JAK2 WT Research Articles

JAK2V617F-dependent down regulation of SHP-1 expression participates in the selection of myeloproliferative neoplasm cells in the presence of TGF-β.

Myeloproliferative neoplasms (MPNs) are characterized by an increased production of blood cells due to the acquisition of mutations such as JAK2V617F. TGF-β, whose secretion is increased in MPN patients, is known to negatively regulate haematopoietic stem cell (HSC) proliferation. Using an isogenic JAK2V617F or JAK2 wild-type UT-7 cell line we observed that JAK2V617F cells resist to TGF-β antiproliferative activity. Although TGF-β receptors and SMAD2/3 expressions are similar in both cell types, TGF-β-induced phosphorylation of SMAD2/3 is reduced in UT-7 JAK2V617F cells compared with JAK2 WT cells. We confirmed that JAK2V617F mutated cells are resistant to the antiproliferative effect of TGF-β in a competitive assay as we observed a positive selection of JAK2V617F cells when exposed to TGF-β. Using cell lines, CD34-positive cells from MPN patients and bone marrow cells from JAK2V617F knock-in mice we identified a down regulation of the SHP-1 phosphatase, which is required for the regulation of HSC quiescence by TGF-β. The transduction of SHP-1 cDNA (but not a phosphatase inactive cDNA) restores the antiproliferative effect of TGF-β in JAK2V617F mutated cells. Finally, SC-1, a known agonist of SHP-1, antagonized the selection of JAK2V617F mutated cells in the presence of TGF-β. In conclusion, we show a JAK2-dependent down regulation of SHP-1 in MPN patients' cells which is related to their resistance to the antiproliferative effect of TGF-β. This may participate in the clonal selection of cancer cells in MPNs.

Interferon-Alpha Impairs Neoplastic Vasculogenesis in a 3D iPSC-Based Model of JAK2V617F+ Myeloproliferative Neoplasms

Overt Myelofibrosis (MF) is an end-stage subtype of BCR-ABL1 negative myeloproliferative neoplasms (MPN), which is characterized by progressive bone marrow (BM) fibrosis resulting in loss of normal hematopoietic support as well as inflammatory and vascular complications. While aberrant neoangiogenesis is known to participate in the pathogenesis of MF, Erba et al. (Am J Pathol, 2017) suggested the occurrence of endothelial-to-mesenchymal transition (EndMT) in BM and spleen of MF patients (pts). Given the relevance of endothelial cells (EC) for MF development and the occurrence of JAK2V617F-mutated EC in MPN pts, targeting of the vascular niche has emerged as a promising therapeutic strategy. To untangle the impact of the JAK2V617F mutation on human vasculogenesis, we leveraged high-purity and clonal patient-specific induced pluripotent stem cell (iPSC)-derived EC (iEC) harboring either the JAK2V617F mutation in hetero- (JAK2V617F HET) or homozygosis (JAK2V617F HOM). JAK2 WT and CRISPR-repaired isogenic JAK2 WT (both referred to as JAK2 WT) were used as controls. In a first set of in vitro experiments, JAK2V617F mutated iEC showed a significantly increased proliferation rate in comparison to JAK2 WT iEC. We next evaluated the effect of selected drugs on iEC viability to assess their vascular toxicity. While treatment with 1 µM each of fedratinib, anagrelide or venetoclax led to a decrease in iEC viability (all **p ≤ .01), no significant alterations were observed with 1 µM ruxolitinib (RUX) or 1000 U/ml interferon-alpha (IFNa). Overall, no differences between the 3 genotypes were observed, pointing towards a genotype-independent mode of action. Given the favorable vascular toxicity profile of IFNa, as shown above, its anti-angiogenic potential was then assessed via endothelial tube formation assays. Pre-incubation with 1000 U/ml IFNa prior to seeding of the cells onto a reconstituted basement membrane matrix led to a significant inhibition of capillary-like structures formation in JAK2 WT and JAK2V617F HET or HOM iEC. One of the limitations of 2D culture assays is the lack of 3D homotypic cellular organization, compromising its translational value. Therefore, we established an adaptive 3D fibrin-based model to recapitulate, in vitro, the critical EC-pericyte crosstalk required for formation and maturation of blood vessels. Recent studies highlighted the capacity of BM mesenchymal stem cells (MSC) to differentiate into pericytes. Therefore, we seeded JAK2 WT and JAK2V617F HETor HOM iEC with or without MSC in fibrin hydrogels. MSC and iEC were visualized using different PKH dyes as fluorescent cell linkers to enable live tracking of vascular tree formation. Although self-assembly of JAK2 WT and JAK2V617F HET or HOM iEC into vessel-like structures was observed in the generated monocultures, the presence of MSC in the cocultures led to a significant increase of the formed network area (Fig. 1), hence recapitulating the critical support of pericytes. While acute IFNa exposure did not alter vascular tree formation (Fig. 1), a separate pre-treatment of iEC and MSC prior to cocultivation, significantly impaired neoplastic vasculogenesis (Fig. 2). Interestingly, interactive microscopy image analysis revealed an increased MSC/iEC interaction in JAK2V617F HOM pre-treated cocultures (*p ≤ .05). Bulk RNA-sequencing of JAK2 WT and JAK2V617F HET or HOM iEC treated for 24h with 1000 U/ml showed a strong response to IFNa. Future analysis may unravel a JAK2V617F specific transcription signature. Moreover, we induced EndMT in our 3D iEC-MSC coculture model by treatment with pro-inflammatory cytokines, reflected by the loss of EC markers (e.g. Claudin 5) and upregulation of mesenchymal markers (e.g. aSMA) by immunofluorescence. We are currently evaluating the capacity of RUX vs IFNa to inhibit EndMT. In summary, we show that IFNa impairs neoplastic vasculogenesis in a robust in vitro 3D coculture system that enables precise control of cellularity and genotype. Our model is a valuable tool to decipher the poorly investigated impact of IFNa on the BM microenvironment. In addition, its translational and scalable properties (e.g. by integrating primary or iPSC-derived hematopoietic cells) will allow us to study the impact of additional drugs on MPN-associated fibrotic transformation of the BM.

Fedratinib Overcomes Ruxolitinib Resistance through Inhibition of the Interferon Signaling Pathway

Background: Myelofibrosis (MF) is a clonal myeloproliferative neoplasm characterized by hyperactive JAK-STAT signaling, bone marrow fibrosis, extramedullary hematopoiesis and splenomegaly. Ruxolitinib (RUX) and Fedratinib are both JAK inhibitors used for the treatment of patients with intermediate or higher risk MF. While RUX is the first line treatment for patients with MF, about 50% of the patients discontinue treatment due to lack of efficacy or intolerance (Harrison et al., Ann. Hematol. 2020) after 3 years. Fedratinib is used as a second line therapy in MF based on efficacy from the JAKARTA2 study. The recently concluded FREEDOM and FREEDOM2 studies further support this. In this work, using in vitro approaches, we have explored the mechanistic basis of fedratinib's ability to overcome RUX resistance. Pharmacologically, Fedratinib binds with high affinity to the substrate-binding site and with low affinity to the ATP-binding site of JAK2(Kesarwani etal., Sci. Rep. 2015) while RUX binds only to the ATP-binding site. Previous studies on RUX resistance have shown acquired mutations driving resistance to RUX but not to Fedratinib. Here, we use RUX sensitive and resistant BaF3 (expressing human JAK2 WT or V617F) cell lines to examine the mechanism of resistance to RUX and the mechanism of efficacy of Fedratinib in RUX resistance. Methods: BaF3 cell lines overexpressing JAK2 WT or JAK2 activating mutation V617F were incrementally exposed to RUX over 3 weeks (up to IC 90) to induce resistance. Cell proliferation and phosphorylation of STAT5 in Fedratinib and RUX-treated cells were measured using celltiter glo and immunoblotting, respectively. RNAseq and ATAC-seq analyses were performed in triplicates by treating the relevant cell lines with DMSO, RUX or Fedratinib at two timepoints (6 and 15 hours). Pathway analysis was performed on RNAseq data using Metacore®. Results and Discussion: Ruxolitinib resistant BaF3 JAK2V617F cells showed minimal inhibition of proliferation (<10%) when treated with RUX but maintained sensitivity towards Fedratinib (IC 50 650nM vs 1552nM in RUX sensitive and resistant cells respectively) in 24-hour proliferation assays. Biochemical analysis revealed that Fedratinib inhibited JAK-STAT signaling in the RUX resistant cell line while RUX was unable to do so as measured by p-STAT5. Similar results were observed in BaF3-JAK2-WT cells as well. To further understand the mechanism of RUX-resistance and differential activity of RUX and Fedratinib, we performed RNAseq analysis on our RUX sensitive and resistant models before and after RUX and Fedratinib treatment. Myelofibrosis is associated with aberrant cytokine production and pro-inflammatory signaling that are mediated through activation of TNF-α and NF-κB signaling (Dunbar et al., Blood 2023). Consistent with this, RNAseq analysis of BaF3 JAK2 V617F cells treated with RUX or Fedratinib showed effective inhibition of these pathways by both agents as early as 6 hours. In the RUX resistant cells, while RUX did not have any significant pathway level changes, the top enriched pathways upon treatment with Fedratinib were predominantly interferon signaling pathways which includes JAK-STAT signaling suggesting that Fedratinib's efficacy in MF is not impacted by RUX resistance. To understand the mechanism of resistance, comparison of gene expression data from RUX senstive and RUX resistant cells without any drug treatment revealed minimal gene expression/pathway level differences suggesting that the mechanism of resistance could be driven by genetic (Downes et al. NPJ Precis Oncol, 2021) or epigenetic changes(Dunbar et al., Blood 2023). Ongoing analyses includes whole genome sequencing and ATAC-seq to investigate potential mutations and chromatin accessibility changes during resistance acquisition to RUX and how Fedratinib may overcome these. Conclusion: Our stable RUX resistant preclinical model demonstrates that Fedratinib is effective in overcoming RUX resistance by continuing to target key pathways in MPN biology including interferon signaling. Resistance to RUX on the other hand seems to be driven through potential genetic or epigenetic changes based on the minimal changes observed in gene expression profiles. Our ongoing analyses will reveal these potential genetic and epigenetic modifications associated with RUX resistance and explain the differential activity of RUX versus Fedratinib.

Tagraxofusp Shows Promising Anti-Tumoral Efficacy in Preclinical in Vitro Models of Myelofibrosis, Both As a Single Agent and in Combination with Janus Kinase Inhibitors

BACKGROUND: Myelofibrosis (MF) is an aggressive myeloproliferative neoplasm. Abnormal growth of hematopoietic stem and progenitor cells in MF is driven by somatic mutations affecting the Janus Kinase (JAK) signaling pathway with JAK2 V617F occurring in 50-60% of patients (Cardoso et al, Plos One 2015). The role of the JAK/Signal transducer and activator of transcriptions (STAT) pathway in MF has led to the recent approval of three different JAK2 inhibitors (ruxolitinib, pacritinib, and fedratinib) for MF treatment. However, despite limiting the symptoms of the disease, JAK2 inhibitors do not result in remission due to persistence of JAK2 activation and MF-driving cells (Pandey et al, Blood Cancer J 2022). Thus, there remains a high unmet need for treatment strategies for patients with MF. Tagraxofusp, a first-in-class CD123-directed therapy, is a recombinant fusion protein consisting of human interleukin-3 conjugated to a truncated diphtheria toxin payload approved (US/EU) for the treatment of Blastic Plasmacytoid Dendritic Cell Neoplasm. In MF, CD123 is expressed on malignant cells and bone marrow (BM) accessory cells that support the proliferation of neoplastic cells (Bao et al, Hematol Oncol Stem Cell Ther 2019; Lasho et al, Blood 2014). Based on CD123 expression, tagraxofusp is a potential, novel approach for MF treatment either alone or in combination. Initial results from a two-stage multicenter, phase 2 study (STML-0401-0314; NCT02268253) of single-agent tagraxofusp in pts with relapsed or refractory (R/R) MF demonstrated encouraging clinical efficacy and a manageable safety profile (Yacoub et al. ASH 2021). We aim to further investigate the antitumoral efficacy of tagraxofusp in MF preclinical models and the potential improvement of antitumoral efficacy by combining tagraxofusp with the JAK inhibitors, ruxolitinib, and pacritinib. METHODS: MF cell lines (SET2, UKE1, HEL: JAK2 V617F; UT7: JAK2 wild type [wt]) and bone marrow mononuclear cell (BMMC) samples from healthy donors (HD) were characterized for the extracellular expression of CD123 by flow cytometry staining. In vitro cytotoxicity experiments in CD123-positive MF cell lines (SET2, UT7, UKE-1) tested tagraxofusp and ruxolitinib or pacritinib either as single agents or in combination (MTS assay; 72 h). The combination index (CI) was calculated according to the Compusyn-Chou method (Chou TC., Pharmacol Rev, 2006) and Combenefit platform. In vitro co-culture experiments were performed with MF cell lines incubated in the presence of human primary HD-BMMCs. Cell death, as measured by LiveDead/AnnV staining, was monitored by FACS analysis through differential staining of MF cells and BMMCs with Cell Trace Violet dye (CTV). A signaling pathway analysis (JAK/STAT, S6, Mcl1) was performed through immunocapillary electrophoresis on MF cell lines treated with single agents or in combination. RESULTS: The MF cell lines showed different levels of CD123 expression. Except for the CD123 negative cell line (HEL), CD123 positive cell lines were all sensitive to tagraxofusp independent of CD123 expression intensity. As expected, both ruxolitinib and pacritinib were active in the presence of JAK2 wt pathway. A synergistic drug interaction was observed (mean CI range: 0.59-1.12) in most combinations of tagraxofusp with JAK inhibitors, as shown in a representative CD123+ cell line in Figure 1. When BMMCs, which are a subpopulation of BM Accessory Cells partially expressing CD123, were co-cultured with MF cell lines, the cytotoxic effect induced by tagraxofusp was enhanced, and this improvement was maintained in combination with JAK2 inhibition. Molecular biomarkers, such as JAK/STAT, S6, and Mcl1, were evaluated in MF cell lines treated with tagraxofusp and JAK inhibitors and supported the MoA of this combination. CONCLUSIONS: Our studies showed high sensitivity of MF cell lines to tagraxofusp as a single agent, which is expected given phase 2 results demonstrated clinical efficacy of single-agent tagraxofusp in R/R MF (Yacoub et al. ASH 2021). In addition, synergism was observed in combination with JAK inhibitors. The in vitro antitumoral effect was more pronounced in the presence of BM accessory cells, suggesting an extra targeting of CD123-positive BM cells by tagraxofusp in vivo. These results support potential clinical development of TAG in combination with JAK inhibitors for the treatment of MF.

JAK2 V617F Impairs Lymphoid Differentiation in Myeloproliferative Neoplasms

Myeloproliferative neoplasms (MPNs) arise from the clonal acquisition of a driver mutation in a hematopoietic stem cell (HSC), often decades before the emergence of clinical disease. A defining characteristic of an HSC is its potential to give rise to both myeloid and lymphoid hematopoietic lineages. Yet the most common MPN driver mutation, JAK2 V617F, has only rarely been observed in lymphocytes. We conducted this study to determine why the JAK2 V617F clone does not contribute to mature lymphocytes. We directly measured myeloid and lymphoid lineage contribution of normal and MPN HSCs in 133 patients with JAK2 V617F MPN. Peripheral blood subpopulations were fractionated via fluorescence-activated cell sorting (FACS) and JAK2 V617F mutation allele frequency (MAF) was measured in each subpopulation by droplet digital PCR. JAK2 V617F was enriched in neutrophils and maintained in erythroid progenitors (EPs) relative to HSCs. In contrast, JAK2 V617F alleles were largely absent in T and B cells relative to HSCs. Lymphopoiesis declines with age, potentially explaining the absence of JAK2 V617F in lymphocytes from older patients with MPN even if JAK2 V617F has no effect on lymphopoiesis. This “JAK2 V617F neutral” hypothesis predicts that patients who acquired the mutation at a younger age, and those who harbor the mutation for a longer duration, have a higher likelihood of accumulating JAK2mutated lymphocytes. However, we found that JAK2 V617F MAF in T and B cells across our cohort does not correlate with either patient age at diagnosis or duration of clinical disease. Thus, predictions of the “JAK2 V617F neutral” hypothesis are not supported by patient data. The alternative explanation is that JAK2 V617F impairs lymphocyte differentiation from mutated progenitors. We tested this “JAK2 V617F adverse” hypothesis by tracking JAK2 V617F during various stages of lymphopoiesis. To assess the relative bias of JAK2 V617F HSCs during the early stages of lymphoid and myeloid commitment, we measured JAK2 V617F MAF in common lymphoid progenitors (CLPs), common myeloid progenitors (CMPs) and HSCs isolated by FACS from the peripheral blood of 79 patients with JAK2 V617F MPN. We found that while myeloid commitment from HSC to CMP was associated with an increase in mean JAK2 V617F MAF, there was no statistically significant change in mean MAF between CLPs and the parent HSCs across the cohort. Therefore, JAK2 V617F HSCs are not completely excluded from the earliest stages of lymphoid differentiation. To further define the extent to which JAK2 V617F affects lymphopoiesis, we used FACS to isolate CLPs and CMPs from 17 patients with JAK2 V617F polycythemia vera (PV) and differentiated each subpopulation in vitro into T cell progenitors (pro- and pre-T cells) and erythroblasts (EBs), respectively. We found that all CLP samples could be differentiated into T cell progenitors. However, we observed a marked depletion of JAK2 V617F alleles in progressively mature T cell progenitors (Figure 1). In contrast, JAK2 V617F alleles were further enriched in progressively mature erythroblasts. The in vitro differentiation potential of MPN progenitors thus supports the “JAK2V617F adverse” hypothesis. To determine the effect of JAK2 V617F on lymphopoiesis in vivo, we performed competitive transplantations of lethally irradiated CD45.1 mice with congenic CD45.2 donor whole bone marrow (WBM) cells harboring JAK2 V617F or wild type JAK2 (JAK2 WT). Measuring CD45.2 chimerism by FACS in lymphocytes and neutrophils from engrafted mice revealed that only CD45.2 cells harboring JAK2 V617F were preferentially depleted in lymphocytes (Figure 2). Therefore, JAK2 V617F HSCs displayed a competitive disadvantage relative to their JAK2 WT counterparts during lymphopoiesis in vivo. Myeloid proliferation in MPN driven by JAK2 V617F is well recognized but does not fully explain certain MPN complications such as increased risk of infections and second malignancies. Our findings demonstrate impaired lymphoid differentiation from JAK2 V617F stem and progenitor cells that may also contribute to clinical MPN phenotypes. Further study is ongoing to define actionable mechanisms through which JAK2 V617F interferes with T cell differentiation.

Circulating CD133+/–CD34– Have Increased c-MYC Expression in Myeloproliferative Neoplasms

Myeloproliferative neoplasms (MPNs) are hematopoietic stem cell (HSC)-originated diseases with clonal myeloproliferation. The constitutive activation of the JAK/STAT pathway is frequently detected in patients with Philadelphia chromosome-negative (Ph–) MPNs with an acquired JAK2V617F mutation. The c-MYC proto-oncogene is associated with malignant growth and cellular transformation, and JAK2V617F was previously shown to induce constitutive expression of c-MYC. This study examines the expressional profile of c-MYC in Ph– MPNs with JAK2V617F and highlights its hierarchical level of activation in circulating hematopoietic stem/progenitor cell (HSPC) subgroups. Mononuclear cells (MNCs) of Ph– MPNs were fluorochrome-labeled in situ with wild-type (wt) JAK2 or JAK2V617F mRNA gold nanoparticle technology and sorted simultaneously. Isolated populations of JAK2wt or JAK2V617F were evaluated for their c-MYC expressions. The MNCs of 14 Ph– MPNs were further isolated for the study of HSPC subgroups regarding their CD34 and CD133 expressions, evaluated for the presence of JAK2V617F, and compared to cord blood (CB) counterparts for the expression of c-MYC. The mRNA-labeled gold nanoparticle-treated MNCs were determined to have the highest ratio of c-MYC relative fold-change expression in the biallelic JAK2V617F compartment compared to JAK2wt. The relative c-MYC expression in MNCs of MPNs was significantly increased compared to CB (p=0.01). The circulating HSPCs of CD133+/–CD34− MPNs had statistically significantly elevated c-MYC expression compared to CB. This is the first study of circulating CD133+/–CD34− cells in Ph– MPNs and it has revealed elevated c-MYC expression levels in HSCs/endothelial progenitor cells (HSCs/EPCs) and EPCs. Furthermore, the steady increase in the expression of c-MYC within MNCs carrying no mutations and monoallelic or biallelic JAK2V617F transcripts was notable. The presence of JAK2V617F with respect to c-MYC expression in the circulating HSCs/EPCs and EPCs of MPNs might provide some evidence for the initiation of JAK2V617F and propagation of disease. Further studies are needed to clarify the implications of increased c-MYC expression in such populations.

Modulation of the Human Thrombopoietin Receptor Conformation Uncouples JAK2 V617F-Driven from Cytokine-Induced Activation

Myeloproliferative Neoplasms are clonal diseases where mutations in hematopoietic stem cells (HSCs) result in cytokine-independent activation of the Thrombopoietin Receptor (TpoR) and aberrant proliferation and differentiation of HSCs and myeloid progenitors1. The most prevalent mutation, JAK2 V617F, is the basis of >95% of Polycythemia Vera (PV) and 60% of Essential Thrombocythemia (ET) and Myelofibrosis cases2. Current treatment strategies have focused on direct inhibition of JAK2 with little or no success and with poor specificity for the V617F mutant. Unlike JAK2, the TpoR is present at the cell surface and is more easily targetable. Using a system of controlled dimerization, we previously reported that different dimeric orientations of murine TpoR result in distinct signaling pathways and variable activation intensity3. Recently, others have demonstrated that extracellular ligands (diabodies) are able to modulate human TpoR conformation to influence downstream signaling4. Here, we investigated the possibility to modulate human TpoR conformation to specifically target JAK2 V617F driven activation while preserving natural cytokine-induced signaling. Using our system of controlled dimerization of TpoR transmembrane (TM) domain, we characterized the active and inactive conformation of human TpoR in presence of JAK2 WT or JAK2 V617F in hematopoietic cells. While murine TpoR is active in almost all (6 out of 7) dimeric conformations, only 3 conformations allowed activation of human TpoR. The discrepancy between murine and human TpoR is due to sequence differences in the TM domain that modulates dimerization. Remarkably, we discovered that different dimeric conformations mediate activation of JAK2 WT and JAK2 V617F in human but not in murine TpoR. Using a cysteine cross-linking assay on live cells, we validated that TpoR dimers exhibit a different conformation in presence of JAK2 V617F than this induced by the thrombopoietin (Tpo). Given the different dimeric conformations of TpoR induced by Tpo and JAK2 V617F, we hypothesize that modulation of TpoR conformation could allow specific inhibition of JAK2 V617F-driven activation while preserving Tpo-induced activation. We probed this by introducing a glycine zipper GxxxG motif to favor dimerization in a conformation that renders JAK2 V617F inactive but allows Tpo induced activation. Conversely, we mutated residues of the TM domain to specifically disrupt the conformation required for JAK2 V617F-mediated activation. Both strategies resulted in complete inactivation of JAK2-V617F driven activation of TpoR but preserved its full activation by Tpo and cell surface localization. Together, these results provide a first demonstration that modulation of TpoR dimeric conformation is a suitable strategy for specific inhibition of the JAK2 V617F-driven activation and opens the road for novel therapeutic avenues. 1. Vainchenker W, Kralovics R. Genetic basis and molecular pathophysiology of classical myeloproliferative neoplasms. Blood. 2017;129(6):667-679. 2. Constantinescu SN, Vainchenker W, Levy G, Papadopoulos N. Functional Consequences of Mutations in Myeloproliferative Neoplasms. Hemasphere. 2021;5(6):e578. 3. Staerk J, Defour JP, Pecquet C, et al. Orientation-specific signalling by thrombopoietin receptor dimers. Embo j. 2011;30(21):4398-4413. 4. Cui L, Moraga I, Lerbs T, et al. Tuning MPL signaling to influence hematopoietic stem cell differentiation and inhibit essential thrombocythemia progenitors. Proceedings of the National Academy of Sciences. 2021;118(2):e2017849118. Figure 1View largeDownload PPTFigure 1View largeDownload PPT Close modal

Erythroid and Megakaryocytic Differentiation Program in JAK2-Mutated Acute Myeloid Leukemia with or without Antecedent Myeloproliferative Neoplasm

JAK2 V617F is the most common driver mutation for Ph-negative MPN and related blast transformation (MPN-BP). In contrast, JAK2 V617F is rare (<5%) in acute myeloid leukemia (AML) lacking an antecedent MPN (nonMPN AML). JAK2 V617F skews the differentiation of hematopoietic stem/progenitor cells (HSPC) towards erythroid and megakaryocytic lineages and in conjunction with TP53 deficiency/mutations results in acute erythroid leukemia in mice (Rampal et al., PNAS 2014). These phenotypes, although also reported in patients with MPN-BP (Mesa et al., Blood 2005), remain incompletely understood. In this study, we set out to study these phenotypes at the protein, epigenomic and transcriptomic levels in JAK2-mutated AML comparing MPN-BP to nonMPN AML. The study cohort included 58 MPN-BP (16 PV, 16 ET, 14 PMF and 12 MPN-U) and 27 nonMPN AML patients (11 de novo AML, 12 AML-MRC, 4 t-AML) with JAK2 V617F mutation (Table 1). In addition, 100 patients with JAK2 WT AML were also included as control group. The median age at diagnosis in MPN-BP and nonMPN AML subgroups as well as in JAK2 WT AML was similar (70 years vs 71 years vs 70 years, respectively) and showed a male preponderance. The median interval between MPN diagnosis and blast transformation was 4.3 years (range, 0-20.5 years). Overall survival was inferior in MPN-BP compared to nonMPN AML (median survival: 7.4 vs 19.3 months). MPN-BP showed a higher rate of MF-2 or 3 myelofibrosis (78% vs 33%, p<0.05), osteosclerosis (50% vs 15%, p<0.05) and marrow necrosis (7% vs 0%) on bone marrow biopsies than nonMPN AML. On immunohistochemistry, 28% (10/32) MPN-BP cases (vs 0% nonMPN AML, p<0.05) were classified as acute leukemia with erythroid/megakaryocytic differentiation (M6b and/or M7) based on expression of E-cadherin, CD71, Glycophorin A, and CD61 on leukemic blasts. ERG, FLI1, and LMO2, transcription factors that are often expressed in HSPCs, were also expressed in leukemic blasts of MPN-BP and nonMPN AML. Aberrant TP53 expression by immunohistochemistry was seen in 31% of MPN-BP (5/9 and 5/21 cases with and without erythroid/megakaryocytic differentiation, respectively) and 17% (3/18) of nonMPN AML. The most common mutations on next-generation sequencing included TP53, RUNX1, ASXL1, TET2, DNMT3A, SRSF2, IDH1, IDH2 and EZH2 in each group. However, of these, TP53 mutations were more frequent in MPN-BP (34% vs 11% in nonMPN AML, p<0.05) and TET2 mutations more frequent in nonMPN AML (48% vs 24% in MPN-BP, p<0.05). Interestingly, NPM1 and FLT3, which are among the most mutated genes in AML were not detected in either MPN-BP or nonMPN AML. Based on the over-representation of erythroid/megakaryocytic phenotype in MPN-BP seen on immunohistochemistry, we hypothesized that MPN-BP blasts might exhibit an upregulated epigenetic and transcriptional program involved in erythroid/megakaryocytic differentiation. RNA-seq and ATAC-seq analysis were performed on flow cytometry sorted CD34+ leukemic blast populations from 11 MPN-BP and 6 nonMPN AML, as well as similar sorted populations from normal controls and JAK2 WT AML. Principal component analysis showed overlapping MPN-BP and nonMPN AML samples, both of which were separated from JAK2 WT AML. Differential expression analysis confirmed nearly identical gene expression between blasts from MPN-BP and nonMPN AML. In addition, consistent with findings by immunohistochemistry, the expression of genes specifically involved in erythroid/megakaryocytic differentiation (TFRC, GYPA, HBB, SPTA1, EPOR, ITGA2B, ITGB3, PF4, ALAS2, GP1BA, ENG and MPL) were upregulated in MPN-BP. Hierarchical clustering using Ward's method on both RNA-seq and ATAC-seq normalized counts revealed that the JAK2 WT AML cluster was distinct from the MPN-BP and nonMPN AML cluster (Figure 1). Lineage deconvolution of ATAC-seq using hematopoietic differentiation stages (Glass et al., Blood 2017, Dilip et al., Blood 2021) to evaluate lineage characteristics of the blasts showed hematopoietic stem cell (HSC) predominance in JAK2 WT AML while MPN-BP and nonMPN AML displayed enrichment of multipotent progenitor (MPP), megakaryocyte-erythrocyte progenitor (MEP) and erythroid signatures. Collectively, our data demonstrates enrichment of erythroid and megakaryocytic programs in both JAK2-mutated MPN-BP and JAK2-mutated nonMPN AML, suggesting that chromatin accessibility and transcription may be driven by mutated JAK2 regardless of antecedent MPN. Figure 1View largeDownload PPTFigure 1View largeDownload PPT Close modal

Insights on JAK2 Modulation by Potent, Selective, and Cell-Permeable Pseudokinase-Domain Ligands.

JAK2 is a non-receptor tyrosine kinase that regulates hematopoiesis through the JAK-STAT pathway. The pseudokinase domain (JH2) is an important regulator of the activity of the kinase domain (JH1). V617F mutation in JH2 has been associated with the pathogenesis of various myeloproliferative neoplasms, but JAK2 JH2 has been poorly explored as a pharmacological target. In light of this, we aimed to develop JAK2 JH2 binders that could selectively target JH2 over JH1 and test their capacity to modulate JAK2 activity in cells. Toward this goal, we optimized a diaminotriazole lead compound into potent, selective, and cell-permeable JH2 binders leveraging computational design, synthesis, binding affinity measurements for the JH1, JH2 WT, and JH2 V617F domains, permeability measurements, crystallography, and cell assays. Optimized diaminotriazoles are capable of inhibiting STAT5 phosphorylation in both WT and V617F JAK2 in cells.

The SH2 domain and kinase activity of JAK2 target JAK2 to centrosome and regulate cell growth and centrosome amplification.

JAK2 is cytokine-activated non-receptor tyrosine kinase. Although JAK2 is mainly localized at the plasma membrane, it is also present on the centrosome. In this study, we demonstrated that JAK2 localization to the centrosome depends on the SH2 domain and intact kinase activity. We created JAK2 mutants deficient in centrosomal localization ΔSH2, K882E and (ΔSH2, K882E). We showed that JAK2 WT clone strongly enhances cell proliferation as compared to control cells while JAK2 clones ΔSH2, K882E and (ΔSH2, K882E) proliferate slower than JAK2 WT cells. These mutant clones also progress much slower through the cell cycle as compared to JAK2 WT clone and the enhanced proliferation of JAK2 WT cells is accompanied by increased S −> G2 progression. Both the SH2 domain and the kinase activity of JAK2 play a role in prolactin-dependent activation of JAK2 substrate STAT5. We showed that JAK2 is an important regulator of centrosome function as the SH2 domain of JAK2 regulates centrosome amplification. The cells overexpressing ΔSH2 and (ΔSH2, K-E) JAK2 have almost three-fold the amplified centrosomes of WT cells. In contrast, the kinase activity of JAK2 is dispensable for centrosome amplification. Our observations provide novel insight into the role of SH2 domain and kinase activity of JAK2 in centrosome localization of JAK2 and in the regulation of cell growth and centrosome biogenesis.

Genetic Landscape and Clonal Evolution Patterns of CEBPA-Mutated Acute Myeloid Leukaemia Based on Next-Generation Sequencing: A Retrospective Analysis

IntroductionAlthough CCAAT/enhancer binding protein alpha double mutated (CEBPADM) acute myeloid leukemia (AML) is considered a low-risk form of AML according to 2017 ELN recommendations, relapse remains a major cause of death. To assess the broader prognostic impact of other cancer-associated genes, we sequenced a panel of 40 myeloid disorders-related genes in a 25 patient cohort.Methods16 CEBPADM AML diagnosis samples along with 9 CEBPAsingle mutated (SM) were analyzed by targeted next-generation sequencing (Ion Torrent) using Oncomine Myeloid Research Assay. 4 CEBPADM and 2 CEBPASM AML relapse samples were analyzed as well. All patients received intensive chemotherapy according to 2017 ELN recommendations.ResultsWith a median follow-up of 3.2 years (range 0.4-12) 5y OS was 61% and 14% for CEBPADM and CEBPASM patients respectively. Overall, a median of 3 concurrent mutations were present at diagnosis, slightly more in CEBPA SM patients (4 vs 3 in CEBPA DMpatients). The number of somatic mutations influenced both PFS and OS (p = 0.04 and p < 0.01 respectively) independently of CEBPA mutational status. Each single unitary increase in the number of mutations increased the hazard for death of 27.7% (95% CI: -1.4-+65; p = 0.064) while passing from 4 to 5 mutations increased the odds of death by 367%. 5y OS in patients with 5 or more concurrent mutation was 14.3% vs 61.8% in patients with less than 5 co-mutated genes; 5y PFS was 0% vs 38.6%.Mutational landscape of CEBPADMand CEBPASMAML differed significantly, with GATA2, FLT3, DNMT3A and TET2 being the most frequently mutated genes in CEBPADM vs NPM1, FLT3, DNMT3A and WT1 in CEBPASM patients. NPM1(77.8% vs 6.7%; p < 0.01) and ASXL1 mutations (44.4% vs 0%; p = 0.02) were more frequent in CEBPASM patients, confirming they are mutually exclusive with CEBPA biallelic lesions. DNA methylation was the most frequently mutated pathway in biallelic patients (87%) while chromatin/cohesin complex (88%) was the most frequently mutated one in CEBPA monoallelic patients. Mutations of CEBPA, NPM1, DNMT3A, WT1, STAG2, TET2, ASXL1, IDH2, SRSF2, CALR, PRPF8, NF1, TP53, RUNX1 had the highest median variant allele frequency (VAF), more often representing founding mutations. GATA2, IDH1, KRAS, BCOR, MPL, IKZ2F1 and PTPN11 had a more borderline median VAF, variably being clonal or subclonal. Mutations in the 3 tyrosine kinases genes FLT3-ITD , CSF3R, NRAS were only subclonal.Mutations in WT1 and FLT3 were associated with increased relapse rate (p = 0.02 and p = 0.01 respectively), while patients with GATA2 mutations had a strong trend towards better 5y OS (83% vs 32%, p = 0.053).We also identified a not previously described allelic variant in the SH2B3 gene (ATGGGG/A INDEL) with an overall prevalence in our population of 58.3% (46.7% of CEBPA DM and 77.8% of CEBPA SMpatients). Patients with the SH2B3allelic variant had a significantly lower bone marrow blast percentage at diagnosis (p = 0.014) and a strong trend towards a higher number of concurrent mutations (p = 0.056). Moreover, when present at diagnosis, SH2B3 variant persisted at relapse with the same VAF. By real time PCR we demonstrated that this SH2B3 allelic variant leads to a dramatic reduction of the corresponding transcript. This gene encodes for a negative regulator of many crucial signaling pathways (SCF/c-KIT, erythropoietin/JAK2, thrombopoietin/MPL WT/ W515L, JAK2 WT/ V617F, GM-CSFR and FLT3-WT/ITD) of the hematopoietic stem cell.Matched diagnosis and relapse samples analysis suggested different features of clonal evolution: while mutations of SH2B3, WT1, DNMT3A, NPM1, and IDH1 consistently persisted at relapse, CEBPA and GATA2 mutations were unstable during disease course. ZRSR2 and PRPF8 mutations were found in relapse samples only.SummaryOur study offers insights into the genetic landscape of CEBPADM mutated AML as compared to CEBPASM AML, highlighting the contribution of NGS to risk stratification. In fact, our data show that the number and the type of concurrent mutation has a prognostic impact, possibly identifying patients eligible to first line allogeneic stem cell transplantation. We identified an allelic variant of SH2B3 that had never been functionally characterized nor associated with AML and that could represent a marker for genetic instability and a potential new target in AML treatment strategies. [Display omitted] DisclosuresNo relevant conflicts of interest to declare.

TGF-β1 protein trap AVID200 beneficially affects hematopoiesis and bone marrow fibrosis in myelofibrosis

Myelofibrosis (MF) is a progressive chronic myeloproliferative neoplasm characterized by hyperactivation of JAK/STAT signaling and dysregulation of the transcription factor GATA1 in megakaryocytes (MKs). TGF-β plays a pivotal role in the pathobiology of MF by promoting BM fibrosis and collagen deposition and by enhancing the dormancy of normal hematopoietic stem cells (HSCs). In this study, we show that MF-MKs elaborated significantly greater levels of TGF-β1 than TGF-β2 and TGF-β3 to a varying degree, and we evaluated the ability of AVID200, a potent TGF-β1/TGF-β3 protein trap, to block the excessive TGF-β signaling. Treatment of human mesenchymal stromal cells with AVID200 significantly reduced their proliferation, decreased phosphorylation of SMAD2, and interfered with the ability of TGF-β1 to induce collagen expression. Moreover, treatment of MF mononuclear cells with AVID200 led to increased numbers of progenitor cells (PCs) with WT JAK2 rather than mutated JAK2V617F. This effect of AVID200 on MF PCs was attributed to its ability to block TGF-β1–induced p57Kip2 expression and SMAD2 activation, thereby allowing normal rather than MF PCs to preferentially proliferate and form hematopoietic colonies. To assess the in vivo effects of AVID200, Gata1lo mice, a murine model of MF, were treated with AVID200, resulting in the reduction in BM fibrosis and an increase in BM cellularity. AVID200 treatment also increased the frequency and numbers of murine progenitor cells as well as short-term and long-term HSCs. Collectively, these data provide the rationale for TGF-β1 blockade, with AVID200 as a therapeutic strategy for patients with MF.

Indoloxytriazines as binding molecules for the JAK2 JH2 pseudokinase domain and its V617F variant

Small molecules that selectively bind to the pseudokinase JH2 domain over the JH1 kinase domain of JAK2 kinase are sought. Virtual screening led to the purchase of 17 compounds among which 9 were found to bind to V617F JAK2 JH2 with affinities of 40 – 300 μM in a fluorogenic assay. Ten analogues were then purchased yielding 9 additional active compounds. Aminoanilinyltriazine 22 was particularly notable as it shows no detectable binding to JAK2 JH1, and it has a 65-μM dissociation constant Kd with V617F JAK2 JH2. A crystal structure for 22 in complex with wild-type JAK2 JH2 was obtained to elucidate the binding mode. Additional de novo design led to the synthesis of 19 analogues of 22 with the most potent being 33n with Kd values of 2–3 μM for WT and V617F JAK2 JH2, and with 16-fold selectivity relative to binding with WT JAK2 JH1.

Fedratinib (FEDR) in myelofibrosis (MF) patients previously treated with ruxolitinib (RUX): A reanalysis of the JAKARTA-2 study.

7057 Background: MF is a life-threatening MPN for which RUX is the only approved treatment (Tx) option. Patients (pts) who are relapsed/refractory (R/R) or intolerant to RUX have a particularly high unmet medical need. FEDR is an oral selective JAK2 inhibitor active against wt and mut JAK2. The JAKARTA-2 study demonstrated ≥35% spleen volume responses (SVR) in pts resistant or intolerant to RUX per investigator assessment. This JAKARTA-2 reanalysis employs a more stringent definition of RUX failure than used in the previous analysis. Methods: Adult pts previously treated with RUX with intermediate or high-risk primary, post-PV, or post-ET MF, palpable splenomegaly, ECOG PS ≤2, and platelet counts ≥50 × 109/L received FEDR 400 mg QD in continuous 28-day cycles. The primary endpoint was spleen volume response rate (SVRR): ≥35% SVR from baseline (BL) at cycle 6 end per blinded central review of MRI/CT scans. A key secondary endpoint was symptom RR (≥50% decrease in total symptom score from BL). Results: 79/97 enrolled pts (81%) met the more stringent criteria for RUX R/R (n=65, 82%) or intolerance (n=14, 18%). Median BL spleen volume was 2946 mL (~14× normal). Median prior RUX Tx duration was 11.5 mo (range 1.0–62.4). Median number of FEDR Tx cycles was 7 (1–20). SVRR with FEDR was 30% (95% CI 21, 42). KM estimated median spleen response duration was not estimable (95% CI 7.2 mo, NE). Symptom RR was 27%. Safety was consistent with prior reports. Conclusions: FEDR provided clinically meaningful reductions in splenomegaly and symptom burden in pts with MF who met more stringent criteria for R/R or intolerance to RUX. Clinical trial information: NCT01523171. [Table: see text]

JAK2 R1063H Variant Enhances V617F Constitutive Signaling and Favors Development of Essential Thrombocythemia with Increased Hemoglobin and Neutrophils

The clinical consequences of the highly prevalent JAK2 V617F mutation in Ph-negative myeloproliferative neoplasms (MPNs) are well studied. However, the impact on the MPN phenotype of co-occurring JAK2 variants is less well characterized. In this study, we focused on JAK2 R1063H, a variant previously reported in a pediatric case of hereditary erythrocytosis (Kapralova et al, Blood 2016) and in 3 out of 93 polycythemia vera patients that were JAK2 V617F-positive (Levine et al, Cancer Cell 2005). It has been described as a very rare polymorphism in population (frequency 0.004377).In order to gain insight into the clinical and functional relevance of coexisting JAK2 mutations, we tested a cohort of 390 JAK2 V617F-positive MPN patients for JAK2 R1063H and identified 14 carriers of both mutations. From the clinical perspective, the double-positive patients exhibited predominantly an essential thrombocythemia phenotype that was accompanied by significantly higher neutrophil granulocyte counts and hemoglobin values, when compared to those harboring only JAK2 V617F mutation. By employing targeted NGS for genes known to be involved in myeloid malignancies we found that the mutational profile and the number of additional somatic mutations in double-mutant patients seemed to be comparable to previous studies of JAK2 V617F-positive MPNs.Next, we used digital droplet PCR (ddPCR) for JAK2 R1063H and JAK2 V617F allele burden assessment. Quantification of JAK2 R1063H allele indicated that in 8 patients the variant was heterozygous, likely inherited (the percentage of the mutant allele being around 50%). In 3 patients with a high JAK2 V617F allelic burden a nearly homozygous status for R1063H was identified (a fractional abundance >80%) suggesting that one R1063H allele was inherited and the second one was acquired by uniparental disomy, as a result of mitotic recombination. In 3 other patients, the JAK2 R1063H mutation was most likely acquired due to the low percentage of the mutant allele (between 20.7% and 31.5%). For cis/trans configuration analysis of JAK2 V617F and R1063H mutations, we performed single-colony sequencing of subcloned JAK2 cDNA from 9 out of 14 patients. A cis configuration of the mutations was detected in 7 and a trans configuration in 2 cases. In one patient, the cis configuration of JAK2 mutations combined with results obtained by ddPCR (the allele burdens for R1063H and V617F were 31.5% and 52.8%, respectively) suggested that R1063H was acquired, following V617F acquisition.To assess the effect on JAK2 signaling of coexisting JAK2 V617F and R1063H mutations in cis or trans, we created human cDNA JAK2 mutants (V617F, R1063H and V617F/R1063H). STAT5 transcriptional activity of the JAK2 WT and JAK2 mutants in the presence of myeloid dimeric cytokine receptors (EPOR, given the higher hemoglobin levels, G-CSFR, given the observed neutrophilia in patients carrying JAK2 V617F and R1063H, and TPOR) measured in JAK2-deficient γ-2A cells by dual luciferase assay revealed a significantly higher constitutive activity of JAK2 V617F/R1063H (cis mutant) compared to that of JAK2 V617F, in both homozygous and heterozygous configurations and with either of the dimeric myeloid cytokine receptors. In the trans configuration of JAK2 V617F and JAK2 R1063H, we did not observe a significant difference compared to combination of JAK2 V617F and JAK2 WT. Western blot analysis assessing activated phosphorylated forms of JAK2, STAT5 and ERK1/2 demonstrated a higher level of constitutive activation of JAK2 and STAT5 generated by the double V617F/R1063H mutant versus JAK2 V617F and similar effect was shown also for ERK1/2 signaling.Our results suggest that either an acquisition of V617F in cis on a germline R1063H allele or a gain of additional JAK2 R1063H on JAK2 V617F will lead to an increase in downstream signaling by the driver JAK2 V617F. The oncogenic signaling of JAK2 V617F enhanced by the R1063H mutation has a clinical effect, leading to a significantly increased number of neutrophilic granulocytes and hemoglobin values, in agreement with higher levels of signaling via EPOR and G-CSFR.The work was supported by projects COP A1.1.4. ID: P_37_798, Contract 149/26.10.2016, (MySMIS2014+: 106774), MyeloAL, WELBIO F 44/8/5-MCF/UIG-10955, ARC: N° 16/21-073, by Ludwig Institute for Cancer Research, Salus Sanguinis Foundation, Fondation Les avions de Sébastien, Belgium and by GACR 17-05988S and MSMT LTAUSA17142. DisclosuresNo relevant conflicts of interest to declare.

Modeling Calreticulin-Mutant Myeloproliferative Neoplasms with Isogenic Induced Pluripotent Stem Cells

Myeloproliferative neoplasms (MPN) are characterized by the excessive production of one or more myeloid lineages and a propensity to progress to acute leukemia. In 2013, mutations in the CALR gene, encoding calreticulin, were identified in patients with MPN, mutually exclusive to the previously identified JAK2 and MPL (TPO-R) mutations. CALR mutations are frameshift mutations - typically a 52-bp deletion (type 1) or a 5-bp insertion (type 2) - that result in a novel C-terminus. The discovery of mutations in a ubiquitously expressed multifunctional protein like calreticulin was unanticipated. Subsequent studies found that CALR mutations lead to activation of JAK/STAT, mediated through aberrant interactions between mutant CALR and MPL, thus presenting an excellent opportunity for targeted therapy. However, the mechanism of MPL activation remains largely unexplained with prior studies using cell lines with exogenous expression of CALR and MPL following transfection.To create a more physiological cellular model to study the effects of CALR mutations, we established multiple iPSC lines from two patients with CALR-mutant MPN - one type 1-like (del34) and one type 2 (ins5) -, as well as from one patient with JAK2V617F MPN. All iPSC lines were confirmed to harbour the CALR or JAK2V617F mutation found in the corresponding patient, to express mutant calreticulin, as detected by flow cytometry using an antibody which specifically recognizes the novel calreticulin C-terminus, and to be karyotypically normal. Genetically matched iPSC lines with WT JAK2 could also be generated from the JAK2V617F (but not the CALR-mutant) patient cells in the same reprogramming round. CRISPR gene editing was used to generate isogenic CALR-corrected lines from both CALR-mutant patients. Furthermore, in order to facilitate biochemical studies, we used CRISPR to introduce a V5 epitope tag in one allele of the endogenous mutant or WT CALR gene, in mutant and isogenic corrected iPSC lines, respectively. We optimized an in vitro differentiation protocol for efficient derivation of megakaryocyte (MK) progenitors from iPSCs and found disease-relevant phenotypes, mainly TPO-independent MK colony formation in semi-solid media, which is the phenotypic hallmark of ex vivo primary MPN cells. In the absence of TPO, JAK2 V617F, CALR-mutant type 1-like and CALR-mutant type 2 iPSCs generated 52.1%, 58.7±22.2% and 59.8±3.6%, respectively, of the number of MK colonies generated in the presence of TPO, as opposed to 10%, 8.8±1.8% and 0.5±0.9%, respectively, for the matched WT JAK2, the corrected CALR-mutant type 1-like and the corrected CALR-mutant type 2 iPSCs. Isolated CALR mutant iPSC-derived CD41a+ MK progenitors had increased phosphorylation of STAT5 following cytokine starvation as compared to isogenic corrected and non-isogenic normal cells. CALR-mutant cells expressed equal transcript levels of the WT and mutant CALR alleles. However, mutant CALR protein levels were severely reduced, at levels 1~12% of those of the WT protein. This is consistent with previous studies documenting instability of mutant calreticulin. Transcriptomics (RNA-seq) and proteomics analyses of CD41a+-sorted MK progenitors derived from CALR mutant and isogenic corrected iPSCs are ongoing.These iPSC models offer the opportunity to study the effects of CALR mutations in a cellular context with both MPL and CALR (WT or mutant) expressed from their endogenous loci. They thus provide a powerful platform to investigate the disease mechanisms underlying CALR-mutant MPNs and to perform small molecule and genetic (CRISPR) screens to identify new therapeutic targets. DisclosuresIancu-Rubin:Merck: Research Funding; Incyte: Research Funding; Summer Road, LLC: Research Funding; Formation Biologics: Research Funding. Hoffman:Incyte: Research Funding; Merus: Research Funding; Formation Biologics: Research Funding; Janssen: Research Funding; Summer Road: Research Funding.

Endothelial JAK2V617F Expression Drives Inflammation and Cellular Senescence; New Evidence for the Roles of Endothelial Cells in MPN-Related Clotting Abnormalities?

JAK2V617F is the primary pathogenic mutation in patients with Philadelphia chromosome-negative myeloproliferative neoplasms (MPNs), a heterogeneous group of haematological malignancies characterised by increases in one or more myeloid lineages. The most common cause of morbidity and mortality in patients with MPNs are thrombohemorrhagic incidents, although the events leading to these clotting abnormalities remain unclear. It is now known that a subset of MPN patients express JAK2V617F in endothelial cells (ECs) which has been linked with a poorer prognosis and an increase in the incidence of thrombosis. Furthermore, using transgenic mouse models we have recently shown that expression of JAK2V617F in ECs plays a critical role in aberrant thrombosis in vivo (PNAS, 2014. 111(6): p. 2295-300). Here, we describe the transcriptomic and phenotypic profiles of JAK2V617F-positive ECs.Lentivirally transduced human umbilical vein ECs (HUVECs) showed a sustained increase in levels of activated JAK2, STAT3 and STAT1 in JAK2V617F-expressing cells compared to WT JAK2. In addition, JAK2V617F also caused a significant increase in expression of total STAT1 protein. Subsequent RNAseq analysis revealed an upregulation of several mRNAs related to inflammation and permeability in JAK2V617F-expressing cells. Expression of interferon-related genes, including serine protease inhibitor B2 (SERPINB2) and early growth response protein 1 (EGR1), and chemokine ligand 2 (CCL2) were also increased in JAK2V617F cells. SERPINB2 and EGR1 have roles in initiating cellular senescence and CCL2 was previously reported to induce permeability. These data, combined with increased expression of total STAT1, suggest that JAK2V617F expression in ECs induces an inflammatory response in this cell type. Moreover, in vitro permeability assays show increased permeability in HUVECs transduced with JAK2V617Fcompared to WT JAK2, which was significantly amplified following stimulation with thrombin.We then determined whether any functional differences existed between patient-derived normal and JAK2V617F-expressing ECs. For this we obtained blood from JAK2V617F-positive polycythaemia vera patients undergoing therapeutic phlebotomy and isolated blood outgrowth endothelial cells (BOECs) by repeated culture of PBMCs. Allelic burden of JAK2V617F, determined by quantitative allele-specific amplification (QuASA) was shown to vary between patient samples, (0% to 59.8% (n=9)) and increased expression of JAK2V617F appeared to correlate with decreased proliferation and significant changes in cellular morphology.In order to quantify these differences, ptychography, a novel label-free imaging technology was used to generate quantitative phase images of BOECs in time-lapse, to measure phenotypic changes in cell morphology, motility and proliferation. These results confirmed that JAK2V617F allelic burden was inversely related to BOEC proliferation. Cells which expressed only WT JAK2 or low levels of JAK2V617F exhibited exponential growth. However, an increase in JAK2V617F resulted in a decrease in BOEC proliferation. Indeed, cells which expressed high JAK2V617F levels (30-60%) failed to proliferate over the 48 hour imaging period. Furthermore, JAK2V617F-high BOECs were also shown to be larger in size, multi-nucleated and less motile compared to JAK2V617F-low. The increase in cell size and decreases in cell motility and proliferation suggest that increased JAK2V617F expression induces senescence in BOECs.These data describe features of ECs which are commonly associated with dysfunctional endothelium: sustained inflammation/permeability, limited cell growth and senescence. The critical role of ECs in maintaining hemostasis and the observed features of EC dysfunction in cells which express JAK2V617F, implicate mutation burden in ECs as a contributing factor towards the thrombotic abnormalities observed in patients with MPNs. DisclosuresNo relevant conflicts of interest to declare.

Clinical and Biological Features in Patients with Ph-Negative Chronic Myeloproliferative Neoplasm Showing Different Molecular Pattern. Comparative Study in 596 Patients of the Registro Italiano Trombocitemie (RIT)

Background. In patients with Ph-negative chronic myeloproliferative neoplasm (MPN) the molecular pattern, beside other characteristics at diagnosis, has been related to the disease prognosis.Aim. To compare clinical and biological features at diagnosis and during the follow-up in gender/age-matched MPN patients showing different molecular pattern.Material and methods. The Registro Italiano Trombocitemie (RIT) is a web-based registry that includes thrombocythemic MPN patients diagnosed according to PVSG or WHO criteria, registered after 2005, and then prospectically followed. The JAK2 V617F mutation (JAK2+) was reported in 941 (58.4%) out of 1610 tested patients. On the basis of subsequent tests, performed in part of the JAK2 WT (JAK2-) subjects, 103 patients were JAK2-/CALR+ (CALR+), 14 patients were JAK2-/CALR-/MPL+ (MPL+), and 46 were JAK2-/CALR-/MPL- (3NEG).The 103 CALR+ patients were compared with 309 (103 x 3) JAK2+ patients matched for gender, age, and revised diagnosis (WHO 2008 criteria). A similar comparison was done between 46 3NEG patients and other 138 (46 x 3) JAK2+ matched patients.Results. CALR+ and matched JAK2+ patients had, as expected, the same gender distribution (males 41%), the same median age (51 years), and no significant difference (p 0.42) in the WHO diagnosis distribution.CALR+ patients, as compared with JAK2+ patients, showed at diagnosis: higher median platelet (PLT) count (839 vs 718 x109/L, p<0.001); lower median white blood cell (WBC) count (7.3 vs 8.9 x109/L, p<0.001); lower median hemoglobin (Hb, 14.2 vs 14.8 g/dL in males, p 0.01; 12.9 vs 14.0 g/dL in females, p<0.001); lower median hematocrit (HCT, 42.4 vs 45.0 %, p 0.002 in males; 38.7 vs 42.2 in females, p<0.001); lower rate of low (<5) serum erythropoietin (0 vs 32%, p 0.003); lower rate of prior thrombosis (PrTh, 5/103, 4.9% vs 60/309, 19.4%, p<0.001), observed for both arterial and venous PrTh; lower rate of high/intermediate thrombotic risk (IPSET, 37% vs 55%, p 0.003).CARL+ and JAK2+ patients had the same rate of antiplatelet and cytoreductive treatment (96% vs 96%, and 86% vs 84%, respectively). During the follow-up the incidence of thrombotic and hemorrhagic events was not significantly different (1.3 vs 1.1/100 pt-years, and 1.0 vs 0.6/100 pt-years, respectively). Moreover, no significant difference was observed in the incidence of evolution to overt primary myelofibrosis (PMF, 0.76 vs 0.61/100 pt-years), polycythemia vera (PV, 0 vs 0.24/100 pt-years), and AL/MDS (0.08 vs 0.10/100 pt-years).Finally, the same overall survival was found after 5, 10. 15, and 20 years (99, 97, 94, 93%, respectively).3NEG patients, as compared with JAK2+ matched patients, showed at diagnosis: lower median WBC count (7.9 vs 10.9 x 109/L, p 0.03); lower Hb and/or HCT level (p 0.006); lower rate of splenomegaly (7% vs 28%, p 0.003); lower rate of symptoms (35% vs 51%, p 0.049). No significant difference was found in: median PLT count (700 vs 720 x 109/L, p 0.61); PrTh (7% vs 16%, p 0.11); prior hemorrhage (4.7% vs 7.5%, p 0.52); high/intermediate thrombotic risk (IPSET, 36 vs 48%, p 0.37). Moreover, no significant difference was observed during the follow-up in: antiplatelet and cytoreductive treatment; thrombosis and hemorrhage rate; PMF, PV, and AL/MDS evolution; overall survival.Conclusion. CALR+ patients, as compared with JAK2+ matched patients, although showed a lower thrombotic risk (lower WBC and HCT levels, lower PrTh rate), received the same antiplatelet and cytoreductive treatment, had the same incidence of adverse events during the follow-up (vascular complications and disease evolution/transformation), and had the same overall survival.3NEG patients, as compared with JAK2+ matched patients, showed results similar to those observed by comparing CALR+ and JAK2+ matched patients.To better define the role of the precise definition of molecular pattern in Ph-MPN patients, new prospective controlled studies seem necessary. DisclosuresDe Stefano:Janssen Cilag: Research Funding; Roche: Research Funding; Novartis: Research Funding, Speakers Bureau; Amgen: Speakers Bureau; Bruno Farmaceutici: Research Funding; Celgene: Speakers Bureau; GlaxoSmithKline: Speakers Bureau; Shire: Speakers Bureau. Passamonti:Novartis: Consultancy, Honoraria, Speakers Bureau.

P49: Macrophages with JAK2V617F mutation are activated by platelets: Role in pathogenesis of myelofibrosis

Background Myelofibrosis with myeloid metaplasia is chronic mieloproliferative disease with ineffective erythropoiesis, dysplastic-megakaryocyte hyperplasia, and an increase in the ratio of immature granulocytes to total granulocytes. The incidence rate is 3.7–5.7 per 100,000 and the median survival is estimated to be between three and six years. The pathogenesis of myelofibrosis might be explained, in part, by a somatic point mutation on exon 14 (V617F) of the JAK2 kinase gene that is located on chromosome 9p24. However, mechanisms of regulation of microenvironment in bone marrow fibrosis are not clear. Considered that main producents of pro-fibrotic factors are the megakaryocytes and the macrophages. In this study we investigated influence of platelet factors on the cellular characteristics of the macrophages with oncogenic mutation JAK2 V617F. Materials and Methods THP-1 cell line was modified by lentiviral modification. Two cell lines established: one with expression of JAK2 with oncogenic mutation JAK2 V617F, another wild type JAK2. Cells were cultured in RPMI-1640 containing 0.1% gentamicin, 1% l-glutamine and 10% fetal bovine serum (FBS) or 5%, 10% and 15% platelet lysate (PL). Cell lines were differentiated into the macrophages using 50 ng/ml phorbolmyristate acetate (PMA). Level of expression transforming growth factor β (TGF β ), galectin-3, matrix metalloproteinases (MMP) 2, 9, 12, 13 and tissue inhibitors of matrix metalloproteinase (TIMP) accessed by specific RT-qPCR. Results We observed increased expression level of galectin-3, MMP-2, MMP-13 and TIMP-3 in JAK2 V617F expressing macrophages cultured with 10% FBS compare to ones with WT JAK2. Also macrophages containing mutation JAK2 V617F has increased level of expression of MMP-2, TIMP-3 and TIMP-4 when cultured with 10% platelet lysate. Also, cell line containing mutation JAK2 V617F has increased levels of expression of MMP-12 for cells cultured with 15% PL. We did not observe any significant difference in expression of TGF β , MMP-9, TIMP-1 and -2. Conclusion Increased levels of expression of galectin-3, MMP-12 and -13 suggest that platelet factors induce macrophages with JAK2 V617F mutation to release of profibrotic factors. On the other hand, increase of antifibrotic MMP-2 and TIMP-3 revealing complex nature of melofibrosis development and need to be analyzed further. This study confirms the idea of interaction between macrophages and platelets in pathogenesis of primary myelofibrosis.

Calreticulin Mutation Status Predicts Improved Disease Outcome in Prefibrotic Primary Myelofibrosis but Not in WHO-Defined Essential Thrombocythemia

The genomic landscape of myeloproliferative neoplasms (MPNs) has altered dramatically due to the recent discovery of somatic mutations of calreticulin (CALR). This discovery enables for the first time molecular information, in addition to JAK2V617F, to be used in the majority of MPN patients as an affirmative variable to discriminate MPNs from reactive myeloid proliferations. The clinical course of essential thrombocythemia (ET) or primary myelofibrosis (PMF) in patients carrying the CALR mutation was reported to be more indolent than in JAK2 positive patients and was associated with increased survival. Our aim was to investigate whether the impact of CALR expression on prognosis and clinical outcome is different in prefibrotic/early PMF (prePMF) compared to WHO-ET.In a cohort of 348 adult patients with the clinical diagnosis of either ET or PMF mutational analysis for CALR was available. Eligibility criteria for the study included: availability of mutation analysis for JAK2, MPL and CALR; availability of representative, treatment-naive bone marrow biopsy (BM); availability of a histological and clinical consensus on the diagnosis; complete long-term documentation of clinical data and outcome.Consenting clinico-pathological findings in our cohort were consistent with 115 cases showing WHO-ET and 85 patients with prePMF. In comparison to WHO-ET, prePMF revealed minor/borderline age- and gender-matched anemia, slight increase in serum LDH level and leukocyte count, minor to slight splenomegaly, and an occasional left shift in granulo- and erythropoiesis with occurrence of a few myelo-and/or erythroblasts (table 1). An accurate differentiation between both MPN entities was shown to exert a significant difference in terms of overall and relative survival and hematologic transformation into overt PMF and AL. The present study revealed a different CALR mutation frequency in ET in contrast to most of the investigations published recently. We observed CALR mutations in 18% of WHO-ET; JAK2, MPL and CALR wildtype (wt) was observed in 13% of WHO-ET. The discrepancy in the frequencies of CALR positivity in our ET cohort to most of the recently published studies may be due to our strict adherence to the WHO criteria for diagnosis of ET. Regarding prePMF, we observed CALR mutations in 39% of the patients. 92% of the JAK2 and MPL wt subgroup carried the CALR mutation, with JAK2, MPL and CALR wt being observed in only 3% of prePMF.The most remarkable differences between WHO-ET and prePMF were seen in the comparison of the overall survival (figure 1). While the CALR mutation did not have any beneficial influence on survival in WHO-ET, it was associated with a superior overall survival in prePMF. Such a striking difference was not seen at the time of transformation into overt myelofibrosis, and there was only a slightly shorter time to progression to fibrosis in CALR wt prePMFs. There was a trend showing that CALR mutated prePMF patients have shorter thrombosis-free survival compared to CALR wt prePMF patients. There was no impact of the CALR mutations on thrombosis-free survival in WHO-ET.The present data confirm that WHO-ET and prePMF are biologically different sub-entities of MPNs. In prePMF, almost 100% of patients are now associated with a known disease-causing mutation. Our data support the classical clinical approach in the diagnosis of thrombocytosis, using BM histology to differentiate WHO-ET from prePMF and to estimate the outcome of the disease more accurately. Table 1:Total cohort (N=200)WHO-ET (N=115)prePMF (N=85)PAge at diagnosis, years0,04median58,8556,460,7range19-8819-8427-88Sex0,587male784335female1227250Hb, g/dL<0,001median14,114,313,5range8,1-17,68,8-17,68,1-16,6WBC, x109/l0,054median9,258,999,6range4,01-24,544,92-22,34,01-24,54Platelets, x109/l0,739median770770794range78-2530414-249078-2530Palpable splenomegaly<0,001No.51 (12 unk, 1 splenectomy)16 (7 unk, 1 splenectomy)35 (5 unk)%25,513,941,2Fibrotic transformations<0,001No.24 (10 unk)6 (1 unk)18 (9 unk)%125,221,2Thrombotic events0,439No.39 (1 unk)22 (1 unk)17%19,51920prev thrombosis0,042No.503416%2530,418,8JAK2 V617F pos0,08No.1207545%6065,252,9CALR pos0,001No.542133%2718,338,8MPL pos0,189No.844%43,54,7JAK2/MPL/CALR wt0,0029No.18153%9133,5 [Display omitted] DisclosuresThiele:AOP Orphan Pharmaceuticals: Consultancy, Honoraria; Incyte Corporation: Consultancy, Honoraria; Novartis: Consultancy, Honoraria, Research Funding; Shire: Consultancy, Honoraria, Research Funding; Sanofi: Consultancy, Honoraria.