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International audience; Three reports address the protection of the vulnerable population of patients with hematologic malignancies in the face of the ongoing COVID pandemic. The reports suggest that some patients who fail to mount a B-cell response to vaccine may nevertheless have protective T cell responses. As a group, these reports suggest that patients should continue to be immunized with additional doses to attempt to improve immune response but that they need to maintain the precautions recommended for the unvaccinated.

The blood system serves as a key model for cell differentiation and cancer. It is orchestrated by precise spatiotemporal expression of crucial transcription factors. One of the key master regulators in the hematopoietic systems is PU.1. Reduced levels of PU.1 are characteristic for human acute myeloid leukemia (AML) and are known to induce AML in mouse models. Here, we show that transcriptional downregulation of PU.1 is an active process involving an alternative promoter in intron 3 that is induced by RUNX transcription factors driving noncoding antisense transcription. Core-binding factor (CBF) fusions RUNX1-ETO and CBFβ-MYH11 in t(8;21) and inv(16) AML, respectively, activate the PU.1 antisense promoter that results in a shift from sense toward antisense transcription and myeloid differentiation blockade. In patients with CBF-AML, we found that an elevated antisense/sense transcript and promoter accessibility ratio represents a hallmark compared with normal karyotype AML or healthy CD34+ cells. Competitive interaction of an enhancer with the proximal or the antisense promoter forms a binary on/off switch for either myeloid or T-cell development. Leukemic CBF fusions thus use a physiological mechanism used by T cells to decrease sense transcription. Our study is the first example of a sense/antisense promoter competition as a crucial functional switch for gene expression perturbation by oncogenes. Hence, this disease mechanism reveals a previously unknown Achilles heel for future precise therapeutic targeting of oncogene-induced chromatin remodeling.

Oncogenic RAS mutations pose substantial challenges for rational drug discovery. Sequence variations within the hypervariable region of Ras isoforms underlie differential posttranslational modification and subcellular trafficking, potentially resulting in selective vulnerabilities. Specifically, inhibiting the palmitoylation/depalmitoylation cycle is an appealing strategy for treating NRAS mutant cancers, particularly as normal tissues would retain K-Ras4b function for physiologic signaling. The role of endogenous N-Ras(G12D) palmitoylation in signal transduction, hematopoietic differentiation, and myeloid transformation is unknown, and addressing these key questions will inform efforts to develop mechanism-based therapies. To evaluate the palmitoylation/depalmitoylation cycle as a candidate drug target in an in vivo disease-relevant model system, we introduced a C181S mutation into a conditional Nras(G12D) “knock-in” allele. The C181S second-site amino acid substitution abrogated myeloid transformation by Nras(G12D), which was associated with mislocalization of the nonpalmitoylated N-Ras mutant protein, reduced Raf/MEK/ERK signaling, and alterations in hematopoietic stem and progenitor populations. Furthermore, hematologic malignancies arising in Nras(G12D/G12D,C181S) compound heterozygous mice invariably acquired revertant mutations that restored cysteine 181. Together, these studies validate the palmitoylation cycle as a promising therapeutic target in NRAS mutant cancers.

Introduction. The boundaries between MDS and AML are still a matter of debate. In the 2001 WHO Classification, the myeloblast count distinguishing AML and MDS was lowered from 30% to 20% of the bone marrow (BM) cells or peripheral blood (PB) leukocytes. It was justified on the basis that treating patients with 20-29% BM blasts with intensive chemotherapy showed a similar outcome to those with > 30% BM blasts. However, the better knowledge of the biology of both diseases is showing that in several cases AML and high risk MDS share identical genetic profiles, as it is well known in AML with myelodysplasia- related changes (AML-MRC). Currently there are new therapeutic options, less toxic, and suitable for elderly people.The threshold of 20% BM blast is artificial, but it is still the main criterion used in clinical trials and also in real life to discriminate patients that probably belong to the spectrum of the same biological entity. Treatment of patients with MDS or AML is widely based in this relatively arbitrary condition. Objective: To study if the threshold of 20% bone marrow blasts, distinguishing MDS with excess of blasts type 2 (MDS EB 2) and AML, is reproducible among different observers. Methods. 120 bone marrow samples from patients previously diagnosed with MDS-EB-2, AML or therapy-related myeloid neoplasms (t-MN) according to 2016 WHO classification were included. The diagnosis of MDS required cytogenetics and/or FISH, and the cases with AML should have been classified following the 2017 ELN recommendations, regarding immunophenotyping, cytogenetics and molecular biology. The design of the study was established to include cases with Results. Finally 116/120 samples were considered suitable for the study. Regarding 2016 WHO categories, 55 cases showed MDS EB-2, 44 AML-MRC, 8 t-MN, 4 AML- NOS, 2 NPM1-mutated AML, 2 RUNX1-RUNX1T1 AML, 1 BCR-ABL1+ AML. Next generation sequencing was performed in 79 cases. Discordance was observed in 34/116 cases (29.3%). 14 cases with MDS-EB2 (1 NPM1+) were classified as AML-MRC by the second observer, 16 AML cases as MDS EB-2, 3 MDS EB-2 as MDS- EB1 and 1 AML as MDS- EB1. The genetic and /or molecular profile of the discordant cases was heterogeneous. Regarding the threshold of 20% BM blasts, discrepancies were 31/116 (26.7%, I Kappa test = 0.46, moderate agreement). The agreement between MDS-EB-2 and AML-MRC, with discordance in 28/98 cases (28.6%), was moderate-fair (Kappa test= 0.42). Conclusion. The threshold of 20% BM blasts did not accurately separate AML from MDS EB-2. Particularly less concordance was seen for AML-MRC. Incorporation of genetic and molecular characteristics to the morphologic diagnosis is needed to optimize the definition of both entities. Acknowledgment: Angel Cedillo, Secretaría Técnica AMHH. Disclosures Font Lopez: GILEAD: Membership on an entity's Board of Directors or advisory committees; CELGENE-BMS: Consultancy, Membership on an entity's Board of Directors or advisory committees; Pfizer: Membership on an entity's Board of Directors or advisory committees. Loscertales: Janssen, Abbvie, Astra-Zeneca, Beigene, Roche, Gilead: Consultancy; Janssen, Abbvie, Roche, Gilead: Speakers Bureau. Cedena: Janssen, Celgene and Abbvie: Honoraria.

INTRODUCTION Immunocompromised patients have been excluded from initial trials evaluating SARS-CoV-2 mRNA vaccines and there is a critical need to warrant vaccine efficacy in hematopoietic stem cell transplant (HSCT) recipients. In this study, we evaluated antibody responses to 2 doses mRNA SARS-CoV-2 vaccine in allogeneic HSCT recipients. METHODS We retrospectively enrolled successive hematopoietic cell transplant recipients across France who completed the 2-dose SARS-CoV-2 mRNA vaccine (BNT162b2 or mRNA-1273) between January 1 st and July 15 th 2021. All included patients had an available semi-quantitative antispike serologic testing after the second dose (from Roche, DiaSorin, Abbott or Siemens). We excluded patients with a prior COVID-19 confirmed by serology or PCR. For detectable antibody, we calculated the binding antibody units per milliliter (BAU/mL) according to the WHO International Standard by applying conversion factors given by the manufacturers (Kristiansen et al. , The Lancet 2021). Antibody response was categorized as "weak" or "good" with a threshold of 264 BAU/mL which has been associated to an estimate of 80% of mRNA vaccine-induced protection against symptomatic COVID-19 in immunocompetent patients (Feng S. et al., medRxiv 2021). We built a multivariate logistic regression model to assess factors independently associated with the absence of antibody response after the second dose of mRNA vaccination. RESULTS Overall, 620 allogeneic HSCT recipients from 12 hospitals across France were included in the analysis (60% male with a median age of 59 years old [IQR 47-66]), most with a myeloid (69%) or lymphoid (26%) malignancies. Donors were matched unrelated for 51%, HLA-identical sibling for 31% and haplo-identical for 18%. Thirty-one percent of HSCT recipients underwent a myeloablative conditioning, while 69% received a reduced intensity conditioning. The two doses of vaccines were given one month apart and the median time between transplantation and the initiation of vaccination was 29 months [IQR 14-58]. At a median of 33 [IQR 27-50] days after dose 2, an antibody response was detectable in 496 patients (80% [95CI: 77 to 83%]). Median [IQR] antibody levels was 243 BAU/mL [29.4-1391]. We classified detectable antibody responses as "weak" in 189 patients (30% [95CI 27 to 34%]) and as "good" in 306 (49% [95CI: 45 to 53%]). In the multivariate analysis including 533 patients (420 with detectable antibodies), factors associated with the absence of humoral responses were a time-interval from HSCT < 12 months (ajusted Odds-Ratio (aOR) 2.8 [95CI 1.6 to 4.8]), absolute lymphocyte count CONCLUSION After 2 dose mRNA vaccination, the majority of allogeneic HSCT recipients developed an antibody response although a significant proportion of these responses may be insufficient. Studies are still needed to investigate the effect of a third vaccine dose in patients with a null or weak humoral response. Disclosures Loschi: Servier: Ended employment in the past 24 months, Honoraria; Novartis: Ended employment in the past 24 months, Honoraria; Gilead: Ended employment in the past 24 months, Honoraria; AbbVie: Ended employment in the past 24 months, Honoraria; CELGENE/BMS: Honoraria; MSD: Honoraria.

Patients with Familial Platelet disorder (FPD) have a germline RUNX1 mutation and are at high risk to developing hematologic malignancies (HM), primarily myelodysplastic syndrome and acute myeloid leukemia (lifetime risk~40%). To understand how germline RUNX1 mutations predispose to HM in vivo, we developed a Runx1 R188Q/+ mouse strain , mimicking the FPD-associated R201Q missense mutation. Analysis of the bone marrow cells in Runx1 R188Q/+ mice revealed a significant increase in the total number of bone marrow cells. Immunophenotypic analysis using Sca-1 and Cd86 markers revealed a significant increase in Sca-1 expression in hematopoietic stem and multi-potential progenitor cells, indicating a systemic inflammation in the bone marrow. In addition, the frequency of common-myeloid, granulocytic-monocytic and granulocytic progenitor cells were found significantly increased in the Runx1 R188Q/+ bone marrow. Accordingly, their colony-forming unit capacity was increased when compared to wildtype controls (wt/Runx1 R188Q/+ CFU average = 45/85), indicating a myeloid bias. The number and size of platelets were not altered in Runx1 R188Q/+ mice. However, platelet function was significantly reduced. The activation of the Cd41/Cd61 fibrinogen receptor complex in membrane after thrombin treatment was reduced in Runx1 R188Q/+ platelets. Similarly, the translocation of P-selectin by alpha granules and the secretion of serotonin by the dense granules were also reduced. Hematopoietic progenitor cells isolated from Runx1 R188Q/+ mice revealed a significant reduction in DNA-damage repair response in vitro. Quantitative analysis of nuclei with 53bp1-positive foci in response to ionizing radiation showed a marked increase in 53bp1-positive foci in Runx1 R188Q/+ nuclei, suggesting that Runx1 R188Q/+ cells have a defective repair of double strand DNA breaks. Furthermore, expression of DNA-damage repair pathway-associated Pmaip1 (Noxa) was significantly reduced in irradiated Runx1 R188Q/+ hematopoietic progenitor cells. To understand underlying mechanism responsible for the observed myeloid bias in Runx1 R188Q/+ cells, transcription profiling analysis was performed in myeloid progenitors from wildtype and Runx1 R188Q/+ mice, utilizing RNA-sequencing. A total of 39 genes were significantly deregulated (> 1.5 FC; FDR To study the FPD-associated pre-leukemic process in vivo, wildtype and Runx1 R188Q/+ mice were monitored for 20 months. Although Runx1 R188Q/+ mice remained healthy for 18 months, somatic mutations in their leukocytes were evident at 12 months. Targeted sequencing of 578 cancer genes (mIMPACT panel) in leukocyte DNA of two Runx1 R188Q/+ mice identified somatic mutations in Kdm6a, Setd1b, Amer1, and Esco1 (variant allele frequencies between 0.5% and 2.8%). These mutations were confirmed at stable frequency for eight following months. Since loss of the second Runx1 allele is a frequent somatic event in progression to FPD/HM, we evaluated the predisposition to HM in Mx1Cre-Runx1 R188Q/fl mice over time. Unlike Runx1 R188Q/+ mice, Runx1 R188Q/Δ mice succumbed to myeloid leukemia with a median latency of 37.5 weeks and full penetrance. In addition, the expression of oncogenic Nras-G12D, in Runx1 R188Q/Δ mice reduced the median latency to 14.7 weeks. These studies demonstrate that FPD-associated Runx1 germline mutations induce inflammation in hematopoietic stem cells, induce myeloid expansion with defective DNA-damage response and predispose to HM over time. These studies suggest that anti-inflammatory therapies in pre-symptomatic FPD patients may reduce clonal expansion and predisposition to HM. Disclosures Ebert: Exo Therapeutics: Membership on an entity's Board of Directors or advisory committees; Skyhawk Therapeutics: Membership on an entity's Board of Directors or advisory committees; Celgene: Research Funding; Deerfield: Research Funding; GRAIL: Consultancy. Levine: Isoplexis: Membership on an entity's Board of Directors or advisory committees; Auron: Membership on an entity's Board of Directors or advisory committees; C4 Therapeutics: Membership on an entity's Board of Directors or advisory committees; Zentalis: Membership on an entity's Board of Directors or advisory committees; Celgene: Research Funding; QIAGEN: Membership on an entity's Board of Directors or advisory committees; Ajax: Membership on an entity's Board of Directors or advisory committees; Imago: Membership on an entity's Board of Directors or advisory committees; Mission Bio: Membership on an entity's Board of Directors or advisory committees; Gilead: Honoraria; Prelude: Membership on an entity's Board of Directors or advisory committees; Janssen: Consultancy; Lilly: Honoraria; Morphosys: Consultancy; Roche: Honoraria, Research Funding; Incyte: Consultancy; Astellas: Consultancy; Amgen: Honoraria.

Signal transduction pathways regulate the proliferation and viability of acute myeloid leukemia (AML) blasts. The regulation in the expression of cytokine receptors in AML is not well understood. In this study, we investigated how the CBFβ-SMMHC fusion protein regulates expression of cytokine receptors in inv(16) AML, with focus on the co-receptor Neuropilin-1 (NRP1). Knock-down of CBFβ-SMMHC expression, utilizing shRNA transduction, induced G1 phase of cell cycle arrest and reduced the viability of inv(16) ME-1 cells in culture. Expression profile analysis of CBFβ-SMMHC knock-down cells revealed a significant repression of genes associated with transmembrane receptor protein kinase pathways, including NRP1 (-5 fold), FGFR1 (-4.2 fold) , FLT3 (-2 fold) and TGFBR2 (-1.2 fold). The expression of NRP1 was significantly upregulated in inv(16) AML cases when compared to other AML sub-types and to hematopoietic stem and progenitor cells. Functionally, NRP1 knock-down reduced the viability of ME-1 cells with a similar dynamics as when using CBFβ-SMMHC shRNAs. In addition, the proliferation of inv(16) AML cells was reduced 4.1-fold when treated with a function-blocking antibody for the FV/VIII extracellular NRP1 domain, while having no effect in non-inv(16) AML cells or when using blocking antibody for the CUB extracellular domain. Furthermore, deletion of Nrp1 by gene editing reduced the colony-forming unit capacity of primary mouse Cbfb +/MYH11 leukemic cells and extended the median leukemia latency in vivo. To identify the genes regulated by NRP1 in inv(16) AML, we analysed the transcription profile of NRP1 knock-down in ME-1 cells. Gene Set Enrichment and Pathway Analysis revealed a repression in STAT5 pathway, and in signalling receptor activity, including FLT3 (-1.8 fold) and TGFBR2 (-1.8 fold) expression, indicating that NRP1 mediates transcriptional regulation of FLT3 and TGFBR2 expression in inv(16) AML. Furthermore, the regulation of FLT3 and TGFB2 expression by CBFβ-SMMHC and by NRP1 was validated by gene editing in inv(16) AML blasts. Accordingly, NRP1 knock-down in AML cells reduced SMAD2/3 phosphorylation. The repression of RUNX1/CBFβ function, using small molecule inhibitors, in inv(16) AML cells with CBFβ-SMMHC knockdown restored NRP1 expression, suggesting that RUNX1 may repress NRP1 expression in AML cells. To evaluate if RUNX1 directly regulates NRP1 expression, we tested RUNX1 binding in the NRP1 locus of AML cells with CBFβ-SMMHC knockdown. RUNX1 binding at one of six sites with RUNX1 occupancy identified by chromatin immunoprecipitation followed by sequencing (RE5, regulatory element 5) was increased in the CBFβ-SMMHC knock-down cells. The RE5 is located 178kb upstream of the NRP1 transcription start site and it is evolutionarily conserved in vertebrates. The deletion of RE5 by gene editing (~50% editing efficiency) increased NRP1 expression 1.8-fold, suggesting that RUNX1 may repress NRP1 expression at by binding to the RE5 enhancer. Taken together, these studies demonstrate that CBFβ-SMMHC regulates expression of cytokine receptors in inv(16) AML. Specifically, it directly regulates expression of the co-receptor NRP1, which is essential for AML survival, acting (at least in part) by regulating FLT3 and TGFB pathways. Disclosures Guzman: SeqRx: Consultancy; BridgeMedicines: Consultancy; Cellectis: Membership on an entity's Board of Directors or advisory committees; Daiichi Sankyo: Membership on an entity's Board of Directors or advisory committees.

Introduction: Alterations of core binding factors (CBF), Runx1 and CBFβ are frequent mutational targets in acute myeloid leukemia (AML). Chromosomal translocations t(8;21)(q22;q22) and inv(16)(p13q22), creating the fusion proteins RUNX1-ETO and CBFβ-MYH11 respectively, account for 15% and thus the largest sub-group of AML called CBF-AML. CBF oncogenes induce global changes in chromatin structure and gene regulation, which lead to differentiation blockade. A critical leukemic event could be the inactivation of PU.1 transcription factor. Normal myeloid differentiation needs PU.1 levels to increase, failure to do so leads to a stop of differentiation and AML development. In contrast, T-cell differentiation requires PU.1 to be completely switched off. The exact mechanism of PU.1 suppression, physiological for T-lymphopoiesis or pathological for leukemia, remains elusive. Results: We assessed the activation of the PU.1 locus throughout human hematopoietic differentiation stages using the assay for transposase-accessible chromatin with high-throughput sequencing (ATAC-seq) and reverse-transcription quantitative polymerase chain reaction (RT-qPCR). Interestingly, we observed high accessibility of a previously identified antisense promoter (AsPr) in intron 3 and antisense transcript (asRNA) expression during early lymphopoiesis which preceded locus shutdown in T cells. The ratio of AsPr/PrPr accessibility and of antisense/sense transcription clearly indicated cellular fate during hematopoiesis (Figure 1A). T-lymphoid differentiation was related to the timely expression of RUNX transcription factors. RUNX1, RUNX3, and the CBF fusions RUNX1-ETO and CBFβ-MYH11 were capable to transactivate PU.1 AsPr. In CBF-AML patient samples we strikingly found elevated asRNA/mRNA ratios compared to normal karyotype AML or healthy CD34+ cells (Figure 1B) and increased AsPr/PrPr ratios unsing DNaseI-seq data in RUNX1-ETO AML patients. Functionally we found that PU.1 asRNA depletion in t(8;21) xenografted immune-deficient (NOD/SCID) mice restored a normal survival (Figure 1C) demonstrating that PU.1 antisense transcripts are required for CBF leukemia outgrow in vivo. To further dissect the mechanism of how CBFs could drive PU.1 antisense transcription we applied active RNA polymerase mapping (PRO-seq) and chromatin accessibility (ATAC-seq) and found a shift from PU.1 antisense to sense transcription after RUNX1-ETO depletion. Using chromosomal conformation capture sequencing (3C, Hi-C and CHiC) in T-lymphoid, myeloid and RUNX1-ETO cells combined with transcript quantification we observed that competitive interaction of an upstream enhancer with the proximal or the antisense promoter are at the heart of differential PU.1 expression during myeloid and T-cell development (Figure 2A). Leukemic CBF fusions thus utilize a physiologic mechanism employed by T-cells to decrease sense PU.1 transcription (Figure 2B). Conclusion: The data suggest that silencing transcription factor PU.1 is an active process that requires a specific chromosome formation that is induced by CBF fusions. Sense/antisense promoter competition represents a crucial functional switch for gene expression perturbation by oncogenes and provide a potential strategy for future precise therapeutic targeting of oncogene-induced chromatin remodeling. Disclosures Valent: Allcyte GmbH: Research Funding; Pfizer: Honoraria; Cellgene: Honoraria, Research Funding. Staber:Roche: Consultancy, Honoraria, Research Funding; AbbVie: Consultancy, Honoraria; Gilead: Consultancy, Honoraria; Janssen: Consultancy, Honoraria; Astra Zeneca: Consultancy, Honoraria; Takeda: Consultancy, Honoraria; msd: Consultancy, Honoraria; Celgene/ BMS: Consultancy, Honoraria.

The gene encoding the RUNX1 transcription factor is mutated in a subset of T-cell acute lymphoblastic leukemia (T-ALL) patients, and RUNX1 mutations are associated with a poor prognosis. These mutations cluster in the DNA-binding Runt domain and are thought to represent loss-of-function mutations, indicating that RUNX1 suppresses T-cell transformation. RUNX1 has been proposed to have tumor suppressor roles in T-cell leukemia homeobox 1/3-transformed human T-ALL cell lines and NOTCH1 T-ALL mouse models. Yet, retroviral insertional mutagenesis screens identify RUNX genes as collaborating oncogenes in MYC-driven leukemia mouse models. To elucidate RUNX1 function(s) in leukemogenesis, we generated Tal1/Lmo2/Rosa26-CreERT2Runx1f/f mice and examined leukemia progression in the presence of vehicle or tamoxifen. We found that Runx1 deletion inhibits mouse leukemic growth in vivo and that RUNX silencing in human T-ALL cells triggers apoptosis. We demonstrate that a small molecule inhibitor, designed to interfere with CBFβ binding to RUNX proteins, impairs the growth of human T-ALL cell lines and primary patient samples. We demonstrate that a RUNX1 deficiency alters the expression of a crucial subset of TAL1- and NOTCH1-regulated genes, including the MYB and MYC oncogenes, respectively. These studies provide genetic and pharmacologic evidence that RUNX1 has oncogenic roles and reveal RUNX1 as a novel therapeutic target in T-ALL.