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This study was funded by the Spanish ISCIII Health Research Fund and the European Regional Development Fund (FEDER) through research grants PI12/01097, PI15/02015, PI18/00337 (F.M.) PIE16-00045 (J.A.M.), DTS19/00145 (J.A.M.), and PI18/00330 (K.B.) The CECEyU and CSyF of the Junta de Andalucia FEDER/European Cohesion Fund (FSE) for Andalusia provided the following research grants: 2016000073391-TRA, 2016000073332-TRA, PI-57069, and PAIDI-Bio326 (F.M.) and PI0014-2016 (K.B.). K.B. also held a Nicolas Monardes regional Ministry of Health contract (0006/2018). M.T.-M. and N.M.-P. are funded by the Spanish Ministry of Science and Innovation (SMSI) through fellowships FPU16/05467 and FPU17/02268, respectively. M.C.-G is funded by the SMSI through fellowship (PEJ-2018-001760-A). The Ramon y Cajal grant RYC-2015-18382 to P.J.R. founded by the Ministry of Economy and Competitiveness. L.L.-O. is supported by the University of Granada doctoral program (2017). M.C.-G., M.T.-M. and N.M.-P. are University of Granada Biomedicine PhD students., Integration-deficient lentiviral vectors (IDLVs) have recently generated increasing interest, not only as a tool for transient gene delivery, but also as a technique for detecting off-target cleavage in gene-editing methodologies which rely on customized endonucleases (ENs). Despite their broad potential applications, the efficacy of IDLVs has historically been limited by low transgene expression and by the reduced sensitivity to detect low-frequency off -target events. We have previously reported that the incorporation of the chimeric sequence element IS2 into the long terminal repeat (LTR) of IDLVs increases gene expression levels, while also reducing the episome yield inside transduced cells. Our study demonstrates that the effectiveness of IDLVs relies on the balance between two parameters which can be modulated by the inclusion of IS2 sequences. In the present study, we explore new IDLV configurations harboring several elements based on IS2 modifications engineered to mediate more efficient transgene expression without affecting the targeted cell load. Of all the insulators and configurations analysed, the insertion of the IS2 into the 30LTR produced the best results. After demonstrating a DAPI-low nuclear gene repositioning of IS2-containing episomes, we determined whether, in addition to a positive effect on transcription, the IS2 could improve the capture of IDLVs on double strand breaks (DSBs). Thus, DSBs were randomly generated, using the etoposide or locus-specific CRISPR-Cas9. Our results show that the IS2 element improved the efficacy of IDLV DSB detection. Altogether, our data indicate that the insertion of IS2 into the LTR of IDLVs improved, not only their transgene expression levels, but also their ability to be inserted into existing DSBs. This could have significant implications for the development of an unbiased detection tool for off-target cleavage sites from different specific nucleases., Spanish ISCIII Health Research Fund, European Commission PI12/01097 PI15/02015 PI18/00337 PIE16-00045 DTS19/00145 PI18/00330, Junta de Andalucia 2016000073391-TRA 2016000073332-TRA PI-57069 PAIDI-Bio326 PI0014-2016, Nicolas Monardes regional Ministry of Health 0006/2018, Spanish Government FPU16/05467, SMSI through fellowship PEJ-2018-001760-A, Spanish Government RYC-2015-18382, Ministry of Economy and Competitiveness, University of Granada doctoral program

When self-renewing pluripotent cells receive a differentiation signal, ongoing cell duplication needs to be coordinated with entry into a differentiation program. Accordingly, transcriptional activation of lineage specifier genes and cell differentiation is confined to the G1 phase of the cell cycle by unknown mechanisms. We found that Polycomb repressive complex 2 (PRC2) subunits are differentially recruited to lineage specifier gene promoters across cell cycle in mouse embryonic stem cells (mESCs). Jarid2 and the catalytic subunit Ezh2 are markedly accumulated at target promoters during S and G2 phases, while the transcriptionally activating subunits EPOP and EloB are enriched during G1 phase. Fluctuations in the recruitment of PRC2 subunits promote changes in RNA synthesis and RNA polymerase II binding that are compromised in Jarid2 −/− mESCs. Overall, we show that differential recruitment of PRC2 subunits across cell cycle enables the establishment of a chromatin state that facilitates the induction of cell differentiation in G1 phase., This study was supported by the Spanish Ministry of Economy and Competitiveness (SAF2013-40891-R and BFU2016-75233-P) and the Andalusian Regional Government (PC-0246-2017). D.L. is a Ramón y Cajal researcher of the Spanish Ministry of Economy and Competitiveness (RYC-2012-10019).

Runt-related transcription factor 1 (Runx1) is a master hematopoietic transcription factor essential for hematopoietic stem cell (HSC) emergence. Runx1-deficient mice die during early embryogenesis due to the inability to establish definitive hematopoiesis. Here, we have used human pluripotent stem cells (hPSCs) as model to study the role of RUNX1 in human embryonic hematopoiesis. Although the three RUNX1 isoforms a, b, and c were induced in CD45+ hematopoietic cells, RUNX1c was the only isoform induced in hematoendothelial progenitors (HEPs)/hemogenic endothelium. Constitutive expression of RUNX1c in human embryonic stem cells enhanced the appearance of HEPs, including hemogenic (CD43+) HEPs and promoted subsequent differentiation into blood cells. Conversely, specific deletion of RUNX1c dramatically reduced the generation of hematopoietic cells from HEPs, indicating that RUNX1c is a master regulator of human hematopoietic development. Gene expression profiling of HEPs revealed a RUNX1c-induced proinflammatory molecular signature, supporting previous studies demonstrating proinflammatory signaling as a regulator of HSC emergence. Collectively, RUNX1c orchestrates hematopoietic specification of hPSCs, possibly in cooperation with proinflammatory signaling.

Bernard Soulier Syndrome (BSS) is a rare autosomal platelet disorder characterized by mutations in the von Willebrand factor platelet receptor complex GPIb-V-IX. In this work we have generated an induced pluripotent stem cell (BSS3-PBMC-iPS4F8) from peripheral blood mononuclear cells of a BSS patient with a p.Phe55Ser mutation in the GPIX gene. Characterization of BSS3-PBMC-iPS4F8 showed that these cells maintained the original mutation present in the BSS patient, expressed pluripotent stem cell markers and were able to differentiate into the three germline layers. This new iPSC line will contribute to better understand the biology of BSS disease.

Intravital microscopy of the calvarium is the only noninvasive method for high-resolution imaging of the bone marrow (BM) and hematopoietic stem cell (HSC) niches. However, it is unclear if the calvarium is representative of all BM compartments. Using the combination of whole body optical imaging, intravital microscopy, and "in vivo fluorescence trapping," a thorough comparison of HSCs and putative HSC niches in the calvaria, epiphyses, and diaphyses, at steady state or after HSC transplantation, can be made. We report substantial heterogeneity between different BM compartments in terms of bone-remodeling activity (BRA), blood volume fraction (BVF), and hypoxia. Although BVF is high in all BM compartments, including areas adjacent to the endosteum, we found that compartments displaying the highest BVF and BRA were preferentially seeded and engrafted upon HSC transplantation. Unexpectedly, the macroanatomical distribution of HSCs at steady state is homogeneous across these 3 areas and independent of these 2 parameters and suggests the existence of "reconstituting niches," which are distinct from "homeostatic niches." Both types of niches were observed in the calvarium, indicating that endochondral ossification, the process needed for the formation of HSC niches during embryogenesis, is dispensable for the formation of HSC niches during adulthood.

Bernard Soulier Syndrome (BSS) is an inherited rare platelet disorder characterized by mutations in the platelet glycoprotein complex GPIb-IX-V. We generated an induced pluripotent stem cell (iPSC) line from a BSS patient with a mutation p.Asn45Ser in the GPIX locus (BSS2-PBMC-iPS4F24). Peripheral blood mononuclear cells were reprogrammed using non-integrative viral transduction. Characterization of BSS2-PBMC-iPS4F24 included mutational analysis of GPIX locus, analysis of conventional pluripotency-associated factors at mRNA and protein level and in vitro and in vivo differentiation studies. This iPSC line will provide a powerful tool to study the biology of BSS disease.

Abstract 25 Introduction: Hematopoietic stem cells (HSCs) reside in specialized bone marrow (BM) microenvironments called stem cell niches (HSCNs). Two types of HSCNs have been reported, involving osteoblasts (osteoblastic niche) and blood sinusoids (vascular niche). Various hematopoietic and non-hematopoietic cell types are contributing to the regulation of HSCs and HSCNs, however, the bone marrow microenvironment has not yet been precisely characterized and the fine localization, composition and regulation of the niches remain highly controversial. Intravital microscopy of the calvarium (IVMC) is the only non-invasive option for high resolution imaging of bone marrow HSCNs. However it is unclear if IVMC provides data representative of all BM compartments. Indeed it has been shown that endochondral ossification, the process used for long bones growth, is required for the formation of HSCNs. By contrast, the calvarium is a flat bone which develops through intra-membranous ossifications, therefore questioning the presence of HSCNs in this bone (Chan CK et al, Nature 2009). Methods: Combined confocal and multiphoton excitation intravital microscopy was used for multiparametric observation of live bone marrow in intact bones after in-vivo contrasting procedures. Bioluminescence imaging was used to quantify the systemic trafficking and proliferation of murine HSCs and human leukemia. Flow cytometry and histology were used to sort specific populations of cells, analyze the frequency of mouse and human stem cells or leukemic cells at steady state or after transplantation, quantify perfusion efficiency and hypoxia and cross-validate in-vivo imaging procedures. Results: Thanks to advanced imaging modalities we realized a thorough study of potential niche markers and HSCs distribution at homeostasis and during hematopoietic reconstitution in calavia, epiphyses and diaphyses. We report important heterogeneity between these compartments in terms of bone remodelling activity (BRA) and blood vessel fraction (BVF). Although BVF was surprisingly high in any compartment, including in very close proximity to any endosteal surface, we found that compartments displaying the highest BVF and BRA were preferentially seeded and engrafted following mouse and human HSC transplantation. Unexpectedly, the macro-anatomical distribution of mouse and human HSCs at steady state is homogeneous across the skeleton and independent of these 2 markers. These data suggest the existence of “reconstituting niches” which would be distinct from “homeostatic niches”. Importantly, this study provides the first evidence that both types of niches are fully functional in the calvarium, including for mouse and human HSCs, indicating that endochondral ossification is dispensable for adult HSCNs. The model is currently being exploited to analyze leukemia/microenvironment interactions in live bone marrow. Conclusions: This study confirms and extends our recent statement considering the critical need for multimodal imaging (Lassailly F et al, Blood 2010). Further more, it demonstrates that combination of different imaging modalities for in-vivo and ex-vivo analysis is a powerful strategy allowing to shed a new light on the structure of the bone marrow microenvironment and improve our understanding of stem cells/niches interactions in health and disease. Disclosures: No relevant conflicts of interest to declare.

Summary Hematopoietic stem and progenitor cells (HSPCs) are exposed to low levels of oxygen in the bone marrow niche, and hypoxia-inducible factors (HIFs) are the main regulators of cellular responses to oxygen variation. Recent studies using conditional knockout mouse models have unveiled a major role for HIF-1α in the maintenance of murine HSCs; however, the role of HIF-2α is still unclear. Here, we show that knockdown of HIF-2α, and to a much lesser extent HIF-1α, impedes the long-term repopulating ability of human CD34 + umbilical cord blood cells. HIF-2α-deficient HSPCs display increased production of reactive oxygen species (ROS), which subsequently stimulates endoplasmic reticulum (ER) stress and triggers apoptosis by activation of the unfolded-protein-response (UPR) pathway. HIF-2α deregulation also significantly decreased engraftment ability of human acute myeloid leukemia (AML) cells. Overall, our data demonstrate a key role for HIF-2α in the maintenance of human HSPCs and in the survival of primary AML cells.