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Patients with relapsed/refractory (R/R) higher-risk myelodysplastic syndromes (MDS) have a dismal median overall survival (OS) after failing hypomethylating agent (HMA) treatment. There is no standard of care for patients after HMA therapy failure; hence, there is a critical need for effective therapeutic strategies. Herein, we present the safety and efficacy of venetoclax + azacitidine in patients with R/R MDS. This phase 1b, open-label, multicenter study enrolled patients ≥18 years. Patients were treated with escalating doses of oral venetoclax: 100, 200, or 400 mg daily for 14 days every 28-day cycle. Azacitidine was administered on Days 1-7 every cycle at 75 mg/m

Primary analyses of cohort 1a of the REFINE trial showed that addition of navitoclax to ruxolitinib induced a 35% or greater reduction in spleen volume (SVRREFINE is a phase 2, multicentre, open-label trial designed to assess the activity and safety of navitoclax alone or in combination with ruxolitinib in patients with primary or secondary (post-polycythaemia vera or post-essential thrombocythaemia) myelofibrosis. Cohort 1a of the study included patients who had disease progression or suboptimal response on stable ruxolitinib monotherapy. Patients in cohort 1a, who had previously received ruxolitinib for 12 weeks or more, continued their current stable dose, and navitoclax was orally administered at 50 mg per day and escalated weekly to a maximum of 300 mg per day, based on tolerability. The primary activity endpoint was SVRBetween Nov 14, 2017, and April 10, 2019, 34 patients in cohort 1a received at least one dose of navitoclax plus ruxolitinib. 23 (68%) patients were male, with 32 (94%) being White. At data cutoff (May 6, 2021), the median follow-up for survivors was 26·2 months (IQR 21·9-32·3). 33 patients were evaluable for biomarker analyses; 19 (58%) had high molecular risk mutations. Five (31%) of 16 patients had SVRThese biomarker analyses reveal clinically meaningful splenic responses independent of high molecular risk mutation status in patients treated with navitoclax plus ruxolitinib who were not benefiting from ruxolitinib monotherapy. Furthermore, the overall survival benefit observed in those with an improvement in fibrosis or a reduction in variant allele frequency is suggestive of disease modification, implying the therapeutic potential of adding navitoclax to ruxolitinib for patients with myelofibrosis who had disease progression or suboptimal response to ruxolitinib monotherapy.AbbVie.

Figure S4. MIR300 anti-proliferative activity accounts for BMM-induced LSC entry into quiescence. A, Structure of 14q32 DLK1-DIO3 genomic imprinted locus hosting the MEG3-regulated human MIR300. B, MIR300 levels in 5-Aza- or DMSO-treated (24h) Ph+ cells. C, Effect of hypoxia on proliferation of CFSE+CD34+ CML-BC cells. D, left: Effect of MSC (HS-5)-derived CM on LAMA-84 proliferation expressed as fold changes of CFSE mean of fluorescence intensity (MFI)+/-SEM; middle: pro-apoptotic effect of PAD (FTY720; 2.5uM) and DMSO (control) on HS-5-cultured LAMA-84 cells; right: Effect of MSC (HS-5)-derived CM BCR-ABL1 expression (anti-ABL1) and activity anti-PY), phospho-BCR-ABL1, JAK2 expression and activity JAK2 Y1007/1008, PP2A activity (pPP2AY307 inactive form) and GRB2 used as a control (blots are representative of three independent experiments). E, Levels of C/EBPbeta and GRB2 mRNA and protein in HS-5 cells exposed to hypoxia (48h; 1% O2). F, Effect of neutralizing TGFbeta antibody (anti-TGFb Ab; 48h, 1.25 μg/ml) on MIR300 levels in CD34+ CML-BC cells. G, Effect of ectopic C/EBPalpha (MigR1-deltauORF-C/EBPalpha-HA) and C/EBPbeta (MigR1-C/EBPB-ERTAM) on MIR300 levels in K562 cells. Immunoblot shows levels of C/EBPbeta and GRB2 in normoxic and hypoxic K562 cells.

Range values of controls for the indicated experiments

Figure S3. MIR300 acts as master PP2A activator and inhibitor of G1/S transition. A, (left) Additional effects of MIR300 on SET, CCND2 and CDK6 expression; (right) KEGG/GO analysis of MIR300 effects on signal transduction pathways. B, CSmiRTar (filters: bone marrow normal and myeloid leukemia cells) and miRDIP-ComiR integrated analyses show functional clustering of predicted/validated MIR300 targets.

Figure S6. Selective suppression of MIR300 pro-apoptotic but not anti-proliferative activity by TUG1 lncRNA in quiescent LSCs. A, BloodSpot array-based TUG1 expression levels during normal myelopoiesis and in myeloid neoplams. B, GEO Profiles show TUG1 levels in in lineage-negative (Lin-) and -positive (Lin+) CD34- and CD34human stem/progenitor cells from healthy individuals. C, Experimental data- and current literature-based graphic representation of signaling network controlling CML LSC quiescence and survival through the BMM-C/EBPbeta-MIR300 and BMM-TGFbeta-FoxM1 pathways. Dotted lines indicate inactive pathways, line thickness indicates relevance of the signal for LSC quiescence. Red lines indicate signals increasing MIR300 levels. Black lines signals increasing TUG1 levels. (bottom) effects of different TUG1 levels on CML leukemic stem (LSC) and progenitor (LPC) cell fate.

Figure S2. KEGG/GO analysis of MIR300 on PP2A-regulated signal transduction pathways. Cartoon shows the SET-dependent PP2A Inhibitory pathway in CML and the pleiotropic inhibitory effect of PP2A activation on validated and predicted MIR300 targets regulating G1/S cell cycle transition, Wnt-beta-catenin, TGFbeta, JAK-STAT, PI-3K-Akt, RAS-MAPK and Notch signaling pathways.

Figure S5. BMM-induced MIR300 anti-proliferative and PP2A-activating functions impair NK cell immune-response. A, PP2A-dependent regulation of MIR300 and miR-155 validated pathways predicted to occur also in the bone marrow endosteal niche. B, Effect of hypoxia (1% O2, 7 days) on CFSE+NK cell proliferation (% CFSE mean of fluorescence). C, HS-5 exosomal miRNAs reported as negative regulators of NK cell proliferation/activity and their experimentally validated targets. miRNA RNAseq was performed on an Illumina platform using libraries derived from 100 ng RNA/sample from HS-5 exosome purifications (n=3). D, Precursor (pre-miR-155) miR-155 (BIC) levels in resting and IL-12/IL-18 (18h)-stimulated NK cells exposed (48h) to HS-5 exosomes. E, Effect of hypoxia (1% O2) and HS-5 CM (48h) on TUG1 expression in CD56+CD3- primary NK and NK-92 cells. F, Effect of CpG-TUG1-shRNA and CpG-scramble (200-500 nM, 5 days) on IL-2-induced NK cell proliferation (% cell number). Data are represented as mean+/-SEM.

Figure S1. MIR300 activity in quiescent leukemic stem and progenitor cells. CFC-replating assays shows effects of lentiviral-mediated ectopic MIR300 expression, 250 nM and 500 nM CpG-miR-300 on serial replating activity (2nd replating) of leukemic chronic and acute CML and normal UCB CD34+CD38- HSC-enriched cell fractions. Infection with lentiviral empty vector and treatment with CpG-anti-MIR300 and CpG-scramble served as controls.

Persistence of drug-resistant quiescent leukemic stem cells (LSC) and impaired natural killer (NK) cell immune response account for relapse of chronic myelogenous leukemia (CML). Inactivation of protein phosphatase 2A (PP2A) is essential for CML-quiescent LSC survival and NK cell antitumor activity. Here we show that

Persistence of drug-resistant quiescent leukemic stem cells (LSC) and impaired natural killer (NK) cell immune response account for relapse of chronic myelogenous leukemia (CML). Inactivation of protein phosphatase 2A (PP2A) is essential for CML-quiescent LSC survival and NK cell antitumor activity. Here we show that MIR300 has antiproliferative and PP2A-activating functions that are dose dependently differentially induced by CCND2/CDK6 and SET inhibition, respectively. MIR300 is upregulated in CML LSCs and NK cells by bone marrow microenvironment (BMM) signals to induce quiescence and impair immune response, respectively. Conversely, BCR-ABL1 downregulates MIR300 in CML progenitors to prevent growth arrest and PP2A-mediated apoptosis. Quiescent LSCs escape apoptosis by upregulating TUG1 long noncoding RNA that uncouples and limits MIR300 function to cytostasis. Genetic and pharmacologic MIR300 modulation and/or PP2A-activating drug treatment restore NK cell activity, inhibit BMM-induced growth arrest, and selectively trigger LSC apoptosis in vitro and in patient-derived xenografts; hence, the importance of MIR300 and PP2A activity for CML development and therapy. Significance: Tumor-naïve microenvironment–induced MIR300 is the only tumor suppressor miRNA that induces CML LSC quiescence while inhibiting NK cell antitumor immune response, and CML LSC/progenitor cell apoptosis through its anti-proliferative and PP2A-activating functions, respectively. Thus, the importance of MIR300 and PP2A-activating drugs for formation/survival and eradication of drug-resistant CML LSCs, respectively. See related commentary by Broxmeyer, p. 13. This article is highlighted in the In This Issue feature, p. 5

Drug-resistance of tumor-initiating cells, impaired NK cell immune-response, PP2A loss-of-function and aberrant miRNA expression are cancer features resulting from microenvironmental- and tumor-specific signals. Here we report that genomic-imprintedMIR300is a cell context-independent dual function tumor suppressor which is upregulated in quiescent leukemic stem (LSC) and NK cells by microenvironmental signals to induce quiescence and impair immune-response, respectively, but inhibited in CML and AML proliferating blasts to prevent PP2A-induced apoptosis.MIR300anti-proliferative and PP2A-activating functions are differentially activated through dose-dependent CCND2/CDK6 and SET inhibition, respectively. LSCs escape PP2A-mediated apoptosis through TUG1 lncRNA that uncouples and limitsMIR300functions to cytostasis by regulating unbound-MIR300levels. HaltingMIR300homeostasis restores NK cell activity and suppresses leukemic but not normal hematopoiesis by eradicating nearly all LSCs. Thus,MIR300tumor suppressor activity is essential and therapeutically important for LSC-driven leukemias.

The success of tyrosine kinase inhibitors (TKIs) in treating chronic myeloid leukemia (CML) depends on the requirement for BCR-ABL1 kinase activity in CML progenitors. However, CML quiescent HSCs are TKI resistant and represent a BCR-ABL1 kinase–independent disease reservoir. Here we have shown that persistence of leukemic HSCs in BM requires inhibition of the tumor suppressor protein phosphatase 2A (PP2A) and expression — but not activity — of the BCR-ABL1 oncogene. Examination of HSCs from CML patients and healthy individuals revealed that PP2A activity was suppressed in CML compared with normal HSCs. TKI-resistant CML quiescent HSCs showed increased levels of BCR-ABL1, but very low kinase activity. BCR-ABL1 expression, but not kinase function, was required for recruitment of JAK2, activation of a JAK2/β-catenin survival/self-renewal pathway, and inhibition of PP2A. PP2A-activating drugs (PADs) markedly reduced survival and self-renewal of CML quiescent HSCs, but not normal quiescent HSCs, through BCR-ABL1 kinase–independent and PP2A-mediated inhibition of JAK2 and β-catenin. This led to suppression of human leukemic, but not normal, HSC/progenitor survival in BM xenografts and interference with long-term maintenance of BCR-ABL1–positive HSCs in serial transplantation assays. Targeting the JAK2/PP2A/β-catenin network in quiescent HSCs with PADs (e.g., FTY720) has the potential to treat TKI-refractory CML and relieve lifelong patient dependence on TKIs.

The use of first- and second-generation tyrosine kinase inhibitors (TKIs) significantly improves prognosis for patients with early chronic phase chronic myeloid leukemia (CML) and efficiently counteracts leukemia in most patients with CML bearing a disease characterized by the expression of BCR–ABL1 mutants. However, the so-called ‘tinib’ TKIs (e.g. imatinib, nilotinib, dasatinib, and bosutinib) are both ineffective in patients who undergo blastic transformation and unable to eradicate CML at the stem cell level. This raises a few important questions. Is BCR–ABL1 expression and/or activity essential for blastic transformation? Is blastic transformation the result of genetic or epigenetic events that occur at the stem cell level which only become apparent in the granulocyte-macrophage progenitor (GMP) cell pool, or does it arise directly at the GMP level? As altered mRNA metabolism contributes to the phenotype of blast crisis CML progenitors (decreased translation of tumor suppressor genes and transcription factors essential for terminal differentiation and increased translation of anti-apoptotic genes), one attractive concept is to restore levels of these essential molecules to their normal levels. In this review, we discuss the mechanisms by which mRNA processing, translation, and degradation are deregulated in BCR–ABL1 myeloid blast crisis CML progenitors, and present encouraging results from studies with pharmacologic inhibitors which support their inclusion in the clinic.

Inhibition of protein phosphatase 2A (PP2A) tumor suppressor is essential for chronic myelogenous leukemia (CML) stem cell (LSC) maintenance and disease development. Persistence of drug-resistant quiescent LSCs depends on cell-autonomous and bone marrow (BM) signals. Herein, we identified miR-300 as an miRNA inhibited in CD34+ CML progenitors and during blastic transformation (BC) through the BCR-ABL1-dependent inhibition of C/EBPβ-induced miR-300 transcription. In CML progenitors, ectopic mir-300 expression directly targets the PP2A inhibitory pathway (i.e., Jak2, hnRNPA1, SET) and other factors essential for LSC maintenance and disease progression (e.g., CCND1/2, b-catenin, Myc, Twist-1). In leukemic but not normal CD34+ cells, miR-300 acts as a potent tumor suppressor by inducing cell cycle exit and promoting apoptosis. Conversely, hypoxia-induced BCR-ABL1 inhibition and induction of C/EBPβ were found essential for increased miR-300 levels in quiescent LSCs, although mesenchymal stromal cells (MSC)-derived exosomal miR-300 also contributed to it. Moreover, low O2 levels and MSC-derived exosomes induced quiescence of CD34+ CML cells. Notably, expression of an anti-miR-300 in MSCs prevented exosome-induced CD34+ CML growth arrest. LSCs escaped miR-300-induced apoptosis through the autocrine/paracrine TGFβ1-induced expression of TUG1, a lncRNA acting as an miR-300 sponge. In fact, TUG1 or TGFβ1 inhibition decreased quiescent (CFSEMAX) LSC number. By contrast, miR-300 inhibition did not alter LSC survival/self-renewal, further supporting a role for TUG1 as an miR-300 sponge. Accordingly, TUG1 was markedly induced in CFSEMAX but not dividing CD34+ CML cells. In fact, low levels of ectopic miR-300 induced growth arrest (decreased LTC-IC and CFC/replating activity) without affecting quiescent LSC number. By contrast, high doses of miR-300 but not scramble CpG-ODN impaired LSC survival (LTC-IC) and self-renewal (CFC/replating), induced marked killing of quiescent LSCs and dividing progenitors, and impaired CML engraftment in NRG-SGM3 mice. Such effects were further enhanced when CPG-miR-300 and CPG-anti-TUG1 were combined. By contrast, high CpG-miR-300 levels did not affect normal CD34+ cell survival/self-renewal likely because of high TUG1 expression. Altogether our results indicate that while miR-300 loss is essential for survival/proliferation of leukemic progenitors, increased miR-300 levels are required for LSC maintenance. Thus, induction of TUG1 may occur to preserve LSC survival in the BM endosteal niche where quiescence is induced by MSCs and low O2 levels through abnormal miR-300 induction. Thus, disrupting the miR-300/TUG1 balance may represent a potential therapeutic approach for treatment/eradication of LSC-derived leukemias. This work is supported in part by NIH-NCI R01CA163800. Citation Format: Giovannino Silvestri, Lorenzo Stramucci, Justin Ellis, Jason Harb, Paolo Neviani, Bin Zhang, Klara Srutova, Gabriel Pineda, Catriona Jamieson, Bruno Calabretta, Fabio Stagno, Paolo Vigneri, Georgios Nteliopoulos, Philippa May, Alistar Reid, Ramiro Garzon, Denis-Claude Roy, Martin Guimond, Peter Hokland, Michael Deininger, Garrett Fitzgerald, Chris Harman, Francesco Dazzi, Dragana Milojkovic, Jane Apperley, Guido Marcucci, Jianfei Qi, Katerina Machova-Polakova, Xiaoxuan Fan, Maria Baer, Rossana Trotta, Danilo Perrotti. The tumor suppressor activity of miR-300 is detrimental for leukemia development but required for leukemia stem cell maintenance [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1134.

An improved pressurized liquid extraction (PLE) method for polycyclic aromatic hydrocarbons (PAHs) in freshwater sediment is described. The optimized method is simple, fast, efficient, and requires small amounts of sample and generates small amounts of secondary waste. Five optimization factors were chosen for study: extraction temperature, extraction time, sample mass, solvent volume, and post‐extraction cooling time. Method development was performed using authentic PAH‐contaminated (“aged”) sediment samples; optimal conditions were identified based upon a response function (recovery/precision) for three representative PAHs (naphthalene, pyrene, and benzo[g,h,i]perylene). In the optimized method, 3.00 mL of n‐hexane and 0.500 g of sediment were heated in a closed stainless steel extraction cell for 60.0 min at 200°C. The cell was then placed in an ice bath for 5.0 min, and the PAH content of the collected extract was determined by gas chromatography‐mass spectrometry (GC‐MS). Detection limits fo...

The dismal outcome of blast crisis chronic myelogenous leukemia (CML-BC) patients underscores the need for a better understanding of the mechanisms responsible for the development of drug resistance. Altered expression of the anti-apoptoticBcl-xL has been correlated with BCR-ABL leukemogenesis; however, its involvement in the pathogenesis and evolution of CML has not been formally demonstrated yet. Thus, we generated an inducible mouse model in which simultaneous expression of p210-BCR-ABL1 and deletion of bcl-x occurs within hematopoietic stem and progenitor cells. Absence of Bcl-xL did not affect development of the chronic phase-like myeloproliferative disease, but none of the deficient mice progressed to an advanced phenotype, suggesting the importance of Bcl-xL in survival of progressing early progenitor cells. Indeed, pharmacological antagonism of Bcl-xL, with ABT-263, combined with PP242-induced activation of BAD markedly augmented apoptosis of CML-BC cell lines and primary CD34(+) progenitors but not those from healthy donors, regardless of drug resistance induced by bone marrow stromal cell-generated signals. Moreover, studies in which BAD or Bcl-xL expression was molecularly altered strongly support their involvement in ABT-263/PP242-induced apoptosis of CML-BC progenitors. Thus, suppression of the antiapoptotic potential of Bcl-xL together with BAD activation represents an effective pharmacological approach for patients undergoing blastic transformation.

FTY720 (Fingolimod, Gilenya) is a sphingosine analog used as an immunosuppressant in multiple sclerosis patients. FTY720 is also a potent protein phosphatase 2A (PP2A)-activating drug (PAD). PP2A is a tumor suppressor found inactivated in different types of cancer. We show here that PP2A is inactive in polycythemia vera (PV) and other myeloproliferative neoplasms characterized by the expression of the transforming Jak2(V617F) oncogene. PP2A inactivation occurs in a Jak2(V617F) dose/kinase-dependent manner through the PI-3Kγ-PKC-induced phosphorylation of the PP2A inhibitor SET. Genetic or PAD-mediated PP2A reactivation induces Jak2(V617F) inactivation/downregulation and impairs clonogenic potential of Jak2(V617F) cell lines and PV but not normal CD34(+) progenitors. Likewise, FTY720 decreases leukemic allelic burden, reduces splenomegaly, and significantly increases survival of Jak2(V617F) leukemic mice without adverse effects. Mechanistically, we show that in Jak2(V617F) cells, FTY720 antileukemic activity requires neither FTY720 phosphorylation (FTY720-P) nor SET dimerization or ceramide induction but depends on interaction with SET K209. Moreover, we show that Jak2(V617F) also utilizes an alternative sphingosine kinase-1-mediated pathway to inhibit PP2A and that FTY720-P, acting as a sphingosine-1-phosphate-receptor-1 agonist, elicits signals leading to the Jak2-PI-3Kγ-PKC-SET-mediated PP2A inhibition. Thus, PADs (eg, FTY720) represent suitable therapeutic alternatives for Jak2(V617F) MPNs.

Abstract 3727 Objective. The molecular events leading to the evolution of BCR/ABL+ chronic myeloid leukemia (CML) from the chronic phase (CP) to the advanced phase (blast crisis-BP) are poorly understood. The aggressiveness of BP and the poor over-all survival of BP patients needs deep investigation of the biological basis of blastic transformation. We here present the results obtained from the analysis of paired (CP)/(BP) CML samples from patients that underwent progression after standard therapy. The access to matched CP/BP samples will render this data highly valuable. Methods. We performed whole-exome sequencing analysis using high-throughput technologies (Illumina Genome Analyzer IIx) from genomic DNA of four paired samples. The cross-match between BP and CP exomes was performed with dedicated in-house C#software. Gross chromosomal rearrangements were evaluated using CEQer(Comparative-Exonic-Quantification-Analyzer) software from whole-exome sequencing data. We also evaluated microRNA(miRNA) differential expression extracting RNA from five paired samples, using Nanostring nCounter miRNA expression assay. Differentially-expressed miRNAs with a p-value Results. By comparing exome-sequences of four paired CP (used as a control) and BP samples we found a total of 8 single nucleotide somatic mutations. Among the 8 variants identified, 4 of them ranked >1 in the GenRanker cancer scoring system (http://cbio.mskcc.org/tcga-generanker). We show here that, unexpectedly, blast crisis samples have a limited number of acquired mutations compared to chronic phases (average=2 mutations/patient) with patient number two and four displaying the lowest and the higher frequencies, respectively (patient n.2=0 mutations, patient n.4=4 mutations). CEQer analysis of whole exome data showed that 3/4 patients present gross chromosomal rearrangements of at least 2 chromosomes (bulky alteration of chromosome7 present in 2/4 patients); critical regulators of cell cycle control (e.g. CDKN2A and p53) have also been shown to be deleted in patient number 1 and 4, respectively. The individual mutations and rearrangements identified will be presented at the meeting. Differential expression of miRNAs showed that miR-106a, miR-17, miR-20a and miR-20b were significantly down-regulated while miR-148a was significantly up-regulated in all the blast crisis compared to chronic phase samples. In-silico analysis of the putative deregulated targets revealed a strong enrichment of genes involved in molecular mechanisms of cancer, with the 20% of these genes involved in cell cycle regulation. The integrated analysis of these informative data will help to understand the molecular mechanisms responsible for blast crisis progression. Disclosures: No relevant conflicts of interest to declare.

MiR-300 is a microRNA predicted to target multiple components of the BCR-ABL1 / JAK2 / hnRNPA1 / SET / PP2A / β-catenin pathway, which is essential for survival/self-renewal of leukemic progenitors and quiescent TKI-resistant Ph+ hematopoietic stem cells (HSCs). Nanostring arrays analysis of bone marrow (BM) cells from healthy individuals (n=5) and CML patients (n=10) showed gradual inhibition of miR-300 expression (CML-CPmiR-300>CML-BCmiR-300). MiR-300 transduction in CMLCD34+ cells and BCR-ABL1+ cell lines decreased JAK2, β-catenin, hnRNPA1 and SET expression and increased PP2A activity. Targets were confirmed by miR-300 expression in BCR-ABL1+ cells expressing Flag-tagged miR-300-targets lacking or carrying a wild-type or mutated 3'UTR. Restored miR-300 expression in CMLCD34+ cells and/or BCR-ABL1+ cell lines impaired cell cycle progression, proliferation and clonogenic potential, markedly reduced LTC-ICs, and increased TKI sensitivity. Notably, miR-300 expression was inhibited by BCR-ABL1 in proliferating cells. Accordingly, imatinib restored miR-300 expression in CD34+ dividing progenitors and BCR-ABL1+ cell lines without altering miR-300 levels in quiescent (CFSEMAX) CMLCD34+ cells (n=3), consistent with the BCR-ABL1 kinase-dependent activation of the Jak2/SET/PP2A/β-catenin pathway in CML progenitors but not quiescent Ph+ HSCs. Indeed, ectopic SET expression counteracted the negative effects of mir-300 on cell proliferation and survival. Surprisingly, miR-300 levels were increased in CD34+CD38- compared to CD34+CD38+ CML cells, and >20-fold higher in CFSEMAX compared to dividing CMLCD34+ cells (n=4). To determine whether enhanced miR-300 expression in quiescent cells depends on cell autonomous events or is induced by the BM microenvironment, we exposed BCR-ABL+ cells to conditioned medium (CM) of HS-5 or hTERT mesenchymal stem cells (MSC). CM strongly decreased proliferation, induced imatinib but not FTY720 (PP2A activator) resistance, increased miR-300 levels, decreased BCR-ABL1 activity and Jak2 expression but not its activity, and did not alter b-catenin levels or PP2A activity. Interestingly, miR-300 was found in MSC-derived exosomes, and its expression increased in BCR-ABL1+ cells exposed to exosomes. Accordingly, proliferation of CML-BCCD34+ and LAMA-84 cells was strongly reduced upon exposure to MSC-derived exosomes. These effects were abolished when we used CM from MSCs transduced with a miR-300 antagomir. Altogether our results indicate that downregulation of miR-300 appears necessary for the activation of JAK2/SET/PP2A/b-catenin survival signals in CML progenitors. Conversely, increased miR-300 levels (endogenous and MSC-derived) seem to be required for HSC quiescence. Disclosures Deininger: Novartis: Other: Consulting or Advisory Role, Research Funding; BMS: Other: Consulting & Advisory Role, Research Funding; Celgene: Research Funding; Genzyme: Research Funding; Gilead: Research Funding; ARIAD Pharmaceutical Inc.: Other: Consulting or Advisory Role; Incyte: Other: Consulting or Advisory Role; Pfizer: Other: Consulting or Advisory Role. Milojkovic:BMS: Honoraria; ARIAD Pharmaceuticals Inc.: Honoraria; Novartis: Honoraria; Pfizer: Honoraria.

We showed that Eμ-MiR-155 transgenic mice develop acute lymphoblastic leukemia/high-grade lymphoma. Most of these leukemias start at approximately 9 months irrespective of the mouse strain. They are preceded by a polyclonal pre–B-cell proliferation, have variable clinical presentation, are transplantable, and develop oligo/monoclonal expansion. In this study, we show that in these transgenic mice the B-cell precursors have the highest MiR-155 transgene expression and are at the origin of the leukemias. We determine that Src homology 2 domain–containing inositol-5-phosphatase (SHIP) and CCAAT enhancer-binding protein β (C/EBPβ), 2 important regulators of the interleukin-6 signaling pathway, are direct targets of MiR-155 and become gradually more down-regulated in the leukemic than in the preleukemic mice. We hypothesize that miR-155, by down-modulating Ship and C/EBPβ, initiates a chain of events that leads to the accumulation of large pre-B cells and acute lymphoblastic leukemia/high-grade lymphoma.

Udgivelsesdato: 2010-Mar-5 MicroRNAs and heterogeneous ribonucleoproteins (hnRNPs) are posttranscriptional gene regulators that bind mRNA in a sequence-specific manner. Here, we report that loss of miR-328 occurs in blast crisis chronic myelogenous leukemia (CML-BC) in a BCR/ABL dose- and kinase-dependent manner through the MAPK-hnRNP E2 pathway. Restoration of miR-328 expression rescues differentiation and impairs survival of leukemic blasts by simultaneously interacting with the translational regulator poly(rC)-binding protein hnRNP E2 and with the mRNA encoding the survival factor PIM1, respectively. The interaction with hnRNP E2 is independent of the microRNA's seed sequence and it leads to release of CEBPA mRNA from hnRNP E2-mediated translational inhibition. Altogether, these data reveal the dual ability of a microRNA to control cell fate both through base pairing with mRNA targets and through a decoy activity that interferes with the function of regulatory proteins.

The kinase inhibitors imatinib mesylate and dasatinib are the preferred treatment for Philadelphia chromosome–positive (Ph+) leukemias, and they are highly successful in the chronic phase of chronic myeloid leukemia (CML). However, they are not efficient in Ph+ B-cell acute lymphoblastic leukemia (B-ALL). Ph+ leukemia cells are highly resistant to apoptosis, and evidence from cell lines and primary cells suggest Bcl-xL as a critical mediator of resistance to apoptosis: however, this concept has never been rigorously tested in an animal model. To clarify the role of Bcl-xL in Ph+ B-ALL, we generated 2 mouse models. In the first model, Ph+ B-ALL and loss of Bcl-xL expression are coinduced; in the second model, leukemia is induced with expression of Bcl-xL protein well above the levels found in wild-type lymphoblasts. Deletion of Bcl-xL did not inhibit leukemogenesis or affect apoptosis, but increased cellular proliferation. Consistent with this result, overexpression of Bcl-xL led to decreased cellular proliferation. These models reveal an unexpected role for Bcl-xL in cell-cycle entry and the proliferation of tumor cells.

e18027 Identification of Additional Therapeutic Options for de novo Acute Myeloid Leukemia (AML) Patient enabled by Next Generation Sequencing (NGS) Jason G. Harb1, Amber N Essenmacher2, Karen-Sue ...

The persistence of tyrosine kinase inhibitor (TKI)-resistant malignant Philadelphia-positive (Ph+) hematopoietic stem cells (HSC) in chronic myelogenous leukemia (CML) TKI-treated patients in complete molecular remission, and the dismal prognosis of blast crisis CML indicate that the molecular mechanisms underlying its emergence, maintenance and progression are not totally dependent on the unrestrained kinase activity of BCR-ABL1 and might rely on other cell autonomous and/or microenvironmental signal capable of sustaining survival and self-renewal of the chronic phase and blast crisis Ph+ HSC compartment(s). We recently demonstrated that the Jak2/SET-PP2A/β-catenin pathway is essential for survival and self-renewal of quiescent TKI-resistant CD34+CD38- Ph+ HSC and that activation of such oncogenic signals requires the expression but not the activity of BCR-ABL1. Because microRNAs (miRNAs) are likely to control in a canonical and/or decoy manner the expression of different components of the Jak2 signalosome, this makes them an attractive target for further understanding the mechanisms of leukemogenesis and, perhaps, for developing new alternative therapies that selectively eradicate quiescent leukemic HSCs. In silico analysis revealed that a specific miR subset shares multiple targets of the BCR-ABL1/Jak2/SET-PP2A signalosome. Nanostring Array analysis performed on primary bone marrow cells from normal individuals and chronic phase or blast crisis CML patients revealed that expression of some of these miRNA is altered in CML. For example, expression of miR-300 and miR-101, which are predicted to simultaneously modulate directly Jak2, hnRNP-A1 and β-catenin and, indirectly, other molecules of the BCR-ABL1/Jak2/SET-PP2A/β-catenin pathway, is significantly inhibited in chronic phase CML and further decreases in advanced CML samples. Additionally, miR-300 expression is several folds lower in dividing (div. 1) compared to quiescent (CFSEMAX) CD34+ CML cells. Lentiviral-transduction of miR-300 in human BCR-ABL+ cell lines resulted in marked downmodulation of JAK2, β-Catenin hnRNPA1 and SET and, as expected, in increased PP2A activity. Moreover, ectopic miR-300 expression decreased reduced clonogenic potential and proliferation, and increased susceptibility to TKI (e.g. Imatinib) induced apoptosis. Interestingly, it appears that forced BCR-ABL1 expression in TF-1 leukemic cells decreases miR-300, consistent with the reported activation in these cells of the Jak2-SET-PP2A-β-catenin pathway. Altogether our results suggest that miR-300 expression and, potentially, that of other deregulated non-coding RNAs might be dispensable or deleterious for the phenotype of Ph+ progenitors and/or indispensable for that of Ph+ HSCs. Thus, experiments in BCR-ABL1 cell lines as well as primary stem and progenitor cell fractions are currently ongoing to assess the role of this and other miRNAs in survival, self-renewal/proliferation of CML stem and progenitor cells. Disclosures No relevant conflicts of interest to declare.

We describe the development of a novel method for real-time in situ characterization of polycyclic aromatic hydrocarbons (PAHs) in submerged freshwater sediments. Laser-induced fluorescence (LIF) spectroscopy, a mature technique for PAH characterization in terrestrial sediments, was adapted for shipboard use. A cone penetrometer-type apparatus was designed for probe penetration at a constant rate (1 cm/s) to a depth of 3 m. A field-portable LIF system was used for in situ measurements in which the output of a pulsed excimer laser was transmitted by optical fiber to a sapphire window (6.4-mm o.d.) in the probe wall; fluorescent emission was collected by a separate optical fiber for transmission to the spectrometer on deck. Four wavelengths (340, 390, 440, 490 nm) were selected via optical delay lines, and multiple-wavelength waveforms were created. These multiple-wavelength waveforms contain information on the fluorescence frequency, intensity, and emission decay rate. Field testing was conducted at 10 sites in Milwaukee Harbor (total PAH concentrations ranged from approximately 10 to 650 microg/g); conventional sediment core samples were collected concurrently. The core samples were analyzed by EPA methods 3545 (pressurized fluid extraction, PFE) and 8270C (gas chromatography-mass spectrometry, GC-MS) for PAHs. A partial least-squares regression (PLSR) model wasthen created based on laboratory LIF measurements and PFE-GC-MS of the core samples. The PLSR model was applied to the in situ field test data, and 13 of the 16 EPA-regulated PAHs were quantified with a relative error of

The dismal outcome of blast crisis chronic myelogenous leukemia (CML-BC) patients underscores the need for a better understanding of the mechanisms responsible for the development of drug-resistance and disease progression. We generated an inducible mouse model in which simultaneous expression of p210 BCR-ABL1 and deletion of bcl-x occurs within hematopoietic stem and progenitor cells. Absence of Bcl-xL did not affect the development of the chronic phase-like myeloproliferative disease; however, none of the deficient mice progressed to an advanced phenotype, suggesting the importance of the anti-apoptotic Bcl-xL signaling network for the progressing early progenitor cell survival. Indeed, pharmacologic inhibition of Bcl-xL with the Bcl-xL/Bcl-2 antagonist ABT-263, combined with activation of the Bcl-xL-regulator BAD, through the PP242-mediated inhibition of mTORC1/2, markedly augmented apoptosis of BCR-ABL1+ cell lines and primary CD34+ progenitors from CML-BC but not from healthy donors, regardless of the drug-resistance effect induced by bone marrow stromal cell-generated signals. Accordingly, shRNA-mediated downregulation of BAD and the BCR-ABL1-target hnRNP A1, which promotes translation of Bcl-xL but not Bcl-2, impaired the ability of PP242 to potentiate the effect of ABT-263, and mimicked the anti-leukemic activity of ABT-263, respectively, in primary leukemic CD34+ progenitors and/or CML-BC cell lines. Thus, suppression of the antiapoptotic potential of Bcl-xL together with activation of Bad represents a novel potential pharmacologic approach for treatment of those patients undergoing blastic transformation. Citation Format: Jason Harb, Paolo Neviani, Justin E. Ellis, Gregory J. Ferenchak, Brenda I. Chyla, Peter Hokland, Denis Claude Roy, Michael A. Caligiuri, Guido Marcucci, Claudia S. Huettner, Danilo Perrotti. Bcl-xL anti-apoptotic network is dispensable for emergence and maintenance of CML but required for disease progression, and represents an alternative target for halting survival of blast crisis CML progenitors. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3454. doi:10.1158/1538-7445.AM2013-3454

Abstract 35 As tyrosine kinase inhibitors (TKIs) do not induce long-term response in blast crisis chronic myeloid leukemia (CML-BC) or Philadelphia chromosome-positive acute lymphoblastic leukemia (Ph+ ALL), and are unable to kill quiescent Ph+ hematopoietic stem cells, alternative therapies targeting dysregulated pathways in BCR-ABL1+ acute leukemias are needed. CRM1, a karyopherin aberrantly overexpressed in several cancers, controls the nuclear export of proteins (e.g. ABL1, SET, p53, p21, FOXO and RB) that regulate normal and malignant hematopoietic cell survival, self-renewal and proliferation. Here we show that enhanced CRM1 expression also occurs in Ph+ leukemias and the clinical stage small molecule inhibitor of CRM1, KPT-330, has a detrimental effect on malignant but not normal hematopoietic progenitors. Specifically, a substantial upregulation (≥ 65% increase) in the expression levels of CRM1 was detected by immunoblot in CD34+ progenitors from bone marrow (BM) of leukemic primary samples (n=3), when compared to CD34+ progenitors from healthy (n=3) donors. Interestingly, CRM1 protein levels in CML CD34+ cells were markedly but not totally reduced by treatment with the ABL1 kinase inhibitor Imatinib (1μM, 72h), suggesting that BCR-ABL1-driven pathways are not the only factor contributing to CRM1 upregulation. Additionally, treatment of primary CML cells with KPT-330 (1μM, 72h) resulted in a 75% reduction in BCR-ABL1 kinase activity, consistent with an interrelationship between BCR-ABL1 and CRM1 activities. Finally, the ability of BCR-ABL1 to induce CRM1 expression was demonstrated upon ectopic BCR-ABL1 expression in myeloid 32Dcl3 precursor cells in which CRM1 protein levels became ∼10-fold higher. To determine the therapeutic relevance of CRM1 inhibition, cell survival was assessed in MACS-purified CD34+ progenitors. Treatment with KPT-330 resulted in ≥80% induction of apoptosis in Ph+ (n=5) CML (2μM, 72hr) and B-ALL (500nM, 72h) CD34+ cells. Conversely, KPT-330 exerted only a minimal effect on the survival of CD34+ progenitors from healthy donor BM (15–30% Annexin V positive, KPT-330 0.5–2μM, 72h). Consistent with the existence of BCR-ABL1-independent signaling leading to increased CRM1 expression, marked apoptosis was also observed in KPT-330-treated CD34+ progenitors isolated from a Ph-negative B-ALL patient. Thus, CRM1 inhibitor-based therapies might also benefit BCR-ABL1-negative leukemias. To formally determine the effects of KPT-330 on leukemic cell survival, methylcellulose-based clonogenic assays were performed. KPT-330 treatment (1μM) resulted in almost total suppression (97% reduction) of the clonogenic potential of leukemic but not normal (30% reduction) CD34+ progenitors. Because of evidence that CRM1 directly interacts with the protein phosphatase 2A (PP2A) inhibitor SET, and that CRM1 inhibition alters trafficking of hnRNP A1, a direct regulator of the SET-PP2A interplay in Ph+ leukemias, it is conceivable that the anti-leukemic activity of KPT-330 is, at least in part, mediated by PP2A activation. Indeed, KPT-330 treatment (1μM, 48hr) of BCR-ABL1+ cell lines resulted in full restoration of PP2A activity, comparable to levels observed in BCR-ABL1-negative cells. Accordingly, KPT-330 treatment (1μM, 12h) of 32Dcl3BCR-ABL1 cells caused a nuclear accumulation (4-fold increase) and cytoplasmic decrease (95% lower) of SET protein levels, as indicated by both confocal microscopy and immunoblots of subcellular fractions. Moreover, KPT-330-treated cells showed altered hnRNP A1 cellular distribution and overall downregulation. However, unexpectedly, hnRNP A1 protein levels were proportionally higher in the cytoplasm and lower in the nucleus, suggesting that the deleterious effect of CRM1 inhibition on hnRNP A1 might not depend on direct inhibition of hnRNP-A1 nuclear export. Because KPT-330 displays favorable ADME properties and has been shown to alleviate leukemic burden using in vivo models of different cancers, we are currently testing the anti-leukemic activity of this compound in a mouse model of CML-BC. We do expect a profound inhibition of leukemogenesis, and all treated mice are currently alive with no signs of toxicity. Thus, CRM1 inhibition by KPT-330 represents a potential new therapeutic avenue which could easily be exploited in malignancies like CML-BC and Ph+ ALL that show a dismal outcome to current available therapies. Disclosures: Walker: Karyopharm Therapeutics Inc: Research Funding. Landesman:Karyopharm Therapeutics Inc: Employment. Shacham:Karyopharm Therapeutics: Employment. Kauffman:Karyopharm Therapeutics Inc: Employment. Perrotti:Karyopharm Therapeutics Inc: Research Funding.

Tyrosine kinase inhibitor (TKI)-based therapies do not induce long-term response in myeloid or lymphoid blast crisis (BC) chronic myelogenous leukemia (CML) and Ph+ acute lymphoblastic leukemia (ALL). Increased expression/activity of nucleocytoplasmic-shuttling heterogeneous nuclear ribonucleoproteins (hnRNPs) A1, E2 and K, are critical for the expression of factors (e.g. SET/PP2A, C/EBPβ/miR-328, and c-Myc) regulating proliferation and survival of Ph+ progenitors. Because the karyopherin CRM1 controls nuclear export of hnRNPs, we assessed the therapeutic potential of CRM1 inhibitors in CML-BC and Ph+ ALL models. Thus, myeloid 32D-p210BCR-ABL1 and lymphoid Baf3-p190BCR-ABL1 progenitors were exposed to the CRM1 selective & potent inhibitors of nuclear export (SINE) KPT-185 and KPT-207. MTT viability assays revealed that KPT-185 and KPT-207 decreased cell viability by ∼80% at concentrations ranging from 150-350 nM. The KPT-SINE not only induced killing, but also affected cytokine-independent growth of BCR-ABL1+ cells: proliferation was inhibited 89% and 81% by KPT-185 and KPT-207, respectively. Notably, growth and survival of non-transformed 32Dcl3 and BaF3 cells was not affected (70-100% viable cells) by KPT-SINE. As expected, treatment (1 μM; 48h) with these inhibitors altered the nuclear/cytoplasmic ratio of hnRNPs important for BCR-ABL1 leukemogenesis. As a result, treatment of BCR-ABL1+ cells with KPT-185 and KPT-207 (1 μM; 48h) resulted in 75% and 50% suppression of BCR-ABL1 expression and kinase activity, respectively. Furthermore, KPT-207 also reduced Myc expression in 32D-p210BCR-ABL1 cells; this is consistent with the potential interference of KPT-207 with the proliferation/survival signals triggered by the BCR-ABL1/MAPK/hnRNP K/Myc pathway in CML-BC progenitors. Because both KPT-185 and KPT-207 significantly alter hnRNP A1 localization, which is important for regulation of the PP2A inhibitor SET and, therefore, for BCR-ABL1 leukemogenesis, we assessed whether KPT-207 and KPT-185 negatively regulate PP2A activity. Indeed, treatment with KPT-207 and KPT-185 (250 nM; 48h) restored PP2A activity in 32D-p210BCR-ABL1 cells to levels similar to those detected in non-transformed 32Dcl3 cells. Although further investigation of KPT-207 and KPT-185 mechanism of action and assessment of their biologic/therapeutic effects in CML-BC and Ph+ ALL mouse models and primary leukemic and normal progenitors is currently ongoing, it is safe to conclude that selective nuclear export (SINE CRM1) inhibitors represent potentially powerful therapeutic tools that, if used alone or in combination with TKIs, might lead to sustained complete molecular remission in CML-BC and Ph+ ALL patients. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3839. doi:1538-7445.AM2012-3839

Abstract 3758 Despite the remarkable response in chronic phase CML, tyrosine kinase inhibitor (TKI)-based therapies do not induce long-term response in myeloid or lymphoid blast crisis CML (CML-BC) and Ph+ ALL, and are unable to kill quiescent Ph+ hematopoietic stem cells. We reported that CML disease progression is characterized by a BCR-ABL1 dose- and kinase-dependent increase in the expression/activity of nucleocytoplasmic-shuttling of heterogeneous nuclear ribonucleoproteins (hnRNPs) A1, E2 and K, which are essential post-transcriptional and translational modulators of critical regulators of cell proliferation, survival and differentiation of CD34+ CML-BC and/or CD34+/CD19+ Ph+ ALL progenitors including SET/PP2A, E2F3, Bcl-xL, C/EBPa, miR-328, and c-Myc. The karyopherin (a class of proteins involved in regulating nuclear import/export) chromosome region maintenance 1 (CRM1) controls the nuclear export of several hnRNPs, and because growth/survival of CML-BC and Ph+ ALL progenitors requires the aberrant cytoplasmic activity of hnRNPs A1, E2 and/or K, we have explored the therapeutic potential of CRM1 inhibitors in p210 and p190 BCR-ABL1+ cell line models of CML-BC and Ph+ ALL, respectively. Thus, the cytokine-dependent myeloid 32Dcl3 and lymphoid BaF3, and the derived cytokine-independent 32D-p210BCR-ABL1 and Baf3-p190BCR-ABL1 mouse progenitors were exposed for 48 hr to different concentrations (0–10 μM) of the CRM1 selective & potent inhibitors of nuclear export (SINE) KPT-185 and KPT-207. MTT viability assays (n=3) revealed that KPT-185 and KPT-207 decreased viability ∼80% in 32D-p210BCR-ABL1 cells (KPT-185≥207) at concentrations ranging from 150–350 nM. Similar results were obtained in Baf3-p190BCR-ABL1 cells as the EC50 was 300nM for KPT-185 and KPT-207. The KPT-SINE not only induced killing, but also affected cytokine-independent growth of BCR-ABL1+ cells: proliferation was inhibited 89% and 81% by KPT-185 and KPT-207, respectively. Notably, growth and survival of non-transformed 32Dcl3 and BaF3 cells was not affected (70–100% viable cells) at concentrations (150–350 nM) of KPT-185 and KPT-207 that impair survival of BCR-ABL1+ cells. Mechanistically, we found that KPT-SINE CRM1 inhibitors altered the nuclear/cytoplasmic ratio of hnRNPs important for BCR-ABL1 leukemogenesis. Indeed, in KPT-207-treated (1 μM; 48h) BCR-ABL1+ cells hnRNP A1, E2 and K accumulated in the cytoplasm. Likewise, KPT-185 treatment (1 μM; 48h) resulted in hnRNP A1 being sequestered in the nucleus whereas hnRNP E2 distribution was not altered and, unexpectedly, levels of hnRNP K expression were markedly decreased in both subcellular compartments. Interestingly, treatment of BCR-ABL1+ cells with KPT-185 and KPT-207 (1 μM; 48h) resulted in 75% and 50% suppression of BCR-ABL1 expression and kinase activity, respectively. Furthermore, KPT-207 also reduced Myc expression in 32D-p210BCR-ABL1 cells; this is consistent with the potential interference of KPT-207 with the proliferation/survival signals triggered by the BCR-ABL1/MAPK/hnRNP K/Myc pathway in CML-BC progenitors. Because both KPT-185 and KPT-207 significantly alter hnRNP A1 localization in addition to impairing BCR-ABL1 expression/activity, and suppression of PP2A tumor suppressor activity is essential for both p210 and p190 BCR-ABL1-driven leukemogenesis, it is highly plausible that KPT-207 and KPT-185 negatively regulate the BCR-ABL1-dependent and hnRNP A1-mediated induction of the PP2A inhibitor SET thereby rescuing PP2A phosphatase activity, which in turn decreases BCR-ABL1 expression and kinase activity, and triggers in vitro and in vivo apoptosis of primary CML-BC and Ph+ ALL progenitors. Indeed, treatment with KPT-207 and KPT-185 (250 nM; 48h) restored PP2A activity in 32D-p210BCR-ABL1 cells to levels similar to those detected in non-transformed 32Dcl3 cells. Although further investigation of KPT-207 and KPT-185 mechanism of action and assessment of their biologic/therapeutic effects in CML-BC and Ph+ ALL mouse models and primary leukemic and normal progenitors is currently ongoing, it is safe to conclude that selective nuclear export (SINE CRM1) inhibitors represent potentially powerful therapeutic tools that, if used alone or in combination with TKIs, might lead to sustained complete molecular remission in CML-BC and Ph+ ALL patients. Disclosures: Shacham: Karyopharm: Equity Ownership. Kauffman:Karyopharm: Equity Ownership.

Abstract 2494 In CD34+ bone marrow (BM) progenitors from Polycythemia Vera (PV) patients and Jak2V617F-expressing cell lines, loss of protein phosphatase 2A (PP2A) tumor suppressor activity is essential for their survival and clonogenic potential, and it depends on signals generated by the mutated Jak2 oncogenic kinase that, in turn, is also negatively regulated by PP2A. Additionally, we showed (Oaks J.J. et al., ASH 2010) that suppression of PP2A requires expression of the Jak2-regulated PP2A inhibitor SET, and it can be reverted by clinically-relevant Jak2 inhibitors or by the sphingosine analog FTY720, which reactivates PP2A and inhibits Jak2V617F expression/activity upon binding/sequestration of SET. Here we report that, unlike the effect of BCR-ABL1 in CML, SET levels do not increase in normal CD34+ BM progenitors engineered to express Jak2V617F; however, SET knock-down restores PP2A activity in both Jak2V617F cell lines and CD34+ PV progenitors, suggesting a post-translational control of SET activity. By using Jak2 and PI-3K kinase inhibitors and SET mutants, we found that PP2A inactivation in PV depends on the Jak2V617F- and PI-3Kγ-induced SET phosphorylation on serine 24. Indeed, a ∼93% restoration of PP2A activity and suppression of SET phosphorylation is achieved upon exposure of Jak2V617F+ cells to the specific PI-3Kγ (AS-604850, 1μM) inhibitor. Likewise, ∼80% loss of SET inhibitory activity is observed upon transduction of S24A but not S9A SET mutant that, albeit essential for PP2A inhibition in other cell types, behaved as wild type SET. Because both Jak2 and PI-3Kγ may regulate nitric oxide synthase (NOS2) that, in turn, inhibits PP2A activity upon induction of peroxynitrate (PN)-mediated nitration of PP2A tyrosine residues with negative regulatory function, we assessed the effect of NOS/PN modulation on the Jak2V617F-dependent inactivation of PP2A. Treatment of Jak2V617F- expressing BaF3 cells with NO donors (1 mM DPTA NONOate and 10μM SIN-1) or PN (500 μM) resulted in 2.7- and 28-fold induction of PP2A activity, respectively. Accordingly, 80% inhibition of PP2A activity was observed in parental BaF3 cells upon inhibition of NOS2 by 3-bromo-7-nitroindazole (5μM). Thus, Jak2V617F inhibits PP2A by inducing SET phosphorylation and inhibiting PP2A tyrosine nitration. Notably, co-existence of both mechanisms of Jak2V617F-dependent PP2A inactivation is also supported by the evidence that NOS2 levels increase upon shRNA-mediated downregulation of SET expression, and by the marked decrease in SET phosphorylation and increase in PP2A tyrosine nitration detected in Jak2V617F cells treated with FTY720 (2.5 mM). Interestingly, similar effects were observed upon exposure of Jak2V617F cells to non-phosphorylatable and non-immunosuppressive FTY720 derivatives (e.g. OSU-2S, QC-FTY720 and SR-FTY720) but not in cells treated with the immunosuppressive phosphorylated FTY720 (FTY720-P). In agreement with our findings indicating that the anti-leukemic activity of FTY720 in Jak2V617F+ primary progenitors and cell lines does not require the sphingosine kinase 2-dependent conversion of FTY720 into FTY720-P and, consequently, its interaction with the S1PR1 receptor (Oaks JJ. et al., ASH 2010), we have now evidence showing that a synthetic FTY720-P, unlike FTY720, impairs (81% inhibition) PP2A activity in normal hematopoietic progenitors, and that the major part of the intracellular FTY720 remains non-phosphorylated. By using S1PR1-selective agonists and S1PR1 receptor knock-down, we also found that inhibition of PP2A activity by synthetic FTY720-P is mediated by the S1PR1-dependent triggering and activation of Jak2 and PI-3Kγ. Accordingly, co-treatment of normal hematopoietic progenitor cells with FTY720-P and either the Jak2 inhibitor AG490 or the PI-3Kγ inhibitor AS-604850 antagonized the inhibitory effects of FTY720-P and restored PP2A activity to levels similar to those detected in untreated cells. Altogether our data indicate that the anti-leukemic effects of FTY720 in CD34+ PV progenitors depends on its ability to antagonize the Jak2V617F- and S1PR1-induced and PI-3Kγ-mediated inactivation of PP2A through inhibition of the PI-3Kγ-dependent SET serine 24 phosphorylation, and via binding/sequestration of phosphorylated SET that, in turn, leads to enhancement of NOS/PN-mediated PP2A activation by tyrosine nitration. Disclosures: No relevant conflicts of interest to declare.

Abstract 2738 Tyrosine kinase inhibitors (TKIs) have become frontline therapy for CML; however, alternative therapies are required, as TKIs do not induce long-term response in CML patients undergoing blastic transformation and are ineffective against Philadelphia-positive (Ph+) quiescent stem cells, which show innate resistance to BCR-ABL1 kinase inhibitors. Therapeutic targets of interest are survival factors conferring resistance to TKI-induced apoptosis and/or those increasing proliferation of leukemic progenitors. We previously reported (Harb JG et al., ASH 2007) that genetic inactivation of Bcl-x did not inhibit BCR-ABL1 leukemogenesis in an inducible mouse model of CML. Thus, we hypothesize that BCR/ABL mediated post-translational modification and inactivation of pro-apoptotic BAD negates the requirement for the anti-apoptotic function of Bcl-xL in stem/progenitor cells from SCLtTA-BCR/ABL1/Bcl-x−/− mice. Following this rationale, we tested if simultaneous pharmacologic BAD activation and Bcl-xL inhibition may be an efficient way of killing CML stem/progenitor cells. To test this, loss of Bcl-xL function with increased levels of active BAD was achieved by expressing Bcl-x shRNA in 32D-BCR/ABL mouse myeloid precursors that were then treated with LY294002 (LY), which suppresses the inhibitory effects of PI-3K/Akt activation on BAD. Flow cytometric analysis of Annexin V+ cells revealed that levels of apoptosis were three times higher in BCR-ABL1+ cells expressing the Bcl-x shRNA when compared with vector-transduced BCR-ABL1+ cells. As expected, increased levels of dephosphorylated (active) BAD at the mitochondrial membrane were found in LY-treated BCR-ABL+ cells. Interestingly, co-treatment of Bcl-x shRNA-expressing BCR-ABL1+ cells with LY and the Bcl-xL/Bcl-2 antagonist ABT-263 (ABT) did not further promote apoptosis, suggesting that decreased survival of BCR-ABL1+ cells was dependent on downregulation of Bcl-xL and not Bcl-2. To determine efficacy of combined pharmacologic Bcl-xL inhibition and BAD activation, 32D-BCR/ABL and K562 cells were treated with compounds expected to activate BAD upon inhibition of PI-3K/Akt/mTOR-generated signals, used alone or in combination with ABT. Individually, at suboptimal doses, LY, Rapamycin (RAP), mTORC1/2 inhibitor PP242, and ABT were tolerated with apoptosis levels lower than 20%. Notably, when combined with ABT, all three efficiently induced apoptosis (∼90% Annexin V+) of BCR-ABL1+ cells. As with LY, increased levels of active BAD were found at the mitochondrial membrane of RAP- and PP242-treated BCR-ABL1+ cells. We found that PP242 downregulated p-Akt (92%), Mcl-1 (67%) and Bcl-xL (51%) more efficiently than RAP or LY. It has been shown that PP242 impairs the clonogenic potential of TKI-resistant mononuclear BM CML-BC cells; however, its effects when used alone or in combination with ABT on survival of normal and leukemic hematopoietic stem (HSCs) and progenitor cells is still unknown. Thus, HSC-enriched (CD34+/CD38-) and progenitor (CD34+) CML-BC cell fractions were isolated from bone marrow and peripheral blood and used in colony forming (CFC) assays with ABT, PP242 or ABT/PP242. ABT alone did not suppress colony formation of Ph+ CD34+/CD38− cells, while PP242 reduced it by nearly 50%. Conversely, ABT/PP242 combination decreased Ph+ stem and progenitor colony formation by ∼80%. Furthermore, the self-renewal of Ph+ CD34+/CD38− cells was markedly impaired by ABT/PP242 as demonstrated by the 80% decrease in replating efficiency. To assess if non-leukemic stem cells would tolerate ABT/PP242, colony assays were performed with LSK from wild type mice treated with ABT, PP242, RAP and ABT/PP242. We did not find a significant effect of ABT or PP242 on clonogenic potential when given as single agents. More importantly, combined treatment decreased CFC output by only 35% while RAP, which has an acceptable toxicity profile as it has been used in clinical trials for patients unresponsive to TKIs, decreased LSK colony forming potential by 50%. In summary, our data showing that combined treatment with the mTORC1/2 inhibitor/BAD activator PP242 and the BCl-xL/Bcl-2 antagonist ABT-263 markedly induces apoptosis of BCR-ABL+ cell lines, in HSCs and in progenitors from CML-BC patients. This approach warrants further pre-clinical investigation aimed at inclusion in clinical protocols for treating blast crisis CML. Disclosures: No relevant conflicts of interest to declare.

Background: The success of tyrosine kinase inhibitors (TKIs) depends on the addiction of Philadelphia-positive (Ph+) CML progenitors to BCR-ABL1 kinase activity. However, CML quiescent hematopoietic stem cells (HSC) are TKI-resistant and represent an active disease reservoir. We hypothesize that this innate drug-resistance depends on inhibition of the tumor suppressor protein phosphatase 2A (PP2A). PP2A can be reactivated by FTY720, a drug that targets CML but not normal progenitors. Here we investigated the mechanism controlling survival/self-renewal of quiescent leukemic HSCs and their sensitivity to PP2A-activating drugs. Methods: HSCs from CML (n=68) and healthy (n=12) donors were FACS-isolated, and the biologic importance of PP2A inhibition and pharmacologic PP2A activation on their survival/self-renewal was assessed by BM serial transplantation; CFSE and Annexin-V staining; LTC-IC and CFC/replating assays; lentiviral shRNA/cDNA-transduction; LEF/TCF and proximity-ligation assays; Western blot, confocal microscopy and FACS analyses. Results: We observed increased BCR-ABL1 expression with impaired kinase activity in quiescent CML HSCs, in which BCR-ABL1 per se is required for induction of JAK2 that subsequently activated β-catenin and inhibited PP2A. In fact, PP2A was suppressed in CML but not normal CD34+/CD38−/CD90+ HSCs. FTY720 and/or its non-immunosuppressive (S)-FTY720-OMe derivative markedly reduced survival and self-renewal of CML but not normal quiescent HSCs through BCR-ABL1 kinase-independent and PP2A-mediated JAK2 and β-catenin inhibition. Importantly, FTY720 also strongly diminished BCR-ABL1+ LT-HSC frequency in serial BM transplantation assays. Conclusions: The pharmacologic targeting of the newly-identified BCR-ABL1 kinase-independent JAK2/β-catenin interplay in quiescent HSCs with FTY720 and its derivatives, might lead to cessation of lifelong patient dependence on TKIs. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr LB-109. doi:10.1158/1538-7445.AM2011-LB-109

Abstract 775 We have shown (Oaks JJ et al. ASH 2009) that the tumor suppressor Protein Phosphatase 2A (PP2A) is functionally inactivated by Jak2V617F in cell line models of Jak2V617F myeloproliferative disorders (MPD) and Jak2V617F-transduced primary mouse bone marrow cells. Inhibition of Jak2 (600 nM Jak Inhibitor I; 50 μM AG490; 10h) or treatment with the PP2A activator FTY720 (2.5μM, 24 hours) restored PP2A activity that caused loss of Jak2V617F protein/activity, impaired Jak2V617F-driven colony formation, and induced apoptosis of Jak2V617F+ but not normal myeloid cells. Notably, FTY720 is a sphingosine analog suggested by the FDA to treat patients with Multiple Sclerosis due to its immunosuppressive activity when phosphorylated by sphingosine kinase 2 (SPHK2). Here we show that FTY720 treatment of CD34+ primary bone marrow cells from JakV617F+ PV patients (n=3) also rescued PP2A activity, induced Jak2 downregulation and significantly impaired cytokine-dependent clonogenic potential. Thus, FTY720 could be used as an alternative to Jak2 inhibitors, as in vitro and in animal assays showed that FTY720 (2.5μM) is not toxic against normal human myeloid progenitors while decreasing survival of CD34+ progenitors from MPD patients. To find out whether FTY720 uses the same mechanism to exert its immunosuppressive and anti-leukemic activities, we determined if the conversion of FTY720 into its phosphorylated form is important for rescuing PP2A activity in Jak2V617F-expressing cells. Impaired FTY720-P conversion by exposure to the SPHK inhibitor dimethylsphingosine (2.5μM, 6 hours) did not affect the ability of FTY720 to activate PP2A. Also, a synthetically phosphorylated FTY720 (FTY720-P, 2.5μM, 6 hours) was unable to activate PP2A or exert any anti-leukemic activity, suggesting that the anti-proliferative and pro-apoptotic effects of FTY720 are independent of its phosphorylation and interaction with the S1PR1 receptor. We found that activation of S1PR1 through the specific agonist SEW2871 (10μM), FTY720-P (2.5μM), or sphingosine-1-phosphate (100nM) markedly suppresses (~60% inhibition) rather than activates PP2A in normal myeloid progenitors. As expected, knockdown of S1PR1 had no effect on FTY720-mediated PP2A activation in Jak2V617F-transformed cells. Mechanistically we found that Jak2V617F and PP2Ac were found in a ternary complex with the PP2A inhibitor SET. SET knockdown by shRNA restored PP2A activity in Jak2V617F+ Ba/F3 cells to levels similar to those found in non-transformed cells, and led to an 84% decrease in Jak2V617F+-driven colony formation. In addition, co-immunoprecipitation assays revealed that FTY720 (10μM) disrupts Jak2-PP2A, PP2A-SET and Jak2-SET interactions, suggesting that SET may be the target of FTY720. Consistently, affinity chromatography showed that FTY720 efficiently interferes with the ability of C6-ceramide (10μM) to bind SET as the amount of SET eluted from the biotin-labeled C6-ceramide was significantly reduced by exposure of the cell lysate to FTY720. As well, lentiviral-mediated expression of wild type or K209D SET mutant (ceramide binding deficient) in Ba/F3 cells impaired PP2A activity (≥80% decrease), which could be totally rescued by FTY720 only in cells transduced with wild type but not K209D SET. The formal demonstration that FTY720 activates PP2A by displacing SET came when we found SET in anti-NBD immunoprecipitates from Jak2V617F-expressing Ba/F3 cells treated with FTY720-phenoxy-NBD (10μM; 30 min). Together, our data show that FTY720 has the potential to be an effective therapeutic agent for MPD patients by virtue of its low toxicity and ability to activate PP2A by displacing SET; however, FTY720 still retains the ability to become phosphorylated and inhibit, at least in part, PP2A. Thus, we developed non-phosphorylatable FTY720 derivatives and assessed them for their ability to: activate PP2A; induce downregulation/inactivation of targeted kinases (e.g. Jak2, BCR-ABL1, Akt); act as anti-proliferative and pro-apoptotic agent to leukemic but not normal myeloid/lymphoid progenitors; do not interact with S1PR1; and show no in vivo effects on B220+/CD19+ and CD4 or CD8 cellular compartments. These FTY720 derivatives were found to be not immunosuppressive but able to mirror FTY720 in terms of inducing Jak2V617F downregulation and cell killing while retaining the parent compound's minimal toxicity towards untransformed cells. Disclosures: Verstovsek: Incyte Corporation: Research Funding.

Abstract 1222 The molecular mechanism leading to disease progression of chronic myelogenous leukemia (CML) still remains to be identified although enhanced BCR/ABL expression and activity seems to play an important role in controlling genomic stability, differentiation, and self renewal of the leukemic cell clone undergoing blastic tranformation by affecting expression and function of RNA binding proteins (RBPs) like hnRNP A1, hnRNP E2, and hnRNP K (Perrotti D. et al. J. Clin Invest. 2010). We previously reported (Harb J. et al., ASH 2009) that a BCR-ABL1 dosage-dependence and hierarchical organization exists for the expression of hnRNP A1, hnRNP E2, and hnRNP K in cell line models of CML. In fact, as BCR/ABL levels increase, upregulated expression of hnRNP A1 is observed, followed by increased expression of hnRNP E2 and hnRNP K. HnRNP A1 and hnRNP K were also temporally expressed within Lineage- Sca-1+ c-kit+ (LSK), common myeloid progenitors (CMP), and granulocyte macrophage progenitors (GMP) in a mouse model (SCLtTA TRE-BCR/ABL) of chronic myeloid leukemia. Interestingly, hnRNP A1 and hnRNP K levels in BCR/ABL+ mouse progenitors correlated with disease severity as mice with higher levels of these RBPs presented a more progressed phenotype characterized by increased mixed lineage B220+/Mac-1+ progenitors in bone marrow and spleen when compared with mice that developed a CML-CP-like phenotype. Here we show that hnRNP A1, hnRNP E2, and hnRNP K expression levels, as well as BCR/ABL activity are different in HSC (CD34+/CD38-), CMP (CD34+/CD38+/CD45+/IL-3Ra-), and GMP (CD34+/CD38+/CD45+/IL-3Ra+) isolated from peripheral blood of patients in chronic phase (CML-CP) at diagnosis, untreated accelerated phase (CML-AP), and blast crisis (CML-BC). Interestingly, in CML-CP, the highest expression of hnRNP A1 was found in the CD34+/CD38- stem cell fraction and it gradually decreased in the more mature CMP and GMP progenitors (55% and 65% lower, respectively). By contrast, consistent with the role of hnRNP A1 as regulator of progenitor cell proliferation and survival, hnRNP A1 expression progressively increased in the HSC, CMP and GMP fractions isolated from patients in CML-AP and CML-BC. Unlike hnRNP A1, hnRNP E2 and hnRNP K were barely detected in the CD34+/CD38- from CML-CP patients but their expression was markedly pronounced in the HSC fraction of progressed CML patients. In agreement with our cell line data, expression of hnRNP A1 and hnRNP E2 in advanced CML (CML-AP and CML-BC) increases when CD34+/CD38- stem cells undergo maturation toward CMP. Conversely, it appears that hnRNP K expression is the last to increase in CML-BC, suggesting a hierarchical regulation of RBP expression during differentiation and lineage commitment. Expectedly, levels of hnRNP A1 in CMPs increase during disease progression (CP Taken together, these data further implicate RBPs hnRNP A1, hnRNP E2, and hnRNP K in CML disease progression and suggest their possible role in the control of survival of stem and primitive CML-CP progenitors. Disclosures: No relevant conflicts of interest to declare.

Abstract 2901 FTY720 is a sphingosine analog proposed by the FDA for treating Multiple Sclerosis patients because of its immunosuppressive activity, which depends on its ability to prevent lymphocyte egress into the peripheral blood. To act as an immunosuppressive drug, FTY720 undergoes sphingosine kinase 2 (SPHK2) phosphorylation and internalization upon interaction with the sphingosine-1-phosphate receptor 1 (S1PR1). FTY720 also acts as a potent activator of protein phosphatase 2A (PP2A), a tumor suppressor found inactivated in chronic and blast crisis CML with wild type or imatinib/dasatinib-resistant BCR-ABL1, Ph+ B-ALL, KitD816V AML, Jak2V617F+ MPDs and other leukemias/lymphomas. FTY720 treatment of cell lines and primary progenitors isolated from bone marrow of patients with these malignancies, markedly suppressed leukemic cell proliferation/survival and induced apoptosis in a PP2A-dependent manner. Notably, long-term treatment with FTY720 of mice carrying these hematopoietic malignancies significantly prolonged survival and restored normal myelopoiesis without exerting any toxic effect in hematopoietic and non-hematopoietic organs. However, in vivo administration of FTY720 strongly, albeit reversibly, decreases the number of circulating B and T lymphocytes. Here we report that a synthetically phosphorylated FTY720 (FTY720-P) is unable to induce neither PP2A activation nor apoptosis of BCR-ABL1-, Jak2V617F-, or KitD816V-expressing myeloid precursors, indicating that FTY720 phosphorylation is dispensable for its anti-leukemic activity. Thus, we functionally characterize two FTY720 derivatives, QC-FTYSM and OSU-2S, which were synthesized as molecules unable to undergo SPHK2 phosphorylation. Treatment (2.5 uM; 24h) of FTY720-sensitive 32D-BCR/ABL cells with QC-FTYSM and OSU2S results in ∼80% and 40%, respectively, more efficient suppression of BCR-ABL1 expression and kinase activity than that observed with FTY720. Moreover, QC-FTYSM, OSU-2S and FTY720 (2.5uM; 0–60h) induce a progressive block of proliferation and marked induction of apoptosis of 32D-BCR/ABL cells. In fact, a 96%, 98%, and 79% decrease in viability is observed after treatment with QC-FTYSM, OSU-2S and FTY720, respectively. Notably, viability of non-transformed myeloid 32Dcl3 cells is not significantly affected by treatment with FTY720 or its derivatives. Consistent with the ability of FTY720 to induce apoptosis through rescue of PP2A activity, phosphatase assays show identical ability of FTY720, QC-FTYSM and OSU-2S to restore PP2A functionality. In fact, comparable and marked decrease in the amount of inactive Y307-phosphorylated PP2Ac was detected in 32D-BCR/ABL cells treated with FTY720 or its derivatives. To formally demonstrate that QC-FTYSM and OSU-2S lack immunosuppressive activity, we first assessed their ability to be internalized upon interaction/association with the S1PR1 receptor. Thus, cells were transduced with a GFP-tagged S1PR1 and treated with either QC-FTYSM, OSU-2S, or, as positive control, FTY720-P. Confocal microscopy revealed that treatment FTY720-P resulted in a strong S1PR1 internalization. Conversely, exposure of the cells to QC-FTYSM and OSU-2S did not alter the S1PR1 membrane localization, indicating that these molecules did not undergo SHPK2 phosphorylation. Further demonstration of the inability of these compounds to act as immunosuppressive molecules was gained upon in vivo administration of a single dose of FTY720, QC-FTYSM or OSU-2C (10 mg/kg) to wild type FVB/N mice. As expected, percentage of B220+/CD19+ circulating B-cells decreased of ∼90% in FTY720-treated animals. Conversely, the percentage of B-cells after exposure to QC-FTYSM and OSU-2S remained unchanged (≤ 1% decrease). Likewise, the number or CD4+ and CD8+ cells also was not affect by treatment with the QC-FTYSM compound. Note that effect of OSU-2S on T-cells and the toxicity profile and anti-leukemic activity of these drugs in healthy animals and mouse models of deadly leukemias (e.g. T315I+ and blast crisis CML and Ph+ ALL) as well as Ph− MPDs are currently being assessed. Altogether our data indicate that QC-FTYSM and OSU-2S represent two potentially powerful and safe drugs which could be introduced in the current therapeutic protocols for different types of hematopoietic and non-hematopoietic malignancies characterized by functional inactivation of the PP2A tumor suppressor. Disclosures: No relevant conflicts of interest to declare.

Abstract 515 The success of tyrosine kinase inhibitors as first line therapy for t(9;22) Chronic Myelogenous Leukemia (CML) depends on the addiction that Philadelphia-positive (Ph+) hematopoietic progenitors, but not quiescent Ph+ stem cells, have for BCR-ABL1 tyrosine kinase activity. We reported that the activity of the tumor suppressor PP2A is inhibited in a SET-dependent manner in CML progenitors and CD34+/CD38- stem cells from chronic phase (CP) and, to a greater extent, blast crisis (BC) CML patients (Neviani P. et al.: Cancer Cell 2005, J. Clin. Invest 2007, and ASH 2008). Restoration of PP2A activity by the immunosuppressive sphingosine analogue FTY720 markedly decreases the number of Ph+ but not normal long-term culture-initiating cells (LTC-IC) and quiescent stem cells (CFSEMAX/CD34+) by suppressing the BCR/ABL kinase-independent enhancement of b-catenin expression/transcriptional activity (Oaks JJ., et al., ASH 2009). Here we report that FTY720 induces apoptosis of Ph+ CD34+/CD38- cells independent from its phosphorylation as treatment with a phosphorylated FTY720 did not alter the number of Ph+ CFSEMAX/CD34+ cells. By contrast, two non phosphorylatable and non immunosuppressive FTY720 derivatives did significantly affect survival of Ph+ stem/progenitor cells. Interestingly, we also noted that the activity but not expression of BCR-ABL1 is considerably lower in quiescent CFSEMAX/CD34+ than CFSE+/CD34+ cells that underwent at least one division (∼80% decrease; n=3). Conversely, BCR-ABL1 expression is significantly higher in quiescent than proliferating CFSE+/CD34+ cells, suggesting that BCR-ABL1 might serve as a scaffold for other kinase(s) able to sustain survival and quiescence of Ph+ stem cells. Indeed, we found that expression of the K1172R kinase-deficient BCR-ABL1 mutant enhances expression and activity of Jak2, a kinase that is not only associated with BCR-ABL1 but is also capable of inactivating and being inactivated by PP2A. Accordingly, lentiviral shRNA-mediated BCR-ABL1 downregulation in Ph+ CD34+/CD38- stem cells resulted in marked (≥70% inhibition, P Thus, expression but not activity of the oncogenic product of the t(9;22) translocation is important for recruiting and allowing SET-mediated inhibition of PP2A and activation of Jak2; two events important for Ph+ stem cell survival and self renewal. Moreover, the ability of FTY720 and of its non-immunosuppressive derivatives to induce apoptosis of Ph+ progenitors and Ph+ but not normal quiescent stem cells emphasizes the notion that FTY720 and its derivatives represent strong and non-toxic anti-leukemic agents potentially useful not only for the treatment but, perhaps, for eradicating Ph+ leukemias. Disclosures: Holyoake: Bristol-Myers Squibb: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees.

MicroRNAs (miRs) and heterogeneous ribonucleoproteins (hnRNPs) are post-transcriptional gene regulators that bind to mRNA in a sequence-specific manner. We showed that hnRNP-E2 inhibits myeloid maturation of bone marrow (BM) progenitors from chronic myelogenous leukemia patients in myeloid blast crisis (CML-BC) by suppressing CEBPA mRNA translation. We report here that loss of miR-328 is induced by BCR/ABL and specifically occurs in CML-BC, and its restored expression rescues differentiation and impairs clonogenic potential of BCR/ABL+ BM progenitors. Accordingly, miR-328 increases during granulocytic differentiation of human CD34+ and mouse LSK BM stem/progenitor cells. Mechanistically, BCR/ABL uses the MAPK-hnRNP-E2 pathway to suppress C/EBPα and miR-328 expression as pharmacologic inhibition of and/or shRNAs against these molecules efficiently restore miR-328 expression. Interestingly, two functional C/EBPα binding sites are present in the miR-328 promoter and positively regulate its transcription. We also show that maturation of differentiation-arrested BCR/ABL+ blasts requires direct interaction of hnRNP-E2 with the miR-328 C-rich regions. Moreover, imatinib treatment restores miR-328 expression, thus allowing its direct binding to hnRNP E2 independent from the RISC complex. Importantly, physiological miR-328 expression decreased hnRNP E2 binding to the uORF/spacer region of endogenous CEBPA mRNA (decoy activity). This, in turn, releases CEBPA mRNA from hnRNP E2 translation inhibition and allows in vitro and in vivo BCR/ABL+ cell differentiation. Although hnRNP E2 was not found in complex with the basic RISC components in BCR/ABL+ cells, miR-328 was found associated to Dicer and Ago2, suggesting that miR-328 also acts through base-pairing with the 3′UTR of mRNA targets in a RISC-dependent manner. In fact, miR-328 suppresses PIM1 protein but not mRNA expression and this effect requires the integrity of the PIM1 3′UTR. Indeed, forced expression of a wild type, but not a kinase-deficient, PIM1 lacking the 3′UTR into miR-328-expressing cells fully rescues BCR/ABL clonogenicity, suggesting that miR-328-induced inhibition of PIM1 accounts for reduced survival of miR-328-infected CML-BCCD34+ blasts. To demonstrate that miR-328 acts on PIM1 in a RISC-dependent manner, we mutated the miR-328 in the seed sequence (miR-328-Mut) while retaining its C-rich character. As expected, miR-328-Mut interacted with hnRNP-E2 and rescued C/EBPα-mediated differentiation, but did not silence PIM1 expression. Thus, the discovery of dual activities for miR-328 which affect myeloid differentiation and survival not only adds a layer to the complexity of mechanisms regulating CML-BC but also highlights the ability of miRNAs to alter mRNA metabolism by acting as molecular decoys for RNA binding proteins. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1950.

Abstract 855 MicroRNAs (miRs) and heterogeneous ribonucleoproteins (hnRNPs) are post-transcriptional gene regulators that bind mRNA in a sequence-specific manner. We have reported that a) hnRNP-E2 suppresses CEBPA mRNA translation and inhibits myeloid maturation of bone marrow (BM) progenitors from chronic myelogenous leukemia patients in myeloid blast crisis (CML-BCCD34+; Perrotti et al, Nat Genet 2002); and b) miR-328 expression is lost in myeloid CML-BCCD34+ progenitors (n=6) and its restored expression at physiological levels rescues granulocytic differentiation and impairs clonogenic potential of primary BCR/ABL+ blasts (Eiring et al, ASH 2007). Here we show by Northern blot, real-time PCR, and microarray analyses that miR-328 levels increase during granulocytic differentiation of normal human CD34+ and mouse Lin− BM progenitors, but not during differentiation towards erythroid, megakaryocytic or monocytic lineages. BCR/ABL uses the same MAPKERK1/2-hnRNP-E2 signaling pathway to suppress both C/EBPα and miR-328, as pharmacologic or shRNA-mediated inhibition of these molecules restored miR-328 expression in BCR/ABL+ cells. In fact, two functional C/EBPα binding sites are present in the miR-328 promoter region and C/EBPα interacts in vivo with these regulatory elements to enhance miR-328 transcription. Importantly, we also show that restored maturation of BCR/ABL+ blasts requires direct interaction of hnRNP-E2 with the C-rich regions of miR-328. Indeed, RNA-immunoprecipitation (RIP) assays demonstrated that miR-328 directly binds to hnRNP-E2 independent of the RNA-induced silencing complex (RISC). Furthermore, ectopic miR-328, but not miR-181b, resulted in decreased in vivo binding of hnRNP-E2 to the uORF/spacer region of CEBPA mRNA, thereby releasing CEBPA from hnRNP-E2 translation inhibition and rescuing C/EBPa-driven neutrophil maturation (decoy activity). Differentiation of miR-328-expressing CML-BCCD34+ blasts (88.8±2.4% post-mitotic cells) correlated with induction of C/EBPa protein expression, whereas CEBPA mRNA and hnRNP E2 protein levels remained unchanged. The existence of a direct miR-328/hnRNP-E2/CEBPA interplay was formally demonstrated in vitro using RRL-directed translation assays and in vivo using the 6.15 clone of 32D-BCR/ABL cells that do not express endogenous CEBPA mRNA and require ectopic C/EBPα (wt-uORF-CEBPA) for differentiation. Addition of miR-328, but not miR-330, to hnRNP-E2-containing RRL reactions increased newly synthesized 35S-C/EBPa levels by >100%. Likewise, forced miR-328 expression in vivo resulted in decreased hnRNP-E2 binding to CEBPA mRNA, induction of C/EBPa protein but not mRNA and rescued granulocytic differentiation of 6.15-wt-uORF-CEBPA but not vector-transduced 6.15 cells. While hnRNP-E2 was not found in complex with basic RISC components (Dicer, TRBP2 and Ago2), RIP assays detected miR-328 associated to Dicer and Ago2 in miR-328-expressing cells, suggesting that it also acts through canonical RISC-dependent base-pairing with mRNA targets. Indeed, we identified the BCR/ABL-regulated PIM1 serine-threonine kinase as a bona fide miR-328 target in BCR/ABL+ cells. Ectopic miR-328 suppressed PIM1 protein but not mRNA levels, and this effect required integrity of the miR-328 binding site present in the PIM1 3'UTR. Forced expression of a wild-type but not kinase-deficient PIM1 lacking the 3'UTR into miR-328-expressing cells fully rescued BCR/ABL clonogenicity, suggesting that miR-328-induced PIM1 suppression accounts for reduced survival of miR-328-infected BCR/ABL+ blasts. To show that miR-328 acts on PIM1 in a RISC-dependent manner, we mutated the miR-328 seed sequence (miR-328-Mut) while retaining its C-rich character. Similar to wild-type miR-328, miR-328-Mut efficiently interacted with hnRNP-E2, restored C/EBPa protein expression and rescued granulocytic differentiation, but was unable to silence PIM1 in 32D-BCR/ABL cells, indicating that the C-rich character of miR-328 is essential for its decoy activity, while its seed sequence integrity is necessary for RISC-dependent pairing to mRNA targets. Thus, the discovery of dual activities for miR-328 not only adds a new layer of complexity to the mechanisms regulating CML disease progression, but also highlights the ability of miRNAs to alter mRNA metabolism by acting as molecular decoys for RNA-binding proteins. Disclosures: Cortes: Novartis: Research Funding.

Abstract 191 The molecular mechanism leading to the progression of chronic myelogenous leukemia (CML) from the indolent chronic phase (CML-CP) to the rapidly fatal blast crisis (CML-BC) are still unclear although a plausible assumption is that enhanced expression of BCR/ABL, as that observed in most of patients undergoing progression, represents the factor promoting clonal evolution of CML. Given that a) BCR/ABL levels are increased in the CML-BC stem/leukemia-initiating cell population; b) a causal relationship exists between levels and activity of the BCR/ABL oncoprotein and aberrant mRNA processing, nuclear export and/or translation; and c) molecular and/or pharmacologic interference with the expression and/or activity of the BCR/ABL-regulated RNA binding proteins (hnRNP A1, hnRNP K and hnRNP E2) antagonizes both in vitro and in vivo BCR/ABL leukemogenesis by impairing proliferation, inhibiting survival and/or restoring differentiation of BCR/ABL+ hematopoietic progenitors; we hypothesized that BCR/ABL initiates a hierarchical activation of signals leading to a temporally-organized increase in the expression/function of specific RNA binding proteins (RBPs), and that this represents an essential step for disease progression. To determine whether expression of hnRNP A1, K and E2 is hierarchically regulated by BCR/ABL and at which stage of the CML stem/progenitor cell development it occurs, we first transduced 32Dcl3 cells with the MigR1-BCR/ABL construct and allowed the clones from a 32D-BCR/ABLlow cell population to become BCR/ABLhigh/hnRNP E2high /C/EBPa− within 21 days of culture in the presence of IL-3. Western blots indicate that BCR/ABL-dependent full induction of hnRNP A1 expression precedes that of hnRNP K and E2 which occurs only after BCR/ABL levels and activity increase to levels capable of conferring cytokine-independent growth and differentiation arrest, suggesting that hnRNP A1 may have the role of a “gatekeeper”, as it allows the increase of BCR/ABL expression through inhibition of PP2A. This, in turn, will promote disease progression in part by inducing expression and activity of hnRNP K and E2 that, as we previously reported, regulate survival, proliferation and differentiation of CD34+ CML-BC progenitors. Notably, we observed a similar pattern of hnRNP induction in the lymphoid BCR/ABL-inducible TonB210157 cells. We also have evidence that differences in hnRNP A1, hnRNP K and hnRNP E2 expression exist between the stem and progenitor cell fractions of BCR/ABL+ primary cells and that they are differentially regulated in leukemic and normal cells. In fact, FACS analysis followed by intracellular protein staining performed on bone marrow- and spleen-derived LSK (Lin−/Sca-1+/c-kit+), common myeloid progenitors (CMPs) (Lin−/Sca-1−/c-kit+/CD34+, FCgRII/IIIdim) and granulocyte monocyte progenitors (GMPs) (Lin−/Sca-1−/c-kit+/CD34+/FCgRII/IIIbright) from leukemic SCLtTA-BCR/ABL double-transgenic mice showed that levels of hnRNP A1 were 3 to 5-fold higher in CMP/GMP than LSK cell fractions (LSKCMP>GMP). Likewise, hnRNP K and E2 levels were 2-fold increased in the progenitors compared to the LSK of leukemic animals, whereas an opposite trend in the expression of hnRNP K was observed in the LSK and CMPs/GMPs of non-induced animals. Moreover, hnRNP K levels in the leukemic CMPs/GMPs were 30 to 40-fold higher than those detected in the same cell fractions from non-induced animals. Interestingly, the highest levels of hnRNP A1 and K in the CMP/GMP fractions correlated with the development of a lymphoid blast crisis-like phenotype as determined by the 30% increase in splenic B220+/Mac-1+ cells. Altogether these data suggest that hierarchical and temporal changes in the expression of hnRNPs occur upon BCR/ABL transformation in stem and progenitor cells and during disease progression. Furthermore, these results are consistent with the reported role of the BCR/ABL-regulated hnRNP A1, K and E2 as a positive regulator of BCR/ABL stability through the SET-dependent inhibition of PP2A, a direct enhancer of Myc translation, and as an inhibitor of C/EBPa-dependent myeloid maturation of blast crisis CML progenitors, respectively. Disclosures: No relevant conflicts of interest to declare.

Abl kinase inhibitors are the preferred treatment for Chronic Myelogenous Leukemia (CML) and they are highly efficient at inducing remission but continued treatment is required as not all BCR/ABL positive cells are eradicated. Survival of tumor cells may be mediated by deregulation of apoptosis pathways as cells transformed by BCR/ABL are highly resistant to a variety of apoptotic stimuli. Resistance to drug-induced apoptosis in CD34+ CML patient cells and BCR/ABL+ cell lines is mediated by downstream signaling pathways activated by BCR/ABL. The signal transducer and activator of transcription 5 pathway which regulates the expression of the anti-apoptotic Bcl-x gene is constitutively activated in BCR/ABL-positive cells. Several lines of evidence led to the hypothesis that Bcl-x is a critical mediator in evasion of apoptosis required to sustain leukemia. In order to examine the in vivo role of Bcl-x in CML we generated a tetracycline-inducible transgenic mouse model in which BCR/ABL is expressed in hematopoietic stem cells and myeloid progenitor cells resulting in a CML-like phenotype, while at the same time the Bcl-x gene is excised. Efficient recombination and loss of Bcl-x expression in this model was verified by Three Primer PCR analysis in whole bone marrow and spleen. DNA from Gr-1+/Mac-1+ cells showed 80% recombination of the Bcl-x locus and Real-Time PCR confirmed reduction of Bcl-x expression in LSK cells (Lin−,Sca-1+, c-Kit+) isolated from bone marrow. Induction of transgenic expression demonstrated increased mortality within the first 3 weeks in Bclx−/−;CML mice with 50% (8/16) of the animals succumbing to disease, whereas only 28.5% (4/14) of CML mice expired. Analysis of peripheral blood revealed increased neutrophila in the remaining Bclx−/−;CML mice compared to CML mice. Loss of Bcl-x exacerbated infiltration of myeloid cells into non-hematopoietic tissue leading to severe damage of kidneys in 100% (8/8) of the Bclx−/−;CML mice that lived for 3 months compared to 25% of CML mice. Furthermore, 37% of Bclx−/−;CML mice that survived longer than 3 months progressed to an accelerated disease characterized by up to 30-fold increase of LSK cells in the bone marrow compared to wild type mice. Massive infiltration of the liver with c-Kit+/Mac-1+ cells, characteristic of immature myeloid cells, accompanied this phenotype. Real-time PCR data revealed that Bcl-x expression decreases as cell differentiate from common myeloid progenitors (CMP) to granulocyte/monocyte progenitor cells (GMP). Thus, loss of Bcl-x may bias differentiation to the myeloid lineage leading to the higher neutrophil counts observed in Bclx−/−;CML mice. One characteristic of the inducible transgenic CML model is progression to B lymphoblastic leukemia in 30% of mice after 8 weeks; deletion of Bcl-x resulted in loss of this phenotype. The Bcl-x gene plays an important role in development of B cell lineage and it is possible that its deletion reduced the pool of cells available for transformation by BCR/ABL. In addition, the promoter/enhancer construct used to direct expression of transgenes in this animal model displays weak activity in immature B cells. An alternative explanation assumes an essential role of Bcl-x in the maintenance of a myeloid/B-cell bipotent progenitor cell. Taken together, our results suggest that Bcl-x is not required for the generation and maintenance of CML-like disease, and may in fact exacerbate the myeloproliferation, due to biased lineage allocation, however, it may be required for progression to lymphoid blast crisis.

While treatment with tyrosine kinase inhibitors is highly successful for patients diagnosed in the chronic phase of chronic myeloid leukemia, these drugs are inefficient for BCR/ABL associated B-cell acute lymphocytic leukemia (B-ALL). Therefore, it is necessary to identify molecular targets downstream of BCR/ABL to develop additional therapeutic approaches. Cells transformed by BCR/ABL are resistant to a wide variety of apoptotic stimuli and therapeutic strategies aimed at reinstating the apoptotic pathway appear as an attractive concept. Bcl-xL is an antiapoptotic member of the Bcl-2 family of proteins and studies employing cell lines, as well as primary cells have linked BCR/ABL expression with increased levels of Bcl-xL, resulting in resistance to chemotherapeutic agents. To define the role of Bcl-xL in BCR/ABL associated B-ALL, we generated two inducible transgenic mouse models. In the first model, BCR/ABL and loss of Bcl-x expression are co-induced, and in the second model, leukemia is induced with expression of Bcl-xL protein well above the levels found in wildtype lymphoblasts. Surprisingly, we found that deletion of Bcl-xL did not inhibit leukemogenesis or affect apoptosis. Bcl-x deficient B-ALL mice rapidly succumbed to a B-ALL like disease with Bcl-x deficient B-ALL animals being moribund as early as 17 days after induction. By day 28, all mice (n=10) had died or had to be euthanized. Necropsy of animals suffering from Bcl-x deficient leukemia revealed massive lymphadenopathy, pleural effusion, and splenomegaly. While loss of Bcl-x in our B-ALL model led to a more severe phenotype with considerable tumor burden, no statistically significant difference was found between the survival time in Bcl-x deficient and wild type B-ALL animals due to development of pleural effusion in both models. The most prominent difference was the presence of mitotic figures in the peripheral blood, lymph node, and spleen of Bcl-x deficient B-ALL animals, suggestive of increased proliferation of Bcl-x deficient lymphoblasts. Cell cycle analysis of leukemic cells isolated from pleural effusion and spleen of Bcl-x deficient B-ALL mice demonstrated a significant increase of cells in S/G2/M phase (p ≤ 0.05) compared to wildtype lymphoblasts. Thus, loss of Bcl-xL results in increased passage through the cell cycle, while expression of the protein limits the proliferation rate. To test this hypothesis, we generated a second model in which Bcl-xL is expressed at higher levels than in wild type lymphoblasts. Overexpression of Bcl-xL in BCR/ABL positive mice led to reduced proliferation as significantly fewer leukemic cells were present in the S phase than in controls substantiating a role for in Bcl-xL proliferation of lymphoblasts. Initial studies performed to determine the mechanisms by which loss of Bcl-x leads to increased proliferation suggest that the protein may indirectly regulate stability of p27Kip1. Our data show that cells from Bcl-x deficient B-ALL mice in G1 and S phase contain less p27, as a consequence of proteosomal degradation. Clearly, our model systems demonstrate an unexpected and novel role for Bcl-xL in the context of BCR/ABL associated B-ALL. Ongoing studies are aimed at the identification of the mechanism and molecules through which Bcl-xL is linked to cell cycle and proliferation of BCR/ABL transformed lymphoblasts.

Germline deletion of bcl-x demonstrated the importance of this gene for hematopoiesis. Bcl-x deficient embryos failed to thrive due to massive apoptosis of immature hematopoietic and neural cells. Deletion of this gene in the adult has uncovered its requirement for the final maturation of erythroid cells, the transition of pre-B cells to the pro-B cell stage and recently, we showed an important role in the development of natural killer cells. While the role of bcl-x in hematopoietic stem cells (HSC) has yet to be addressed, a role for other members of anti-apoptotic bcl-2 family of proteins has been established. Overexpression of bcl-2 led to accumulation of HSC, whereas genetic deletion of mcl-1 resulted in ablation of HSC. In the present study, we determined by quantitative Real-Time PCR, that in addition to mcl-1, bcl-x is also highly expressed in hematopoietic stem cells, whereas the level of bcl-2 is threefold lower. Moreover, we found the expression of bcl-x to be down regulated with differentiation of HSC into progenitor populations (i.e. GMP, CMP and MEP). Therefore, we established a model to delete bcl-x in the stem cells of adult mice. We bred the Mx1-cre transgenic line with mice that carry the bcl-x gene flanked by loxP sites. Deletion of bcl-x occurred following administration of pIpC. A total of 10 bcl-x deficient mice and 7 age-matched control animals (Mxi-cre/wildtype bcl-x) were examined. Quantitative real-time PCR revealed a 55 – 95% reduction of bcl-x mRNA in Lin−, Sca-1+, c-kit+ cells 8 days post the first administration of pIpC. This population consists of phenotypically and functionally defined long-term HSC (LT-HSC), short-term HSC (ST-HSC) and multipotent progenitors. We used FACS analysis to identify LT-HSC versus ST-HSC populations and to further assess the effects of bcl-x deletion. LT-HSC were defined by staining, for the following cell surface markers as Lin−, Sca-1+, c-kit+, Flt3−, CD34−; ST-HSC were characterized as Lin−, Sca-1+, c-kit+, Flt3−, CD34+. We found a 2 fold shift in the ratio of LT-HSC versus ST-HSC in bcl-x deficient mice compared to the ratio in control animals. This result raises the hypothesis that bcl-x regulates entry of HSC into the cell cycle. A notion that is supported by the observation that overexpression of bcl-x in fibroblasts delays transition from the Go/G1 to the S phase of the cell cycle. Future experiments will determine if the observed phenomenon is mediated through the anti-apoptotic function or cell cycle activity of the bcl-x protein.

The size of the Natural Killer (NK) cell pool is maintained through production and subsequently export from the bone marrow, peripheral survival and proliferation, and ultimately cell death. While there exists considerable knowledge about developmental stages of lymphoid, myeloid and erythroid cells, comparably little is known about NK cell intermediates and the genes required for their development. Most of the models of NK cell differentiation have been based on in vitro culture systems where NK cells could be generated from multipotent HSC precursors. However, this approach suffers from the problems inherent to in vitro cell manipulations. We have utilized conditional gene targeting in adult mice to examine the role of the bcl-x gene in the development of NK cells in bone marrow and after their export to the spleen. Bcl-x is an important member of the anti-apoptotic member of the Bcl-2 gene family, and a critical role for this gene in the survival of hematopoietic cells was demonstrated in bcl-x-deficient mice causing embryonic death due to massive apoptosis of immature hematopoietic cells and of neurons. Conditional deletion in erythroid cells lead to hemolytic anemia and extensive splenomegaly. Furthermore, bcl-x is critical for the maturation of pre-B cells to the pro B cell stage, while it is not essential for the development of effector and memory T cells. We have conditionally deleted the gene in the stem cells of adult mice by cross breeding them with the Mxi-cre deleter strain, which allows for induced expression of cre recombinase by injection with pIpC. As early as 9 days after the first injection of pIpC, the number of NK cells in the bone marrow of mice started to decline as demonstrated by multi-color FACS analysis staining for IL2 Receptor beta (CD122) and NKG2D, among other markers. Cultures of bone marrow and spleen cells in the presence of cytokines to generate lymphokine activated killer cells failed, while no such effect was observed in cultures from Mxi-cre mice that were subjected to pIpC injections and carried along as controls. Analysis of animals after 3 weeks of pIpC administration revealed absence of NK cell precursors in the bone marrow as demonstrated by the lack of CD122+/Lin- negative cells. This phenomenon was accompanied by a reduction in the number of mature NK cells in the spleen. To date, six stages of NK cell maturation are described with the acquisition of IL2 receptor beta expression marking commitment to the NK-cell lineage. IL2 Receptor beta as well as NKG2D are expressed throughout NK cell development and at all stages. In order to characterize the specific stage at which expression of bcl-x is essential for NK cell maturation, we employed multi-color FACS analysis staining for CD122, NKG2D, CD49b, and the integrins CD43 and Mac-1 after depletion of lineage positive cells, followed by sorting for defined populations. Real Time PCR on sorted cells demonstrated that Bcl-x mRNA is highly expressed throughout all stages of NK cell development. Taken together, the gradual reduction in the number of NK cell precursors eventually leading to complete loss of this lineage in the bone marrow and peripheral sites suggests that bcl-x is indispensable for the development of NK cells presumably from the earliest time point of commitment to this lineage.