Growth Of Hematopoietic Progenitor Cells Research Articles

SHP2 inhibition displays efficacy as a monotherapy and in combination with JAK2 inhibition in preclinical models of myeloproliferative neoplasms.

Myeloproliferative neoplasms (MPNs), including polycythemia vera, essential thrombocytosis, and primary myelofibrosis, are clonal hematopoietic neoplasms driven by mutationally activated signaling by the JAK2 tyrosine kinase. Although JAK2 inhibitors can improve MPN patients' quality of life, they do not induce complete remission as disease-driving cells persistently survive therapy. ERK activation has been highlighted as contributing to JAK2 inhibitor persistent cell survival. As ERK is a component of signaling by activated RAS proteins and by JAK2 activation, we sought to inhibit RAS activation to enhance responses to JAK2 inhibition in preclinical MPN models. We found the SHP2 inhibitor RMC-4550 significantly enhanced growth inhibition of MPN cell lines in combination with the JAK2 inhibitor ruxolitinib, effectively preventing ruxolitinib persistent growth, and the growth and viability of established ruxolitinib persistent cells remained sensitive to SHP2 inhibition. Both SHP2 and JAK2 inhibition diminished cellular RAS-GTP levels, and their concomitant inhibition enhanced ERK inactivation and increased apoptosis. Inhibition of SHP2 inhibited the neoplastic growth of MPN patient hematopoietic progenitor cells and exhibited synergy with ruxolitinib. RMC-4550 antagonized MPN phenotypes and increased survival of an MPN mouse model driven by MPL-W515L. The combination of RMC-4550 and ruxolitinib, which was safe and tolerated in healthy mice, further inhibited disease compared to ruxolitinib monotherapy, including extending survival. Given SHP2 inhibitors are undergoing clinical evaluation in patients with solid tumors, our preclinical findings suggest that SHP2 is a candidate therapeutic target with potential for rapid translation to clinical assessment to improve current targeted therapies for MPN patients.

SHP2 Inhibition Enhances JAK2 Inhibitor Therapy in Preclinical Models of Myeloproliferative Neoplasms

The classic myeloproliferative neoplasms (MPNs) polycythemia vera, essential thrombocythemia, and primary myelofibrosis are driven by aberrant activation of the JAK2 kinase, primarily by mutations in 3 different genes, JAK2, MPL, and CALR. JAK2 inhibitor therapy is generally effective at reducing MPN patient constitutional symptoms and thus can improve quality of life but has minimal effect on reducing mutant driver allele burden and does not readily alter the course of disease, reflecting the ability of MPN-driving cells to persist during therapy. Thus, the ability of current anti-JAK2 therapies to significantly modify underlying disease biology is limited. Long-term studies of the JAK2 inhibitor ruxolitinib in myelofibrosis patients have, however, identified survival benefits, yet this may be due to anti-inflammatory and spleen reducing impacts. Ultimately, the inability of ruxolitinib and other approved JAK2 inhibitors to effectively antagonize allele burden indicates alternative therapeutic approaches are needed to overcome the persistent survival of MPN-driving cells during JAK2 inhibitor therapy. Efforts to identify mechanisms that can thwart the effects of JAK2 inhibition in MPN cells have highlighted activation of the RAS-ERK pathway, including mechanisms of RAS-ERK signaling that are independent of JAK2. ERK or MEK inhibition have been shown to enhance the activity of ruxolitinib in MPN mouse models, but monotherapeutic inhibition of these kinases displayed limited efficacy. The SHP2 (Src homology region 2 domain-containing phosphatase-2) tyrosine phosphatase mediates RAS activation and subsequent MEK/ERK signaling downstream of cytokine and growth factor receptors. SHP2 plays a role in oncogenic signaling and in resistance to kinase targeted therapies, exemplified in both solid tumor and leukemia models. Given the evidence that activation of RAS-ERK signaling can antagonize the effects of JAK2 inhibition and the fact that SHP2 mediates RAS activation via multiple mechanisms downstream of numerous growth factor and cytokine receptors, we hypothesized that SHP2 may contribute to the survival of MPN cells during JAK2 inhibitor treatment and may provide a therapeutic target to improve current anti-JAK2 therapies. Using preclinical MPN models, we aimed to block RAS activation by inhibiting SHP2 to assess the potential SHP2 inhibition may have as a monotherapy or in combination with JAK2 inhibition. We found that the SHP2 inhibitors RMC-4550 and SHP099 enhanced growth inhibition of MPN model cell lines (e.g., SET2 and UKE1) in combination with ruxolitinib, effectively preventing ruxolitinib persistent growth. Concomitant treatment with both inhibitors led to enhanced inhibition of RAS-GTP levels and ERK activation and increased apoptosis compared to single agents. SHP2 inhibition antagonized the neoplastic growth of primary hematopoietic progenitor cells from MPN patients and synergized with ruxolitinib in all patient samples assessed. In an MPN mouse model driven by MPL-W515L, RMC-4550 antagonized MPN phenotypes, including leukocytosis, hepatosplenomegaly, and allele burden. RMC-4550 treatment diminished levels of a subset of pro-inflammatory cytokines, including IL-1β, CXCL9, and TNFα, among others. The combination of RMC-4550 with ruxolitinib not only suppressed advancing leukocytosis but led to white blood cell levels lower than at treatment initiation (Panel A) and suppressed thrombocytosis compared to ruxolitinib alone. The combination also led to enhanced inhibition of hepatomegaly, extramedullary hyperplasia, megakaryocyte hyperplasia, and allele burden, and a trend toward improved fibrosis. The SHP2 inhibitor RMC-4550 enhanced the survival of mice (following termination of treatment analyses) as both a monotherapy (p < 0.0001 compared to vehicle) and in combination with ruxolitinib (p < 0.01 compared to ruxolitinib monotherapy). Importantly, the combination of SHP2 inhibition using RMC-4550 with JAK2 inhibition using ruxolitinib for 4 weeks in wildtype mice was well tolerated with respect to hematologic parameters and exemplified by no effect on body weight (Panel B). Given SHP2 inhibitors are already undergoing clinical evaluation in patients with solid tumors, our findings suggest that SHP2 is a therapeutic target with potential to be rapidly translated to clinical assessment for MPN patients.

Tagraxofusp Shows Promising Anti-Tumoral Efficacy in Preclinical in Vitro Models of Myelofibrosis, Both As a Single Agent and in Combination with Janus Kinase Inhibitors

BACKGROUND: Myelofibrosis (MF) is an aggressive myeloproliferative neoplasm. Abnormal growth of hematopoietic stem and progenitor cells in MF is driven by somatic mutations affecting the Janus Kinase (JAK) signaling pathway with JAK2 V617F occurring in 50-60% of patients (Cardoso et al, Plos One 2015). The role of the JAK/Signal transducer and activator of transcriptions (STAT) pathway in MF has led to the recent approval of three different JAK2 inhibitors (ruxolitinib, pacritinib, and fedratinib) for MF treatment. However, despite limiting the symptoms of the disease, JAK2 inhibitors do not result in remission due to persistence of JAK2 activation and MF-driving cells (Pandey et al, Blood Cancer J 2022). Thus, there remains a high unmet need for treatment strategies for patients with MF. Tagraxofusp, a first-in-class CD123-directed therapy, is a recombinant fusion protein consisting of human interleukin-3 conjugated to a truncated diphtheria toxin payload approved (US/EU) for the treatment of Blastic Plasmacytoid Dendritic Cell Neoplasm. In MF, CD123 is expressed on malignant cells and bone marrow (BM) accessory cells that support the proliferation of neoplastic cells (Bao et al, Hematol Oncol Stem Cell Ther 2019; Lasho et al, Blood 2014). Based on CD123 expression, tagraxofusp is a potential, novel approach for MF treatment either alone or in combination. Initial results from a two-stage multicenter, phase 2 study (STML-0401-0314; NCT02268253) of single-agent tagraxofusp in pts with relapsed or refractory (R/R) MF demonstrated encouraging clinical efficacy and a manageable safety profile (Yacoub et al. ASH 2021). We aim to further investigate the antitumoral efficacy of tagraxofusp in MF preclinical models and the potential improvement of antitumoral efficacy by combining tagraxofusp with the JAK inhibitors, ruxolitinib, and pacritinib. METHODS: MF cell lines (SET2, UKE1, HEL: JAK2 V617F; UT7: JAK2 wild type [wt]) and bone marrow mononuclear cell (BMMC) samples from healthy donors (HD) were characterized for the extracellular expression of CD123 by flow cytometry staining. In vitro cytotoxicity experiments in CD123-positive MF cell lines (SET2, UT7, UKE-1) tested tagraxofusp and ruxolitinib or pacritinib either as single agents or in combination (MTS assay; 72 h). The combination index (CI) was calculated according to the Compusyn-Chou method (Chou TC., Pharmacol Rev, 2006) and Combenefit platform. In vitro co-culture experiments were performed with MF cell lines incubated in the presence of human primary HD-BMMCs. Cell death, as measured by LiveDead/AnnV staining, was monitored by FACS analysis through differential staining of MF cells and BMMCs with Cell Trace Violet dye (CTV). A signaling pathway analysis (JAK/STAT, S6, Mcl1) was performed through immunocapillary electrophoresis on MF cell lines treated with single agents or in combination. RESULTS: The MF cell lines showed different levels of CD123 expression. Except for the CD123 negative cell line (HEL), CD123 positive cell lines were all sensitive to tagraxofusp independent of CD123 expression intensity. As expected, both ruxolitinib and pacritinib were active in the presence of JAK2 wt pathway. A synergistic drug interaction was observed (mean CI range: 0.59-1.12) in most combinations of tagraxofusp with JAK inhibitors, as shown in a representative CD123+ cell line in Figure 1. When BMMCs, which are a subpopulation of BM Accessory Cells partially expressing CD123, were co-cultured with MF cell lines, the cytotoxic effect induced by tagraxofusp was enhanced, and this improvement was maintained in combination with JAK2 inhibition. Molecular biomarkers, such as JAK/STAT, S6, and Mcl1, were evaluated in MF cell lines treated with tagraxofusp and JAK inhibitors and supported the MoA of this combination. CONCLUSIONS: Our studies showed high sensitivity of MF cell lines to tagraxofusp as a single agent, which is expected given phase 2 results demonstrated clinical efficacy of single-agent tagraxofusp in R/R MF (Yacoub et al. ASH 2021). In addition, synergism was observed in combination with JAK inhibitors. The in vitro antitumoral effect was more pronounced in the presence of BM accessory cells, suggesting an extra targeting of CD123-positive BM cells by tagraxofusp in vivo. These results support potential clinical development of TAG in combination with JAK inhibitors for the treatment of MF.

Similar but distinct: The impact of biomechanical forces and culture age on the production, cargo loading, and biological efficacy of human megakaryocytic extracellular vesicles for applications in cell and gene therapies.

Megakaryocytic extracellular vesicles (MkEVs) promote the growth and megakaryopoiesis of hematopoietic stem and progenitor cells (HSPCs) largely through endogenous miR-486-5p and miR-22-3p cargo. Here, we examine the impact of biomechanical force and culture age/differentiation on the formation, properties, and biological efficacy of MkEVs. We applied biomechanical force to Mks using two methods: shake flask cultures and a syringe pump system. Force increased MkEV production in a magnitude-dependent manner, with similar trends emerging regardless of whether flow cytometry or nanoparticle tracking analysis was used for MkEV counting. Both methods produced MkEVs that were relatively depleted of miR-486-5p and miR-22-3p cargo. However, while the shake flask-derived MkEVs were correspondingly less effective in promoting megakaryocytic differentiation of HSPCs, the syringe pump-derived MkEVs were more effective in doing so, suggesting the presence of unique, unidentified miRNA cargo components. Higher numbers of MkEVs were also produced by "older" Mk cultures, though miRNA cargo levels and MkEV bioactivity were unaffected by culture age. A reduction in MkEV production by Mks derived from late-differentiating HSPCs was also noted. Taken together, our results demonstrate that biomechanical force has an underappreciated and deeply influential role in MkEV biology, though that role may vary significantly depending on the nature of the force. Given the ubiquity of biomechanical force in vivo and in biomanufacturing, this phenomenon must be grappled with before MkEVs can attain clinical relevance.

EFFECTS OF TRAUMA PLASMA-DERIVED EXOSOMES ON HEMATOPOIETIC PROGENITOR CELLS.

Background: Severe trauma disrupts bone marrow function resulting in persistent anemia and immunosuppression. Exosomes are extracellular vesicles implicated in disease, cellular functions, and immunomodulation. The effects of trauma plasma-derived exosomes on bone marrow hematopoiesis are unstudied; we hypothesized that trauma plasma-derived exosomes suppress bone marrow hematopoietic progenitor cell (HPC) growth and contribute to increased inflammatory cytokines and HPC mobilization. Methods: Plasma was collected from a prospective, cohort study of trauma patients (n = 15) with hip and/or femur fractures and an ISS > 15 and elective total hip arthroplasty (THA) patients (n = 15). Exosomes were isolated from both groups using the Invitrogen Total Exosome Isolation Kit. Healthy bone marrow was cultured with 2% plasma, 50 μg, 100 μg, or 200 μg of exosomal protein and HPC (granulocyte, erythrocyte, monocyte, megakaryocyte colony-forming units [CFU-GEMM], erythroid burst-forming units [BFU-E], and macrophage colony-forming units [CFU-GM]) growth assessed. After culturing healthy bone marrow stroma with 100 μg of exosomal protein, expression of cytokines and factors influencing HPC mobilization were assessed by qPCR. Differences were compared using the ANOVA, with significance defined as P < 0.05. Results: The only demographic difference was age; trauma patients were significantly younger than THA (mean 44 vs. 63 years). In vitro exposure to trauma plasma significantly decreased growth of all HPCs. In vitro exposure to 100 μg or 200 μg of trauma exosomal protein significantly decreased growth of BFU-E and CFU-GM, whereas 50 μg had no effect. Culture of trauma exosomal protein with bone marrow stromal cells resulted in increased expression of IFN-γ, IL-1α, TNF-α, G-CSF, CXCR4, SDF-1, and VCAM-1 in bone marrow stroma. Conclusions: Both plasma and plasma-derived exosomes from trauma patients adversely affect bone marrow function. Plasma-derived exosomes may contribute to altered hematopoiesis after severe trauma; analysis of exosomal content may improve our understanding of altered bone marrow function.

DOT1L Methyltransferase Regulates Calcium Influx in Erythroid Progenitor Cells in Response to Erythropoietin.

Erythropoietin (EPO) signaling plays a vital role in erythropoiesis by regulating proliferation and lineage-specific differentiation of murine hematopoietic progenitor cells (HPCs). An important downstream response of EPO signaling is calcium (Ca2+) influx, which is regulated by transient receptor potential channel (TRPC) proteins, particularly TRPC2 and TRPC6. While EPO induces Ca2+ influx through TRPC2, TRPC6 inhibits the function of TRPC2. Thus, interactions between TRPC2 and TRPC6 regulate the rate of Ca2+ influx in EPO-induced erythropoiesis. In this study, we observed that the expression of TRPC6 in KIT-positive erythroid progenitor cells was regulated by DOT1L. DOT1L is a methyltransferase that plays an important role in many biological processes during embryonic development including early erythropoiesis. We previously reported that Dot1l knockout (Dot1lKO) HPCs in the yolk sac failed to develop properly, which resulted in lethal anemia. In this study, we detected a marked downregulation of Trpc6 gene expression in Dot1lKO progenitor cells in the yolk sac compared to the wild type (WT). The promoter and the proximal regions of the Trpc6 gene locus exhibited an enrichment of H3K79 methylation, which is mediated solely by DOT1L. However, the expression of Trpc2, the positive regulator of Ca2+ influx, remained unchanged, resulting in an increased TRPC2/TRPC6 ratio. As the loss of DOT1L decreased TRPC6, which inhibited Ca2+ influx by TRPC2, Dot1lKO HPCs in the yolk sac exhibited accelerated and sustained elevated levels of Ca2+ influx. Such heightened Ca2+ levels might have detrimental effects on the growth and proliferation of HPCs in response to EPO.

DOT1L Mediated Gene Repression in Extensively Self-Renewing Erythroblasts.

DOT1L is essential for embryonic hematopoiesis but the precise mechanisms of its action remain unclear. The only recognized function of DOT1L is histone H3 lysine 79 (H3K79) methylation, which has been implicated in both transcriptional activation and repression. We observed that deletion of the mouse Dot1L gene (Dot1L-KO) or selective mutation of its methyltransferase domain (Dot1L-MM) can differentially affect early embryonic erythropoiesis. However, both mutations result in embryonic lethality by mid-gestation and growth of hematopoietic progenitor cells (HPCs) is similarly affected in extensively self-renewing erythroblast (ESRE) cultures established from yolk sac cells. To understand DOT1L-mediated gene regulation and to clarify the role of H3K79 methylation, we analyzed whole transcriptomes of wildtype and Dot1L-mutant ESRE cells. We observed that more than 80% of the differentially expressed genes (DEGs) were upregulated in the mutant ESRE cells either lacking the DOT1L protein or the DOT1L methyltransferase activity. However, approximately 45% of the DEGs were unique to either mutant group, indicating that DOT1L possesses both methyltransferase-dependent and -independent gene regulatory functions. Analyses of Gene Ontology and signaling pathways for the DEGs were consistent, with DEGs that were found to be common or unique to either mutant group. Genes related to proliferation of HPCs were primarily impacted in Dot1L-KO cells, while genes related to HPC development were affected in the Dot1L-MM cells. A subset of genes related to differentiation of HPCs were affected in both mutant groups of ESREs. Our findings suggest that DOT1L primarily acts to repress gene expression in HPCs, and this function can be independent of its methyltransferase activity.

Adrenergic Modulation of Erythropoiesis After Trauma.

Severe traumatic injury results in a cascade of systemic changes which negatively affect normal erythropoiesis. Immediately after injury, acute blood loss leads to anemia, however, patients can remain anemic for as long as 6 months after injury. Research on the underlying mechanisms of such alterations of erythropoiesis after trauma has focused on the prolonged hypercatecholaminemia seen after trauma. Supraphysiologic elevation of catecholamines leads to an inhibitive effect on erythropoiesis. There is evidence to show that alleviation of the neuroendocrine stress response following trauma reduces these inhibitory effects. Both beta blockade and alpha-2 adrenergic receptor stimulation have demonstrated increased growth of hematopoietic progenitor cells as well as increased pro-erythropoietic cytokines after trauma. This review will describe prior research on the neuroendocrine stress response after trauma and its consequences on erythropoiesis, which offer insight into underlying mechanisms of prolonged anemia postinjury. We will then discuss the beneficial effects of adrenergic modulation to improve erythropoiesis following injury and propose future directions for the field.

Changes in Subcellular Localization of GSK3β As a Novel Driver of AML

Novel therapeutic approaches are urgently needed for Acute Myeloid Leukemia (AML). Currently only 20% of patients over age 56 survive 2 years leading to an enormous unmet need for novel approaches. Unfortunately, there has not been a change in standard AML agents in over 40 years. We and others previously identified that targeting GSK3 in AML is a promising therapeutic strategy as it has high activity on leukemic cells, but does not impair the growth of normal hematopoietic progenitor cells. One major reason for the selective responsiveness of AML and normal hematopoietic cells may be due to the fact that GSK3β protein is markedly different between these two conditions. In AML, GSK3β is highly overexpressed, activated, and can be found at high levels in the nucleus. Our cell, animal and patient biomarker studies suggest that this nuclear pool of GSK3β that is not found in normal hematopoietic cells can drive AML progression and drug resistance.GSK3β protein is markedly elevated universally and aberrantly localized to the nucleus in all AML samples tested (>100 samples) by flow cytometry as well as western blot. Nuclear GSK3β has unique access to specific target proteins such as nuclear transcription factors leading to different effects of nuclear and cytoplasmic GSK3β on cellular processes. In order to simultaneously quantify the nuclear localization and expression of GSK3β in AML (n=86) as compared to normal hematopoietic cells (n=10), imaging cytometry was utilized. This cytometric analysis along with traditional fluorescent microscopy identified a large pool of nuclear GSK3β that does not exist in normal hematopoietic cells. To characterize the function of nuclear GSK3β, we developed AML cell lines that expressed inducible and targeted (nuclear or cytoplasmic) forms of GSK3b. Interestingly, the expression of nuclear, but not cytoplasmic targeted GSK3β increased colony formation of AML cells (ex. 38% more colonies formed in HL-60 cells with nuclear as opposed to cytoplasmic GSK3β induction, p<0.001). To further analyze the impact of GSK3β localization in AML, we investigated the ability of nuclear and cytoplasmic GSK3β to both promote AML disease progression and chemotherapy resistance in vivo in immunodeficient mice . Nuclear, but not cytoplasmic GSK3β induction in vivo led to enhanced AML growth and doxorubicin resistance using two AML cell lines (ex. See figure). Finally, nuclear GSK3β localization has clinical significance as it strongly correlates to worse patient survival (n=86 HR=2.2 p<0.01) using a panel of clinically annotated AML samples. [Display omitted] Disclosuresde Lima:Pfizer: Membership on an entity's Board of Directors or advisory committees; Celgene Corporation: Research Funding.

BRAFV600E-induced senescence drives Langerhans cell histiocytosis pathophysiology.

Langerhans cell histiocytosis (LCH) is a potentially fatal condition characterized by granulomatous lesions with characteristic clonal mononuclear phagocytes (MNPs) harboring activating somatic mutations in mitogen-activated protein kinase (MAPK) pathway genes, most notably BRAFV600E. We recently discovered that the BRAFV600E mutation can also affect multipotent hematopoietic progenitor cells (HPCs) in multisystem LCH disease. How the BRAFV600E mutation in HPCs leads to LCH is not known. Here we show that enforced expression of the BRAFV600E mutation in early mouse and human multipotent HPCs induced a senescence program that led to HPC growth arrest, apoptosis resistance and a senescence-associated secretory phenotype (SASP). SASP, in turn, promoted HPC skewing toward the MNP lineage, leading to the accumulation of senescent MNPs in tissue and the formation of LCH lesions. Accordingly, elimination of senescent cells using INK-ATTAC transgenic mice, as well as pharmacologic blockade of SASP, improved LCH disease in mice. These results identify senescent cells as a new target for the treatment of LCH.

Isoform-specific and signaling-dependent propagation of acute myeloid leukemia by Wilms tumor 1.

Acute myeloid leukemia (AML) is caused by recurrent mutations in members of the gene regulatory and signaling machinery that control hematopoietic progenitor cell growth and differentiation. Here, we show that the transcription factor WT1 forms a major node in the rewired mutation-specific gene regulatory networks of multiple AML subtypes. WT1 is frequently either mutated or upregulated in AML, and its expression is predictive for relapse. The WT1 protein exists as multiple isoforms. For two main AML subtypes, we demonstrate that these isoforms exhibit differential patterns of binding and support contrasting biological activities, including enhanced proliferation. We also show that WT1 responds to oncogenic signaling and is part of a signaling-responsive transcription factor hub that controls AML growth. WT1 therefore plays a central and widespread role in AML biology.

Aberrantly elevated suprabasin in the bone marrow as a candidate biomarker of advanced disease state in myelodysplastic syndromes.

Myelodysplastic syndromes (MDS) are preleukemic disorders characterized by clonal growth of mutant hematopoietic stem and progenitor cells. MDS are associated with proinflammatory signaling, dysregulated immune response, and cell death in the bone marrow (BM). Aging, autoinflammation and autoimmunity are crucial features of disease progression, concordant with promoting growth of malignant clones and accumulation of mutations. Suprabasin (SBSN), a recently proposed proto-oncogene of unknown function, physiologically expressed in stratified epithelia, is associated with poor prognosis of several human malignancies. Here, we showed that SBSN is expressed in the BM by myeloid cell subpopulations, including myeloid-derived suppressor cells, and is secreted into BM plasma and peripheral blood of MDS patients. The highest expression of SBSN was present in a patient group with poor prognosis. SBSN levels in the BM correlated positively with blast percentage and negatively with CCL2 chemokine levels and lymphocyte count. Invitro treatment of leukemic cells with interferon-gamma and demethylating agent 5-azacytidine (5-AC) induced SBSN expression. This indicated that aberrant cytokine levels in the BM and epigenetic landscape modifications in MDS patients may underlie ectopic expression of SBSN. Our findings suggest SBSN as a candidate biomarker of high-risk MDS with a possible role in disease progression and therapy resistance.

Abstract B49: Modeling a pathogenic SAMD9 mutation in human induced pluripotent stem cells

Abstract Germline mutations in SAMD9 are the molecular determinants of some pedigrees with familial monosomy 7 and myelodysplastic syndrome (MDS) (OMIM 252270), MIRAGE syndrome (myelodysplasia, infection, restriction of growth, adrenal hypoplasia, genital phenotypes, enteropathy), and other nonsyndromic pediatric MDS. Mutations identified thus far have been associated with decreased growth of hematopoietic stem and progenitor cells (HSPCs) and bone marrow hypocellularity. Previous studies suggest that the wild-type function of SAMD9 is growth restrictive; therefore, the mutations identified in pediatric MDS have been deemed gain-of-function (GoF), given the exaggerated antiproliferative phenotype demonstrated in overexpression systems within nonhematopoietic cell lines, most commonly HEK-293T. To date, no model systems are available to study the impact of these mutations in their endogenous locus, a locus known to be induced by inflammatory stimuli, namely interferon (IFN). Here we report our development, using CRISPR/Cas9 genomic engineering, of two separate isogenic human induced pluripotent stem cell (iPSC) lines, derived from a non-neoplastic iPSC line (BJFF.6), containing a known pathogenic SAMD9 mutation (c.3406G&amp;gt;C; p.E1136Q), which we previously identified in a family with monosomy 7 and MDS. From this mutated iPSC line we are able to successfully differentiate hematopoietic precursors and myeloid cells that contain a pathogenic SAMD9 mutation. Using our novel pediatric MDS model system, we demonstrate that in the presence of IFN, viability of HSPCs (CD34+) and differentiated myeloid cells (CD13+) is significantly decreased when the SAMD9 p.E1136Q mutation is present (CD34+; WT=16.7% vs Mut=11.5% (p=0.05), CD13+; WT=22.6% vs. Mut=3.5% (p&amp;lt;0.0001)). Furthermore, we show that the genomically engineered SAMD9 locus of our iPSC-derived CD34+ and CD13+ cells remains robustly responsive to IFN stimulation. The basal level of SAMD9 expression is higher in differentiated myeloid cells than in HSPCs, a finding consistent with expression levels within normal hematopoiesis. These data suggest that we have a SAMD9 mutant pediatric MDS model system that provides a plentiful source of cells in which the effect of specific mutations on hematopoietic differentiation, proliferation, and viability may be investigated. Citation Format: Jason R. Schwartz, Jon P. Connelly, Shondra M. Pruett-Miller, Jeffery M. Klco. Modeling a pathogenic SAMD9 mutation in human induced pluripotent stem cells [abstract]. In: Proceedings of the AACR Special Conference on the Advances in Pediatric Cancer Research; 2019 Sep 17-20; Montreal, QC, Canada. Philadelphia (PA): AACR; Cancer Res 2020;80(14 Suppl):Abstract nr B49.

A Phase II Trial Evaluating the Efficacy and Safety of Sargramostim Post Infusion of T -Replete HLA Mismatched Peripheral Blood Haploidentical Hematopoietic Stem Cells with Post Transplant Cyclophosphamide

Phase II trial evaluating the Efficacy and Safety of Sargramostim Post Infusion of T -Replete HLA Mismatched Peripheral Blood Haploidentical Hematopoietic Stem Cells with Post-Transplant Cyclophosphamide Background: T-replete HLA mismatched haploidentical transplantation (Haplo) with post-transplant cyclophosphamide (PTCY) is widely used for patients lacking a suitably matched donor. Patients who receive haplo with PTCY usually start growth factor support (G-CSF) on day +5 for neutropenia prophylaxis. G-CSF is a specific stimulator of the growth and differentiation of hematopoietic progenitor cells committed to the neutrophil lineage. Granulocyte-macrophage colony-stimulating factor (GM-CSF) on the other hand, is not restricted to neutrophils because it also affects monocytes and eosinophils and plays a central role in antifungal host response. In a prior randomized study among 206 matched allogeneic HCT patients (wang et al, J Clin Oncol. 2015;33(34):3999-4006), neutropenia prophylaxis with GM-CSF +/- G-CSF had significantly lower 100-day TRM, lower 100-day cumulative mortality, lower infection related mortality and lower 600-day invasive fungal disease related mortality compared to G-CSF. Given the increase number of HLA mismatched haplos with PTCY performed each year and the high risk of infectious complications associated with this type of transplant, we suggest that GM-CSF administration post haplo and PTCY can yield similar count recovery rate to GCSF with a potential of lowering risk of infectious complications. Endpoints : The primary endpoint is to establish equivalent effectiveness of sargramostim to a matched control cohort of G-CSF treated patients in time to neutrophil engraftment (ANC&gt;500 x 3 days) post haplo with PTCY. Secondary endpoints: estimates of OS, DFS, relapse rate, GVHD and TRM. Donor chimerism at days 30, 60, 100 and 6 months post-transplant. Incidence and grade of cytokine release syndrome between day 0 and 30. Incidence of infectious complications and infection related mortality at day 100. Immune profiling assay "CYTOF" will be measured at days 30, 60, 90 and 180 for functional and phenotypic immune analysis. Other secondary endpoints include length of hospital stay and charges-based cost comparison during the first 100 days between sargramostim and G-CSF matched controls. All primary and secondary endpoints will be reported for the study group and compared to a matched control cohort from our database containing patients who received haplo with PTCY using G-CSF for neutropenia prophylaxis. A set of 20 patients, who are not on the Sargramostim study and receiving haplo transplant with G-CSF at the same time of the study accrual period, will be consented for immunoprofiling assay at days 30, 60, 100 and 180. Methods: Patients aged 18-78 years with an available 5/10-8/10 HLA- matched related donor with high risk hematological malignancy with adequate organ function will be enrolled for the study. Patients will receive Sargramostim 250mg/m2 /day subcutaneous starting day +5 post haplo/PTCY and continue till ANC&gt;1000x 3 days or &gt;1500x 1 day. If grade 3 or 4 adverse reactions occur, reduce dose by 50% or temporarily discontinue the dose until the reaction abates. Conditioning regimens to be used include previously published and currently used at our center (Flu/Cy/TBI, Flu/high dose TBI or Flu/Mel). Treatment will be in the outpatient setting and admissions if clinically indicated. Sample Size Determination : Time to ANC&gt;500 will be evaluated between the prospectively enrolled patients and matched historical controls. Based on the data of haplo transplants done at Northside in 2010-2016, the standard deviations (SD) was 3 days for time to ANC500. The correlation in measurements between prospective patient and matched control is conservatively assumed to be 0.1. SD for differences between matched pairs would be 3.55 days for time to ANC500. To control the type I error to be 5%, the 90% paired t confidence interval for difference in time to ANC500 should be constructed and its upper boundary will be compared to the non-inferiority margin 2 days. The size of 35 prospective patients provides the power 90% for concluding that time to ANC500 for Sargramostim is no more than 2 days longer than historical controls. The actual enrollment size is decided to be 38 to accommodate 10% of drop off rate. Enrollment: The study is expect to start enrolling patients October 2019. Disclosures No relevant conflicts of interest to declare.

Functional Analysis of MYC Deregulation By Diverse Genetic Mechanisms during Hematopoiesis

Alterations of the c-MYC (MYC) locus, mainly in the form of translocations or amplifications, are common in hematologic malignancies and result in deregulated MYC expression, disrupting cellular proliferation and differentiation and promoting leukemogenesis. While amplified MYC loci typically maintain all native cis-regulatory elements, translocated loci are instead driven by heterologous regulatory regions and are therefore expected to be unresponsive to external signals. However, no studies have directly compared the effects of different modes of MYC deregulation in cellular phenotypes and functions.Consistent with the idea that different modes of MYC deregulation may respond differentially to external mitogenic signals, we found that treatment with DMSO resulted in rapid downregulation of MYC protein expression in hematopoietic cell lines with amplified MYC loci (HL-60, BJAB), but failed to do so in cell lines harboring a t(8;14) MYC translocation (Ramos, LY8) despite the induction of cell cycle arrest in all lines. To precisely model these two genetic mechanisms of MYC deregulation during hematopoiesis and study their effects in phenotypes relevant to oncogenesis (differentiation and proliferation), we used genetic engineering of human embryonic stem cells (hESCs). First, MYC deregulation upon translocation was modeled using a doxycycline (DOX)-inducible lentiviral construct to ectopically express MYC with GFP (MYC-DoxOE). Transduced single cell clones of the H1 hESC line were selected to ensure uniform MYC expression. Second, we created a conditional MYC gene amplification model in H1 hESCs by using CRISPR/Cas9 to insert a floxed cassette consisting of three tandem MYC cDNA copies and GFP in the MYC locus (MYC-Amp). Correctly edited clones were identified via PCR and Southern Blot, and one clone with bi-allelic targeting (yielding 8 inducible MYC copies) was selected. All clones were confirmed to be karyotypically normal.Upon in vitro hematopoietic differentiation of unmodified H1 hESCs, we identified a peak of MYC expression (day 12) coinciding with the emergence of CD34+/CD45+ hematopoietic progenitor cells (HPCs). MYC expression subsequently declined as cells matured further down the myeloid lineage and lost CD34 expression (days 14-20). To test the effects of MYC deregulation in the two models we induced MYC expression by doxycycline (MYC-DoxOE) or cell-permeant TAT-Cre recombinase (MYC-Amp) on day 10 of hematopoietic differentiation. Induction of MYC expression above normal levels was found to be comparable between the two models during peak MYC expression (day 12). However, while ectopically expressed MYC in the MYC-DoxOE model maintained high levels throughout the culture period, amplified MYC in the MYC-Amp model - which is subject to regulation by the native MYC locus cis regulatory elements - was downregulated after day 12, in a pattern similar to that observed in uninduced H1 cells.MYC induction in the MYC-DoxOE model caused a block in differentiation with accumulation of CD34+ HPCs and reduction of more mature CD34-/CD45+ hematopoietic cells. Wright-Giemsa staining corroborated these results showing a predominance of morphologically immature myeloid cells. Strikingly, MYC-DoxOE HPCs could be maintained in culture for over 90 days, while H1-derived HPCs without doxycycline treatment completely arrested their proliferation after approximately 28 days of culture. This phenotype is dramatically distinct from the finite growth potential of HPCs derived from normal human pluripotent stem cells, documented in multiple hESC and induced pluripotent stem cell (iPSC) lines, and reminiscent of the extended proliferation we previously reported in HPCs derived from iPSCs generated from patients with acute myeloid leukemia (AML) (Kotini et al. Cell Stem Cell 2017). Induction of MYC in the MYC-Amp model had similar effects in differentiation and proliferation, albeit much less pronounced, extending the growth of HPCs to approximately 50 days. This milder phenotypic perturbation is likely due to the downregulation of amplified MYC, as opposed to the continuous high expression of the ectopically expressed MYC in the MYC-DoxOE model.In summary, we have generated a novel model that offers for the first time the opportunity to study the effects of MYC deregulation by different genetic mechanisms in human hematopoiesis in a dynamic fashion. DisclosuresNo relevant conflicts of interest to declare.

Effects of VEGFR1+ hematopoietic progenitor cells on pre-metastatic niche formation and in vivo metastasis of breast cancer cells

The pre-metastatic niche has been shown to play a critical role in tumor metastasis, and its formation is closely related to the tumor microenvironment. However, the underlying molecular mechanisms remain unclear. In the present study, we successfully established a mouse model of lung metastasis using luciferase-expressing MDA-MB-435s cells. In this model, recruitment of vascular endothelial growth factor receptor-1 (VEGFR1)+CD133+ hematopoietic progenitor cells (HPCs) was gradually increased in lung but gradually decreased after the formation of tumor colonies in lung. We also established a highly metastatic MDA-MB-435s (MDA-MB-435s-HM) cell line from the mouse model. Changes in protein profiles in different culture conditions were investigated by protein microarray analysis. The levels of CXC chemokine ligand 16, interleukin (IL)-2Rα, IL-2Rγ, matrix metalloproteinase (MMP)-1, MMP-9, platelet-derived growth factor receptor (PDGFR)-α, stromal cell-derived factor (SDF)-1α, transforming growth factor (TGF)-β, platelet endothelial cell adhesion molecule (PECAM)-1 and vascular endothelial (VE)-cadherin were significantly greater (> fivefold) in the culture medium from MDA-MB-435s-HM cells than in that from MDA-MB-435s cells. Moreover, the levels of MMP-9, PDGFR-α, and PECAM-1 were significantly greater in the co-culture medium of MDA-MB-435s-HM cells and CD133+ HPCs than in that from MDA-MB-435s-HM cells. Differentially expressed proteins were validated by enzyme-linked immunosorbent assay, and expression of their transcripts was confirmed by quantitative real-time polymerase chain reaction. Moreover, inhibition of MMP-9, PDGFR-α, and PECAM-1 by their specific inhibitors or antibodies significantly decreased cell migration, delayed lung metastasis, and decreased recruitment of VEGFR1+CD133+ HPCs into lung. Intra-hepatic growth of HPCs enhanced the invasive growth of MDA-MB-435s-HM cells in the liver. Our data indicate that VEGFR1+CD133+ HPCs contribute to lung metastasis.

Potent Dual BET Bromodomain-Kinase Inhibitors as Value-Added Multitargeted Chemical Probes and Cancer Therapeutics.

Synergistic action of kinase and BET bromodomain inhibitors in cell killing has been reported for a variety of cancers. Using the chemical scaffold of the JAK2 inhibitor TG101348, we developed and characterized single agents which potently and simultaneously inhibit BRD4 and a specific set of oncogenic tyrosine kinases including JAK2, FLT3, RET, and ROS1. Lead compounds showed on-target inhibition in several blood cancer cell lines and were highly efficacious at inhibiting the growth of hematopoietic progenitor cells from patients with myeloproliferative neoplasm. Screening across 931 cancer cell lines revealed differential growth inhibitory potential with highest activity against bone and blood cancers and greatly enhanced activity over the single BET inhibitor JQ1. Gene drug sensitivity analyses and drug combination studies indicate synergism of BRD4 and kinase inhibition as a plausible reason for the superior potency in cell killing. Combined, our findings indicate promising potential of these agents as novel chemical probes and cancer therapeutics. Mol Cancer Ther; 16(6); 1054-67. ©2017 AACR.

Bone Marrow Stromal Cells Derived at Diagnosis and Remission Preferentially Support AML Cell Proliferation in Vitro

Introduction: Acute myeloid leukemia (AML) is a heterogeneous clonal disorder of hematopoietic progenitor cells characterized by excessive proliferation of early progenitor cells in the bone marrow (BM). The BM niche is a specialized type of microenvironment (ME) that actively supports growth, maturation and maintenance of hematopoietic stem and progenitor cells. They include a complex network of extracellular matrix proteins, soluble growth factors, and cytokines. The information regarding the clinical significance of leukemic microenvironment for AML initiation and propagation is scarce. The aim of the current study was to characterize the AML BM ME and its crosstalk with leukemic progenitor cells and define the role of BM stromal cells (SCs) in supporting leukemic cell growth in active disease as compared to the complete remission (CR) state.Methods: We developed a unique model of patient own stroma (POS) by growing SCs obtained at diagnosis, CR and relapse in a specific medium, including cytokines and proteins derived from human plasma solution. The growth rate and morphological changes of AML SCs were assessed and their ability to support AML cell expansion was studied by monitoring immunophenotyping alterations of CD34 and CD45 expression using FACS analysis. Levels of secreted MCP-1, IL6, GM-CSF, G-CSF and TNF in the co-culture medium were measured using the bead-based immunoassay CBA (BD Biosciences). In addition, real-time PCR (RT-PCR) amplification was performed in the co-culture to quantify changes aiming to assess the expression of specific genes associated with the maintenance of hematopoietic cells, self-renewal, quiescence regulation and mobilization. The following genes were amplified: CXCL12 (SDF1), transforming growth factor beta 1 (TGFβ), secreted phosphoprotein 1 (SPP1) and angiopoietin 1 (ANGP1).Results: The developed POS model allowed revealing high diversity in the morphological appearance of leukemic SCs obtained from different patients and at various disease stages. Patient SCs were found to grow slower than those of healthy volunteers (fig. 1A). Moreover, the growth rate differed between the patients and disease stages, being highest at relapse and lowest at diagnosis (fig. 1A). Levels of SDF-1, TGF-1 and SPP1 were elevated (X50, X50 and X 300 fold, respectively) in CR stroma compared to that of healthy volunteers. Of note, the level of MCP-1 was increased (2200 pg/ml) in the stroma derived at active disease stages (diagnosis and relapse) compared to CR or normal SCs (1050 pg/ml). Additionally, the expansion of CD34+ leukemic progenitor cells was higher in the presence of CR-derived stroma compared to healthy SCs (fig. 1B). In the co-culture, we observed selective ability of POS to support CD34+ leukemic cell expansion, as detected by flow cytometry (1.5 million cells more in POS than in healthy stroma), as well as differential expression of MCP-1 (x3000 pg/ml) and IL-6 (x9000 pg/ml) cytokines in co-cultures of AML cells and POS. These cytokines are known to be involved in the regulation of cell migration and inflammation.Conclusions: AML BM SCs are heterogeneous and differ from healthy BM SCs in their growth rate, morphology, immunophenotype features and the ability to support expansion of early progenitor CD34+ cells. Moreover, even an AML CR-derived stroma, which is considered "healthy", can provide an advantageous ME to leukemogenesis, as evident by increased proliferation of CD34+ leukemic cells in a co-culture with CR SCs compared to healthy SCs. Additionally, POS was found to selectively support the expansion of AML cells of the same patient. These findings, suggesting a cross-talk between leukemic progenitor cells and their specific POS, may lead to better understanding of the mechanisms underlying AML initiation and relapse. [Display omitted] DisclosuresNo relevant conflicts of interest to declare.

Fbxw11 Variants Control the Quiescence of HSCs

Ubiquitination is a posttranslational mechanism that controls diverse cellular processes. Fbxw11, a constituent of the SCF (Skp1-Cul1-F-box) ubiquitin ligase complex, targets for degradation of several important transcription factors, including NF-κB, β-cantenin and ATF4. Fbxw11 may play pivotal roles in many aspects of hematopoiesis through regulating various signal transduction pathways. However, the role of Fbxw11 on HSCs quiescence remains largely unknown. In this study, we cloned three transcript variants (Fbxw11a, Fbxw11c and Fbxw11d) to study the biological function of Fbxw11 in hematopoiesis.To elucidate Fbxw11 distribution in hematopoietic system, hematopoietic cell populations at different stages of differentiation were isolated from bone marrow of 8 week-old mice and Fbxw11 expression was studied by real-time PCR. Expression of Fbxw11 were lower in purified long term hematopoietic stem cells (LT-HSC, LSK CD34- Flt3-), but higher in short term hematopoietic stem cells (ST-HSC, LSK CD34+ Flt3-), hematopoietic stem and progenitor cells (LSK), and various hematopoietic progenitor cells. The results reveal that Fbxw11 is preferentially expressed in more mature progenitor cells. The expression of Fbxw11 in mature blood cells was also studied showing that Fbxw11 was expressed at lower level in neutrophils, higher level in B and T lymphocytes, and moderate level in monocytes.To assess the impact of Fbxw11 on reconstitution capacity of LT-HSCs, we cloned Fbxw11a, Fbxw11c and Fbxw11d into retrovirus system, respectively. LSK cells were infected with MSCV-Fbxw11a/Fbxw11c/Fbxw11d-IRES-GFP or the blank control vector MSCV-GFP. Competitive repopulation assays we performed 48h later after infection, and reconstitution in peripheral blood (PB) was analyzed every 4 weeks. Repopulation of donor cells expressing high level of Fbxw11 variants was significantly lower than those infected with control vector at 1 and 4 months in PB and at 4 months in BM after transplantation. These data indicate that Fbxw11 is negative for the long-term repopulating capacity of HSCs.To further confirm the effects of Fbxw11 variants in hematopoiesis, the effect of Fbxw11 variants on the growth and enumeration of hematopoietic progenitor cells was detected by colony-forming cell assay (CFC). The number of CFU-G, CFU-GM, CFU-GEMM and the total number of CFU were lower in LSK over-expressing Fbxw11 variants when compared with LSK control. To determine the cell-cycle distribution of HSC cells, Hoechst 33342 and Ki67 staining were performed showing that G0 phase LSK cells were decreased when they over-expressing Fbxw11 variants.In conclusion, our data reveal unrecognized roles for Fbxw11 in the regulation of HSPCs. Our findings suggest that Fbxw11 variants have negative effect on reconstitution capacity of LT-HSCs. Fbxw11 variants decrease the reconstitution capacity through promoting cell proliferation, which results in loss of hematopoietic stem cell quiescence. We anticipate that our experiments will facilitate the understanding of hematopoiesis through which Fbxw11-mediated signals control HSC quiescence and functions.The work was supported by the Grants 81300376, 81370634, 81570153 from the National Natural Science Foundation of China (NSFC); 14JCQNJC10600 from the Tianjin Science and Technology Programs; DisclosuresNo relevant conflicts of interest to declare.

Abstract 1117: Dissection of the major hematopoietic quantitative trait locus in chromosome 6q23.3 identifies miR-3662 as a player in hematopoiesis and AML

Abstract Background: Over 15 genome-wide association studies (GWASs) have identified a narrow region in chromosome 6q23.3 as a major hematopoietic quantitative trait locus (QTL) for many hematologic traits. The associated phenotypes have yet to be fully explained by genes in the region, including MYB. We noted a newly annotated microRNA within the QTL, miR-3662, and sought to characterize its role in the QTL by exploring its function in normal and malignant hematopoiesis, specifically acute myeloid leukemia (AML). Results: To assess the association between the QTL-defining SNPs and miR-3662 abundance, we genotyped 10 GWAS SNPs in 200 blood samples from non-leukemic individuals. The risk alleles of all 10 SNPs associated with higher miR-3662 abundance. Homozygosity for rs66650371, a three base pair (bp) deletion, showed the strongest association (P&amp;lt;.0001). Reinserting the three bp in cells homozygous for rs66650371 with CRISPR/Cas9 technology lowered miR-3662 abundance by 20%, suggesting rs66650371 directly affects miR-3662 transcription. MiR-3662 was nearly absent in primitive hematopoietic progenitor cells (HPCs) cells, whereafter its abundance increased throughout differentiation. MiR-3662 overexpression increased cell growth and colony formation of HPCs, particularly of the erythroid lineage. To explore the role of miR-3662 in malignant hematopoiesis, we overexpressed miR-3662 in AML cell lines (Kasumi1, KG1a, MV4-11) and primary AML patient blasts (n = 12). Overexpression of miR-3662 reduced cell growth and viability in vitro, and significantly reduced tumor size in a murine xenograft model (scramble vs. miR-3662; n = 9 mice/group). We used targeted RNA sequencing to identify differentially expressed genes between forced miR-3662-expresssing- and control AML patient blasts using a panel of 361 candidate genes (TruSeq RNA platform, Illumina). IKBKB was identified as a promising target, and direct regulation of IKBKB's 3′-UTR by miR-3662 was confirmed via luciferase assay. MiR-3662 overexpression lowered IKBKB mRNA and protein expression in both HPCs and AML patient blasts. As IKBKB promotes activation and nuclear localization of NF-κB, overexpression of miR-3662 led to decreased NF-κB transactivation potential via luciferase assay. Confocal microscopy of HPCs and AML cells, as well as subcellular fraction immunoblots, revealed decreased NF-κB nuclear localization upon miR-3662 overexpression. Re-introduction of IKBKB partly rescued miR-3662's phenotype by increasing nuclear localization of NF-κB, decreasing growth of HPCs, and increasing viability of AML cells. Conclusion: The risk alleles of the 6q23.3 region associated with higher miR-3662 abundance, and miR-3662 abundance was functionally linked to rs66650371. We characterized miR-3662 as an agent in normal and malignant hematopoiesis, and demonstrated that miR-3662 reduces NF-κB activity by targeting IKBKB. Citation Format: Sophia E. Maharry, Christopher J. Walker, Sandya Liyanarachchi, Sujay Mehta, James S. Blachly, Denis C. Guttridge, Clara D. Bloomfield, Albert de la Chapelle, Ann-Kathrin Eisfeld. Dissection of the major hematopoietic quantitative trait locus in chromosome 6q23.3 identifies miR-3662 as a player in hematopoiesis and AML. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1117.