The Diagnostic Role of Flow Cytometry in Idiopathic Cytopenia of Unknown Significance (ICUS): A Single-Center Analysis of 79 Patients

Abstract Introduction and aims The term ICUS is used to describe cases with persistent cytopenia (more than 6 months) without evidence of dysplasia in the bone marrow (BM) smear, and normal cytogenetics. Flow cytometry is currently a standard tool for the diagnosis of myelodysplastic syndromes (1). Our goal was to evaluate the efficacy of flow cytometry in detecting bone marrow dysplasia in the absence of abnormal morphology. Materials and methods A total of 79 patients with ICUS and 12 patients with MDS were analyzed with a standardized flow cytometry panel (2). Data were analyzed using two different algorithms; the Ogata algorithm (3) and Wells algorithm (4). For each of the algorithms, samples were classified as suggestive for dysplasia when scoring equal to or higher than 2. Furthermore, all ICUS patients were screened for mutations using targeted sequencing of 20 genes (DNMT31, TET2, EZH2, SRSF2, CBL1, SF3B1, UAF1, GATA2, IDH1, IDH2, CEBPa1, ZRSR2, TP53, KRAS, NRAS, ETV6, RUNX1, JAK2 and ASXL1) and 57/71 samples were additionally evaluated for signs of dysplasia by two independent hematopathologists. Results The diagnostic value of flow cytometry in MDS was confirmed by our data; the Wells algorithm was slightly superior to Ogata algorithm in predicting MDS (10/12 patients (83.3%) vs. 7/12 patients (58.3%) respectively, fig. 1A) and was therefore the main algorithm used for the further analysis of the ICUS patients. Flow cytometry was suggestive of dysplasia in 34/79 (43%) patients with ICUS. Interestingly, in patients with at least one detected mutation, flow cytometry was positive in 23/42 (54.8%) of the cases, while it was positive in 8/9 (88.9%) patients with more than two mutations (fig. 1C). There was no higher frequency of abnormal flow cytometry in patients with higher risk mutations. Additionally, for the 57 patients evaluated for dysplastic changes, flow cytometric abnormalities were more frequent (p= 0.05) in the 22 patients with moderate/severe dysplasia (15/22 patients or 68.2%, fig. 1D). After a median follow-up of 20 months (range: 3 - 90), a total of eight patients progressed to either MDS, CMML or AML. Interestingly, these eight patients had flow cytometric abnormalities suggestive of dysplasia before developing hematological malignancy. In conclusion, flow cytometry can identify a subgroup of ICUs patients with a higher mutational burden, dysplastic changes and a higher risk for progression to MDS or a more aggressive myeloid disease and has a place in the diagnostic evaluation of patients with idiopathic cytopenia. References: Porwit A, Van De Loosdrecht AA, Bettelheim P, Eidenschink Brodersen L, Burbury K, Cremers E, et al. Revisiting guidelines for integration of flow cytometry results in the WHO classification of myelodysplastic syndromes - Proposal from the International/European LeukemiaNet Working Group for Flow Cytometry in MDS. Leukemia. 2014;28(9):1793-8. van Dongen JJM, Lhermitte L, Böttcher S, Almeida J, van der Velden VHJ, Flores-Montero J, et al. EuroFlow antibody panels for standardized n-dimensional flow cytometric immunophenotyping of normal, reactive and malignant leukocytes. Leukemia [Internet]. 2012 Sep [cited 2014 Nov 28];26(9):1908-75. Available from: http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=3437410&tool=pmcentrez&rendertype=abstract Porta MGD, Picone C, Pascutto C, Malcovati L, Tamura H, Handa H, et al. Multicenter validation of a reproducible flow cytometric score for the diagnosis of low-grade myelodysplastic syndromes: Results of a European LeukemiaNET study. Haematologica. 2012;97(8):1209-17. Wells DA, Benesch M, Loken MR, Vallejo C, Myerson D, Leisenring WM, et al. Myeloid and monocytic dyspoiesis as determined by flow cytometric scoring in myelodysplastic syndrome correlates with the IPSS and with outcome after hematopoietic stem cell transplantation. Blood. 2003;102(1):394-403. Figure 1. Figure 1. Disclosures Hansen: Otsuka Pharma: Membership on an entity's Board of Directors or advisory committees. Grønbæk:Celgene: Membership on an entity's Board of Directors or advisory committees; Janssen Pharma: Membership on an entity's Board of Directors or advisory committees; Otsuka Pharma: Membership on an entity's Board of Directors or advisory committees.

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Decreased Expression of CD62L and CD54 Correlates with Poor Cytogenetic Risk Group In Myelodysplastic Syndromes

Blood ◽

10.1182/blood.v116.21.2915.2915 ◽

2010 ◽

Vol 116 (21) ◽

pp. 2915-2915

Author(s):

Canan Alhan ◽

Theresia M. Westers ◽

Claudia Cali ◽

Floortje L. Kessler ◽

Monique Terwijn ◽

...

Keyword(s):

Bone Marrow ◽

Flow Cytometry ◽

Healthy Volunteers ◽

Board Of Directors ◽

Myelodysplastic Syndromes ◽

Research Funding ◽

Risk Category ◽

Advisory Committees ◽

Cd34 Cells

Abstract Abstract 2915 Interactions in the bone marrow (BM) between haematopoietic progenitor cells (HPC) and the BM micro environment are important for the regulation of cell adhesion, proliferation, differentiation and survival. Expression of both CD62L (L-selectin) and CD54 (ICAM-1) on HPC demonstrated to play a role in signal transduction routes for proliferation and growth regulation. Especially CD54 is involved in uncontrolled proliferation and block of apoptosis. Previously, it was described that decreased expression of CD62L in acute myeloid leukemia (AML) was associated with a poor cytogenetic risk profile and an adverse clinical outcome (Graf M et al, Eur J Haematol 2003) Myelodysplastic syndromes are a group of clonal HPC disorders characterized by ineffective hematopoiesis and a propensity to evolve into AML. The International Prognostic Scoring System (IPSS) provides information on both survival and risk of development of an AML. The purpose of our study was to evaluate CD62L and CD54 expression on CD34+ cells in MDS patients by flow cytometry and to assess the value of a CD62L/CD54 ratio for prognostication. Bone marrow samples of 30 newly diagnosed MDS patients (3 RA(RS)/18 RCMD(RS), the <5% blasts group; 5 RAEB-1, 4 RAEB-2, the >5% blasts group), 16 AML patients with prior MDS and 26 healthy volunteers were analyzed for CD62L and CD54 expression on CD34+ cells by using flow cytometry. An adhesion index was calculated as a ratio of the percentage and MFI of CD62L and CD54 positive cells (as was reported by Buccisano et al, Eur J Haematol 2007). The CD62L/CD54 ratio was significantly decreased in MDS with <5% blasts (median 79.09 p<0.0001) as compared to healthy volunteers (median 480.4) and even more decreased in high risk MDS (median 14.67 p<0.0001 and p=0.001 as compared to healthy volunteers and MDS with <5% blasts, respectively) and AML with prior MDS (median 12.54, p<0.0001 and p=0.009 as compared to healthy volunteers and MDS with <5% blasts, respectively). The MDS patients were assigned to the good, intermediate or poor IPSS cytogenetic risk category. Cytogenetics was available for 22 MDS patients. The CD62L/CD54 ratio was significantly lower in the cytogenetic poor risk category compared with the good risk category (median 5.4 and median 70.79 respectively, p=0.018). Moreover, a low CD62L/CD54 ratio correlated significantly with poor cytogenetics, p=0.006. In the group of MDS patients with <5% blasts, 4 developed a refractory anemia with excess of blasts or AML within a follow up period of 12 months. There was a trend for a lower CD62L/CD54 ratio for MDS patients who developed an AML compared with patients who did not. In conclusion, the CD62L/CD54 ratio is significantly decreased in MDS compared with healthy volunteers and even more decreased in AML with prior MDS. Both CD62L and CD54 are involved in regulation of proliferation and apoptosis of the HPC. A decreased adhesion ratio in low risk MDS patients might reflect HPC damage at an early stage of the disease with an increased proliferative capacity and a decreased apoptotic profile. Interestingly, a low CD62L/CD54 ratio showed a significant inverse correlation with the IPSS cytogenetic risk category. Due to an absence of metaphases in a proportion of MDS patients, cytogenetics is not always available. The CD62L/CD54 ratio might serve as a surrogate marker for poor prognosis cytogenetics in case no karyotype is available. Low risk MDS patients who developed an AML within 12 months tended to have a lower CD62L/CD54 ratio. Although these results are promising, sample size and follow up period needs to be extended. The CD62L/CD54 ratio might add to prognostication of MDS patients and might identify MDS patients with <5% blasts who are at risk for development of an AML. Disclosures: Ossenkoppele: Celgene: Membership on an entity's Board of Directors or advisory committees, Research Funding. Van de Loosdrecht:Celgene: Membership on an entity's Board of Directors or advisory committees, Research Funding.

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Inducible SBDS Deficiency Impairs Bone Marrow Niche Function to Engraft Donor Hematopoietic Stem Cell after Transplantation

Blood ◽

10.1182/blood-2019-121763 ◽

2019 ◽

Vol 134 (Supplement_1) ◽

pp. 3199-3199

Author(s):

Ji Zha ◽

Lori Kunselman ◽

Hongbo Michael Xie ◽

Brian Ennis ◽

Jian-Meng Fan ◽

...

Keyword(s):

Bone Marrow ◽

Stem Cell ◽

Mouse Model ◽

Board Of Directors ◽

Hematopoietic Stem Cell ◽

Graft Failure ◽

Advisory Committees ◽

Hematopoietic Stem ◽

Bm Niche ◽

Deficient Mice

Hematopoietic stem cell (HSC) transplantation (HSCT) is required for curative therapy for patients with high-risk hematologic malignancies, and a number of non-malignant disorders including inherited bone marrow failure syndromes (iBMFS). Strategies to enhance bone marrow (BM) niche capacity to engraft donor HSC have the potential to improve HSCT outcome by decreasing graft failure rates and enabling reduction in conditioning intensity and regimen-associated complications. Several studies in animal models of iBMFS have demonstrated that BM niche dysfunction contributes to both the pathogenesis of iBMFS, as well as impaired graft function after HSCT. We hypothesize that such iBMFS mouse models are useful tools for discovering targetable niche elements critical for donor engraftment after HSCT. Here, we report the development of a novel mouse model of Shwachman-Diamond Syndrome (SDS) driven by conditional Sbds deletion, which demonstrates profound impairment of healthy donor hematopoietic engraftment after HSCT due to pathway-specific dysfunctional signaling within SBDS-deficient recipient niches. We first attempted to delete Sbds specifically in mature osteoblasts by crossing Sbdsfl/flmice with Col1a1Cre+mice. However, the Col1a1CreSbdsExc progenies are embryonic lethal at E12-E15 stage due to developmental musculoskeletal abnormalities. Alternatively, we generated an inducible SDS mouse model by crossing Sbdsfl/flmice with Mx1Cre+ mice, and inducing Sbds deletion in Mx1-inducible BM hematopoietic and osteolineage niche cells by polyinosinic-polycytidilic acid (pIpC) administration. Compared with Sbdsfl/flcontrols, Mx1CreSbdsExc mice develop significantly decreased platelet counts, an inverted peripheral blood myeloid/lymphoid cell ratio, and reduced long-term HSC within BM, consistent with stress hematopoiesis seen in BMF and myelodysplastic syndromes. To assess whether inducible SBDS deficiency impacts niche function to engraft donor HSC, we transplanted GFP+ wildtype donor BM into pIpC-treated Mx1CreSbdsExc mice and Sbdsfl/flcontrols after 1100 cGy of total body irradiation (TBI). Following transplantation, Mx1CreSbdsExc recipient mice exhibit significantly higher mortality than controls (Figure 1). The decreased survival was related to primary graft failure, as Mx1CreSbdsExc mice exhibit persistent BM aplasia after HSCT and decreased GFP+ reconstitution in competitive secondary transplantation assays. We next sought to identify the molecular and cellular defects within BM niche cells that contribute to the engraftment deficits in SBDS-deficient mice. We performed RNA-seq analysis on the BM stromal cells from irradiated Mx1CreSbdsExc mice versus controls, and the results revealed that SBDS deficiency in BM niche cells caused disrupted gene expression within osteoclast differentiation, FcγR-mediated phagocytosis, and VEGF signaling pathways. Multiplex ELISA assays showed that the BM niche of irradiated Mx1CreSbdsExc mice expresses lower levels of CXCL12, P-selectin and IGF-1, along with higher levels of G-CSF, CCL3, osteopontin and CCL9 than controls. Together, these results suggest that poor donor HSC engraftment in SBDS-deficient mice is likely caused by alterations in niche-mediated donor HSC homing/retention, bone metabolism, host monocyte survival, signaling within IGF-1 and VEGF pathways, and an increased inflammatory state within BM niches. Moreover, flow cytometry analysis showed that compared to controls, the BM niche of irradiated Mx1CreSbdsExc mice contained far fewer megakaryocytes, a hematopoietic cell component of BM niches that we previously demonstrated to be critical in promoting osteoblastic niche expansion and donor HSC engraftment. Taken together, our data demonstrated that SBDS deficiency in BM niches results in reduced capacity to engraft donor HSC. We have identified multiple molecular and cellular defects in the SBDS-deficient niche contributing to this phenotype. Such niche signaling pathway-specific deficits implicate these pathways as critical for donor engraftment during HSCT, and suggest their potential role as targets of therapeutic approaches to enhance donor engraftment and improve HSCT outcome in any condition for which HSCT is required for cure. Disclosures Olson: Merck: Membership on an entity's Board of Directors or advisory committees; Bluebird Bio: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees; Miltenyi: Honoraria.

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Diagnostic Utility of Flow Cytometry in Myelodysplastic Syndrome: A Retrospective Validation From a Single Centre.

Blood ◽

10.1182/blood.v114.22.4846.4846 ◽

2009 ◽

Vol 114 (22) ◽

pp. 4846-4846

Author(s):

Pervin Topcuoglu ◽

Klara Dalva ◽

Sule Mine Bakanay ◽

Sinem Civriz Bozdag ◽

Onder Arslan ◽

...

Keyword(s):

Bone Marrow ◽

Flow Cytometry ◽

Myelodysplastic Syndrome ◽

Antigen Expression ◽

Pathological Examination ◽

Light Scatter ◽

Myeloid Lineage ◽

Hematopoietic Stem ◽

Altered Expression ◽

Flow Cytometric

Abstract Abstract 4846 Myelodysplastic syndrome (MDS) is heterogeneous clonal hematopoietic stem cell disorder characterized by cytopenia(s) and dysplasia in one or more cell lineage. Though flow cytometry (FCM) is an important diagnostic tool in hematopoietic cell disorders, a prominent immunophenotyping feature in MDS may not be determined. In this study, we retrospectively evaluated flow cytometric features of bone marrow samples diagnosed as MDS with clinical and hematological findings. Patients-Method Between Feb 2004 and March 2009, flow cytometric parameters of 73 patients (M/F: 50/23) with MDS were re-analyzed. Median age was 59 years (17-89 ys). Our general principles are to evaluate quality of bone marrow samples, to determine proportion of the cells and features of their light scatter, and to give percentage of the blast. When detected a finding of dysplasia in the first analysis, the second step includes the determination of the maturation of the cells and the presence of the aberrant antigen expression. Results The samples were interpreted as MDS in % 76.7, MDS-RAEB-1 or RAEB-2 in %16.4, myeloproliferative disorder in %1.4 and non-diagnostic in %6.8 of the cases by flow cytometric examination. We detected variable degrees of hypogranulation in myeloid lineage in %82.2 of the samples by the light scatter features of the cells: 85% of severe and 15% of moderate or mild hypogranulation. The ratio of myeloid and lymphoid was changing from 0.3 to 17.5 (median 2). The decreasing of this ratio (<1) was observed in 19.4% of the samples. We detected altered expression of mature granulocyte. These included decreasing or lack of expression in CD15 45/73 (61.4%), CD13 38/70 (54.3%), CD16 53/67 (79.1%), CD11b 51/71 (71.8%), CD24 44/69 (65.2%), CD10 23/72 (31.9%) and MPO 14/72 (19.4%). Besides, bright expression of CD33 in 53.5% of the samples was observed. CD36 and CD56 in myeloid lineage were co-expressed in about 50 % of the samples. In 80.8 % of the samples dysplasia in erythroid compartment could be evaluated: Expression of CD71 according to glycophorin A (ratio <1) was decreased in 23.7 %. When we made similar analysis in the samples without RAEB-1 and -2 as pathological examination of bone marrow, 13.4 % of the samples could not be evaluated in favor of dysplasia. Of the samples with dysplasia hypogranulation, aberrant antigen expression of myeloid lineage and eryhtroid dysplasia were observed in 92.1%, 34.1% and 31.5%, respectively. In conclusion, FCM events may help to the differantial diagnosis of MDS especially when combining with clinical events. Improving of the analysis by focusing on the blast characteristics may be a standard approach to evaluate for low risk MDS. Disclosures No relevant conflicts of interest to declare.

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CLL2007FMP, a Phase III Randomized Multicentric Trial of the French Cooperative Group On CLL and WM (FCGCLL/MW) and the “Groupe Ouest-Est D'etudes Des Leucémies Aigües Et Autres Maladies Du Sang” (GOELAMS): Immunochemotherapy with Fludarabine (F), Cyclophosphamide (C), and Rituximab (R) (FCR) Yields a Significantly Better Response Than Fludarabine (F), Cyclophosphamide (C) and MabCampath (Cam) (FCCam) In Previously Untreated B-Chronic Lymphocytic Leukemia Patients as Evaluated by a Sensitive 6 Color Flow Cytometry MRD

Blood ◽

10.1182/blood.v116.21.698.698 ◽

2010 ◽

Vol 116 (21) ◽

pp. 698-698 ◽

Cited By ~ 2

Author(s):

Letestu Remi ◽

Stéphane Leprêtre ◽

Arnoulet Christine ◽

Baseggio Lucille ◽

Campos Lydia ◽

...

Keyword(s):

Bone Marrow ◽

Flow Cytometry ◽

Board Of Directors ◽

Residual Disease ◽

Phase Iii ◽

Advisory Committees ◽

Color Flow ◽

Phase Iii Trial ◽

Mutational Status

Abstract Abstract 698 Introduction: Between 11/2007 and 01/2009, the FCGCLL/MW and the GOELAMS conducted a multicenter phase III trial, CLL2007FMP, to evaluate the efficacy of FCCam versus FCR in previously untreated medically fit patients. PFS was the primary-end-point of this trial. The trial was discontinued after randomisation of 165 patients for unacceptable toxicity in the FCCam arm. PFS and OS are not yet evaluable but as a sensitive 6 color flow cytometry technique was used to assess MRD in blood and bone marrow at month 9, we were able to evaluate the quality of the response in both arms. Methods and patients: A cohort of 178 medically fit patients (cumulative illness rating scale (CIRS) score < or = 6 and creatinine clearance ≥ 60 ml/min), younger than 65 years old, were enrolled. 165 patients were randomized to receive six oral courses of FC (F 40mg/m2 d1-3 and C 250 mg/m2 d1–3; q 28 days) in combination with either R (n=83; 375 mg/m2 i.v. d 0 at first cycle and 500 mg/m2 d1 all subsequent cycles; q 28 days) or Cam (n=82; 30 mg s/c d1-3; q 28 days). Patients were stratified according to IGHV mutational status and presence of 11q deletion. Cases with 17p deletion were excluded. The trial recruitment was discontinued because of an excess of mortality in the FCCam arm (6 deaths versus 0 in FCR arm), and the last 13 patients enrolled were not randomized. Clinical response was evaluated based on IWCLL criteria. We established a sensitive and specific approach for the evaluation of minimal residual disease (MRD) at month 9, using a 6-color flow cytometry technique with 3 combinations including characteristic markers and light chain expression. We determined the limit of detection (LOD) of the assay by studying normal blood samples, LOD varied from 0.5 to 0.7×10-5 depending on the combination considered. Result: The Overall Response Rate (ORR) was 91% in the FCR arm and 85% in the FCCam arm (ns). Clinical responses were as follows: CR (FCR: 56/80=70%, FCCam: 45/79 =59%, ns), CR I (FCR:13/FCCam: 11), PR (FCR:5/FCCam:15), and stable and progressive cases (FCR: 6/FCCam:8). When considering together CR and CR I, response rate appeared significantly higher in the FCR arm (86%) than in FCCam arm (70%) (p=0.03). MRD was assessed both in blood and bone marrow at month 9, and was undetectable in 58% patients in blood and only in 36 % in bone marrow. No patient had an undetectable MRD in marrow when detectable in blood. Similarly, MRD was detectable in bone marrow in 15 cases with histologically normal bone marrow biopsy, whereas no nodal PR had undetectable MRD. Therefore, flow cytometry MRD in bone marrow appears as the most sensitive technique for the evaluation of response. Of note, 9 patients had a very good PR with presence of a residual lymphnode, and had undetectable blood and bone marrow MRD (4 in FCCAm arm and 5 in FCR arm). When considering MRD independently from clinical response, the number of MRD negative cases was not significantly different between the two arms (FCR: 45%/FCCam: 26 %) arm. But when combining MRD negativity with clinical complete response, the number of MRD negative CR was significantly higher with FCR (40%) than with FCCam (15%)(p= 0.029). The number of courses of chemotherapy received was slightly but significantly lower in the FCCam arm. Nonetheless, this difference was not accountable for the difference in the quality of response as the number of MRD negative CR remained significantly higher with FCR than with FCCam when considering only the patients having received at least 4 courses of either chemotherapy. The quality of response was not influenced by either presence of deletion11q or mutational status as well. In conclusion, detection of MRD in bone marrow by flow cytometry is a sensitive technique for the evaluation of minimal residual disease. Combining clinical CR with flow cytometry bone marrow MRD allows detecting the best responders. In this randomized phase III trial, besides leading to a lower rate of toxicity, the FCR regimen yielded a significantly higher rate of MRD negative CR than FCCam, and therefore had a positive impact on the quality of the response. Disclosures: Veronique: roche: Consultancy, Membership on an entity's Board of Directors or advisory committees; mundipharma: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; genzyme: Membership on an entity's Board of Directors or advisory committees; celgene: Membership on an entity's Board of Directors or advisory committees; jansen: Membership on an entity's Board of Directors or advisory committees.

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Deregulation of TNFRSF18 (GITR) Through Promoter CpG Island Methylation Induces Tumor Proliferation in Multiple Myeloma

Blood ◽

10.1182/blood.v118.21.2424.2424 ◽

2011 ◽

Vol 118 (21) ◽

pp. 2424-2424

Author(s):

Yang Liu ◽

Yong Zhang ◽

Phong Quang ◽

Hai T Ngo ◽

Feda Azab ◽

...

Keyword(s):

Gene Expression ◽

Bone Marrow ◽

Flow Cytometry ◽

Board Of Directors ◽

Research Funding ◽

Plasma Cells ◽

Cpg Island ◽

Advisory Committees ◽

Brdu Assay

Abstract Abstract 2424 Introduction Tumor necrosis factor receptor super families (TNFRSFs) play an important role in activation of lymphocyte and cell apoptosis. However the function of TNFRSFs in multiple myeloma (MM) remains unknown. Loss of function mutation of Fas antigen (TNFRSF6) was identified in MM cells, thus suggesting the possible role of TNFRSFs in regulating MM pathogenesis. We therefore investigated the epigenetic mechanisms that may mediate inactivation of TNFRSFs and its functional role in MM. Methods Dchip software was utilized for analyzing gene expression dataset. DNA was extracted from both primary CD138+ MM plasma cells and MM cell lines using blood & tissue DNA isolation kit (Qiagen, Inc.). Expression of GITR in primary CD138+ plasma cells was detected by Imunohistochemistry (IHC) DNA methylation was analyzed by methylated DNA immunoprecipitation (Medip) assay and bisulfate sequencing. 5'azacytidine was used to demethylate genomic DNA. Gene expression was detected by qRT-PCR and confirmed at the protein level by flow cytometry and western-blot. Over-expression of GITR was obtained in MM1.S cells by using GITR recombinant plasmid and electroporation. Apoptosis was determined using Annexin/PI staining and flow cytometry analysis. Activation of apoptotic signaling was studied by western blot. Cell survival and proliferation were analyzed by MTT and BrdU assay, respectively. Recombinant GITR-lentivirus was obtained from the supernatant of culture medium after 72 hours transfection in 293 cells. GFP positive MM cells were sorted and analyzed by flow cytometry. In vivo effect of GITR on MM tumor growth was determined by injection of GITR over-expressing MM cells in null mice. Mice skull, femur and vertebrae were isolated after 4 weeks injection. Anti-human CD138+ mAb microbead was used to detect MM cells extracted from mice tissue by flow cytometry. Results Gene-expression profiling showed down-regulation of TNFRSFs, including TNFRSF11A, TNFRSF11B, TNFRSF8, TNFRSF10C, TNFRSF9, TNFRSF21, TNFRSF1B, TNFRSF1A and TNFRSF18, compared to normal plasma cells. Moreover, Our IHC results also showed that GITR expression was positive in primary CD138+ plasma cells from 9 normal bone marrow, but negative in 9 MM samples. Importantly, we found that low GITR expression significantly correlated with MM progression. Indeed, GITR gene levels were lower in smoldering and active MM patients compared to MGUS patients and normal donors. Promoter CpG island (CGI) methylation of GITR was indentified in 5 out of 7 MM primary bone marrow (BM)-derived CD138+ cells but not in normal BM-derived plasma cells. Bisulfate sequencing and Medip assay showed that methylation of GITR was significantly associated with GITR expression in 5 MM cell lines, including MM1.S, OPM1, U266, RPMI and INA6. Promoter CGI of GITR was highly methylated leading to complete silencing of GITR in MM1.S cell line. GITR expression was significantly up-regulated in MM cells upon treatment with the 5'azacytidine. MTT and BrdU assay revealed that the proliferation and survival of MM1.S cells was disrupted in the GITR over-expressing MM1.S cells, notably with inhibition of cell proliferation compared to control vector infected cells. Moreover induction of cytotoxicity in GITR over-expressing cells was confirmed by using GFP competition assay. GITR-induced apoptosis was supported by induction of caspase 8 and 3 cleavage. The inhibition of human CD138+ plasma cell growth in the bone marrow of SCID mice using a disseminated MM xenograft model was observed in the experimental group injected with GITR expressing cells compared to the control group after 4 weeks injection. Conclusion Our findings uncovered a novel epigenetic mechanism contributing to MM pathogenesis, showing the role of GITR methylation as a key regulator of MM cell survival. Disclosures: Roccaro: Roche:. Ghobrial:Novartis: Membership on an entity's Board of Directors or advisory committees; Celgene: Membership on an entity's Board of Directors or advisory committees; Millennium: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Bristol-Myers Squibb: Research Funding; Noxxon: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding.

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Screening Patients with Myelodysplastic Syndrome and Aplastic Anemia for Paroxysmal Nocturnal Hemoglobinuria Clones: A Retrospective Study,

Blood ◽

10.1182/blood.v118.21.3426.3426 ◽

2011 ◽

Vol 118 (21) ◽

pp. 3426-3426 ◽

Cited By ~ 1

Author(s):

Andrew Shih ◽

Ian H. Chin-Yee ◽

Ben Hedley ◽

Mike Keeney ◽

Richard A. Wells ◽

...

Keyword(s):

Bone Marrow ◽

Retrospective Study ◽

Aplastic Anemia ◽

Board Of Directors ◽

Paroxysmal Nocturnal Hemoglobinuria ◽

Bone Marrow Failure ◽

Cell Surface Expression ◽

Advisory Committees ◽

Hematopoietic Stem ◽

Pnh Clone

Abstract Abstract 3426 Introduction: Paroxysmal Nocturnal Hemoglobinuria (PNH) is a rare disorder due to a somatic mutation in the hematopoietic stem cell. The introduction of highly sensitive flow cytometric and aerolysin testing have shown the presence of PNH clones in patients with a variety of other hematological disorders such as aplastic anemia (AA) and myelodysplasic syndrome (MDS). It is hypothesized that patients with these disorders and PNH clones may share an immunologic basis for marrow failure with relative protection of the PNH clone, due to their lack of cell surface expression of immune accessory proteins. This is supported by the literature showing responsiveness in AA and MDS to immunosuppressive treatments. Preliminary results from a recent multicenter trial, EXPLORE, notes that PNH clones can be seen in 70% of AA and 55% of MDS patients, and therefore there may be utility in the general screening of all patients with bone marrow failure (BMF) syndromes. Furthermore, it has been suggested that the presence of PNH cells in MDS is a predictive biomarker that is clinically important for response to immunosuppressive therapy. Methods: Our retrospective cohort study in a tertiary care center used a high sensitivity RBC and FLAER assay to detect PNH clones as small as 0.01%. Of all patients screened with this method, those with bone marrow biopsy and aspirate proven MDS, AA, or other BMF syndromes (defined as unexplained cytopenias) were analysed. Results from PNH assays were compared to other clinical and laboratory parameters such as LDH. Results: Overall, 102 patients were initially screened over a 12 month period at our center. 30 patients were excluded as they did not have biopsy or aspirate proven MDS, AA, or other BMF syndromes. Of the remaining 72 patients, four patients were found to have PNH clones, where 2/51 had MDS (both RCMD, IPSS 0) [3.92%] and 2/4 had AA [50%]. The PNH clone sizes of these four patients were 0.01%, 0.01%, 0.02%, and 1.7%. None of the MDS patients with known recurrent karyotypic abnormalities had PNH clones present. Only one of the four patients had a markedly increased serum LDH level. Conclusions: Our retrospective study indicates much lower incidence of PNH clones in MDS patients or any patients with BMF syndromes when compared to the preliminary data from the EXPLORE trial. There is also significant disagreement in other smaller cohorts in regards to the incidence of PNH in AA and MDS. Screening for PNH clones in patients with bone marrow failure needs further study before adoption of widespread use. Disclosures: Keeney: Alexion Pharmaceuticals Canada Inc.: Consultancy, Membership on an entity's Board of Directors or advisory committees. Wells:Alexion Pharmaceuticals Canada Inc: Honoraria. Sutherland:Alexion Pharmaceuticals Canada Inc.: Consultancy, Membership on an entity's Board of Directors or advisory committees.

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Epoxyeicosatrienoic Acids Regulate Hematopoietic Stem/Progenitor Cell Fate Decision During Stress Response and Embryonic Hematopoiesis

Blood ◽

10.1182/blood.v118.21.860.860 ◽

2011 ◽

Vol 118 (21) ◽

pp. 860-860

Author(s):

Pulin Li ◽

Emily K Pugach ◽

Elizabeth B Riley ◽

Dipak Panigrahy ◽

Garrett C Heffner ◽

...

Keyword(s):

Bone Marrow ◽

Board Of Directors ◽

Marrow Transplantation ◽

Mouse Bone Marrow ◽

Zebrafish Embryos ◽

Advisory Committees ◽

Hematopoietic Stem ◽

Equity Ownership ◽

Runx1 Expression

Abstract Abstract 860 During bone marrow transplantation, hematopoietic stem/progenitor cells (HSPCs) are exposed to various stress signals, and undergo homing, rapid proliferation and differentiation in order to achieve engraftment. To explore how fate decisions are made under such stress conditions, we developed a novel imaging-based competitive marrow transplantation in zebrafish. The feasibility of handling hundreds of zebrafish for transplantation per day allowed us to screen a library of 480 small molecules with known bioactivity, aimed at identifying new drugs and pathways regulating HSPC engraftment. Two structurally related eicosanoids, 11,12-epoxyeicosatrienoic acid (EET) and 14,15-EET, were able to enhance GFP+ marrow engraftment compared to DsRed2+ engraftment in zebrafish. This remarkable effect of EETs on adult marrow prompted us to study the effect of EETs in embryonic hematopoiesis. Treating zebrafish embryos with 11,12-EET during definitive hematopoiesis increased the HSPC marker Runx1 expression in the AGM (Aorta-Gonad-Mesonephros), resulting in a significant increase of HSPC in the next hematopoietic site, caudal hematopoietic tissue, the equivalent of fetal liver/placenta in mammals. The same treatment condition also induced ectopic Runx1 expression in the tail mesenchyme, a non-hematopoietic tissue. Microarray analysis on EET-treated zebrafish embryos revealed an upregulation of genes involved in stress response, especially Activator Protein 1 (AP-1) family members. Genetic knockdown experiments confirmed AP-1 members, especially JunB and its binding partners, cFos and Fosl2, are required for Runx1 induction. Motif analysis also predicted several conserved AP-1 binding sites in the Runx1 enhancer regions. To understand how EETs induced AP-1 expression, a suppressor screen was performed in zebrafish embryos. The screen revealed that activation of both PI3K/Akt and Stat3 are required for induced AP-1 expression, and therefore Runx1 upregulation. Similarly, ex vivo treatment of mouse whole bone marrow with 11,12-EET resulted in a 2-fold increase of long-term repopulating units. Microarray data had previously shown that Cyp2j6, one of the cytochrome P450 enzymes involved in EET biosynthesis from arachidonic acid, is enriched in quiescent mouse long-term HSCs. To further increase the EET levels in HSPCs, human CYP2C8 enzyme was over-expressed in transgenic mice using the Tie2 promoter. These transgenic mice have a 4-fold increase of long-term multi-lineage repopulating unit compared to their wild-type siblings. In purified mouse HSPCs, EETs directly and cell-autonomously activate PI3K/AKT pathway. Co-treatment of mouse bone marrow with EET and a PI3K inhibitor, LY294,002, completely blocked EET-induced enhancement of mouse bone marrow engraftment. In conclusion, we performed the first competitive marrow transplantation-based chemical screen, leading to the discovery of arachidonic acid-cytochrome P450-EETs as a novel modulator of HSC cell fate decision. PI3K/Akt and Stat3 pathways activated by EETs are required for adult HSPC engraftment and/or embryonic HSC specification, partially through transcriptional regulation of AP-1. We also demonstrated the requirement of AP-1 family members for Runx1 expression during embryonic development. This discovery may have clinical application in marrow or cord blood transplantation. Disclosures: Daley: iPierian, Inc: Equity Ownership, Membership on an entity's Board of Directors or advisory committees; Epizyme, Inc: Equity Ownership, Membership on an entity's Board of Directors or advisory committees; Verastem, Inc: Equity Ownership, Membership on an entity's Board of Directors or advisory committees; Solasia, KK: Equity Ownership, Membership on an entity's Board of Directors or advisory committees; MPM Capital, Inc: Consultancy, Membership on an entity's Board of Directors or advisory committees; Johnson & Johnson: Membership on an entity's Board of Directors or advisory committees. Zon:Fate Therapeutics: Founder; Stemgent: Consultancy.

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Mutational Studies Using Next Generation Sequencing in High Risk Myelodysplastic Syndromes and Secondary Acute Myeloid Leukemia Patients Treated with Azacitidine (High risk MDS 2009 protocol from CETLAM Group)

Blood ◽

10.1182/blood.v126.23.2905.2905 ◽

2015 ◽

Vol 126 (23) ◽

pp. 2905-2905

Author(s):

Marta Cabezon ◽

Joan Bargay ◽

Blanca Xicoy ◽

Laura Palomo ◽

Sílvia Marcé ◽

...

Keyword(s):

Acute Myeloid Leukemia ◽

High Risk ◽

Treatment Response ◽

Board Of Directors ◽

Myelodysplastic Syndromes ◽

Myeloid Leukemia ◽

Advisory Committees ◽

Hematopoietic Stem ◽

Increased Risk ◽

Acute Myeloid

Abstract INTRODUCTION: Myelodysplastic syndromes (MDS) are a group of myeloid neoplasms originated in hematopoietic stem cells, characterized by citopenias, dysplasia in one or more cell lines, ineffective hematopoiesis and an increased risk of progression to acute myeloid leukemia (AML). Treatment of MDS depends on subtype and prognostic category. DNA methyltranferase inhibitors are approved for high risk MDS. Over the past decade, the application of new high-throughput technologies to the study of MDS has led to the identification of several recurrently mutated genes. These include genes producing proteins involved in RNA splicing, DNA methylation, chromatin modification, transcription, DNA repair control, cohesin function, RAS pathway, and DNA replication. There is a significant overlap between the genes mutated commonly in MDS with those found in AML. Mutation status is not widely used to select treatment in MDS. The aim of this study is to define the mutational status of MDS and secondary AML (sAML) patients at diagnosis that have been treated with azacitidine (AZA) to see if it could help to discriminate which patients will respond from those who will not. MATERIAL AND METHODS: A prospective study was performed on 36 patients with MDS and sAML treated with AZA. Genomic DNA was obtained from bone marrow at diagnosis. SeqCap EZ and KAPA Library Preparation Kit (Roche) reagents have been used to enrich DNA of 83 genes implicated in myeloid neoplasm. The customized panel has been analyzed in MiSeq Illumina platform with 150bp paired-end reads. Samples were preliminary analyzed using Illumina MiSeq Reporter and Variant Studio softwares. Data from response to treatment and survival have been collected from all patients. RESULTS:The mean depth of the targeted resequencing per base was 685-fold. After filtering all the variations obtained for quality, biological consequence and discard the known SNPs, we have obtained 162 variations, including 145 single nucleotide variants (SNV) and 17 insertions/deletions. All patients harbored at least 1 alteration with a mean of 4.5 variants per sample. The average of alterations detected in each cytological category can be observed in Table 1.Table 1.Average abnormalities detected by cytological category.Nº patientsAverage of alterations detected for patient (range)sAML104,8 (1-8)RAEB-274,9 (2-8)RAEB-1123,7 (1-6)RCDM54,4 (3-7)RCDM-RS16RARs11The most frequent altered genes have been TP53, TET2 and DNMT3A. The numbers of variations detected for each gene are represented in Table 2.Complete results, including correlation with treatment response will be presented in the meeting.Table 2.Number of variations in each gene.GeneNº of variations foundNº of diferent variationsNº of patients with variationsFrequency of variationsTP5322191952,8%TET214101027,8%DNMT3A88822,2%CREBBP75719,4%SRSF271719,4%ASXL165616,7%U2AF162616,7%EP30053513,9%STAG255513,9%CUX144411,1%ETV643411,1%MLL (KMT2A)43411,1%RUNX14438,3%BCOR3338,3%CDH133338,3%CTNNA13238,3%EZH23338,3%GCAT3338,3%MLL2 (KMT2D)3338,3%NF13338,3%PDGFRB3338,3%SH2B33338,3%TGM23238,3%UMODL13338,3%CEBPA2125,6%CSF3R2225,6%GATA22125,6%PHLPP12225,6%RAD212225,6%SF3B12125,6%SUZ122225,6%TIMM502125,6%Others*1112,8%*ABL1, BCORL1, CALR, CDH3, IDH2, KRAS, LUC7L2, NPM1, NRAS, PHF6, SF3A1, SFPQ, SMC3, TERT, WT1, ZRSR2. CONCLUSIONS: Targeted deep-sequencing technique is a good tool to study mutational profile in MDS and sAML. SNV are the most frequent type of alteration found in our cohort. The patients with sAML and RAEB-2 present more variations than patients with RAEB-1. The rest of groups are less representing to be evaluated. The most affected genes match with those described in the literature, with some exceptions that need to be studied in more detail. We expect to predict in advance which patients are going to respond when we study the correlation of mutational analysis with treatment response. Acknowledgments: Instituto de Salud Carlos III, Ministerio de Sanidad y Consumo, Spain (PI 11/02519); 2014 SGR225 (GRE) Generalitat de Catalunya; Fundació Josep Carreras, Obra Social "La Caixa" and Celgene Spain. Diana Domínguez for her technical assistance Disclosures Valcarcel: Amgen: Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; GSK: Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Novartis: Honoraria, Speakers Bureau; Celgene: Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau.

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RON Kinase Is a Novel Therapeutic Target for Philadelphia-Negative Myeloproliferative Neoplasms

Blood ◽

10.1182/blood-2021-148192 ◽

2021 ◽

Vol 138 (Supplement 1) ◽

pp. 1462-1462

Author(s):

Lindsay Meg Gurska ◽

Rachel Okabe ◽

Meng Maxine Tong ◽

Daniel Choi ◽

Kristina Ames ◽

...

Keyword(s):

Bone Marrow ◽

Board Of Directors ◽

Research Funding ◽

Myeloproliferative Neoplasms ◽

Marrow Transplant ◽

Jak Inhibitor ◽

Advisory Committees ◽

Hematopoietic Stem ◽

Stat Signaling ◽

Genetic Deletion

Abstract The Philadelphia-chromosome negative myeloproliferative neoplasms (MPNs), including polycythemia vera (PV), essential thrombocytosis (ET), and primary myelofibrosis (PMF), are clonal hematopoietic stem cell disorders characterized by the proliferation of one or more myeloid lineage compartments. Activation of JAK/STAT signaling is a major driver of all Ph-negative MPNs. During disease progression, MPN patients experience increased pro-inflammatory cytokine secretion, leading to remodeling of the bone marrow microenvironment and subsequent fibrosis. The JAK inhibitor ruxolitinib is an approved targeted therapy for MPN patients and has shown promise in its ability to reduce splenomegaly and the cytokine storm observed in patients. However, JAK inhibitors alone are not sufficient to reduce bone marrow fibrosis or to eliminate the JAK2-mutated clone. Furthermore, JAK inhibitor persistence, or reactivation of JAK/STAT signaling upon chronic JAK inhibitor treatment, has been observed in both MPN mouse models and MPN patients. Therefore, there is an urgent need for new treatment options in MPN. The tyrosine kinase RON, a member of the MET kinase family, has well-characterized roles in erythroblast proliferation and pro-inflammatory cytokine production. RON can be phosphorylated by JAK2 to stimulate erythroblast proliferation. However, the role of RON in MPN pathogenesis is unknown. We found that the ALK/MET/RON/ROS1 inhibitor crizotinib inhibited colony formation by MPN patient CD34+ cells, regardless of their disease subtype, mutation status, or JAK2 inhibitor treatment history (Figure 1A). To determine whether this is due to inhibition of the JAK/STAT signaling pathway, we performed phospho-flow cytometry of STAT3 and STAT5 in myelofibrosis patient erythroblasts treated with crizotinib ex vivo as well as Western blot analysis in the JAK2-mutated cell lines SET2 and HEL. We found that crizotinib inhibits the phosphorylation of JAK2, STAT3, and STAT5 (Figure 1B). Since crizotinib has not been reported to directly inhibit JAK2, we asked whether these effects of crizotinib in MPN cells could be explained by RON inhibition. Consistent with this hypothesis, we observed that shRNA knockdown of multiple RON isoforms also decreases the phosphorylation of JAK2, STAT5, and STAT3 in HEL cells (Figure 1C-D). To determine whether crizotinib can alter the MPN disease course in vivo, we tested crizotinib by oral gavage in the MPLW515L bone marrow transplant murine model of myelofibrosis at 100mg/kg daily for 2 weeks. We showed that crizotinib decreased the disease burden of MPL-W515L mice, as evidenced by decreased spleen and liver weights (Figure 1E). To determine the effects of RON genetic deletion on MPN pathogenesis, we tested whether genetic deletion of Stk (mouse gene for RON) impairs disease progression in the JAK2V617F bone marrow transplant MPN model by transplanting Stk-/- c-Kit+ bone marrow cells transduced with the JAK2V617F-GFP retrovirus into lethally irradiated recipients. We observed a significant delay in disease onset in Stk-/- transplant recipients compared to WT controls (Figure 1F). However, we found that Stk-/- mice have normal numbers of hematopoietic stem and progenitor cells, and normal bone marrow myeloid colony forming capacity, suggesting that RON is a safe therapeutic target. To determine whether RON plays a role in the JAK inhibitor persistence phenotype, we generated persistent cells by treating SET2 cells with increasing doses of ruxolitinib over 8 weeks, and confirmed persistent proliferation and JAK/STAT activation. Interestingly, we found that RON phosphorylation is enhanced in JAK inhibitor persistent cells, and that dual inhibition of RON and JAK2 overcomes JAK inhibitor persistence in SET2 cells (Figure 1G-H), suggesting that RON may potentiate the JAK2 persistence phenotype in response to ruxolitinib. Importantly, we showed by immunoprecipitation that phospho-RON and phospho-JAK2 physically interact in JAK inhibitor persistent SET2 cells, and that this interaction is disrupted by crizotinib (Figure 1I). In summary, our data demonstrate that RON kinase is a novel mediator of JAK/STAT signaling in MPNs, and that it plays a particularly important role in JAK inhibitor persistence. Our work suggests that therapeutic strategies to inhibit RON, such as crizotinib, should be investigated in MPN patients. Figure 1 Figure 1. Disclosures Halmos: Guardant Health: Membership on an entity's Board of Directors or advisory committees; Apollomics: Membership on an entity's Board of Directors or advisory committees; TPT: Membership on an entity's Board of Directors or advisory committees; Eli-Lilly: Research Funding; Advaxis: Research Funding; Blueprint: Research Funding; Elevation: Research Funding; Mirati: Research Funding; Pfizer: Membership on an entity's Board of Directors or advisory committees, Research Funding; GSK: Research Funding; Novartis: Membership on an entity's Board of Directors or advisory committees, Research Funding; Boehringer-Ingelheim: Membership on an entity's Board of Directors or advisory committees, Research Funding; AbbVie: Research Funding; Amgen: Membership on an entity's Board of Directors or advisory committees, Research Funding; Astra-Zeneca: Membership on an entity's Board of Directors or advisory committees, Research Funding; Bristol Myers Squibb: Membership on an entity's Board of Directors or advisory committees, Research Funding; Merck: Membership on an entity's Board of Directors or advisory committees, Research Funding. Gritsman: iOnctura: Research Funding.

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Dissecting Signaling Network Responses to PAK4 Allosteric Modulators (PAMs) in Cell Subsets within Primary Human Acute Myeloid Leukemia Samples

Blood ◽

10.1182/blood.v126.23.3686.3686 ◽

2015 ◽

Vol 126 (23) ◽

pp. 3686-3686

Author(s):

Paul Brent Ferrell ◽

William Senapedis ◽

Alexander Cook ◽

Erkan Baloglu ◽

Yosef Landesman ◽

...

Keyword(s):

Bone Marrow ◽

Flow Cytometry ◽

Board Of Directors ◽

Cell Lines ◽

Myeloid Leukemia ◽

Research Funding ◽

Leukemia Stem Cells ◽

Allosteric Modulators ◽

Advisory Committees ◽

Acute Myeloid

Abstract Background: Acute myeloid leukemia (AML) is the most common acute leukemia in adults and has a poor outcome with limited treatment options in patients with relapsed or resistant disease. Therapy resistance in AML is likely related to the inadequacy of therapy within leukemia cell subsets, including leukemia stem cells (LSCs). The p21-activated kinase (PAK) family of proteins was shown to be overexpressed in cancer cells and to play a key role in proliferation, survival, and maintenance of cellular structure. The series of orally bioavailable PAK4 allosteric modulators (PAM) have previously been shown to have activity in hematological cancer cell lines, including those derived from acute myeloid leukemia (AML) (Senapedis et al. Blood124, 2208-2208). Understanding how therapies target cellular subsets within primary patient samples could aid drug development by revealing any subset specific drug effects. In this project, we studied the effects of p21-activated kinase 4 (PAK4) modulation in AML samples. PAK4 modulation has been shown to have significant effects on many intracellular signaling pathways, including PI3K/AKT, MAPK/ERK and WNT/β-catenin pathways (Senapedis et al. Blood124, 2208-2208). It is unknown whether PAMs will have similar activity in primary leukemia cells. Likewise, it is currently unclear to what extent PAMs will differentially impact primary cell subsets including leukemia stem cells and non-malignant cell subsets that may be critical to recovery of bone marrow functions. We have previously shown that the single cell biology platform of flow cytometry is well-suited for dissecting clinically relevant signaling network mechanisms in primary human AML (Irish et al. Cell, 118(2):217-28). Methods: Flow cytometry was used to dissect the impact of an orally bioavailable PAM in AML cell lines and primary patient tissue. Cell lines chosen for this study included NRAS mutant KG-1 and Kasumi-1, which carry t(8;21) and express the AML1:ETO fusion protein. Primary AML biopsies were acquired from bone marrow or blood prior to any treatment and patients were identified and consented for this study according to a local Institutional Review Board-approved protocol. AML tissue samples were viably cryopreserved and then assayed ex vivo. Established protocols were used for phospho-specific flow cytometry, fluorescent cell barcoding, and data analysis in Cytobank (Irish et al. Cell, 118(2):217-28, Doxie and Irish, Curr Top Microbiol Immunol. 377:1-21). Results: Differential effects of PAK4 inhibition were observed between cell lines and among cell subsets from AML patient bone marrow. In leukemia cell lines and patient samples, p-ERK and p-S6 showed marked inhibition via PAM, though degree of inhibition varied. In AML patient samples, PAMs blocked signaling responses in p-ERK specifically in AML blasts, but spared normal CD45hi mononuclear cells (0.88 vs. 0.29-fold reduction (arcsinh scale) in p-ERK at 10 nM). Within the AML blast population, CD34+ CD38- and CD34+ CD38+ AML subsets showed similar PAM dose response via p-ERK. Conclusions: Single cell analysis effectively distinguishes effects of PAK4 inhibition via a series of allosteric modulators of PAK4 (PAMs) on leukemia and non-leukemia subsets in the same sample. PAM reduced immediate p-ERK and p-S6 levels in primary leukemia and cell lines. Notably, inhibition in various subsets within human AML was successfully measured by phospho-flow cytometry. Signaling changes in p-ERK were minimal within non-leukemic mature CD45+ mononuclear cells found in primary patient biopsies. Analysis of CD34+ CD38- cells indicates that PAMs could have activity within leukemia stem cells, and, at least, effect the AML progenitors. These findings support further investigation into the mechanism of action and treatment potential of PAMs in AML. Disclosures Senapedis: Karyopharm Therapeutics, Inc.: Employment, Patents & Royalties. Baloglu:Karyopharm Therapeutics Inc.: Employment, Equity Ownership. Landesman:Karyopharm: Employment. Irish:Novartis: Honoraria; Cytobank, Inc.: Equity Ownership, Membership on an entity's Board of Directors or advisory committees; Karyopharm: Research Funding; InCyte: Research Funding. Savona:Gilead: Membership on an entity's Board of Directors or advisory committees; Celgene: Membership on an entity's Board of Directors or advisory committees; Incyte: Membership on an entity's Board of Directors or advisory committees, Research Funding; Karyopharm: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding.

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