Rspo-LGR4 Cooperates with HOXA9 to Sustain Self-Renewal in Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is associated with high relapse rates and poor survival, with limited response to conventional cancer therapy and lacking effective targeting of highly self-renewing leukemic stem cells (LSCs). The mechanism underlying the high self-renewal activity of LSCs that determines the aggressiveness of disease remains poorly understood. Although we and others have previously demonstrated the clinical significance of aberrant WNT/β-catenin signaling in AML (Science, 327:1650-1653, 2010; Cancer Cell, 18:606-618, 2010), its pharmacologically tractable components essential for the regulation of LSC self-renewal have not yet been determined. Our studies discover, for the first time, a critical link between R-spondin (RSPO)-LGR4/HOXA9 and WNT/β-catenin pathways in AML LSCs. Microarray data analysis of 183 AML patient samples showed a significant positive correlation between expression of LGR4 and HOXA9 (r=0.546, P<0.0001). LGR4 exerted a cell-of-origin-specific function in promoting aberrant self-renewal and AML progression in vivo through cooperating with HOXA9, a poor prognostic predictor. We observed that LGR4 itself was not able to fully transform normal hematopoietic stem/progenitor cells (HSPCs), but instead cooperated with HOXA9 in HSPCs to accelerate disease onset producing a highly aggressive short latency AML in vivo. LGR4 and HOXA9 were epigenetically upregulated and their coexpression was an essential determinant of RSPO-LGR4 oncogenic activity. RSPO/WNT3 ligands could serve as stem cell growth factors to sustain myeloid differentiation block and to promote proliferation of CD34+ LSC-enriched subpopulations in primary AML patient specimens co-expressing LGR4 and HOXA9. Conversely, CRISPR/Cas9-mediated knockout of LGR4 not only suppressed RSPO/WNT3 signals and markedly decreased nuclear active β-catenin, but also reduced tumor burden in a patient-derived xenograft (PDX) mouse model of relapsed AML. Importantly, this study is the first to demonstrate that pharmacological inhibition of RSPO3-LGR4 signaling by a clinical-grade anti-RSPO3 monoclonal antibody induced LSC differentiation and consequently prevented tumor growth in AML PDX mice but did not affect normal human stem cell compartment in NSG mice. Together, our findings support a critical role for RSPO-LGR4 in the Wnt/β-catenin signaling pathway to promote AML leukemogenesis. Aberrant activation of RSPO-LGR4 is crucial for enhancing the self-renewal potential and myeloid differentiation block, which contribute to an aggressive leukemia phenotype through cooperating with HOXA9. Genetic and pharmacological targeting of this pathway impairs LSC self-renewal and survival and impedes AML development in murine models and patient-derived xenografts, highlighting the therapeutic value of targeting RSPO-LGR4 signaling in AML. References: Wang Y, et al. The Wnt/beta-catenin pathway is required for the development of leukemia stem cells in AML. Science. 2010;327:1650-1653. Yeung J, et al. Beta-catenin mediates the establishment and drug resistance of MLL leukemic stem cells. Cancer Cell. 2010;18:606-618. Disclosures No relevant conflicts of interest to declare.

Download Full-text

GADD45a Controls Self-Renewal in Acute Myeloid Leukemia Stem Cells

Blood ◽

10.1182/blood-2020-142389 ◽

2020 ◽

Vol 136 (Supplement 1) ◽

pp. 31-31

Author(s):

Nunki Hassan ◽

Hangyu Yi ◽

Lucie Gaspard-Boulinc ◽

Franklin Chen ◽

Jayvee Datuin ◽

...

Keyword(s):

Stem Cells ◽

Acute Myeloid Leukemia ◽

Cancer Cell ◽

Myeloid Leukemia ◽

Conflicts Of Interest ◽

Leukemia Stem Cells ◽

Beta Catenin ◽

Self Renewal ◽

Acute Myeloid

Acute myeloid leukemia (AML) is a heterogenous malignancy, where the persistence of chemo-resistant leukemia stem cells (LSCs) contributes to disease relapse. We have previously demonstrated the clinical significance of WNT/β-catenin signaling in driving AML LSCs (Science, 327:1650-1653, 2010; Cancer Cell, 38:1-16, 2020). In this study, we uncover that GADD45a (growth arrest and DNA-damage inducible protein) is an essential regulator of β-catenin signaling pathway and its loss promotes LSC function and leukemia progression. Transgenic knockout of Gadd45a led to a progressive increase in aberrant self-renewal and leukemogenesis in vivo. Gadd45a-/- leukemic cells developed a more aggressive leukemia with a shorter latency than Gadd45a+/+ cells in mice, indicating the involvement of Gadd45a loss in AML initiation and progression. Subsequent serial transplantation experiments showed that Gadd45a deletion enhanced LSC self-renewal in vivo. In agreement with our findings in murine LSCs, deletion of GADD45a by CRISPR/Cas9 in AML patient-derived xenograft (PDX) cells revealed increased engraftment and tumor burden in NSG mice. Consistent with our phenotypic observations, knockout of GADD45a increased βcatenin activity and key WNT/self-renewal target genes in human AML cells. In addition, our cellular indexing of transcriptomes and epitopes by sequencing (CITE-seq) data showed that GADD45a deletion in patient-derived LSCs was associated with cell metabolism, reactive oxygen species and tumor progression, as well as poor patient outcomes in AML. Further studies are being conducted to evaluate transcriptional mechanisms discovered by our single-cell sequencing. Taken together, this study is the first to demonstrate that GADD45a loss promotes LSC potential and consequently enhances tumor growth in murine and PDX models of AML, thus showcasing GADD45a as a promising therapeutic target in AML. References: Wang Y, Krivtsov AV, Sinha AU, et al. The Wnt/beta-catenin pathway is required for the development of leukemia stem cells in AML. Science. 2010;327:1650-1653. Salik B, Yi H, Hassan N, et al. Targeting RSPO-LGR4 signaling for leukemia stem cell eradication in acute myeloid leukemia. Cancer Cell. 2020; 38:1-16. Disclosures No relevant conflicts of interest to declare.

Download Full-text

Novel Therapeutics That Can Target and Eradicate Leukemic Stem Cells in Acute Myeloid Leukemia: Investigating FDA-Approved Anti-Inflammatory Compounds

Blood ◽

10.1182/blood-2019-125338 ◽

2019 ◽

Vol 134 (Supplement_1) ◽

pp. 5052-5052

Author(s):

Isabella Iasenza ◽

Meaghan Boileau ◽

Andrea Neumann ◽

Héloïse Frison ◽

Mark D. Minden ◽

...

Keyword(s):

Stem Cells ◽

Acute Myeloid Leukemia ◽

Mechanism Of Action ◽

Myeloid Leukemia ◽

Terminal Differentiation ◽

Dependent Manner ◽

Leukemic Stem Cells ◽

Anti Inflammatory ◽

Acute Myeloid

Acute myeloid leukemia (AML) is an aggressive form of blood cancer defined by the uncontrolled proliferation of immature myeloblast cells in the blood and bone marrow, leading to hematopoietic failure. The 5-year survival rate is 28% in patients aged 20 years and older and 64% in patients aged 19 years and younger (SEER 2019). A large portion of these patients succumb to the disease partially due to the chemo-resistant nature of leukemic stem cells (LSCs). Hence, novel therapies targeting unique LSC biology that spare hematopoietic stem cells (HSCs) are needed to eliminate and avoid reoccurrence of this disease. We had previously identified FDA-approved anti-inflammatory glucocorticoids mometasone, halcinonide, and budesonide as compounds that induce terminal differentiation of the LSC (CD34+CD38-) and progenitor cell (CD34+CD38+) populations to leukemic blast cells (CD15+CD34-) in refractory human AML (Laverdière & Boileau et al., Blood Can. J. 2018). Following the paradigm of successful differentiation treatment in AML (acute promyelocytic leukemia with all-trans retinoic acid), the effects and mechanism of action of the glucocorticoids on LSCs need to be further investigated for other AML subtypes. Furthermore, dexamethasone, a glucocorticoid currently used to successfully treat acute lymphoblastic leukemia (ALL), is being studied in a Phase II clinical trial for induction and post-remission chemotherapy in older patients with de novo or therapy-related AML (clinicalTrials.gov, NCT03609060). To identify the subtypes of AML that are sensitive to steroid-induced LSC differentiation, we began by screening a panel of cell lines (F36P, MOLM-13, Kasumi-6, Kasumi-1 and K562) and observed that only Kasumi-1, a pediatric leukemia carrying the t(8;21) mutation leading to the fused RUNX1-RUNX1T1 gene, was responsive to glucocorticoid treatment, although without differentiation. This is consistent with the finding of Simon et al. who observed a loss of bulk AML cells in RUNX1 AML samples following dexamethasone treatment (Simon et al., Clin Cancer Res. 2017). However, we observed expansion of bulk cells following differentiation of LSCs in primary AML, indicating different mechanisms of steroid response in different samples: differentiation of LSCs or overall loss of AML cells. We will further investigate these compounds in a panel of 10 genetically defined primary AML samples to classify which oncogenetic drivers or subtypes of AML are linked to sensitivity to the three glucocorticoids, including which drive cell death vs LSC differentiation. We will perform ex vivo and in vivo studies of the glucocorticoids to assess the extent of engraftment in treated versus DMSO treated samples. This additional data will be presented at the annual meeting. In addition, to explore the mechanism of action of these steroids in AML, we investigated the roles of the cytokines interleukin-3 (IL-3), interleukin-6 (IL-6), stem cell factor (SCF), granulocyte colony stimulating factor (GCSF), thrombopoietin (TPO) and FMS-like tyrosine kinase 3 ligand (FLT3L), used to culture AML, on the differentiation effects induced by the glucocorticoids. We observed that only FLT3L was required for the complete differentiation of LSCs. In summary, we have observed that the three glucocorticoid steroids (mometasone, halcinonide, and budesonide), as well as dexamethasone to a lesser extent, can induce two different responses in a sample-dependent manner: terminal differentiation of LSCs or overall cell loss. We have also observed that the differentiation response requires FLT3L for maturation of the AML cells. Our current studies involve in vivo and genomic assays to determine the effect on functional LSCs and the genetic markers of sensitivity and we will present these results. Disclosures Minden: Trillium Therapetuics: Other: licensing agreement.

Download Full-text

Identification and Purification of Leukemic Stem-Like Cells in Acute Myeloid Leukemia Cell Line

Blood ◽

10.1182/blood.v112.11.4734.4734 ◽

2008 ◽

Vol 112 (11) ◽

pp. 4734-4734

Author(s):

Miaorong She ◽

Xingqing Niu ◽

Xilin Chen ◽

Guo Kunyuan ◽

Maohua Zhou ◽

...

Keyword(s):

Stem Cells ◽

Acute Myeloid Leukemia ◽

Cell Line ◽

Myeloid Leukemia ◽

Critical Role ◽

Leukemia Cell ◽

Leukemia Cell Line ◽

Leukemic Stem Cells ◽

Self Renewal ◽

Acute Myeloid

Abstract Acute myeloid leukemia (AML) is initiated and maintained by a rare population of (leukemic stem cells) LSCs. LSCs play the central role in the relapse and refractory of AML and highlight the critical need for the new therapeutic strategies to directly target the LSC population for ultimately curing leukemia which is it is important to identify and study LSCs. However, relatively little is known about the unique molecular mechanisms of survival and self-renewal of LSCs because of very small number of LSCs in bone marrow. In this study, we investigated whether established leukemia cell lines contain LSCs. We showed that leukemia cell line contain leukemic stem-like cells which have been phenotypically restricted within the CD34+CD38− fraction. We demonstrated that CD34+CD38− cells could generate CD34+CD38+ cells in culture medium and had proliferation function. Moreover, CD34+CD38− cells had self-renewal potential both in vitro soft agar colonies formation assay and in vivo NOD/SCID mouse xenotransplant model serial transplantation. Furthermore, CD34+CD38− cells isolated from leukemia cell line were found resistant to conventional chemotherapy and NK cells-mediated cytotoxicity and these were related to up-regulation of ABCG2 and MRP-1 and antiapoptotic proteins of Bcl2. Down-regulation of NKG2D ligand also played a critical role in NK cytotoxicity resistance. Taken together, our studies provide a novel cell model for leukemic stem cells research. Our data also shed light on mechanism of double resistant to resistant to chemotherapy and NK cell immunotherapy, which was helpful for developing novel effective strategies for LSCs.

Download Full-text

439 Dual modes of action for anti-TIM-3 antibody MBG453 in myelodysplastic syndromes (MDS) and acute myeloid leukemia (AML): preclinical evidence for immune-mediated and anti-leukemic activity

Journal for ImmunoTherapy of Cancer ◽

10.1136/jitc-2020-sitc2020.0439 ◽

2020 ◽

Vol 8 (Suppl 3) ◽

pp. A465-A465

Author(s):

Catherine Sabatos-Peyton ◽

Tyler Longmire ◽

Lisa Baker ◽

Nidhi Patel ◽

Anne-Sophie Wavreille ◽

...

Keyword(s):

Stem Cells ◽

Acute Myeloid Leukemia ◽

Stem Cell ◽

High Risk ◽

Myeloid Leukemia ◽

Leukemic Stem Cells ◽

Self Renewal ◽

Immune Mediated ◽

Acute Myeloid ◽

Cell Stem Cell

BackgroundTIM-3 is expressed on leukemic stem cells (LSCs) and blasts in AML,1 2 and TIM-3 expression on MDS blasts correlates with disease progression.3 Functional evidence for TIM-3 in AML was established with an anti-TIM-3 antibody which inhibited engraftment and development of human AML in immuno-deficient murine hosts.1 TIM-3 promotes an autocrine stimulatory loop via the TIM-3/Galectin-9 interaction, supporting LSC self-renewal.4 In addition to its cell-autonomous role on LSCs/blasts, TIM-3 also has a critical role in immune system regulation, in adaptive (CD4+ and CD8+ T effector cells, regulatory T cells) and innate (macrophages, dendritic cells, NK cells) immune responses.5 MBG453 is a high-affinity, humanized anti-TIM-3 IgG4 antibody (Ab) (stabilized hinge, S228P), which blocks the binding of TIM-3 to phosphatidylserine (PtdSer). Recent results from a multi-center, open label phase Ib dose-escalation study (NCT03066648) in patients with high-risk MDS and no prior hypomethylating agent therapy evaluating MBG453 in combination with decitabine demonstrated encouraging preliminary efficacy with an overall response rate of 58%,6 and MBG453 combined with azacitidine also showed encouraging response rates.7 Preclinical experiments were undertaken to define the mechanism of action of the hypomethylating agent and anti-TIM-3 combination.MethodsTHP-1 cells (a human monocytic AML cell line) were pre-treated with decitabine and co-cultured with anti-CD3 activated healthy human donor peripheral blood mononuclear cells (PBMCs) in an Incucyte-based assay to measure cell killing. The ability of MBG453 to mediate antibody-dependent cellular phagocytosis (ADCP) was measured by determining the phagocytic uptake of an engineered TIM-3-overexpressing Raji cell line in the presence of MBG453 by phorbol 12-myristate 13-acetate (PMA)-activated THP-1 cells. Patient-derived AML xenograft studies were undertaken in immune-deficient murine hosts to evaluate the combination of decitabine and MBG453.ResultsMBG453 was determined to partially block the TIM-3/Galectin-9 interaction in a plate-based MSD (Meso Scale Discovery) assay, supported by a crystal structure of human TIM-3.8 Pre-treatment of THP-1 cells with decitabine enhanced sensitivity to immune-mediated killing in the presence of MBG453. MBG453 was determined to mediate modest ADCP, relative to controls. MBG453 did not enhance the anti-leukemic activity of decitabine in patient-derived xenograft studies in immuno-deficient hosts.ConclusionsTaken together, these results support both direct anti-leukemic effects and immune-mediated modulation by MBG453. Further studies are ongoing to determine: (1) whether MBG453 can mediate physiologically relevant ADCP of TIM-3-expressing leukemic cells; and (2) the potential of MBG453 to impact the autocrine feedback loop of TIM-3/Galectin-9.Ethics ApprovalThe human tissue used in these studies was under the Novartis Institutes of BioMedical Research Ethics Board IRB, Approval Number 201252867.ReferencesKikushige Y, Shima T, Takayanagi S, et al. TIM-3 is a promising target to selectively kill acute myeloid leukemia stem cells. Cell Stem Cell 2010;7(6):708–717.Jan M, Chao MP, Cha AC, et al. Prospective separation of normal and leukemic stem cells based on differential expression of TIM3, a human acute myeloid leukemia stem cell marker. Proc Natl Acad Sci USA 2011; 108(12): 5009–5014.Asayama T, Tamura H, Ishibashi M, et al. Functional expression of Tim-3 on blasts and clinical impact of its ligand galectin-9 in myelodysplastic syndromes. Oncotarget 2017;8(51): 88904–88917.Kikushige Y, Miyamoto T, Yuda J, et al. A TIM-3/Gal-9 autocrine stimulatory loop drives self-renewal of human myeloid leukemia stem cells and leukemic progression. Cell Stem Cell 2015; 17(3):341–352.Acharya N, Sabatos-Peyton C, Anderson AC. Tim-3 finds its place in the cancer immunotherapy landscape. J Immunother Cancer 2020; 8(1):e000911.Borate U, Esteve J, Porkka K, et al. Phase Ib Study of the Anti-TIM-3 Antibody MBG453 in combination with decitabine in patients with high-risk myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML). Blood 2019;134 (Supplement_1):570.Borate U, Esteve J, Porkka K, et al. Abstract S185: Anti-TIM-3 antibody MBG453 in combination with hypomethylating agents (HMAs) in patients (pts) with high-risk myelodysplastic syndrome (HR-MDS) and acute myeloid leukemia (AML): a Phase 1 study. EHA 2020.Sabatos-Peyton C. MBG453: A high affinity, ligand-blocking anti-TIM-3 monoclonal Ab. AACR 2016.

Download Full-text

PP2A is a therapeutically targetable driver of cell fate decisions via a c-Myc/p21 axis in Acute Myeloid Leukemia

Blood ◽

10.1182/blood.2020010344 ◽

2021 ◽

Author(s):

Swagata Goswami ◽

Rajeswaran Mani ◽

Jessica Nunes ◽

Chi-ling Chiang ◽

Kevan Zapolnik ◽

...

Keyword(s):

Acute Myeloid Leukemia ◽

Cell Fate ◽

Myeloid Leukemia ◽

Myeloid Differentiation ◽

Cell Fate Decisions ◽

Phosphatase 2A ◽

Differentiation Block ◽

Pp2a Activation ◽

Acute Myeloid

Dysregulated cellular differentiation is a hallmark of acute leukemogenesis. Phosphatases are widely suppressed in cancers but have not been traditionally associated with differentiation. Herein, we identified that the silencing of Protein Phosphatase 2A (PP2A) directly contributes to differentiation block in acute myeloid leukemia (AML). Gene expression and mass cytometric profiling reveal that PP2A activation modulates cell cycle and transcriptional regulators that program terminal myeloid differentiation. Using a novel pharmacological agent OSU-2S in parallel with genetic approaches, we discovered that PP2A enforces c-Myc and p21 dependent terminal differentiation, proliferation arrest and apoptosis in AML. Finally, we demonstrate that PP2A activation decreases leukemia initiating stem cells, increases leukemic blast maturation, and improves overall survival in murine Tet2-/-Flt3ITD/WT and human AML models in-vivo. Our findings identify the PP2A/c-Myc/p21 axis as a critical regulator of the differentiation/proliferation switch in AML that can be therapeutically targeted in malignancies with dysregulated maturation fate.

Download Full-text

The Role of Gata2 in Murine Models of Acute Myeloid Leukemia

Blood ◽

10.1182/blood.v128.22.1516.1516 ◽

2016 ◽

Vol 128 (22) ◽

pp. 1516-1516

Author(s):

Taylor Yamauchi ◽

Etienne Danis ◽

Xi Zhang ◽

Simone Riedel ◽

Hua Huang ◽

...

Keyword(s):

Stem Cells ◽

Acute Myeloid Leukemia ◽

Myeloid Leukemia ◽

Murine Models ◽

Self Renewal ◽

Genetic Inactivation ◽

Acute Myeloid

Abstract The importance of stem cell and self-renewal programs in Acute Myeloid Leukemia (AML) is generally accepted, but the molecular details are incompletely understood. The master transcriptional regulator GATA2 is highly expressed in hematopoietic stem cells (HSCs) and has critically important roles in the hematopoietic system. Gata2 is required for murine HSC development and maintenance, and heterozygous loss of Gata2 compromises murine HSC- and progenitor cell-function. High levels of GATA2-expression have been correlated with adverse prognosis in human AML. GATA2 is also overexpressed in human chronic myeloid leukemia. These data suggest an important role for GATA2 in normal stem cells and in leukemia. However, genetic lesions resulting in compromised GATA2 function can lead to MDS and in some cases AML. In a murine AML model driven by Flt3-ITD and inactivation of Tet2, Gata2 is strongly downregulated. Furthermore, mouse models of leukemia suggest that high-level forced expression of Gata2 can have a tumor suppressor role. To clarify the role of Gata2in AML we used homozygous genetic inactivation in established murine models of leukemia, using a a conditional allele. We initially tested the role of Gata2 in a murine leukemia mediated by forced expression of Meningioma1 (MN1). This model has a HoxA9/Meis1 transcriptional program. We recently found that MN1-driven leukemia depends on the histone methyltransferase Dot1l (J Clin Invest. 2016 Feb 29. pii: 80825). Lineage marker negative (Lin-), Sca1+, Kit-positive (LSK) bone marrow cells from mice with a floxed exon 5 in the Gata2 gene, and a ROSA26-YFP Cre-reporter allele were transduced with an MSCV-based ecotropic retroviral vector expressing MN1 and linked via an internal ribosomal entry site (IRES) the selectable marker GFP. Floxed Gata2-sequences were excised using transduction with a self-excising Cre-expressing vector (HR-Cre). Cells were sorted and plated in methylcellulose. The GFP/YFP double positive Gata2ko cells showed a replating defect compared to GFP single positive Gata2-floxed cells, both with regard to colony number and colony size. Next, we tested the role of Gata2 in disease maintenance in vivo. We established MN1 Gata2ff leukemias in primary recipients. Primary leukemias were transduced with Cre-expressing vector and Gata2ko and Gata2ff MN1 cells were transplanted. While all mice in the Gata2ffgroup developed leukemia with a median survival of 35 days, the mice in the Gata2ko cohort developed leukemia with incomplete penetrance with a latency of 249 days (p=0.0005). These data suggest an important role for Gata2in MN1 leukemia in vitro and in vivo. Genetic inactivation of Gata2 resulted in increased protein levels of p53 in vitro as detected by Western blot. Furthermore, MN1-transduced cells showed accentuated p53 stabilization and apoptosis in response to the Mdm2-antagonist and p53 stabilizer Nutlin3. We next tested the role of Gata2 in leukemia driven by the oncogenic fusion MLL-AF9. In contrast to the MN1 model, recipients of Gata2koleukemias only showed a trend towards prolonged latency in secondary recipients (median survival Gata2ff=48 days vs. Gata2ko=62 days, p=0.09). In this model, we also did not observe a substantial effect of Gata2-inactivation on p53 activation. We are currently characterizing the underlying molecular mechanisms. Our data document an important role for Gata2 in AML mediated by MN1 and to a lesser degree, MLL-AF9. The role of Gata2 in leukemia is complex and depends on expression levels and cellular context. A more detailed understanding of leukemic self-renewal, including the role of Gata2, will inform the development of more efficacious and less toxic therapies for this difficult-to-treat malignancy. Disclosures Bernt: Epizyme: Patents & Royalties: patent filed. Neff:Epizyme: Patents & Royalties: patent filed; Bristol Myers Squibb: Other: Travel; Janssen: Other: Travel.

Download Full-text

Cytosine Arabinoside Chemotherapy Does Not Enrich For Leukemic Stem Cells In Xenotransplantation Model Of Human Acute Myeloid Leukemia

Blood ◽

10.1182/blood.v122.21.1651.1651 ◽

2013 ◽

Vol 122 (21) ◽

pp. 1651-1651 ◽

Cited By ~ 2

Author(s):

Fabienne de Toni ◽

Robin L. Perry ◽

Estelle Saland ◽

Mayumi Sugita ◽

Marion David ◽

...

Keyword(s):

Cell Cycle ◽

Stem Cells ◽

Acute Myeloid Leukemia ◽

Myeloid Leukemia ◽

Leukemic Cells ◽

Leukemic Stem Cells ◽

Frequent Relapses ◽

Cell Cycle Status ◽

Acute Myeloid

Abstract Despite a high rate of complete remission after treatment with conventional genotoxic agents, the overall survival of patients with acute myeloid leukemia (AML) is poor due to frequent relapses caused by the chemoresistance of rare leukemic stem cells (LSCs, also called Scid-Leukemia Initiating Cells). This unfavorable situation leads to a strong need to characterize those cells in order to target them with new specific therapies. Using a robust immunodeficient mouse model (NOD/LtSz-scid IL2Rγchainnull or NSG), we have previously shown that these LSCs were rare and not restricted to the CD34+CD38- immature compartment. This phenotypical heterogeneity of LSCs suggests that pharmacological targeting of LSC will not work if solely based on their cell surface markers. A better understanding of the mechanisms underlying the in vivo chemoresistance is required for the development of innovative targeted therapies. Aracytine (Ara-C, a pyrimidine analog), the most clinically used chemotherapeutic agents for AML patients, inhibits DNA synthesis and, therefore, targets and kills cycling AML cells in S phase of the cell cycle. Based on this mechanism of action, we hypothesized that Ara-C treatment will spare and enrich quiescent LSCs in vivo. We analyzed the response to Ara-C and residual disease in NSG mice engrafted with primary AML cells from 13 patients in two clinical centers (University of Pennsylvania, Philadelphia, USA and Purpan Hospital, Toulouse, France). A sub-lethal treatment of 60 mg/kg Ara-C given daily for five days induced a 5- to 50- fold reduction of peripheral blood blasts and total tumor burden in spleen and bone marrow in all patients tested. For 5 patients, we observed relapse within 4 to 6 weeks post-chemotherapy. Surprisingly, residual viable cells after Ara-C treatment showed no significant enrichment in quiescent cells and CD34+CD38- cells for the majority of primary samples tested (12 and 10 out of 13 total tested, respectively). Of note, the largest fraction (70-90%) of leukemic cells is in G0/G1 phase (including 0.5-20% in G0) in untreated engrafted mice. Moreover, we observed no significant changes in cell cycle profile of residual leukemic cells during the time course of the disease progression for 3 out of 4 patients. Finally, we assessed the frequency of LSCs in Ara-C-treated and control mice using transplantation and limiting dilution analysis in secondary recipients. Interestingly, we observed that Ara-C treatment did not increase the frequency of SL-ICs in residual cells, suggesting that blasts and LSC were equally sensitive to Ara-C in vivo. Our results show that sub-lethal regimen of Ara-C does not lead to enrichment of LSCs and induces cell death of both leukemic bulk and stem/progenitor cells independently of their cell cycle status probably through another in vivo mechanism such as apoptosis, autophagy or necroptosis. This study also suggests that further characterization of chemoresistant leukemic cells beyond phenotype and cell cycle status must rely on more functional properties in order to better elucidate the molecular basis of resistance in AML. Disclosures: Perry: MERCK: Employment. Carroll:Leukemia and Lymphoma Society: Research Funding. Sarry:AFFICHEM SA: Membership on an entity’s Board of Directors or advisory committees.

Download Full-text