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Classical BCR-ABL–negative myeloproliferative neoplasms (MPNs) are clonal disorders of hematopoietic stem cells (HSCs) caused mainly by recurrent mutations in genes encoding JAK2 (JAK2), calreticulin (CALR), or the thrombopoietin receptor (MPL). Interferon α (IFNα) has demonstrated some efficacy in inducing molecular remission in MPNs. To determine factors that influence molecular response rate, we evaluated the long-term molecular efficacy of IFNα in patients with MPN by monitoring the fate of cells carrying driver mutations in a prospective observational and longitudinal study of 48 patients over more than 5 years. We measured the clonal architecture of early and late hematopoietic progenitors (84 845 measurements) and the global variant allele frequency in mature cells (409 measurements) several times per year. Using mathematical modeling and hierarchical Bayesian inference, we further inferred the dynamics of IFNα-targeted mutated HSCs. Our data support the hypothesis that IFNα targets JAK2V617F HSCs by inducing their exit from quiescence and differentiation into progenitors. Our observations indicate that treatment efficacy is higher in homozygous than heterozygous JAK2V617F HSCs and increases with high IFNα dose in heterozygous JAK2V617F HSCs. We also found that the molecular responses of CALRm HSCs to IFNα were heterogeneous, varying between type 1 and type 2 CALRm, and a high dose of IFNα correlates with worse outcomes. Our work indicates that the long-term molecular efficacy of IFNα implies an HSC exhaustion mechanism and depends on both the driver mutation type and IFNα dose.

INTRODUCTION Primary and post-ET/PV myelofibrosis are myeloproliferative neoplasms harboring in most cases driving mutations in JAK2, CALR or MPL, and a variable number of additional mutations in other genes. Molecular analysis represents a powerful tool to guide prognosis and clinical management. Only about 10% of patients with myelofibrosis harbor alterations in MPL gene. No data are available about the transplantation outcome in the specific MPL-mutated group. PATIENTS We collected the data of 18 myelofibrosis patients(primary: 14; post-ET: 4) transplanted in 4 EBMT centers (Hamburg, Paris, Essen, and Hannover) between 2005 and 2016. RESULTS Before the transplant, we explored the molecular profile by NGS and reported the frequency of mutations occurring in a panel of genes including JAK2, MPL, CALR, U2AF1, SRSF2, SF3B1, ASXL1, IDH1, IDH2, CBL, DNMT3A, TET2, EZH2, TP53, IKZF1, NRAS, KRAS, FLT3, SH2B3, and RUNX1. The 1-year transplant-related mortality was 16.5%, 5-years overall survival and 5-y relapse-free survival 83.5%. The only relapse occurred in a patient who harbored mutations in both ASXL1 and EZH2 genes. CONCLUSION These retrospective data suggest that MPL-mutated myelofibrosis patients have a favorable outcome after allogeneic transplantation with very low rate of disease relapse (5.5%) in comparison with the available historical controls regarding myelofibrosis in all.

Myelodysplastic syndromes and acute myeloid leukemia with TP53 mutations are characterized by frequent relapses, poor or short responses, and poor survival with the currently available therapies including chemotherapy and 5-azacitidine (AZA). PRIMA-1Met(APR-246,APR) is a methylated derivative of PRIMA-1, which induces apoptosis in human tumor cells through restoration of the transcriptional transactivation function of mutant p53. Here we show that low doses of APR on its own or in combination with AZA reactivate the p53 pathway and induce an apoptosis program. Functionally, we demonstrate that APR exerts these activities on its own and that it synergizes with AZA in TP53-mutated myelodysplastic syndromes (MDS)/acute myeloid leukemia (AML) cell lines and in TP53-mutated primary cells from MDS/AML patients. Low doses of APR on its own or in combination with AZA also show significant efficacy in vivo Lastly, using transcriptomic analysis, we found that the APR + AZA synergy was mediated by downregulation of the FLT3 pathway in drug-treated cells. Activation of the FLT3 pathway by FLT3 ligand reversed the inhibition of cell proliferation by APR + AZA. These data suggest that TP53-mutated MDS/AML may be better targeted by the addition of APR-246 to conventional treatments.

Allogeneic hematopoietic stem cell transplantation is curative in myelofibrosis, and current prognostic scoring systems aim to select patients for transplantation. Here, we aimed to develop a prognostic score to determine prognosis after transplantation itself, using clinical, molecular, and transplant-specific information from a total of 361 patients with myelofibrosis. Of these, 205 patients were used as a training cohort to create a clinical-molecular myelofibrosis transplant scoring system (MTSS), which was then externally validated in a cohort of 156 patients. Multivariable analysis on survival identified age at least 57 years, Karnofsky performance status lower than 90%, platelet count lower than 150 × 109/L, leukocyte count higher than 25 × 109/L before transplantation, HLA-mismatched unrelated donor, ASXL1 mutation, and non-CALR/MPL driver mutation genotype being independent predictors of outcome. The uncorrected concordance index for the final survival model was 0.723, and bias-corrected indices were similar. Risk factors were incorporated into a 4-level MTSS: low (score, 0-2), intermediate (score, 3-4), high (score, 5), and very high (score, >5). The 5-year survival according to risk groups in the validation cohort was 83% (95% confidence interval [CI], 71%-95%), 64% (95% CI, 53%-75%), 37% (95% CI, 17%-57%), and 22% (95% CI, 4%-39%), respectively (P < .001). Increasing score was predictive of nonrelapse mortality (P < .001) and remained applicable to primary (0.718) and post-essential thrombocythemia (ET)/polycythemia vera (PV) myelofibrosis (0.701) improving prognostic ability in comparison with all currently available disease-specific systems. In conclusion, this MTSS predicts outcome of patients with primary and post-ET/PV myelofibrosis undergoing allogeneic stem cell transplantation.

Key Clinical Message Although uncommon, clinicians should be aware that polycythemia vera may be masked due to hemolysis. The report of such associations could help them in clinical practice to establish an early and accurate diagnosis that may be challenging in atypical presentations of myeloproliferative neoplasms.

In acute myeloid leukemia (AML), acquired genetic aberrations carry prognostic implications and guide therapeutic decisions. Clinical algorithms have been improved by the incorporation of novel aberrations. Here, we report the presence and functional characterization of mutations in the transcription factor NFE2 in patients with AML and in a patient with myelosarcoma. We previously described NFE2 mutations in patients with myeloproliferative neoplasms and demonstrated that expression of mutant NFE2 in mice causes a myeloproliferative phenotype. Now, we show that, during follow-up, 34% of these mice transform to leukemia presenting with or without concomitant myelosarcomas, or develop isolated myelosarcomas. These myelosarcomas and leukemias acquired AML-specific alterations, including the murine equivalent of trisomy 8, loss of the AML commonly deleted region on chromosome 5q, and mutations in the tumor suppressor Trp53. Our data show that mutations in NFE2 predispose to the acquisition of secondary changes promoting the development of myelosarcoma and/or AML.

Introduction Les maladies auto-immunes et inflammatoires (MAI) sont associees a un sur-risque de survenue de neoplasies myeloproliferatives (NMP) [1] . Chez les patients presentant un syndrome myelodysplasique (SMD) ou une leucemie myelomonocytaire chronique (LMMC), les mutations des regulateurs epigenetiques TET2 et IDH1/2 etaient significativement plus frequentes chez les patients porteurs de MAI comparativement a une cohorte temoin de patients SMD/LMMC sans MAI [2] . Dans le contexte des NMP, le phenotype des MAI associees et leur statut mutationnel ont ete peu decrits. Les objectifs de notre etude etaient de decrire les caracteristiques cliniques et genetiques des patients presentant une NMP avec une MAI et d’evaluer leur impact pronostique. Patients et Methodes Dans notre centre, 1541 patients ont ete diagnostiques d’une NMP selon les criteres OMS, entre janvier 2011 et janvier 2021. Des analyses moleculaires par sequencage haut debit (NGS) ciblant un panel de 36 genes mutes dans les NMP ont ete realisees pour 998 d’entre eux, au diagnostic ou durant le suivi. Tous les cas de MAI ont ete revus par un interniste et les diagnostics evalues selon les criteres internationaux correspondants. Les diagnostics de MAI post-interferon ont ete exclues. Resultats Notre cohorte comprenait 522 (34 %) polyglobulie de Vaquez (PV), 709 (46 %) thrombocytemie essentielle (TE) et 229 (15 %) myelofibrose primitive (MFP). Cent (6.6 %) patients presentaient une MAI associee et les 1441 patients restants ont constitue notre population temoin. L’âge median de nos patients dans la cohorte globale etait de 54 ans [IQR 40.4–63.2] et il y avait une predominance de femmes dans le groupe avec MAI associee (66 (66 %) versus 769 (53 %), p = 0.019). Il n’y avait pas de difference significative entre les 2 groupes concernant les sous-types de NMP, les mutations « driver » (JAK2, MPL et CALR) ou encore l’hemogramme au diagnostic des NMP. L’incidence d’episodes thrombotiques ou hemorragiques etait aussi similaire dans les 2 groupes (44 (44 %) versus 564 (39 %), p = 0.356). Le diagnostic de MAI etait anterieur a celui de NMP dans 34 % des cas, concomitant dans 12 % et ulterieur dans 31 %. Sur les 100 MAI, 45 etait des maladies auto-immunes specifiques d’organe, principalement des thyroidites auto-immunes, 13 rhumatismes inflammatoires chroniques, 9 connectivites, 8 dermatoses inflammatoires, 7 vascularites et 18 inclasses. Au total, 70 % des maladies auto-immunes remplissaient leurs criteres de classification. Parmi les patients ayant beneficie d’une analyse moleculaire, 62 % avaient une mutation additionnelle a la mutation « driver ». Les mutations de TET2 etaient plus frequemment retrouvees chez les patients avec une MAI associee (24 (34 %) versus 205 (22 %), OR = 1.84 95 %CI [1.08–3.07], p = 0.028), mais elle n’etait pas associee a un sous-type de MAI particulier. Il existait une tendance a une surrepresentation des mutations IDH1/2 dans le groupe MAI (4 (6 %) versus 27 (3 %), OR = 2.02 95 %CI[0.74–5.51], p = 0.27), bien que non statistiquement significatif. Nous n’avons pas retrouve d’autres mutations de facteurs epigenetiques, de transcription, d’epissage ou de mutation a haut risque moleculaire, associees preferentiellement au groupe NMP avec MAI associee. Apres un suivi median de 8.3 ans dans la cohorte globale, la presence d’une MAI n’avait pas d’impact defavorable sur la survie globale (p = 0.82), ni sur la survenue de myelofibrose secondaire (p = 0.98), ou la survenue de transformation en SMD/LAM (p = 0.53). Conclusion Au sein de cette large cohorte retrospective de NMP decrite sur le plan clinique et moleculaire, la prevalence de MAI est de 6.6 %, similaire a celle de la population generale. Une sur-prevalence de la mutation de TET2 a ete mise en evidence dans le groupe de patients presentant une MAI associee. Nos resultats suggerent une susceptibilite genetique commune, secondaire a la mutation de regulateur epigenetique comme TET2, potentiellement responsable des phenotypes inflammatoire et hematologique.

International audience; Myeloproliferative neoplasms (MPNs) are a group of disorders characterized by clonal expansion of abnormal hematopoietic stem cells leading to hyperproliferation of one or more myeloid lineages. The main complications in MPNs are high risk of thrombosis and progression to myelofibrosis and leukemia. MPN patients with high risk scores are treated by hydroxyurea (HU), interferon-α, or ruxolitinib, a tyrosine kinase inhibitor. Polycythemia vera (PV) is an MPN characterized by overproduction of red blood cells (RBCs). ABCG2 is a member of the ATP-binding cassette superfamily transporters known to play a crucial role in multidrug resistance development. Proteome analysis showed higher ABCG2 levels in PV RBCs compared to RBCs from healthy controls and an additional increase of these levels in PV patients treated with HU, suggesting that ABCG2 might play a role in multidrug resistance in MPNs. In this work, we explored the role of ABCG2 in the transport of ruxolitinib and HU using human cell lines, RBCs, and in vitro differentiated erythroid progenitors. Using stopped-flow analysis, we showed that HU is not a substrate for ABCG2. Using transfected K562 cells expressing three different levels of recombinant ABCG2, MPN RBCs, and cultured erythroblasts, we showed that ABCG2 potentiates ruxolitinib-induced cytotoxicity that was blocked by the ABCG2-specific inhibitor KO143 suggesting ruxolitinib intracellular import by ABCG2. In silico modeling analysis identified possible ruxolitinib-binding site locations within the cavities of ABCG2. Our study opens new perspectives in ruxolitinib efficacy research targeting cell types depending on ABCG2 expression and polymorphisms among patients.

We aimed to study the prognostic impact of the mutational landscape in primary and secondary myelofibrosis. The study included 479 patients with myelofibrosis recruited from 24 French Intergroup of Myeloproliferative Neoplasms (FIM) centers. The molecular landscape was studied by high-throughput sequencing of 77 genes. A Bayesian network allowed the identification of genomic groups whose prognostic impact was studied in a multistate model considering transitions from the 3 conditions: myelofibrosis, acute leukemia, and death. Results were validated using an independent, previously published cohort (n = 276). Four genomic groups were identified: patients with TP53 mutation; patients with ≥1 mutation in EZH2, CBL, U2AF1, SRSF2, IDH1, IDH2, NRAS, or KRAS (high-risk group); patients with ASXL1-only mutation (ie, no associated mutation in TP53 or high-risk genes); and other patients. A multistate model found that both TP53 and high-risk groups were associated with leukemic transformation (hazard ratios [HRs] [95% confidence interval], 8.68 [3.32-22.73] and 3.24 [1.58-6.64], respectively) and death from myelofibrosis (HRs, 3.03 [1.66-5.56] and 1.77 [1.18-2.67], respectively). ASXL1-only mutations had no prognostic value that was confirmed in the validation cohort. However, ASXL1 mutations conferred a worse prognosis when associated with a mutation in TP53 or high-risk genes. This study provides a new definition of adverse mutations in myelofibrosis with the addition of TP53, CBL, NRAS, KRAS, and U2AF1 to previously described genes. Furthermore, our results argue that ASXL1 mutations alone cannot be considered detrimental.

International audience; Molecular tests have become an indispensable tool for the diagnosis and prognosis of hematological malignancies and are subject to accreditation according to the International Standard ISO 15189. National standardization of these techniques is essential to ensure that patients throughout France benefit from the same care. We report here on the experience of the GBMHM (Groupe des Biologistes Moléculaires des Hémopathies Malignes). By organizing External Evaluation of Quality (EEQ) programs and training meetings, the GBMHM has contributed to improvement and standardization of molecular tests in 64 French laboratories. A retrospective analysis of the quality-control results of 11 national campaigns spanning 10 years was performed for the 3 most frequently prescribed tests: BCR-ABL1, JAK2 V617F, and lymphoid clonality. For each test, particular attention was placed on comparing methodologies and their evolution throughout the period. The establishment of the BCR-ABL1, JAK2 V617F, and lymphoid clonality EEQ programs and the associated training meetings have initiated a process of collective standardization concerning the methods of implementation (JAK2 V617F) and the interpretation and formulation of results (lymphoid clonality). In addition, it resulted in objective improvement in technical performance (BCR-ABL1). Our evaluation of the impact of these EEQ programs demonstrates that it is possible to obtain reproducible values across different laboratories in France by applying national recommendations. To our knowledge, this is the first publication that evaluates the impact of a national quality assurance program on improving molecular results in hematology.

International audience; No abstract available

International audience; No abstract available

Juvenile myelomonocytic leukemia (JMML) is a rare aggressive myelodysplastic/myeloproliferative neoplasm of early childhood, initiated by RAS-activating mutations. Genomic analyses have recently described JMML mutational landscape; however, the nature of JMML-propagating cells (JMML-PCs) and the clonal architecture of the disease remained until now elusive. Combining genomic (exome, RNA-seq), Colony forming assay and xenograft studies, we detect the presence of JMML-PCs that faithfully reproduce JMML features including the complex/nonlinear organization of dominant/minor clones, both at diagnosis and relapse. Further integrated analysis also reveals that although the mutations are acquired in hematopoietic stem cells, JMML-PCs are not always restricted to this compartment, highlighting the heterogeneity of the disease during the initiation steps. We show that the hematopoietic stem/progenitor cell phenotype is globally maintained in JMML despite overexpression of CD90/THY-1 in a subset of patients. This study shed new lights into the ontogeny of JMML, and the identity of JMML-PCs, and provides robust models to monitor the disease and test novel therapeutic approaches.

BCR-ABL-negative myeloproliferative neoplasms (MPNs) in transformation have a dismal prognosis, and allogeneic hematopoietic stem cell transplantation (allo-HSCT) is considered the sole curative therapeutic option. We retrospectively analyzed 53 molecularly annotated patients treated at Saint Louis Hospital, Paris, diagnosed with MPN in transformation between 2008 and 2018. The median patient age was 65 years, and the median interval between MPN diagnosis and MPN transformation was 46 months. The median overall survival (OS) of the entire cohort after transformation was 7.1 months. OS was better for patients treated with hypomethylating agents (HMAs) or with chemotherapy compared than for those treated by best supportive care or single-agent targeted therapy (median, 9.1 months versus 1.5 months; P.001). Patients treated with chemotherapy more often achieved complete remission compared with those treated with HMAs (68% versus 29%; P = .02), and were more often candidates for transplantation (59% versus 14%; P = .02), but the median OS was similar in the 2 groups. We then compared the outcomes in transplant recipients and nonrecipients using the Mantel-Byar methodology and found that allo-HSCT did not improve survival. In multivariate analysis, independent factors in prognosis were performance status at transformation (P.01), initial treatment with HMAs or chemotherapy (P = .02), and the ability to achieve complete remission during follow-up (P.01). Our data demonstrate that the indication for allo-HSCT for high-risk MPN should be discussed before transformation, because transplantation rescues few patients after transformation.

Myeloproliferative neoplasms (MPNs) are the most frequent underlying causes of splanchnic vein thromboses (SVTs). MPN patients with SVTs (MPN-SVT) often have a unique presentation including younger age, female predominance, and low Janus kinase 2 (JAK2) mutation allele burden. This study aimed at identifying risk factors for adverse hematologic outcomes in MPN-SVT patients. We performed a retrospective study of a fully characterized cohort of MPN-SVT patients. The primary outcome was the incidence of evolution to myelofibrosis, acute leukemia, or death. Eighty patients were included in the testing cohort. Median follow-up was 11 years. Most of the patients were women with a mean age of 42 years and a diagnosis of polycythemia vera. The primary outcome was met in 13% of the patients and was associated with a JAK2V617F allele burden ≥50% (odds ratio [OR], 14.7) and presence of additional mutations in genes affecting chromatin/spliceosome (OR, 9). We identified high-risk patients (29% of the cohort) as those harboring at least 1 molecular risk factor: JAK2-mutant allele burden ≥50%, presence of chromatin/spliceosome/TP53 mutation. High-risk patients had worse event-free survival (81% vs 100%; P = .001) and overall survival at 10 years (89% vs 100%; P = .01) than low-risk patients. These results were confirmed in an independent validation cohort of 30 MPN-SVT patients. In conclusion, molecular profiling identified MPN-SVT patients with dismal outcome. In this high-risk population, a disease-modifying therapy should be taken into consideration to minimize the probability of transformation.

Mutations in the MPL gene encoding the human thrombopoietin receptor (TpoR) drive sporadic and familial essential thrombocythemias (ETs). We identified 2 ET patients harboring double mutations in cis in MPL, namely, L498W-H499C and H499Y-S505N. Using biochemical and signaling assays along with partial saturation mutagenesis, we showed that L498W is an activating mutation potentiated by H499C and that H499C and H499Y enhance the activity of the canonical S505N mutation. L498W and H499C can activate a truncated TpoR mutant, which lacks the extracellular domain, indicating these mutations act on the transmembrane (TM) cytosolic domain. Using a protein complementation assay, we showed that L498W and H499C strongly drive dimerization of TpoR. Activation by tryptophan substitution is exquisitely specific for position 498. Using structure-guided mutagenesis, we identified upstream amino acid W491 as a key residue required for activation by L498W or canonical activating mutations such as S505N and W515K, as well as by eltrombopag. Structural data point to a common dimerization and activation path for TpoR via its TM domain that is shared between the small-molecule agonist eltrombopag and canonical and novel activating TpoR mutations that all depend on W491, a potentially accessible extracellular residue that could become a target for therapeutic intervention.

Introduction The pathogenesis of myeloproliferative neoplasms (MPNs) is closely related to the acquisition of specific molecular alterations in JAK2, MPL, or CALR genes, the presence of which represent major diagnostic criteria in the WHO classification. The CALR exon 9 insertions and deletions are very heterogeneous, and their detection mainly relies on polymerase chain reaction (PCR) fragment length analysis. Methods We report on the rare nonclassical profiles that we observed among the 1382 patients analyzed by PCR fragment length analysis. In difficult cases, we tested germline DNA and performed NGS analysis. Results We faced some troubling results because of the presence of several unexpected peaks. Our investigations showed that they resulted from a mix of isolated or double somatic mutations combined with germline alterations which could be misleading for a correct diagnosis. The precise interpretation of such difficult cases is mandatory as a misinterpretation could lead to the prescription of cytoreductive drugs to nondiseased persons or to an absence of treatment in true MPN patients. Conclusion Our observations showed that every mutation should be verified by direct Sanger sequencing, and we show that sometimes it may be necessary to study germline DNA and to complement with NGS analysis to precisely interpret the molecular alterations.

Polycythemia vera is characterized by the acquisition of the JAK2V617F mutation. Recommended treatments include hydroxyurea and interferon-alpha. Several groups have reported a reduction in the JAK2 mutant allele burden in interferon-treated patients, but significance of this observation is questioned. We characterized the activity of ropeginterferon alpha-2b, a novel form of interferon-alpha recently shown to be safe and efficacious in polycythemia vera. Ropeginterferon was able to inhibit the proliferation of the HEL, UKE-1, and UT-7 JAK2-mutant cell lines while sparing JAK2-wild-type UT-7 and normal CD34+ cells growth. In vitro treatment of erythroid progenitors derived from PV patients showed that ropeginterferon could considerably inhibit the growth of endogenous erythroid colonies, a hallmark of polycythemia vera. Finally, we could study in sequential samples the clonal architecture of erythroid progenitors derived from patients included in a randomized study comparing hydroxyurea to ropeginterferon. After 1 year of treatment with ropeginterferon, the ratio of JAK2-mutated to wild-type colonies grown from bone marrow progenitors was reduced by 64%, compared to 25% in patients receiving hydroxyurea. This study shows that ropeginterferon has a potent targeted activity against JAK2-mutant cells and is able to drastically reduce the proportion of malignant progenitors in patients treated with this drug.

Background Circulating peripheral blasts ≥1% have long been considered an unfavorable feature for patients with primary myelofibrosis. Recent findings (Masarova et al. Cancer 2020) suggested more differentiated impact of blasts on outcome. However, accelerated-phase (AP) myelofibrosis, which is currently defined by circulating blasts 10-19%, usually confers worse outcome. The outcome of allogeneic stem cell transplantation for AP myelofibrosis has not been evaluated yet. Patients and Methods Thirty-five out of 349 patients with primary or secondary myelofibrosis undergoing reduced intensity allogeneic stem cell transplantation were reported as AP (10-19% blasts) at time of transplantation. Outcome of these patients was compared to patients with circulating blasts: 0% (n=135), 1-4% (n=146), and 5-9% (n=33). Conditioning consisted of busulfan/fludarabine, fludarabine/melphalan, or fludarabine/TBI2Gy. Results Characteristics. The median blast percentage in the AP group was 14% (10-19%). More patients in the AP group appeared to have secondary myelofibrosis (40%) compared with patients with the 0% blasts (21%), the 1-4% blasts (31%), and 5-9% blasts group (30%, P=0.08). The median hemoglobin levels decreased with blast group: 9.7g/dl (0%), 9.5g/dl (1-4%), 9.4g/dl (5-9%), and 9.0g/dl (AP). In contrast, median leukocyte counts appeared to be increased: 7.2, 9.5, 16.9, and 13.6 x 10 9/l, respectively. More patients in the AP group presented with constitutional symptoms (71%). There was no difference in frequency of driver mutations (P=0.57) and presence of high-risk mutation profile (defined as ASXL1, SRSF2, IDH1/2, EZH2; P=0.93). Most patients in each group received matched unrelated donor transplants (P=0.61). Survival. The median follow-up of all patients was 6 years. Follow-up was similar across the 4 groups (P=0.30). 5-year overall survival (95% confidence interval) according to blast group was 66% (58-73%) for the 0%, 62% (53-71%) for the 1-4%, 66% (50-81%) for the 5-9%, and 68% (53-83%) for the AP group (P=0.92). Median overall survival was not reached for all groups, except for 1-4% blasts group (17.1 years). 10-year long-term follow-up showed survival rates of 64% for the 0%, 58% for the 1-4%, 66% for the 5-9%, and 68% for the AP group. In terms of relapse-free survival, 5-year outcome was comparable (P=0.95) showing 57% (48-66%) for the 0%, 52% (43-60%) for the 1-4%, 55% (37-73%) for the 5-9%, and 52% (34-69%) for the AP group. Median relapse-free survival was 7.9 years, 5.7 years, 6.5 years, and 9.2 years, respectively. Taking blasts as continuous variable in spline function analyses on survival, no significant effect was identified overall, while after 15% blasts, risk for death appeared to increase consistently. Non-relapse mortality and relapse. In terms of non-relapse mortality, no difference was found between the groups (P=0.33). 5-year outcome was 25% (19-31%) for the 0%, 33% (25-40%) for the 1-4%, 31% (15-47%) for the 5-9%, and 17% (5-30%) for the AP group. In terms of cumulative incidence of relapse, the AP group showed 5-year outcome of 31% (15-47%) compared to 18% (12-24%) for the 0%, 16% (10-22%) for the 1-4%, and 14% (1-28%) for the 5-9% group (P=0.17). Pairwise comparison showed significant difference between AP and the other groups (P=0.03). Spline function analyses using blasts as continuous variable and 3 knots showed no significant effect on non-relapse mortality but suggested increased risk of relapse for AP myelofibrosis (P=0.04). Multivariable analysis. Adjusting for clinical (diagnosis, leukocyte and platelet counts, age, constitutional symptoms) and molecular-genetic (driver mutation genotype, high-risk mutations), Cox model on survival (with the 0% group as reference) showed comparable risk of death across blast groups with hazard ratios of 1.06 (0.70-1.62) for the 1-4%, 1.11 (0.56-2.32) for the 5-9%, and 0.95 (0.49-1.86) for the AP group. Independent factors for worse outcome were age, platelet and leukocyte counts, and CALR/MPL-unmutated genotype. Conclusion Reduced intensity allogeneic stem cell transplantation for AP myelofibrosis was associated with excellent and similar survival and non-relapse mortality in comparison with other blast groups and after adjustment for other risk factors, offering long-term survival (>10 years) for more than half of patients at AP. Relapse incidence appeared to be increased for AP myelofibrosis. Disclosures Heuser: Karyopharm: Research Funding; Tolremo: Membership on an entity's Board of Directors or advisory committees; Daiichi Sankyo: Membership on an entity's Board of Directors or advisory committees, Research Funding; Novartis: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Janssen: Honoraria; Jazz: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Astellas: Research Funding; Pfizer: Membership on an entity's Board of Directors or advisory committees, Research Funding; AbbVie: Membership on an entity's Board of Directors or advisory committees, Research Funding; BergenBio: Research Funding; Roche: Membership on an entity's Board of Directors or advisory committees, Research Funding; Bayer Pharma AG: Research Funding; BMS/Celgene: Membership on an entity's Board of Directors or advisory committees, Research Funding. Thol: Pfizer: Honoraria; Abbvie: Honoraria; Astellas: Honoraria; BMS/Celgene: Honoraria, Research Funding; Jazz: Honoraria; Novartis: Honoraria. Scott: Bristol Myers Squibb: Consultancy, Honoraria, Research Funding. Kroeger: Neovii: Honoraria, Research Funding; Sanofi: Honoraria; Jazz: Honoraria, Research Funding; Celgene: Honoraria, Research Funding; Riemser: Honoraria, Research Funding; Gilead/Kite: Honoraria; AOP Pharma: Honoraria; Novartis: Honoraria.

Introduction: Although myeloproliferative neoplasms (MPN) are driven by three mutually exclusive driver mutations (JAK2, CALR and MPL), targeted deep sequencing studies identified multiple additional somatic mutations potentially impacting MPN evolution. Presence of a high molecular risk (HMR: ASXL1, EZH2, SRSF2 and IDH1/2) or a TP53 mutations has been associated with adverse prognosis. However, to date, the effect of clonal evolution (CEv) on MPN patients' outcome has not been evaluated, as most of the studies assessed mutational-based prognosis stratification from single baseline molecular genotyping. The objective of our study was to describe the clinical and molecular characteristics of patients with CEv in a large cohort of MPN patients and analyze its impact on patients' outcome. Methods: A total of 1538 consecutive patients were diagnosed with MPN according to WHO criteria and followed in our hospital between January 2011 and January 2021. From this large retrospective cohort, we included in this study 446 patients who had at least 2 molecular analyses during the chronic phase of MPN, performed at diagnosis and/or during follow-up using next generation sequencing (NGS), targeting a panel of 36 genes involved in myeloid malignancies. Significant variants were retained with a sensitivity of 1%. CEv was defined as the acquisition of a new additional non-driver mutation between baseline and subsequent NGS evaluation. Statistical analyses were performed using the STATA software (STATA 17.0 for Mac Corporation, College Station, TX). Results: The median age at MPN diagnosis in our whole cohort was 51 years [IQR 41 - 60]. Our cohort included 167 (37%), 205 (46%) and 64 (14%) patients with polycythemia vera, essential thrombocythemia and primary myelofibrosis (MF) respectively. With a median interval of 1.6 years [IQR 1.0 - 2.8] between the first and the second NGS analysis in the whole cohort, CEv occurred in 128 patients (29%). Patients with CEv were significantly older compared to patients without CEv (n=318) (p=0.03). MPN diagnosis, the type of driver mutation and complete blood counts at MPN diagnosis did not differ between the 2 groups. Eighty-one (63%) and 198 (62%) patients with or without CEv respectively had at least one additional non-driver mutation at baseline NGS (p=0.59), while the rate of HMR (n=25 (20%) versus n=79 (25%)) or TP53 (n=7 (5%) versus n=20 (6%)) mutations at baseline NGS did not differ between the 2 groups. Thirty six out of 128 (28%) of patients with CEv had more than 1 acquired mutation. Most recurrently acquired mutations involved the epigenetic regulators TET2 and DNMT3A that were mutated in respectively 33% and 25% of patients with CEv (Figure 1A). Moreover, 38% of CEv patients acquired HMR (ASXL1 (14%), EZH2 (6%), SRSF2 (3%), IDH1/2 (2%)) or TP53 (13%) mutations. After a median follow up of 10.8 years [IQR 6.6 - 17.2] in the whole cohort representing a total of 5635 patient years, 32 (7%) patients died, and 11 (2.5%) and 11 (2.5%) patients with at least 2 NGS performed during MPN chronic phase transformed respectively into secondary MF or myelodysplastic syndrome / acute myeloid leukemia (MDS/AML). Interestingly, CEv (HR 11.27, 95%CI [5.09; 24.96], p Conclusion: Our study on a large retrospective clinically and biologically annotated real-life cohort of MPN patients with long-term follow up shows that CEv independently adversely impacts OS, MDS/AML and secondary MF free survivals. CEv occurred in a clinically relevant proportion of MPN patients (28%) and was associated with patients' age. Acquired mutations mainly involved epigenetic regulators, HMR and TP53 genes. These results suggest that serial molecular monitoring using NGS could be routinely implemented in MPN patients follow up, to assess more accurately disease evolution and potentially update therapeutic management. Figure 1 Figure 1. Disclosures Raffoux: PFIZER: Consultancy; CELGENE/BMS: Consultancy; ABBVIE: Consultancy; ASTELLAS: Consultancy. Kiladjian: Novartis: Membership on an entity's Board of Directors or advisory committees; Taiho Oncology, Inc.: Research Funding; Bristol Myers Squibb: Membership on an entity's Board of Directors or advisory committees; Incyte Corporation: Membership on an entity's Board of Directors or advisory committees; PharmaEssentia: Other: Personal fees; AOP Orphan: Membership on an entity's Board of Directors or advisory committees; AbbVie: Membership on an entity's Board of Directors or advisory committees. Benajiba: Gilead: Research Funding; Pfizer: Research Funding.

Introduction: Auto-immune and Inflammatory Diseases (AID) have been associated with myeloproliferative neoplasms (MPN) in a large population-based study (Kristinsson et al. Haematologica 2010). In myelodysplastic syndromes (MDS) and chronic myelomonocytic leukemia (CMML), epigenetic regulators TET2 and IDH1/2 were more frequently mutated in patients with AID, suggesting potentially common pathogenesis pathways (Zhao et al. Leukemia 2021). However, in the context of MPN, AID specific features remain poorly characterized, and no study has reported to date the mutational landscape of MPN patients with AID. The objectives of our study were to describe the clinical and molecular characteristics of MPN patients with associated AID and evaluate its impact on patient's outcome. Methods: A total of 1541 patients were diagnosed with MPN according to WHO criteria between January 2011 and January 2021 in our center, of whom 998 had a molecular analysis by next generation sequencing (NGS) targeting a panel of 36 genes involved in MPN, performed at diagnosis and/or during follow-up. AID diagnosis was based on international criteria, and all cases have been reviewed by internal medicine experts. Patients with AID induced by interferon-alpha treatment were not included. Results: The median age of our whole cohort was 51.3 years IQR[40.4-63.2]. Our cohort included 522 (34%), 709 (46%) and 229 (15%) diagnosis of polycythemia vera (PV), essential thrombocytemia (ET) and primary myelofibrosis (MF) respectively. A total of 100 patients (6.6%) had AID and were compared to the remaining 1441 MPN patients without AID. There were more females (66 (66%) versus 769 (53%), p=0.019) within the AID group compared to non-AID patients. MPN subtype, driver mutation, complete blood counts at diagnosis did not differ between the two groups. Occurrence of thrombosis and hemorrhage episodes did not vary either (44 (44%) versus 564 (39%), p=0.356). AID diagnosis was prior to MPN in 34% of cases, concomitant in 12% and posterior in 31% of cases. AID diagnosis included 45 (45%) organ-specific AID (mainly autoimmune thyroiditis, n=34), 13 (13%) inflammatory arthritis, 9 (9%) connective tissue diseases, 8 (8%) inflammatory dermatosis, 7 (7%) systemic vasculitis and 18 (18%) unclassified AID (Figure 1A). The AID fulfilled the required classification criteria in 70 (70%) cases, while complete criteria were not reached in 30 (30%) cases. The median interval of time between MPN diagnosis and NGS was 7.2 years IQR[2.1-13.3] in the whole cohort. Among patients with available molecular analysis, 44 (62%) and 571 (62%) patients had at least one additional non-driver mutated gene in the AID and control groups respectively. Interestingly, TET2 mutations were more frequent in MPN patients with AID (24 (34%) versus 205 (22%), OR=1.84 95%CI[1.08-3.07], p=0.028, Fig 1B). The prevalence of TET2 mutations did not significantly differ between the AID categories. When focusing on IDH1/2 mutations, as they act on the same biological epigenetic pathway as TET2, IDH1/2 mutations were more frequent in the AID cohort although not statistically significant (4 (6%) versus 27 (3%), OR=2.02 95%CI[0.74-5.51], p=0.27). No other mutations including other epigenetic factors, splicing regulators, transcription factors or high molecular risk mutations, were significantly associated with AID. After a median follow up of 8.3 years IQR[3.7-14.3] in the whole cohort, 10 (10%) and 122 (8%) patients died in the AID and control groups respectively. The presence of AID did not impact overall survival (p=0.82), secondary myelofibrosis free (p=0.98) or MDS/AML transformation free (p=0.53) survivals. Conclusion Our study reports on a large retrospective clinically and molecularly annotated cohort the prevalence of AID in MPN patients (6.6%). This prevalence did not differ from that of the general population. Interestingly, our data emphasize a high prevalence of TET2 mutations in patients with both AID and MPN, compared to MPN patients without AID. Although other studies are warranted to better define the causal relationship between MPN and AID, our results may suggest a common pathophysiology as it has been proposed in MDS patients, based on shared genetic susceptibilities with mutations in TET2 that could occur within early hematopoietic progenitors and give rise to both the inflammatory phenotype and myeloid malignancy. DE and LPZ contributed equally to this work. Figure 1 Figure 1. Disclosures Fenaux: Celgene/BMS: Honoraria, Research Funding; Novartis: Honoraria, Research Funding; JAZZ: Honoraria, Research Funding; Abbvie: Honoraria, Research Funding; Takeda: Honoraria, Research Funding; Janssen: Honoraria, Research Funding; Syros Pharmaceuticals: Honoraria. Ades: Novartis: Honoraria; Takeda: Honoraria; Abbvie: Honoraria; JAZZ: Honoraria; Celgene: Honoraria, Research Funding. Raffoux: PFIZER: Consultancy; CELGENE/BMS: Consultancy; ABBVIE: Consultancy; ASTELLAS: Consultancy. Kiladjian: Incyte Corporation: Membership on an entity's Board of Directors or advisory committees; AOP Orphan: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees; Bristol Myers Squibb: Membership on an entity's Board of Directors or advisory committees; AbbVie: Membership on an entity's Board of Directors or advisory committees; Taiho Oncology, Inc.: Research Funding; PharmaEssentia: Other: Personal fees. Benajiba: Pfizer: Research Funding; Gilead: Research Funding.

Background: Although patients with MPN may achieve complete hematological remission (CHR) with cytoreductive therapy (CRT), lifelong treatment represents a major burden for these chronically ill patients. IFN has emerged as a major MPN therapy thought to positively alter the natural history of MPN and appears to be the only drug that can provide long-term CHR after discontinuation in some patients (Hasselbalch HC et al., Seminars in Immunopathology, 2019). In this study, we aimed to identify clinical and molecular factors associated with long-term CHR after IFN treatment discontinuation and to compare clinical outcome of patients who discontinued therapy, to patients who continued IFN treatment despite achieving a CHR. Methods: All MPN patients followed between January 2000 and May 2020 in our institution who received at least 3 months of IFN were included. Logistic regression analysis was performed to identify variables associated with long-term CHR after discontinuation. Cumulative incidence of relapse was calculated from time of IFN discontinuation, considering death without relapse as a competitive event. Overall survival (OS) and thrombotic/hemorrhagic events free survival (EFS) curves were estimated using the Kaplan-Meier method and compared by cause-specific hazard Cox models. Results: 381 patients treated with IFN were included in the study. Median age at MPN diagnosis was 44 years [interquartile range: 33-54]. 171 had polycythemia vera (PV), 169 essential thrombocythemia (ET) and 34 primary myelofibrosis (PMF). JAK2V617F was the most frequent driver mutation (78.8% of patients), while CALR and MPL were mutated in 15.5%, and 2.9% of patients respectively. Median driver variant allele frequency (VAF) at time of IFN therapy initiation was 34% [12; 51]. Reasons to start IFN included age younger than 50 years in 44.9%, intolerance/resistance to previous therapies in 17.8% or pregnancy in 1.8% patients. CHR was achieved with IFN in 77.2% of patients. After a median follow-up of 72.4 months [28.4; 119.7] from IFN initiation, 131 patients were still on IFN treatment, while 250 patients had discontinued therapy. No significant difference was observed between continued and discontinued IFN patients in terms of MPN subtype, initial clinical, biological or molecular characteristics. Reasons for discontinuation were toxicity in 128 (50.4%), prolonged hematological CHR in 76 (29.9%), failure in 16 (6.3%) and other in 30 (11.8%) patients. At time of IFN discontinuation, 170 (66.9%) patients were in CHR and the median driver mutation VAF was 12% [3; 35]. Of note, IFN was re-introduced in 61 patients who lost CHR with a second CHR rate of 83.6%, arguing for the absence of development of IFN resistance in post-discontinuation relapses. In multivariate logistic regression analysis, stopping for prolonged CHR (OR 3.52, 95%CI [1.18; 10.56], p=0.024) was associated with a higher CHR without CRT rate, while VAF ≥ 10% at time of IFN discontinuation (OR 0.26, 95%CI [0.1; 0.65], p=0.004) was associated with lower long-term CHR without CRT. Accordingly, VAF ≥ 10% at time of IFN discontinuation (HR 3.06 [1.59; 5.90], p=0.001) was independently associated with a higher cumulative incidence of relapse after IFN discontinuation (Figure A). Driver VAF at IFN discontinuation time was available for 117 patients. 53 patients (45.3%) had a driver VAF < 10% at IFN discontinuation. Finally, we compared discontinued IFN patients who stopped IFN for CHR (n=76) to patients who obtained CHR and are still on IFN therapy (n=116). Hematological transformation could not be evaluated due to very low incidence of events: 2 patients from each group progressed to myelofibrosis while leukemic progression was only observed in 1 patient. Importantly, OS (HR 0.23, 95%CI [0.5; 1.14], p=0.07) and EFS (HR 0.53, 95%CI [0.19; 1.45], p=0.217) were not significantly different between patients who discontinued and those who continued IFN. Conclusion: Overall, our study demonstrates that IFN discontinuation represents a safe strategy in MPN patients who achieved CHR, and particularly in patients with a driver VAF lower than 10% at time of discontinuation. Importantly, relapsed patients did not develop IFN resistance. Our data contributes to set the frame for future clinical trials evaluating the possibility of IFN treatment holidays in good responding MPN patients. Disclosures Benajiba: Gilead Foundation: Research Funding. Kiladjian:Abbvie: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees; BMS: Membership on an entity's Board of Directors or advisory committees; AOP Orphan: Membership on an entity's Board of Directors or advisory committees.

Background and Aims Myeloproliferative neoplasms (MPN) are the leading cause of splanchnic vein thrombosis (SVT). Janus kinase 2 gene ( JAK2 ) V617F mutations are found in 80 to 90% of patients with SVT and MPN. Mutations of the calreticulin ( CALR ) gene have also been reported. However, as their prevalence ranges from 0 to 2%, the utility of routine testing is questionable. This study aimed to identify a group of patients with SVT at high risk of harboring CALR mutations and thus requiring this genetic testing. Methods CALR , JAK2 V617F and thrombopoietin receptor gene ( MPL ) mutations were analysed in a test cohort that included 312 patients with SVT. Criteria to identify patients at high risk of CALR mutations in this test cohort was used and evaluated in a validation cohort that included 209 patients with SVT. Results In the test cohort, 59 patients had JAK2 V617F , five had CALR and none had MPL mutations. Patients with CALR mutations had higher spleen height and platelet count than patients without these mutations. All patients with CALR mutations had a spleen height ⩾16cm and platelet count >200×10 9 /L. These criteria had a positive predictive value of 56% (5/9) and a negative predictive value of 100% (0/233) for the identification of CALR mutations. In the validation cohort, these criteria had a positive predictive value of 33% (2/6) and a negative predictive value of 99% (1/96). Conclusion CALR mutations should be tested in patients with SVT, a spleen height ⩾16cm, platelet count >200×10 9 /L, and no JAK2 V617F . This strategy avoids 96% of unnecessary CALR mutations testing. Lay summary: Mutations of the CALR gene are detected in 0 to 2% of patients with SVT, thus the utility of systematic CALR mutation testing to diagnose MPN is questionable. This study demonstrates that CALR mutations testing can be restricted to patients with SVT, a spleen height ⩾16cm, a platelet count >200×10 9 /L, and no JAK2 V617F . This strategy avoids 96% of unnecessary CALR mutations testing.

An emerging noninvasive approach to assess tissue proliferation uses the PET tracer 3'-deoxy-3'-18F-fluorothymidine (18F-FLT). To evaluate the diagnostic value of this technique in myelofibrosis, 18F-FLT PET imaging results were compared with bone marrow histology and bone marrow scintigraphy (BMS), the gold standard techniques in this clinical situation. Methods: Fifteen patients with histology-proven myelofibrosis were included consecutively in the study. Tracers' distributions were assessed using a visual grading assessment score of the uptake in the axial skeleton, proximal and distal limbs, liver, and spleen. This visual score was used to define patterns of tracer distribution and to compare the information provided either by PET or by BMS. A semiquantitative analysis with determination of SUVmax in the same localizations was performed for 18F-FLT PET. Results: The histology grade of fibrosis correlated with the SUVmax in the axial skeleton (spine and iliac crests) and proximal limbs. 18F-FLT uptake in these areas was much lower in patients with grade 3 fibrosis than in patients with grade 1 or 2 fibrosis. 18F-FLT PET showed the same distribution of uptake as BMS in 13 of 14 patients (1 patient did not undergo BMS). In 1 patient, 18F-FLT PET clearly showed an intense abnormal splenic uptake, whereas spleen uptake was inconclusive with BMS. Conclusion:18F-FLT PET appears to be a reliable and convenient technique to assess hematopoietic activity in bone marrow. It yields results close to those observed with BMS. In our study population, 18F-FLT uptake in the axial skeleton and proximal limbs assessed by SUVmax correlated with the grade of fibrosis. Thus, 18F-FLT PET may be a useful tool to measure the severity of myelofibrosis, and to monitor noninvasively the patients' status during follow-up. Finally, 18F-FLT PET may be foreseen as an alternative to BMS.

Genes encoding the RNA splicing factors SF3B1, SRSF2, and U2AF1 are subject to frequent missense mutations in clonal hematopoiesis and diverse neoplastic diseases. Most "spliceosomal" mutations affect specific hotspot residues, resulting in splicing changes that promote disease pathophysiology. However, a subset of patients carry spliceosomal mutations that affect non-hotspot residues, whose potential functional contributions to disease are unstudied. Here, we undertook a systematic characterization of diverse rare and private spliceosomal mutations to infer their likely disease relevance. We utilized isogenic cell lines and primary patient materials to discover that 11 of 14 studied rare and private mutations in SRSF2 and U2AF1 induced distinct splicing alterations, including partially or completely phenocopying the alterations in exon and splice site recognition induced by hotspot mutations or driving "dual" phenocopies that mimicked two co-occurring hotspot mutations. Our data suggest that many rare and private spliceosomal mutations contribute to disease pathogenesis and illustrate the utility of molecular assays to inform precision medicine by inferring the potential disease relevance of newly discovered mutations.

Purpose: The genesis of all cancers results from an accumulation of mutations, constitutional and/or acquired when induced by external mutagenic factors. High-speed technologies for genome sequencing have completely changed the study of disease genetics, but with limited knowledge of the functional value of most genetic changes. Experimental Design: Here, we proposed an innovative individual approach by studying tissue samples from a young woman with an unusual association of breast cancer, polycythemia vera, and rheumatoid arthritis. We performed genomic analyses for copy number variations and point mutations on laser-microdissected tumor cells from the breast cancer, and on CD34+ cells sorted from bone marrow aspiration, to identify gene abnormalities common to these two types of cell populations. Results: Using ONCOSCAN technology, we identified a constitutional pR988C, c2962C>T mutation of MET. Using CRISPR-Cas9 technology, we established pR988C MET-mutated transgenic mice, which reproduced the autoimmune diseases and myeloproliferation found in our index-case; one of the transgenic mice spontaneously developed a skin squamous cell carcinoma. We also showed that additional mutagenic factors were required to induce cancers, including skin squamous cell carcinoma and thyroid cancer. Using an anti-MET drug, cabozantinib, we demonstrated for the first time the functional role of this mutation in the maintenance of myeloproliferation and rheumatoid arthritis, and in cancer genesis. Conclusions: Our study opens a considerable field of application in the domain of constitutional genetics, to establish genetic links between cancers and other very different severe diseases.

IF 2.845; International audience; External quality assurance (EQA) programs are vital to ensure high quality and standardized results in molecular diagnostics. It is important that EQA for quantitative analysis takes into account the variation in methodology. Results cannot be expected to be more accurate than limits of the technology used, and it is essential to recognize factors causing substantial outlier results. The present study aimed to identify parameters of specific importance for JAK2 V617F quantification by quantitative PCR, using different starting materials, assays, and technical platforms. Sixteen samples were issued to participating laboratories in two EQA rounds. In the first round, 19 laboratories from 11 European countries analyzing JAK2 V617F as part of their routine diagnostics returned results from in-house assays. In the second round, 25 laboratories from 17 countries participated. Despite variations in starting material, assay setup and instrumentation the laboratories were generally well aligned in the EQA program. However, EQA based on a single technology appears to be a valuable tool to achieve standardization of the quantification of JAK2 V617F allelic burden.

Introduction: Ruxolitinib (RUX) is a JAK 1/2 inhibitor approved for the treatment of intermediate and high-risk primary or secondary MF. In addition to the intrinsic immune system modulation in MF patients, JAK inhibition exerts an immunosuppressive effect that may increase the risk of infections (Heine A et al., Blood, 2013). We aimed to investigate the association between treatment with RUX and infectious complications in a large monocentric cohort of MF patients. Methods: We performed a retrospective study including all patients diagnosed with primary, post-polycythemia vera or post-essential thrombocythemia MF in our MPN clinic between January 2011 and April 2020. Patients were considered RUX treated when exposed to treatment for at least two consecutive months. Clinical and biological characteristics at time of MF diagnosis and follow-up were collected from medical charts and electronic medical records. We used logistic regression models to identify specific infection subtypes associated with RUX treatment. Univariate and multivariate analysis assessing the impact of categorical and continuous variables on infectious events cumulative incidence were performed using Cox models. Results: 213 MF patients were included in the study. 114 (51.1%) patients were treated with RUX while 99 didn't receive RUX. Overall 102 patients exhibited a total of 231 infectious episodes. To search for factors associated with occurrence of infectious events, we evaluated the characteristics of MF, potential clinical and biological immune suppression factors (immunosuppressive treatment, prior non-MPN cancer, diabetes, hepatic/renal failure, neutropenia, lymphopenia, hypo-gammaglobulinemia, complement deficiency, prior history of infection), use of infection prophylaxis and RUX treatment using Cox models. In this cohort of MF patients, we found that RUX treatment (HR 5.91, 95%CI[3.48; 10.04], p 2 (HR 4.91, 95%CI[2.62; 9.21], p 41 (22.2%) and 16 (34.0%) infections were of grade 3-4, 61 (33.1%) and 19 (40.4%) required hospitalization, while 8 (4.3%) and 2 (4.2%) resulted in patients' death, in RUX treated and RUX naïve patients, respectively (all non-significantly different). Bacterial infections represented 48.0%, viral infections 16.0% and fungal infections 4.8% of the total infectious events. The most frequently infected sites were lungs/respiratory tract (28.1%) and skin/soft tissue (24.2%). RUX treatment was significantly associated with recurrence of HSV-1/2 or VZV (OR 7.57, 95%CI[1.01-57.30], p=0.050), and skin/soft tissue infections including the above herpetic recurrences (OR 3.33, 95%CI[1.13-9.78], p=0.029). 17 (14.9%) patients presented with HSV/VZV recurrence in the RUX treated group compared to 1 (1.0%) in non-treated patients (Figure B). We then thought to evaluate the impact of potential infections risk factors in RUX treated patients at time of treatment initiation in order to inform clinicians' therapeutic decision and follow-up guidance. For this analysis, we evaluated the characteristics of MF, potential clinical and biological immune suppression factors, treatment lines prior to RUX, dose and cumulative exposition to RUX. No clinical or biological factor was significantly associated with the overall risk of infectious events. However, hypo-gammaglobulinemia at time of RUX initiation was independently associated with HSV and/or VZV recurrence (HR 4.57, 95%CI[1.29-16.16], p=0.018). The higher rate of infections in RUX treated patients was associated with a not statistically significant trend towards shorter OS (HR 1.97, 95%CI[0.98; 3.94], p=0.055). Conclusion: This retrospective study shows an association between the occurrence of infectious complications and Ruxolitinib treatment in MF patients, and in particular, a higher incidence of HSV/VZV recurrence and skin/soft tissue infection. Patients with hypo-gammaglobulinemia at time of RUX initiation were at higher risk of HSV/VZV recurrence, suggesting that protein electrophoresis should be performed prior to treatment initiation and primary antiviral prophylaxis proposed to patients with hypo-gammaglobulinemia. Disclosures Rea: BMS: Membership on an entity's Board of Directors or advisory committees; Pfizer: Honoraria, Membership on an entity's Board of Directors or advisory committees; Novartis: Honoraria, Membership on an entity's Board of Directors or advisory committees; Incyte: Honoraria, Membership on an entity's Board of Directors or advisory committees. Kiladjian:AbbVie: Membership on an entity's Board of Directors or advisory committees; AOP Orphan: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees; Bristol Myers Squibb: Membership on an entity's Board of Directors or advisory committees. Benajiba:Gilead Foundation: Research Funding.

Introduction: MPN are a heterogeneous group of chronic hematological malignancies often resulting from a combination of a driver gene mutation (JAK2, MPL or CALR) and a variety of somatic mutations harboring diverse prognosis values. A subset of MPN patients carry somatic mutations in the hematopoietic transcription factor NFE2 (nuclear factor erythroid 2) resulting in a functionally enhanced truncated form of NFE2 (Jutzi JS et al., JEM, 2013). Moreover, epigenetically induced overexpression of NFE2 has recently been reported in the majority of MPN patients (Peeken JC et al., Blood, 2018). In transgenic murine models, NFE2 overexpression results in an MPN phenotype (thrombocytosis, leukocytosis, EPO-independent colony formation, characteristic bone marrow histology and expansion of stem and progenitor compartments) and has recently been shown to predispose to the acquisition of additional genetic abnormalities and subsequent leukemic transformation (Kaufmann KB et al., JEM, 2012) (Jutzi JS et al., Blood, 2019). However, clinical impact of NFE2 mutations in MPN patients remains unknown. The aim of this study was to evaluate the phenotypic characteristics and prognostic impact of NFE2 somatic mutations in a large mono-centric cohort of MPN patients. Methods: A total of 1243 consecutive patients were diagnosed with MPN according to WHO criteria and followed in our hospital between January 2011 and May 2020. This study included 707 of them in whom a next-generation sequencing (NGS) molecular analysis targeting 36 myeloid genes was performed at diagnosis and/or during follow-up. Clinical and biological characteristics at time of diagnosis and follow-up were collected from medical charts and electronic medical records. Statistical analyses were performed using the STATA software (STATA 15.0 Corporation, College Station, TX). Results: In our cohort, 411 patients presented with polycythemia vera (PV), 577 with essential thrombocythemia (ET), 184 with primary or pre-fibrotic myelofibrosis (PMF), 59 with unclassified MPN and 12 with MDS/MPN. Median age at diagnosis was 51 years [40-63]. 73.1% patients had a JAK2V617F mutation, 14.1% a CALR mutation and 3.3% a MPL mutation. Overall, 64 (9.05%) patients harbored a NFE2 mutation with a variant allelic frequency (VAF) ≥ 0.5% and 36 had a VAF ≥ 5%, the latest were considered as NFE2 mutated for the rest of the study as VAF Conclusion: In this retrospective study we show that presence of NFE2 mutations with a VAF ≥5% is independently associated with an increased risk of leukemic transformation and shorter overall survival. These findings are in line with recently reported animal models and suggest that NFE2 mutations screening should be routinely performed in MPN patients. Disclosures Rea: Incyte: Honoraria, Membership on an entity's Board of Directors or advisory committees; Novartis: Honoraria, Membership on an entity's Board of Directors or advisory committees; Pfizer: Honoraria, Membership on an entity's Board of Directors or advisory committees; BMS: Membership on an entity's Board of Directors or advisory committees. Kiladjian:AOP Orphan: Membership on an entity's Board of Directors or advisory committees; AbbVie: Membership on an entity's Board of Directors or advisory committees; Bristol Myers Squibb: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees. Benajiba:Gilead Foundation: Research Funding.

Introduction: Next generation sequencing (NGS) studies identified additional somatic mutations impacting disease evolution and prognosis in MPN. SF3B1, a component of the U2 small nuclear ribonucleoprotein splicing complex, is frequently mutated in myelodysplastic syndromes where it has been proposed to define a new entity (Malcovati et. al. Blood 2020). In MPN, SF3B1 is mutated in approximately 10% of patients with primary myelofibrosis (PMF) and 3-5% with polycytemia vera or essential thrombocytemia (ET). Recent reports suggested that spliceosome mutations may adversely affect myelofibrosis free survival (MFS) in ET (Tefferi et. al. Br J Haematol. 2020). The main objective of this study was to evaluate the impact of the concomitant presence of driver (JAK2, MPLor CALR) and SF3B1 clonal or sub-clonal mutations on MPN phenotype and evolution in a large single center cohort of MPN patients. Methods: A total of 1243 consecutive patients were diagnosed with MPN according to WHO criteria between January 2011 and May 2020 in our center, of whom 707 had molecular analysis by NGS targeting a panel of 36 myeloid genes performed at diagnosis and/or during follow-up. Significant variants were retained with a sensitivity of 0.5%. Patients were grouped according to variant allele frequencies (VAF) determined by NGS as "driver" SF3B1mutated patients when driver and SF3B1 mutations VAF were similar (double mutated clone), and as "non-driver" SF3B1 mutants when SF3B1VAF was lower than that of the driver mutation, suggestive of a sub-clone. 4 patients with SF3B1 mutations but no driver mutation were excluded. We then compared the characteristics and outcomes of 3 groups of patients according to their SF3B1 mutational status: wild type (WT), driver and non-driver SF3B1 mutations. Results: A total of 39/703 (5.6%) patients had SF3B1 mutations, of whom 11/39 (28.2%) and 28/39 (71.8%) harbored driver and non-driver SF3B1mutations, respectively. Driver SF3B1 mutations were associated with PMF (OR 6.1, 95%CI [1.1; 33.6], p= 0.039) and MPN unclassified (OR 15.6,95%CI [1.1; 116.5], p= 0.007) subtypes, presence of immature myeloid cells ≥ 2% (OR 9.3, CI [2.6; 32.8], p= 0.001) and peripheral blasts ≥ 1% (OR 5.0,95%CI [1.0; 24.7], p= 0.047) at diagnosis (Figure A). Other variables were not significantly different between patients with driver, non-driver and WT SF3B1, including age, driver mutation type, MPN-related symptoms, cytogenetics and high molecular risk mutations (ASXL1, EZH2, SRSF2, IDH1/2or U2AF1). There was no significant difference in the response to therapy: complete hematological response was seen in 5/11 (45.5%), 10/28 (35.7%) and 327/664 (49.3%) of patients with driver, non-driver and WT SF3B1 respectively. After a median follow-up of 103.7 months IQR [47.2; 175.6], evolution to myelofibrosis occurred in 5/7 (71.4%), 6/20 (30.0%) and 99/564 (17.6%) of patients with driver, non-driver and WT SF3B1 respectively. Interestingly, driver SF3B1 but not non-driver SF3B1 mutational status adversely impacted MFS (OR 7.56,95%CI[2.95; 19.38], p Conclusion: This study in a large cohort of MPN patients highlights for the first time to our knowledge the adverse impact on MFS of SF3B1 and MPN-driver co-mutated clones. In contrast, presence of an SF3B1 mutation at sub-clonal level didn't increase the risk of MF development. In line with our findings, a recent study reported an association between rapid progression to myelofibrosis and SF3B1 mutations in patients with age-related clonal hematopoiesis (Bartels et al. Leukemia 2020). Further studies are warranted to confirm our results on independent cohorts and to investigate the mechanisms of bone marrow fibrosis development in patients with SF3B1 and MPN-drivermutations. Disclosures Rea: Incyte: Honoraria, Membership on an entity's Board of Directors or advisory committees; Novartis: Honoraria, Membership on an entity's Board of Directors or advisory committees; Pfizer: Honoraria, Membership on an entity's Board of Directors or advisory committees; BMS: Membership on an entity's Board of Directors or advisory committees. Kiladjian:BMS: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees; AOP Orphan: Membership on an entity's Board of Directors or advisory committees; Abbvie: Membership on an entity's Board of Directors or advisory committees. Benajiba:Gilead Foundation: Research Funding.

International audience; Myeloproliferative neoplasms are clonal disorders characterized by the presence of several gene mutations associated with particular hematologic parameters, clinical evolution, and prognosis. Few therapeutic options are available, among which interferon α (IFNα) presents interesting properties like the ability to induce hematologic responses (HRs) and molecular responses (MRs) in patients with JAK2 mutation. We report on the response to IFNα therapy in a cohort of 31 essential thrombocythemia (ET) patients with CALR mutations (mean follow-up of 11.8 years). HR was achieved in all patients. Median CALR mutant allelic burden (%CALR) significantly decreased from 41% at baseline to 26% after treatment, and 2 patients even achieved complete MR. In contrast, %CALR was not significantly modified in ET patients treated with hydroxyurea or aspirin only. Next-generation sequencing identified additional mutations in 6 patients (affecting TET2, ASXL1, IDH2, and TP53 genes). The presence of additional mutations was associated with poorer MR on CALR mutant clones, with only minor or no MRs in this subset of patients. Analysis of the evolution of the different variant allele frequencies showed that the mutated clones had a differential sensitivity to IFNα in a given patient, but no new mutation emerged during treatment. In all, this study shows that IFNα induces high rates of HRs and MRs in CALR-mutated ET, and that the presence of additional nondriver mutations may influence the MR to therapy.

Interferon alpha (IFN-α) has been used for over 30 years to treat myeloproliferative neoplasms (MPNs). IFN-α was shown to induce clinical, hematological, molecular and histopathological responses in small clinical studies. Such combined efficacy has never been achieved with any other drug to date in such a significant proportion of patients. However, toxicity remains a limitation to its broader use despite the development of pegylated forms with better tolerance. Several on going phase 3 studies of peg- IFN-α versus hydroxyurea will help to define its exact place in MPN management. IFN-α efficacy is likely the consequence of a broad range of biological properties, including enhancement of immune response, direct effects on malignant cells and ability to cycle dormant malignant stem cells. However, comprehensive elucidation of its mechanism of action is still lacking. Sustained clinical, molecular and morphological responses after IFN-α discontinuation raised the hope that this drug could eradicate MPN. There is now consistent evidence showing that IFN-α is able to eliminate malignant clones harboring JAK2V617F or Calreticulin mutations. However, the molecular complexity of these diseases could hamper IFN-α efficacy, as the presence of additional non-driver mutations, like in the TET2 gene, could be associated with resistance to IFN-α. Therefore, combined therapy with another targeted agent could be required to eradicate MPN, and the best IFN-α companion for achieving this challenge remains to be determined.

Classical BCR-ABL-negative myeloproliferative neoplasms (MPN) are acquired clonal disorders of hematopoietic stem cells (HSC) leading to the hyperplasia of one or several myeloid lineages. They are caused by recurrent mutations in 3 three main genes: JAK2, mainly JAK2V617F, calreticulin (CALR) and thrombopoietin receptor (MPL). Most therapies have limited effect on the malignant clone, except the interferon alpha (IFN) that has demonstrated some efficacy induction of a molecular remission. However, since its mechanism of action is still largely unknown, we have followed two strategies: First, we studied how the IFN affects the different MPN hematopoietic cell compartment by following a longitudinal observational cohort of 50 JAK2V617F and CALRmut patients treated with IFN for 4-5 years. At 4-month intervals, we performed the clonal architecture in early and late hematopoietic progenitors and in mature cells. We also used a mathematical model to infer the behavior of IFN-targeted mutated HSC from those of mutated progenitors and mature cells. Our results predict that IFN slowly exhausts the mutated HSC by inducing their differentiation into progenitors and mature cells. In addition, we show that this effect was increased with high IFN doses and in patients with homozygous JAK2V617F or very low proportion of heterozygous JAK2V617F and CALRmut HSC. Second, we searched to improve IFN therapy and focused on PML, an IFNα downstream effector whose targeting by arsenic (AS) was implicated in acute promyelocytic leukemia eradication. In a JAK2V617F mouse model, the combination improved hematological and molecular responses at the level of early progenitors leading to disease-initiating cell eradication. The effects of this combination required PML and were associated with features of senescence. Collectively, these studies identify HSC exhaustion as a mechanism of IFN disease eradication, may help to stratify patients for IFN therapy and show that a combination with AS will improve its clinical efficacy.

Juvenile myelomonocytic leukemia (JMML) is a rare aggressive myelodysplastic/myeloproliferative neoplasm of early childhood, initiated by RAS-activating mutations. Genomic analyses have recently described JMML mutational landscape; however, the nature of JMML-propagating cells (JMML-PCs) and the clonal architecture of the disease remained until now elusive. Combining genomic (exome, RNA-seq), Colony forming assay and xenograft studies, we detect the presence of JMML-PCs that faithfully reproduce JMML features including the complex/nonlinear organization of dominant/minor clones, both at diagnosis and relapse. Further integrated analysis also reveals that although the mutations are acquired in hematopoietic stem cells, JMML-PCs are not always restricted to this compartment, highlighting the heterogeneity of the disease during the initiation steps. We show that the hematopoietic stem/progenitor cell phenotype is globally maintained in JMML despite overexpression of CD90/THY-1 in a subset of patients. This study shed new lights into the ontogeny of JMML, and the identity of JMML-PCs, and provides robust models to monitor the disease and test novel therapeutic approaches.

The genesis of all cancers results from an accumulation of mutations, constitutional and/or acquired when induced by external mutagenic factors. High-speed technologies for genome sequencing have completely changed the study of disease genetics, but with limited knowledge of the functional value of most genetic changes.Here, we proposed an innovative individual approach by studying tissue samples from a young woman with an unusual association of breast cancer, polycythemia vera, and rheumatoid arthritis. We performed genomic analyses for copy number variations and point mutations on laser-microdissected tumor cells from the breast cancer, and on CD34Using ONCOSCAN technology, we identified a constitutional pR988C, c2962CT mutation ofOur study opens a considerable field of application in the domain of constitutional genetics, to establish genetic links between cancers and other very different severe diseases.

The JAK2V617F mutation is part of the major criteria for diagnosis of myeloproliferative neoplasms (MPN). Allele-specific quantitative PCR (qPCR) is the most prevalent method used in laboratories but with the advent of next-generation sequencing (NGS) techniques, we felt necessary to evaluate this approach for JAK2 mutations testing. Among DNA samples from 427 patients analyzed by qPCR and NGS, we found an excellent concordance between both methods when allelic burden was superior to 2% (the detection limit of our NGS assay). Only one sample among 298 was found negative by NGS while allelic burden by qPCR was 3%. Because NGS detection limit is higher, sensitivity was lower as exemplified by 21 samples found negative whereas qPCR measured allelic burdens between 0.1 and 1%. Importantly, quantitative data of samples found positive by both techniques were highly correlated (R2 = 0.9477). We also evaluated 40 samples tested for JAK2 exon 12 mutations by HRM. The concordance with NGS was of 100%. Using NGS, the full coding region of JAK2 was analyzed leading to identification of several variants outside of exon 12 and 14 which were previously described or not. Interestingly, we found one somatic mutation (c.1034A>T p.H345L) which induced constitutive activation of the JAK/STAT pathway leading to an increased proliferation of BaF/3 cells with low-dose EPO. This study showed that NGS is a robust method highly correlated to qPCR, although less sensitive, but providing the opportunity to identify other JAK2 variants with potential impact on disease initiation or evolution.

International audience

Thromboembolic events are the main cause of mortality in BCR-ABL1-negative myeloproliferative neoplasms (MPNs) but their underlying mechanisms are largely unrecognized. The Janus kinase 2 (JAK2)V617F mutation is the most frequent genetic alteration leading to MPN. Usually found in haematopoietic progenitors and stem cells, this mutation has also been described in endothelial cells (ECs) of MPN patients. In this study, we have questioned the impact of the JAK2V617F mutation on EC phenotype and functions. We developed an induced pluripotent stem cells strategy to compare JAK2 mutant and wild-type ECs. Transcriptomic assays showed that several genes and pathways involved in inflammation, cell adhesion and thrombotic events were over-represented in JAK2V617F ECs and expression levels of von Willebrand factor and P-selectin (CD62P) proteins were increased. Finally, we found that leucocytes from MPN patients adhere more tightly to JAK2V617F ECs. Our results show that JAK2V617F ECs have a pro-inflammatory and pro-thrombotic phenotype and were functionally pro-adherent.

Introduction: Myeloproliferative neoplasms (MPN) are the most frequent underlying causes of splanchnic vein thromboses (SVT), including Budd-Chiari syndrome (BCS), portal, splenic and mesenteric vein thromboses. This subgroup of MPN patients with SVT (MPN/SVT) has been shown to have distinct clinical and molecular features like younger age and lower JAK2V617Fmutant allele burden (the most predominant driver mutation in MPN/SVT patients) and are often considered as early stages of MPN. However, there is no study that has investigated the contribution of NGS data to the risk stratification of these MPN/SVT patients. Main objective of this study was to evaluate the impact of the presence of additional mutations to the driver mutation in the long term outcome of MPN patients with SVT. Patients and methods: Over the past 10 years, a total of 286 patients with SVT have been referred to our center for diagnostic assessment of an underlying MPN. Among them, a diagnosis of MPN (WHO criteria) was finally made in 197. Full molecular analyses by NGS was available in 61 of these MPN/SVT patients. The molecular profiling of patients was performed using a Capture-based custom NGS panel provided by Sophia Genetics. This panel comprised the coding sequences of 36 genes involved in MPN. Bioinformatics analysis was performed by Sophia Genetics, with a detection limit of variants set at 1%. We compared (i) MPN/SVT patients to our local cohort of 1371 MPN patients without SVT with full NGS data available; (ii) MPN/SVT patients with and without additional mutation to the driver mutation. Results: Median follow-up of MPN/SVT patients was 11 years, 62% were females, 33% had BCS and 67% portal vein thrombosis (Table 1). All patients received anticoagulants and 58 (95%) a cytoreductive therapy after the diagnosis of MPN. The type of MPN was polycythemia vera in 69%, essential thrombocythemia in 25% and myelofibrosis in 6%. The driver mutation was JAK2V617F in 57 (93%), and CALR mutation in 4 patients, respectively, and no patient had MPL mutation. By NGS, an additional mutation was identified in 27 (44%) patients, while 34 patients had only the driver mutation. The most frequent additional mutations were in TET2 (23%), ASXL1 (8%), DNMT3A (7%), IDH1/2 (5%), and LNK (5%) genes. EZH2, SRSF2 and TP53 mutations were found in only 1 patient, respectively. When compared to NGS data from 1371 MPN patients, there was no significant difference in the frequency of each additional mutation, except a slightly lower incidence of ASXL1 mutations in MPN/SVT patients (8% vs 23% in MPN patients, p=0.07). Of note, MPN/SVT patients were younger at MPN diagnosis than the control MPN cohort (44 vs 61 years, p< 10-5). Clinical characteristics were not significantly different between MPN/SVT patients with and without additional mutations (Table 1), including age at SVT or at MPN diagnosis, type of MPN or type of SVT, use of cytoreductive therapy, and median follow-up (11 years in both groups). In contrast, MPN/SVT patients with additional mutations had a significantly higher median JAK2V617F mutant allele burden than those with only JAK2 mutation (34% vs 11%, p=0.003, Figure 1); and higher risk of hematological transformation to myelofibrosis or acute leukemia or death (30% vs 6%, p=0.017). Only few patients experienced recurrence of thrombosis during follow up, with no difference according to the mutational profile (3 (11%) and 6 (17%) in patients with and without additional mutations, respectively). Conclusion: This is to our knowledge the first study assessing the impact of molecular abnormalities in the long term outcome of MPN/SVT patients showing a possible prognostic role for NGS data in these patients. Our results suggest that MPN/SVT patients with only JAK2V617F mutation and a low mutant allele burden (below 50%) are at low risk of adverse hematological outcome. In contrast, NGS may identify a group of patients with additional mutations at very high long term risk of unfavorable hematological evolution (30% transformation or death after 11 years of median follow-up, that is unexpected in such a young MPN population). In this group, a disease modifying therapy like interferon or other targeted therapies should be proposed when possible to reduce this risk of transformation. Disclosures Kiladjian: Novartis: Honoraria, Research Funding; Celgene: Consultancy; AOP Orphan: Honoraria, Research Funding.

Background Atypical CML (aCML) is a rare myeloid neoplasm with molecular heterogeneity and overlapping features of MDS and MPN. Distinction from unclassified MDS/MPNu based on WHO criteria remains difficult, and the management of these closely related entities remains ill-codified. Most patients (pts) are managed with cytoreductive agents, but small series have reported responses to hypomethylating agents or tyrosine kinase inhibitors (TKI), notably ruxolitinib. Allogeneic stem cell transplantation (SCT) is considered the only curative option. To instruct clinical trials with novel agents in this rare and heterogeneous population, real-life cohorts must i) provide prognostic factors and molecular characterization able to stratify patients, and ii) benchmark outcomes with current treatment options. Methods The French National observational study of rare MDS/MPNs performed a retrospective analysis of adult patients with MDS/MPN from 35 centers. Cases were centrally reclassified according to the 2016 WHO criteria to exclude CMML, classical MPNs and CNL. All statistical analyses were done without dichotomizing continuous clinical or biological variables. The prognostic influence of treatments was analyzed considering onset of treatment as a time-dependent covariate (Mantel-Byar method). Results The study population included 136 pts (M/F 83/53), with a median age of 72 years. Only 43 (31.6%) met WHO 2016 criteria for aCML while the remaining 93 were classified as MDS/MPNu. At diagnosis, spleen enlargement or other tumor symptoms were present in 36% of pts, while 32% had constitutional symptoms. Mutations in ASXL1, splice genes (U2AF1, SF3B1, SRSF2 or ZRSR2), SETBP1, EZH2, CSF3R, JAK2 and ETNK1 were present in 68.8%, 50.0%, 30.3%, 15.9%, 12.7%, 12.6% and 7.4% of 63 tested cases, respectively. 25 pts had an AML transformation. With a median follow-up of 29.8 months (0.5-276.4) median overall survival (OS) and AML-free survival (AMLFS) were 25.6 and 20.6 months, resp. Median OS was 20.2 versus (vs) 29.7 months in aCML vs MDS/MPNu, resp (log rank test p=0.2) and median AMLFS was 16.6 vs 27.4 months, resp (p=0.09). In univariate analysis, higher WBC (p=0.003) and lower Hb level (p0.1). Patients with EZH2 mutations had shorter OS (median 9.9 vs 20.6 months in EZH2 wt pts, p=0.03), while other gene mutations had no significant prognostic impact (all p>0.1). All variables with p At any time during follow-up, 89 (65.4%) pts received cytoreductive agents (mostly hydroxyurea), while 23 (16.9%) received TKI (ruxolitinib in 12, imatinib in 8, dasatinib in 3 pts) and 18 (13.2%) HMA, resp. Of note, 8/18 pts received HMA at transformation to AML, while all pts received TKIs prior to AML. Finally, 19 pts (14.0%) received an SCT and 35.3% received ≥2 of the above-mentioned types of treatment. Median time to SCT, HMA and TKI were 10.2, 11.9 and 4.7 months, resp. In multivariate Cox models adjusting for baseline Hb level and dyserythropoieisis, neither cytoreduction (p=0.6) nor SCT (p=0.5) were associated with a significant OS improvement. Treatment with HMA was associated with a significantly worse OS (HR=3.0, p=0.001), but this effect was confounded when transformation to AML, as a time-dependent event, was included in the model (HMA: HR= 1.1, p=0.7, AML: HR=5.1, p Conclusions Anemia and dyserythropoiesis are important prognostic factors in aCML and MDS/MPNu that should be used to stratify pts included in clinical trials. There is currently no standard of care for these overlap syndromes. Treatment with HMAs and TKIs should be restricted to clinical trials. Disclosures Rea: Incyte Biosciences: Honoraria; Pfizer: Honoraria, Membership on an entity's Board of Directors or advisory committees; Novartis: Honoraria, Membership on an entity's Board of Directors or advisory committees; BMS: Honoraria. Roy:Incyte Biosciences: Consultancy. Caers:Takeda: Honoraria, Membership on an entity's Board of Directors or advisory committees; Janssen: Honoraria, Membership on an entity's Board of Directors or advisory committees; Celgene: Honoraria, Membership on an entity's Board of Directors or advisory committees; Amgen: Honoraria, Membership on an entity's Board of Directors or advisory committees. Nicolini:Incyte Biosciences: Honoraria, Research Funding, Speakers Bureau; Novartis: Research Funding, Speakers Bureau; Sun Pharma Ltd: Consultancy. Legros:Novartis: Honoraria; BMS: Honoraria; Incyte Biosciences: Honoraria, Research Funding; Pfizer: Honoraria, Research Funding.

Treatment of polycythemia vera (PV) with the Murine Double Minute 2 (MDM2) antagonist, idasanutlin, in a phase 1 trial was reported by our group to be well tolerated with a high overall response rate (Mascarenhas et al, Blood. 2019 Jun 5). A global, phase 2 trial is currently underway evaluating idasanutlin in hydroxyurea (HU) resistant/intolerant PV patients (NCT03287245). MDM2, a negative regulator of TP53 is upregulated in PV CD34+ cells and inhibition of MDM2 targets PV hematopoietic stem/progenitor cells (HSPC) (Lu et al, Blood. 2014;124(5):771-90). Additional trials of MDM2 antagonists have shown promise, however, there is concern that these agents have the potential to induce TP53 mutations or promote expansion of TP53 mutated clones. Resistance to MDM2 inhibitors has been evaluated in solid tumor cell lines and attributed to either the emergence of de novoTP53 mutations or the selection of TP53 mutated clones. (Michaelis et al, Cell Death Dis. 2011;2:e243; Skalniak et al, Cancers. 2018;10(11)). The effect of MDM2 inhibition on TP53 mutant clones is of particular interest in myeloproliferative neoplasms (MPNs). TP53 mutations have been reported with a low allele burden in ~15% of chronic MPN patients (Kubesova et al, Leukemia. 2018;32(2):450-61), however, TP53 loss of heterozygosity and rapid expansion of TP53 mutant clones is associated with transformation to blast phase (Lundberg et al, Blood 2014,123:2220-8). As reported, in the idasanutlin PV trial, 1/13 patients were identified to have a baseline pathogenic TP53 mutation in hematopoietic cells (VAF 5.5%), using a deep sequencing assay with a limit of detection (LOD) of VAF 0.5%. This patient was a non-responder to idasanutlin and upon treatment had an increasing JAK2V617F and TP53 mutant VAF. End of study hematopoietic cell specimens of all patients were deep sequenced (LOD 0.1%) and revealed that 4 additional patients harbored detectable TP53 mutations after idasanutlin treatment with VAF ranging from 1-12%. In each sample, 1-5 unique TP53 mutations were identified, all within the hotspot domain of the TP53 gene. Deeper sequencing of baseline and follow-up samples revealed these mutations were present at a subclonal level (VAF 0.1-5.5%) and increased over time, indicating that treatment with the MDM2 antagonist promoted expansion of already existing TP53 mutant clones (Table 1, Figure 1). None of the patients who lacked a TP53 mutation at baseline developed a TP53 mutant clone with idasanutlin treatment. There was no clear association of presence of TP53 mutations with prior HU, anagrelide or interferon exposure. There has been careful monitoring of patients to determine whether the expanding TP53 clone has clinical ramifications. Patients were on study for a median of 54 weeks (23-131). The only patient who exemplified resistance to idasanutlin was the single patient with a high burden TP53 mutation (37%). All other patients were taken off study due to patient choice/toxicity. Furthermore, all TP53 mutant and non-mutant patients have had stable disease with no evidence of progression to MF or AML. Sequencing of 2 patients post-discontinuation of idasanutlin revealed that the VAF of the TP53 mutant clones decreased since the agent was discontinued. Updated patient molecular data post-treatment discontinuation will be reported at the meeting. To investigate whether idasanutlin induces de novo TP53 mutations in PV myeloid cells we performed long term HSPC cultures. Mononuclear cells from 6 distinct PV patients were treated continuously with idasanutlin (500 nM) over ~6 weeks and DNA from both treated and untreated colonies were analyzed using next generation sequencing with a LOD of 2% VAF and no TP53 mutations were detected. The combined in vitro and clinical data reveals that treatment with an MDM2 antagonist is not associated with the emergence of de novoTP53 mutations but rather the expansion of prior existing TP53 clones. This does not appear to have clinical repercussions, however, close monitoring of these patients is essential. We recommend that patients be screened for TP53 mutations prior to treatment with an MDM2 antagonist and that if present the TP53 mutant VAF be followed during their treatment course. Resistance to MDM2 inhibition is likely dependent on the TP53 mutant VAF and further studies will need to clarify the ideal dosing schedule of MDM2 antagonists and/or combinatorial therapy to prohibit TP53 mutant clonal expansion. Disclosures Houldsworth: Cancer Genertics: Other: stock in; Sema4: Employment. Rossi:Sema4: Employment. Kiladjian:AOP Orphan: Honoraria, Research Funding; Celgene: Consultancy; Novartis: Honoraria, Research Funding. Rampal:Agios, Apexx, Blueprint Medicines, Celgene, Constellation, and Jazz: Consultancy; Constellation, Incyte, and Stemline Therapeutics: Research Funding. Mascarenhas:Incyte: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Novartis: Research Funding; Roche: Consultancy, Research Funding; Merck: Research Funding; Celgene: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; CTI Biopharma: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Janssen: Research Funding; Promedior: Research Funding; Merus: Research Funding; Pharmaessentia: Consultancy, Membership on an entity's Board of Directors or advisory committees. Hoffman:Merus: Research Funding.

Introduction Classical BCR-ABL-negative myeloproliferative neoplasms (MPN) include Polycythemia Vera (PV), Essential Thrombocythemia (ET) and Primary Myelofibrosis (PMF). These are acquired clonal disorders of hematopoietic stem cells (HSC) leading to the hyperplasia of one or several myeloid lineages. MPN are caused by three main recurrent mutations: JAK2V617F, mutations in the calreticulin (CALR) and thrombopoietin receptor (MPL) genes. Interferon alpha (IFNα) treatment induces not only a hematological response in around 70% of ET, PV and early myelofibrosis, but also a significant molecular response on both JAK2V617F- and CALR-mutated cells. However, a complete molecular response is only achieved in around 20% of patients. Our aim is to predict the long-term efficacy of IFNα in JAK2V617F- and CALR-mutated patients by monitoring the fate of the disease-initiating mutated HSC in order to better stratify the molecular responders. Methods A longitudinal observational study (3-5 years) was performed in 46 IFNα-treated patients. The MPN disease distribution was 42% ET, 47% PV and 11% PMF. We detected 33 patients with JAK2V617F mutation, 11 with CALR mutations (7 type 1/type 1-like and 4 type 2/type 2-like), 1 with both JAK2V617F and CALR mutation and 1 with JAK2V617F, CALR mutation and MPLS505N. At 4-month intervals, the JAK2V617For CALR mutation variant allele frequency was measured in mature cells (granulocytes, platelets). Simultaneously, the clonal architecture was determined by studying the presence of the mutations in colonies derived from the different hematopoietic stem and progenitor cell (HSPC) populations (CD90+CD34+CD38-HSC-enriched, CD90-CD34+CD38- immature and CD34+CD38+committed progenitors). We used a combination of mathematical modeling (Michor et al., Nature, 2005) and Bayesian analysis to infer the long-term behavior of mutated HSC. Results After a median follow-up of 40 months, IFNα targeted more efficiently and more rapidly the HSPC, particularly the HSC-enriched progenitors, than the mature blood cells in JAK2V617Fpatients (p Since it is very difficult to purify true HSC from patients, we used a combination of mathematical and statistical modeling to infer the behavior and the kinetics of IFNα-targeted mutated HSC. The model gave a good fit to the data and indicated that mutated HSC are exhausted slowly (> 1 year) with concomitant increase in mutated HSPC and granulocytes in well-responding patients. We calculated the rate of HSC decrease for each patient. Rates of decrease are very low for heterozygous JAK2V617F and CALR-mutated HSC and greater for homozygous JAK2V617FHSC, but all increase with high IFNα dose (>100 µg/week). Moreover, very low proportion of heterozygous mutated HSC compared to high proportion can be targeted more easily in patients. The associated mutations at diagnosis and at the last timepoint were also investigated using an NGS-targeted myeloid panel. Results indicate that IFNα does not induce any further mutations on additional genes and the mathematical approach predicts that associated mutations have no major impact on the ratio of HSC decrease. Conclusion Altogether, using a rigorous method of statistical inference, our results show that IFNα exhaust the human mutated HSC by differentiation in HSPC and mature cells. This is likely due to IFNα inducing a stronger proliferation of mutated compared to wild-type HSC, as previously shown in a mouse model (Mullally et al., Blood, 2013). Our study predicts that IFNα can slowly eradicate the mutated HSC, but this beneficial effect would be more efficient: i) in patients with homozygous JAK2V617F versus those with heterozygous JAK2V617F or CALR-mutated, ii) with high IFNα dose, iii) in patients with very low proportion of heterozygous JAK2V617F and CALR-mutated HSC. Thus, this study will help to stratify patients for IFNα treatment.These results might also explain the different outcomes in current IFNα clinical trials. Disclosures Constantinescu: Novartis: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; AlsaTech: Other: Co-Founde; AgenDix GmbH: Other: Co-Founder, MyeloPro Research and Diagnostics; Wiley & Sons: Other: Editor in Chief, Journal of Cellular and Molecular Medicine. Kiladjian:AOP Orphan: Honoraria, Research Funding; Novartis: Honoraria, Research Funding; Celgene: Consultancy.

Background The dynamic International Prognostic Scoring System (DIPSS) is commonly applied to predict survival among patients with primary myelofibrosis (PMF) but has been shown to perform less precisely in secondary myelofibrosis (SMF) and after transplantation. Furthermore, the prognostic relevance of mutation profile resulted in the mutation-enhanced IPSS (MIPSS) in PMF, as well as in a model specific to SMF (MYSEC-PM) after essential thrombocythemia (ET) or polycythemia vera (PV). The aim of the current study was to develop a comprehensive prognostic system including clinical and molecular information, specifically in myelofibrosis undergoing transplantation. Methods Previously published methods were used to sequence myelofibrosis-associated genes (i.a. CALR1/2, JAK2, MPL,ASXL1, SRSF2, EZH2, IDH1/2, DNMT3A, TET2, TP53). Outcome was calculated from date of transplant (95% confidence interval). Variables associated with overall survival (OS) constructed a Cox regression with a stepwise selection procedure. Hazard ratios (HR) were used as weights for model development. Validation was done using repeated random subsampling. Performance of the model was verified via Harrel's concordance index C and was also tested in predefined cohorts: disease (PMF, SMF), conditioning, and ruxolitinib pretreatment. Results Population. The total cohort consisted of 361 patients from four different centers in Germany and France (260 PMF, 101 SMF). Median age at transplant was 57 years (range, 22-75), 58% were male and 42% had a Karnofsky performance score (KPS) Factors on outcome. In univariate analysis, mutations in CALR and MPL showed better OS (79% and 76%) vs. JAK2 (53%) and triple negative (50%; p=0.001). Outcome was similar according to CALR type (p=0.99). ASXL1 and DNMT3A mutations also entered the multivariate model. The following eight clinical, molecular and transplant-related variables were identified (corresponding HR): leukocytes >25x109/l (1.71), platelets 57 years (1.69), recipient/donor CMV serostatus (+/- vs. other, 1.68), ASXL1 (1.74), JAK2/triple negative (2.10), and MMUD (2.11). Myelofibrosis Transplant Scoring System (MTSS). A weighted score of 1 was assigned to leukocytosis, thrombocytopenia, KPS 57, recipient/donor CMV serostatus (+/-), and ASXL1 mutation, whereas 2 points were assigned to JAK2/triple negative and MMUD. Four risk groups constructed the MTSS: low (score 0-2), intermediate (score 3-4), high (score 5-6), and very high (score 7-9). The 5-year OS according to risk groups was 88%, 71%, 50%, and 20% (Figure 1). The hazard for death (with low-risk as reference) was 2.36 for intermediate-risk, 4.65 for high-risk, and 9.72 for very high-risk. The score was predictive of OS overall as well as for PMF and SMF (p Conclusions The new MTSS includes modern disease- and transplant-associated risk variables pertinent to both PMF and SMF. This proposed system consistently predicts outcome facilitating posttransplant decision-making and can be applied to different conditioning settings and to patients receiving ruxolitinib pretreatment. Figure 1. Figure 1. Disclosures Beelen: Medac: Consultancy, Other: Travel Support. Kroeger:Novartis: Honoraria, Research Funding; Riemser: Honoraria, Research Funding; Neovii: Honoraria, Research Funding; Sanofi: Honoraria; JAZZ: Honoraria; Celgene: Honoraria, Research Funding.

In 2008, a European registry of relapsed acute promyelocytic leukemia was established by the European LeukemiaNet. Outcome data were available for 155 patients treated with arsenic trioxide in first relapse. In hematological relapse (n=104), 91% of the patients entered complete hematological remission (CR), 7% had induction death and 2% resistance, 27% developed differentiation syndrome and 39% leukocytosis, whereas no death or side effects occurred in patients treated in molecular relapse (n=40). The rate of molecular (m)CR was 74% in hematological and 62% in molecular relapse (P=0.3). All patients with extramedullary relapse (n=11) entered clinical and mCR. After 3.2 years median follow-up, the 3-year overall survival (OS) and cumulative incidence of second relapse were 68% and 41% in hematological relapse, 66% and 48% in molecular relapse and 90 and 11% in extramedullary relapse, respectively. After allogeneic or autologous transplantation in second CR (n=93), the 3-year OS was 80% compared with 59% without transplantation (n=55) (P=0.03). Multivariable analysis demonstrated the favorable prognostic impact of first remission duration ⩾1.5 years, achievement of mCR and allogeneic or autologous transplantation on OS of patients alive after induction (P=0.03, P=0.01, P=0.01) and on leukemia-free survival (P=0.006, P