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The 1,287 genes targeted by the shRNA of the leukemia library.

Standard hematologic parameters in SIRT5-/- mice and SIRT5+/+ littermates.

Plasmids.

Patient sample information.

Antibodies.

We discovered that the survival and growth of many primary acute myeloid leukemia (AML) samples and cell lines, but not normal CD34+ cells, are dependent on SIRT5, a lysine deacylase implicated in regulating multiple metabolic pathways. Dependence on SIRT5 is genotype agnostic and extends to RAS- and p53-mutated AML. Results were comparable between SIRT5 knockdown and SIRT5 inhibition using NRD167, a potent and selective SIRT5 inhibitor. Apoptosis induced by SIRT5 disruption is preceded by reductions in oxidative phosphorylation and glutamine utilization, and an increase in mitochondrial superoxide that is attenuated by ectopic superoxide dismutase 2. These data indicate that SIRT5 controls and coordinates several key metabolic pathways in AML and implicate SIRT5 as a vulnerability in AML.Significance:Reducing SIRT5 activity is detrimental to the survival of AML cells regardless of genotype, yet well tolerated by healthy hematopoietic cells. In mouse models, disrupting SIRT5 inhibits AML progression. SIRT5 controls several metabolic pathways that are required for leukemia cell survival. These results identify SIRT5 as a therapeutic target in AML.See related commentary by Li and Melnick, p. 198.

All supplemental figures and legends.

Next generation sequencing for 52 myeloid malignancies-related genes.

Oligonucleotide sequences.

Ranked list of 1,287 genes from shRNA library screen in primary AML cells. The 1,287 genes assessed with an shRNA library screen were sorted by the second highest percentile fold change present in 2 shRNA and across 2 samples, with the 34 genes showing a fold change in the top 2 percent in more than 2 samples listed first.

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Supplementary Methods, Legends

Supplementary Table 1

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Supplementary Figure 2

Purpose:Myelofibrosis is a hematopoietic stem cell neoplasm characterized by bone marrow reticulin fibrosis, extramedullary hematopoiesis, and frequent transformation to acute myeloid leukemia. Constitutive activation of JAK/STAT signaling through mutations in JAK2, CALR, or MPL is central to myelofibrosis pathogenesis. JAK inhibitors such as ruxolitinib reduce symptoms and improve quality of life, but are not curative and do not prevent leukemic transformation, defining a need to identify better therapeutic targets in myelofibrosis.Experimental Design:A short hairpin RNA library screening was performed on JAK2V617F-mutant HEL cells. Nuclear–cytoplasmic transport (NCT) genes including RAN and RANBP2 were among top candidates. JAK2V617F-mutant cell lines, human primary myelofibrosis CD34+ cells, and a retroviral JAK2V617F-driven myeloproliferative neoplasms mouse model were used to determine the effects of inhibiting NCT with selective inhibitors of nuclear export compounds KPT-330 (selinexor) or KPT-8602 (eltanexor).Results:JAK2V617F-mutant HEL, SET-2, and HEL cells resistant to JAK inhibition are exquisitely sensitive to RAN knockdown or pharmacologic inhibition by KPT-330 or KPT-8602. Inhibition of NCT selectively decreased viable cells and colony formation by myelofibrosis compared with cord blood CD34+ cells and enhanced ruxolitinib-mediated growth inhibition and apoptosis, both in newly diagnosed and ruxolitinib-exposed myelofibrosis cells. Inhibition of NCT in myelofibrosis CD34+ cells led to nuclear accumulation of p53. KPT-330 in combination with ruxolitinib-normalized white blood cells, hematocrit, spleen size, and architecture, and selectively reduced JAK2V617F-mutant cells in vivo.Conclusions:Our data implicate NCT as a potential therapeutic target in myelofibrosis and provide a rationale for clinical evaluation in ruxolitinib-exposed patients with myelofibrosis.

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PDF file, 421K, Supplementary Figure 1: Specificity of imatinib, dasatinib, PPY-A, BAW667, and a SCF-blocking antibody on Mo7ep210BCR-ABL1 cells. Supplementary Figure 2: Comparison of CFU-GM colony formation upon sole BCR-ABL1 inhibition (PPY-A), sole KIT inhibition or dual BCR-ABL1/KIT inhibition. Supplementary Figure 3: Inhibition of KIT in human cells through use of a lentiviral vector for simultaneous expression of shKIT and GFP . Supplementary Figure 4: Analysis of CML CFU-GM colony growth following removal of the individual cytokines. Supplementary Figure 5: BCR-ABL1 and KIT signaling influence activation of AKT associated with a reduction of Foxo3A. Supplementary Table 1. Activity profile of BAW667.

Imatinib and other BCR-ABL1 inhibitors are effective therapies for chronic myelogenous leukemia (CML), but these inhibitors target additional kinases including KIT, raising the question of whether off-target effects contribute to clinical efficacy. On the basis of its involvement in CML pathogenesis, we hypothesized that KIT may govern responses of CML cells to imatinib. To test this, we assessed the growth of primary CML progenitor cells under conditions of sole BCR-ABL1, sole KIT, and dual BCR-ABL1/KIT inhibition. Sole BCR-ABL1 inhibition suppressed mature CML progenitor cells, but these effects were largely abolished by stem cell factor (SCF) and maximal suppression required dual BCR-ABL1/KIT inhibition. In contrast, KIT inhibition did not add to the effects of BCR-ABL1 inhibition in primitive progenitors, represented by CD34+38− cells. Long-term culture-initiating cell assays on murine stroma revealed profound depletion of primitive CML cells by sole BCR-ABL1 inhibition despite the presence of SCF, suggesting that primitive CML cells are unable to use SCF as a survival factor upon BCR-ABL1 inhibition. In CD34+38+ cells, SCF strongly induced pAKTS473 in a phosphoinositide 3-kinase (PI3K)–dependent manner, which was further enhanced by inhibition of BCR-ABL1 and associated with increased colony survival. In contrast, pAKTS473 levels remained low in CD34+38− cells cultured under the same conditions. Consistent with reduced response to SCF, KIT surface expression was significantly lower on CD34+38− compared with CD34+38+ CML cells, suggesting a possible mechanism for the differential effects of SCF on mature and primitive CML progenitor cells. Cancer Res; 73(18); 5775–86. ©2013 AACR.

We discovered that the survival and growth of many primary acute myeloid leukemia (AML) samples and cell lines, but not normal CD34+ cells, are dependent on SIRT5, a lysine deacylase implicated in regulating multiple metabolic pathways. Dependence on SIRT5 is genotype agnostic and extends to RAS- and p53-mutated AML. Results were comparable between SIRT5 knockdown and SIRT5 inhibition using NRD167, a potent and selective SIRT5 inhibitor. Apoptosis induced by SIRT5 disruption is preceded by reductions in oxidative phosphorylation and glutamine utilization, and an increase in mitochondrial superoxide that is attenuated by ectopic superoxide dismutase 2. These data indicate that SIRT5 controls and coordinates several key metabolic pathways in AML and implicate SIRT5 as a vulnerability in AML. Significance: Reducing SIRT5 activity is detrimental to the survival of AML cells regardless of genotype, yet well tolerated by healthy hematopoietic cells. In mouse models, disrupting SIRT5 inhibits AML progression. SIRT5 controls several metabolic pathways that are required for leukemia cell survival. These results identify SIRT5 as a therapeutic target in AML. See related commentary by Li and Melnick, p. 198.

Normal human bone marrow cells are critical for studies of hematopoiesis and as controls to assess toxicity. As cells from commercial vendors are expensive, many laboratories resort to cancer-free bone marrow specimens obtained during staging or to umbilical cord blood cells, which may be abnormal or reflect a much younger age group compared to the disease samples under study. We piloted the use of femoral heads as an alternative and inexpensive source of normal bone marrow. Femoral heads were obtained from 21 successive patients undergoing elective hip arthroplasty. Mononuclear cells (MNCs) were purified with Ficoll, and CD3+, CD14+, and CD34+ cells were purified with antibody-coated microbeads. The median yield of MNCs was 8.95 × 107 (range, 1.62 × 105–2.52 × 108), and the median yield of CD34+ cells was 1.40 × 106 (range, 3.60 × 105–9.90 × 106). Results of downstream applications including qRT-PCR, colony-forming assays, and ex vivo proliferation analysis were of high quality and comparable to those obtained with standard bone marrow aspirates. We conclude that femoral heads currently discarded as medical waste are a cost-efficient source of bone marrow cells for research use.

Therapy resistance is responsible for most cancer-related death and is mediated by the unique ability of cancer cells to leverage metabolic conditions, signaling molecules, redox status, and other pathways for their survival. Interestingly, many cancer survival pathways are susceptible to disturbances in cellular reactive oxygen species (ROS) and may therefore be disrupted by exogenous ROS. Here, we explore whether trident cold atmospheric plasma (Tri-CAP), a gas discharge with exceptionally low-level ROS, could inhibit multiple cancer survival pathways together in a murine cell line model of therapy-resistant chronic myeloid leukemia (CML). We show that Tri-CAP simultaneously disrupts three cancer survival pathways of redox deregulation, glycolysis, and proliferative AKT/mTOR/HIF-1α signaling in this cancer model. Significantly, Tri-CAP blockade induces a very high rate of apoptotic death in CML cell lines and in primary CD34(+) hematopoietic stem and progenitor cells from CML patients, both harboring the therapy-resistant T315I mutation. In contrast, nonmalignant controls are minimally affected by Tri-CAP, suggesting it selectively targets resistant cancer cells. We further demonstrate that Tri-CAP elicits similar lethality in human melanoma, breast cancer, and CML cells with disparate, resistant mechanisms and that it both reduces tumor formation in two mouse models and improves survival of tumor-bearing mice. For use in patients, administration of Tri-CAP may be extracorporeal for hematopoietic stem cell transplantation therapy, transdermal, or through its activated solution for infusion therapy. Collectively, our results suggest that Tri-CAP represents a potent strategy for disrupting cancer survival pathways and overcoming therapy resistance in a variety of malignancies.

Chronic myelomonocytic leukemia (CMML) is an aggressive myeloid neoplasm of older individuals characterized by persistent monocytosis. Somatic mutations in CMML are heterogeneous and only partially explain the variability in clinical outcomes. Recent data suggest that cardiovascular morbidity is increased in CMML and contributes to reduced survival. Clonal hematopoiesis of indeterminate potential (CHIP), the presence of mutated blood cells in hematologically normal individuals, is a precursor of age-related myeloid neoplasms and associated with increased cardiovascular risk. To isolate CMML-specific alterations from those related to aging, we performed RNA sequencing and DNA methylation profiling on purified monocytes from CMML patients and from age-matched (old) and young healthy controls. We found that the transcriptional signature of CMML monocytes is highly proinflammatory, with upregulation of multiple inflammatory pathways, including tumor necrosis factor and interleukin (IL)-6 and -17 signaling, whereas age per se does not significantly contribute to this pattern. We observed no consistent correlations between aberrant gene expression and CpG island methylation, suggesting that proinflammatory signaling in CMML monocytes is governed by multiple and complex regulatory mechanisms. We propose that proinflammatory monocytes contribute to cardiovascular morbidity in CMML patients and promote progression by selection of mutated cell clones. Our data raise questions of whether asymptomatic patients with CMML benefit from monocyte-depleting or anti-inflammatory therapies.

Purpose: Myelofibrosis is a hematopoietic stem cell neoplasm characterized by bone marrow reticulin fibrosis, extramedullary hematopoiesis, and frequent transformation to acute myeloid leukemia. Constitutive activation of JAK/STAT signaling through mutations in JAK2, CALR, or MPL is central to myelofibrosis pathogenesis. JAK inhibitors such as ruxolitinib reduce symptoms and improve quality of life, but are not curative and do not prevent leukemic transformation, defining a need to identify better therapeutic targets in myelofibrosis. Experimental Design: A short hairpin RNA library screening was performed on JAK2V617F-mutant HEL cells. Nuclear–cytoplasmic transport (NCT) genes including RAN and RANBP2 were among top candidates. JAK2V617F-mutant cell lines, human primary myelofibrosis CD34+ cells, and a retroviral JAK2V617F-driven myeloproliferative neoplasms mouse model were used to determine the effects of inhibiting NCT with selective inhibitors of nuclear export compounds KPT-330 (selinexor) or KPT-8602 (eltanexor). Results: JAK2V617F-mutant HEL, SET-2, and HEL cells resistant to JAK inhibition are exquisitely sensitive to RAN knockdown or pharmacologic inhibition by KPT-330 or KPT-8602. Inhibition of NCT selectively decreased viable cells and colony formation by myelofibrosis compared with cord blood CD34+ cells and enhanced ruxolitinib-mediated growth inhibition and apoptosis, both in newly diagnosed and ruxolitinib-exposed myelofibrosis cells. Inhibition of NCT in myelofibrosis CD34+ cells led to nuclear accumulation of p53. KPT-330 in combination with ruxolitinib-normalized white blood cells, hematocrit, spleen size, and architecture, and selectively reduced JAK2V617F-mutant cells in vivo. Conclusions: Our data implicate NCT as a potential therapeutic target in myelofibrosis and provide a rationale for clinical evaluation in ruxolitinib-exposed patients with myelofibrosis.

Acute myeloid leukemia (AML) is a cancer of myeloid-lineage cells with limited therapeutic options. We previously combined ex vivo drug sensitivity with genomic, transcriptomic, and clinical annotations for a large cohort of AML patients, which facilitated discovery of functional genomic correlates. Here, we present a dataset that has been harmonized with our initial report to yield a cumulative cohort of 805 patients (942 specimens). We show strong cross-cohort concordance and identify features of drug response. Further, deconvoluting transcriptomic data shows that drug sensitivity is governed broadly by AML cell differentiation state, sometimes conditionally affecting other correlates of response. Finally, modeling of clinical outcome reveals a single gene, PEAR1, to be among the strongest predictors of patient survival, especially for young patients. Collectively, this report expands a large functional genomic resource, offers avenues for mechanistic exploration and drug development, and reveals tools for predicting outcome in AML.

FLT3-ITD mutations occur in 20-30% of AML patients and are associated with aggressive disease. Patients with relapsed FLT3-mutated disease respond well to 2nd generation FLT3 TKIs but inevitably relapse within a short timeframe. In this setting, until overt relapse occurs, the bone marrow microenvironment facilitates leukemia cell survival despite continued on-target inhibition. We demonstrate that human bone marrow derived conditioned medium (CM) protects FLT3-ITD+ AML cells from the 2nd generation FLT3 TKI quizartinib and activates STAT3 and STAT5 in leukemia cells. Extrinsic activation of STAT5 by CM is the primary mediator of leukemia cell resistance to FLT3 inhibition. Combination treatment with quizartinib and dasatinib abolishes STAT5 activation and significantly reduces the IC50 of quizartinib in FLT3-ITD+ AML cells cultured in CM. We demonstrate that CM protects FLT3-ITD+ AML cells from the inhibitory effects of quizartinib on glycolysis and that this is partially reversed by treating cells with the combination of quizartinib and dasatinib. Using a doxycycline-inducible STAT5 knockdown in the FLT3-ITD+ MOLM-13 cell line, we show that dasatinib-mediated suppression of leukemia cell glycolytic activity is STAT5-independent and provide a preclinical rationale for combination treatment with quizartinib and dasatinib in FLT3-ITD+ AML.

Chronic myelomonocytic leukemia (CMML) is a clinically heterogeneous neoplasm in which JAK2 inhibition has demonstrated reductions in inflammatory cytokines and promising clinical activity. We hypothesize that annotation of inflammatory cytokines may uncover mutation-independent cytokine subsets associated with novel CMML prognostic features. A Luminex cytokine profiling assay was utilized to profile cryopreserved peripheral blood plasma from 215 CMML cases from three academic centers, along with center-specific, age-matched plasma controls. Significant differences were observed between CMML patients and healthy controls in 23 out of 45 cytokines including increased cytokine levels in IL-8, IP-10, IL-1RA, TNF-α, IL-6, MCP-1/CCL2, hepatocyte growth factor (HGF), M-CSF, VEGF, IL-4, and IL-2RA. Cytokine associations were identified with clinical and genetic features, and Euclidian cluster analysis identified three distinct cluster groups associated with important clinical and genetic features in CMML. CMML patients with decreased IL-10 expression had a poor overall survival when compared to CMML patients with elevated expression of IL-10 (P = 0.017), even when adjusted for ASXL1 mutation and other prognostic features. Incorporating IL-10 with the Mayo Molecular Model statistically improved the prognostic ability of the model. These established cytokines, such as IL-10, as prognostically relevant and represent the first comprehensive study exploring the clinical implications of the CMML inflammatory state.

// Ami B. Patel 1, * , Thoralf Lange 5, * , Anthony D. Pomicter 2, * , Christopher J. Conley 2 , Christina A. Harrington 4 , Kimberly R. Reynolds 2 , Todd W. Kelley 3 , Thomas O’Hare 1, 2 and Michael W. Deininger 1, 2 1 Division of Hematology and Hematologic Malignancies, The University of Utah, Salt Lake City, UT, USA 2 Huntsman Cancer Institute, The University of Utah, Salt Lake City, UT, USA 3 Department of Pathology, The University of Utah, Salt Lake City, UT, USA 4 Integrated Genomics Laboratory, Oregon Health and Science University, Portland, OR, USA 5 University of Leipzig, Division of Haematology and Oncology, Leipzig, Germany * These authors contributed equally to this work Correspondence to: Michael W. Deininger, email: Michael.Deininger@hci.utah.edu Keywords: chronic myeloid leukemia; BCR-ABL1; deep molecular response; treatment-free remission Received: January 20, 2018 Accepted: March 15, 2018 Published: April 03, 2018 ABSTRACT The life expectancy of patients with chronic phase chronic myeloid leukemia on tyrosine kinase inhibitor therapy now approaches that of the general population. Approximately 60% of patients treated with second generation tyrosine kinase inhibitors achieve a deep molecular response, the prerequisite for a trial of treatment-free remission. Those patients unlikely to achieve deep molecular response may benefit from more intensive therapy up front. To identify biomarkers predicting deep molecular response we performed transcriptional profiling on CD34 + progenitor cells from newly diagnosed chronic phase chronic myeloid leukemia patients treated with nilotinib on a prospective clinical trial. Using unsupervised and targeted analytical strategies, we show that gene expression profiles are similar in patients with and without subsequent deep molecular response. This result is in contrast to the distinct expression signature of CD34 + chronic phase chronic myeloid leukemia patients failing to achieve a cytogenetic response on imatinib and suggests that deep molecular response to second-generation tyrosine kinase inhibitors is governed by the biology of more primitive chronic myeloid leukemia cells or extrinsic factors.

BCR-ABL1 transcripts at imatinib cessation were quantified by droplet digital PCR (ddPCR) for 175 patients on the STIM2 trial. Patients with BCR-ABL1 transcripts below a defined cutoff had a 12-month molecular recurrence rate of 46% versus 68% for those above the cutoff. Implications of using ddPCR in forecasting successful imatinib cessation are discussed. See related article by Nicolini et al., p. 6606

The oncogenic BCR-ABL1 tyrosine kinase is the driver of chronic myeloid leukemia (CML) and Philadelphia chromosome-positive acute lymphoblastic leukemia (Ph+ALL). Tyrosine kinase inhibitors (TKIs) targeting ABL kinase are generally effective, but subsets of patients treated with single-agent TKIs develop resistance due to mutations in BCR-ABL1 that impair TKI binding. We have previously reported that BCR-ABL1 compound mutants (exhibiting two mutations within the same BCR-ABL molecule) that include the T315I gatekeeper mutation confer a high degree of resistance to all clinical ABL TKIs used as single agents, including ponatinib and the allosteric inhibitor asciminib. However, combining asciminib with ponatinib provides an effective strategy for overcoming compound mutation-based resistance (Eide et al. Cancer Cell 2019). As the clinical utility of ponatinib is limited by cardiovascular toxicity, including arterial occlusive events (AOEs), we decided to search for alternative molecules for use in combination with asciminib. To identify functional ponatinib analogs, we performed Quantum Similarity Modeling (QSM) on the reported crystal structure of T315I mutant ABL1 kinase in complex with nilotinib and asciminib (5MO4) (Wylie et al. Nature 2017) to search for other molecules. Compared to conventional computational modeling, QSM identifies novel classes of structurally distinct compounds that are comparable on a quantum level by precisely defining their interaction with the target. Affinity inferred by close complementarity with the shared ligand-protein surface in the region of the surveyed binding site is mapped, using multiple weak local associations. Our in silico QSM platform combines quantum methods with machine learning to investigate extensive chemical spaces. We screened several million compounds against BCR-ABL1 and identified 51 potential candidates predicted effectively to block T315I mutant BCR-ABL1 when combined with asciminib. To prioritize potent and non-toxic drug combinations for further development against compound mutants, we initially profiled all 51 compounds for their efficacy against Ba/F3 BCR-ABL T315I cells, alone and in combination with asciminib (1 nM). Of 51 compounds, LY3009120, a pan-RAF inhibitor that is currently in phase I clinical development for advanced solid malignancies (Sullivan et al. Mol Cancer Ther 2020), showed strong activity against BCR-ABL T315I when combined with asciminib. These data provided proof of principle for the QSM approach. We next tested the efficacy of all 51 candidates ± asciminib against Ba/F3 cells harboring T315I-inclusive BCR-ABL1 compound mutants, including Y253H/T315I, E255V/T315I, H396R/T315I, G250E/T315I, and T315L as the most resistant mutants. Neither single agent showed any effect. However, LY3009120 strongly inhibited BCR-ABL1 compound mutants when combined with asciminib. No toxicity was observed against Ba/F3 parental cells, confirming that the effects of the combinations are mediated by inhibition of BCR-ABL1. Synergy quantification of the dose-response matrix for the LY3009120/asciminib combination using the Zero Interaction Potency model demonstrated highly synergistic interactions (Synergy score > 10) between the two inhibitors. To directly assess the binding affinity of LY3009120 to the ABL1 kinase domain, we used the cell-based NanoBRET intracellular ABL1 kinase assay on HEK-293 cells expressing luciferase-tagged ABL1. The NanoBRET assay uses energy transfer to quantify the affinity of test compounds by competitive displacement of a cell-permeable fluorescent tracer that is reversibly bound to an ABL1-luciferase fusion protein. We found that LY3009120 competes off the fluorescent tracer at a low micromolar range (EC 50 = 0.75 μM), confirming direct binding of LY3009120 to the kinase domain of ABL1. We hypothesize that the binding of LY3009120 to the ABL1 kinase domain induces a conformational change that re-establishes asciminib binding to the myristoyl binding pocket, allowing for synergy. Studies to quantify the binding affinity of LY3009120 and asciminib to BCR-ABL1 mutants are underway, and data will be presented. In summary, our findings validate QSM as a novel in silico approach to identify TKI combinations. Combining LY3009120 with asciminib may be an effective, low-risk strategy to target BCR-ABL1 compound mutants in patients with clinical TKI resistance. Disclosures Deininger: SPARC, DisperSol, Leukemia & Lymphoma Society: Research Funding; Sangamo: Consultancy, Membership on an entity's Board of Directors or advisory committees; Incyte: Consultancy, Honoraria, Research Funding; Fusion Pharma, Medscape, DisperSol: Consultancy; Novartis: Consultancy, Research Funding; Blueprint Medicines Corporation: Consultancy, Membership on an entity's Board of Directors or advisory committees, Other: Part of a Study Management Committee, Research Funding; Takeda: Consultancy, Membership on an entity's Board of Directors or advisory committees, Other: Part of a Study Management Committee, Research Funding.

Background: Selinexor is an oral, small molecule, selective inhibitor of nuclear export (SINE) compound that specifically blocks the karyopherin protein exportin 1 (XPO1, CRM1). In an shRNA library screen, we discovered that the survival of JAK2V167F mutant HEL cells is dependent on XPO1-mediated nuclear-cytoplasmic transport. Selinexor selectively suppressed primary myelofibrosis (MF) cells as compared with normal progenitor cells and induced hematologic responses in an MPN mouse model. Methods: An open label, prospective, investigator-initiated single center study is ongoing in adults with primary or secondary MF with resistance or intolerance to JAK inhibitor (JAKi) therapy with platelets > 30 K/μL and neutrophils > 500/μL. Selinexor was given orally once a week. Spleen volume was assessed by MRI at week 12 and week 24. The study was amended to include additional MRIs every 12 weeks in the year 1 and 24 weeks in the year 2. Primary end point is spleen response, defined as ≥ 35% spleen volume reduction (SVR) by MRI or CT, where applicable) at week 24. Bone marrow was evaluated at baseline and at week 24. The projected sample size of 24 will provide 83% power to reject a response rate of 15% and allow for up to a 25% dropout rate. We provide an interim report after completing 50% enrollment. Results: Between May 2019 and February 2021, 12 patients (pts) were enrolled. JAK2, CALR and MPL mutations were present in 7 (58.3%), 4 (33.3%) and 1 (8.3%) pts respectively. Eight pts (66.6%) had at least one high molecular risk mutation at baseline (Table 1). Median duration of prior JAKi therapy was 22 months (0.5 to 96 months) and 11 out of 12 were refractory to ruxolitinib at study enrollment. Median baseline spleen volume was 1454 cm 3(range 835 to 5792). Selinexor starting dose was 80 mg weekly in the first 6 pts and 60 mg for subsequent pts. At data cutoff, median duration of selinexor therapy was 36 weeks (range 11-114 weeks). One pt was not response evaluable and died due to liver abscess at week 12 (unrelated). One pt discontinued selinexor at week 18 due to grade 3 fatigue and was not evaluable for the primary end point. Of the 11 pts who had week 12 MRI or CT, 6 showed ≥ 10% SVR, 3 showed ≥ 25% SVR and 1 pt had early progression (Figure 1). At week 24, 5/9 (56%) pts had ≥ 25% SVR and 2/9 (22%) had ≥ 35% SVR (Figure 1). In 9 pts who had ≥24 weeks of selinexor, SVR ≥ 25% and ≥ 35% occurred at any point during study treatment in 4 (44%) and 3 (33%) pts, respectively. Two pts were red cell transfusion dependent at baseline; 1 became transfusion independent after 36 weeks of treatment, has not required transfusion for 49 weeks and remains on study treatment to date (114 weeks). Six pts (50%) discontinued selinexor. Reasons for treatment discontinuation are death in 1 pt, progressive disease in 1 pt, alternative treatment in 2 pts, and toxicity in 2 pts. Ten pts required dose reduction due to fatigue (1pt), anemia (1 pt), thrombocytopenia (2 pts), abdominal pain (1pt) and weight loss (5 pts). The most common treatment related adverse event was weight loss (grade 2 in 4 pts and grade 3 in 1 pt). This was manageable with treatment interruption and dose reduction, except in one pt who discontinued selinexor. As yet no changes in reticulin fibrosis MF grade were observed among 9 patients who received at least 24 weeks of treatment. Conclusions: Once weekly, oral selinexor showed single agent activity with sustained spleen responses in pts with JAKi refractory MF. Long-term administration of selinexor was well tolerated over time in MF pts. Correlatives studies including circulating inflammatory cytokine levels and mutant allele burden, as well as clonality studies by X-chromosome inactivation studies in woman, are underway and will be presented. Figure 1 Figure 1. Disclosures Tantravahi: BMS: Research Funding; Novartis: Research Funding; CTI BioPharma: Research Funding; Abbvie Inc.: Research Funding; Karyopharm Therapeutics Inc.: Consultancy, Honoraria, Research Funding. Patel: Stemline: Research Funding; Genentech: Research Funding; Roche: Research Funding. Chamoun: Karyopharm Therapeutics Inc.: Current Employment, Current equity holder in publicly-traded company. Shah: Karyopharm: Current Employment. George: Celgene: Consultancy; Bristol Meyers Squibb: Consultancy; Incyte Corporation: Consultancy; Blueprint Medicines: Consultancy. Deininger: Fusion Pharma, Medscape, DisperSol: Consultancy; SPARC, DisperSol, Leukemia & Lymphoma Society: Research Funding; Takeda: Consultancy, Membership on an entity's Board of Directors or advisory committees, Other: Part of a Study Management Committee, Research Funding; Blueprint Medicines Corporation: Consultancy, Membership on an entity's Board of Directors or advisory committees, Other: Part of a Study Management Committee, Research Funding; Novartis: Consultancy, Research Funding; Sangamo: Consultancy, Membership on an entity's Board of Directors or advisory committees; Incyte: Consultancy, Honoraria, Research Funding.

Background Chronic phase chronic myeloid leukemia (CP-CML) is characterized by overproduction of differentiating myeloid cells, while blast phase CML (BP-CML) cells exhibit a differentiation block. Tyrosine kinase inhibitors (TKIs) are effective in CP-CML, but resistance is common in BP-CML and can occur without explanatory BCR-ABL1 kinase mutations (BCR-ABL1-independent resistance). Similarly, CML stem/progenitor cells (LSPCs) are insensitive to TKIs, and residual leukemia persists in the majority of CML patients on TKI therapy. We previously reported overlap between the transcriptomes of CD34 + cells from BP-CML and TKI-naïve CP-CML patients with primary TKI resistance, pointing to commonalities between LSPC quiescence, BCR-ABL1-independent TKI resistance, and BP-CML. Results To identify common mechanisms, we performed a meta-analysis of published CML transcriptomes. We identified a small set of genes with consistently low expression in LSPC quiescence, BCR-ABL1-independent TKI resistance, and BP-CML, including Membrane Spanning 4-Domains A3 (MS4A3), a signaling protein previously reported to inhibit hematopoietic cell cycle progression. Low MS4A3 in CD34 + cells from TKI-naïve CP-CML patients was associated with shorter survival on subsequent TKI therapy, suggesting that MS4A3 governs TKI response. To understand the function of MS4A3, we lentivirally introduced MS4A3 shRNA or an MS4A3 expression vector into CML CD34 + LSPCs. MS4A3 knockdown increased clonogenicity and imatinib resistance, while ectopic MS4A3 expression had opposite effects. MS4A3 KD also increased LSPC persistence ex vivo in LTC-IC assays, and in vivo in NSG mice xenografts, while modulating MS4A3 expression had no effect on normal CD34 + cells. We next generated Ms4a3+/+│-/-; Scl-tTA+; TRE-BCR-ABL1+ compound transgenic mice. Upon BCR-ABL1 induction, Ms4a3-/-; Scl-tTA+; TRE-BCR-ABL1+ mice developed leukocytosis comparable to Ms4a3+/+ controls. However, BM of Ms4a3-/-; Scl-tTA+; TRE-BCR-ABL1+ mice showed increased short-term HSCs and multipotent progenitor cells, and reduced granulocyte-macrophage progenitors. When Lin - BM cells from leukemic mice were transplanted into irradiated recipients, Ms4a3-/-; Scl-tTA+; TRE-BCR-ABL1+ cells showed increased engraftment and myeloid leukocytosis, validating our observations in human cells. To determine how MS4A3 is downregulated in CML, we expressed BCR-ABL1in 32D-cl3 cells. p210 BCR-ABL1 drastically reduced Ms4a3 expression, while kinase-inactive p210 BCR-ABL1-K271R had no effect. Moreover, we found that suppression of C/EBPε by MECOM reduces MS4A3, consistent with previous reports of MECOM as a driver of TKI resistance and progression to BP. Treatment of CML CD34 + cells with a library of epigenetic pathway inhibitors revealed that MS4A3 is suppressed by both DNA methylation and PRC2/EZH2-mediated H3K27 trimethylation, which was confirmed by patch-PCR and ChIPseq. These data indicate that multi-levelled mechanisms cooperate in the suppression of MS4A3 in CML. To determine how MS4A3 regulates clonogenicity and TKI response, we expressed MS4A3-EGFP fusion protein in LAMA-84 CML cells. We found that MS4A3 resides on the plasma membrane and in endosomes. Surface protein biotin labelling and tandem mass spectrometry ± MS4A3 KD showed that MS4A3 controls endocytosis of membrane proteins, including common β chain (βc) cytokine receptors. Specifically, MS4A3 promotes endocytosis of βc cytokine receptors upon GM-CSF/IL-3 stimulation of primary LSPCs and enhances downstream signaling and differentiation, suggesting that restoring MS4A3 expression has therapeutic efficacy. To test this, we manufactured a prototype MS4A3 protein-loaded liposomal nanoparticle (NP) using coating with the CD34 CD62L for targeted delivery to CD34 + cells. Compared to MS4A3-free NPs, MS4A3 NPs increased CD34 +CD38 + and CD34 -CD38 + at the expense of CD34 +CD38 - cells, reduced clonogenicity, and increased sensitivity to TKIs, mimicking ectopic MS4A3 expression. Conclusion MS4A3 governs response to differentiating myeloid cytokines, providing a unifying mechanism for the differentiation block characteristic of primitive LSPCs and BP-CML cells. We posit that LSPCs downregulate MS4A3 to evade βc cytokine-induced differentiation to maintain a primitive, TKI-insensitive state. MS4A3 re-expression or delivery of ectopic MS4A3 may eliminate LSPCs. Figure 1 Figure 1. Disclosures Druker: Aptose Therapeutics: Consultancy, Current equity holder in publicly-traded company, Membership on an entity's Board of Directors or advisory committees; Cepheid: Consultancy, Membership on an entity's Board of Directors or advisory committees; EnLiven: Consultancy, Research Funding; Blueprint Medicines: Consultancy, Current equity holder in publicly-traded company, Membership on an entity's Board of Directors or advisory committees; Aileron: Membership on an entity's Board of Directors or advisory committees; Amgen: Current equity holder in publicly-traded company, Membership on an entity's Board of Directors or advisory committees; Bristol-Myers Squibb: Research Funding; ALLCRON: Consultancy, Membership on an entity's Board of Directors or advisory committees; GRAIL: Current equity holder in publicly-traded company; Iterion Therapeutics: Membership on an entity's Board of Directors or advisory committees; Merck & Co: Patents & Royalties; Nemucore Medical Innovations, Inc.: Consultancy; Novartis Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees, Patents & Royalties; Pfizer: Research Funding; Recludix Pharma, Inc.: Consultancy; The RUNX1 Research Program: Membership on an entity's Board of Directors or advisory committees; Third Coast Therapeutics: Membership on an entity's Board of Directors or advisory committees; VB Therapeutics: Membership on an entity's Board of Directors or advisory committees; Vincerx Pharma: Current equity holder in publicly-traded company, Membership on an entity's Board of Directors or advisory committees; Vivid Biosciences: Membership on an entity's Board of Directors or advisory committees. Tyner: Agios: Research Funding; Astrazeneca: Research Funding; Array: Research Funding; Genentech: Research Funding; Janssen: Research Funding; Takeda: Research Funding; Gilead: Research Funding; Incyte: Research Funding; Petra: Research Funding; Seattle Genetics: Research Funding; Constellation: Research Funding; Schrodinger: Research Funding. Oehler: BMS: Consultancy; OncLive: Honoraria; Pfizer: Research Funding; Takeda: Consultancy; Blueprint Medicines: Consultancy. Radich: BMS: Membership on an entity's Board of Directors or advisory committees; Genentech: Membership on an entity's Board of Directors or advisory committees; Amgen: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees. Deininger: Sangamo: Consultancy, Membership on an entity's Board of Directors or advisory committees; Takeda: Consultancy, Membership on an entity's Board of Directors or advisory committees, Other: Part of a Study Management Committee, Research Funding; Incyte: Consultancy, Honoraria, Research Funding; Fusion Pharma, Medscape, DisperSol: Consultancy; Novartis: Consultancy, Research Funding; SPARC, DisperSol, Leukemia & Lymphoma Society: Research Funding; Blueprint Medicines Corporation: Consultancy, Membership on an entity's Board of Directors or advisory committees, Other: Part of a Study Management Committee, Research Funding.

Standard of care for AML includes chemotherapy and stem cell transplant, with 5-year survival rates Citation Format: Dongqing Yan, Anca Franzini, Anthony D. Pomicter, Brayden J. Halverson, Orlando Antelope, Clinton C. Mason, Jonathan M. Ahmann, Anna V. Senina, Courtney L. L. Jones, Matthew S. Zabriskie, Hein Than, Michael J. Xiao, Alexandria van Scoyk, Ami B. Patel, William L. L. Heaton, Shawn C. Owen, Joshua L. Andersen, Christina M. Egbert, Julie A. Reisz, Angelo D'Alessandro, James E. Cox, Kevin C. Gantz, Hannah M. Redwine, Siddharth M. Iyer, Jamshid S. Khorashad, Nima Rajabi, Christian A. Olsen, Thomas O'Hare, Michael W. Deininger. A critical role for SIRT5 in acute myeloid leukemia metabolism [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr LB109.

Philadelphia chromosome-positive (Ph+) B-cell precursor acute lymphoblastic leukemia (ALL) expressing BCR-ABL1 oncoprotein is a major subclass of ALL with poor prognosis. BCR-ABL1-expressing leukemic cells are highly dependent on double-strand break (DSB) repair signals for their survival. Here we report that a first-in-class HDAC1,2 selective inhibitor and doxorubicin (a hyper-CVAD chemotherapy regimen component) impair DSB repair networks in Ph+ B-cell precursor ALL cells using common as well as distinct mechanisms. The HDAC1,2 inhibitor but not doxorubicin alters nucleosomal occupancy to impact chromatin structure, as revealed by MNase-Seq. Quantitative mass spectrometry of the chromatin proteome along with functional assays showed that the HDAC1,2 inhibitor and doxorubicin either alone or in combination impair the central hub of DNA repair, the Mre11–Rad51–DNA ligase 1 axis, involved in BCR-ABL1-specific DSB repair signaling in Ph+ B-cell precursor ALL cells. HDAC1,2 inhibitor and doxorubicin interfere with DISC (DNA damage-induced transcriptional silencing in cis)) or transcriptional silencing program in cis around DSB sites via chromatin remodeler-dependent and -independent mechanisms, respectively, to further impair DSB repair. HDAC1,2 inhibitor either alone or when combined with doxorubicin decreases leukemia burden in vivo in refractory Ph+ B-cell precursor ALL patient-derived xenograft mouse models. Overall, our novel mechanistic and preclinical studies together demonstrate that HDAC1,2 selective inhibition can overcome DSB repair ‘addiction’ and provide an effective therapeutic option for Ph+ B-cell precursor ALL.

The V617F mutation in the JH2 domain of Janus kinase 2 (JAK2) is an oncogenic driver in several myeloproliferative neoplasms (MPNs), including essential thrombocythemia, myelofibrosis, and polycythemia vera (PV). Other mutations in JAK2 have been identified in MPNs, most notably exon 12 mutations in PV. Here, we describe a novel recurrent mutation characterized by a common 4-amino-acid deletion and variable 1-amino-acid insertion (Leu583-Ala586DelInsSer/Gln/Pro) within the JH2 domain of JAK2. All 4 affected patients had eosinophilia, and both patients with Leu583-Ala586DelInsSer fulfilled diagnostic criteria of both PV and chronic eosinophilic leukemia (CEL). Computational and functional studies revealed that Leu583-Ala586DelInsSer (herein referred to as JAK2ex13InDel) deregulates JAK2 through a mechanism similar to JAK2V617F, activates signal transducer and activator of transcription 5 and extracellular signal-regulated kinase, and transforms parental Ba/F3 cells to growth factor independence. In contrast to JAK2V617F, JAK2ex13InDel does not require an exogenous homodimeric type 1 cytokine receptor to transform Ba/F3 cells and is capable of activating β common chain family cytokine receptor (interleukin-3 receptor [IL-3R], IL-5R, and granulocyte-macrophage colony stimulating factor receptor) signaling in the absence of ligand, with the maximum effect observed for IL-5R, consistent with the clinical phenotype of eosinophilia. Recognizing this new PV/CEL-overlap MPN has significant clinical implications, as both PV and CEL patients are at high risk for thrombosis, and concomitant cytoreduction of red cells, neutrophils, and eosinophils may be required for prevention of thromboembolic events. Targeted next-generation sequencing for genes recurrently mutated in myeloid malignancies in patients with unexplained eosinophilia may reveal additional cases of Leu583-Ala586DelInsSer/Gln/Pro, allowing for complete characterization of this unique MPN.

Rapid conversion of chronic myeloid leukemia to chronic myelomonocytic leukemia in a patient on imatinib therapy

Chronic myelomonocytic leukemia (CMML) is a hematologic malignancy nearly confined to the elderly. Previous studies to determine incidence and prognostic significance of somatic mutations in CMML have relied on candidate gene sequencing, although an unbiased mutational search has not been conducted. As many of the genes commonly mutated in CMML were recently associated with age-related clonal hematopoiesis (ARCH) and aged hematopoiesis is characterized by a myelomonocytic differentiation bias, we hypothesized that CMML and aged hematopoiesis may be closely related. We initially established the somatic mutation landscape of CMML by whole exome sequencing followed by gene-targeted validation. Genes mutated in ⩾ 10% of patients were SRSF2, TET2, ASXL1, RUNX1, SETBP1, KRAS, EZH2, CBL and NRAS, as well as the novel CMML genes FAT4, ARIH1, DNAH2 and CSMD1. Most CMML patients (71%) had mutations in ⩾ 2 ARCH genes and 52% had ⩾ 7 mutations overall. Higher mutation burden was associated with shorter survival. Age-adjusted population incidence and reported ARCH mutation rates are consistent with a model in which clinical CMML ensues when a sufficient number of stochastically acquired age-related mutations has accumulated, suggesting that CMML represents the leukemic conversion of the myelomonocytic-lineage-biased aged hematopoietic system.

Extreme mutational selectivity of axitinib limits its potential use as a targeted therapeutic for BCR-ABL1-positive leukemia

Tumor necrosis factor alpha (TNF) is increased in myelofibrosis (MF) and promotes survival of malignant over normal cells. The mechanisms altering TNF responsiveness in MF cells are unknown. We show that the proportion of marrow (BM) cells expressing TNF is increased in MF compared to controls, with the largest differential in primitive cells. Blockade of TNF receptor 2 (TNFR2), but not TNFR1, selectively inhibited colony formation by MF CD34+ and mouse JAK2V617F progenitor cells. Microarray of mouse MPN revealed reduced expression of X-linked inhibitor of apoptosis (Xiap) and mitogen-activated protein kinase 8 (Mapk8) in JAK2V617F relative to JAK2WT cells, which were normalized by TNFR2 but not TNFR1 blockade. XIAP and MAPK8 were also reduced in MF CD34+ cells compared to normal BM, and their ectopic expression induced apoptosis. Unlike XIAP, expression of cellular IAP (cIAP) protein was increased in MF CD34+ cells. Consistent with cIAP's role in NF-κB activation, TNF-induced NF-κB activity was higher in MF vs. normal BM CD34+ cells. This suggests that JAK2V617F reprograms TNF response toward survival by downregulating XIAP and MAPK8 through TNFR2. Our results reveal an unexpected pro-apoptotic role for XIAP in MF and identify TNFR2 as a key mediator of TNF-induced clonal expansion.