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Background: Myelodysplastic Syndromes (MDS) are genetically and hematologically diverse stem cell malignancies pathogenetically linked to constitutive innate immune activation. Other than rare germline mutations and age, the only known predispositions to adult MDS include prior cytotoxic therapy, clonal hematopoiesis, and autoimmune or chronic inflammatory disorders. Few studies investigating the genetic susceptibility to MDS have been performed owing to the limitations of SNP-array sample size. Here, we report the results from the first unbiased genome wide analysis of germline polymorphisms associated with non-del(5q) MDS using a multinational curated data set. Methods: Association analyses were performed on 2 sample sets (set 1: 555 cases, 2,964 controls; set 2: 352 cases, 2,640 controls) and combined by meta-analysis. Standard SNP- and sample-level QC was applied. Haplotype reference consortium (HRC) imputation was done by the Michigan imputation server (Rsq>0.4) providing 23,278,269 markers for analysis. Gene expression sequencing was performed on an independent sample set from the National Taiwan University Hospital (213 MDS cases, 20 healthy donors; HumanHT-12 v4 Expression BeadChip; Chuang et al. Leukemia 2015). Functional analyses were performed as described. Results: Eight MDS associated loci were identified with lead variants, rs6683416, rs34539210, rs341274, rs1634783, rs7099032, rs2947170, rs4404050, and rs1206818, at 1q31.1 (PLA2G4A), 3p14.1 (FAM19A4), 5q21.3 (EFNA5), 6p21.33, 10q23.1 (GRID1), 12q24.32, 15q26.1, and 20q13.12 (EYA2), respctively. Odds ratio (OR) and p-values of each are listed in Table 1. Using gene expression profiling in an independent MDS sample set, we found expression of these five candidate genes was significantly increased in MDS vs controls (p Conclusion: We describe here the first MDS susceptibility loci ever identified the majority of which have a direct relationship to innate immune activation, a driver of MDS pathogenesis. Expression of genes housing these loci is increased in MDS with demonstrable prognostic and biological relevance. Further, functional studies implicate EYA2 as a novel, biologically rational target for MDS treatment. The direct functions of each polymorphism as well as the potential relationship between these predisposition loci to age-related clonal hematopoiesis merits further investigation. Disclosures Cluzeau: Amgen: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Jazz Pharma: Consultancy, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Sanofi: Speakers Bureau; AbbVie: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Menarini: Consultancy; Celgene: Consultancy, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Pfizer: Speakers Bureau. Sallman:Celgene: Research Funding, Speakers Bureau. Sokol:Spectrum Pharmaceuticals: Consultancy; Seattle Genetics: Consultancy; Mallinckrodt Pharmaceuticals: Consultancy. Maciejewski:Ra Pharmaceuticals, Inc: Consultancy; Alexion Pharmaceuticals, Inc.: Consultancy, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Apellis Pharmaceuticals: Consultancy; Ra Pharmaceuticals, Inc: Consultancy; Apellis Pharmaceuticals: Consultancy; Alexion Pharmaceuticals, Inc.: Consultancy, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau. List:Celgene: Research Funding.

Introduction: TP53 mutant (mTP53) MDS and AML represent a distinct molecular cohort with poor outcomes. Hypomethylating agents (HMA) have emerged as preferred treatment for these patients with CR rate of 20-30% and median OS of 6-12 months. APR-246 is a mutant p53 activator with single agent activity in mTP53 AML. We report initial phase 1b results of APR-246 + AZA in mTP53 MDS/AML. Methods: Eligible pts included HMA naïve mTP53 MDS and oligoblastic AML (≤ 30% blasts) ≥ 18 years of age. Pts received APR-246 in a 3+3 dose escalation design (50, 75, 100 mg/kg lean body weight) IV daily over 4 days in a lead-in phase (days -14 to -10) followed by the same dose of APR-246 (days 1-4) + AZA 75 mg/m2 SC/IV over 7 days (days 4-10 or 4-5 and 8-12) in 28 day cycles. Primary objective was safety with AEs graded by CTCAE v4.03 and DLT assessment over 6 weeks. Secondary endpoints included response by IWG 2006 criteria as well as serial next generation sequencing (NGS) and p53 IHC for evaluation of clonal suppression and remission depth. Results: As of Jan 1, 2017, 9 pts (33% male; median age 65 years (39-73)) have enrolled with 3 pts per cohort. Three pts had AML-MRC and 6 had MDS; all pts had poor risk cytogenetics (11% poor, 89% very poor) and higher risk disease by IPSS-R (22% high, 78% very high). Median BM blasts were 18% (4-30). Seven pts (78%) remain on study: 2 pts in the 50mg/kg cohort discontinued treatment (Tx), 1 pt due to infection during C2 who later died of sepsis unrelated to Tx, and 1 pt electively discontinued in durable marrow CR (mCR) after 5 cycles of therapy. Median time on study is 106 days (14-221). Tx related AEs during the lead-in phase (all G1) included ataxia (n=1), dizziness (n=1), and facial numbness (n=1). AEs occurring in > 1 pt included dizziness (n=3), nausea (n=3), neutropenia (n=3), thrombocytopenia (n=3), infection (n=3), headache (n=2), pain (n=2), weakness (n=2), falls (n=2), facial numbness (n=2), and ataxia (n=2); all G1/G2 except neutropenia/thrombocytopenia (G4). No Tx-related SAEs or DLTs have occurred to date. Five of six pts were response evaluable with 1 pt discontinuing tx prior to 1st disease assessment. ORR by IWG was 100% with 4 CR (80%, 3/3 in DL2) and 1 mCR. All CR patients achieved complete cytogenetic response. One CR patient achieved a mCR and partial cytogenetic response after APR-246 lead-in prior to combination therapy. All CR pts had high p53 positivity by IHC at baseline (55-70%) which normalized on serial assessment ( Conclusions: APR-246 + AZA combination is well tolerated in mTP53 MDS/AML. Responses have been achieved in all pts (80% CR) accompanied by deep molecular remissions. The maximum tolerated dose has not been reached and dose escalation is ongoing. Citation Format: David A. Sallman, Amy DeZern, Kendra Sweet, David P. Steensma, Thomas Cluzeau, Mikkael Sekeres, Guillermo Garcia-Manero, Gail Roboz, Amy McLemore, Kathy McGraw, John Puskas, Ling Zhang, Chirag Bhagat, Armin Graber, Najla H. Al Ali, Eric Padron, Roger Tell, Jeffrey E. Lancet, Pierre Fenaux, Alan F. List, Rami S. Komrokji. Phase Ib/II combination study of APR-246 and azacitidine (AZA) in patients with TP53 mutant myelodysplastic syndromes (MDS) and acute myeloid leukemia (AML) [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 CT068.

Anemia is the predominant clinical manifestation of myelodysplastic syndromes (MDS). Loss or deletion of chromosome 7 is commonly seen in MDS and leads to a poor prognosis. However, the identity of functionally relevant, dysplasia-causing, genes on 7q remains unclear. Dedicator of cytokinesis 4 (DOCK4) is a GTPase exchange factor, and its gene maps to the commonly deleted 7q region. We demonstrate that DOCK4 is underexpressed in MDS bone marrow samples and that the reduced expression is associated with decreased overall survival in patients. We show that depletion of DOCK4 levels leads to erythroid cells with dysplastic morphology both in vivo and in vitro. We established a novel single-cell assay to quantify disrupted F-actin filament network in erythroblasts and demonstrate that reduced expression of DOCK4 leads to disruption of the actin filaments, resulting in erythroid dysplasia that phenocopies the red blood cell (RBC) defects seen in samples from MDS patients. Reexpression of DOCK4 in -7q MDS patient erythroblasts resulted in significant erythropoietic improvements. Mechanisms underlying F-actin disruption revealed that DOCK4 knockdown reduces ras-related C3 botulinum toxin substrate 1 (RAC1) GTPase activation, leading to increased phosphorylation of the actin-stabilizing protein ADDUCIN in MDS samples. These data identify DOCK4 as a putative 7q gene whose reduced expression can lead to erythroid dysplasia.

Granulocyte-macrophage–colony-stimulating factor (GM-CSF) hypersensitivity is a hallmark of juvenile myelomonocytic leukemia (JMML) but has not been systematically shown in the related human disease chronic myelomonocytic leukemia (CMML). We find that primary CMML samples demonstrate GM-CSF–dependent hypersensitivity by hematopoietic colony formation assays and phospho-STAT5 (pSTAT5) flow cytometry compared with healthy donors. Among CMML patients, the pSTAT5 hypersensitive response positively correlated with high-risk disease, peripheral leukocytes, monocytes, and signaling-associated mutations. When compared with IL-3 and G-CSF, GM-CSF hypersensitivity was cytokine specific and thus a possible target for intervention in CMML. To explore this possibility, we treated primary CMML cells with KB003, a novel monoclonal anti–GM-CSF antibody, and JAK2 inhibitors. We found that an elevated proportion of immature GM-CSF receptor-α(R) subunit-expressing cells were present in the bone marrow myeloid compartment of CMML. In survival assays, we found that myeloid and monocytic progenitors were sensitive to GM-CSF signal inhibition. Our data indicate that a committed myeloid precursor expressing CD38 may represent the progenitor population with enhanced GM-CSF dependence in CMML, consistent with results in JMML. These preclinical data indicate that GM-CSF signaling inhibitors merit further investigation in CMML and that GM-CSFR expression on myeloid progenitors may be a biomarker for this therapy.

Introduction: MDS encompasses a heterogeneous group of clonal neoplastic hematopoietic stem cell diseases characterized by ineffective hematopoiesis and frequent progression to acute myeloid leukemia (AML). While the advent of hypomethylating agent (HMA) therapy has modestly improved survival in higher-risk MDS, outcomes following HMA failure are dismal. The hedgehog (HH) signaling pathway is an important regulator of hematopoietic cell survival and differentiation, with upregulation of HH target genes and signaling proteins a frequent occurrence in MDS and AML. PF-04449913 (PF-04) is an orally bioavailable inhibitor of Smoothened (SMO), which demonstrated antileukemic activity in a prior phase 1 study. We performed a phase 2 study of PF-004449913 in patients with MDS following HMA failure. Objectives: 1) primary: to estimate overall IWG-2006 response rate (CR + PR + HI + marrow CR) to PF-04449913 in patients with relapsed/refractory MDS and CMML; 2) secondary: to assess safety , overall survival, time to AML transformation, and effect of PF-04 on HH target gene expression. Methods: This was a single center, open-label phase 2 study. Key eligibility criteria included: 1) patients age ≥ 18 with MDS, CMML, or AML (20-29% blasts) following failure of HMA; 2) ECOG 0-2; 3) Adequate kidney and hepatic function. Treatment consisted of PF-04 at 100 mg/day in 4-week cycles for 4 cycles, with continuation allowed for achievement of stable disease or better. Dose increase to 200 mg/day was permitted after cycle 2 for patients with stable disease. Bone marrow biopsy for response assessment occurred after cycle 2, cycle 4, and every 4th cycle thereafter. For correlative studies, total cellular RNA was isolated from ficolled mononuclear cells in baseline marrow samples, and qPCR performed for expression of GLI1, PTCH1, and SMO). Results: Thirty five patients were enrolled, 24M and 11F. Median age was 75 years (range 55-88). A majority of patients had higher-risk (54%) vs lower-risk (43%) MDS by IPSS at time of enrollment. A total of thirty four patients (97%) had received prior azacitidine for a median of 9 cycles (range 1-88), while 6 patients had received decitabine for a median of 4 cycles (5 of whom had also received azacitidine). The median number of cycles of PF-04 received was 3 (range 1-11). All 35 patients were evaluable for response. Two of 35 patients (6%) achieved response per MDS IWG 2006 criteria, including one patient with HI-N/HI-P and another with marrow CR and HI-N. Nineteen additional patients (54%) had stable disease following cycle 2 (8 weeks). With a median follow-up of 10.1 months, the median survival for all treated patients was 10.2 months (95% CI 6.8-13.6 mo)(Figure 1). Median survival for patients with low/INT-1 risk MDS was longer than for INT-2/high risk patients (22.4 mo vs 7.5 mo, p=0.013). Six patients (17%) died while on-treatment, but none of the deaths were considered related to PF-04. The most common treatment-emergent adverse events included myalgia/muscle cramps (65%), dysgeusia (60%), nausea (31%), anorexia (31%), oral mucositis (25%), and AST elevation (22%). The vast majority of these events were grade 1/2. Except for infection (11%), there were no treatment-emergent grade 3/4 events in > 10% of patients. Only 5 patients (14%) required dose reduction to 50 mg/day due to intolerance. In the pharmacodynamics analysis, responding patients had higher baseline expression of SMO, PTC ,and GLI1 than non-responders, while non-responders had higher post-treatment increases in these transcripts compared with responders. Conclusions: PF-04 was well tolerated in patients with advanced, HMA refractory MDS, but had limited efficacy in this unselected, heavily pre-treated patient population. The pharmacodynamic analysis suggests predictive potential of HH pathway gene expression for response warranting further investigation. Figure 1 Figure 1. Disclosures Komrokji: Celgene: Membership on an entity's Board of Directors or advisory committees, Research Funding; Novartis: Consultancy, Speakers Bureau; Incyte: Consultancy. Sweet:Novartis: Consultancy, Speakers Bureau; Ariad: Consultancy, Speakers Bureau; Pfizer: Speakers Bureau; Incyte Corporation: Research Funding; Karyopharm: Honoraria, Research Funding.

Background: Treatment with rhu-Epo ameliorates anemia in a subset of LR-MDS patients, however, effective salvage therapy is limited. LEN promotes erythroid lineage competence and expansion of primitive erythroid precursors in vitro. In the MDS-002 and MDS-005 trials, treatment with LEN improved erythropoiesis, yielding RBC transfusion-independence in 26% of azanucleoside-naïve, transfusion-dependent (TD) LR, non-del(5q) MDS patients for a median of 10.2 and 7.75 months, respectively. We previously reported that LEN restores Epo-responsiveness in MDS progenitors by inducing formation of lipid rafts enriched for signaling competent JAK2/Epo-receptor complexes and excluding large isoforms of the JAK2/lyn kinase-phosphatase CD45 (McGraw K, et. al. PLoS One 2014; Basiorka A, et. al. Cancer Res 2016). In a pilot study of Epo-refractory MDS patients, addition of EA yielded erythroid responses in 28% of patients who were unresponsive to LEN alone, suggesting that LEN may overcome resistance and augment response to rhEpo (Komrokji R, et. al. Blood 2012). To test this hypothesis, we performed a randomized phase III trial comparing treatment with LEN to LEN+EA in LR non-del(5q) MDS patients who were refractory to, or not candidates for treatment with rhEpo. Methods: Patients with Low or Intermediate-1 (Int-1) risk IPSS MDS with hemoglobin 2 units/mo) with serum Epo >500mU/mL were eligible for study. Patients were stratified by serum Epo level and prior rhEpo (EA vs. darbepoetin vs. none) then randomized to treatment with LEN 10 mg/d x21d q4wk (Arm A) or LEN + EA 60,000U SC/wk (Arm B). Primary endpoint was IWG 2006 major erythroid response (MER) rate after 4 cycles. Arm A non-responders were offered cross-over to combined therapy. Secondary endpoints included analysis of response biomarkers. Results: Between April 2009 and May 2016, 248 patients were enrolled and 195 were randomized and will be included in the primary analysis. Interim analysis of 163 patients (Arm A, 81; B, 82) accrued before July 2015 showed that the study met predefined stopping criteria. Baseline characteristics were balanced between arms. Median age was 74 years (range, 47-89) receiving a median of 2 RBC units/mo (0-8). Overall, 64 (39%) patients had Low IPSS risk and 90 (55%) Int-1 risk. Among these, 150 received prior rhuEpo (92%) and 27, azanucleosides (17%). In an ITT analysis, MER rate was significantly higher with combination therapy, Arm B 25.6% (n=21) vs. Arm A 9.9% (n=8) (P=0.015). Among 116 patients evaluable at week 16, 33.3% (20/60) and 14.3% (8/56) achieved MER, respectively (P=0.018), with a median response duration of 25.4 months vs. not reached in Arm A responders. Response to combined treatment was associated with baseline CD45-isoform distribution in erythroid precursors. Patients achieving MER had a significantly lower CD45 RA+RB:RO ratio (median, 1.51) compared to non-responders (median, 4.21; P=0.04), favoring homo-dimerization of the short CD45-RO isoform and inhibition of phosphatase activity. MER rate in Arm B patients with a low isoform ratio (< median) was 72.7% vs. 18.2% in the high ratio group (P=0.03). Thirty-four Arm A non-responders crossed over to combination-therapy with only 1 MER. There was no difference in the frequency or distribution of >Grade 3, non-hematologic AEs. Conclusions: LEN restores sensitivity to rhEpo in Epo-refractory LR-non-del(5q) MDS patients to yield durable and significantly higher rates of erythroid response to combination treatment without added toxicity. Erythroid CD45 isoform profile may serve as a response biomarker for selection of candidates for combination therapy. Disclosures Bennett: Celgne: Membership on an entity's Board of Directors or advisory committees. Altman:Syros: Honoraria; Janssen: Honoraria; BMS: Honoraria; Novartis: Honoraria. Komrokji:Celgene: Membership on an entity's Board of Directors or advisory committees, Research Funding; Novartis: Consultancy, Speakers Bureau. Schiffer:Teva: Other: DSMB member; BMS: Research Funding; Ariad: Research Funding; Pfizer: Other: DSMB member.

Background: The EZH2 gene (Enhancer of Zester homolog 2), located on chromosome (chr) 7q, is mutated in ~10% of patients with myelodysplastic syndromes (MDS) or MDS/myeloproliferative neoplasm (MDS/MPN). EZH2 gene mutations are associated with negative clinical outcomes; however, the role of EZH2 protein in MDS or MDS/MPN is largely unknown. In contrast to lymphomas or solid tumors in which expression is upregulated, decreased EZH2 mRNA expression was recently reported in 47% of MDS patients compared to normal controls and was more prevalent in patients with chr7 abnormalities. Furthermore, patients with decreased EZH2 expression lacking chr7 abnormalities had better overall survival (OS) (Cabrero M et al 2016). This study aims to explore EZH2 protein expression by immunohistochemistry (IHC) in MDS and MDS/MPN and the relationship to gene mutation and clinical outcome. Methods: Retrospective analysis (Institutional Review Board approved) of MDS or MDS/MPN cases seen at Moffitt Cancer Center between 7/2013-5/2015 with myeloid targeted, next generation sequencing (NGS) panel was performed. Bone marrow biopsies corresponding to NGS date were used for IHC. Nuclear EZH2 expression was assessed semi-quantitatively using an IHC score calculated by multiplying signal strength (0-3+) with percent positivity in hematopoietic cells. OS was considered duration between date of diagnosis and date of death. P-values were determined using Chi-squared, student t-test, ANOVA, or log-rank analysis with p Results: EZH2 mutations were identified in 69/620 MDS and MDS/MPN patients (11.13%). Of these, IHC was performed on 44 patients with EZH2 mutations including 33 MDS and 11 MDS/MPN [median age 74 years (y), male to female (M: F) ratio 2:1], and on 25 EZH2 wild type (WT) patients (median age 70.5 y, M:F ratio 2.1:1, including 22 MDS and 3 MDS/MPN). While mean EZH2 IHC score was reduced in all MDS cases compared to hematologically normal, WT controls (n=5), more controls are required to reach statistical significance (MDS IHC score 77.60±8.09 control score 102.50±14.00, p=0.167). However, there was significantly decreased expression in EZH2 mutant compared to EZH2 WT cases (IHC score, 0.5716±0.09 vs 1.113±0.11, p Conclusion: EZH2 mutation is associated with reduced protein expression, chr7 abnormalities and inferior OS in MDS and MDS/MPN. Analysis of a larger cohort is warranted to explore the role of EZH2 protein in the pathogenesis and natural disease history of MDS and MDS/MPN. Figure OS of patients with MDS and MDS/MPN with or without EZH2 mutation Figure. OS of patients with MDS and MDS/MPN with or without EZH2 mutation Disclosures Komrokji: Novartis: Consultancy, Speakers Bureau; Celgene: Membership on an entity's Board of Directors or advisory committees, Research Funding.

Background: We reported that ineffective hematopoiesis in MDS results from caspase-1-dependent, pyroptotic cell death. Pyroptosis, which culminates in cytolysis, is initiated by the NOD-like receptor NLRP3, a redox-sensitive, cytosolic sensor of danger signals. Upon activation, NLRP3 recruits the ASC (apoptosis-associated speck-like protein containing a CARD (caspase activation and recruitment domain)) adapter protein that polymerizes to create large cytoplasmic filaments. Exposure of the ASCCARDdomain on the filament surface fosters docking of pro-caspase-1 that initiates processing of pro-IL1β and -IL18. Large cytoplasmic aggregates formed by filament clusters are known as ASC specks. Upon cytolysis, ASC specks are released into the extracellular space where they retain catalytic activity and propagate inflammation. Given the magnitude of pyroptosis in the bone marrow (BM) and maturing cells, we hypothesized that detection of ASC specks in peripheral blood (PB) plasma may serve as a biologically-rational MDS biomarker. Methods: We optimized a flow cytometry assay for detection of extracellular ASC specks averaging 1µm in size. Following protein quantitation, 300µg was aliquoted from each donor and stained with rabbit-anti-ASC 1o-Ab at a 1:1500 dilution for 60' at 37°C. 1:1500 dilution 2o-Ab was added and incubated for 30' at 37°C. Sample acquisitions were made on a BD FACSCalibur flow cytometer. PB plasma samples were obtained from normal donors (ND, n=40) and patients with MDS (n=220), de novo AML (n=16), 2o AML (n=26), CMML (n=20), CLL (n=50), CML (n=52), ALL (n=7), ET (n=20), PV (n=20), myeloma (n=20) and patients with Type-2 diabetes (T2D, n=25) on IRB-approved protocols, providing >90% power to detect significant differences between diseases. The diagnostic biomarker is defined as % of PB plasma ASC specks adjusted for glucose. Sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV) were used to evaluate classification accuracy of the biomarker at varied cutoff points. Results: The % PB-ASC specks was significantly increased in lower-risk (LR) MDS vs. ND (n=196, 27.7±1.5 vs. 7.2±0.7, respectively; p=2.8x10-32). No significant difference was observed between ND and higher-risk (HR) MDS (12.4±3.0), however, % of PB-ASC specks was significantly increased in LR (n=196) vs. HR (n=16) MDS (p= 2.0x10-5). Because hyperglycemia stimulates NLRP3 inflammasome assembly and activation, we measured plasma glucose by colorimetric assay to adjust for this potentially confounding variable. % PB-ASC specks were normalized to plasma glucose concentration and corrected for volume. Glucose-adjusted speck % in LR-MDS remained significantly higher vs. ND (1.2±0.2 and 0.02±3.0x10-3, respectively; p=3.7x10-6). Notably, the corrected % PB specks was not significantly greater in HR-MDS (0.6±0.5) vs. ND or between LR-MDS vs. HR-MDS, although the HR-MDS sample size is small. Importantly, LR-MDS samples (1.2±0.2) displayed significantly greater corrected % ASC specks compared to de novo AML (0.3±0.2, p=2.3x10-3), 2o-AML (0.16±0.05, p=5.5x10-5), CMML (0.03±0.01; p=4.4x10-6), CLL (0.03±4.4x10-3, p=4.4x10-6), CML (0.04±0.01; p=5.2x10-6), ALL (0.2±0.09, p=9.9x10-5), ET (0.17±0.07, p=8.7x10-5), PV (0.12±0.06, p=3.8x10-5), myeloma (0.14±0.04, p=3.8x10-5) and PB from non-cancer patients with T2D (p=4.8x10-6), suggesting specificity for MDS (see Figure). A cut off of 0.053 was selected to minimize total misclassification error (false positive + false negative). With this cutoff, the biomarker achieves 99% sensitivity and 81% specificity in classifying MDS from ND. The corresponding PPV and NPV are both 95%. In a preliminary cohort of lenalidomide-treated LR-MDS patients, PB-ASC specks decreased a mean of 48% (range, 3-69%) at week 16, suggesting that specks may serve as a biomarker index of ineffective hematopoiesis. Conclusion: ASC specks are readily quantified by flow cytometry and profoundly increased in PB plasma of MDS patients compared to ND and other hematologic malignancies. ASC specks are a sensitive and specific diagnostic biomarker for MDS that may also serve as an index of disease activity and emerging treatment response. Disclosures Pinilla-Ibarz: Novartis: Consultancy; Janssen: Consultancy, Honoraria; Abbvie: Consultancy, Speakers Bureau; Pharmacyclics: Consultancy, Speakers Bureau; Novartis: Consultancy; Gilead: Consultancy, Speakers Bureau; Gilead: Consultancy, Speakers Bureau. Komrokji:Novartis: Consultancy, Speakers Bureau; Celgene: Membership on an entity's Board of Directors or advisory committees, Research Funding.

Background: The myelodysplastic syndromes (MDS) are senescence-dependent stem cell malignancies. Although germ line mutations in RUNX1, CEBPA, ETV6, or GATA2 may underlie familial cases, inherited genetic polymorphisms influencing MDS susceptibility has not been evaluated. We performed the first genome-wide association study (GWAS) to identify single nucleotide polymorphisms (SNPs) linked to MDS predisposition. Methods: DNA from 1361 MDS patients from 9 international centers was genotyped on the Affymetrix Genome-Wide Human SNP Array 6.0; all patients signed informed consent. Data on 4597 healthy controls genotyped on the same platform were obtained from the Database of Genotypes and Phenotypes (dbGaP). Applying standard quality control metrics and limiting to individuals of European ancestry, we analysed 999 MDS patients and 4,309 controls at 545,924 markers. We used logistic regression to determine associations with MDS after appropriate adjustment assuming an additive genetic model. Results: We identified 15 SNPs at 9 loci associated with MDS, including rs6780298 (3q13; p = 5.0×10-6) and rs1206819 (20q13; p = 1.54×10-6) that mapped to the intragenic regions of MORC1 and EYA2, respectively; and rs2473784 (1p31; p = 3.32×10-7) that mapped downstream of DEPDC1. Other hits (p≤5×10-6) at 4q28, 5p14, 9p22, 6p22, 10q21 are in or near SLC7A11, GUSBP1, SH3GL2, MOG and TRIM27, respectively. Using publicly available gene expression profiling data, we confirmed all of these genes are expressed in the hematopoietic compartment and MORC1, EYA2, DEPDC1, and SH3GL2 are increased in leukemic samples suggesting that variation in these genes may be functionally relevant in myeloid malignancies. Notably, EYA2 (20q13) flanks the commonly deleted region in del(20q) MDS and has oncogenic properties in solid tumors. SH3GL2 is very highly upregulated in leukemic samples and interacts with Dynamin-1, a GTP-binding protein involved in cellular trafficking and is similarly associated with solid tumorigenesis. To validate our observed associations, we analysed the 15 SNP markers in an independent set of 12,385 individuals of whom 117 developed hematologic malignancy (including MDS) during follow up. The markers at 1p31 including rs2473784 and 4 others in strong linkage disequilibrium were associated with individuals who developed hematologic malignancy (p Conclusions: These data provide the first genome-wide identification of germline variants associated with MDS. Current work is underway to replicate these findings in an independent sample set of MDS patients and healthy controls. If confirmed, these SNPs may serve as biomarkers to identify individuals at risk for MDS and potentially support new prevention strategies. Citation Format: Kathy McGraw, Chia-Ho Cheng, Ann Chen, Giulio Genovese, Thomas Cluzeau, Hui-Yi Lin, Bartlomiej Przychodzen, Mar Mallo, Leonor Arenillas, Azim Mohamedali, Lionel Ades, Ashley Basiorka, Brittany Irvine, David Sallman, Eric Padron, Lubomir Sokol, Andrea Pellagatti, Chimene Brest, Sophie Raynaud, Bjorn Nilsson, Jacqueline Boultwood, Benjamin Ebert, Francesc Sole, Pierre Fenaux, Ghulam Mufti, Jaroslaw Maciejewski, Peter Kanetsky, Alan List. Identification of genetic polymorphisms associated with myelodysplastic syndromes by genome-wide association study. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2570.

Aberrant JAK2 signalling plays an important role in the aetiology of myeloproliferative neoplasms (MPNs). JAK2 inhibitors, however, do not readily eliminate neoplastic MPN cells and thus do not induce patient remission. Further understanding JAK2 signalling in MPNs may uncover novel avenues for therapeutic intervention. Recent work has suggested a potential role for cellular cholesterol in the activation of JAK2 by the erythropoietin receptor and in the development of an MPN-like disorder in mice. Our study demonstrates for the first time that the MPN-associated JAK2-V617F kinase localizes to lipid rafts and that JAK2-V617F-dependent signalling is inhibited by lipid raft disrupting agents, which target membrane cholesterol, a critical component of rafts. We also show for the first time that statins, 3-hydroxy-3-methyl-glutaryl coenzyme A (HMG-CoA) reductase inhibitors, widely used to treat hypercholesterolaemia, induce apoptosis and inhibit JAK2-V617F-dependent cell growth. These cells are more sensitive to statin treatment than non-JAK2-V617F-dependent cells. Importantly, statin treatment inhibited erythropoietin-independent erythroid colony formation of primary cells from MPN patients, but had no effect on erythroid colony formation from healthy individuals. Our study is the first to demonstrate that JAK2-V617F signalling is dependent on lipid rafts and that statins may be effective in a potential therapeutic approach for MPNs.

Transmembrane drug export mediated by the ATP-binding cassette (ABC) transporter P-glycoprotein contributes to clinical resistance to antineoplastics. In this study, we identified the substituted quinoline HG-829 as a novel, noncompetitive, and potent P-glycoprotein inhibitor that overcomes in vitro and in vivo drug resistance. We found that nontoxic concentrations of HG-829 restored sensitivity to P-glycoprotein oncolytic substrates. In ABCB1-overexpressing cell lines, HG-829 significantly enhanced cytotoxicity to daunorubicin, paclitaxel, vinblastine, vincristine, and etoposide. Coadministration of HG-829 fully restored in vivo antitumor activity of daunorubicin in mice without added toxicity. Functional assays showed that HG-829 is not a Pgp substrate or competitive inhibitor of Pgp-mediated drug efflux but rather acts as a noncompetitive modulator of P-glycoprotein transport function. Taken together, our findings indicate that HG-829 is a potent, long-acting, and noncompetitive modulator of P-glycoprotein export function that may offer therapeutic promise for multidrug-resistant malignancies. Cancer Res; 72(16); 4204–13. ©2012 AACR.

Background While lenalidomide (LEN) shows high efficacy in myelodysplastic syndromes (MDS) with del[5q], responses can be also seen in patients presenting without del[5q]. We hypothesized that improved detection of chromosomal abnormalities with new karyotyping tools may better predict response to LEN. Design and methods We have studied clinical, molecular and cytogenetic features of 42 patients with MDS, myeloproliferative neoplasms (MPN), MDS/MPN overlap syndromes and secondary acute myeloid leukemia (sAML) without del[5q] by metaphase cytogenetics (MC) who underwent therapy with LEN. Results Fluorescence in situ hybridization (FISH) or single nucleotide polymorphism array (SNP-A)-based karyotyping marginally increased the diagnostic yield over MC, detecting 2/42 (4.8%) additional cases with del[5q], one of whom were responded to LEN. Responses were more often observed in patients with a normal karyotype by MC (60% vs abnormal MC; 17%, p = .08) and those with gain of chromosome 8 material by either of all 3 karyotyping methods (83% vs all other chromosomal abnormalities; 44% p = .11). However, 5 out of those 6 patients received combined LEN/AZA therapy and it may also suggest those with gain of chromosome 8 material respond well to AZA. The addition of FISH or SNP-A did not improve the predictive value of normal cytogenetics by MC. Mutational analysis of TET2, UTX, CBL, EZH2, ASXL1, TP53, RAS, IDH1/2, and DNMT-3A was performed on 21 of 41 patients, and revealed 13 mutations in 11 patients, but did not show any molecular markers of responsiveness to LEN. Conclusions Normal karyotype and gain of chromosome 8 material was predictive of response to LEN in non-del[5q] patients with myeloid malignancies.

Introduction: Accumulating evidence implicates innate immune activation in the pathobiology of myelodysplastic syndromes (MDS) and its inflammatory bone marrow microenvironment. Excess elaboration of S100A9 accompanied by secondary induction of TNFα and IL-1β, characterize the inflammatory milieu found in lower risk MDS. Erythroid stimulating agents (ESA) and lenalidomide (LEN) are erythropoietic promoters with known anti-inflammatory properties. To date, the role of such inflammation parameters in endogenous erythropoietin (Epo) regulation and response to such treatments has not been investigated. Herein, we investigated the role of these inflammatory cytokines on Epo elaboration and response to ESA and LEN in MDS. Materials and Methods: The HepG2 hepatoma cell line was used to investigate Epo elaboration in vitro. Serum collected from 311 MDS patients from 3 centers (AOU Careggi, University of Firenze; Saint Louis hospital in Paris; H. Lee Moffitt Cancer Center in Tampa) were investigated. 125 were obtained prior to ESA treatment and 186 prior to LEN or LEN+EPO treatment. ELISAs for S100A9, S100A8, TNFα, IL1β and EPO were performed. Clinical data were analyzed from all patients. Spearman's correlation, Mann-Withney and Jonckheere-Terpstra tests were used for analysis of continuous variables. Chi-square test was used for analysis of non-continuous variables. Results: Median age of the patients was 76 years [41-94]. IPSS was low, intermediate-1, intermediate-2 and high risk in 47%, 49%, 3% and 1% of patients, respectively; whereas 28%, 42%, 22%, 7% and 1% of patients were very low, low, intermediate, high and very high risk according to IPSS-R. Serum S100A9 concentration was significantly higher in patients with lower risk versus higher risk MDS (17,798 pg/ml vs 15,291 pg/ml, p=0.01). No significant difference was observed for TNFα and IL1 β according to IPSS or IPSS-R. ELISA quantitation following 24h in vitro stimulation of HepG2 cells with S100A9 (10 to 20 µg/ml), TNFα (10 to 100 ng/ml) or IL1β (10 to 100ng/ml) showed that each cytokine significantly suppressed Epo elaboration in a concentration-dependent manner. To validate these findings, we assessed the relationship between serum concentration of these inflammatory cytokines and Epo level in MDS patients. We observed a significant negative correlation between TNFα and Epo concentration (r=-0.158, p=0.01), and a corresponding significant negative correlation between S100A9 and Epo (r=-0.143, p=0.01). We also found a significant positive correlation between S100A9 and S100A8 (r=0.757, p Conclusion: These findings demonstrate that S100A9 and its NFκ-B transcriptional target, TNFα, directly suppress Epo elaboration and endocrine response to anemia in MDS. These inflammatory proteins may serve as rational biomarkers for response to lenalidomide and/or ESA treatment and merit prospective validation. Therapeutic strategies that either neutralize S100A9 or suppress elaboration of this alarmin may improve erythropoiesis in MDS by restoring Epo response and suppressing innate immune effectors. Disclosures Fenaux: Janssen: Honoraria, Research Funding; Celgene Corporation: Honoraria, Research Funding; Amgen: Honoraria, Research Funding; Novartis: Honoraria, Research Funding. Santini:celgene, Janssen, Novartis, Onconova: Honoraria, Research Funding. List:Celgene Corporation: Honoraria, Research Funding.

Autoimmune disorders (AIDs) are commonly observed in up to 30% of myelodysplastic syndromes (MDS) patients. Clinical manifestations such as acute neutrophilic febrile dermatosis, rheumatoid arthritis, inflammatory bowel disease, pulmonary infiltrates and peripheral polyneuropathy, precede or accompany the diagnosis of MDS. In fact, large-scale epidemiologic studies have suggested that patients with AIDs have an elevated risk of developing MDS. To gain more insight into the association of MDS and AIDs, bone marrow mononuclear cells available from 202 patients were sequenced for 'inflammsome' pathway genes NLRP3 and MYD88, which have been shown to carry mutations in various AIDs. According to WHO classification, the patient cohort included 49% RA/RARS/RCMD, 24% RAEB, 6% 5q syndrome, 10% AML, 6% MPD/MDS and 5% tMDS. Median age of the cohort was 62 years (IQR 54.8-62.3). Autoimmune data was available on 129 patients and 49/129 patients had evidence of AID. 31 patients were diagnosed with MDS-associated 'Sweets Syndrome' AID. Myeloid neoplasm-related 22 gene panel mutation data was available for 171 patients. Initially all protein coding exons of NLRP3 and MYD88 genes were sequenced in 31 cases diagnosed with MDS-Sweets syndrome. Targeted sequencing (Miseq, Illumina) revealed NLRP3 variants in 6/31 cases. NLRP3 heterozygous variants detected were V200M (n=1/31, 3%), Q705K (n=4/31, 13%) and T954M (n=1/31, 3%). Two of these variants (V200M and Q705K) have been previously described as pathogenic and linked with various autoimmune disorders1,2. Variant T954M is reported in COSMIC v73 database as a confirmed somatic mutation. The frequency of these variants in general European population (NHLBI ESP 2015) varies in-between 0.6% to 5% (V200, rs121908147 MAF= 0.7%; Q705, rs35829419 MAF= 5% and T954, MAF= NLRP3 heterozygous variants V200M and Q705K were observed in 2.3% (n=4/171) and 12% (n= 20/171), of the patients screened, respectively. T954M variant was not detected in the follow-up cohort. Sequencing of the paired constitutional DNA (CD3+ lymphocytes/ Skin biopsy), where available (n=10), confirmed that V200M and Q705K are inherited SNPs. The frequency of these NLRP3 variants observed among our study population is marginally higher than previously described in the general European population (Pearson chi-square test, P >0.05) In order to detect the effects of the NLRP3 variant on the inflammasome pathway, we quantified the serum cytokine (IL1β, IL18, TNFα and INFγ) levels using ProcartaPlex Human Cytokine & Chemokine Panel 1A (34 plex, eBioscience) kit in patients with NLRP3 variants (MDS/sweets syndrome, n =3) vs NLRP3 wildtype (MDS-Sweets syndrome, n=5; MDS/non-sweets syndrome, n=2). No difference was observed in the serum cytokine levels between two patient groups. Next, we investigated the mRNA levels of the NLRP3 and other genes that lie downstream of the NLRP3 inflammasome pathway (IL1β, IL18, TNFα, CASPASE-1 and INGγ). 34 patients (NLRP3 variant, n= 22 and NLRP3 Wildtype, n= 12) were included in this analysis. An increased trend in the IL18 mRNA levels was observed in patients with NLRP3 variant vs NLRP3 wildtype. In conclusion, our study shows that the NLRP3 variants are enriched in MDS patients with AIDs compared to historical data on the general population. Patients with NLRP3 variants have an increased levels of IL18 mRNA levels. A larger study to explore the inflammasome pathway is needed to define the molecular pathogenesis of the autoimmunity associated with MDS. Identifying the casual genomic variants of AIDs in MDS may provide relevance to development of novel therapeutic strategies. References 1. Verma D, Sarndahl E, Andersson H, et al. The Q705K polymorphism in NLRP3 is a gain-of-function alteration leading to excessive interleukin-1beta and IL-18 production. PLoS One. 2012;7(4):e34977. 2 . Hoffman HM, Mueller JL, Broide DH, Wanderer AA, Kolodner RD. Mutation of a new gene encoding a putative pyrin-like protein causes familial cold autoinflammatory syndrome and Muckle-Wells syndrome. Nat Genet. 2001;29(3):301-305. Disclosures Kulasekararaj: Alexion: Consultancy. List:Celgene Corporation: Honoraria, Research Funding.

Recent studies suggest that aging-associated inflammation, or "inflammaging", contributes to genetic instability and MDS predisposition. Although innumerable somatic genetic events have been annotated in recent years, including many that are not unique to MDS, they are not sufficient for disease initiation. The precise underlying mechanisms conducive to the emergence of these genetic events also remain to be delineated. We reported that bone marrow (BM) myeloid derived suppressor cells (MDSC) activated by the damage associated molecular pattern (DAMP) protein S100A9, promote ineffective hematopoiesis and the development of MDS. Inflammaging associated alterations in metabolism have been implicated in predisposition to cancer development with age. Here we report that S100A9 and ROS-induced inflammaging are associated with insulin resistance and hyperglycemia in the BM microenvironment that triggers activation of adaptive oncogenic pathways and genomic instability in HSPC. Glucose concentrations were markedly elevated in MDS BM plasma vs. age-matched controls, and directly correlated with S100A9 concentration (r=0.513, P=0.003, n=41). The magnitude of BM-glucose elevation significantly exceeded that in the peripheral blood and negatively correlated with the proportion of HSPC while directly correlating with BM MDSC percentage. S100A9 transgenic (Tg) mice displayed age-dependent elevation of glucose in peripheral blood and BM when compared to wild type mice accompanied by accumulation of somatic mutations (SM) by sequencing in aged S100A9-Tg compared to younger counterparts. NGS of 38 primary MDS BM specimens showed that SM common to MDS, such as those involving ASXL1, U2AF1 and DNMT3A, we re present only in high glucose stratified MDS-BM specimens (glucose > 110 ug/ml). Furthermore, there was a strong correlation between the cellular ROS/nuclear-β-catenin to DNA damage (γH2AX+ cells) linking S100A9-induced ROS accumulation to genetic instability. Elevation in BM plasma glucose was specifically associated with upregulation of the fat mass and obesity associated (FTO) transcript and protein in both MDS BM and S100A9Tg mice. FTO is a risk factor for type 2 diabetes, and encodes an α-ketoglutarate-dependent dioxygenase that functions as an RNA demethylase specific for N 6-methyladenosine (m6A) residues, targeted by splicing factors. Both human and murine MDS specimens displayed decreased m6A mRNA methylation compared to controls. FTO knockdown with CRISPR increased mRNA m6A methylation in MDS primary specimens, whereas overexpression of FTO led to a corresponding decrease that was enhanced by S100A9 stimulation. Moreover, S100A9 treatment induced FTO and demethylation of m6A mRNA in human and murine BM cells. These effects were accompanied by disruption of spliceosomes in the nucleus, as demonstrated by delocalization of SRSF2 from nuclear speckles into the cytoplasm where they colocalize with FTO in MDS patients. Interestingly, from the S100A9Tg mouse sequencing studies, we discovered that the mutation hotspots were in locations specific for histone H3K27 acetylation which have been previously linked to genomic instability, as well as splicing regulation, and regulated by histone deacetylase 1 (HDAC1). Reduced HDAC1 levels are known to make cells hypersensitive to DNA-damaging insults, such as those inducing ROS, similar to what we observed with treatment of S100A9 in healthy human bone marrow, MDS patient samples and S100A9Tg mice. Furthermore, this correlated with increased levels of DNA damage in our patient samples and in our murine S100A9Tg model as measured by phosphorylated γH2AX histones. Our studies provide a biological rationale for the initiation of DNA-genetic damage under inflammatory senescence conditions in MDS. These findings demonstrate that S100A9-induced inflammation activates a signaling cascade that enhances development of splicing variants and somatic gene mutations critical to MDS pathogenesis. Disclosures List: Celgene Corporation: Honoraria, Research Funding.

Background: TP53 mutations have been reported in 5-10% of patients with myelodysplastic syndromes (MDS) with a higher frequency in those who harbor del(5q) and complex cytogenetics. Next generation sequencing (NGS) data in large clinical cohorts has revealed TP53 mutation is a strong independent prognostic factor. Limited studies have shown an association of p53 protein overexpression with TP53 mutation as well as other clinical characteristics, such as blast count, cytogenetics, and mutant TP53 variant allele frequency (VAF). The study aims to validate the association of p53 overexpression by immunohistochemical (IHC) staining with TP53 mutation, and to explore its relationship to clinical outcome and mutation burden in MDS. Methods: We retrospectively analyzed MDS patients with or without transformation to acute myeloid leukemia with myelodysplasia-related changes (AML-MRC) diagnosed between 7/2013 and 5/2015. Only those cases with available bone marrow (BM) biopsies with good quality (>1.0 cm in length) corresponding to the NGS testing date were included. Five normal BMs were used as controls. IHC was performed according to our institutional standard protocol and modified in accordance with the manufacturer. Nuclear expression of p53 was assessed semi-quantitatively using an IHC score calculated by multiplying IHC stain intensity with percent positivity in hematopoietic cells. Paired t-test was used for numerical parameters. Hazard ratios were generated using the standard cox proportional harzard model. Survival was analyzed by the Kaplan-Meier method and overall survival (OS) was defined as the duration between date of diagnosis and date of death. Results: Of 201 patients with myeloid malignancies, 29 (14%) harbored clinically significant mutations by NGS testing. Twenty two of these (9 AML-MRC,13 MDS) had available BM biopsies. An additional 32 patients (27 MDS, 5 AML) with wild type (WT) TP53 were included. Clinicopathologic differences of these groups are summarized in Figure 1. IHC analysis showed significantly (p=1.21x10-6) elevated nuclear p53 expression in TP53 mutated patients (mean score, 1.11± 0.164) compared to WT (0.037 ±0.014). A higher p53 IHC score in mutated patients correlated with a complex karyotype (n=14) compared to those without (n=6) (1.235 ± 0.206, and 0.580 ± 0.257, respectively, p=0.054). A significant positive correlation between IHC score and cytogenetic risk group according to revised international prognostic scoring system ( R-IPSS) existed in our cohort (p1.0 [16 month (95% CI, 8-23)] (p= 0.007). Similarly, with a p53 IHC score cutoff of 0.5, median OS was again not reached in those patients with 0.5 (p=0.015). HR for IHC score > 0.5 was 3 (95% CI, 1.2-8), p=0.02. HR for IHC score >1.0 was 3.5 [(95% CI, 1.3-9), p=0.01]. Conclusion: p53 overexpression in MDS patients correlates with mutated TP53 and mutation burden. A higher IHC score was associated with poorer clinical features and outcomes. A larger cohort is warranted to define its diagnostic or prognostic utility. Figure 1. Clinicopathologic Characteristics in Our MDS/AML-MRC Cohort Figure 1. Clinicopathologic Characteristics in Our MDS/AML-MRC Cohort Disclosures Komrokji: Celgene: Consultancy, Research Funding; Pharmacylics: Speakers Bureau; Incyte: Consultancy; Novartis: Research Funding, Speakers Bureau. Padron:Incyte: Research Funding; Novartis: Speakers Bureau. Lancet:Celgene: Consultancy, Research Funding; Amgen: Consultancy; Pfizer: Consultancy; Boehringer-Ingelheim: Consultancy; Kalo-Bios: Consultancy; Seattle Genetics: Consultancy. List:Celgene Corporation: Honoraria, Research Funding.

Myelodysplastic syndrome (MDS) is one of the major types of acquired bone marrow failure characterized by impaired peripheral blood cell production (anemia and cytopenias). Although much has been learned regarding the molecular genetic events involved in the pathogenesis of the MDS hematopoietic stem/progenitor cell compartment (HSC/HPC), investigations of the environmental pressures underlying disease initiation have been limited. Prior investigations have shown that inflammation in the bone marrow microenvironment contributes to hematopoietic impairment, with inflammatory molecules providing regulatory cues driving the proliferation and apoptotic death of HSC/HPC. We have reported that, the accumulation of myeloid-derived suppressor cells (MDSCs) in the local inflammatory bone marrow microenvironment plays a major role in the direct pathogenesis of MDS. MDSCs function by producing mediators and inflammatory cytokines capable of suppressing hematopoiesis and are defined by the lack of all lineage markers and only one key surface receptor, CD33 (CD33+HLA-DR−Lin−). We recently uncovered that this receptor is greatly expressed in MDSC isolated from patients with MDS and plays an important role in MDSC-mediated hematopoietic suppressive function. We also found that this ITIM (immune-receptor tyrosine-based inhibitor motif) containing molecule could induce suppressive cytokines after its engagement with its newly identified ligand S100A9. Therefore, we tested the hypothesis that with a fully human IgG1 monoclonal antibody against CD33 (mAb33.1) we could prevent the engagement of this ligand/receptor pair as well as induce ADCC of pathogenic MDSC through the reduction in the accumulation of CD33+ MDSC, the immune suppression and the restoration of hematopoiesis in MDS bone marrow specimens. After testing primary specimens with mAb33.1 we saw a significant increase in ADCC activity via NK as evidenced by significant reduction in the proportion of MDSC in culture as compared to isotype control treated cells. This decrease correlated with an increase in CD107a granule mobilization as well as an increased cytotoxicity in a killing assay without changing the proportion of NK cells in the culture. Functionally, the concentration of secreted IL-10, TGFb and VEGF were decreased, as was the gene expression of these suppressive cytokines after treatment with mAb33.1 but was restored when the antibody was cross-linked with an anti-human IgG antibody demonstrating the blocking ability of the antibody in preventing CD33 downstream signaling. Importantly, all of these observations correlated with the restoration of hematopoiesis, as there was a significant increase in the formation of CFU-GM and BFU-E colonies on a methylcellulose assay (n=9). In addition mAb33.1 also displayed a protective effect on HSC by blocking ROS production and reducing DNA damage, as demonstrated by comet and H2AX assays. This work provides the ground for the development of a novel group of therapies directly aimed at the suppressive MDSC and blockage of their signaling, rather than directly targeting the malignant clone, with the long term goal of improving the local microenvironment. This strategy will provide the background to assess its clinical potential to serve as a therapeutic target in MDS and AML. Disclosures No relevant conflicts of interest to declare.

Erythropoietin receptor (EpoR) signaling is impaired in patients with Myelodysplastic Syndromes (MDS) despite appropriate growth factor production and cellular receptor display. We previously reported that EpoR signaling is dependent upon receptor localization within membrane lipid raft microdomains, and that disruption of raft integrity abolishes signaling capacity (McGraw KL, et al. PLoS One 2012). Here, we show that MDS erythroid progenitors display markedly diminished raft assembly (p=0.005) and smaller raft aggregates (p=0.023) compared to normal controls. Because lenalidomide triggers raft coalescence in T-lymphocytes to promote immune synapse formation, we assessed the effects of lenalidomide on raft assembly in MDS erythroid precursors and UT7 cells. Lipid rafts were isolated from UT7 cells using ultracentrifugation and identified by GM-1 dot blot and Lyn kinase western blot. Lenalidomide rapidly induced lipid raft formation in UT7 cells which was confirmed by confocal microscopy visualization of GM-1 fluorescence. Lenalidomide also significantly induced lipid raft formation in pooled MDS erythroid progenitors (CD71+, cKit+) from 11 patients [mean raft size, control (n=569) vs. lenalidomide treatment (n=659), p Disclosures List: Celgene: Consultancy.

Abstract 920 Background: Stabilization of p53 as a result of ribosomal stress is a key effector of the hypoplastic anemia in del(5q)MDS. We recently reported that lenalidomide (LEN) stabilizes MDM2 to promote p53 degradation, fostering cell cycle transition and arrest of del(5q) progenitors in G2/M (Wei S, et. al. Oncogene 2012). P53 expression in erythroid precursors (EP) decreased in responders, but was up-regulated upon emergence of resistance to LEN (Wei S, et. al. PNAS 2010). We hypothesized that targeted suppression of p53 may be an effective strategy to restore erythropoiesis in del(5q)MDS EP after LEN-treatment failure. Methods: To test this hypothesis, we first investigated the in vitro effects of cenersen (Aezea®, Eleos Inc. Plymouth, MN), a 20mer phosphorothioate antisense oligonucleotide complementary to the exon 10 coding sequence of the TP53 gene transcript, on cellular p53 expression and colony forming capacity (CFC) in EP. Cenersen, through RNase H-mediated site specific mRNA cleavage, down regulates wild type & mutant TP53 mRNA and protein expression. Using a 2-stage EP differentiation assay, CD34+ cells from MDS patient specimens (del5q, n=11; non-del5q, n=4; normal donor, n=5) were cultured in StemSpan® SFEM expansion medium (StemCell Technologies, Vancouver, Canada) supplemented with SCF, EPO, and dexamethasone for 3–10 days, then transfected with 40nM cenersen or scrambled oligodeoxynucleotides using Lipofectamine RNAiMAX (Invitrogen, Carlsbad, CA). Cells were harvested after 3 days for analysis of p53 expression by fluorescence microscopy, FISH and methylcellulose cultures. CFC was assessed after 7–14 days incubation. Results: Del(5q) EP displayed greater nuclear p53 fluorescence intensity (FI) than non-del(5q)MDS or controls [del(5q) mean =82.12±11.4, non-del(5q)=41.55±13.05, controls=63.58±10.14, P=0.07; del(5q) vs. controls, P=0.33]. Treatment with cenersen significantly reduced nuclear (> 50%) and cytoplasmic p53 expression (> 59%) in EP in 9 of 11 del(5q) specimens, (mean nuclear FI, cenersen=41.75±9.02 vs. non-sense=86.84±13.12, P=0.01; mean cytoplasmic FI, cenersen= 23.38±3.14 vs. non-sense= 41.34±6.21, P=0.02). Similar but non-significant trends in nuclear p53 were observed in control (P=0.19) and non-del5qMDS (P=0.29) EP, whereas cytoplasmic p53 was significantly reduced only in controls (>52%, P=0.03; non-del5q, P=0.35). Cenersen treatment significantly increased recovery of BFU-E and CFU-E (n=15, P=0.01; n=13,P=0.0001, respectively) in MDS specimens, whereas there were no consistent changes in granulocyte-macrophage or mixed lineage colony yields. Similar but less robust improvements in CFC were observed in control samples with cenersen treatment (CFU-E, n=4, P=1.0; BFU-E, n=5, P=0.43). The magnitude of improvement in BFU-E in MDS specimens highly correlated with the extent of p53 suppression (nuclear Spearman r= -0.64, P=0.009; cytoplasmic r= -0.65, P=0.008). Changes in CFU-E showed no significant correlation with changes in nuclear (P=0.65) or cytoplasmic (P= 0.89) p53 expression. FISH analysis of cytospin preparations following oligonucleotide treatment of del(5q) EP using the EGR1 5q31/5p15.31 deletion probe (Cytocell, Cambridge UK) showed no significant differences between cenersen and non-sense treated cells (P=0.55, n=10). To determine if in vivo p53 suppression can restore transfusion independence (TI) after secondary LEN treatment failure, we treated 8 LEN-resistant, transfusion-dependent del(5q) MDS patients with dexamethasone (Dex) 20 mg weekly and continued LEN 5–10 mg/day. The glucocorticoid receptor acts as a ligand-dependent transcription factor and mutual antagonist of p53. All patients had Conclusions: Targeted suppression of p53 restores effective erythropoiesis in del(5q) MDS in the absence of clonal suppression. A clinical trial testing treatment with cenersen +Dex in LEN-resistant patients is currently in development. Disclosures: Smith: Smith Holdings LLC: Equity Ownership, Smith Holdings owns rights to the drug involved in this study. Patents & Royalties; Eleos, Inc: Eleos has licensed rights to the drug from Smith Holdings. Patents & Royalties. Daugherty:Eleos, Inc: Employment, Equity Ownership, Membership on an entity's Board of Directors or advisory committees. Wells:Alexion: Honoraria; Janssen: Honoraria; Celgene : Honoraria; Novartis: Honoraria, Research Funding. List:Celgene: Consultancy. Off Label Use: lenalidomide combined with Dexamethasone in lenalidomide refractory patients .

Abstract 3793 Introduction: Chronic Myelomonocytic Leukemia (CMML) is a genetically diverse hematologic malignancy characterized by cytopenias with or without leukocytosis, marrow dysplasia, monocytosis, splenomegaly, and a propensity to transform to Acute Myeloid Leukemia (AML). It is among the most aggressive chronic myeloid malignancies with a three year overall survival approximating twenty percent. The current treatment armamentarium is generally ineffective and mostly derived from studies that focused on Myelodysplastic Syndromes (MDS). As has been well defined in Juvenile Myelomonocytic Leukemia (JMML), we previously reported that primary CMML bone marrow mononuclear cells exhibit GM-CSF-dependent pSTAT5 hypersensitivity. To determine whether this signaling pathway is a viable therapeutic target, we present the preclinical characterization of KB003 activity on primary CMML bone marrow mononuclear cells. Methods: KB003 was engineered and provided by KaloBios Pharmaceuticals. The ability of KB003 to neutralize GM-CSF was tested in two ways using doses that ranged from .01 to 10,000 ng/ml of KB003. First, 10 ng/ml of GM-CSF was used to induce IL-8 production by the U937 cell line as measured by ELISA. Second, the GM-CSF-dependent cell line, MO7e, was cultured in the absence and in the presence of 0.1, 1, and 10 ng/ml of GM-CSF with or without KB003. Apoptosis and viability was assessed by annexin and 4′,6-diamidino-2-phenylindole (DAPI) staining by flow cytometry. To determine anti-GM-CSF sensitivity in CMML, primary cryoperserved bone marrow samples from 10 patients were reconstituted in prewarmed StemSpan H3000 with 10% FBS and microcultured in a 96 well plate with increasing doses of GM-CSF and KB003. After a 48-hour incubation period, cells were stained with myeloid-specific antibodies (CD34, CD38, CD14, CD33) and measured by multi-color flow cytometry. DAPI was used to measure cell viability. MethoCult hematopoietic progenitor assays were used to quantify the expansion and differentiation of CMML mononuclear cells in the presence of GM-CSF and escalating concentrations of KB003 in those samples with sufficient cell number for analysis (n=7). Results: KB003 effectively neutralized GM-CSF-induced IL-8 secretion in U937 cells with an IC50 of 48.2 ng/ml, as shown in Figure 1a. A dose of 0.1, 1, and 10 ng/ml of GM-CSF was used to protect MO7e cells from apoptosis and a survival benefit was achieved at each dose tested (Fig 1b). In this assay, KB003 neutralized 0.1 ng/ml of GM-CSF, but was unable to neutralize GM-CSF at higher doses (10 ng/ml). Using CMML samples, doses ranging from 10 to 100 μg/ml of KB003 were tested in apoptosis and in colony formation assays. Using the gating strategy shown in Figure 2a, the percentage of mature (CD33+/CD14+) and immature (CD33+/CD14-) myeloid subpopulations within the viable gate after KB003 treatment was decreased (Figure 2b). As expected, KB003 had no effect on CD3+ T-cells in the mixed culture populations. In addition to myeloid subpopulations identified by CD14, the CD33+/CD38+ cells were also more sensitive than CD33+/CD38- or CD38+/CD34+ inhibition by KB003 (Figure 2c, p Conclusion: GM-CSF neutralization using KB003 suppresses CMML progenitor survival in vitro. These preclinical data suggest that KB003 and other GM-CSF signaling axis inhibitors merit further investigation in CMML. A more committed myeloid precursor expressing CD38 may represent the progenitor population with enhanced GM-CSF dependence in CMML. This is consistent with the results showing CD38 to be a marker of GM-CSF-dependent pSTAT5 sensitivity in JMML and may prove to be predictive biomarker for anti-GM-CSF therapy in CMML. Disclosures: No relevant conflicts of interest to declare.

Abstract 3455 Background: Lenalidomide (LEN) and its analogue, pomalidomide, promote erythroid lineage competence and in vitro colony-forming capacity. In patients with non-del(5q) myelodysplastic syndrome (MDS), LEN restores erythropoiesis in a subset of patients (List et al. N Eng J Med 2005;352:549). Investigations by Ebert et al. showed that such responders to LEN treatment display repression of erythroid specific genes and that LEN restored transcriptional response to erythropoietin (Epo) (Ebert BL et al. PLoS Medicine 2008;5(2):e35), suggesting that LEN enhances Epo receptor (R) signal fidelity. We previously reported that LEN induces cellular expression of JAK2 associated EpoR in a concentration-dependent manner, however, the mechanism of regulation is unclear (Basiorka et al. Blood 2011,118: 2382a). Recent investigations implicated inhibition of the cereblon RING (really interesting new gene) finger domain containing E3-ubiquitin ligase complex as a key target of the immunomodulatory drugs (IMiDs) responsible for the teratogenic effects of thalidomide and the cytotoxic effects of LEN in multiple myeloma (Ito T et al. Science 2010; 327:1345–50; Zhu YW et al. Blood 2011;118:4771–9). We recently showed that LEN also interacts with the RING finger E3 ubiquitin ligase, murine double minute 2 (MDM2) to inhibit ligase ubiquitination and stabilize the protein (Wei et al. Oncogene, MS#ONC-2011-01840R, 2012). Because EpoR turnover is regulated by ubiquitination and proteasomal degradation, we evaluated the effects of LEN on the E3-ubiquitin ligase, RNF41, which regulates steady state or ligand independent, Janus kinase (JAK2) associated Type I receptor internalization (Wauman et al. J Cell Science. 2011;124:921–932). We hypothesized that LEN upregulates JAK2/EpoR expression through inhibition of RNF41 function, thereby increasing EpoR expression and enhancing JAK2 competent receptor signaling. Methods and Results: Treatment of the UT-7 erythroid progenitor cell line with cycloheximide ±1μM LEN showed that LEN stabilized cellular EpoR (T1/2, LEN >72h vs. 56h). To determine if the effects of LEN on receptor turnover are restricted to Type 1 cytokine receptors, we examined the effects of LEN on cellular expression of IL3-R (Type 1) and c-Kit (Type 2). LEN up-regulated IL3-R expression in a concentration-dependent fashion, whereas c-Kit expression was unchanged, confirming Type 1 receptor specificity. To determine if LEN alters EpoR/RNF41 interaction, we assessed protein association after LEN treatment. Immunoprecipitation (IP) of either EpoR or RNF41 followed by immunoblot (IB) for the binding partner showed that LEN promoted EpoR/RNF41 association in a concentration dependent manner. To investigate the effects of LEN on RNF41 function, we assessed protein specific ubiquitination after proteasomal inhibition with bortezomib followed by LEN treatment. IP of RNF41 and EpoR followed by ubiquitin IB showed that LEN inhibited RNF41 auto-ubiquitination in a concentration-dependent fashion accompanied by a corresponding decrease in EpoR ubiquitination, suggesting that LEN inhibits RNF41 ubiquitination to increase EpoR accumulation. To confirm that RNF41 is the principal target of LEN responsible for EpoR stabilization, we transfected HEK293T cells with EpoR and/or RNF41 expression vectors using the calcium phosphate method. Steady state EpoR expression was lower in EpoR/RNF41 cells compared with cells transfected with EpoR alone. Moreover, EpoR upregulation by LEN was abrogated in EpoR/RNF41 cells indicating that cellular RNF41 is a critical determinant of EpoR upregulation by LEN. Immunohistochemical staining of 16 bone marrow biopsies from non-del(5q) LEN-treated MDS patients are in progress to determine the relationship between cellular RNF41 level in erythroid precursors and clinical response. Conclusion: Our findings suggest that LEN acts as a broad RING finger E3-ubiquitin ligase inhibitor, whose targets extend to the Type 1 cytokine receptor specific, RNF41. RNF41 inhibition by LEN promotes accumulation of signaling competent JAK2/EpoR complexes that may augment Epo responsiveness. Further investigation is warranted to determine if erythroid expression level of RNF41 may serve as a biomarker for response to LEN in patients with non-del(5q) MDS. Disclosures: List: Celgene: Consultancy.

Abstract 788 Understanding the pathophysiology of myelodysplastic syndrome (MDS) is limited by a complex molecular mechanism and lack of an adequate animal model that recapitulates the role of inflammation in the abnormal hematopoiesis. We recently showed that patients with MDS have expansion of inflammation-related hematopoietic suppressive cells called immature myeloid-derived suppressor cells (MDSC) that display direct cytotoxic and suppressive effects on autologous hematopoietic progenitor cells (HPCs). Expansion of bone marrow (BM) MDSCs contributed to the production of inflammatory cytokines and reduced HPC survival underlying BM failure in lower risk patients. Here we provide evidence that MDSC activation, expansion and development is driven by overexpression of inflammatory-related signaling molecules, myeloid-related protein 8 (MRP8, encoded by S100A8) and MRP14 (encoded by S100A9). Both MRP proteins serve as the native endogenous ligands for Toll-like receptor 4 (TLR4), which is an important damage-associated molecular pattern (DAMP) mediating inflammatory response. We found higher expression of MRP8 and MRP14 in BM mononuclear cells from MDS patients compared to healthy donors, in whom these proteins were not detectable. High surface expression of both TLR2 and TLR4 in MDS MDSCs compared to healthy donor MDSCs confirmed that this signaling pathway is activated in MDS. Inhibition of MRP8/MRP14 proteins in MDSCs using specific shRNAs dramatically attenuated IL-10 and TGF-β production and rescued BFU-E and CFU-GM colony formation of autologous bone marrow progenitors. These data show that inflammation-associated MRP8/MRP14 expression plays a critical role in the suppressive activities of MDS MDSCs. We therefore generated S100A9 transgenic mice (S100A9Tg) overexpressing the murine MRP14 homologue and investigated the role of this protein in bone marrow failure. Significant MDSC accumulation was evident in the BM of S100A9Tg mice by 6 weeks, but not in S100 knockout (KO) or wild type (WT) mice. Similar to human MDS, MDSCs from S100A9Tg mice, but not S100KO or WT mice, significantly inhibited BFU-E colony formation. Depletion of MDSCs in vitro rescued BM colony formation in the S100A9Tg mice indicating that the BM suppression is mediated by MDSC cells. TGF-β and IL-10 secretion was significantly increased in S100A9Tg mice, substantiating the role of S100A9 as an essential inflammatory factor that regulating MDSC suppressive activity. Analogous to human MDS, 6-month old S100A9Tg mice developed ineffective hematopoiesis with severe anemia, leukopenia, and thrombocytopenia accompanied by MDS-like morphological features. BM aspirates and core biopsies from S100A9Tg mice were hypercellular with trilineage cytological dysplasia characteristic of MDS. Treatment with ATRA, which induced the differentiation of MDSCs rescued hematopoiesis in S100A9Tg mice. Our findings indicate that primary BM expansion of MDSC is sufficient to perturb hematopoiesis and result in the development of MDS, supporting the notion of microenvironment-conducive oncogenesis. S100A9Tg transgenic mice provide a novel in vivo model of human MDS for target discovery and testing of novel therapeutics. Disclosures: No relevant conflicts of interest to declare.

Abstract 2382 The erythropoietin receptor (EpoR) is a type 1 cytokine receptor and key effector of Epo signaling. Maturation and shuttling of receptor to the plasma membrane is dependent on its association with Janus Kinase 2 (JAK2). Upon ligand binding, EpoR homo-dimerizes to activate JAK2, which in turn phosphorylates tyrosine residues on the receptor cytoplasmic tail permitting recruitment and phosphorylation of the signal transduction and activator of transcription (STAT)-5. Lenalidomide (LEN), a second generation immunomodulatory drug (IMiD), promotes the expansion of primitive erythroid progenitors in vitro and induces the expression of genes involved in erythroid lineage commitment (JCI 2008;118(1):248-58; PLoS medicine 2008;5(2):e35). In del(5q) clones, LEN is cytotoxic by inhibiting the haplodeficient serine/threonine phosphatase PP2A (Wei S, et. al PNAS 2009). To investigate the mechanism by which LEN promotes erythropoiesis, we investigated its action on EpoR cellular dynamics. Using the human erythroleukemia cell line, UT-7, treatment with LEN elicited concentration dependent upregulation of EpoR in whole cell lysates demonstrable by immunoblot (IB) analysis at concentrations ranging from 0.1–10μM, reaching a 2-fold increase vs. controls. Treatment with pomalidomide, an IMiD with superior erythropoietic activity, displayed greater potency, increasing cellular EpoR at concentrations as low as 1nM; whereas thalidomide was ineffective. Upregulation of EpoR was apparent as early as 1 hour (h) after LEN exposure, reaching maximal receptor induction at 8h. Isolation of membrane and cystosolic fractions followed by IB showed that EpoR was upregulated predominantly in the membrane fraction, increasing 4.3-fold. IB of detergent insoluble membrane raft (MR) fractions showed that LEN promoted lipid raft assembly and F-actin polymerization accompanied by translocation of EpoR into MR microdomains with JAK2, STAT-5 and Lyn signal intermediates. Pretreatment with the Rho-GTPase inhibitor, Y27632, suppressed LEN effects on MR and F-actin. Immunoprecipitation (IP) of EpoR showed that JAK2 co-precipitated with the receptor in LEN treated cells, suggesting drug induced release of pre-assembled EpoR/JAK2 complexes from golgi and endosomal compartments. Q-PCR showed no changes in EpoR gene mRNA expression up to 24 hours after drug exposure, supporting post translational drug effects. To confirm that the observed changes in EpoR dynamics in UT-7 cells extends to normal erythroid progenitors, we assessed EpoR expression and MR assembly (cholera toxin B labeled GM1) by fluorescence microscopy in CD71+ erythroid precursors from three normal bone marrow donors. Expression of EpoR increased a mean of 17% (range:0-68%) after 1h LEN [1μM] exposure (p=0.003), accompanied by an increase in MR aggregates. Because the LEN inhibitable target PP2A is a key regulator of Rho-GTPase guided receptor trafficking and AP-1 directed formation of MR targeted clathrin-coated vesicles, we examined its effects on EpoR induction. Pretreatment of UT-7 cells with the PP2A activator FTY720 antagonized the upregulation of EpoR by LEN, whereas treatment with the PP2A inhibitor cantharadin [0.5μM] induced EpoR protein expression. Inhibition of dynamin activity, a PP2A sensitive GTPase involved in type 1 receptor recycling from membrane to endosomes, recapitulated the effects of cantharadin, inducing EpoR expression. In summary, these findings indicate that LEN modulates EpoR equilibrium, mediated in part through its inhibitory effect on PP2A. LEN induces rapid translocation of mature EpoR/JAK2 complexes to the plasma membrane while promoting the assembly of MRs with signaling competent receptor platforms to optimize cytokine signal integrity. Disclosures: List: Celgene: Consultancy.

Abstract 2816 JAK2 is a non-receptor tyrosine kinase that acts as an important signal transducer in cytokine signaling, promoting growth, survival, and differentiation of various cell types. Somatic mutations in JAK2 are commonly found in myeloproliferative neoplasms (MPNs). JAK2-V617F is frequently found in patients with classical MPNs: polycythemia vera (>95%), essential thrombocythemia (∼50%), and myelofibrosis (∼50%). JAK2-V617F is accepted to be a key contributing mutation to MPN development and its expression recapitulates an MPN phenotype in mouse models. While clinical trials treating MPN patients with JAK inhibitors have shown positive results, success has primarily been due to improved patient symptomology without the induction of clinical remission. Therefore, additional advances in understanding MPN cell biology could be utilized to further improve clinical outcomes. Normally, JAK2 associates with membrane-bound cytokine receptors and upon cytokine stimulation phosphorylates the receptor, allowing for signaling molecules like STAT5 to associate and be activated by JAK2 phosphorylation. While cytokine stimulation is not required to initiate signaling by JAK2-V617F, its interaction with cytokine receptors is thought to be required for deregulated signaling. We have previously shown that wild-type JAK2 resides in lipid rafts and that erythropoietin receptor signaling is dependent upon intact lipid rafts (Blood 2009, 114: 2506). Lipid rafts are nano-sized microdomains of the plasma membrane that are enriched in cholesterol and sphingolipids. Their functions include regulating membrane trafficking and regulating localization of proteins as a mechanism of facilitating specific protein complex formation in the membrane. Our ongoing work demonstrates that JAK2-V617F co-localizes with lipid rafts and that JAK2-V617F/STAT signaling is inhibited with lipid raft disrupting agents. We hypothesize that targeting lipid rafts, perhaps through alteration of cholesterol levels, will negatively impact JAK2-V617F signaling. The statin drugs, commonly used to treat hypercholesterolemia, inhibit HMG-CoA reductase, the rate-limiting enzyme in the mevalonate pathway, which leads to cholesterol biosynthesis. In our current study, we demonstrate that multiple statins (simvastatin, lovastatin, and atorvastatin) are effective at inhibiting JAK2-V617F-dependent cell growth and viability, through the use of JAK2-V617F-dependent MPN model cell lines (HEL, SET-2, and Uke1). In addition, we show that simvastatin treatment decreases primary MPN cell colony formation in a dose-dependent manner. Most impressively, we show cooperation between statins and JAK inhibitors (JAK inhibitor I as well as INCB018424), leading to an enhanced reduction of JAK2-V617F-positive cell growth over single agent treatment. Concurrently, we show that lipid rafts are altered and JAK2 protein no longer associates with rafts after low dose atorvastatin treatment. In addition to reducing cholesterol levels, statins can also alter other lipid-dependent processes, such as protein prenylation, and thus there may be additional statin effects that are contributing to decreased JAK2-V617F-dependent cell growth in our studies. Nonetheless cholesterol-dependent lipid rafts may facilitate proper JAK2-V617F signaling complexes required for the transforming signals of the JAK2-V617F oncoprotein. This hypothesis is consistent with recent work that has demonstrated that inhibition of membrane cholesterol efflux in hematopoietic progenitor cells promotes MPN formation in mice, and that removal of membrane cholesterol reverses this effect. Utilizing cholesterol-lowering statins in combination with JAK inhibitor treatment may provide a more effective therapeutic strategy for MPN patients than single agent JAK inhibitor use alone. Such combination therapies may also allow for lower doses of JAK inhibitor to be effective in MPN patients, potentially avoiding side effects and complications of higher dose therapy and increasing the number of patients who could tolerate long term JAK inhibitor therapy. Disclosures: No relevant conflicts of interest to declare.

Abstract 4191 JAK2 is a cytoplasmic tyrosine kinase that plays an important role in signaling following activation of various cytokine receptors. JAK2 activation promotes growth, survival, and differentiation of various cell types. Mutation of JAK2 is seen in numerous hematopoietic diseases, most notably in myeloproliferative neoplasms (MPNs). JAK2-V617F is a frequent mutation found in the classical MPNs: polycythemia vera, essential thrombocythemia, and primary myelofibrosis. The single amino acid change of valine to phenylalanine occurs in the pseudokinase domain of JAK2, relieving auto-inhibition of the kinase domain and allowing constitutive kinase activity. Numerous mouse models have demonstrated that JAK2-V617F can induce MPN-like disorders in mice. Thus, this point mutation, as well as other less common JAK2 mutations, is believed to play an important etiologic role in the development of MPNs in humans. The development and use of JAK2 inhibitors in clinical trials has shown promising results, again demonstrating the important role JAK2 plays in MPNs. While the JAK2-V617F mutation, as well as other JAK2 mutations, decreases auto-inhibition of JAK2 kinase activity, it is clear that mutated JAK2 still requires the expression of cytokine receptors to induce activation of transforming signals in hematopoietic cells. Normally, JAK2 binds to homodimeric and heterodimeric cytokine receptors through specific receptor motifs and is activated by various structural changes induced by cytokine stimulation. Following activation it utilizes receptor tyrosines as substrates for phosphorylation, leading to recruitment of downstream signaling molecules, such as STAT5, among others. JAK2 then activates STAT5 via phosphorylation and STAT5 then translocates to the nucleus to regulate transcription of target genes. JAK2-V617F does not require ligand for activation, but still requires the scaffolding function of cytokine receptors to facilitate its full activation and activation of downstream signaling via phosphorylation. Lipid rafts are microdomains of the plasma membrane that are enriched in cholesterol and sphingolipids. They have gained appreciation in signal transduction as sites of localization of signaling mediators, including membrane-bound receptors. Congregation of signaling proteins in lipid rafts within the plasma membrane promotes complex formation and signaling cascade activation. We have recently demonstrated that JAK2 is present in lipid rafts during erythropoietin signaling and that lipid raft integrity is required for erythropoietin-mediated signal transduction (Blood 2009, 114: 292). In our current study, we demonstrate that constitutive JAK-STAT signaling driven by JAK2-V617F is sensitive to lipid raft disruption. Human erythroleukemia (HEL) cells express constitutive activation of JAK-STAT signaling due to the presence of JAK2-V617F. Treatment of these cells with methyl-beta-cyclodextrin to disrupt lipid rafts abolished JAK2, STAT5, and STAT3 activation. Similar results are obtained in other cell lines harboring JAK2-V617F and that exhibit JAK-STAT activation that is dependent on this activated form of JAK2. We also demonstrate that JAK2-V617F co-localizes with lipid rafts, as shown by immunofluorescence, and that this co-localization is abolished by lipid raft disruption. This suggests the loss of JAK2-V617F-mediated JAK-STAT activation we observe following lipid raft disruption may be due to an inhibition of properly localized protein complex formation in the plasma membrane that is necessary for JAK2-V617F signaling. Lipid rafts may provide a site for an accumulation of JAK2-V617F-containing signaling complexes and may be necessary for the cellular signals initiated by JAK2-V617F. Our data show JAK2-V617F-driven JAK-STAT pathway activation is vulnerable to lipid raft disrupting agents and suggest lipid raft integrity as a potential therapeutic target for JAK2-V617F positive neoplasms. Targeting lipid rafts in combination with JAK2 kinase inhibitors may allow for more effective kinase inhibition at lower doses, potentially decreasing undesirable side effects associated with kinase inhibitor treatment. Disclosures: No relevant conflicts of interest to declare.

Abstract 2823 Background: Mutations in TP53, or less often its regulators, increases risk for malignant transformation. Murine double minute protein 2 (MDM2), an E3 ubiquitin ligase, targets p53 for proteasomal degradation and is the most well studied negative regulator of p53. Recent investigations have highlighted the emerging importance of p53 in MDS. Haploinsufficiency for ribosomal protein S14 in deletion 5q MDS liberates free ribosomal proteins that bind to and promote degradation of MDM2, thereby activating p53 in erythroid precursors. A single nucleotide polymorphism (SNP) in an MDM2 promoter (SNP309) is linked to younger age of onset of several solid tumors and an increased risk for acute myeloid leukemia (AML) [Knappskog and Lonning. 2011. Transcription 2:207, Xiang et al. 2009. Leuk Res. 33:1454]. The thymine (T) to guanine (G) substitution introduces an additional Sp1 transcription factor binding site causing upregulation of MDM2 transcription. A second SNP in this promoter (SNP285) has also been linked to cancer susceptibility, where a guanine (G) to cytosine (C) exchange is associated with decreased ovarian and breast cancer risk (Knappskog and Lonning. 2011. Oncotarget. 2:251). The C-allele of SNP285 has diminished Sp1 promoter binding compared to the G-allele decreasing MDM2 expression. In this study we investigated genotype distribution of MDM2 SNPs in del(5q) and non-del(5q) MDS patients and compared results to healthy controls. Methods and Results: Using Sanger sequencing, we compared allele and genotype frequencies for SNP285 and SNP309 in 155 healthy controls, 97 non-del(5q), and 119 del(5q) MDS patients. For SNP285, we found no significant difference in genotype or allele frequency among non-del(5q) or del(5q) cases compared to controls (p=0.25 and 0.26, respectively). Although there was no difference in age at diagnosis by genotype in del(5q) MDS (p=0.82), there was a significant difference among non-del(5q) MDS cases [p Conclusions: MDM2 SNP309 may be linked to MDS susceptibility, as well as LEN responsiveness and PFS in del(5q) and non-del(5q) MDS, respectively. These data warrant validation in a larger patient cohort. Investigation of the interaction between MDM2 SNPs and the well described TP53 R72P SNP is underway. Disclosures: List: Celgene: Consultancy.

Approximately 30-40% of patients with myelodysplastic syndromes (MDS) develop acute myeloid leukemia (AML). Several recurrent mutations have been identified in MDS using next-generation sequencing (NGS) technology and recent studies have greatly illuminated the molecular landscape of this disorder. However, the molecular events driving MDS progression to AML remain poorly understood. In order to investigate the genetic basis of leukemic transformation in MDS during disease progression, we evaluated the frequency and chronology of the acquisition of mutations using a targeted NGS myeloid gene panel on serial (paired) samples from 41 MDS patients before (pre-progression) and after disease progression (post-progression) to a more advanced subtype (n=7) or to AML (n=34). The mutational profile was characterized using a TruSeq Custom Amplicon panel (Illumina) targeting the hotspots of 31 recurrently (>1%) mutated genes in myeloid malignancies. Samples were run on an Illumina MiSeq and variants were annotated and filtered using Illumina VariantStudio v2.1.36, and interpreted according to the ACMG recommendations. The proportion of sequencing reads reporting a given mutation (variant allele frequency, VAF) was used to estimate the fraction of tumor cells carrying that mutation, and to determine whether mutations are clonal (in all tumor cells) or subclonal (in a fraction of tumor cells). A total of 100 and 123 mutations across 23 genes were identified in pre- and post-progression samples, respectively. The number of mutations was generally higher in the post-progression samples: the number of cases with one or two mutations was 24 in pre-progression samples and 16 in post-progression samples, while the number of cases with three or four mutations was 12 in pre-progression samples and 18 in post-progression samples. Several cases showed ≥5 mutations. The most frequently mutated genes (in >15% of samples) were ASXL1, TET2, SRSF2, U2AF1, RUNX1 and TP53; ASXL1 was the top ranking mutated gene with a frequency of 44% in pre-progression samples and 46% in post-progression samples. SF3B1, the most frequently mutated gene in MDS, was mutated in only two cases in our cohort. This finding is consistent with the strong association of SF3B1 mutation with the low-risk MDS subtype RARS, and its status as a good prognostic marker. Our patient cohort is highly selected, comprised only of patients whose disease has progressed and our data thus indicate that ASXL1 mutations are strongly associated with MDS cases that show disease progression and conversely that SF3B1 mutations are rarely associated with MDS disease progression. Mutations of genes involved in splicing (U2AF1, SRSF2), chromatin modification (EZH2, ASXL1) and DNA methylation (TET2, IDH1/2) were present in the pre-progression samples for almost all cases with mutations of these genes; these mutations often occur in founding clones (VAF >40%) and may play a role in disease initiation. Mutations of genes involved in transcriptional regulation (RUNX1, ETV6, PHF6) and signal transduction (NRAS, KRAS) were found in many cases in the post-progression sample only, suggesting that these are often late events that may co-operate with early events to drive disease progression. Interestingly, co-occurrence of NRAS and ASXL1 mutations was a frequent event in post-progression samples (n=5) in our cohort. It has been reported that NRas mutation and Asxl1 loss co-operate to drive myeloid proliferation and myeloid leukemia in mice, and our data support this observation. The average VAF of some mutations changed markedly during disease progression, with NRAS showing the largest VAF fold increase among genes mutated in >5 cases. The majority of subclones containing mutations in RUNX1 or ASXL1 expanded with disease progression. EZH2 mutations were mainly in the founding clone, while TP53 mutations were mainly in a subclone. For six cases we sequenced an additional serial sample, thus giving more precise information on the mutational profile evolution during disease progression. This is the first study to investigate the mutation status of a large group of MDS patients showing disease progression by the study of serial samples using a NGS myeloid gene panel. This study informs the timing of mutation acquisition during disease initiation and progression in MDS and closely related conditions, illuminating the genetic basis of leukemic transformation in these disorders. Disclosures No relevant conflicts of interest to declare.