Abstract: Introduction: Myeloproliferative neoplasms (MPN) are a heterogeneous group of chronic myeloid malignancies resulting from the combination of a driver mutated gene (JAK2, MPL or CALR) and a variety of acquired additional somatic mutations. Although next-generation sequencing (NGS) has identified high molecular risk mutations associated with adverse prognosis (Vannucchi et al., Leukemia, 2013 , Guglielmelli et al., Leukemia, 2014 ), the clonal evolution of these mutations remains poorly described. Chronic exposure to cytoreductive treatment, especially genotoxic drugs such as hydroxyurea (HU), could impact clonal evolution. A previous study suggested that Interferon-α (IFN) could limit the accumulation of cytogenetic abnormalities compared to HU (Mondello et al., Blood, 2018). The objective of our study was to describe the long-term evolution of the mutational landscape in the era of NGS in a large cohort of MPN patients. 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. This study included 1039 of them in whom a NGS molecular analysis targeting 36 myeloid genes with a sensitivity of 1% was performed at diagnosis and/or during follow-up. Patients with only one NGS (n=588), AML/MDS transformation at either the first (n=3) or the second NGS (n=2) were excluded from the analysis. Serial NGS data obtained in chronic MPN phase were thus analyzed for 446 patients. Clinical and biological characteristics at time of diagnosis and follow-up were collected from medical charts and electronic medical records. Mutation rates per year were calculated for each gene as the difference in the number of mutations between first and last NGS divided by the time interval (in years) between both NGS. Results : 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 (PV), Essential thrombocythemia (ET) and primary myelofibrosis (MF) respectively. 279 patients (63%) had at least one additional mutation at first NGS, and respectively 27 (6%) and 104 (23%) patients had TP53 and high molecular risk mutations. Median interval between MPN diagnosis and the first NGS was 6.5 years [IQR 1.7-13] while median time between the first and the last NGS was 2.5 years [IQR 1.6-4, range 0.3-14.3]. Overall, 178 patients (39.9%) acquired an additional mutation at last NGS evaluation, most frequently involving TET2, DNMT3A, ASXL1, TP53 and NFE2 genes . To study the impact of chronic MPN therapy on clonal evolution, we focused on patients who electively received HU (n=112) or IFN (n=92) as a monotherapy, or did not receive any cytoreductive treatment (n=119) between the first and the last NGS. The remaining patients received ruxolitinib (n=44), anagrelide (n=10), vercyte (n=7) or polytherapy (n=62). At last follow-up, 74 patients receiving IFN (80.4%) and 65 (58%) treated with HU had a complete hematological response. When combining all additional mutations, the global mutation rate per year did not significantly differ between treatment groups. When analyzing individual genes, TP53 mutation rate was higher in patients treated with HU compared to the patients receiving IFN (p=0.014) or not treated (p=0.008) (Figure). MDS/AML evolution occurred in 4 patients (3.6%) treated with HU, 2 (1.7%) without cytoreductive therapy versus none of the 92 patients treated with IFN (ns). In the whole cohort, MDS/AML evolution was significantly increased in patients harboring TP53 mutations (p= 0.004). In contrast, DNMT3A mutation rate was significantly increased in patients receiving IFN compared to patients treated with HU (p=0.045) (Figure). The latest result is in line with previous observations showing that loss of DNMT3A could confer resistance to IFN in a JAK2-V617F mouse model (Stetka et al., Blood, 2020). Conclusion: Our results highlight the impact of chronic cytoreductive therapy on clonal evolution shaping in MPN. IFN limits the emergence of TP53 mutated clones compared to HU, thus potentially reducing the risk of leukemogenesis. Emergence of DNMT3A mutated clones under IFN therapy requires further exploration and could potentially play a role in therapeutic resistance. This study on a large clinically and biologically annotated cohort illustrates how serial NGS analysis may guide therapeutic options for MPN patients. Figure 1 Figure 1. Disclosures Raffoux: PFIZER: Consultancy; ASTELLAS: Consultancy; ABBVIE: Consultancy; CELGENE/BMS: Consultancy. 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; 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; Novartis: 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.

Abstract: 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 & lt;0.001) (Figure 1B), age at MPN diagnosis (HR 1.11, 95%CI [1.07; 1.15], p & lt;0.001) and the presence of HMR mutations at baseline NGS (HR 4.48, 95%CI [2.05; 9.77], p & lt;0.001) independently adversely impacted OS in a COX regression multivariate analysis. CEv also independently adversely impacted MDS/AML free survival (HR 13.15, 95%CI [3.88; 44.47], p & lt;0.001) and secondary MF free survival (HR 21.13, 95%CI [6.18; 72.20], p & lt;0.001) in a COX regression multivariate analysis. 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.

Abstract: Several studies have demonstrated the impact of VTd on response rates and PFS either as induction or consolidation regimen. However, studies have also showed that VGPR and greater was associated with a better survival prognostic at completion of intensification regimen, and that these patients benefited most from consolidation. Questions remain as to the benefit of consolidation using VTd in patients that reached solely PR at completion of VTd induction, patients that display features of resistance to VTd partially. We aimed to assess the efficacy and safety of VTd as consolidation therapy in the context of VTd as induction regimen followed by a single auto (VTd-auto-VTd regimen) in patients that reached PR at completion of Induction with VTd. Method This study has included a group of 121 newly diagnosed MM patients that underwent auto from 2009 to 2011 across 9 IFM centers. Patients were to be eligible for auto in first-line of treatment, aged less than 65 years old and treated with VTd-auto-VTd regimen. We then studied the 54 patients that reached PR (n=54) at completion of VTd induction. Results The median age was 57 years, the sex ratio was 1,25, 58% had ISS 2 and 3, 25% had adverse FISH, including t(4;14) and/or del17p (similar in the 2 groups). Overall, 37 (68%) responded (at least VGPR) at completion of consolidation, including 33% and 35% that reached VGPR and CR, respectively. 55% and 37.5% of patients improved response rate after auto and consolidation, respectively. We then studied the role of consolidation whether or not patients responded to auto. Out of the 15 patients that remained on PR after auto (no benefit from auto), 35% responded at completion of consolidation. Out of the 31 patients that improved response rate to auto (at least VGPR), 32% further improved to CR at completion of consolidation. The median duration of response was prolonged from 10 (CI95% 10-14) months to 16 (13-20) months for patients in PR at completion of induction with VTd that further responded to consolidation VTd (p=0.24). With a median follow-up of 36 months, 30% and 11% had relapsed and died, respectively. The incidence rate of relapse was greater in patients that did not benefit from consolidation, 86% versus 14%, (p=0.27). The median TTP was 26 (95%CI 13-38) months in patients that did not benefit from consolidation versus not reached yet for the second group (p=0.012); the 3-year TTP was 18% and 87%. The 3-year PFS was 18% and 58%, respectively (p=0.03). The median OS was not reached in either group, and the 2-years OS was 75%. In univariate analysis, the variables that shortened TTP were high serum beta2m greater than 5mg/L (p=0.014), adverse FISH (p=0.05), elevated serum LDH (p 〈 0.0001), absence of response following auto (p=0.022) and absence of response following consolidation (p=0.012). We have then sought for independent variables that impacted TTP in multivariate analysis after exclusion of LDH, beta2m and FISH known to be the most powerful predicators of short TTP and OS. The response to consolidation was then the most important marker to predict shorten median TTP [OR = 4.4, 95%CI = 1-21; p = 0.039]. The safety profile of VTd consolidation in this population was similar to reports in patients that reached VGPR and greater at completion of induction with no excess toxicity. The incidence rate of hematological EIs of grade 3 and 4 was 2% with no excess and non hematological EIs related to consolidation. The incidence rate of neuropathy all grades was 7%, but only 2% occurred during the consolidation. We have also observed 9 (9%) thromboembolic events (TE), with none occurring during the consolidation phase. Conclusion This study showed an improved response rate in relation to the VTd consolidation phase in two-third of patients that only partially responded to the induction VTd regimen. This improved quality of responses translated into a lower relapse rate with a prolonged PFS. This study demonstrates that VTd consolidation in the context of a the VTd-auto-VTd regimen upfront should be recommended to all responders to the induction phase, including patients in PR characterized with a lower sensitivity to the VTd regimen. Disclosures: Leleu: Amgen: Honoraria; Millennium : Honoraria; Leopharma: Consultancy, Honoraria; Onyx: Consultancy, Honoraria; Celgene: Consultancy, Honoraria; Janssen: Consultancy, Honoraria; Novartis: Honoraria. Roussel:CELGENE: Honoraria; JANSSEN: Honoraria. Facon:JANSSEN: Honoraria, Speakers Bureau; CELGENE: Honoraria, Speakers Bureau. Attal:JANSSEN: Honoraria, Speakers Bureau; CELGENE: Honoraria, Speakers Bureau. Moreau:JANSSEN: Honoraria, Speakers Bureau; CELGENE: Honoraria, Speakers Bureau.

Abstract: Background: Many studies have evaluated the impact of cytomegalovirus (CMV) reactivation after allogeneic hematopoietic stem cell transplantation (allo-HSCT) showing a significant association with reduced risk of relapse while other studies failed to show the same results. On the other hand, other frequent viral infections or reactivations like Epstein-Barr virus (EBV), Human Herpesvirus 6 (HHV6) and BK virus (BK-V) have not been evaluated in the same context and/or together with CMV. The aim of this study is to evaluate the impact of CMV, EBV, HHV6 and BK-V reactivation in acute leukemia (AL) patients receiving allo-HSCT in complete remission (CR) and evaluate their impact alone or associated on disease relapse. Patients and methods: We evaluated 158 consecutive AL patients who received allo-HSCT at our center between January 2008 and June 2015; 95 (60%) were males, median age was 46 years (range: 18-67), 101 (64%) had acute myeloid leukemia (AML) and 57 (36%) had acute lymphoblastic leukemia (ALL). At transplantation, 114 (72%) patients were in first complete response (CR1) and 44 (28%) were in second CR (CR2). Conditioning regimen was myeloablative for 97 (61%) patients and of reduced intensity in 61 (39%) patients. DNA levels of CMV, EBV, HHV6 and BK-V were detected by quantitative RQ-PCR after weekly monitoring up to 3 months after allo-HSCT. CMV-DNA, EBV-DNA, HHV6-DNA or BK-V-DNA was considered positive when the copies exceeded 1000 copies/ml. Results: Among 158 patients, 45 (28%) patients had CMV reactivation after a median time of 40 days (6-72) after allo-HSCT; 53 (34%) patients had EBV reactivation after a median time of 42 days (18-91) after allo-HSCT; 33 (21%) patients had HHV6 reactivation after a median time of 24 days (6-69) and 30 (19%) patients had BK-V reactivation after a median time of 32 days (6-60) after allo-HSCT. There was 61 (39%) patients who did not have any viral reactivation, 46 (29%) patients who had only one reactivation (no matter the type), and 51 (32%) patients who had at least two or more co-reactivations (group 1). When evaluating relapse incidence, we found that patients who had only one reactivation were not significantly different from those who did not have any reactivation, with a comparable relapse incidence, thus, we analyzed those patients as one group (group 2, n=107). The cumulative incidence of relapse at 6 months for group 1 and group 2 was 2% (95% CI: 0.2-9.4) and 15.9 % (95%CI: 9.7-23.5) respectively, this result remained significantly different when stratifying on the type of conditioning, in RIC: 15.8% (95%CI: 3.7-35.7) vs 21.4% (95%CI: 10.5-35) respectively; and in MAC: 0 % vs 15.4% (95%CI: 7.8-25.3) respectively, p=0.04. This finding was confirmed independently of the presence of GVHD or not. Conclusion: We demonstrate here for the first time, in this homogeneous population, that co-reactivation of multiple viruses after allo-HSCT could lead to a lower incidence of relapse in AL patients in CR; this can be explained as a stimulation of the immune system via both the functional activation and amplification of NK compartment. This could also explain some conflicting results in studies evaluating single viral reactivation alone without consideration of other reactivations. These encouraging results should be validated at a larger multicenter level. Disclosures No relevant conflicts of interest to declare.

Abstract: 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 & lt;0.001)(Figure B). Other variables independently associated with adverse MFS in multivariate COX regression analysis included age at MPN diagnosis (OR 1.02, 95%CI[1.00; 1.04], p=0.003), JAK2V617F allele burden (OR 1.03, 95%CI[1.02; 1.04] , p & lt;0.001), MPL (OR 13.94, 95%CI[4.90; 39.70], p & lt;0.001) and CALR (OR 7.06, 95%CI[3.69; 13.51], p & lt;0.001) mutations. SF3B1 mutational status had no impact on overall survival, transformation to MDS/AML or thrombotic/hemorrhagic events free survival. 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.

Abstract: 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 & lt;5% may refer to a minor clone without clinical relevance. NFE2 mutations were present in 7.3%, 5.3% and 3.6% of PV, PMF and ET patients respectively. No significant association between the presence of NFE2 mutation and clinical/molecular MPN characteristics (driver mutation, constitutional symptoms, splenomegaly, blood counts, cytogenetic and other molecular features) was observed using a logistic regression association model. Median follow-up was 103.8 months, IQR [47.2; 175.6] . In terms of response to therapy, 52.8% of patients achieved complete response, complete hematological response or clinical improvement in NFE2 mutated vs 61.7% in non-mutated patients (p= 0.026). Interestingly, presence of a NFE2 mutation (HR 9.92, 95%CI[3.21; 30.64], p & lt; 0.001), age at diagnosis (HR 1.09, 95%CI[1.05; 1.12], p & lt; 0.001), PMF subtype (HR 6.92, 95%CI[2.81; 17.06], p & lt; 0.001) and high-risk mutations (ASXL1, EZH2, SRSF2, IDH1/2 and U2AF1) (HR 2.45, 95%CI[1.14; 5.28], p=0.021) were independently associated with AML/MDS transformation free survival (TFS) in a COX regression multivariate analysis (Figure A). Presence of a NFE2 mutation was also independently associated with overall survival (OS) (HR 9.37, 95%CI [4.18; 21.03] , p & lt;0.001) (Figure B). Median TFS were 216.1 months and not reached, while median OS were 144.2 months and not reached for NFE2 mutated and non-mutated patients, respectively. No difference was observed in terms of thrombo-hemorrhagic events (HR 0.73; 95%CI [0.10; 5.21], p=0.752) and secondary myelofibrosis free survivals (HR 0.67; 95%CI [0.09; 4.87] , p=0.693). 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.

Abstract: 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. Occurren ce 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.

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