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PDF file - 146K, Figure S1. Analysis of microRNA profiles in various MLL-associated leukemias. Figure S2. Analysis of MLL-AF9 expression cells transformed from bone marrow stem/progenitors. Figure S3. Phenotypic analysis of leukemia from mice transplanted with MLL-AF9 cells. Figure S4. Quantitative PCR analysis of miR-150 expression in various cell types. Figure S5. Wild type and mutated binding sites of miR-150 in Myb luciferase constructs. Figure S6. Wild type and mutated binding sites of miR-150 in CBL and Egr2 luciferase constructs.

The microRNA miR-150, a critical regulator of hematopoiesis, is downregulated in mixed-lineage leukemia (MLL). In this study, miR-150 acts as a potent leukemic tumor suppressor by blocking the oncogenic properties of leukemic cells. By using MLL-AF9–transformed cells, we demonstrate that ectopic expression of miR-150 inhibits blast colony formation, cell growth, and increases apoptosis in vitro. More importantly, ectopic expression of miR-150 in MLL-AF9–transformed cells completely blocked the development of myeloid leukemia in transplanted mice. Furthermore, gene expression profiling revealed that miR-150 altered the expression levels of more than 30 “stem cell signature” genes and many others that are involved in critical cancer pathways. In addition to the known miR-150 target Myb, we also identified Cbl and Egr2 as bona fide targets and shRNA-mediated suppression of these genes recapitulated the pro-apoptotic effects observed in leukemic cells with miR-150 ectopic expression. In conclusion, we demonstrate that miR-150 is a potent leukemic tumor suppressor that regulates multiple oncogenes.Implications: These data establish new, key players for the development of therapeutic strategies to treat MLL-AF9–related leukemia. Mol Cancer Res; 11(8); 912–22. ©2013 AACR.

The response to immunotherapy could be better predicted by using a wide set of biomarkers, including serum tumor markers; however, robust immune markers associated with efficacy have yet to be validated. In this study, changes in immune cell subsets from NSCLC patients treated with anti-PD1 therapy were longitudinally monitored by high-dimensional cytometry by time of flight (CyTOF). The frequencies of circulating CD8+ and CD8+CD101hiTIM3+ (CCT T) subsets were significantly correlated with clinical response and survival. Enrichment of these populations in peripheral blood mononuclear cells (PBMCs) indicated a poor clinical response to ICB therapy. Cell function assays revealed that these subsets were remarkably impaired, which supported the poor outcomes observed. Additionally, longitudinal analysis showed that KLRG1 expression and cytokines were associated with the response to therapy. Overall, our results provide novel potential biomarkers for guiding the management of NSCLC patients eligible to anti-PD-1 therapy, and contribute insights for new therapeutic strategies. Funding: The research leading to these results has received funding from the Macau Science and Technology Development Fund (Project no: 0096/2018/A3 & 001/2020/ALC), NSFC overseas and Hong Kong and Macao scholars cooperative research fund project (Project no: 81828013) and Janssen therapeutic fund (Project code: ICD#1101175) as well as The 2020 Guangdong Provincial Science and Technology Innovation Strategy Special Fund (Guangdong- Hong Kong-Macau Joint Lab) (Project no: 2020B1212030006). Declaration of Interest: None to declare. Ethical Approval: This study was approved by Kiang Wu Hospital under the approval number 2018-007.

The scope and roles of regulated isoform gene expression during erythroid terminal development are poorly understood. We identified hundreds of differentiation-associated isoform changes during terminal erythropoiesis. Sequences surrounding cassette exons of skipped exon events are enriched for motifs bound by the Muscleblind-like (MBNL) family of splicing factors. Knockdown of Mbnl1 in cultured murine fetal liver erythroid progenitors resulted in a strong block in erythroid differentiation and disrupted the developmentally regulated exon skipping of Ndel1 mRNA, which is bound by MBNL1 and critical for erythroid terminal proliferation. These findings reveal an unanticipated scope of the alternative splicing program and the importance of Mbnl1 during erythroid terminal differentiation.

Differentiation of erythroblasts to mature red blood cells involves dynamic changes of the membrane and cytoskeleton networks that are not fully characterized. Using a mouse fetal liver erythroblast culture system and a targeted shRNA functional screening strategy, we identified a critical role of pleckstrin-2 in actin dynamics and protection of early stage terminal erythroblasts from oxidative damage. Knockdown of pleckstrin-2 in the early stage of terminal erythropoiesis disrupted the actin cytoskeleton and led to differentiation inhibition and apoptosis. This pro-survival and differentiation function of pleckstrin-2 was mediated through its interaction with cofilin, by preventing cofilin's mitochondrial entry when the intracellular level of reactive oxygen species was higher in the early stage of terminal erythropoiesis. Treatment of the cells with a scavenger of reactive oxygen species rescued cofilin's mitochondrial entry and differentiation inhibition induced by pleckstrin-2 knockdown. In contrast, pleckstrin-2 knockdown in late stage terminal erythroblasts had no effect on survival or differentiation but blocked enucleation due to disorganized actin cytoskeleton. Thus, our study identified a dual function of pleckstrin-2 in the early and late stages of terminal erythropoiesis through its regulations of actin dynamics and cofilin's mitochondrial localization, which reflects intracellular level of reactive oxygen species in different developmental stages.

The microRNA miR-150, a critical regulator of hematopoiesis, is downregulated in mixed-lineage leukemia (MLL). In this study, miR-150 acts as a potent leukemic tumor suppressor by blocking the oncogenic properties of leukemic cells. By using MLL-AF9–transformed cells, we demonstrate that ectopic expression of miR-150 inhibits blast colony formation, cell growth, and increases apoptosis in vitro. More importantly, ectopic expression of miR-150 in MLL-AF9–transformed cells completely blocked the development of myeloid leukemia in transplanted mice. Furthermore, gene expression profiling revealed that miR-150 altered the expression levels of more than 30 “stem cell signature” genes and many others that are involved in critical cancer pathways. In addition to the known miR-150 target Myb, we also identified Cbl and Egr2 as bona fide targets and shRNA-mediated suppression of these genes recapitulated the pro-apoptotic effects observed in leukemic cells with miR-150 ectopic expression. In conclusion, we demonstrate that miR-150 is a potent leukemic tumor suppressor that regulates multiple oncogenes. Implications: These data establish new, key players for the development of therapeutic strategies to treat MLL-AF9–related leukemia. Mol Cancer Res; 11(8); 912–22. ©2013 AACR.

This article reviews the regulation of production of RBCs at several levels. We focus on the regulated expansion of burst-forming unit-erythroid erythroid progenitors by glucocorticoids and other factors that occur during chronic anemia, inflammation, and other conditions of stress. We also highlight the rapid production of RBCs by the coordinated regulation of terminal proliferation and differentiation of committed erythroid colony-forming unit-erythroid progenitors by external signals, such as erythropoietin and adhesion to a fibronectin matrix. We discuss the complex intracellular networks of coordinated gene regulation by transcription factors, chromatin modifiers, and miRNAs that regulate the different stages of erythropoiesis.

It is unclear how epigenetic changes regulate the induction of erythroid-specific genes during terminal erythropoiesis. Here we use global mRNA sequencing (mRNA-seq) and chromatin immunoprecipitation coupled to high-throughput sequencing (CHIP-seq) to investigate the changes that occur in mRNA levels, RNA polymerase II (Pol II) occupancy, and multiple posttranslational histone modifications when erythroid progenitors differentiate into late erythroblasts. Among genes induced during this developmental transition, there was an increase in the occupancy of Pol II, the activation marks H3K4me2, H3K4me3, H3K9Ac, and H4K16Ac, and the elongation methylation mark H3K79me2. In contrast, genes that were repressed during differentiation showed relative decreases in H3K79me2 levels yet had levels of Pol II binding and active histone marks similar to those in erythroid progenitors. We also found that relative changes in histone modification levels, in particular, H3K79me2 and H4K16ac, were most predictive of gene expression patterns. Our results suggest that in terminal erythropoiesis both promoter and elongation-associated marks contribute to the induction of erythroid genes, whereas gene repression is marked by changes in histone modifications mediating Pol II elongation. Our data map the epigenetic landscape of terminal erythropoiesis and suggest that control of transcription elongation regulates gene expression during terminal erythroid differentiation.

Using RNA-seq technology, we found that the majority of microRNAs (miRNAs) present in CFU-E erythroid progenitors are down-regulated during terminal erythroid differentiation. Of the developmentally down-regulated miRNAs, ectopic overexpression of miR-191 blocks erythroid enucleation but has minor effects on proliferation and differentiation. We identified two erythroid-enriched and developmentally up-regulated genes, Riok3 and Mxi1, as direct targets of miR-191. Knockdown of either Riok3 or Mxi1 blocks enucleation, and either physiological overexpression of miR-191 or knockdown of Riok3 or Mxi1 blocks chromatin condensation. Thus, down-regulation of miR-191 is essential for erythroid chromatin condensation and enucleation by allowing up-regulation of Riok3 and Mxi1.

This work started out with our discovery of a pattern of throughput distributions among links in IEEE 802.11 networks from experimental results. This pattern gives rise to an easy computation method, which we term back-of-the-envelop (BoE) computation. For many network configurations, very accurate results can be obtained by BoE within minutes, if not seconds, by simple hand computation. This allows us to make shortcuts in performance evaluation, bypassing complicated stochastic analysis. To explain BoE, we construct a theory based on the model of an “ideal CSMA network” (ICN). The BoE computation method emerges from ICN when we take the limit c → 0, where c is the ratio of the mean backoff countdown time to the mean transmission time in the CSMA protocol. Importantly, we derive a new mathematical result: the link throughputs of ICN are insensitive to the distributions of the backoff countdown time and transmission time (packet duration) given the ratio of their means c. This insensitivity result explains why BoE works so well for practical 802.11 networks, in which the backoff countdown process is one that has memory, and in which the packet size can be arbitrarily distributed. Our results indicate that BoE is a good approximation technique for modest-size networks such as those typically seen in 802.11 deployments. Beyond explaining BoE, the theoretical framework of ICN is also a foundation for fundamental understanding of very-large-scale CSMA networks. In particular, ICN is similar to the Ising model in statistical physics used to explain phenomena arising out of the interactions of a large number of entities. Many new research directions arise out of the ICN model.

In wild-type Arabidopsis, levels of ASN1 mRNA and asparagine (Asn) are tightly regulated by environmental factors and metabolites. Because Asn serves as an important nitrogen storage and transport compound used to allocate nitrogen resources between source and sink organs, we tested whether overexpression of the major expressed gene for Asn synthetase, ASN1, would lead to changes in nitrogen status in the ultimate storage organ for metabolites—seeds. Transgenic Arabidopsis constitutively overexpressing ASN1 under the cauliflower mosaic virus 35S promoter were constructed (35S-ASN1). In seeds of the 35S-ASN1 lines, three observations support the notion that the nitrogen status was enhanced: (a) elevations of soluble seed protein contents, (b) elevations of total protein contents from acid-hydrolyzed seeds, and (c) higher tolerance of young seedlings when grown on nitrogen-limiting media. Besides quantitative differences, changes in the relative composition of the seed amino acid were also observed. The change in seed nitrogen status was accompanied by an increase of total free amino acids (mainly Asn) allocated to flowers and developing siliques. In 35S-ASN1 lines, sink tissues such as flowers and developing siliques exhibit a higher level of free Asn than source tissues such as leaves and stems, despite significantly higher levels of ASN1 mRNA observed in the source tissues. This was at least partially due to an enhanced transport of Asn from source to sink via the phloem, as demonstrated by the increased levels of Asn in phloem exudates of the 35S-ASN1 plants.

CD47, also known as integrin-associated protein, is over-expressed in several tumor types, including AML, NHL, breast cancer and multiple myeloma. Elevated expression of CD47 on the cell surface protects tumors from phagocytosis by macrophages through binding to signal regulatory protein alpha (SIRPα) on the surface of macrophages. The CD47-SIRPα interaction triggers events that culminate in the inhibition of the phagocytic process. Macrophages are one of the immune cell types frequently found in the tumor microenvironment and exist as a heterogeneous population that includes both M1 and M2 macrophages. While the spectrum of macrophage subpopulations are likely quite diverse, traditionally, it is thought that M1 macrophages are pro-inflammatory, enhancing immune responses against tumor cells, while M2 macrophages are pro-tumor, since they express a wide array of anti-inflammatory molecules, cytokines and growth factors that promote tumor growth, angiogenesis and an immunosuppressive microenvironment. We evaluated CC-90002 efficacy in the RPMI-8226 multiple myeloma and MDA-MB-231 breast cancer xenograft models and enumerated mouse M1/M2-like macrophage populations within in the tumor before and after C-90002 treatment. In both RPMI-8226 and MDA-MB-231 models, M2 macrophages were the primary resident macrophage. Our studies show that when mice bearing RPMI-8226 multiple myeloma xenografts were treated with CC-90002 (a humanized anti-CD47 antibody), tumor regression was preceded by infiltration of macrophages into the xenograft. In the MDA-MB-231 breast cancer model, resident macrophages appeared to mediate the CC-90002 anti-tumor efficacy in vivo without additional macrophage trafficking. Interestingly, in both models, M1 macrophages appear to mediate CC-90002 efficacy regardless of whether macrophages infiltrate the tumor or are tumor-resident. In the MDA-MB-231 breast cancer model where CC-90002 did not induce infiltration, it is possible that resident tumor macrophages were re-educated to an M1 phenotype. In in vitro experiments using human monocyte-derived macrophages, both M1 and M2 macrophages are able to promote phagocytosis and additionally, we observed that CC-90002 selectively inhibited migration of M2 macrophages toward tumor cell conditioned media. This would presumably shift the overall balance of tumor-associated macrophages toward the M1 phenotype and suggests that inhibiting CD47 can both promote tumor phagocytosis and skew tumor macrophage subpopulations toward an anti-tumor phenotype. CC-90002 is currently being tested in two ongoing Phase I clinical studies in subjects with advanced solid and hematologic cancers (NCT02367196, NCT02641002). Citation Format: Bing Zheng, Piu Wong, WenQing Yang, Rama Narla, Michael Burgess, Laure Escoubet, Heather Raymon, Kandasamy Hariharan, John Boylan, Kristen Hege, Victoria Sung. CC-90002 (anti-CD47 antibody) in vivo anti-tumor activity is associated with an increase in M1-polarized macrophages [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 2009. doi:10.1158/1538-7445.AM2017-2009

Sotatercept (ACE-011), a recombinant human fusion protein containing the extracellular domain of the human Activin receptor IIA, binds to and inhibits activin and other members of the transforming growth factor -β (TGF-β) superfamily. Administration of sotatercept led to a rapid and sustained increase in red blood cell (RBC) count and haemoglobin (Hb) in healthy volunteers (phase I clinical trials), but the mechanism is not fully understood. Mice treated with RAP-011 (murine ortholog of ACE-011) respond with a rapid (within 24 h) increase in haematocrit, Hb, and RBC count. These effects are accompanied by an equally rapid stimulation of late-stage erythroid precursors in the bone marrow (BM). RAP-011 also induces a significant increase in erythroid burst-forming units and erythropoietin, which could contribute to additional, sustained effects on RBC production. Further in vitro co-culture studies demonstrate that BM accessory cells are required for RAP-011 effects. To better understand which TGF-β family ligand(s) mediate RAP-011 effects, we evaluated the impact of several of these ligands on erythroid differentiation. Our data suggest that RAP-011 may act to rescue growth differentiation factor 11/Activin A-induced inhibition of late-stage erythropoiesis. These data define the mechanism of action of a novel agent that regulates RBC differentiation and provide the rationale to develop sotatercept for the treatment of anaemia and ineffective erythropoiesis.

Background: 3D echocardiography derived volumetric data can be used to generate left ventricular volume time curves and has the potential to elucidate aspects of contractility, systolic and diastolic function in normal and disease states. We sought to establish the validity and reproducibility of these volume-time curves and apply novel parameters derived from these curves to a clinical population. Methods: Consecutive patients, who underwent cardiac MRI and echocardiography within 12 hours, were used to compare volume-time curves. Inter- and intra observer variability of the 3D echocardiography volume-time curves was assessed. Peak emptying rates, peak filling rates, and peak systolic acceleration were measured in a normal population (normal Ejection Fraction (EF) and no significant valvular disease) and their change with age was evaluated. Results: 16 patients with an average EF of 55% ± 10% underwent cardiac MRI and 3D echocardiography. Therewas no significant difference between the systolic and diastolic slopes of the volume-time curves derived by the two methods with good correlation for both systole (r=0.62, p < 0.0001) and diastole (r=0.43, p=0.0025). In 50 normal patients aged 19-91, peak emptying rate, peak filling rate, and peak systolic acceleration all decreased with age. Conclusion: 3D echocardiography volume-time curves correlate well with the gold standard of cardiac MRI. The novel 3D parameters of peak emptying rate, peak filling rate, and peak systolic acceleration may prove to be helpful in the assessment of systolic and diastolic function and provide insight into ventricular performance.

Despite advances in defining the critical molecular determinants for leukemia stem cell (LSC) generation and maintenance, little is known about the roles of microRNAs in LSC biology. Here, we identify microRNAs that are differentially expressed in LSC-enriched cell fractions (c-kit+) in a mouse model of MLL leukemia. Members of the miR-17 family were notably more abundant in LSCs compared with their normal counterpart granulocyte-macrophage progenitors and myeloblast precursors. Expression of miR-17 family microRNAs was substantially reduced concomitant with leukemia cell differentiation and loss of self-renewal, whereas forced expression of a polycistron construct encoding miR-17-19b miRNAs significantly shortened the latency for MLL leukemia development. Leukemias expressing increased levels of the miR-17-19b construct displayed a higher frequency of LSCs, more stringent block of differentiation, and enhanced proliferation associated with reduced expression of p21, a cyclin-dependent kinase inhibitor previously implicated as a direct target of miR-17 microRNAs. Knockdown of p21 in MLL-transformed cells phenocopied the overexpression of the miR-17 polycistron, including a significant decrease in leukemia latency, validating p21 as a biologically relevant and direct in vivo target of the miR-17 polycistron in MLL leukemia. Expression of c-myc, a crucial upstream regulator of the miR-17 polycistron, correlated with miR-17-92 levels, enhanced self-renewal, and LSC potential. Thus, microRNAs quantitatively regulate LSC self-renewal in MLL-associated leukemia in part by modulating the expression of p21, a known regulator of normal stem cell function. Cancer Res; 70(9); 3833–42. ©2010 AACR.

Differential expression of Hox genes is associated with normal hematopoiesis, whereas inappropriate maintenance of Hox gene expression, particularly Hoxa7 and Hoxa9, is a feature of leukemias harboring mixed-lineage leukemia (MLL) mutations. To understand the pathogenic roles of Hox genes in MLL leukemias, we assessed the impact of Hoxa7 or Hoxa9 nullizygosity on hematopoietic progenitor compartments and their susceptibility to MLL-induced leukemias. Selective reductions in the absolute numbers of committed progenitors, but not of hematopoietic stem cells, distinguished Hoxa7- and Hoxa9-deficient mice. Megakaryocytic/erythroid progenitor (MEP) reductions in Hoxa7-/-mice correlated with reticulocytosis and thrombocytopenia without anemia. Conversely, Hoxa9-/- mice displayed marked lymphopenia and substantial reductions of common lymphoid progenitors (CLPs) and lymphoid precursors, in addition to significant reductions of common myeloid progenitors (CMPs) and granulocyte/monocyte progenitors (GMPs). In retroviral transduction/transplantation assays, Hoxa7- and Hoxa9-deficient progenitors remained susceptible to transformation by MLL-GAS7, which activates MLL through a dimerization-dependent mechanism. However, Hoxa7-/- or Hoxa9-/-progenitors were less efficient in generating transformed blast colony-forming units (CFUs) in vitro and induced leukemias with longer disease latencies, reduced penetrance, and less mature phenotypes. Thus, Hoxa7 and Hoxa9 contribute to hematopoietic progenitor homeostasis but are not necessary for MLL-GAS7–mediated leukemogenesis, yet they appear to affect disease latency, penetrance, and phenotypes consistent with their critical roles as downstream targets of MLL fusion proteins. (Blood. 2004;103:3192-3199)

Leukemic stem cells (LSCs) are derived from hematopoietic stem or progenitor cells and often share gene expression patterns and specific pathways. Characterization and mechanistic studies of LSCs are critical as they are responsible for the initiation and potential relapse of leukemias, however the overall framework, including epigenetic regulation, is not yet clear. We previously identified microRNA-150 (miR-150) as a critical regulator of mixed lineage leukemia (MLL) -associated leukemias by targeting oncogenes. Our additional results suggest that miR-150 can inhibit LSC survival and disease initiating capacity by suppressing more than 30% of “stem cell signature genes,” hence altering multiple cancer pathways and/or stem cell identities. MLL-AF9 cells derived from miR-150 deficient hematopoietic stem/progenitor cells displayed significant proliferating advantage and enhanced leukemic colony formation. Whereas, with ectopic miR-150 expression, the MLL-AF9 associated LSC population (defined as Lin-ckit+sca1- cells) was significantly decreased in culture. This is further confirmed by decreased blast leukemic colony formation in vitro. Furthermore, restoration of miR-150 levels in transformed MLL-AF9 cells, which often display loss of miR-150 expression in AML patients with MLL-fusion protein expressing, completely blocked the myeloid leukemia development in a transplantation mouse model. Gene profiling analysis demonstrated that an increased level of miR-150 expression down regulates 30 of 114 stem cell signature genes by more than 1.5 fold, partially mediated by the suppressive effects of miR-150 on CBL, c-Myb and Egr2 oncogenes. In conclusion, our results suggest that miR-150 is a potent MLL-AF9 leukemic inhibitor that may act by suppressing the survival and leukemic initiating potency of MLL-AF9 LSCs. Disclosures: No relevant conflicts of interest to declare.

Erythropoiesis, the process of cell proliferation and differentiation that produces erythrocytes, is a tightly regulated process, but apart from early progenitor development and the EPO-dependent response, very little is known about other molecular signals which control cellular fate during RBC production. Members of the transforming growth factor beta (TGFβ) superfamily have been studied as potential regulators of erythropoiesis, iron regulation and globin expression. Sotatercept, an ActRIIA ligand trap, binds to and inhibits activin and other members of the TGFβ superfamily to induce a rapid increase in red cell number and hemoglobin in healthy volunteers. Pharmacological findings demonstrate that RAP-011, a murine ortholog of sotatercept, stimulates RBC parameters in mice through a mechanism distinct from EPO. We conducted the current study to evaluate if RAP-011 may stimulate expansion of a late-stage erythroblast population that is not normally expanded and/or may induce faster differentiation of erythroid precursors. In order to determine if RAP-011 promotes proliferation or differentiation during erythropoiesis, the number of cell divisions was quantified by CFSE staining. During in vitro erythroid differentiation, RAP-011 did not appear to alter the number of cell divisions; however, the percentage of cells that underwent the last division was higher in cultures treated with RAP-011, suggesting that the drug induced faster cellular maturation/differentiation. We also analyzed cell viability of GPA+ cells at the end of the differentiation process and observed that the percentage of apoptotic death was higher in control vs. RAP-011-treated cells. This suggests that RAP-011 may promote survival of late-stage precursors. To assess potential candidates which may mediate the erythropoietic effects of RAP-011, we selected three high affinity RAP-011 ligands, Activin A, Activin B and GDF-11, and proceeded to evaluate their effects on Smad signaling and on erythroid differentiation of human bone marrow progenitors. First, we observed that RAP-011 blocked ligand-induced Smad2/3 phosphorylation in the bone marrow-derived cells. Secondly, RAP-011 rescued activin A-induced inhibition of BFU-E colony formation. Finally, when mature CD36+ cells were differentiated in liquid media containing each of the three ligands, RAP-011 was able to reverse GDF-11- and Activin A-induced inhibition of of erythroid cell proliferation. GDF-11 and Activin A also significantly decreased the percentage of GPA-positive cells in culture, while significantly increasing the percentage of CD45-positive cells. Consistent with proliferation results, RAP-011 blocked these ligand effects. Treatment of CD36+ cells with Activin B did not alter growth or differentiation. These data suggest that GDF-11 and Activin A may contribute, in part, to the erythropoietic stimulatory effects of RAP-011. Several members of the TGFβ superfamily of ligands have been implicated as negative growth regulators, or “chalones”, functioning in homeostasis to maintain specific, mature tissue size. The results from our studies using the ActRIIA-Fc ligand trap, RAP-011, suggest that GDF-11 and Activin A, as well as other sotatercept ligands, may also be “chalones” for the blood, specifically regulating homeostasis of mature RBCs. We suggest that sotatercept increases red blood cell maturation and survival by blocking the negative growth regulation by TGFβ members. In pathologic states such as ineffective erythropoiesis, sotatercept may have an even greater impact than in the healthy, homeostatically-balanced environment. Disclosures: Carrancio: Celgene Corp.: Employment. Markovics:Celgene Corp.: Employment. Wong:Celgene Corp.: Employment. Heise:Celgene: Employment, Equity Ownership. Daniel:Celgene Corp.: Employment, Equity Ownership. Chopra:Celgene: Employment, Equity Ownership. Sung:Celgene Corp.: Employment.

Abstract 372 The regulation of erythropoiesis requires stem cell factor and erythropoietin (EPO) for the proliferation and survival of erythroid progenitor and early precursor cells. While recombinant EPO is widely used for treating various types of anemia, it often lacks efficacy in cases of anemia due to ineffective erythropoiesis in which immature erythroid precursors undergo apoptosis. Thus, there is an need for new therapies to treat the later stages of erythropoiesis. Members of the transforming growth factor beta (TGFβ) superfamily have been studied as potential regulators of erythropoiesis, iron regulation and globin expression. Sotatercept (ACE-011), a recombinant fusion protein consisting of the extracellular domain of the human activin receptor IIA (ActRIIA) linked to the human immunoglobulin G1 (IgG1) Fc domain, is a ligand trap which binds a number of TGFβ superfamily ligands including activin A, activin B, growth differentiation factor-11 (GDF-11) and bone morphogenetic protein-10 (BMP-10). Administration of sotatercept led to substantial increases in red cell number and hemoglobin in human subjects, but the mechanism is not fully understood. We utilized both mouse in vivo and human in vitro models to investigate the mechanism of sotatercept in promoting erythropoiesis. In order to compare the effects of RAP-011 (the murine version of sotatercept) to EPO on red blood cell (RBC) parameters, C57/Bl mice were dosed with RAP-011, EPO or control vehicle. RAP-011-treated mice had a rapid and statistically significant increase in hematocrit, hemoglobin, and RBC number in less than 72-hours. As rapidly as 24 hours after treatment, RAP-011 induced a significant increase in RNA-negative, enucleated cells in the bone marrow (BM). RAP-011 also rapidly increased BM BFU-e and CFU-e erythroid progenitors, while EPO was more effective on spleen-derived progenitors. These data suggest that RAP-011 acts primarily on both bone marrow progenitor cells and late erythroblasts to promote erythropoiesis. In order to investigate the cellular mechanism by which RAP-011 increases red blood cell parameters, we conducted a series of in vitro experiments and found no evidence to support direct effects of RAP-011 on human CD34+ cells assessed in colony formation assays and in erythroid differentiation in liquid culture. As both clinical and pharmacological findings point to a clear role for RAP-011 in stimulating RBC parameters, we hypothesized that RAP-011 effects may be mediated by accessory cells in the BM microenvironment. Human CD36+ cells, which are highly enriched for erythroid progenitors, were co-cultured with long-term BM cultures and erythroid differentiation was assessed following 6 days of culture in EPO (2U/mL)-supplemented media. At day 6 the output of these cultures was predominantly characterized as EryA (∼basophilic erythroblast) but with the addition of RAP-011 (50μM), a significant fraction of CD36+ cells matured into EryB/C cells (polychromatic/orthochromatic erythroblasts), suggesting that factors produced by BM accessory cells mediate RAP-011 erythropoietic effects and that, in contrast to EPO, RAP-011 may play a role in the latter stages of erythroblast maturation. To identify cytokines that may mediate RAP-011 effects, CD36+ cells were treated with several activin receptor IIA ligands. GDF-11 treatment significantly decreased proliferation of GPA+ cells during the differentiation process and RAP-011 effectively reversed this effect, but had no consequence on untreated cells. These data suggest that GDF-11 may mediate the erythroipoietic stimulatory effects of RAP-011. In summary, RAP-011 induced a rapid increase in RBC parameters in mice (reflected in the number of enucleated cells found in the bone marrow), likely mediated by BM accessory cells. Our data also suggest that effects of sotatercept may be mediated at least partly by GDF-11, acting as a potential negative regulator of the terminal stages of erythropoiesis. The ability of sotatercept to reverse this inhibition would lead to a rapid release of terminal erythroid cells into the circulation. These data support the rationale to develop sotatercept for the treatment of anemia and ineffective erythropoiesis, especially in patients who may not respond to EPO. Disclosures: Carrancio: Celgene Corporation: Employment. Markovics:Celgene Corporation: Employment. Wong:Celgene Corporation: Employment. Leisten:Celgene Corporation: Employment. Groza:Celgene Corporation: Employment. Raymon:Celgene Corporation: Employment. Heise:Celgene Corporation: Employment. Chopra:Celgene Corp: Employment, Equity Ownership. Daniel:Celgene Corporation: Employment. Sung:Celgene Corporation: Employment.

Abstract 2093 Mammalian erythropoiesis is globally regulated by erythropoietin (Epo). Epo binds to its receptor on the cell surface of erythroid precursors. This induces a series of downstream pathways that promote cell differentiation and inhibit apoptosis. A recent genome wide transcriptional profile study demonstrated that over 600 genes are up-regulated during erythropoiesis. Surprisingly, the roles that most of the identified genes play in erythroid cells are still unknown. Understanding the functions of these unknown genes in the erythroid cells is necessary to elucidate the pathogenesis of red cell disorders such as congenital dyserythropoietic anemias, fanconi anemia, aplastic anemia, megaloblastic anemia, as well as leukemia and myelodysplastic syndromes with leukemic or dysplastic erythroid features. The goal of our study is to identify novel genes involved in different stages of erythropoiesis. To achieve this goal, we developed a high-throughput flow cytometry based assay that simultaneously detects erythroid cell differentiation and enucleation. In this assay, mouse fetal liver erythroblasts were purified and infected with lentiviruses containing a mammalian shRNA knockdown library obtained from the Broad Institute. The infected cells were cultured in a 96-well plate. Over the following two days the unaffected cells fully differentiate with approximately 60% of the cells enucleated. However, those cells in which shRNAs have knocked down genes critical for erythropoiesis are expected to show alterations in differentiation and/or enucleation. The system was validated using lentiviruses expressing shRNAs against Gata1 and mDia2, known proteins that are involved in the early and late stages of erythropoiesis, which showed inhibitions of differentiation and enucleation, respectively. We have pre-screened the most up-regulated 100 genes that play unknown functions in the erythroid cells. Future studies will be focused on the identified novel genes on their functions in erythroid lineage commitment, Epo mediated signaling pathways, hemoglobin enrichment, chromatin condensation, and enucleation. Disclosures: No relevant conflicts of interest to declare.

The critical molecular determinants for leukemia stem cell (LSC) generation and maintenance are largely unknown. While the majority of studies focus on investigating key transcription factors and cytokines involved in hematological malignancies, very little is known about the roles of recently discovered microRNAs in leukemogenesis. Bioinformatic analysis of the Mouse Retroviral Tagged Cancer Gene Database (Neal Copeland et al.) revealed the 5′ regulatory region of the miR-142 gene as a recurrent target for retroviral integrations in a variety of mouse leukemia and lymphoma cancer models, including MLL-induced leukemia, thereby implicating miR-142 as a potential collaborating oncomir in hematologic malignancies. Subsequent expression analyses of differentially expressed microRNAs in LSC-enriched cell fractions (c-kit+) from MLL leukemias in comparison with various stages of normal hematopoietic progenitors indicated that the miR-17-5p family of microRNAs is highly expressed in LSC-enriched fractions and their possible normal counterparts, granulocyte-macrophage progenitors. Moreover, the expression of miR-142 and miR-17-5p family microRNAs was substantially reduced upon differentiation and loss of self-renewal in cells transformed by the conditional MLL-ENL-ER oncogene following withdrawal of tamoxifen. Forced expression of the miR-17-19b polycistron or miR-142 substantially shortened the latency for MLL leukemia development. In particular, leukemias expressing high levels of the miR-17-19b polycistron displayed a higher frequency of LSCs due to an efficient block of differentiation and enhanced proliferation associated with reduced expression of p21, a cyclin dependent kinase inhibitor previously implicated as a direct target of miR-17-19b microRNAs. Knock down of p21 in MLL oncogene transduced cells recapitulates the MLL-leukemic phenotype induced by over-expression of the miR-17-19b cluster including a significant decrease in MLL leukemia latency. This observation validates p21 as a biologically relevant in vivo target of the miR-17-19b polycistron in MLL leukemia. Thus, microRNAs are pathogenically important for quantitatively modulating the self-renewing LSC compartment in a murine model of MLL leukemia. Potential requirements of the miR-17-19b microRNA cluster for induction and/or maintenance of MLL leukemia are currently under study.

The Mixed Lineage Leukemia (MLL) gene codes for a histone methyltransferase that is required for hematopoietic development. As a consequence of chromosomal translocations, MLL is fused with over 40 different genes to yield in-frame fusion proteins in acute leukemias. AF4, the most common fusion partner, accounts for 40% of MLL leukemias. The AF4-related proteins, LAF4 and AF5q31, are also fusion partners for MLL in rare cases of leukemia, whereas a fourth family member, FMR2, is a candidate protein for fragile X mental retardation syndrome. MLL fusions with AF4 family proteins manifest as acute biphenotypic or lymphoblastic leukemias that are associated with a poor prognosis, but the lack of appropriate disease models has hampered progress in understanding their underlying molecular mechanisms. Here we report that each of the MLL-AF4 family fusion proteins (the leukemia associated MLL-AF4, MLL-LAF4, and MLL-AF5q31 as well as an artificial MLL-FMR2) is capable of altering the growth and self-renewal properties of primary murine hematopoietic stem/progenitor cells in a serial methylcellulose culture myeloid replating assay. However, significant differences were observed in their respective oncogenic potentials. Cells transduced with MLL-LAF4, MLL-AF5q31 and MLL-FMR2 rapidly expanded during serial replatings, adapted to growth in liquid culture, and induced leukemias in syngeneic recipient mice within 6 months. Conversely, bone marrow cells transduced with MLL-AF4 yielded modest numbers of blast-like colonies in the third round of plating, only transiently expanded in vitro, and quickly underwent terminal differentiation. MLL-LAF4 and MLL-AF5q31 were also capable of enhancing the self-renewal of transduced cells with B lymphoid progenitor phenotypes. A structure/function analysis demonstrated that the previously reported transcriptional transactivation domains of AF4 family proteins were neither necessary nor sufficient for in vitro transformation. Furthermore, the recently reported AF9 interaction motif that is conserved in all AF4 family proteins was also shown to be dispensable for in vitro transformation. These data indicate that transactivation per se, and interaction with AF9 in particular, are not absolutely required for the oncogenic actions of MLL-AF4 family proteins. Conversely, a minimal transformation domain was mapped to the highly conserved carboxy-terminal homology domain shared among all AF4 family proteins, including the Drosophila homolog Lilliputian, and shown to be necessary and sufficient for transformation. Taken together, our studies establish transformation models for MLL-AF4 family fusion proteins and provide critical mechanistic insights into their underlying molecular mechanisms.