Runx1 shapes the chromatin landscape via a cascade of direct and indirect targets.

Runt-related transcription factor 1 (Runx1) can act as both an activator and a repressor. Here we show that CRISPR-mediated deletion of Runx1 in mouse metanephric mesenchyme-derived mK4 cells results in large-scale genome-wide changes to chromatin accessibility and gene expression. Open chromatin regions near down-regulated loci enriched for Runx sites in mK4 cells lose chromatin accessibility in Runx1 knockout cells, despite remaining Runx2-bound. Unexpectedly, regions near upregulated genes are depleted of Runx sites and are instead enriched for Zeb transcription factor binding sites. Re-expressing Zeb2 in Runx1 knockout cells restores suppression, and CRISPR mediated deletion of Zeb1 and Zeb2 phenocopies the gained expression and chromatin accessibility changes seen in Runx1KO due in part to subsequent activation of factors like Grhl2. These data confirm that Runx1 activity is uniquely needed to maintain open chromatin at many loci, and demonstrate that Zeb proteins are required and sufficient to maintain Runx1-dependent genome-scale repression.

Developmental trajectory of prehematopoietic stem cell formation from endothelium.

Hematopoietic stem and progenitor cells (HSPCs) in the bone marrow are derived from a small population of hemogenic endothelial (HE) cells located in the major arteries of the mammalian embryo. HE cells undergo an endothelial to hematopoietic cell transition, giving rise to HSPCs that accumulate in intra-arterial clusters (IAC) before colonizing the fetal liver. To examine the cell and molecular transitions between endothelial (E), HE, and IAC cells, and the heterogeneity of HSPCs within IACs, we profiled ∼40 000 cells from the caudal arteries (dorsal aorta, umbilical, vitelline) of 9.5 days post coitus (dpc) to 11.5 dpc mouse embryos by single-cell RNA sequencing and single-cell assay for transposase-accessible chromatin sequencing. We identified a continuous developmental trajectory from E to HE to IAC cells, with identifiable intermediate stages. The intermediate stage most proximal to HE, which we term pre-HE, is characterized by increased accessibility of chromatin enriched for SOX, FOX, GATA, and SMAD motifs. A developmental bottleneck separates pre-HE from HE, with RUNX1 dosage regulating the efficiency of the pre-HE to HE transition. A distal candidate Runx1 enhancer exhibits high chromatin accessibility specifically in pre-HE cells at the bottleneck, but loses accessibility thereafter. Distinct developmental trajectories within IAC cells result in 2 populations of CD45+ HSPCs; an initial wave of lymphomyeloid-biased progenitors, followed by precursors of hematopoietic stem cells (pre-HSCs). This multiomics single-cell atlas significantly expands our understanding of pre-HSC ontogeny.

E2A-PBX1 functions as a coactivator for RUNX1 in acute lymphoblastic leukemia.

E2A, a basic helix-loop-helix transcription factor, plays a crucial role in determining tissue-specific cell fate, including differentiation of B-cell lineages. In 5% of childhood acute lymphoblastic leukemia (ALL), the t(1,19) chromosomal translocation specifically targets the E2A gene and produces an oncogenic E2A-PBX1 fusion protein. Although previous studies have shown the oncogenic functions of E2A-PBX1 in cell and animal models, the E2A-PBX1-enforced cistrome, the E2A-PBX1 interactome, and related mechanisms underlying leukemogenesis remain unclear. Here, by unbiased genomic profiling approaches, we identify the direct target sites of E2A-PBX1 in t(1,19)-positive pre-B ALL cells and show that, compared with normal E2A, E2A-PBX1 preferentially binds to a subset of gene loci cobound by RUNX1 and gene-activating machineries (p300, MED1, and H3K27 acetylation). Using biochemical analyses, we further document a direct interaction of E2A-PBX1, through a region spanning the PBX1 homeodomain, with RUNX1. Our results also show that E2A-PBX1 binding to gene enhancers is dependent on the RUNX1 interaction but not the DNA-binding activity harbored within the PBX1 homeodomain of E2A-PBX1. Transcriptome analyses and cell transformation assays further establish a significant RUNX1 requirement for E2A-PBX1-mediated target gene activation and leukemogenesis. Notably, the RUNX1 locus itself is also directly activated by E2A-PBX1, indicating a multilayered interplay between E2A-PBX1 and RUNX1. Collectively, our study provides the first unbiased profiling of the E2A-PBX1 cistrome in pre-B ALL cells and reveals a previously unappreciated pathway in which E2A-PBX1 acts in concert with RUNX1 to enforce transcriptome alterations for the development of pre-B ALL.

RUNX1 and CBFß-SMMHC transactivate target genes together in abnormal myeloid progenitors for leukemia development.

Inversion of chromosome 16 is a consistent finding in patients with acute myeloid leukemia subtype M4 with eosinophilia, which generates a CBFB-MYH11 fusion gene. It is generally considered that CBFß-SMMHC, the fusion protein encoded by CBFB-MYH11, is a dominant negative repressor of RUNX1. However, recent findings challenge the RUNX1-repression model for CBFß-SMMHC-mediated leukemogenesis. To definitively address the role of Runx1 in CBFB-MYH11-induced leukemia, we crossed conditional Runx1 knockout mice (Runx1f/f) with conditional Cbfb-MYH11 knockin mice (Cbfb+/56M). On Mx1-Cre activation in hematopoietic cells induced by poly (I:C) injection, all Mx1-CreCbfb+/56M mice developed leukemia in 5 months, whereas no leukemia developed in Runx1f/fMx1-CreCbfb+/56M mice, and this effect was cell autonomous. Importantly, the abnormal myeloid progenitors (AMPs), a leukemia-initiating cell population induced by Cbfb-MYH11 in the bone marrow, decreased and disappeared in Runx1f/fMx1-CreCbfb+/56M mice. RNA-seq analysis of AMP cells showed that genes associated with proliferation, differentiation blockage, and leukemia initiation were differentially expressed between Mx1-CreCbfb+/56M and Runx1f/fMx1-CreCbfb+/56M mice. In addition, with the chromatin immunocleavage sequencing assay, we observed a significant enrichment of RUNX1/CBFß-SMMHC target genes in Runx1f/fMx1-CreCbfb+/56M cells, especially among downregulated genes, suggesting that RUNX1 and CBFß-SMMHC mainly function together as activators of gene expression through direct target gene binding. These data indicate that Runx1 is indispensable for Cbfb-MYH11-induced leukemogenesis by working together with CBFß-SMMHC to regulate critical genes associated with the generation of a functional AMP population.

Regulation of RUNX1 dosage is crucial for efficient blood formation from hemogenic endothelium.

During ontogeny, hematopoietic stem and progenitor cells arise from hemogenic endothelium through an endothelial-to-hematopoietic transition that is strictly dependent on the transcription factor RUNX1. Although it is well established that RUNX1 is essential for the onset of hematopoiesis, little is known about the role of RUNX1 dosage specifically in hemogenic endothelium and during the endothelial-to-hematopoietic transition. Here, we used the mouse embryonic stem cell differentiation system to determine if and how RUNX1 dosage affects hemogenic endothelium differentiation. The use of inducible Runx1 expression combined with alterations in the expression of the RUNX1 co-factor CBFß allowed us to evaluate a wide range of RUNX1 levels. We demonstrate that low RUNX1 levels are sufficient and necessary to initiate an effective endothelial-to-hematopoietic transition. Subsequently, RUNX1 is also required to complete the endothelial-to-hematopoietic transition and to generate functional hematopoietic precursors. In contrast, elevated levels of RUNX1 are able to drive an accelerated endothelial-to-hematopoietic transition, but the resulting cells are unable to generate mature hematopoietic cells. Together, our results suggest that RUNX1 dosage plays a pivotal role in hemogenic endothelium maturation and the establishment of the hematopoietic system.

Msx1 cooperates with Runx1 for inhibiting myoblast differentiation.

Myogenesis is an important and complicated biological process, especially during the process of embryonic development. The homeoprotein Msx1 is a crucial transcriptional repressor of myogenesis and maintains myogenic precursor cells in an undifferentiated, proliferative state. However, the molecular mechanism through which Msx1 coordinates myogenesis remains to be elucidated. Here, we determine the interacting partner proteins of Msx1 in myoblast cells by a proteomic screening method. Msx1 is found to interact with 55 proteins, among which our data demonstrate that the cooperation of Runt-related transcription factor 1 (Runx1) with Msx1 is required for myoblast cell differentiation. Our findings provide important insights into the mechanistic roles of Msx1 in myoblast cell differentiation, and lays foundation for the myogenic differentiation process.

CBFß stabilizes HIV Vif to counteract APOBEC3 at the expense of RUNX1 target gene expression.

The HIV-1 accessory protein Vif hijacks a cellular Cullin-RING ubiquitin ligase, CRL5, to promote degradation of the APOBEC3 (A3) family of restriction factors. Recently, the cellular transcription cofactor CBFß was shown to form a complex with CRL5-Vif and to be essential for A3 degradation and viral infectivity. We now demonstrate that CBFß is required for assembling a well-ordered CRL5-Vif complex by inhibiting Vif oligomerization and by activating CRL5-Vif via direct interaction. The CRL5-Vif-CBFß holoenzyme forms a well-defined heterohexamer, indicating that Vif simultaneously hijacks CRL5 and CBFß. Heterodimers of CBFß and RUNX transcription factors contribute toward the regulation of genes, including those with immune system functions. We show that binding of Vif to CBFß is mutually exclusive with RUNX heterodimerization and impacts the expression of genes whose regulatory domains are associated with RUNX1. Our results provide a mechanism by which a pathogen with limited coding capacity uses one factor to hijack multiple host pathways.

Integration of transcriptome and cistrome analysis identifies RUNX1-target genes involved in pancreatic cancer proliferation.

The extremely high proliferation rate of tumor cells contributes to pancreatic cancer (PC) progression. Runt-related transcription factor 1(RUNX1), a key factor in hematopoiesis that was correlated with tumor progression. However, the role of RUNX1 in PC proliferation was still unclear. We found that RUNX1 was significantly upregulated in PC tissues and its expression was negatively associated with prognosis of PC patients in a multicenter analysis according to immunohistochemical (IHC). RUNX1 downregulation in PC resulted in a significantly reduced cell proliferation rate, which was consistent with in vivo subcutaneous tumor formation assay results. RNA-seq and ChIP-seq results revealed that a portion of target genes, including HAP1, GPRC5B, PTPN21, VHL and EN2, were regulated by RUNX1, a finding successfully validated by ChIP-qPCR, qRT-PCR and Western blot. Subsequently, IHC and proliferation assays showed these target genes to be dysregulated in PC, affecting tumor growth. Our data suggest that RUNX1 plays an oncogenic role in tumor proliferation and is a potential prognostic biomarker and therapeutic target for PC.

Cooperating, congenital neutropenia-associated Csf3r and Runx1 mutations activate pro-inflammatory signaling and inhibit myeloid differentiation of mouse HSPCs.

Patients with the pre-leukemia bone marrow failure syndrome called severe congenital neutropenia (CN) have an approximately 15% risk of developing acute myeloid leukemia (AML; called here CN/AML). Most CN/AML patients co-acquire CSF3R and RUNX1 mutations, which play cooperative roles in the development of AML. To establish an in vitro model of leukemogenesis, we utilized bone marrow lin- cells from transgenic C57BL/6-d715 Csf3r mice expressing a CN patient-mimicking truncated CSF3R mutation. We transduced these cells with vectors encoding RUNX1 wild type (WT) or RUNX1 mutant proteins carrying the R139G or R174L mutations. Cells transduced with these RUNX1 mutants showed diminished in vitro myeloid differentiation and elevated replating capacity, compared with those expressing WT RUNX1. mRNA expression analysis showed that cells transduced with the RUNX1 mutants exhibited hyperactivation of inflammatory signaling and innate immunity pathways, including IL-6, TLR, NF-kappaB, IFN, and TREM1 signaling. These data suggest that the expression of mutated RUNX1 in a CSF3R-mutated background may activate the pro-inflammatory cell state and inhibit myeloid differentiation.

Regulation of microglial process elongation, a featured characteristic of microglial plasticity.

Microglia, a type of glia within the brain characterized by a ramified morphology, are essential for removing neuronal debris and restricting the expansion of a lesion site. Upon moderate activation, they undergo a transformation in morphology inducing beneficial responses. However, upon strong stimulation, they mediate neuronal damage via production of pro-inflammatory cytokines. The inhibition of this cascade is considered an effective strategy for neuroinflammation-associated disorder therapy. During this pathological activation microglia also undergo a shortening of process length which contributes to the pathogenesis of such disorders. Thus, microglial plasticity should be considered to have two components: one is the production of inflammatory mediators, and the other is the dynamic changes in their processes. The former role has been well-documented in previous studies, while the latter one remains largely unknown. Recently, we and others have reported that the elongation of microglial process is associated with the transformation of microglia from a pro-inflammatory to an anti-inflammatory state, suggesting that the shortening of process length would make the microglia lose their ability to restrict pathological injury, while the elongation of microglial process would help attenuate neuroinflammation. Compared with the traditional anti-neuroinflammatory strategy, stimulating elongation of microglial process not only reduces the production of pro-inflammatory cytokines, but restores the ability of microglia to scan their surrounding environments, thus rendering their homeostasis regulation more effective. In this review, we provide a discussion of the factors that regulate microglial process elongation in vitro and in vivo, aiming to further drive the understanding of microglial process plasticity.

Supraphysiologic levels of the AML1-ETO isoform AE9a are essential for transformation.

Chromosomal translocation 8;21 is found in 40% of the FAB M2 subtype of acute myeloid leukemia (AML). The resultant in-frame fusion protein AML1-ETO (AE) acts as an initiating oncogene for leukemia development. AE immortalizes human CD34(+) cord blood cells in long-term culture. We assessed the transforming properties of the alternatively spliced AE isoform AE9a (or alternative splicing at exon 9), which is fully transforming in a murine retroviral model, in human cord blood cells. Full activity was realized only upon increased fusion protein expression. This effect was recapitulated in the AE9a murine AML model. Cotransduction of AE and AE9a resulted in a strong selective pressure for AE-expressing cells. In the context of AE, AE9a did not show selection for increased expression, affirming observations of human t(8;21) patient samples where full-length AE is the dominant protein detected. Mechanistically, AE9a showed defective transcriptional regulation of AE target genes that was partially corrected at high expression. Together, these results bring an additional perspective to our understanding of AE function and highlight the contribution of oncogene expression level in t(8;21) experimental models.

Runx1 promotes satellite cell proliferation during ischemia - Induced muscle regeneration.

RUNX1 is a transcription factor that is not expressed in uninjured muscles, but can be detected in denervated muscles, suggesting a role of RUNX1 in muscle's response to injury. However, the role of RUNX1 in muscle's response to ischemia has not been reported. Our study showed that Runx1 is up regulated in skeletal muscle during ischemia reperfusion induced injury. Over-expression of Runx1 in C2C12 cells inhibits myogenic differentiation, but promotes proliferation of myoblasts. Consistent with these findings, we found that Runx1 expression was decreased in differentiated satellite cells. Our results indicate that Runx1 regulates muscle regeneration by promoting proliferation of satellite cells.

Runx1 Transcription Factor Is Required for Myoblasts Proliferation during Muscle Regeneration.

Following myonecrosis, muscle satellite cells proliferate, differentiate and fuse, creating new myofibers. The Runx1 transcription factor is not expressed in naïve developing muscle or in adult muscle tissue. However, it is highly expressed in muscles exposed to myopathic damage yet, the role of Runx1 in muscle regeneration is completely unknown. Our study of Runx1 function in the muscle's response to myonecrosis reveals that this transcription factor is activated and cooperates with the MyoD and AP-1/c-Jun transcription factors to drive the transcription program of muscle regeneration. Mice lacking dystrophin and muscle Runx1 (mdx-/Runx1f/f), exhibit impaired muscle regeneration leading to age-dependent muscle waste, gradual decrease in motor capabilities and a shortened lifespan. Runx1-deficient primary myoblasts are arrested at cell cycle G1 and consequently differentiate. Such premature differentiation disrupts the myoblasts' normal proliferation/differentiation balance, reduces the number and size of regenerating myofibers and impairs muscle regeneration. Our combined Runx1-dependent gene expression, ChIP-seq, ATAC-seq and histone H3K4me1/H3K27ac modification analyses revealed a subset of Runx1-regulated genes that are co-occupied by MyoD and c-Jun in mdx-/Runx1f/f muscle. The data provide unique insights into the transcriptional program driving muscle regeneration and implicate Runx1 as an important participant in the pathology of muscle wasting diseases.

Runt-Related Transcription Factor 1 Regulates LPS-Induced Acute Lung Injury via NF-κB Signaling.

Runt-related transcription factor 1 (RUNX1), a transcription factor expressed in multiple organs, plays important roles in embryonic development and hematopoiesis. Although RUNX1 is highly expressed in pulmonary tissues, its roles in lung function and homeostasis are unknown. We sought to assess the role of RUNX1 in lung development and inflammation after LPS challenge. Expression of RUNX1 was assessed in the developing and postnatal lung. RUNX1 was conditionally deleted in pulmonary epithelial cells. Pulmonary maturation was evaluated in the developing and postnatal lung, and lung inflammation was investigated in adult mice after LPS challenge. Interactions between RUNX1 and inflammatory signaling via NF-κB-IkB kinase ß were assessed in vitro. RUNX1 was expressed in both mesenchymal and epithelial compartments of the developing and postnatal lung. The RUNX1 gene was efficiently deleted from respiratory epithelial cells producing Runx1∆/∆ mice. Although lung maturation was delayed, Runx1∆/∆ mice survived postnatally and subsequent growth and maturation of the lung proceeded normally. Increased respiratory distress, inflammation, and proinflammatory cytokines were observed in the Runx1-deleted mice after pulmonary LPS exposure. RUNX1 deletion was associated with the activation of NF-κB in respiratory epithelial cells. RUNX1 was required for the suppression of NF-κB signaling pathway via inhibition of IkB kinase ß in in vitro studies. RUNX1 plays a critical role in the lung inflammation after LPS-induced injury.

RUNX1 Regulates Migration, Invasion, and Angiogenesis via p38 MAPK Pathway in Human Glioblastoma.

Runt-related transcription factor 1 (RUNX1) is essential for the establishment of fetal and adult hematopoiesis and neuronal development. Aberrant expression of RUNX1 led to proliferation and metastasis of several cancers. The aim of the present study was to investigate the role of RUNX1 in migration, invasion, and angiogenesis of human glioblastoma using IL-1ß-treated U-87 MG human glioblastoma cells as a model. IL-1ß at 10 ng/ml stimulated translocation of RUNX1 into the nucleus with increased expressions of RUNX1, MMP-1, MMP-2, MMP-9, MMP-19, and VEGFA in U-87 MG cells. In addition, silencing of RUNX1 gene significantly suppressed U-87 MG cell migration and invasion abilities. Moreover, knockdown of RUNX1 mRNA in U-87 MG cells reduced the tube formation of human umbilical vein endothelial cells. Further investigation revealed that IL-1ß-induced RUNX1 expression might be mediated via the p38 mitogen-activated protein kinase (MAPK) signaling molecule for the expression of these invasion- and angiogenic-related molecules. Together with an inhibitor of p38 MAPK (SB203580) could decrease RUNX1 mRNA expression. Thus, RUNX1 may be one of the putative molecular targeted therapies against glioma metastasis and angiogenesis through the activation of p38 MAPK signaling pathway.

RUNX1 is essential for mesenchymal stem cell proliferation and myofibroblast differentiation.

Myofibroblasts are a key cell type in wound repair, cardiovascular disease, and fibrosis and in the tumor-promoting microenvironment. The high accumulation of myofibroblasts in reactive stroma is predictive of the rate of cancer progression in many different tumors, yet the cell types of origin and the mechanisms that regulate proliferation and differentiation are unknown. We report here, for the first time to our knowledge, the characterization of normal human prostate-derived mesenchymal stem cells (MSCs) and the TGF-ß1-regulated pathways that modulate MSC proliferation and myofibroblast differentiation. Human prostate MSCs combined with prostate cancer cells expressing TGF-ß1 resulted in commitment to myofibroblasts. TGF-ß1-regulated runt-related transcription factor 1 (RUNX1) was required for cell cycle progression and proliferation of progenitors. RUNX1 also inhibited, yet did not block, differentiation. Knockdown of RUNX1 in prostate or bone marrow-derived MSCs resulted in cell cycle arrest, attenuated proliferation, and constitutive differentiation to myofibroblasts. These data show that RUNX1 is a key transcription factor for MSC proliferation and cell fate commitment in myofibroblast differentiation. This work also shows that the normal human prostate gland contains tissue-derived MSCs that exhibit multilineage differentiation similar to bone marrow-derived MSCs. Targeting RUNX1 pathways may represent a therapeutic approach to affect myofibroblast proliferation and biology in multiple disease states.

Incoherent feed-forward regulatory loops control segregation of C-mechanoreceptors, nociceptors, and pruriceptors.

Mammalian skin is innervated by diverse, unmyelinated C fibers that are associated with senses of pain, itch, temperature, or touch. A key developmental question is how this neuronal cell diversity is generated during development. We reported previously that the runt domain transcription factor Runx1 is required to coordinate the development of these unmyelinated cutaneous sensory neurons, including VGLUT3(+) low-threshold c-mechanoreceptors (CLTMs), MrgprD(+) polymodal nociceptors, MrgprA3(+) pruriceptors, MrgprB4(+) c-mechanoreceptors, and others. However, how these Runx1-dependent cutaneous sensory neurons are further segregated is poorly illustrated. Here, we find that the Runx1-dependent transcription factor gene Zfp521 is expressed in, and required for establishing molecular features that define, VGLUT3(+) CLTMs. Furthermore, Runx1 and Zfp521 form a classic incoherent feedforward loop (I-FFL) in controlling molecular identities that normally belong to MrgprD(+) neurons, with Runx1 and Zfp51 playing activator and repressor roles, respectively (in genetic terms). A knock-out of Zfp521 allows prospective VGLUT3 lineage neurons to acquire MrgprD(+) neuron identities. Furthermore, Runx1 might form other I-FFLs to regulate the expression of MrgprA3 and MrgprB4, a mechanism preventing these genes from being expressed in Runx1-persistent VGLUT3(+) and MrgprD(+) neurons. The evolvement of these I-FFLs provides an explanation for how modality-selective sensory subtypes are formed during development and may also have intriguing implications for sensory neuron evolution and sensory coding.

RUNX1 reshapes the epigenetic landscape at the onset of haematopoiesis.

Cell fate decisions during haematopoiesis are governed by lineage-specific transcription factors, such as RUNX1, SCL/TAL1, FLI1 and C/EBP family members. To gain insight into how these transcription factors regulate the activation of haematopoietic genes during embryonic development, we measured the genome-wide dynamics of transcription factor assembly on their target genes during the RUNX1-dependent transition from haemogenic endothelium (HE) to haematopoietic progenitors. Using a Runx1-/- embryonic stem cell differentiation model expressing an inducible Runx1 gene, we show that in the absence of RUNX1, haematopoietic genes bind SCL/TAL1, FLI1 and C/EBPß and that this early priming is required for correct temporal expression of the myeloid master regulator PU.1 and its downstream targets. After induction, RUNX1 binds to numerous de novo sites, initiating a local increase in histone acetylation and rapid global alterations in the binding patterns of SCL/TAL1 and FLI1. The acquisition of haematopoietic fate controlled by Runx1 therefore does not represent the establishment of a new regulatory layer on top of a pre-existing HE program but instead entails global reorganization of lineage-specific transcription factor assemblies.

Runx1 exon 6-related alternative splicing isoforms differentially regulate hematopoiesis in mice.

RUNX1 is an important transcription factor for hematopoiesis. There are multiple alternatively spliced isoforms of RUNX1. The best known isoforms are RUNX1a from use of exon 7A and RUNX1b and c from use of exon 7B. RUNX1a has unique functions due to its lack of C-terminal regions common to RUNX1b and c. Here, we report that the ortholog of human RUNX1a was only found in primates. Furthermore, we characterized 3 Runx1 isoforms generated by exon 6 alternative splicing. Runx1bEx6(-) (Runx1b without exon 6) and a unique mouse Runx1bEx6e showed higher colony-forming activity than the full-length Runx1b (Runx1bEx6(+)). They also facilitated the transactivation of Runx1bEx6(+). To gain insight into in vivo functions, we analyzed a knock-in (KI) mouse model that lacks isoforms Runx1b/cEx6(-) and Runx1bEx6e. KI mice had significantly fewer lineage-Sca1(+)c-Kit(+) cells, short-term hematopoietic stem cells (HSCs) and multipotent progenitors than controls. In vivo competitive repopulation assays demonstrated a sevenfold difference of functional HSCs between wild-type and KI mice. Together, our results show that Runx1 isoforms involving exon 6 support high self-renewal capacity in vitro, and their loss results in reduction of the HSC pool in vivo, which underscore the importance of fine-tuning RNA splicing in hematopoiesis.

STAT3 mediates oncogenic addiction to TEL-AML1 in t(12;21) acute lymphoblastic leukemia.

The t(12;21)(p13;q22) translocation is the most common chromosomal abnormality in pediatric leukemia. Although this rearrangement involves 2 well-characterized transcription factors, TEL and AML1, the molecular pathways affected by the result of the translocation remain largely unknown. Also in light of recent studies showing genetic and functional heterogeneities in cells responsible for cancer clone maintenance and propagation, targeting a single common deregulated pathway may be critical for the success of novel therapies. Here we describe a novel signaling pathway that is essential for oncogenic addiction in TEL-AML1 leukemia. Our data indicate a direct role for TEL-AML1, via increasing the activity of RAC1, in regulating the phosphorylation of signal transducer and activator of transcription 3 (STAT3), which results in transcriptional induction of MYC. We demonstrate that human leukemic cell lines carrying this translocation are highly sensitive to treatment with S3I-201, a specific STAT3 inhibitor, and, more interestingly, that primary human leukemic samples are also responsive to the drug in the same concentration range. Thus, STAT3 inhibition represents a promising possible therapeutic strategy for the treatment of TEL-AML1 leukemia.