HOTTIP-dependent R-loop formation regulates CTCF boundary activity and TAD integrity in leukemia.

HOTTIP lncRNA is highly expressed in acute myeloid leukemia (AML) driven by MLL rearrangements or NPM1 mutations to mediate HOXA topologically associated domain (TAD) formation and drive aberrant transcription. However, the mechanism through which HOTTIP accesses CCCTC-binding factor (CTCF) chromatin boundaries and regulates CTCF-mediated genome topology remains unknown. Here, we show that HOTTIP directly interacts with and regulates a fraction of CTCF-binding sites (CBSs) in the AML genome by recruiting CTCF/cohesin complex and R-loop-associated regulators to form R-loops. HOTTIP-mediated R-loops reinforce the CTCF boundary and facilitate formation of TADs to drive gene transcription. Either deleting CBS or targeting RNase H to eliminate R-loops in the boundary CBS of ß-catenin TAD impaired CTCF boundary activity, inhibited promoter/enhancer interactions, reduced ß-catenin target expression, and mitigated leukemogenesis in xenograft mouse models with aberrant HOTTIP expression. Thus, HOTTIP-mediated R-loop formation directly reinforces CTCF chromatin boundary activity and TAD integrity to drive oncogene transcription and leukemia development.

Therapeutic Opportunities of Targeting Canonical and Noncanonical PcG/TrxG Functions in Acute Myeloid Leukemia.

Transcriptional deregulation is a key driver of acute myeloid leukemia (AML), a heterogeneous blood cancer with poor survival rates. Polycomb group (PcG) and Trithorax group (TrxG) genes, originally identified in Drosophila melanogaster several decades ago as master regulators of cellular identity and epigenetic memory, not only are important in mammalian development but also play a key role in AML disease biology. In addition to their classical canonical antagonistic transcriptional functions, noncanonical synergistic and nontranscriptional functions of PcG and TrxG are emerging. Here, we review the biochemical properties of major mammalian PcG and TrxG complexes and their roles in AML disease biology, including disease maintenance as well as drug resistance. We summarize current efforts on targeting PcG and TrxG for treatment of AML and propose rational synthetic lethality and drug-induced antagonistic pleiotropy options involving PcG and TrxG as potential new therapeutic avenues for treatment of AML.

Transcriptional memory of cells of origin overrides ß-catenin requirement of MLL cancer stem cells.

While ß-catenin has been demonstrated as an essential molecule and therapeutic target for various cancer stem cells (CSCs) including those driven by MLL fusions, here we show that transcriptional memory from cells of origin predicts AML patient survival and allows ß-catenin-independent transformation in MLL-CSCs derived from hematopoietic stem cell (HSC)-enriched LSK population but not myeloid-granulocyte progenitors. Mechanistically, ß-catenin regulates expression of downstream targets of a key transcriptional memory gene, Hoxa9 that is highly enriched in LSK-derived MLL-CSCs and helps sustain leukemic self-renewal. Suppression of Hoxa9 sensitizes LSK-derived MLL-CSCs to ß-catenin inhibition resulting in abolishment of CSC transcriptional program and transformation ability. In addition, further molecular and functional analyses identified Prmt1 as a key common downstream mediator for ß-catenin/Hoxa9 functions in LSK-derived MLL-CSCs. Together, these findings not only uncover an unexpectedly important role of cells of origin transcriptional memory in regulating CSC self-renewal, but also reveal a novel molecular network mediated by ß-catenin/Hoxa9/Prmt1 in governing leukemic self-renewal.

Interplay between Homeobox proteins and Polycomb repressive complexes in p16INK4a regulation.

The INK4/ARF locus regulates senescence and is frequently altered in cancer. In normal cells, the INK4/ARF locus is found silenced by Polycomb repressive complexes (PRCs). Which are the mechanisms responsible for the recruitment of PRCs to INK4/ARF and their other target genes remains unclear. In a genetic screen for transcription factors regulating senescence, we identified the homeodomain-containing protein HLX1 (H2.0-like homeobox 1). Expression of HLX1 extends cellular lifespan and blunts oncogene-induced senescence. Using quantitative proteomics, we identified p16(INK4a) as the key target mediating the effects of HLX1 in senescence. HLX1 represses p16(INK4a) transcription by recruiting PRCs and HDAC1. This mechanism has broader implications, as HLX1 also regulates a subset of PRC targets besides p16(INK4a). Finally, sampling members of the Homeobox family, we identified multiple genes with ability to repress p16(INK4a). Among them, we found HOXA9 (Homeobox A9), a putative oncogene in leukaemia, which also recruits PRCs and HDAC1 to regulate p16(INK4a). Our results reveal an unexpected and conserved interplay between homeodomain-containing proteins and PRCs with implications in senescence, development and cancer.

Recruitment of RXR by homotetrameric RARalpha fusion proteins is essential for transformation.

While formation of higher-order oncogenic transcriptional complexes is critical for RARalpha fusion proteins in acute promyelocytic leukemia, the essential components and their roles in mediating transformation are still largely unknown. To this end, the present study demonstrates that homodimerization is not sufficient for RARalpha fusion-mediated transformation, which requires higher-order homotetramerization. Surprisingly, intrinsic homo-oligomeric DNA binding by the fusion proteins is also dispensable. Importantly, higher-order RXR/RARalpha fusion hetero-oligomeric complexes that aberrantly recruit transcriptional corepressors to downstream targets are essential for transformation. Intervention of RXR-dependent pathways by panRXR-agonists or RXRalpha shRNAs suppresses RARalpha fusion-mediated transformation. Taken together, these results define the oncogenic threshold for self-association and reveal the pathological significance of higher-order RARalpha fusion/RXR hetero-oligomeric complexes and their potential value as a therapeutic target.

Forced homo-oligomerization of RARalpha leads to transformation of primary hematopoietic cells.

Almost 100% of APL patients carry chimeric transcripts encoding truncated RARalpha fused to homo-oligomerization domains from partner proteins. To gain further insights into the cellular transformation mechanisms mediated by RARalpha fusion proteins, thorough structure/function analyses have been performed and identified the POZ homo-oligomerization domain as the minimal transformation domain that is necessary and sufficient for PLZF-RARalpha-mediated in vitro transformation of primary hematopoietic cells. A transformation-incompetent PLZF-RARalpha mutant defective in homo-oligomerization but not corepressor interaction could be rescued by synthetic FKBP-oligomerization domains. Furthermore, an artificial FKBP-RARalpha construct not only mimicked various biochemical properties of bona fide RARalpha fusion proteins but also mediated an ATRA-dependent transformation. Taken together, these findings endorse an oligomerization-dependent mechanism for RARalpha-mediated transformation and suggest a potential avenue for molecular therapy.

Leukemic transformation by the APL fusion protein PRKAR1A-RAR{alpha} critically depends on recruitment of RXR{alpha}.

PRKAR1A (R1A)-retinoic acid receptor-alpha (R1A-RARalpha) is the sixth RARalpha-containing fusion protein in acute promyelocytic leukemia (APL). Using the murine bone-marrow retroviral transduction/transformation assay, we showed that R1A-RARalpha fusion protein could transform bone-marrow progenitor/stem cells. In gel-shift assays, R1A-RARalpha was able to bind to a panel of retinoic acid response elements both as a homodimer and as a heterodimer with RXRalpha, and demonstrated distinct DNA-binding characteristics compared with wild-type RARalpha/RXRalpha or other X-RARalpha chimeric proteins. The ratio of R1A-RARalpha to RXRalpha proteins affected the retinoic acid response element interaction pattern of R1A-RARalpha/RXRalpha complexes. Studies comparing R1A-RARalpha with R1A-RARalpha(DeltaRIIa) demonstrated that the RIIa protein interaction domain located within R1A was responsible for R1A-RARalpha homodimeric DNA binding and interaction with wild-type R1A protein. However, the RIIa domain was not required for R1A-RARalpha-mediated transformation because its deletion in R1A-RARalpha(DeltaRIIa) did not compromise its transformation capability. In contrast, introduction of point mutations within the RARalpha portion of either R1A-RARalpha or R1A-RARalpha(DeltaRIIa), previously demonstrated to eliminate RXRalpha interaction or treatment of transduced cells with RXRalpha shRNA or a RXRalpha agonist, reduced transformation capability. Thus, leukemic transformation by APL fusion protein PRKAR1A-RARalpha is critically dependent on RXRalpha, which suggests RXRalpha is a promising target for APL.

Transforming activity of AML1-ETO is independent of CBFbeta and ETO interaction but requires formation of homo-oligomeric complexes.

Although both heterodimeric subunits of core binding factors (AML1/RUNX1 and CBFbeta) essential for normal hematopoiesis are frequently mutated to form different chimeric fusion proteins in acute leukemia, the underlying molecular mechanisms and structural domains required for cellular transformation remain largely unknown. Despite the critical role of CBFbeta for wild-type AML1 function and its direct involvement in chromosomal translocation, we demonstrate that both the expression and interaction with CBFbeta are superfluous for AML1-ETO (AE)-mediated transformation of primary hematopoietic cells. Similarly, the hetero-oligomeric interaction with transcriptional repressor ETO family proteins and the highly conserved NHR1 domain in AE fusion are also dispensable for transforming activity. In contrast, AE-mediated transformation is critically dependent on the DNA binding and homo-oligomeric properties of the fusion. Abolishment of homo-oligomerization by a small-molecule inhibitor could specifically suppress AML1 fusion-mediated transformation of primary hematopoietic cells. Together, these results not only identify the essential molecular components but also potential avenues for therapeutic targeting of AE-mediated leukemogenesis.

Reconstruction of Human AML Using Functionally and Immunophenotypically Defined Human Haematopoietic Stem and Progenitor Cells as Targeted Populations.

Acute myeloid leukaemia (AML) is a highly heterogenous blood cancer, in which the expansion of aberrant myeloid blood cells interferes with the generation and function of normal blood cells. Although key driver mutations and their associated inhibitors have been identified in the last decade, they have not been fully translated into better survival rates for AML patients, which remain dismal. In addition to DNA mutation, studies in mouse models strongly suggest that the cell of origin, where the driver mutation (such as MLL fusions) occurs, emerges as an additional factor that determines the treatment outcome in AML. To investigate its functional relevance in human disease, we have recently reported that AML driven by MLL fusions can transform immunophenotypically and functionally distinctive human hematopoietic stem cells (HSCs) or myeloid progenitors resulting in immunophenotypically indistinguishable human AML. Intriguingly, these cells display differential treatment sensitivities to current treatments, attesting the cell of origin as an important determinant governing treatment outcome for AML. To further facilitate this line of investigation, here we describe a comprehensive disease modelling protocol using human primary haematopoietic cells, which covers all the key steps, from the isolation of immunophenotypically defined human primary haematopoietic stem and progenitor populations, to oncogene transfer via viral transduction, the in vitro liquid culture assay, and finally the xenotransplantation into immunocompromised mice.

Functional reconstruction of human AML reveals stem cell origin and vulnerability of treatment-resistant MLL-rearranged leukemia.

Chemoresistance remains the major challenge for successful treatment of acute myeloid leukemia (AML). Although recent mouse studies suggest that treatment response of genetically and immunophenotypically indistinguishable AML can be influenced by their different cells of origin, corresponding evidence in human disease is still largely lacking. By combining prospective disease modeling using highly purified human hematopoietic stem or progenitor cells with retrospective deconvolution study of leukemia stem cells (LSCs) from primary patient samples, we identified human hematopoietic stem cells (HSCs) and common myeloid progenitors (CMPs) as two distinctive origins of human AML driven by Mixed Lineage Leukemia (MLL) gene fusions (MLL-AML). Despite LSCs from either MLL-rearranged HSCs or MLL-rearranged CMPs having a mature CD34-/lo/CD38+ immunophenotype in both a humanized mouse model and primary patient samples, the resulting AML cells exhibited contrasting responses to chemotherapy. HSC-derived MLL-AML was highly resistant to chemotherapy and expressed elevated amounts of the multispecific anion transporter ABCC3. Inhibition of ABCC3 by shRNA-mediated knockdown or with small-molecule inhibitor fidaxomicin, currently used for diarrhea associated with Clostridium difficile infection, effectively resensitized HSC-derived MLL-AML toward standard chemotherapeutic drugs. This study not only functionally established two distinctive origins of human LSCs for MLL-AML and their role in mediating chemoresistance but also identified a potential therapeutic avenue for stem cell-associated treatment resistance by repurposing a well-tolerated antidiarrhea drug already used in the clinic.

Retroviral/Lentiviral transduction and transformation assay.

Non-random chromosomal translocations can be found in about half of acute leukaemia patients and mostly lead to either over-expression of proto-oncogenes or creation of novel fusion genes. To assess the oncogenic potential and characterize the underlying mechanisms mediated by these candidate oncoproteins, a retroviral transduction/transformation assay (RTTA) has been successfully employed to study the biological impacts of a number of proto-oncoproteins and novel fusion proteins in primary hematopoietic cells both in vitro and in vivo. To further widen the application of the RTTA, a lentiviral transduction/transformation assay (LTTA) has also been developed to target the most quiescent hematopoietic stem cells (HSCs). This chapter will cover both the RTTA and LTTA for studying candidate oncogenes involved in human leukaemia.

HOTTIP lncRNA Promotes Hematopoietic Stem Cell Self-Renewal Leading to AML-like Disease in Mice.

Long non-coding RNAs (lncRNAs) are critical for regulating HOX genes, aberration of which is a dominant mechanism for leukemic transformation. How HOX gene-associated lncRNAs regulate hematopoietic stem cell (HSC) function and contribute to leukemogenesis remains elusive. We found that HOTTIP is aberrantly activated in acute myeloid leukemia (AML) to alter HOXA-driven topologically associated domain (TAD) and gene expression. HOTTIP loss attenuates leukemogenesis of transplanted mice, while reactivation of HOTTIP restores leukemic TADs, transcription, and leukemogenesis in the CTCF-boundary-attenuated AML cells. Hottip aberration in mice abnormally promotes HSC self-renewal leading to AML-like disease by altering the homeotic/hematopoietic gene-associated chromatin signature and transcription program. Hottip aberration acts as an oncogenic event to perturb HSC function by reprogramming leukemic-associated chromatin and gene transcription.

Hoxa9 and Meis1 are key targets for MLL-ENL-mediated cellular immortalization.

MLL fusion proteins are oncogenic transcription factors that are associated with aggressive lymphoid and myeloid leukemias. We constructed an inducible MLL fusion, MLL-ENL-ERtm, that rendered the transcriptional and transforming properties of MLL-ENL strictly dependent on the presence of 4-hydroxy-tamoxifen. MLL-ENL-ERtm-immortalized hematopoietic cells required 4-hydroxy-tamoxifen for continuous growth and differentiated terminally upon tamoxifen withdrawal. Microarray analysis performed on these conditionally transformed cells revealed Hoxa9 and Hoxa7 as well as the Hox coregulators Meis1 and Pbx3 among the targets upregulated by MLL-ENL-ERtm. Overexpression of the Hox repressor Bmi-1 inhibited the growth-transforming activity of MLL-ENL. Moreover, the enforced expression of Hoxa9 in combination with Meis1 was sufficient to substitute for MLL-ENL-ERtm function and to maintain a state of continuous proliferation and differentiation arrest. These results suggest that MLL fusion proteins impose a reversible block on myeloid differentiation through aberrant activation of a limited set of homeobox genes and Hox coregulators that are consistently expressed in MLL-associated leukemias.

The eleven-nineteen-leukemia protein ENL connects nuclear MLL fusion partners with chromatin.

Mixed lineage leukemia (MLL) fusion proteins are derived from translocations at 11q23 that occur in aggressive subtypes of leukemia. As a consequence, MLL is joined to different unrelated proteins to form oncogenic transcription factors. Here we demonstrate a direct interaction between several nuclear MLL fusion partners and present evidence for a role of these proteins in histone binding. In two-hybrid studies, ENL interacted with AF4 and AF5q31 as well as with a fragment of AF10. A structure-function analysis revealed that the AF4/AF5q31/AF10 binding domain in ENL coincided with the C-terminus that is essential for transformation by MLL-ENL. The ENL/AF4 association was corroborated by GST-pulldown experiments and by mutual coprecipitation. Both proteins colocalized in vivo in a nuclear speckled pattern. Moreover, AF4 and ENL coeluted on sizing columns together with the known ENL binding partner Polycomb3, suggesting the presence of a multiprotein complex. The overexpression of ENL alone activated a reporter construct and a mutational screen indicated the conserved YEATS domain as essential for this function. Overlay and pulldown-assays finally showed a specific and YEATS domain-dependent association of ENL with histones H3 and H1. In summary, our studies support a common role for nuclear MLL fusion partners in chromatin biology.

Combinatorial Tethering: A Novel Mode to Recruit Non-canonical PRC1 for Normal and Malignant GC B Cell Development.

Polycomb repressive complexes (PRCs) are key to normal development and are frequently deregulated in human cancer. In this issue of Cancer Cell, Béguelin et al. report a mechanism of non-canonical PRC1 recruitment by BCL6 in collaboration with EZH2-mediated H3K27me3 for establishment of stable repressive complexes in germinal center B cells.

Targeting Aberrant Epigenetic Networks Mediated by PRMT1 and KDM4C in Acute Myeloid Leukemia.

Transcriptional deregulation plays a major role in acute myeloid leukemia, and therefore identification of epigenetic modifying enzymes essential for the maintenance of oncogenic transcription programs holds the key to better understanding of the biology and designing effective therapeutic strategies for the disease. Here we provide experimental evidence for the functional involvement and therapeutic potential of targeting PRMT1, an H4R3 methyltransferase, in various MLL and non-MLL leukemias. PRMT1 is necessary but not sufficient for leukemic transformation, which requires co-recruitment of KDM4C, an H3K9 demethylase, by chimeric transcription factors to mediate epigenetic reprogramming. Pharmacological inhibition of KDM4C/PRMT1 suppresses transcription and transformation ability of MLL fusions and MOZ-TIF2, revealing a tractable aberrant epigenetic circuitry mediated by KDM4C and PRMT1 in acute leukemia.

A knockout Combo: eradicating AML Stem Cells with TKI plus SIRT1 inhibition.

SIRT1 inhibition facilitates elimination of CML stem cells by Imatinib, in part via p53 activation. In this issue of Cell Stem Cell, Li et al. (2014) demonstrate a similar role for SIRT1 inhibition in eradicating FLT3-ITD AML stem cells, potentially through a positive feedback loop with c-MYC, highlighting SIRT1 as a potential target in combination cancer therapy.

Linking MLL Leukemia with Integrin Signaling.

Identification of tractable signaling molecules essential for leukemogenesis facilitates the development of effective targeted therapies. In this issue of Cancer Cell, Miller and colleagues report that Integrin Beta 3, which is largely dispensable for normal hematopoiesis, plays an important role and is a potential therapeutic target in mixed lineage leukemia.