ALBERT

Description: Abstract 1848 Poster Board I-874 Multiple myeloma (MM) is the second most common hematologic malignancy and characterized by clonal proliferation of CD138+ bone marrow plasma cells. Despite various treatment options few patients with MM have been cured. Furthermore, high relapse rates and recent evidence from xenogeneic transplantation models and primary MM marrow samples indicate that a rare population of cells or MM cancer stem cells (MM CSCs) within the marrow regenerates itself and may be responsible for drug resistance. These MM CSCs are phenotypically similar to memory B cells (CD138- CD34-CD19+) but differ in that they have the capacity to regenerate themselves or self-renewal. However, most of the reports on MM CSC animal models are established in NOD/SCID mice that require a larger number (1 – 10 × 106) of bead sorted cells for each animal. In addition, the latency of MM induction (4 – 6 months) in NOD/SCID mouse models and lack of in vivo tracking of the malignant clone preclude robust pre-clinical testing of novel therapeutic strategies that target MM CSC. Mononuclear cells were isolated from autologous mobilized peripheral blood of at least four primary MM patients after Ficoll gradient centrifugation followed by immunomagnetic bead depletion of CD34+ and CD138+ cells and/or further sorted using a FACSAria. The CD138-CD34- population was transduced with lentiviral luciferase GFP (GLF) and transplanted (10,000 to 106 cells per mouse) intrahepatically into neonatal RAG2-/- gamma chain-/- (RAG2-/-gc-/-) mice. Engraftment was compared to mice transplanted with either CD34+ or CD138+ cells. Mice were imaged with an in vivo imaging system (IVIS) to detect bioluminescent engraftment. Results showed that a relatively rare CD138- CD27+ population, resembling memory B cells (∼1.2%), persists in MM autografts and can engraft immunocompromised mice more rapidly and effectively than the CD138+ (Lin+) population of mature plasma cells. This data supports the persistence of CSCs despite high dose chemotherapy further underscoring the need for CSC targeted therapy. Bioluminescence was detected in live mice transplanted with as little as 60,000 cells of CD138- CD34- population and as soon as 4 weeks after transplantation. FACS analysis of these mice demonstrated successful engraftment with the presence of CD45+ and CD138+ population in bone marrow, spleen and liver and bioluminescence was also detected in the secondary transplantation of cells from MMCSC primary engraftment demonstrating the self-renewal capacity of this rare CD138- CD27+ population. Our results suggest that by utilizing a lentiviral GFP-luciferase system in a highly immunocompromised mouse strain fewer cells will be required to monitor MM engraftment and perhaps hasten disease development. Further studies to confirm the expression of selected IgG genes from myeloma cells and to characterize the self-renewal capacity with genes involved in developmental signaling such as sonic hedgehog and wnt pathways are underway. Disclosures: Goff: Coronado Biosciences: Research Funding.

Description: Chronic myeloid leukemia (CML) represents an important paradigm for identifying the molecular events that promote malignant reprogramming of progenitors into therapeutically recalcitrant leukemia stem cells (LSC) during blast crisis (BC) transformation. To elucidate mechanisms of human BC LSC generation, whole transcriptome RNA sequencing (RNA Seq), lentiviral BCR-ABL and JAK2 transduction, quantitative RT-PCR (qRT-PCR) and serial xenotransplantation studies were performed. In human BC LSC, RNA seq revealed extensive upregulation of inflammation-responsive genes in conjunction with JAK/STAT signaling pathway activation and splice isoform specific qRT-PCR uncovered a predilection for selective STAT5a isoform expression. While lentiviral BCR-ABL1 expression in cord blood progenitors enhanced JAK2 activation and expression of specific STAT5a splice isoforms, lentiviral human JAK2 overexpression globally activated inflammation-response genes and expression of adenosine deaminase RNA associated (ADAR1), a primate specific RNA editase previously shown to activate self-renewal in response to inflammation. Notably, inhibition of BC LSC self-renewal with dasatinib, a BCR-ABL inhibitor, combined with a potent JAK2 inhibitor, SAR302503, was associated with reduced STAT5a isoform expression and phospho-STAT5 activation as well as ADAR1 expression and activity. These results highlight a novel JAK/STAT pathway driven niche-responsive mechanism of human BC LSC generation that can be targeted, at least in part, with a selective JAK2 inhibitor and may be utilized as an RNA editing-based biomarker of cancer stem cell generation and therapeutic resistance. Disclosures: Jamieson: Sanofi: Consultancy.

Description: Introduction Disease relapse is the leading cause of death in secondary AML (sAML), which evolves from antecedent hematologic disorders like myelodysplastic syndrome (MDS) or myeloproliferative neoplasms (MPNs) or following exposure to chemotherapy. Persistence of therapy-resistant leukemia stem cells (LSC) harboring enhanced survival and self-renewal capacity has been linked to high relapse rates in sAML. Previously, we showed that missplicing of a stem cell regulatory gene, GSK3 b, and splice isoform switching favoring pro-survival BCL2 family isoform expression promoted generation of therapy-resistant LSC (Abrahamsson et al PNAS 2009; Goff et al Cell Stem Cell 2013). However, whether aberrant pre-mRNA splicing promotes sAML LSC generation, in the absence of mutation, and if pharmacological splicing modulation impairs LSC maintenance, in a mutation-independent manner, has not been elucidated. Methods and Results Comparative RNA-sequencing and gene set enrichment analyses revealed significant alterations in splicing factor gene expression in purified progenitors from untreated sAML compared with normal samples. In addition, using an isoform-specific alignment algorithm, we established a sAML LSC splice isoform expression signature that identified increased expression of select transcripts, e.g. CD82 and PTK2B. Thus, we investigated the LSC inhibitory efficacy of a stable, potent splicing modulatory agent, 17S -FD-895, in humanized AML LSC stromal co-culture and primagraft assays. Notably, there was a dose-dependent reduction in AML LSC (n=4) survival and self-renewal after in vitro 17S -FD-895 treatment, with a favorable therapeutic index compared to normal controls (n=3, p

Description: Abstract 1927 Multiple myeloma (MM) is characterized by clonal proliferation of CD138+ plasma cells in the bone marrow (BM) and remains an incurable disease. Recent identification of a rare population of MM cancer stem cells (MM CSC) is phenotypically similar to memory B cells (CD138- CD34- CD19+) but differs in that they have the self-renewal capacity within the BM and may be responsible for drug resistance. Preclinical testing of novel therapeutic strategies that target MM CSC requires animal models that closely resemble human disease and allow quantitative evaluation of the applied therapy. We have previously reported results on establishing a MM animal model by transplanting MM CSC from autologous mobilized peripheral blood of primary MM patients, transduced with lentiviral luciferase GFP (GLF) and transplanted intrahepatically (IH) into neonatal RAG2/gc double knock-out (RG-KO). Here we evaluate engraftment efficiency in consideration of BM microenvironment by comparing CD45+ human cell engraftment in mice transplanted either IH to neonates or intrafemorally (IF) to gamma-irradiated young adult mice. MM CSC were selected from isolated PBMC after Ficoll gradient centrifugation of fresh BM biopsy from two primary MM patients or from a human MM cell line, H929, followed by immunomagnetic bead depletion of CD34+ and CD138+ cells. The cells were transplanted into RG-KO mice ranging from 53,000 to 10⋀6 cells per mouse either IH or IF. Mice transplanted with GLF-transduced MM CSC were imaged with an in vivo imaging system (IVIS) to detect bioluminescent engraftment. Results showed that bioluminescence signal levels were detected in mice transplanted IF with 53,000 MM CSC per mouse even before 3 weeks by ventral view and as early as 5 weeks by lateral view. To date, tumor growth was only discovered in mice transplanted IH with 2 × 10⋀6 unselected MM PBMC from a fresh BM biopsy as early as 10 week post-transplantation. FACS analysis of these mice demonstrated successful engraftment with the presence of CD45+, CD19+ and CD138+ population in tumor, bone marrow, spleen and liver. In addition, expression of clonal light chain restriction in myeloma cells confirmed myeloma engraftment. Future studies will focus on expression of genes involved in sonic hedgehog pathway as analyzed by PCR to confirm the self-renewal capacity. Moreover, investigations on the effect of B cell-activating factor (BAFF) in BM microenvironment by transplanting MM CSC into BAFFxRG-KO mice are in progress. Disclosures: Jamieson: Bristol-Meyers Squibb: Research Funding.

Description: INTRODUCTION Representing ten percent of all hematologic malignancies, multiple myeloma (MM) is typified by clonal plasma cell proliferation in the bone marrow (BM) and may progress to a therapy-resistant stage characterized by circulating malignant plasma cells, which is termed plasma cell leukemia, (PCL). Notably, copy number amplification involving the myeloid cell leukemia (MCL)-1 locus, and translocations at the BCL2 locus have been described in high-risk MM. Moreover, BCL2 family members, including both MCL1 and BCL2, are highly expressed in relapsed MM. Finally, downregulation of BCL2 family members was reported to increase sensitivity to lenalidomide, a standard treatment for MM, suggesting that BCL2-targeted treatments may obviate therapeutic resistance.Sabutoclax, a potent small molecule inhibitor of 6 anti-apoptotic BCL2 family proteins, was shown to inhibit cancer stem cell (CSC) survival in chronic myeloid leukemia (CML) (Goff et al Cell Stem Cell 2013). Thus, we investigated BCL2 pro-survival splice isoform expression in therapy-resistant MM and PCL and evaluated whether Sabutoclax reduces malignant plasma cell burden in humanized primagraft assays. METHODS AND RESULTS BCL2 pro survival expression in primary myeloma samples Mononuclear cells from MM or PCL samples were isolated by Ficoll-Paque density gradient separation and collected for RNA extraction or FACS Aria purified into CD138+ or CD34-/CD138- cancer stem cells (CSC) subsets. To evaluate BCL2 pro-survival family member expression, splice isoform-specific quantitative PCR was performed to measure expression of pro-survival long isoforms compared with short splice isoforms, which are pro-apoptotic. Interestingly, MM and PCL patients displayed higher levels of BCL2-L, MCL1-L, BCLX-L and BLF1-L compared to normal controls. Moreover, prolonged lenalidomide exposure increased BCL2-L and MCL1-L expression in the myeloma cell line H929, compared to untreated cells. Sabutoclax Treatment of a Novel Humanized Plasma Cell Leukemia Primagraft Model Mononuclear cells from three primary PCL patient samples were stably transduced with a GFP-luciferase lentiviral vector and transplanted intrahepatically in newborn RAG2γ-/- c-/- mice. Engraftment was monitored by peripheral blood free light chain ELISA assays. Flow cytometric analyses revealed robust engraftment of PCL cells in bone marrow, spleen, liver and peripheral blood. Once transplanted mice displayed significant tumor burden above background free light chain levels, animals were randomized by ELISA values in vehicle versus Sabutoclax groups. Sabutoclax was selected because this pan-BCL2 targeted compound, unlike related BCL2 inhibitors such as ABT-199, also inhibits MCL1. Sabutoclax (10mg/kg) was administered intravenously twice weekly for two to four doses. Sabutoclax treated PCL mice showed reduced human plasma cell burden in bone marrow and spleen tissues compared to vehicle controls. CONCLUSION Expression of pro-survival BCL2 splice isoforms, including BCL2-L and MCL1-L, portends PCL engraftment in immunocompromised mice. Treatment of human PCL engrafted mice with Sabutoclax reduces malignant plasma cell survival in hematopoietic tissues. Thus, selective targeting of pro-survival isoform expressing CSC with a pan-BCL2 inhibitor may abolish BCL2 and MCL1-dependent therapeutic resistance in MM and PCL. Disclosures Jamieson: GlaxoSmithKline: Research Funding; Johnson & Johnson: Research Funding.

Description: Introduction Human bone marrow aging is typified by decreased cellularity, stem cell exhaustion and myeloid lineage bias that may set the stage for development of myeloid malignancies. Secondary AML (sAML) arises from prior myelodysplastic syndromes (MDS) or myeloproliferative neoplasms (MPN), and occurs in patients with an average age of 〉65. Because of the typically advanced age of this population, patients currently have few effective treatment options available after leukemic transformation. We and others have recently identified a key role for enzymatic RNA editing activity in cancer progression, and in particular in leukemia stem cell (LSC) generation. In hematopoietic stem and progenitor cells, adenosine deaminase acting on dsRNA-1 (ADAR1) is the most abundantly expressed RNA editing gene. However, the role of abnormal RNA editing activity has not been elucidated in healthy human bone marrow aging and age-related MDS with a high risk of transforming to sAML. Therefore, we established whole transcriptome-based RNA editing signatures of benign versus malignant bone marrow progenitor cell aging, which provides novel RNA-based functionally relevant biomarkers of aging, MDS and progression to sAML. Methods Whole transcriptome sequencing (RNA-Seq) was performed on FACS-purified hematopoietic stem (CD34+CD38-Lin- HSC) and progenitor cells (CD34+CD38+Lin- HPC) from aged (average age = 65.9 y/o) versus young (average age = 25.8 y/o) adult healthy bone marrow samples, and in leukemia stem cells (LSC) from patients with sAML (average age = 71.4 y/o) and MDS (average age = 63.8 y/o). Comparative gene set enrichment analyses (GSEA) and RNA editing profiles were identified for normal and malignant progenitor cell aging. Results Aberrant RNA editing activity has recently been shown to be induced in multiple cancers, and has been implicated as a malignant reprogramming factor. Comparative whole transcriptome RNA sequencing (RNA-seq) and single nucleotide variant analyses revealed widespread increases in RNA editing rates in aged versus young HPC, and in human sAML LSC compared with age-matched normal progenitors. Moreover, RNA editing rates, represented as adenosine (A) to inosine/guanosine (G) changes at known RNA editing loci, were increased in sAML compared with MDS progenitors. The differential expression of certain sites is as high as 70%, which can be readily detected by RESS-qPCR. These data suggest that during aging niche-dependent RNA editing deregulation contributes to MDS progression to sAML. Interestingly, the highly edited loci in sAML LSC were distinct from loci that were differentially edited in aged versus young HPC, suggesting that pro-inflammatory conditions in sAML may trigger RNA editing of a unique set of transcripts, including predominantly RNA processing-related gene products and transcription factors. Notably, several loci in transcripts of APOBEC3C/D that we previously found were associated with blast crisis transformation of chronic myeloid leukemia also displayed enhanced editing in sAML LSC, but not aged versus young HPC. Conclusions Detection of aberrant RNA processing provides novel biomarkers as well as potential therapeutic targets for sAML LSC eradication with implications for treatment of a variety of human malignancies and other age-related disorders. We have identified commonly RNA-edited transcripts in multiple hematologic malignancies, which could be developed clinically and as companion diagnostic targets for LSC-targeted therapeutics. Disclosures Jamieson: CTI Biopharma: Research Funding; Johnson & Johnson: Research Funding; GlaxoSmithKline: Research Funding.

Description: Abstract 516 Introduction: Several studies have demonstrated the role of leukemia stem cells (LSC) in the development and maintenance of human chronic myeloid leukemia (CML). These cells, which first develop in chronic phase CML (CP CML) with acquisition of the BCR-ABL fusion protein, are often quiescent and can be highly resistant to apoptosis induced by drugs and radiotherapy that target rapidly dividing cells. Data has also shown that CML LSC become increasingly resistant to BCR-ABL inhibition with progression to blast crisis CML (BC CML). Bcl-2 family proteins are key regulators of apoptosis and have been shown by numerous studies to regulate cancer resistance to chemotherapy. This family of proteins has also been implicated in the development of BC CML, however most studies have focused on CML cell lines and their expression of Bcl-2 family proteins in vitro. Thus, there is relatively little data on expression of Bcl-2 family proteins in primary CML LSC and on the role of these proteins in regulating chemotherapy resistance in CML LSC in vivo. As Bcl-2 family proteins are known regulators of chemotherapy resistance we hypothesized that human BC CML LSC may overexpress these proteins compared to normal hematopoietic stem cells. We analyzed Bcl-2 family mRNA and protein expression in CP CML and BC CML LSC and compared this expression to normal cord blood stem and progenitor cells. We also analyzed whether these cells were sensitive to chemotherapy treatment in vitro. Finally, we tested whether a high potency pan-Bcl-2 inhibitor, 97C1, could effectively kill CML LSC in vitro and in vivo. Methods: Bcl-2 and Mcl-1 protein expression was measured in primary CP CML, BC CML, and normal cord blood cells using intracellular FACS. We also measured Bcl-2, Mcl-1, Bcl-X, and Bfl-1 mRNA expression in FACS sorted CD34+CD38+lin− cells (LSC) from these samples. For all drug studies we used either serially transplanted CD34+ cells derived from primary BC CML patient samples or primary CD34+ normal cord blood cells. In vitro drug responses were tested by culturing CD34+ cells either alone or in co-culture with a mouse bone marrow stromal cell line (SL/M2). Effects on colony formation and replating were also tested by culturing sorted CD34+CD38+lin− cells in methylcellulose in the presence and absence of drug. For in vivo testing of 97C1 we transplanted neonatal RAG2-/-yc-/- mice with CD34+ cells from 3 different BC CML and cord blood samples. Transplanted mice were screened for peripheral blood engraftment at 6–8 weeks post-transplant and engrafted mice were then treated for 2 weeks with 97C1 by IP injection. Following the treatment period the mice were sacrificed and hemotapoietic organs were analyzed for human engraftment by FACS. Results: BC CML progenitors expressed higher levels of Bcl-2 and Mcl-1 protein compared to normal cord blood and chronic phase CML cells. mRNA expression of Mcl-1, Bcl-X, and Bfl-1 was also increased in BC CML progenitors compared to CP CML progenitors. While BC CML LSC cultured in vitro were resistant to etoposide and dasatinib-induced cell death, 97C1 treatment led to a dose-dependent increase in cell death along with a dose-dependent decrease in the frequency of CD34+CD38+lin− cells compared to vehicle treated controls. While cord blood progenitor cells were also sensitive to 97C1 treatment they had an IC50 around 10 times higher than that for the BC CML cells (100nM versus 10nM). Importantly, 97C1 treatment did not inhibit cord blood colony formation or colony replating in vitro. Mice transplanted with BC CML LSC developed CML in 6–8 weeks post-transplant with diffuse myeloid sarcomas and engraftment of human CD34+CD38+lin− cells in the peripheral blood, liver, spleen, and bone marrow. In vivo treatment with 97C1 led to a significant reduction in both total human engraftment and engraftment of CD34+CD38+lin− cells in all hematopoietic organs analyzed. Conclusion: Our results demonstrate that BC CML LSC are resistant to conventional chemotherapy but are sensitive to 97C1 in vitro and in vivo. Broad-spectrum inhibition of Bcl-2 family proteins may help to eliminate CML LSC while sparing normal hematopoietic stem and progenitor cells. Disclosures: Jamieson: CoronadoBiosciences: Research Funding; CIRM: Research Funding.

Description: Background In the development of Myelofibrosis (MF), disease which is often associated with aging, deregulation of stem cell activity by somatic mutations occurs in the setting of deregulation of the bone marrow microenvironment. While somatic mutations in stem and progenitor cells have been shown to promote pathological myeloproliferation, functional involvement of stromal cells in MF pathogenesis is not well defined. Chronic inflammation has long been regarded as a driving force for MF development, which is intensified by continuous release of pro-inflammatory cytokines and chemokines. We have recently reported that inflammatory cytokine signaling in the aging bone marrow niche via aberrant RNA editing and splicing may predispose patients to leukemia stem cell generation. However, little is known about the functional effects of stromal cell driven inflammasome signaling in MF pathogenesis. Here we investigate the impact of the inflammasome on aging of the BM niche and how changes in the activity of BM niche cells contribute to MF pathogenesis. Methods Stromal monolayers were established from CD34-negative bone marrow cells obtained from young ( 60 y/o; a-NBM) healthy donors and myelofibrosis (MF) patient samples. Conditioned media from those stromal cultures was collected after 7 d post confluence and was analyzed for secreted factors by Human multi-analyte (MAP) Luminex-based analysis (Myriad). Some stromal cultures were used for RNA transcriptome analysis. The other stromal cultures were utilized for the co-culture experiments with CD34+ cells (hematopoietic stem cells, HSC) selected from y-NBM, a-NBM and cord blood (CB), which were subsequently subjected to survival and self-renewal assays. Results MF and a-NBM derived stroma severely impaired survival and self-renewal of CB (n=5) and y-NBM (n=5) HSC in co-culture models. In similar experiments HSC from a-NBM (n=6) demonstrated significantly higher survival and self-renewal capacity when co-cultured with y-NBM stroma (n=3) compared to a-NBM stroma (n=3). Conditioned media (CM) from both a-NBM and MF stroma by itself also greatly reduced HSC survival and self-renewal. Notably, co-culture conditions resulted in a greater reduction in survival and self-renewal capacity than CM alone, suggesting that cell-cell contact or unstable secreted factors exacerbate the effects. Focused analysis of 45 inflammation-associated secreted factors in 3 y-NBM and 4 a-NBM stromal CM samples revealed no substantial alterations in inflammatory factors However, a-NBM stroma demonstrated a significant reduction in production of a variety of HSC-regulatory factors (BDNF, IL-17, IL-12p40, SCF, ICAM-1,VEGF, Eotaxin-1, RANTES). Moreover, RNA transcriptome analysis of 3 y-NBM, 4 a-NBM and 3 MF stromal monolayers revealed significant variations in differentially expressed genes (DEG) between y-NBM and a-NBM. Specifically, aged stroma was typified by upregulated expression of inflammation and immune response genes and of genes associated with the lysosome (ELANE, CLU, DEFA1, LBP). Downregulation of collagen genes and energy metabolism genes, particularly mitochondrial genes, as well as alterations in the expression of apoptosis, cell cycle and cellular senescence biomarkers (IL1B, IDS, TPST2, SERPINB2) was observed. Comparisons in gene expression between a-NBM and MF stroma revealed upregulated inflammatory response(PLIN2, IL1B,PDK4,) and TGF-β signaling pathways(PTGDS,) and downregulated ability to support HSC (RARRES2, FGF7, FGF23, and IGF2) Notably, some transcripts (RUNX3, BCL2) were identified within the MF stromal niche, while they were absent in a-NBM stroma or HSC from a-NBM and MF samples. Conclusions These data indicate that MF and aged stroma exhibit a severely compromised ability to maintain normal hematopoiesis. Molecular inflammasome signatures of aging and MF reflect a combination of degenerative processes and transcriptional responses. Targeting this pathological interplay between the MF niche and MF stem cells could represent a novel avenue for the treatment of MF. Disclosures No relevant conflicts of interest to declare.

Description: Compelling murine studies demonstrate that adenosine-to-inosine (A-to-I) RNA editing mediated by adenosine deaminase associated with RNA1 (ADAR1) is vital for both fetal and adult hematopoiesis. While genetic ablation of ADAR1 editase leads to murine embryonic lethality due to severe defects in erythropoiesis, conditional deletion in the hematopoietic system impairs maintenance indicative of cell type and context specific roles for ADAR1 in cell fate specification and self-renewal. By regulating mRNA and microRNA (miRNA) stability, ADAR1 exhibit wide-ranging effects on embryonic development and stem cell regulation. We have previously shown that inflammation-responsive ADAR1 plays important roles in both stem cell differentiation and self-renewal in CML (chronic myeloid leukemia) disease progression. Here, we describe a novel function of ADAR1 in cell cycle regulation of normal hematopoietic stem cell and progenitors (HSPC) by regulation of miRNA biogenesis. Our results demonstrated that ADAR1 induces G0 to G1 phase transition in normal cord blood HSPCs, as demonstrated by elevated expression of Ki67, reduced DiR signal, and enhanced in vivo cord blood engraftment. Cell cycle qRT-qPCR microarray of 84 cell cycle transcripts and whole transcriptome RNA-sequencing analysis of KEGG cell cycle pathway indicate that several cell cycle genes (CDKN1A, CDKN2A, CCNC, CCND1, BRCA2, etc) are differentially expression upon overexpression of ADAR1 WT or an A-to-I editing deficient ADAR1 mutant (ADAR1E912A). We previously demonstrated that impaired biogenesis of let-7 miRNAs by ADAR1 WT induces enhanced self-renewal in cord blood CD34+ HSPCs. To determine the miRNA targets of ADAR1-mediated RNA editing, we performed miRNome miScript PCR array of 1008 miRNA candidates in cord blood CD34+ HSPCs overexpressing ADAR1 WT or ADAR1E912A. Total of 263 miRNAs were differentially expressed (142 upregulated and 121 downregulated) by comparing ADAR1 WT to the backbone control. Interestingly, ADAR1E912A mutant also exhibit A-to-I editing independent regulation of miRNAs (307 upregulated and 59 downregulated). We found that the expression of miR-26a-5p, a miRNA frequently downregulated in leukemia, is inhibited by ADAR1-mediated RNA editing. ADAR1 directly binds and edits the DROSHA cleavage site of primary miR-26a transcript, thereby prevent miR26a-5p maturation. Moreover, lentiviral expression of mature miR26-5p reverses the effect of ADAR1 WT, including enhanced CDKN1A expression, inhibition of cord blood proliferation in vivo, as well as reduced HSC self-renewal as measured by colony-formation assay. Our finding suggests carefully regulated A-to-I editing by ADAR1 is essential for the maintenance of proper cell proliferation in HSC. For future study, it will be interesting to investigate if the elevated expression of ADAR1 in CML BC LSC contributes to false regulation of cell cycle that leads to the expansion of malignant leukemia stem cells. Disclosures Jamieson: CTI Biopharma: Research Funding; GlaxoSmithKline: Research Funding; Johnson & Johnson: Research Funding.

Description: Abstract 1693 The aim of this study is to develop clinical strategies that will HALT progression of CML by reducing leukemia stem cell (LSC) burden using a clinical grade JAK2 inhibitor, SAR302503 (SAR503, Sanofi, Cambridge, MA), alone or in combination with a potent BCR-ABL inhibitor, dasatinib. For this, CML patient samples in blast crisis phase (BC CML) were subjected to immunomagnetic bead CD34 selection or FACS Aria ll sorted to obtain leukemic progenitors (LSC/CD34+CD38+Lin−). Malignant progenitors were then transplanted into neonatal RAG2−/−gc−/− mice, and 8 weeks post-transplant, mice were treated with SAR503, dasatinib and vehicle for 14 days. Following treatment, hematopoietic tissues were analyzed for human engraftment by FACS analysis. Our results revealed that single agent experiments with SAR503 had a cytostatic rather than a cytoreductive effect on BC LSC. The treatment alone (60 mg/kg twice daily administered by oral gavage) did not significantly reduce leukemic progenitor burden in the liver, spleen, bone marrow and peripheral blood. Conversely, combination therapy with SAR503 and dasatinib (50mg/kg/day) significantly reduced LSC progenitors in all tissues examined. Interestingly, we observed that dasatinib alone therapy reduced the LSC burden in the liver, spleen, and peripheral blood, but the bone marrow retained a significant population of BC LSC. Also we found that the GMP population, previously shown to be enriched for BC LSC (Jamieson et al NEJM 2004; Abrahamsson et al PNAS 2009), was preferentially localized in the bone marrow. As shown by our laboratory and others, LSC therapeutic resistance may be influenced by extrinsic cues provided by the niche (e.g. promoting quiescence). Because quiescence has been implicated in driving tyrosine kinase inhibitor resistance and LSC survival and because the bone marrow retains a resistant population, we decide to perform secondary transplantation experiments to determine relapse potential (self-renewal). LSC progenitors were isolated by immunomagnetic bead selection of human CD34+ cells from marrows and spleens of treated mice. After serially transplanting an equal number of this cells into secondary recipients, we observed a significant reduction in LSC serial transplantation only following combination treatment, suggesting that the combination therapy can abolish LSC self-renewal capacity and thereby potentially prevent relapse. To validate drug exposure, we have been performing both genomic and nanoproteomic analysis. Regarding the proteomics validation studies, we analyzed sorted LSC derived from spleen (pooled 5 mice per group) that were treated with vehicle or SAR503 for 14 days. The analysis was performed to detect status of p-JAK2, JAK2, p-STAT5 and B2-microglobulin (loading control). We observed a down regulation on the levels of p-JAk2 (active site Tyr 1007–08) and p-Stat5 (active site Tyr 694) (35% and 42% respectively), while no changes are observed for total JAK2 protein or B2M between both conditions. The full transcriptome sequencing, on sorted LSC treated with SAR503 alone and in combination with dasatinib, identified specific isoform changes in the JAK/STAT pathway that could be used as biomarkers of response and could explain the synergistic effect of the combination therapy. We have also characterized, at an isoform level, biomarkers of resistance that could explain relapse of disease after single agent therapy and we are currently validating these findings. Disclosures: No relevant conflicts of interest to declare.