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The cellular mechanisms required to ensure homeostasis of the hematopoietic niche and the ability of this niche to support hematopoiesis upon stress remain elusive. We here identify Wnt5a in Osterix+ mesenchymal progenitor and stem cells (MSPCs) as a critical factor for niche-dependent hematopoiesis. Mice lacking Wnt5a in MSPCs suffer from stress-related bone marrow (BM) failure and increased mortality. Niche cells devoid of Wnt5a show defective actin stress fiber orientation due to an elevated activity of the small GTPase CDC42. This results in incorrect positioning of autophagosomes and lysosomes, thus reducing autophagy and increasing oxidative stress. In MSPCs from patients from BM failure states which share features of peripheral cytopenia and hypocellular BM, we find similar defects in actin stress fiber orientation, reduced and incorrect colocalization of autophagosomes and lysosomes, and CDC42 activation. Strikingly, a short pharmacological intervention to attenuate elevated CDC42 activation in vivo in mice prevents defective actin-anchored autophagy in MSPCs, salvages hematopoiesis and protects against lethal cytopenia upon stress. In summary, our study identifies Wnt5a as a restriction factor for niche homeostasis by affecting CDC42-regulated actin stress-fiber orientation and autophagy upon stress. Our data further imply a critical role for autophagy in MSPCs for adequate support of hematopoiesis by the niche upon stress and in human diseases characterized by peripheral cytopenias and hypocellular BM.

Inherited bone marrow failure syndromes (IBMFS) are a heterogeneous group of disorders characterized by impaired stem cell function resulting in pancytopenia. Diagnosis of IBMFS presents a major challenge due to limited diagnostic tests and overlapping phenotypes. For that reason, novel and clinical relevant biomarkers and possible targets are urgently needed. Our study defines NIPA as an IBMFS gene, which is significantly downregulated in a distinct subset of MDS-type refractory cytopenia of childhood patients. Mechanistically, NIPA binds FANCD2 and regulates its nuclear abundance. The stabilization of both non- and monoubiquitinated FANCD2 identifies NIPA as an essential player in the Fanconi Anemia (FA) pathway. NIPA thereby prevents MMC hypersensitivity visualized by increased numbers of chromosome radials and reduced cell survival after induction of interstrand crosslinks. To provide proof of principle, re-expression of FANCD2 in Nipa deficient cells restores MMC sensitivity. In a knockout mouse model, Nipa deficiency leads to major cell intrinsic long-term repopulation defects of hematopoietic stem cells (HSCs), with impaired self-renewal in serial transplantations and a bias towards myeloid differentiation. Unresolved DNA damage in Nipa deficient HSCs causes increased sensitivity to cell death and leads to progressive, age-related loss of the HSC pool. Induction of replication stress triggers the phenotypic reduction and functional decline of murine HSCs, resulting in complete bone marrow failure and death of the mice thereby mimicing Fanconi Anemia. Taken together, our study adds NIPA to the short list of FA-associated proteins being essential for a functional DNA repair/FA/BRCA axis and thereby emphasizing its impact as potential diagnostic marker and/or possible target in bone marrow failure syndromes. Disclosures Niemeyer: Celgene: Consultancy.

GATA2 Zinc-Finger (ZF) mutations are associated with specific subgroups of myeloid malignancies. Alterations of the N-terminal ZF1 were identified in AML patients with biallelic CEBPA mutations, whereas the C-terminal ZF2 is typically affected by germline mutations, predisposing to MDS and AML, or by somatic lesions in CML blast crisis. Nevertheless, the context-dependent mechanism underlying GATA2 ZF2 mutations remains mostly unclear. Here, we set out to study the functional consequences of GATA2 ZF mutations in myeloid malignancies. In particular, we performed DNA-pulldown experiments, using FLAG-tagged full length GATA2 ZF WT and mutant proteins after expression in HEK293T cells and incubation of the cell lysates with biotinylated oligonucleotides encoding the binding-motif GATC. These experiments showed disruption of DNA binding for all GATA2 ZF mutants tested in vitro - regardless of the mutant positions within the ZF domains (Figure 1 A). Moreover, we studied the impact of GATA2 ZF mutations on the protein-interaction with Friend of GATA Protein 1 (FOG1; HGNC symbol ZFPM1). The influence of FOG1 on GATA2-dependent transcriptional activation was evaluated using a GATA-specific luciferase reporter. All GATA2 mutants tested were able to activate this reporter, although to variable extent. While co-expression of FOG1 overall counteracted GATA2-dependent transcriptional activation, this effect was significantly reduced for the GATA2 mutants L321F (located in ZF1) and T354M (located in ZF2). This suggested that both ZF domains are involved in the FOG1-interaction. To gain insights into the influence of GATA2 ZF1 mutation on hematopoiesis we performed colony forming cell (CFC)-assays. Lin- primary murine bone marrow cells expressing GATA2 WT or mutants were plated in methylcellulose supplemented with cytokines. Consistent with previous reports, GATA2 WT expression led to a reduced colony number, while this effect was decreased for both mutants A318T (ZF1) and L359V (ZF2). In particular, a higher number of CFU-G colonies were observed for the GATA2 mutant-expressing cells indicating a lineage shift towards granulopoiesis (Figure 1 B). In summary, we have shown that GATA2 mutations influence DNA-binding, protein-interactions and myeloid differentiation. Our findings further suggest that GATA2 ZF1 mutations may contribute to myeloid leukemogenesis through increased proliferation of granulocytic progenitors. Understanding the oncogenic collaboration of GATA2 mutations with other driver genes in distinct patient subgroups is a challenge ahead. Disclosures Hiddemann: Celgene: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; F. Hoffman-La Roche: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Janssen: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Bayer: Consultancy, Research Funding.

Accumulation of DNA damage in hematopoietic stem cells (HSCs) is associated with aging, bone marrow failure and development of hematological malignancies. Although HSCs numerically expand with age, their functional activity declines over time and the protection mechanism from DNA damage accumulation remains to be elucidated. NIPA (Nuclear Interaction Partner of ALK) is highly expressed in hematopoietic stem and progenitor cells, especially in the most primitive long-term repopulating HSCs (CD34-Flt3-Lin-Sca1+cKit+). Loss of NIPA leads to a significant exhaustion of primitive hematopoietic cells, where Lin-Sca1+cKit+ (LSK) cells were reduced to 40% of wildtype (wt) littermates (p To further examine the role of NIPA in HSC maintenance and exhaustion, we performed in vivo repopulationexperiments, where Nipa deletion causes bone marrow failure in case of competition, as Nipako/ko cells contributed to less than 10% of transplanted BM cells 6 month after transplantation (TX). According to this, colony formation assays and limiting dilution transplantation showed a dramatic reduction of competitive repopulation units (p To further investigate the stress response in Nipa-deficient HSCs, we irradiated LSKs with 3 Gy and stained for DNA-Damage foci by pH2ax. Remarkably, loss of NIPA led to significant higher numbers of pH2ax foci in irradiated HSCs (46% > 6 foci vs. 17% > 6 foci in wt cells) and highly increased the rates of apoptotic cells especially in the primitive CD34-LSK population. Taken together our results highlight the importance of the DNA damage response at HSC level for lifelong hematopoiesis and establish NIPA as a novel regulator of aging and stress response of the primitive HSC pool. Disclosures No relevant conflicts of interest to declare.

Hematopoietic stem cells (HSC) are regulated by an interplay of intrinsic and extrinsic signals, the latter of which are mostly transmitted by the niche. The processes involved and their interactions are largely unknown. We studied the dynamic interaction of HSC and niche stromal cells, using co-cultures of HSC (lineage-negative Sca-1+ c-Kit+: LSK) cells and HSC-maintaining UG26-1B6 stromal cells. Microarray analyses from cells prior to co-culture and cells sorted separately from the cultures revealed that most changes in gene expression take place in the first 24 hours of co-culture. Analyses using STEM clustering, LIMMA, and ToppGene databases showed early activation of cell cycle progression In LSK cells and extensive remodeling of chomatin structure and transcriptional activation in both LSK and stromal cells. Interestingly, connective tissue growth factor (Ctgf/Ccn2), which is involved in TGFb, BMP and Wnt signaling, was strongly upregulated in both stromal and LSK cells. To study the role of Ctgf as a stromal mediator, LSK cells were co-cultured with siCTGF knockdown stromal cells. We showed that although short-term HSC activity was unchanged, siCtgf-stromal cells were unable to sustain long-term repopulating ability. To study underlying mechanisms, a Boolean model simulating possible signaling mechanisms leading to cell cycle activation was extracted from the data. We validated this model by co-cultures of LSK cells with control and Ctgf-knockdown stroma, separating LSK and stromal cells and assessing protein levels and phosphorylation using immunocytofluorescence in LSK cells. We found that in the absence of extrinsic Ctgf, expression of Pten was increased in LSK cells. However, phosphorylation of Akt (both p308, and p473) and Erk was unchanged. In contrast, both canonical Wnt (LRP6, Gsk3b, b-catenin) and Tgfb (Smad2/3) signaling were significantly affected in that Wnt signaling was turned off, whereas Tgf signaling was turned on, by the lack of extrinsic Ctgf. This resulted in a downregulation of G1 transition, as was exemplified by downregulation of Cyclin D1, upregulation of p27Kip1 and modulations in the phosphoryalation of both Rb and p53. Equally interestingly, we could show that extrinsic Ctgf deficiency also downregulates induction of Ctgf in HSC, suggesting the existence of intrinsic-extrinsic feedback signaling. In summary, co-culture of LSK cells with stromal cells results in cellular activation of both stromal cells and LSK cells, involving Tgf and canonical Wnt signaling pathways. Furthermore, reduced expression of extrinsic Ctgf, regulates mediators of G1 cell cycle progression in LSK cells an a biochemical level and functionally results in an increased production of myeloid progenitors and decreased long-term repopulating ability. Our studies show the dynamics of reciprocal signaling between HSC and niche stromal cells and give insights how the niche regulates early regenerative responses in hematopoiesis. Disclosures: No relevant conflicts of interest to declare.

Abstract 771 Introduction: Activating mutations of FLT3 are among the most frequent alterations in AML. Two distinct types of mutations have been described: Duplication of the juxtamembraneous domain (ITD) and point mutations of the tyrosine kinase domain (TKD). Although both lead to constitutive FLT3 signaling, FLT3-ITD signals through the STAT5 pathway whereas FLT3-TKD and FLT3 WT activate STAT5 to a much lesser degree. In a murine bone marrow transplantation model, FLT3-ITD leads to a myeloproliferative neoplasm; FLT3-TKD induces a lymphoid disease with significantly longer latency. Interaction of SRC with FLT3-ITD was recently shown to be necessary for STAT5 activation. Methods: Fetal liver cells from constitutive STAT5 knockout mice were infected with FLT3 oncogenes and cultured on methylcellulose. Conditional STAT5 knockout mice were analyzed for phenotype and latency of hematologic disorders after transplantation with FLT3-ITD or FLT3-TKD-infected bone marrow. Expression of STAT5 target genes was determined using Realtime-PCR. Phospho-STAT5 levels were quantified by intracellular staining and flow cytometry. Results: Depletion of STAT5 leads to a dramatic decrease in the ability of fetal liver cells to form colonies in Methylcellulose when infected with FLT3-ITD, but not when infected with FLT3-TKD. In-vivo studies using a murine conditional knockout model proved that STAT5 is necessary for the induction of a myeloproliferative disease by FLT3-ITD, while FLT3-TKD is not dependent on STAT5. Mice with deletion of the STAT5 allele displayed expansion of mainly lymphoid cells with an increased latency similar to the FLT3-TKD disease. In additional studies differential activation of STAT5 in myeloid and lymphoid progenitors was found. The STAT5 target gene Oncostatin M is exclusively expressed in myeloid progenitor cells infected with FLT3-ITD and in mice with intact STAT5 that were transplanted with FLT3-ITD. Furthermore, elevated SRC expression levels could be detected in proliferating myeloid progenitor cells compared to lymphoid progenitors. Conclusion: STAT5 is critical for the induction of a myeloproliferative disease by FLT3-ITD in a murine transplantation model. Interaction of SRC and FLT3-ITD leads to the activation of STAT5 in myeloid progenitors, but not in lymphoid progenitors expressing low levels of SRC. Oncostatin M upregulation is likely to be a downstream process contributing to the phenotype and latency of the disease. Disclosures: No relevant conflicts of interest to declare.

Abstract 2353 Hematopoiesis is maintained throughout life by the constant production of mature blood cells from hematopoietic stem cells (HSC). One mechanism by which the number of HSC is maintained is self-renewal, a cell division in which at least one of the daughter cells is a cell with the same functional potential as the mother cell. The mechanisms of this process are largely unknown. We have described cell lines that maintain self-renewal in culture. To study possible mechanisms and mediators involved in self-renewal, we performed co-cultures of HSC model cells: Lineage-negative Sca-1+ c-Kit+ (LSK) cells and HSC maintaining UG26–1B6 stromal cells. Microarray analyses were performed on cells prior to co-culture and cells sorted from the cultures. STEM clustering analysis of the data revealed that most changes in gene expression were due to early cell activation. Functional enrichment analysis revealed dynamic changes in focal adhesion and mTOR signaling, as well as changes in epigenetic regulators, such as HDAC in stromal cells. In LSK cells, genes whose products are involved in inflammation, Oxygen homeostasis and metabolism were differentially expressed after the co-culture. In addition, genes involved in the regulaton of H3K27 methylation were also affected. Interestingly, connective tissue growth factor (CTGF), which is involved in TGF-b, BMP and Wnt signaling, was upregulated in both stromal and LSK cells in the first day of co-culture. To study a possible extrinsic role of CTGF as a stromal mediator, we co-cultured siCTGF knockdown stromal cells with wild-type LSK cells. Since self-renewal requires cell division, we focused on cell cycle regulation of LSK cells. We found that knockdown of CTGF in stromal cells downregulates CTGF in LSK cells. In addition, knockdown of stromal CTGF downregulated Ccnd1, Cdk2, Cdkn1a (p21), Ep300 and Fos. On the other hand, decreased CTGF in stromal cells upregulates Cdkn1b (p27) and phosphorylation of Smad2/3. These results show that stromal CTGF regulates the cell cycle of LSK cells. On a functional level, we found that decreased stromal CTGF results in an increased production of MPP and myeloid colony-forming cells in 1-week co-cultures. We will present data showing whether and how a decrease in CTGF in stromal cells affects the maintenance of transplantable HSC. In summary, our current results indicate that reduced expression of CTGF in stromal cells regulates mediators of cell cycle and Smad2/3-mediated signaling in LSK cells, resulting in an increased production of myeloid progenitors. Disclosures: No relevant conflicts of interest to declare.

Abstract 2330 Hematopoietic stem cells (HSCs) are characterized by their ability to self-renewal and multilineage differentiation. Since mostly HSCs exist in a quiescent state re-entry into cell cycle is essential for their regeneration and differentiation and the expression of numerous cell cycle regulators must be tightly controlled. We previously characterized NIPA (Nuclear Interaction Partner of ALK) as a F-Box protein that defines an oscillating ubiquitin E3 ligase targeting nuclear cyclin B1 in interphase thus contributing to the timing of mitotic entry. To examine the function of NIPA on vivo, we generated NIPA deficient animals, which are viable but sterile due to a defect in recombination and testis stem cell maintenance. To further characterize the role of NIPA in stem cell maintenance and self-renewal we investigated hematopoiesis in NIPA deficient animals. Peripheral blood counts taken at different ages revealed no apparent difference between NIPA knockout and wild type mice in numbers and differentiation. In contrast, looking at the hematopoietic stem cell pool, FACS analyses of bone marrow showed significantly decreased numbers of Lin-Sca1+cKit+ (LSK) cells in NIPA deficient animals, where LSKs were reduced to 40% of wild type littermates (p=0,0171). This effect was only apparent in older animals, where physiologically higher LSK numbers have to compensate for the exhaustion of the stem cell pool. Additionally, older NIPA deficient mice have only half the amount of multi myeloid progenitors (MMPs) in contrast to wild type animals. To examine efficient activation of stem cells to self-renew in response to myeloid depression, we treated young and old mice with the cytotoxic drug (5-FU) four days before bone marrow harvest. As expected, 5-FU activated hematopoietic progenitors in wild type animals, whereas NIPA deficient progenitors failed to compensate to 5-FU depression, e.g. LSKs of NIPA knockout mice were reduced to 50% of wild type levels (p Taken together our results demonstrate a unique and critical role of NIPA in regulating the primitive hematopoietic compartment as a regulator of self-renewal, cycle capacity and HSC expansion. Disclosures: No relevant conflicts of interest to declare.