Vav-Cre Mice Research Articles

The Roles of Immune Cells Derived from Clonal Hematopoiesis in Colorectal Cancer Metastasis

[Backgrounds] Clonal hematopoiesis (CH) has been observed in over 20% of patients diagnosed with solid cancers, and it has been associated with a poor prognosis in most cases, with the exception of colorectal cancer (CRC) patients who tend to have a favorable prognosis. Among the various gene mutations detected in CH, TET2 mutations are particularly prevalent. Previous research has demonstrated that the impact of TET2-mutated CH immune cells depends on the specific types of cancer tissues involved. Specifically, studies have shown that Tet2-KO myeloid cells can inhibit the progression of melanoma while promoting the growth of hepatoma and lung cancers. However, there is a limited understanding of the roles of TET2-mutated CH immune cells on CRC progression. [ Methods] We conducted an experiment focusing on CRC and CH using CRC organoid cells transplanted into the spleens of different types of Tet2 conditional knockout mice: VAV1Cre (with Tet2 gene deletion in all hematopoietic cells), LysMCre (myeloid cells), CD19Cre (B lymphocytes), and CD4Cre (T lymphocytes). After a period of 30 days, we collected livers for analysis. The extent of liver metastasis tumor burden (LMTB) was determined by counting the number of tumor foci on 10 hematoxylin-stained slides, with an interval of 80 μm between each slide. We utilized immunohistochemistry (IHC) and flow cytometry (FC) to analyze immune cells. The number of positive cells was automatically counted in 10 fields at 20x magnification. Furthermore, we performed whole transcriptome analysis (WTA) on sorted CD4+, CD8+, CD11b+, and CD19+ cells, respectively. [Results] LMTB was significantly lower in VAV1Cre and CD4Cre mice compared to the control mice ( VAV1Cre vs. control: 55.65±35.62 vs. 89.93±34.57 foci/1000 mm², p<0.05; CD4Cre vs. control: 50.19±30.39 vs. 85.98±7.92 foci/1000 mm², p<0.05). However, the LMTB of CD19Cre and LysMCre mice was comparable to that of control mice. The differential gene expression of WTA of CD8+ cells revealed that 243 genes were up-regulated, while 189 genes were down-regulated in both of VAV1Cre and CD4Cre mice compared to the control mice. Among the down-regulated genes, a gene group consisting of Pdcd1, Tigit, Lag3 and Havcr2, which encode inhibitory molecules associated with exhausted T cells exhibited enrichment (Enrichment score =3.77) based on the analysis using DAVID. Ingenuity Pathway Analysis further indicated that these molecules were downregulated by the upstream regulator TOX. Furthermore, T cell exhaustion signaling pathways were downregulated in both VAV1Cre and CD4Cre mice using gene set enrichment analysis. The immunofluorescence results demonstrated that the number of CD8-T cells expressing PDCD1 and HAVCR2 was decreased in VAV1Cre mice and CD4Cre compared to the control mice ( VAV1Cre and CD4Cre vs control; 5.91±2.50 and 12.86±5.3 vs. 35.15±8.21 cells/100 CD8-T cells, p<0.01 for PDCD1; 4.51±4.06 and 13.65±7.84 vs. 24.51±9.94 cells/100 CD8-T cells, p<0.01 for HAVCR2). Moreover, the number of CD8-T cells expressing TOX was also significantly decreased in VAV1Cre mice and CD4Cre compared to the control mice ( VAV1Cre and CD4Cre vs control; 5.19±7.48 and 11.31±14.43 vs. 35.15±16.50 cells/100 CD8-T cells; p<0.01). [ Conclusions] The depletion of the Tet2 gene in hematopoietic cells resulted in a reduction of exhausted CD8+ T cells and suppressed the liver metastasis of CRC organoid cells. These findings provide insights into the favorable prognosis observed in CRC patients with CH. Considering the high prevalence of CH in cancer patients, our results suggest that studying the roles of CH-immune cells could shed new light on understanding the cancer microenvironment from a novel perspective.

Abstract 18831: The Protective Effect of Duffy Antigen Receptor for Chemokines Gene Deletion on Abdominal Aortic Aneurysm Formation: Role of Lysyl Oxidase

Introduction: Epidemiologic studies indicate that Blacks are protected against abdominal aortic aneurysm (AAA), despite similar risk factors as Caucasians. Approximately 70% of Blacks harbor a variant in the Duffy Antigen Receptor for Chemokines (DARC), a non-signaling receptor expressed primarily on erythrocytes which binds and regulates inflammatory chemokines. Since chemokines play a critical role in AAA, we tested the hypothesis that lack of DARC expression on erythrocytes protects against AAA, and we investigated the mechanisms. Methods: Mice lacking DARC on hematopoietic cells were created by mating DARC flox/flox mice with Vav1-Cre mice. At 10-14 weeks of age, both Vav1-Cre positive (DARC HKO ) and Vav1-Cre - mice [wild type (WT) littermate controls] received PCSK9 adeno-associated virus injection and were started on high fat diet. After 1 week, mice were infused with angiotensin II (AngII, 1,000 ng/kg/min) via osmotic minipump for 26 days to induce AAA. Results: Compared to WT, there was no difference in blood pressure or cholesterol levels in AngII-infused DARC HKO mice, which were confirmed to lack DARC on erythrocytes by flow cytometry. Aortic diameter and AAA incidence were significantly decreased in DARC HKO compared to WT mice (n=16-20, p=0.0257), in conjunction with reduced elastin degradation and macrophage infiltration. We did not find differences in levels of DARC-bound chemokines that could account for the protection against AAA. Interestingly, we found that the level and activity of lysyl oxidase (LOX), a cytokine-like enzyme that crosslinks extracellular matrix proteins and protects against AAA, were higher in DARC HKO mice compared to WT mice. Conclusions: Loss of DARC on hematopoietic cells protects against AAA formation, likely by upregulating aortic LOX. These findings provide novel insight into ethnic differences in susceptibility to AAA.

UBC9 Deficiency Accelerates AML Progression By Downregulating PARP1 Sumoylation and Subsequently Activating the NF-ΚB Pathway

Acute myeloid leukemia (AML) is the most common acute leukemia in adults and is a disease of heterogeneous clonal hematopoietic progenitor cells. Despite the relentless development of therapeutic options, including chemotherapy, and identification of many new therapeutic targets, the prognosis of AML is still poor, with a 5-year survival rate of approximately 35% in patients younger than 60 years old and 10% in patients older than 60 years, which may be due to the high degree of AML heterogeneity. Targeting the shared mechanisms of action among subclones has emerged led to find novel strategies, such as targeting protein post post-translational modification. Here we show that the impact of the UBC9 (the only E2 conjugation enzyme) and its related SUMOylation, a critical and reversible posttranslational modification as ubiquitination, on AML progression, to provide novel and potent sight for AML therapy. To identify the role of UBC9 expression in the clinical outcome of AML patients, we measured the expression of key SUMOylation genes, including SAE1, UBA2, UBC9, SUMO1, SUMO2, SUMO3, PIAS1, PIAS2, PIAS3, PIAS4, CBX4, and RANBP2, in the TCGA dataset (n=164) by using a multivariate survival curve based on the online tool SurvExpress. Our results showed that the survival probability curves of the low- and high-risk groups were significantly different due to the differences in the levels of expression of key SUMOylation genes ( p = 0.0011), and we found that the expression level of UBC9 was lower in the high-risk group than in the low-risk group. We next analyzed the UBC9 expression level and the prognosis of AML patients from the Therapeutically Applicable Research To Generate Effective Treatment dataset (n=156), which showed that AML patients with low UBC9 expression showed shorter overall survival ( p = 0.018)(Figure 1). In addition, real-time PCR showed that the UBC9 expression level in bone marrow samples from patients with de novo AML was significantly downregulated in both the high-risk patients based on National Comprehensive Cancer Network guidelines and the liver or spleen enlargement patients ( p = 0.01 and p = 0.001, respectively) (Figure 1). We used RNA interference system to knock out Ubc9 in a mouse model of MLL-AF9 and MLL-ENL induced AML. We observed the survival of AML mice and found that Ubc9 fl/+;Vav-Cre recipient mice showed a significantly shorter survival time than Ubc9 fl/+ mice (Figure 2). In addition, apoptosis experiments revealed reduced apoptosis of leukemia cells and leukemia stem cells in Ubc9 fl/+;Vav-cre mice. HE staining showed significantly higher leukemia cell infiltration in the livers, meninges, and kidneys of Ubc9 fl/+;Vav-cre mice compared to Ubc9 fl/+ mice. Transwell experiments indicated that Ubc9 deficiency resulted in increased migration of leukemia cells. Colony forming assay demonstrated that Ubc9-deficient leukemia cells formed significantly larger colonies. Flow cytometry analysis showed that Ubc9-deficient leukemia cells expressed high levels of c-kit and low levels of Mac1. To further elucidate the role of UBC9 in human AML cells, we constructed UBC9 deficient KG1 and K562 cell lines by using CRISPR-Cas9 technology. The UBC9 KD cell lines presented the same phenotype such as the increase of cell viability and the enhancement of invasiveness. To better understand the role that UBC9 plays in AML cell biology, we performed a system-level approach based on quantitative SUMO proteomic analysis to identify potential UBC9 substrates and then identified PARP1 with SUMOylation site lysine residue 486 (K486) by multiomics analysis. Finally, we validated that UBC9-mediated PARP1 SUMOylation played an important role in AML progression by activating the NF-kappa B pathway via the site-directed mutation of PARP1 (K486R) in UBC9-KO AML cells by using western blot and Co-immunoprecipitation. Together, our study revealed that UBC9 KO accelerated AML progression and enabled us to contrast global SUMOylation profiles of UBC9 in AML cells. In addition, we found that the PARP1 SUMOylation-mediated NF-κB pathway activation is a positive regulatory mechanism in the tumorigenesis of AML. These findings may provide potential clinical implications for AML treatment.

Polycomb Group Protein Yin Yang 1 Controls DNA Methylation in Murine T Lymphocyte Development

Transcription factors and epigenetic regulators play key functions in T lymphocyte lineage commitment and development. Mammalian PcG proteins are major negative regulators of gene expression that typically bind as large complexes (such as Polycomb Repressive Complex 1 (PRC1) and PRC2) to gene regulatory regions selectively targeted by an as yet unknown mechanism. While nearly all mammalian PcG proteins are lack of specific DNA binding capacities, transcription factor Yin Yang 1 (YY1) can bind at the DNA sites specifically and recruit other PcG proteins to specific DNA sequences. A small 25 amino acid YY1 domain (the REPO domain) is necessary and sufficient for the recruitment of PcG proteins, PcG-mediated stable transcriptional repression and histone modification. While YY1ΔREPO is competent for DNA binding, transcriptional activation, transient transcriptional repression and interaction with transcriptional coregulators such as HDACs, YY1ΔREPO is defective in all YY1 PcG functions. Our prior study demonstrated that deletion of YY1 in the hematopoietic system leads to T cell development blockage at the double-negative 3 (DN3) stage. Interestingly, ectopic expression of YY1∆REPO in YY1 null background by retroviral bone marrow transplantation could not rescue YY1 deficiency mediated T cell survival defect. This suggests that the YY1 REPO domain/PcG function plays a vital role in T cell development. Herein, we generated the YY1 REPO domain conditional knockout mouse model ( Yy1 f/∆REPO Vav-Cre) by CRISPR/Cas9 to further dissect the YY1 REPO domain/PcG function in T cell development. Yy1 f/∆REPO Vav-Cre ( Yy1 -/∆REPO) mice have reduced thymus weight and cellularity with medulla aplasia. Compared with wild-type (WT) and YY1 heterozygous ( Yy1 +/-) mice, Yy1 -/∆REPO mice have an increased percentage of DN3 cells with decreased DN4 percentage supporting a developmental blockage at the DN3 stage. Yy1 -/∆REPO DN T cells fail to go through TCRβ rearrangement with increased apoptosis and reduction of TCRβ + cells in the thymus compared with WT and Yy1 +/-cells. Interestingly, RNA-seq analysis in sorted primary DN3 cells revealed the genetic network governing DNA methylation, histone methylation, chromosome maintenance, and translation initiation complex formation are deregulated in Yy1 -/∆REPO mice compared to WT and Yy1 +/-mice. DN T cell development requires precise and dynamic regulation of histone and DNA methylation and demethylation. Interestingly, the DNA demethylase enzymes ten-eleven translocation (TET) family proteins Tet1 and Tet2 are downregulated in Yy1 -/∆REPO DN3 thymocytes compared with WT and Yy1 +/- DN3 cells. Our data show that the YY1 REPO domain/PcG function is required for DN3-DN4 transition via regulating TCRβ rearrangement and the YY1 REPO domain/PcG function plays a critical role in proper DNA methylation status in early T cell development. As PcG proteins modify histone via trimethylation of histone H3 on lysine 27, we will further assess the impact of YY1 REPO domain/PcG function on histone methylation in T cell development. By utilizing a powerful mouse model for identifying YY1 functions as PcG domain-dependent or PcG domain-independent, our study has generated fundamental new insights into the epigenetic landscape in T cell development.

Usp22 Deficiency Leads to Downregulation of PD-L1 and Pathological Activation of CD8+ T Cells and Causes Immunopathology in Response to Acute LCMV Infection.

Ubiquitin-specific peptidase 22 (Usp22) cleaves ubiquitin moieties from numerous proteins, including histone H2B and transcription factors. Recently, it was reported that Usp22 acts as a negative regulator of interferon-dependent responses. In the current study, we investigated the role of Usp22 deficiency in acute viral infection with lymphocytic choriomeningitis virus (LCMV). We found that the lack of Usp22 on bone marrow-derived cells (Usp22fl/fl Vav1-Cre mice) reduced the induction of type I and II interferons. A limited type I interferon response did not influence virus replication. However, restricted expression of PD-L1 led to increased frequencies of functional virus-specific CD8+ T cells and rapid death of Usp22-deficient mice. CD8+ T cell depletion experiments revealed that accelerated CD8+ T cells were responsible for enhanced lethality in Usp22 deficient mice. In conclusion, we found that the lack of Usp22 generated a pathological CD8+ T cell response, which gave rise to severe disease in mice.

Haploinsufficiency of Calreticulin Causes Dysregulation of Hematopoiesis and Results in a Myeloproliferative Phenotype in an in-Vivo Mouse Model

Background.JAK2V617F, MPLW515K/L and CALR exon9 mutations drive the development of Philadelphia negative Myeloproliferative Neoplasms (MPN). While JAK2 and MPL knock out (KO) leads to severe defects of hematopoiesis, the role of Calreticulin (CALR) deregulation is not fully understood, however recent reports demonstrated that CALR deletion leads to neoplastic transformation (Shide K et al Blood, 2020; Balliu M et al Blood Adv., 2021). Aim. To investigate the role of CALR in hematopoiesis and MPN development, we generated and characterized a Calr KO mouse model. Methods. We developed a Calr+/- mouse model by crossing mice carrying a floxed allele targeting Calr intron 3 and 7 (Calr+/flx) with Vav-Cre mice. The phenotypic characterization was performed by blood counts analysis, bone marrow (BM) and spleen histology, flow-cytometry and clonogenic assay by plating 2x104 cells for CFU-GEMM, CFU-GM and BFU-E and 2.5x105 for CFU-Mk. Proteome profiler membrane-based array and RNA-sequencing were employed to profiling BM protein and gene expression. Results. Sanger sequencing of BM cells of Calr+/- mice confirmed the deletion of Calr exons 4-7, resulting in the downregulation of Calr mRNA (-0.56-fold ±0.1) and decreased protein content (-0.47-fold ±0.21) compared to Calr+/flx mice (hence, controls). The productivity, body weight and lifespan were comparable between Calr+/- and controls. Calr+/- mice showed increased platelet count (median: 1360, range: 1156-1986 x109/L), 1.2 to 1.5-fold compared to controls, starting from month 4 (p=0.007) up to the latest time point available at 12 months (p=0.03). Erythrocytes count of Calr+/- increased after month 6 up to month 12 (median:10.3, range:9.9-10.6 x1012/L)(P=0.004) paralleled by an upward trend of hematocrit (median:47%, range:42-51%) and hemoglobin (median:18.3, range:14.2-20.1 g/L) levels. The spleen index (spleen:body weight ratio x100) resulted increased in Calr+/- (mean: 0.38±0.09) compared to controls (mean: 0.3±0.08) (P=0.039). Histology revealed an augmented number of poly-lobulated megakaryocytes, most of which found in clusters, in BM of Calr+/- mice. Most Calr+/- mice showed a progressive loss of spleen architecture with resulting loss of clear demarcation between red and white pulp combined with focal fibrosis, especially in aged mice. Calr haploinsufficiency conferred an augmented clonogenic capacity to BM and, to a lesser extent, splenic progenitors, in semisolid culture assay. BM Calr+/- CFU-GEMM, CFU-GM and CFU-Mk were in median 8.9 (5-15), 102.7 (49-157) and 16 (9.7-13.6) and 5.7 (3 -10) (P=0.007), 80.2 (33-159) (P=0.0031) and 11.9 (12.7-22) (P=0.048) in Calr+/flx, respectively. Of note, BM BFU-E count was 1.3-fold higher in Calr +/- than controls (P=0.009). Calr+/- splenic progenitors showed an enhanced clonogenic potential, strikingly for CFU-GEMM (P=0.0064). We observed that Calr+/- myeloid progenitors were significantly expanded when compared to controls: BM Mk precursors (FcγRlow/cKIT+/CD41+) and both BM and spleen LSK (Lin-/Sca1+/cKIT+) were, respectively, +1.6-fold (P=0.048), +1.4-fold (P=0.042) and +2-fold (P=0.008) than controls. Proteomic array performed on BM cells identified a set of deregulated proteins, mainly represented by cytokines (IL1α, IL4, IL6), chemokines (CXCL5, CXCL12, C1qR, VEGF, Endoglin) and cell adhesion molecules (VCAM, ICAM). These observations were strengthened by the result of transcriptomics analysis on bulk RNA-sequencing from Calr+/- and Calr+/flx mice BM cells highlighting a number of differentially expressed RNAs (increased n=385, decreased n=350) that resulted enriched in signaling pathways regulating chemokines and cell adhesion molecules, including "cell chemotaxis” (GO:0060326), "leukocyte migration” (GO:0050900), "regulation of cell-cell adhesion” (GO:0022407); cytokines, as determined by "Cytokine-cytokine receptor interaction” (KEGG:mmu04060); and Ca2+ homeostasis, including "cellular calcium ion homeostasis” (GO:0006874), "regulation of cytosolic calcium ion concentration” (GO:0051480 ). Conclusion. Finding from this in-vivo model suggest that haploinsufficiency of Calr induces dysregulation of hematopoiesis, ultimately resulting in a mild-ET phenotype. These findings will be validated in larger series and integrated with ongoing experiments to outline the molecular mechanisms underlying the CALR-driven MPN phenotype.

Terminal Erythroid Differentiation Is Regulated By Mitochondrial Translation Via Maintenance of Iron Homeostasis

Erythropoiesis is a process that continuously produces red blood cells, the most abundant cell type in the body which actively synthesize hemoglobin protein. Modification of transfer RNA (tRNA) regulates its function such as protein translation and other cellular processes (Chujo et al, FEBS J. 2021). Lack of mitochondrial tRNA (mt-tRNA) taurine modification mediated by mitochondrial tRNA translation optimization 1 (Mto1) has been recently shown to induce unfolded protein response (UPR) in ES cells (Fakruddin et al, Cell Rep. 2018). However, how UPR is induced by mt-tRNA dysfunction and its relevance in tissue homeostasis remains unclear. Since aberrant protein translation induces cellular stress, we hypothesized that Mto1 dysfunction might result in defective hematopoiesis, especially erythropoiesis which produces a substantial amount of protein such as hemoglobin via mitochondria. Yet, the role of any modification of mt-tRNAs in hematopoiesis including taurine has not been elucidated at all. To elucidate the biological function of Mto1 in hematopoiesis, we generated hematopoietic-specific Mto1 conditional KO (cKO) mice by a cross with Vav-Cre mice. The Mto1cKO mouse was embryonic lethal and the fetus had a paler appearance than the wild-type (WT) control, indicating severe anemia. Total fetal liver cellularity was significantly reduced and erythrocyte-related parameters were marked reduced in a peripheral blood parameter analysis in Mto1 cKO fetuses compared to WT. Further erythroblast subpopulation analysis revealed that absolute number of erythroblasts decreased significantly in almost all differentiation stages in Mto1 KO fetal liver compared to WT, especially most decrease in number at polychromatic erythroblast stage. Analysis in adult hematopoiesis using Mx1-Cre system also showed similar anemic phenotype in Mto1 cKO mouse. Consistent with these results, Mto1 mRNA expression and taurine modification levels were markedly up-regulated at the polychromatic erythroblast stage in the WT fetal liver, suggesting essential role of Mto1 in terminal erythroid differentiation at the polychromatic erythroblast stage. Previous study showed Mto1 dysfunction suppressed mitochondrial translation which impaired the formation of respiratory complexes (Fakruddin et al, Cell Rep. 2018). Consistent with this finding, Mto1 cKO fetal liver polychromatic erythroblasts showed marked suppression of complex-I formation. Impaired formation of complex-I which contains abundant iron-sulfur clusters is supposed to affect intracellular iron distribution, from mitochondria to cytoplasm. In line with this hypothesis, Mto1 cKO fetal liver polychromatic erythroblasts showed cytoplasmic iron accumulation which upregulated ALAS2 protein translation and heme biosynthesis. Proteome analysis of Mto1 cKO fetal liver polychromatic erythroblasts revealed significant upregulation of embryonic hemoglobin proteins expression compared with WT. Upregulation of heme and unstable embryonic hemoglobin biosynthesis is supposed to induce cellular stress. Consistent with this hypothesis, Xbp1 splicing-mediated UPR was significantly upregulated in Mto1 cKO fetal liver polychromatic erythroblasts. Moreover, both specific inhibitor of Xbp1 splicing and iron chelator rescued terminal erythroid differentiation in the Mto1 KO fetal liver in vitro, suggesting that Xbp1 splicing-mediated UPR via cytoplasmic iron overload derived from Mto1 deficiency is a cause of defect in terminal erythroid differentiation in Mto1 cKO mouse. This study demonstrated for the first time, an indispensable role of mt-tRNA taurine modification in erythroid terminal differentiation and suggests the biological role of mitochondria in iron homeostasis and erythropoiesis. Our findings propose a novel, non-energy-related molecular mechanism which links primary mitochondrial protein deficiency with erythroid differentiation block via imbalanced iron homeostasis. This important but unappreciated molecular mechanism might make a breakthrough by bringing a new biological aspect to hematology.

Stromal STAT5-Mediated Trophic Activity Regulates Hematopoietic Multipotent Progenitor Niche Factors

Signal transducer and activator of transcription 5 (STAT5a and STAT5b isoforms) are latent transcription factors that are intrinsically critical for normal hematopoiesis but are also expressed in bone marrow stromal cells. However, despite known STAT5a (prolactin) and STAT5b (growth hormone) functions in breast and liver tissue respectively, hematopoiesis-supporting stromal functions have not been explored. Here, floxed STAT5ab double knockout (KO) was generated with a variety of hematopoietic and stromal Cre-expressing transgenic mouse strains to test whether STAT5a or STAT5b play a role in extrinsic regulation of hematopoiesis. Pan-hematopoietic deletion with Vav1-Cre was the positive control for loss of multipotent hematopoietic function but surprisingly Vav1-Cre also deleted STAT5ab in CD45neg cells, dysregulated stromal niche factor mRNA expression, and altered MSC osteo/chondro-lineage differentiation priming. Single cell transcriptome analysis of bone marrow from wild-type or Vav1-Cre KO mice long-term repopulating hematopoietic stem cells (LT-HSC) showed upregulated oxidative phosphorylation (P<10-10) and protein translation (P<10-39) genes in LT-HSCs and throughout myeloid-primed clusters, while in contrast common lymphoid progenitor-like cells were completely eliminated. Similarly, like Vav1+ cells and based on scRNAseq analysis, Nes+ cells were also detected in both CD45neg and CD45+ cell clusters. Deletion of STAT5ab with Nes-Cre reduced the number of LT-HSCs (30% reduction compared to Nes-Cre control mice, P=0.02, N=4) and caused hematopoietic repopulating defects. To follow up on these promiscuous Cre promoter deletions in CD45+ and CD45neg bone marrow cell populations, additional stroma-specific Cre strains (LepR, Osx, Ocn, Prrx1, Nes-CreERT2) as well as endothelial-specific (Cdh5) strains were generated lacking STAT5ab. Viable mice were assessed for peripheral blood hematology, competitive repopulation, as well as hematopoietic stem/progenitor subsets (LT-HSC, MPP1, MPP2, MPP3, MPP4, KLS) using well-accepted flow cytometry antibody panels. Deletion of STAT5 in stromal cells with LepR-Cre significantly reduced MPP4 (35% reduction, P=0.025, N=4). Overall, extrinsic stromal (but not endothelial) cells were deficient in supporting multipotent and lineage-committed hematopoietic progenitors. However, peripheral blood hematology was rescued through homeostatic production. Gene regulation profiles resembled that of cytokine-starved MPPs. Functional support for niche-supporting activity was also assessed using STAT5ab KO MSCs in vitro. With Lepr-Cre mediated STAT5ab deletion, bone marrow niche factor mRNAs were downregulated (Cxcl12, Angpt1, Igfbp4, Igf1, Il34) and support of multipotent hematopoietic cells was significantly impaired. ELISA assay showed that SDF-1a was reduced 3-fold (P<0.001, N=5) and IGF-1 was reduced 2-fold (P<0.01, N=5) compared to wild type MSCs. Inversely, mRNAs for Kitl and Grem1 were increased in the same stromal cells, consistent with loss of multipotency and mixed HSC vs. MPP stromal support activities. Furthermore, novel scRNAseq analysis on simultaneous bone marrow hematopoietic and stromal cells compared wild-type and KO mice. Differentially expressed gene sets with Maximum Mean Discrepancy (MMD) co-expression revealed a key central role for coordinated STAT5ab/Cish balance as a biomarker of the cellular progenitor activity state (metabolic, protein translation, survival) associated with multipotency, comparing KO to wild-type cells. Importantly, Cish was strongly co-expressed in cells primed for lineage-specific differentiation (MSC - osteo/chondrogenic-primed; HSC - myeloid-primed) with STAT5ab co-expressed genes being associated with multipotent potential. Therefore, novel STAT5ab/Cish-mediated gene regulation programs operate within the bone marrow microenvironment to mediate cytokine production and responsiveness. This work highlights the importance of examining heterotypic cell types within bone marrow to elucidate functional cross-talk underlying hematopoietic homeostasis and defines conserved STAT5ab-mediated gene signatures as potential biomarkers to test in human hematopoietic and immune cell disorders.

Knockout of HMGA1 Promotes ATRA-Induced Differentiation in MLL-Rearranged Leukemia

Introduction: HMGA1 is a small non-histone chromatin protein characterized by 3 AT-hooks, which modulates transcription by altering the chromatin architecture. It tends to be highly expressed in poorly differentiated cells including hematopoietic stem cells. In various kinds of solid tumors, HMGA1 has been reported to play as an oncogene, and its upregulation was reported to be associated with poorer clinical outcomes. However, although recent study demonstrated that HMGA1 was associated with progression of JAK2V617F positive myeloproliferative neoplasms to myelofibrosis or leukemia, the roles of HMGA1 in myeloid malignancies were still unclear. Therefore, we analyzed its function in myeloid malignancies using HMGA1 knockout (KO) mice. We could not detect clear differences in peripheral blood counts or bone marrow cells, and generated various kinds of models of acute myeloid leukemia (AML). Knockout of HMGA1 could not delay the onset of these leukemia, but we could demonstrate that leukemic cells with MLL-ENL fusion gene which lacked in HMGA1 gene were more susceptible to all-trans retinoic acid (ATRA)-induced expression of CD11b, which suggested that HMGA1 could be associated with the differentiation process induced by ATRA. Method: We generated Vav-iCre HMGA1flox/flox mice, and we first analyzed the fraction of peripheral blood cells and bone marrow cells of these mice using flow cytometry. Next, to evaluate roles of HMGA1 in AML, we made various murine models of AML. We collected c-kit positive fractions of bone marrow cells from the KO mice or Vav-iCre(-) HMGA1flox/flox mice and retrovirally introduced MLL-ENL, MOZ-TIF2, BCL2-MYC fusions. Primary transplantation was performed to sublethally irradiated (5Gy) mice with 4.0×105 infected cells for each fusion gene. Secondary transplantation was also performed for MLL-ENL and BCL2-MYC with 1.0×104 cells for each. Blood samples were collected weekly, and disease-free survivals were compared by Kaplan-Meier method with Vav-iCre(-) HMGA1flox/flox mice. Also, leukemic cells infected with MLL-ENL were collected, and these cells were cultured for four days with different concentrations (1μM, 5μM or 10μM) of ATRA. Flow cytometry analysis was performed to evaluate the differentiation induced by ATRA with antibodies to murine CD11b, c-kit and Gr-1. Result: There was no significant difference in birth rate between Vav-iCre(+) and Vav-iCre(-) mice, which suggested that HMGA1 was not essential for fetal hematopoiesis. Peripheral blood cell counts were measured at week 8 and week 20, and fractions of peripheral blood and bone marrow cells were assessed at week 8 using flow cytometry, but no significant differences were observed. As for AML models, there were no significant differences in survival periods after primary or secondary transplantation. Leukemic cells retrovirally induced with MLL-ENL were collected from primary recipients, and these cells were treated with ATRA. Although growth rates were not show significantly different, expression levels of CD11b after exposure to ATRA were significantly higher in Vav-Cre (+) mice (5μM: 70% vs 44%, respectively; p=0.035. 10μM: 80% vs 46%, respectively; p=0.0006) as shown in the following figure. Discussion: We established HMGA1 KO mice and assessed the function of HMGA1 both in normal and leukemic hemopoiesis. Although HMGA1 might have little effect on normal hematogenesis, it seemed that HMGA1 had inhibitory effect on ATRA-induced differentiation process of MLL-ENL leukemic cells. Though more conditions including AML types and differentiation methods should be tested and the molecular mechanisms should be scrutinized, there is a possibility that inhibition of HMGA1 is effective in combination with differentiation treatments. Figure 1View largeDownload PPTFigure 1View largeDownload PPT Close modal

S191: CALRETICULIN-MUTATED HEMATOPOIETIC CELLS ARE VULNERABLE TO THE COMBINED INHIBITION OF THE PROTEASOME AND THE IRE1A-XBP1 AXIS OF THE UNFOLDED PROTEIN RESPONSE

Background: Mutations in the endoplasmic reticulum (ER) chaperone calreticulin (CALR) are frequent and disease-initiating in myeloproliferative neoplasms (MPN). These mutant CALR proteins have impaired chaperone function. Concordant with this, transcriptional upregulation of the unfolded protein response (UPR) has been reported in patients with CALR-mutated MPN. However, it is not understood how CALR-mutated cells counter-balance ER stress resulting from impaired chaperone function. Despite the frequency of CALR mutations in MPN, there are currently no treatment strategies to preferentially target CALR-mutant cells over healthy cells. Aims: To determine the mechanisms by which mutant CALR cells alleviate ER stress and to exploit these mechanisms selectively by pharmacological intervention in vivo. Methods: Long-term HSCs (LT-HSCs) from CalrΔ52 knockin mice were isolated for RNA-Seq. CalrΔ52 bone marrow (BM) was differentiated ex vivo, subsequently enriched for megakaryocytes, and subjected to quantitative proteomic analysis. Pathway analyses of CALR-mutant megakaryocyte progenitors (MkPs) were performed on Genotyping of Transcriptome (GoT) data from patients with Essential Thrombocytosis (ET). CalrΔ52 VAVCre mice and chimeric transplanted CalrΔ52 mice were treated with a proteasome inhibitor, an IRE1a inhibitor, or both. Results: To identify dysregulated pathways in CalrΔ52/Δ52 LT-HSCs, we performed RNA-seq and found that Xbp1s downstream of IRE1a and the proteasome pathway was significantly upregulated in CalrΔ52/Δ52 animals. We confirmed elevated XBP1s protein level in heterozygous CalrΔ52 knockin mice by flow cytometry. Since elevated and abnormal megakaryopoiesis are hallmarks of CALR-mutant MPN, we performed quantitative proteomics on megakaryocyte-enriched cells from CalrΔ52 mice. We confirmed upregulation of both proteasome and ER chaperone proteins in Calr-mutant cells. To verify these findings in human MPN, we interrogated a published GoT data set and found that in addition to XBP1s, the proteasome pathway was also differentially upregulated in CALR-mutated patient MkPs compared to the wildtype MkPs (p<2e-16). To investigate the functional consequences of IRE1a or proteasome inhibition, we treated CALR-mutant hematopoietic cell lines with KIRA6 (IRE1a inhibitor) and/or bortezomib (proteasome inhibitor) and found CALR-mutant cells to be differentially sensitive as compared to isogenic controls. We subsequently found that treatment with bortezomib leads to the accumulation of misfolded and ubiquitinylated proteins in mutant Calr BM, resulting in pro-apoptotic priming. In vivo treatment of CalrΔ52/+ mice with bortezomib resulted in a significant reduction in platelets. Finally, we combined IRE1a and proteasome inhibition in a chimeric BM transplantation in vivo model using CD45.2 CalrΔ52/+ MxCre GFP and CD45.1 wild-type competitor cells. Following combination therapy, we found a significant reduction in Calr-mutant donor chimerism, specifically in LT-HSCs and platelets, accompanied by reductions in LT-HSC frequency and platelet count. Summary/Conclusion: In summary, we have found that disrupted proteasis in CALR-mutated MPN cells results in a dependency on the IRE1-XPB1 axis of the UPR and proteasome activity. Using pre-clinical MPN models, we found that combined inhibition of both pathways in vivo normalizes important features of MPN and preferentially targets CalrΔ52 over wild-type cells. These findings highlight combined inhibition of IRE1 and the proteasome as a promising therapeutic strategy in CALR-mutant MPN.

Abstract 823: ABL1 kinase regulates leukemia phenotype and response to therapy

Abstract The ABL1 proto-oncogene encodes a cytoplasmic and nuclear protein tyrosine kinase. Cytoplasmic ABL1 stimulates, whereas nuclear ABL1 inhibits cell growth. Analysis of the CGAP Mitelman database detected loss of ABL1 allele in hematologic malignancies. Among the top genetic aberrations detected in acute myeloid leukemias were t(8;21) generating fusion protein AML1-ETO and NUP98 translocations, e.g., t(1;11) encoding NUP98-PMX1.Expression of AML1-ETO and NUP98-PMX1 in Lin-cKit+ stem cell enriched bone marrow cells from Abl1-/-;Vav-Cre and Abl1+/+;Vav-Cre mice revealed that oncogene-expressing Abl1-/- cells gained tremendous proliferation advantage. Conversely, overexpression of ABL1 and DPH (ABL1 kinase agonist)-mediated activation of ABL1 kinase resulted in reduced proliferation of AML1-ETO and NUP98-PMX1 cells. Since the role of ABL1 in DNA damage response (DDR) is well established, we applied the inhibitors of ATM (ATMi), ATR (ATRi) and DNA-PKcs (DNA-PKi) to identify potential therapeutic vulnerabilities. AML1-ETO;Abl1-/- cells were more sensitive to all three inhibitors when compared to AML1-ETO;Abl1+/+ cells. Remarkably, NUP98-PMX1;Abl1-/- cells were exceptionally sensitive to ATRi, but not to ATMi and DNA-PKi when compared to NUP98-PMX1;Abl+/+ cells. ABL1 did not affect the sensitivity of transformed cells to DNA damaging agent doxorubicin. To pinpoint mechanisms collaborating with Abl1 deletion in transformation of hematopoietic cells, Abl1 was knockout in 32Dcl3 cells by CRISPR/Cas9 to obtain 32Dcl3-Abl1ko cells. 32Dcl3-Abl1wt and 32Dcl3-Abl1ko cells were challenged to induce transformation by starving them from growth factors (IL3). Only 32Dcl3-Abl1ko cell populations generated growth factor-independent cells. This indicates that the absence of Abl1 may facilitate malignant transformation of hematopoietic cells. RNA-seq was performed to compare gene expression in 32Dcl3-Abl1wt and 32Dcl3-Abl1ko cells in the presence or absence of IL-3. Pathway analysis of FPKM reads (top 30 significant canonical pathways) and Venn diagram analysis followed by KEGG pathway analysis (291 genes uniquely expressed in growth factor-independent 32Dcl3-Abl1ko cells) implicated PI3K-AKT pathway signaling including mTOR and sirtuins (SITR) in 32Dcl3-Abl1ko cells. PI3K inhibitor buparlisib, mTOR inhibitor rapamycin and SIRT2 inhibitor thiomyristoyl exerted selective activity against Lin-cKit+ AML1-ETO;Abl1-/- and/or NUP98-PMX1;Abl1-/- cells when compared to Abl1+/+ counterparts. Moreover, inhibition of ABL1 kinase by imatinib in Lin-cKit+ AML1-ETO;Abl1+/+ and/or NUP98-PMX1;Abl+/+ cells induced the sensitivity to PI3K-ATK pathway inhibitors. In conclusion, we showed that normal ABL1 kinase plays a tumor suppressor role and is a potential therapeutic target in hematological malignancies induced by AML1-ETO and NUP98-PMX1. The latter statement is being currently tested in animal models. Citation Format: Konstantin Golovine, Zhaorui Lian, Kumaraswamy Naidu Chitrala, Gleb Ablakov, Margaret Nieborowska-Skorska, Jian Huang, Tomasz Skorski. ABL1 kinase regulates leukemia phenotype and response to therapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 823.

Characterization of a CLCF1 conditional knock-out mice model

Abstract The Cardiotrophin-like Cytokine Factor 1 (CLCF1) is a cytokine of the IL6 family with important pro-neurotrophic and immuno-modulating functions. However, the mechanisms behind CLCF1 activities and CLCF1 properties in pathological models remain poorly understood. This is partly due to a lack of tools for the study of CLCF1 functions in vivo. Indeed, the complete knock-out of CLCF1 in mice is lethal at P1: underdeveloped motor neurons of the face and jaw prevent the pups from suckling. To overcome this obstacle, we obtained a CLCF1 conditional knock-out mice model generated using the CRISPR-Cas9 technology. We then bred these mice with Vav-Cre mice to induce CLCF1 deletion in hematopoietic cells. We are now conducting immuno-phenotyping experiments of the main immune populations in those animals. Considering the activities of CLCF1 that were previously shown in vitro or in vivo using overexpression models, we hypothesize that the knock-out of CLCF1 in immune cells will lead to decreased numbers of B and myeloid cells. This new CLCF1 conditional knock-out mice model will be a potent tool to confirm and further study the activities, mechanisms, and pathological roles of CLCF1 in vivo. Supported by grants from CIHR (Canadian Institutes of Health Research)

Haploid Kmt2c Deletions Enhance Hematopoietic Stem Cell Mobilization in Response to Granulocyte-Colony Stimulating Factor

KMT2C is one of several tumor suppressor genes deleted in myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML) as part of larger chromosome 7q deletions. These deletions are particularly common in therapy-related MDS/AML, raising the question of whether loss of one or more 7q genes conveys a selective advantage to hematopoietic stem cells (HSCs) in the setting of chemotherapy-induced stress. We recently showed that haploid Kmt2c deletions do indeed enhance HSC self-renewal capacity. However, the deletions do not drive HSCs into cycle; instead, a proliferative stress such as chemotherapy is required to create a context in which Kmt2c deletion convey a selective advantage. We have also identified a mechanism, to explain this phenotype. Kmt2c encodes MLL3, a SET domain protein that binds enhancers and facilitates transcription. We have shown that Kmt2c/MLL3 deletions impairs enhancer recruitment during HSC differentiation, therefore blunting HSC commitment.Our findings suggest that acquired or pre-existing 7q (KMT2C) deletions may select for HSCs that could give rise to MDS/AML in the setting of autologous-transplantation. Granulocyte-colony stimulating factor (G-CSF) is a cytokine that is often used to expedite neutrophil recovery after chemotherapy and to mobilize HSCs for collection and transplantation. We considered the possibility that 7q deletions, and KMT2C deletions in particular, may promote disproportionate mobilization of the mutant HSCs in response to G-CSF. To test this, we treated Kmt2c f/f; Vav1-Cre and Kmt2c f/+; Vav1-Cre mice with G-CSF, and we assessed HSC mobilization to the spleen and bone marrow. A far greater proportion of HSCs with heterozygous and homozygous Kmt2c deletions mobilized in response to G-CSF, relative to wild type HSCs. Kmt2c deletion also enhanced colony forming unit frequency in the peripheral blood after G-CSF treatment. Total body HSC numbers did not change in the body after G-CSF treatment on any genetic background, indicating that Kmt2c deletions enhanced HSC mobilization in response to G-CSF rather than self-renewal.To more faithfully recapitulate clinical conditions, we used Fgd5-CreER to delete a single Kmt2c allele in only a minority of HSCs. We then tested whether the mutant HSCs mobilized more efficiently than wild type HSCs. Surprisingly, Kmt2c deletions did not enhance HSC mobilization in this context. This raised the question of whether Kmt2c deletion in a non-HSC population could promote HSC mobilization in the Kmt2c f/+; Vav1-Cre mice. Indeed, analysis of mice chimeric for wild type and Kmt2c f/+; Vav1-Cre bone marrow suggested that Kmt2c non-cell autonomously regulates HSC mobilization. Finally, Kmt2c deletions did not enhance mobilization following exposure to plerixafor, a CXCR4 antagonist that acts directly on HSCs to promote mobilization rather than indirectly via monocyte populations, as occurs with G-CSF. Additional studies are needed to elucidate the mechanism by which Kmt2c non-cell autonomously regulates HSC mobilization. Our findings provide reassurance that, in a clinical setting, rare KMT2C-mutant HSCs will not disproportionately mobilize prior to apheresis. Furthermore, the data suggest that transient inhibition of MLL3, or its targets, may enhance HSC mobilization and negate selective advantages that 7q-deleted HSCs may acquire after chemotherapy treatment. DisclosuresNo relevant conflicts of interest to declare.

SKIDA1 Sustains MLL-ENL-Expressing Hematopoietic Stem and Progenitor Cells

Chromosomal translocations involving the MLL1 gene often drive infant acute myeloid leukemia (AML). MLL fusion proteins (e.g., MLL-ENL, MLL-AF9, MLL-AF10) activate self-renewal programs in hematopoietic stem and progenitor cells, ultimately leading to transformation. The high frequency of MLL1 rearrangements in infant leukemias suggests that neonatal progenitors are uniquely poised to transform in response to these mutations. Indeed, we have recently shown that MLL-ENL initiates AML more efficiently in neonatal progenitors than in adult progenitors (Okeyo-Owuor et. al. Blood Advances, 2019). This raises the question of whether MLL-ENL induces key effectors of transformation more efficiently in neonatal progenitors than in adult progenitors. We identified Ski/Dach Domain Containing 1 (Skida1) as a gene that is highly induced by MLL-ENL in neonatal, but not adult hematopoietic progenitors. SKIDA1 is also highly expressed in human pediatric AML, and this expression is largely restricted to leukemias with MLL1 rearrangements. These observations suggest that Skida1 may be a critical, neonate-specific effector of MLL-ENL driven leukemogenesis.To test whether SKIDA1 plays a role in hematopoiesis and leukemogenesis, we generated a germline loss-of-function mouse allele. First, we tested whether loss of SKIDA1 perturbed hematopoiesis by flow cytometry. SKIDA1 deletion caused modest expansion of hematopoietic stem cells (HSCs) in neonates at post-natal day 0 (P0) and a modest decrease of hematopoietic committed progenitor cells (HPCs) at P14, but no changes in fetal or adult HSC/HPC numbers were observed. We then assessed HSC function by transplanting HSCs into lethally irradiated mice to assess their multi-lineage repopulating potential. Loss of SKIDA1 did not perturb HSC function at any stage of development, indicating a minor role in normal hematopoiesis. Next, we tested whether SKIDA1 modulates HSC or HPC numbers in the context of MLL-ENL expression. We crossed Skida1-/- mice to mice with a tetracycline inducible MLL-ENL transgene. The resulting mice (TetO-MLL-ENL; Vav1-Cre) express MLL-ENL specifically in hematopoietic cells beginning at embryonic day 10.5 (E10.5) in the absence of doxycycline, concordant with the onset of Vav1-Cre expression. When we induced MLL-ENL expression in Skida1 -/- mice, we observed near complete loss of HSCs and a severe reduction of HPCs. The reduction in HSC and HPC numbers in TetO-MLL-ENL; Vav1-Cre; Skida1 -/- neonates was far more severe than we observed in TetO-MLL-ENL; Vav1-Cre mice alone. Furthermore, this phenotype emerged between P0 and P14, indicating that it requires a neonatal context to manifest. This coincides with the developmental stage at which MLL-ENL most efficiently induces AML (Okeyo-Owuor et. al. Blood Advances, 2019). This data suggest that SKIDA1 helps sustain pre-leukemic MLL-ENL-expressing HSCs and HPCs shortly after birth. Ongoing studies will address the mechanism by which SKIDA1 regulates AML.Our current work raises two important points. First, SKIDA1 appears to have a far more important role in sustaining MLL-ENL-expressing progenitors than normal progenitors, raising the prospect for an excellent therapeutic window. Secondly, our findings show that age-specific MLL-ENL targets can shape pre-leukemic hematopoiesis and potentially AML initiation. This may help explain why MLL1 rearrangements account for a higher percentage of infant and childhood leukemias than adult leukemias. DisclosuresNo relevant conflicts of interest to declare.

Mitochondrial Carrier Homolog 2 (MTCH2) Regulates the Differentiation of AML Cells By Controlling Pyruvate Entry into the Mitochondria, Nuclear Localization of Pyruvate Dehydrogenase Complex and H3 and H4 Histone Acetylation

Mitochondrial carrier homolog 2 (MTCH2) is a mitochondrial outer membrane protein that functions as a receptor for pro-apoptotic BID. MTCH2 also regulates metabolism through undefined mechanisms.We identified MTCH2 as essential for AML growth and viability in a CRISPR screen. OCI-AML2 leukemia cells overexpressing CAS9 were transduced with a library of 91,320 sgRNAs in barcoded lentiviral vectors. We focused on the gRNA that targeted the 1050 mitochondrial proteins and identified MTCH2 within the top 1% of hits. We validated this finding using two individual sgRNAs, where knockout of MTCH2 reduced the growth and viability of CAS9-OCI-AML2 cells by >75%. Additionally, knockdown of MTCH2 with shRNA reduced growth and viability of OCI-AML2 (>90%), TEX (>80%), U937 (>65%), and HL60 (>75%) leukemia cells. Knockdown of MTCH2 targeted the leukemia initiating cells as it abrogated engraftment of TEX cells into immune deficient mice. We also infected hematopoietic cells from wild type and MTCH2 flox/flox Vav1-cre mice with the MLL-AF9 oncogene. Knockout of MTCH2 decreased the leukemogenic potential of these cells and increased survival of the mice.In contrast, knockout of MTCH2 has more subtle effects on normal murine hematopoiesis. flox/flox Vav1-cre MTCH2 knockout mice had normal peripheral blood counts, but a ~15% reduction in the abundance of hematopoietic stem cells with a slight increase in the restricted progenitor populations (Maryanovich et al, Nat Comm 2015).Murine hematopoietic cells from MTCH2 -/- mice have increased mitochondrial mass and increased oxidative phosphorylation. Yet, no changes in basal oxygen consumption, mitochondrial ROS, or respiratory chain activity were seen in AML cells after MTCH2 knockdown.We examined changes in gene expression by RNA-sequencing in OCI-AML-2 cells after MTCH2 knockdown. MTCH2 knockdown increased expression of genes involved in cellular differentiation and decreased genes that maintain stemness. Therefore, we analyzed differentiation of AML cells after MTCH2 knockdown and demonstrated increased non-specific esterase staining and increased expression of CD cell surface markers associated with differentiation.In lung cancer mitochondrial stress induces the translocation of pyruvate dehydrogenase (PDH) from the mitochondria to the nucleus, where it increases conversion of pyruvate to Acetyl CoA which, in turn, acetylates histones. Therefore, we examined changes in PDH localization and histone acetylation after MTCH2 knockdown in OCI-AML-2 and TEX cells. MTCH2 knockdown decreased mitochondrial pyruvate, increased nuclear localization of pyruvate dehydrogenase complex (PDH) and increased acetylation of H3 and H4 histones. Over-expression of the PDH E2 subunit with a nuclear localizing signal mimicked MTCH2 knockdown and decreased cell growth, induced differentiation, and increased H3 and H4 histone acetylation in. Likewise, the acetyltransferase inhibitor, C646, abrogated the effects of MTCH2 knockdown on histone acetylation, growth and differentiation. We also demonstrated that knockdown of MTCH2 decreased levels of mitochondrial pyruvate in OCI-AML-2 cells.The entry of pyruvate into the mitochondria is regulated by the Mitochondrial Pyruvate Carrier (MPC). Therefore, we treated AML cells (OCI-AML2, TEX, HL60, and 8227) with the selective MPC inhibitor UK5099. UK5099 mimicked the effects of MTCH2 knockdown and decreased mitochondrial levels of pyruvate, reduced cell growth and viability, increased levels of acetylated histones, increased nuclear levels of PDH and promoted differentiation of AML cells. Treatment of mice with UK5099 (40mg/kg) decreased growth of OCI-AML2 xenografts. Finally, UK5099 induced cell death by Annexin V/PI staining in undifferentiated primary AML cells, but differentiated primary AML cells (by morphology and CD marker staining) and normal hematopoietic cells were more resistant.In summary, inhibiting entry of pyruvate into the mitochondria is a new mechanism to regulate differentiation of AML cells and stem cells by increasing PDH localization to the nucleus and histone acetylation. Thus, MTCH2 or MPC inhibitors may be novel therapeutics agents for AML. DisclosuresSchimmer:Medivir: Research Funding; Takeda Pharmaceuticals: Research Funding; Novartis Pharmaceuticals: Honoraria.

CARM1/PRMT4 As a Novel Therapeutic Target for AML

Chromatin modifying enzymes, and specifically the protein arginine methyltransferases (PRMTs) have emerged as important targets in cancer. PRMT4, also known as CARM1, is overexpressed in a number of cancers, including breast, prostate, pancreatic, and lung cancer. We have reported the overexpression of PRMT4 in the context of acute myeloid leukemia (AML), showing that more than 70% of cytogenetically normal AML patients have up-regulation of PRMT4. Here, we investigated the role of PRMT4 in normal hematopoiesis and leukemia development.In order to study the role of PRMT4 in normal hematopoiesis, Prmt4 -floxed mice were crossed with Vav1-cre mice purchased from the Jackson Laboratory. Inducible Prmt4 conditional KO mice were generated by crossing Prmt4 -floxed mice with Mx1-Cre mice and Prmt4 gene excision was induced by poly(I:C), given by three i.p. injections at a dose of 10 mg/kg. Using this hematopoietic specific knockout system, we show that loss of PRMT4 has little effect on normal hematopoiesis, but promotes the differentiation of hematopoietic stem and progenitor cells. Next we evaluated the role of PRMT4 in leukemia initiation and maintenance using leukemia transplantation models driven by fusion oncoproteins. Strikingly, the knockout of PRMT4 completely abrogates leukemia initiation and maintenance in fetal liver transplantation models driven by the AML1-ETO or MLL-AF9 fusion proteins.We further characterized the mechanism for the leukemia-specific dependence on PRMT4 using leukemia cell lines lentiviral vectors that express shRNAs directed against PRMT4. We knocked down PRMT4 in four leukemia cell lines or normal human cord-blood derived CD34+ cells and found that knockdown of PRMT4 impairs cell cycle progression, induces apoptosis, and down-regulates E2F target genes, identifying several mechanisms for the leukemia-specific dependence on PRMT4. Gene set enrichment analysis showed that all four leukemia cell lines with knockdown of PRMT4 showed a significant down-regulation of MYC and E2F target genes compared to the scrambled control. Differentially regulated pathways included Cell cycle, DNA replication, and Homologous recombination. Finally, we utilized inhibitors of the Type I arginine methyltransferase family in order to validate the selective inhibition of PRMT4 in leukemia cells and patient cell cells.The PRMT4 conditional knockout mice, PRMT4 knockdown in leukemia cell lines, and chemical inhibition of PRMT4, all suggest that loss of PRMT4 protein levels or activity has a selective effect on leukemia cells compared to normal hematopoietic stem and progenitor cells. Collectively this work suggests that targeting PRMT4 activity may be an effective therapeutic strategy for AML, with minimal effects on normal hematopoiesis. DisclosuresWatts:Jazz Pharmaceuticals: Consultancy, Speakers Bureau.

HnRNP K: A Driver of Myeloid Malignancy

Acute myeloid leukemia (AML) is increasingly defined as a heterogeneous group of malignancies divided largely by recurrent genetic and/or chromosomal aberrations. With this knowledge, development of targeted therapeutics has improved outcomes for some AML subsets. Increasing our understanding of alterations that drive disease development and progression will expand our armamentarium of therapeutic opportunities. To this end, our laboratory has identified a subset of AML that harbors increased expression of heterogeneous nuclear ribonucleoprotein K (HNRNPK; protein- hnRNP K).In a sizeable cohort of patients with AML, we have demonstrated HNRNPK gene amplification, RNA overexpression, and protein overexpression using FISH, qRT-PCR, and reverse phase protein array analyses. Importantly, the patient population with high protein expression of hnRNP K exhibited decreased median overall survival, increased WBC, and increased blast percentage in both bone marrow and peripheral blood. Together, these data suggest that hnRNP K may function as an oncogene in AML when overexpressed.To formally test this hypothesis, we generated a Vav-Cre;HnrnpkTg mouse model. Indeed, these mice overexpress hnRNP K in comparison to Vav-Cre mice or unrecombined littermates. Critically, Vav-Cre;HnrnpkTg mice develop myeloid malignancies. When transplanted, these malignancies have a decreased latency period and are rendered more aggressive. Currently, we are evaluating HSPC proliferation, differentiation potential, and using time of flight mass cytometry (CyTOF) to extensively characterize these transplanted leukemias.Functionally, hnRNP K is an RNA and ssDNA binding protein that can shuttle bidirectionally between the nucleus and cytoplasm. To further understand how hnRNP K can cause AML, we are evaluating subcellular localization of hnRNP K in the context of overexpression. As our preliminary experiments suggest a predominant cytoplasmic localization when hnRNP K is overexpressed, we are performing hnRNP K RNA-immunoprecipitation (RIP) followed by next generation sequencing in human cell lines with inducible overexpression of hnRNP K. In addition to confirming our previous observations of hnRNP K binding to the MYC transcript, we have identified several other putative targets for hnRNP K binding that may potentiate these malignant phenotypes.Additionally, we have performed RNA-Seq on HNRNPK- overexpressing human cell lines. These data suggest a significant role for hnRNP K in the regulation of innate immune response-observations that are consistent with phenotypes seen in our transplant models. Together, these studies provide insight into the pathways most critically altered in hnRNP K-overexpressing AML and will allow for appropriate utilization of therapeutic modalities. DisclosuresNo relevant conflicts of interest to declare.

Blockade of IL-22 signaling reverses erythroid dysfunction in stress-induced anemias.

Patients with myelodysplastic syndromes (MDS) display severe anemia but the mechanisms underlying this phenotype are incompletely understood. Right open-reading-frame kinase 2 (RIOK2) encodes a protein kinase located at 5q15, a region frequently lost in MDS del(5q) patients. Here, we show that hematopoietic cell-specific haploinsufficient deletion of Riok2 (Riok2f/+Vav1cre) led to reduced erythroid precursor frequency leading to anemia. Proteomic analysis of Riok2f/+Vav1cre erythroid precursors suggested immune system activation and transcriptomic analysis revealed an increase in p53-dependent interleukin-22 (IL-22) in Riok2f/+Vav1cre CD4+ T cells (TH22). Further, we discovered that the IL-22 receptor, IL-22RA1, was unexpectedly present on erythroid precursors. Blockade of IL-22 signaling alleviated anemia not only in Riok2f/+Vav1cre mice but also in wild-type mice. Serum concentrations of IL-22 were increased in the subset of del(5q) MDS patients as well as patients with anemia secondary to chronic kidney disease (CKD). This work reveals a possible therapeutic opportunity for reversing many stress-induced anemias by targeting IL-22 signaling.

YY1 Promotes SCF/c-Kit Signaling and HSC Self-Renewal in Fetal Hematopoiesis

Fetal hematopoietic stem cells (HSCs) exhibit markedly different properties as compared to adult HSCs including cell surface marker expression, proliferation state and repopulation capacity. Changes of HSC activity in postnatal mice is defined by a process of decreasing in cell cycle and entering into quiescence. Yin Yang 1 (YY1) is a ubiquitous transcription factor and mammalian Polycomb Group (PcG) Protein with important functions to regulate embryonic development, lineage differentiation and cell proliferation. While homozygote deletion of YY1 in mice results in lethality at the peri-implantation stage, heterozygote deletion of YY1 causes severe developmental defects. By conditionally deleting YY1 in adult hematopoietic system, our previous study showed that YY1 is an essential regulator for adult hematopoiesis by promoting HSC long-term self-renewal and maintaining adult HSC quiescence. In contrast to adult HSCs, in which quiescence is a fundamental characteristic, over 95% of fetal HSCs are in an active cell cycle to rapidly generate homeostatic levels of blood cells for oxygen transport and immune system development in the growing organism. Herein, we assessed whether YY1 was also required for maintaining fetal HSC pool and regulating fetal HSC functions, and what was the underlying mechanism by which YY1 regulated fetal HSCs. To test how loss-of-function of YY1 impacted fetal hematopoiesis, Yy1f/f mice in which the Yy1 promoter region and exon 1 are flanked by loxP sites, were crossed to Vav-Cre mice to generate heterozygous Yy1f/+ Vav-cre mice. The Vav promoter drives Cre recombinase expression specifically in fetal liver hematopoietic cell starting at day E11.5. Yy1f/+ Vav-cre mice were then subsequently bred with Yy1f/f mice to generate homozygous Yy1f/f Vav-cre mice. Among 141 pups resulting from breeding Yy1f/fto Yy1f/+ Vav-Cre, only 7 were Yy1f/f Vav-Cre (n=7) and was significantly lower than the estimated number (n=35) according to the Mendelian ratio (P&amp;lt;0.05). All Yy1f/f Vav-Cre pups died within 72 hours after birth, which supported essential role of YY1 in fetal hematopoiesis and survival. At E14.5 of fetal development, Yy1f/f Vav-Cre fetuses had reduced numbers of hematopoietic stem and progenitor cells in the liver. In addition, YY1 deficient fetal HSCs failed to self-renew in primary and secondary bone marrow transplantation assays. Colony formation assay showed that fetal liver cells from Yy1f/f Vav-Cre mice failed to form CFU-GEMM, CFU-GU and BFU-E compared to Vav-Cre control. While YY1 promotes SCF/c-Kit signaling in adult HSCs, it does not impact c-Kit cell surface expression in early T cell progenitors (unpublished data). To assess YY1 impact on SCF/c-Kit axle in fetal HSCs, c-Kit transcript level, c-Kit median fluorescence intensity and phosphorylated AKT were measured. Similar as its function in adult HSCs, YY1 deficient fetal HSCs had decreased Kit transcript expression, decreased c-Kit cell surface expression and decreased SCF/c-Kit signaling. Our results supported that YY1 is required for maintaining a continuous pool of HSCs in fetal liver and is critical for fetal HSC long-term self-renewal and differentiation. Similar as its function in adult HSCs, YY1 promotes SCF/c-Kit signaling in fetal HSCs. Disclosures No relevant conflicts of interest to declare.

Ifnα Attenuates the Disease Phenotype and Extends Survival in Mouse Models of MPN

The curative potential of Type I interferons for patients suffering from Myeloproliferative Neoplasms (MPNs) has been reported and these are the only class of drugs that can lead to reduction of the mutant allelic burden in patients. However, modelling IFN treatment in mice has been challenging. Here, we report the use of murine pegylated IFNα (murine ropeginterferon-a, mRopeg) developed by PharmaEssentia (Taipei, Taiwan) to model IFN treatment in transgenic MPN mouse models. We started treating JAK2V617Ff/+;vavCre and control vavCre mice (n=6-8) with PBS or mRopeg (600 ng/mouse/week), by subcutaneous injections from the time they were 4 weeks old. The mice were bled every 2 weeks from the facial vein and the blood parameters were monitored. We observed significant normalization of platelet and WBC counts in Jak2-V617F fl/+ vavCre mice to wild type levels. No effect on hematocrit and hemoglobin level was observed in the Jak2-V617F fl/+ vavCre mice. VavCre control animals showed no sign of negative effect such as cytopenia during the entire treatment course. We observed a highly significant prolongation of the survival of mRopeg treated JAK2V617Ff/+;vavCre mice over a duration of 80 days of treatment. While all the PBS treated JAK2V617Ff/+;vavCre mice died within 60 days, all the mRopeg treated mice were still alive till the end of the treatment duration. We also generated a novel transgenic mouse model that conditionally expresses hybrid mutant CALR protein (murine exons 1-8 and human CALR del52 exon9) from the endogenous murine Calr locus. We bred them into vavCre background (in both heterozyhous and homozygous states) to induce expression of CALR-del52 in hematopoietic cells. Upon Cre recombinase expression, the endogenous murine exon 9 is replaced by the human del52 exon 9 and the expression of the humanized Calr-del52 oncoprotein is detectable by Western blot analysis using mutant CALR specific antibodies. Calr-del52 animals develop an essential thrombocythemia (ET) like phenotype when expressed in a heterozygous state with elevated number of hematopoietic stem cells and megakaryocytes in the bone marrow. In the homozygous state, the thrombocythemia is more severe with splenomegaly and older animals show anemia with increased WBC. Bone marrow histology shows megakaryocytic hyperplasia with no sign of fibrosis up to age of one year. We treated a cohort of animals with 600 ng mRopeg/PBS once a week for 4 weeks. Peripheral blood counts were determined at baseline and at regular intervals during treatment. At the end of treatment, mice were sacrificed, and splenic and bone marrow cells were immunophenotyped and quantified by FACS. We observed correction of thrombocythemia in the homozygous Calr-del52 mice but no unspecific decrease of platelet count in the vavCre mRopeg treated animals. We observed significant specific reduction of the long-term hematopoietic stem cells (LT-HSCs/fraction A) in homozygous CALR-del52 mice. In conclusion, Type I IFN treatment significantly reduces platelet counts to normal levels in both JAK2 and CALR mutant driven MPN mouse models. The prolongation of survival of JAK2V617F transgenic mice upon Type I IFN treatment is particularly remarkable; as no survival data is reported until now in any clinical trials or other animal models. Further experiments are required to understand the mechanism of action of this phenomenon. Disclosures No relevant conflicts of interest to declare.