Bernd B. Zeisig

β-Catenin Mediates the Establishment and Drug Resistance of MLL Leukemic Stem Cells

Identification of molecular pathways essential for cancer stem cells is critical for understanding the underlying biology and designing effective cancer therapeutics. Here, we demonstrated that β-catenin was activated during development of MLL leukemic stem cells (LSCs). Suppression of β-catenin reversed LSCs to a pre-LSC-like stage and significantly reduced the growth of human MLL leukemic cells. Conditional deletion of β-catenin completely abolished the oncogenic potential of MLL-transformed cells. In addition, established MLL LSCs that have acquired resistance against GSK3 inhibitors could be resensitized by suppression of β-catenin expression. These results unveil previously unrecognized multifaceted functions of β-catenin in the establishment and drug-resistant properties of MLL stem cells, highlighting it as a potential therapeutic target for an important subset of AMLs.

Functional crosstalk between Bmi1 and MLL/Hoxa9 axis in establishment of normal hematopoietic and leukemic stem cells.

Bmi1 is required for efficient self-renewal of hematopoietic stem cells (HSCs) and leukemic stem cells (LSCs). In this study, we investigated whether leukemia-associated fusion proteins, which differ in their ability to activate Hox expression, could initiate leukemia in the absence of Bmi1. AML1-ETO and PLZF-RARα, which do not activate Hox, triggered senescence in Bmi1(-/-) cells. In contrast, MLL-AF9, which drives expression of Hoxa7 and Hoxa9, readily transformed Bmi1(-/-) cells. MLL-AF9 could not initiate leukemia in Bmi1(-/-)Hoxa9(-/-) mice, which have further compromised HSC functions. But either gene could restore the ability of MLL-AF9 to establish LSCs in the double null background. As reported for Bmi1, Hoxa9 regulates expression of p16(Ink4a)/p19(ARF) locus and could overcome senescence induced by AML1-ETO. Together, these results reveal an important functional interplay between MLL/Hox and Bmi1 in regulating cellular senescence for LSC development, suggesting that a synergistic targeting of both molecules is required to eradicate a broader spectrum of LSCs.

SnapShot: Acute myeloid leukemia.

20%-25% (28%-35%); High blast count; Poor prognosisespecially in cases with high mutant to WT allelic ratio5%-7% (10%-14%); Prognostic impact remains controversial10% (9%-14%); Enriched in CBF AML; Prognosis unknown<5% (<5%); 25-30% in CBF leukaemia<2% (2%); Prognosis unknown25%-30% (40%-65%); M4 blast morphology lacks CD34 expression; Hox gene upregulation; Favorable prognosis inthe presence of

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

Although both heterodimeric subunits of core binding factors (AML1/RUNX1 and CBFβ) 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 CBFβ for wild-type AML1 function and its direct involvement in chromosomal translocation, we demonstrate that both the expression and interaction with CBFβ 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.

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

Summary 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.

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 p16INK4a as the key target mediating the effects of HLX1 in senescence. HLX1 represses p16INK4a transcription by recruiting PRCs and HDAC1. This mechanism has broader implications, as HLX1 also regulates a subset of PRC targets besides p16INK4a. Finally, sampling members of the Homeobox family, we identified multiple genes with ability to repress p16INK4a. Among them, we found HOXA9 (Homeobox A9), a putative oncogene in leukaemia, which also recruits PRCs and HDAC1 to regulate p16INK4a. Our results reveal an unexpected and conserved interplay between homeodomain‐containing proteins and PRCs with implications in senescence, development and cancer.

Leukemic transformation by the APL fusion protein PRKAR1A-RARα critically depends on recruitment of RXRα

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.

The role of CBFβ in AML1-ETO's activity

To the editor: Identification of interacting proteins essential for oncogenic functions of leukemia-associated transcription factors is important for understanding the underlying transformation mechanisms and designing effective cancer therapeutics.[1][1] We have recently found that homo-

Transcriptional memory of cells of origin overrides β‐catenin requirement of MLL cancer stem cells

While β‐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 β‐catenin‐independent transformation in MLL‐CSCs derived from hematopoietic stem cell (HSC)‐enriched LSK population but not myeloid–granulocyte progenitors. Mechanistically, β‐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 β‐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 β‐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 β‐catenin/Hoxa9/Prmt1 in governing leukemic self‐renewal.

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.