenesis of MLL-leukemia, we examined the effects of p27 deletion (p27(-/-)) on MLL-AF9 (MA9)-induced murine AML development. HOXA9/MEIS1 (H/M)-induced, p27 wild-type (p27(+/+)) and p27(-/-) AML were studied in parallel as controls. We found that LCs from both MA9-AML and H/M-AML can be separated into three fractions, a CD117(-)CD11b(hi) differentiated fraction as well as CD117(+)CD11b(hi) and CD117(+)CD11b(lo), two less differentiated fractions. The CD117(+)CD11b(lo) fraction, comprising only 1-3% of total LCs, expresses higher levels of early hematopoietic progenitor markers but lower levels of mature myeloid cell markers compared to other populations of LCs. p27 is expressed and is required for maintaining the quiescent and drug-resistant states of the CD117(+)CD11b(lo) fraction of MA9-LCs but not of H/M-LCs. p27 deletion significantly compromises the leukemogenic capacity of CD117(+)CD11b(lo) MA9-LCs by reducing the frequency of leukemic stem cells (LSCs) but does not do so in H/M-LCs. In addition, we found that p27 is highly expressed and required for cell cycle arrest in the CD117(-)CD11b(hi) fraction in both types of LCs. Furthermore, we found that c-Myc expression is required for maintaining LCs in an undifferentiated state independently of proliferation. We concluded that p27 represses the proliferation of LCs, which is specifically required for maintaining the quiescent and drug-resistant states of a small subset of MA9-LSCs in collaboration with the differentiation blockage function of c-Myc.