In human hematopoietic progenitors, RUNX1 and its partner CBFB are up-regulated at the onset of megakaryocytic differentiation and down-regulated at the onset of erythroid differentiation. The complex of RUNX1 and CBFB cooperates with the transcription factor GATA1 in the transactivation of megakaryocyte-specific genes. In addition, RUNX1 and GATA1 physically interact (Elagib et al. 2003), and this interaction involves the zinc finger domain of GATA1 (Xu et al. 2006). Other components of the RUNX1:CBFB activating complex at megakaryocytic promoters are GATA1 heterodimerization partner, ZFPM1 (FOG1), histone acetyltransferases EP300 (p300) and KAT2B (PCAF), the WDR5-containing histone methyltransferase MLL complex and the arginine methyltransferase PRMT1 (Herglotz et al. 2013). In the absence of PRMT1, the transcriptional repressor complex can form at megakaryocytic promoters, as RUNX1 that is not arginine methylated can bind to SIN3A/SIN3B co-repressors (Zhao et al. 2008). Besides SIN3A/SIN3B, the RUNX1:CBFB repressor complex at megakaryocytic promoters also includes histone deacetylase HDAC1 and histone arginine methyltransferase PRMT6 (Herglotz et al. 2013). Megakaryocytic promoters regulated by the described RUNX1:CBFB activating and repressing complexes include ITGA2B, GP1BA, THBS1 and MIR27A (Herglotz et al. 2013). ITGA2B is only expressed in maturing megakaryocytes and platelets and is involved in platelet aggregation (Block and Poncz 1995). GP1BA is expressed at the cell surface membrane of maturing megakaryocytes and platelets and participates in formation of platelet plugs (Cauwenberghs et al. 2000, Jilma-Stohlawetz et al. 2003, Debili et al. 1990). THBS1 homotrimers contribute to stabilization of the platelet aggregate (Bonnefoy and Hoylaerts 2008). MIR27A is a negative regulator of RUNX1 mRNA translation and may be involved in erythroid/megakaryocytic lineage determination (Ben-Ami et al. 2009). The RUNX1:CBFB complex stimulates transcription of the PF4 gene, encoding a component of platelet alpha granules (Aneja et al. 2011), the NR4A3 gene, associated with the familial platelet disorder (FPD) (Bluteau et al. 2011), the PRKCQ gene, associated with inherited thrombocytopenia (Jalagadugula et al. 2011), the MYL9 gene, involved in thrombopoiesis (Jalagadugula et al. 2010), and the NFE2 gene, a regulator of erythroid and megakaryocytic maturation and differentiation (Wang et al. 2010).
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At the level of transcription, expression of the RUNX1 transcription factor is regulated by two alternative promoters: a distal promoter, P1, and a proximal promoter, P2. P1 is more than 7 kb upstream of P2 (Ghozi et al. 1996). In mice, the Runx1 gene is preferentially transcribed from the proximal P2 promoter during generation of hematopoietic cells from hemogenic endothelium. In fully committed hematopoietic progenitors, the Runx1 gene is preferentially transcribed from the distal P1 promoter (Sroczynska et al. 2009, Bee et al. 2010). In human T cells, RUNX1 is preferentially transcribed from P1 throughout development, while developing natural killer cells transcribe RUNX1 predominantly from P2. Developing B cells transcribe low levels of RUNX1 from both promoters (Telfer and Rothenberg 2001). RUNX1 mRNAs transcribed from alternative promoters differ in their 5'UTRs and splicing isoforms of RUNX1 have also been described. The function of alternative splice isoforms and alternative 5'UTRs has not been fully elucidated (Challen and Goodell 2010, Komeno et al. 2014). During zebrafish hematopoiesis, RUNX1 expression increases in response to NOTCH signaling, but direct transcriptional regulation of RUNX1 by NOTCH has not been demonstrated (Burns et al. 2005). RUNX1 transcription also increases in response to WNT signaling. BothTCF7 and TCF4 bind the RUNX1 promoter (Wu et al. 2012, Hoverter et al. 2012), and RUNX1 transcription driven by the TCF binding element (TBE) in response to WNT3A treatment is inhibited by the dominant-negative mutant of TCF4 (Medina et al. 2016). In developing mouse ovary, Runx1 expression is positively regulated by Wnt4 signaling (Naillat et al. 2015). Studies in mouse hematopoietic stem and progenitor cells imply that RUNX1 may be a direct transcriptional target of HOXB4 (Oshima et al. 2011). Conserved cis-regulatory elements were recently identified in intron 5 of RUNX1. The RUNX1 breakpoints observed in acute myeloid leukemia (AML) with translocation (8;21), which result in expression of a fusion RUNX1-ETO protein, cluster in intron 5, in proximity to these not yet fully characterized cis regulatory elements (Rebolledo-Jaramillo et al. 2014). At the level of translation, RUNX1 expression is regulated by various microRNAs which bind to the 3'UTR of RUNX1 mRNA and inhibit its translation through endonucleolytic and/or nonendonucleolytic mechanisms. MicroRNAs that target RUNX1 include miR-378 (Browne et al. 2016), miR-302b (Ge et al. 2014), miR-18a (Miao et al. 2015), miR-675 (Zhuang et al. 2014), miR-27a (Ben-Ami et al. 2009), miR-17, miR-20a, miR106 (Fontana et al. 2007) and miR-215 (Li et al. 2016). At the posttranslational level, RUNX1 activity is regulated by postranslational modifications and binding to co-factors. SRC family kinases phosphorylate RUNX1 on multiple tyrosine residues in the negative regulatory domain, involved in autoinhibition of RUNX1. RUNX1 tyrosine phosphorylation correlates with reduced binding of RUNX1 to GATA1 and increased binding of RUNX1 to the SWI/SNF complex, leading to inhibition of RUNX1-mediated differentiation of T-cells and megakaryocytes. SHP2 (PTPN11) tyrosine phosphatase binds to RUNX1 and dephosphorylates it (Huang et al. 2012). Formation of the complex with CBFB is necessary for the transcriptional activity of RUNX1 (Wang et al. 1996). Binding of CCND3 and probably other two cyclin D family members, CCND1 and CCND2, to RUNX1 inhibits its association with CBFB (Peterson et al. 2005), while binding to CDK6 interferes with binding of RUNX1 to DNA without affecting formation of the RUNX1:CBFB complex. Binding of RUNX1 to PML plays a role in subnuclear targeting of RUNX1 (Nguyen et al. 2005). RUNX1 activity and protein levels vary during the cell cycle. RUNX1 protein levels increase from G1 to S and from S to G2 phases, with no increase in RUNX1 mRNA levels. CDK1-mediated phosphorylation of RUNX1 at the G2/M transition is implicated in reduction of RUNX1 transactivation potency and may promote RUNX1 protein degradation by the anaphase promoting complex (reviewed by Friedman 2009).
The RNA-induced silencing complex contains an Argonaute (AGO) protein, whose PAZ domain binds the 3' end of the miRNA. The PIWI domain of AGO is responsible for cleavage of target RNAs, that is, RNAs complementary to the miRNA. Only AGO2 (EIF2C2) is capable of cleavage, however. AGO1 (EIF2C1), AGO3 (EIF2C3), and AGO4 (EIF2C4) repress translation of target RNAs by binding without cleavage. In vivo, cleavage by AGO2 and repression of translation by all AGOs require interaction with a TNRC6 protein (GW182 protein) and MOV10. The interaction with TNRC6 proteins is also responsible for localizing the AGO complex to Processing Bodies (P-bodies). Tethering of the C-terminal domain of a TNRC6 protein to a mRNA is sufficient to cause repression of translation.
Protein arginine methyltransferase 1 (PRMT1) methylates arginine residues R206 and R210 of RUNX1. Methylation of R206 and R210 inhibits binding of co-repressors to RUNX1, thus enhancing RUNX1 transcriptional activity (Zhao et al. 2008). In mice, arginine methylation seems to be dispensable for the function of RUNX1 in definitive hematopoiesis and steady-state platelet production, but is needed for the maintenance of the peripheral population of CD4+ T cells (Mizutani et al. 2015).
RUNX1 forms a complex with protein arginine methyltransferase 1 (PRMT1) in a RNA- and DNA-independent manner. The interaction with PRMT1 involves the C-terminus of RUNX1. Since PRMT1 colocalizes with RUNX1 at RUNX1 target promoters, RUNX1 is shown as part of the RUNX1:CBFB complex (Zhao et al. 2008).
The transcriptional co-repressor SIN3A (and possibly SIN3B) can bind to the RUNX1:CBFB complex at the promoter of the ITGA2B (CD41) gene, encoding Integrin alpha IIb. Binding of SIN3A (and probably SIN3B) to RUNX1 is inhibited by PRMT1-mediated arginine methylation of RUNX1 arginine residues R206 and R210 (Zhao et al. 2008). In addition to SIN3A, the RUNX1-containing transcriptional repressor complex at the ITGA2B promoter also includes histone arginine methyltransferase PRMT6 and histone deacetylase HDAC1 (Herglotz et al. 2013). Dimethylation of histone H3 on lysine residue K4 (K5 when taking into account the initiator methionine), known as the H3K4me2 mark, is characteristic of nucleosomes associated with megakaryocyte specific promoters, including the ITGA2B gene, prior to the onset of differentiation (Herglotz et al. 2013).
The RUNX1:CBFB complex binds the promoter of the ITGA2B (CD41) gene, encoding Integrin alpha IIb, and stimulates ITGFA2B transcription. Transcription of ITGA2B is significantly upregulated by PRMT1-dependent arginine methylation of RUNX1, which interferes with the recruitment of the SIN3A (or, possibly, SIN3B) co-repressor (Zhao et al. 2008). The transcription activator complex at the ITGA2B promoter includes the RUNX1:CBFB complex, PRMT1, the GATA1:ZFPM1 complex, histone acetyltransferases p300 (EP300) and PCAF (KAT2B), and the WDR5-containing histone methyltransferase MLL complex. The MLL complex produces the activating H3K4me3 mark on nucleosomes associated with the ITGA2B gene promoter (Herglotz et al. 2013). The transcription repressor complex at the ITGA2B promoter is formed when the SIN3A (or possibly SIN3B) co-repressor binds to the RUNX1:CBFB complex along with histone arginine methyltransferase PRMT6 and histone deacetylase HDAC1. Histone H3 arginine methylation by PRMT6 interferes with methylation of H3K4me2 to generate the activating H3K4me3 mark at the ITGA2B gene promoter, thus contributing to transcriptional repression (Herglotz et al. 2013). ITGA2B, involved in platelet aggregation, is only expressed in maturing megakaryocytes and platelets and is a model gene for megakaryocyte specific expression (Block and Poncz 1995, Jackson 2007).
The RUNX1:CBFB complex can bind to transcriptional co-repressors SIN3A and SIN3B. The interaction with SIN3A has been studied in more detail. Binding to SIN3A leads to transcriptional repression of RUNX1 target genes, which may involve SIN3A-mediated recruitment of histone deacetylases (HDACs) to target promoters (Lutterbach et al. 2000). Arginine methylation of RUNX1 by PRMT1 inhibits association of RUNX1 with SIN3A (Zhao et al. 2008). RUNX1 transcriptional repressor complex with SIN3A also includes histone arginine methyltransferase PRMT6 and HDAC1 (Herglotz et al. 2013).
The RUNX1:CBFB complex can bind to the promoter of the ITGA2B (CD41) gene, encoding Integrin alpha-IIb, both in the absence and in the presence of PRMT1. PRMT1-mediated arginine-methylation significantly increases transcriptional activity of the RUNX1:CBFB complex at the ITGA2B promoter (Zhao et al. 2008). In addition to the RUNX1:CBFB complex, the complex of GATA1 and ZFPM1 (FOG1) (Freson et al. 2003) is also recruited to the ITGA2B promoter (Herglotz et al. 2013), likely through the interaction between GATA1 and RUNX1 (Elagib et al. 2003). The zinc finger domain of GATA1 is involved in binding to RUNX1 (Xu et al. 2006). Along with RUNX1 and GATA1, histone acetyltransferases p300 (EP300) and PCAF (KAT2B), as well as the WDR5-containing histone methyltransferase MLL complexes are also recruited to the ITGA2B promoter (Herglotz et al. 2013). Dimethylation of histone H3 on lysine residue K4 (K5 when taking into account the initiator methionine), known as the H3K4me2 mark, is characteristic of nucleosomes associated with megakaryocyte specific promoters, including the ITGA2B gene, prior to the onset of differentiation (Herglotz et al. 2013).
The WDR5-containing histone methyltransferase MLL complex, recruited to the ITGA2B promoter via RUNX1 (and possibly GATA1), methylates histone H3 on dimethylated lysine residue K4 (K5 when taking into account the initiator methionine), producing the H3K4me3 mark. The H3K4me3 mark is characteristic of nucleosomes associated with transcriptionally active promoters of megakaryocyte-specific genes (Herglotz et al. 2013).
The histone arginine methyltransferase PRMT6 asymmetrically dimethylates histone H3 on arginine residue R2 (R3 when taking into account the initiator methionine), thus creating the H3R2me2a mark on nucleosomes at the ITGA2B gene promoter. Histone H3 arginine methylation by PRMT6 interferes with methylation of H3K4me2 to generate the activating H3K4me3 mark at the ITGA2B gene promoter, thus contributing to transcriptional repression (Herglotz et al. 2013).
The transcriptional co-repressor SIN3A (and possibly SIN3B) can bind to the RUNX1:CBFB complex at the promoter of the GP1BA (CD42b) gene, encoding Platelet glycoprotein Ib alpha chain. Binding of SIN3A (and probably SIN3B) to RUNX1 is inhibited by PRMT1-mediated arginine methylation of RUNX1 arginine residues R206 and R210 (Zhao et al. 2008). In addition to SIN3A, the RUNX1-containing transcriptional repressor complex at the GP1BA promoter also includes histone arginine methyltransferase PRMT6 and histone deacetylase HDAC1 (Herglotz et al. 2013). Dimethylation of histone H3 on lysine residue K4 (K5 when taking into account the initiator methionine), known as the H3K4me2 mark, is characteristic of nucleosomes associated with megakaryocyte specific promoters, including the GP1BA gene, prior to the onset of differentiation (Herglotz et al. 2013).
The histone arginine methyltransferase PRMT6 asymmetrically dimethylates histone H3 on arginine residue R2 (R3 when taking into account the initiator methionine), thus creating the H3R2me2a mark on nucleosomes at the GP1BA gene promoter. Histone H3 arginine methylation by PRMT6 interferes with methylation of H3K4me2 to generate the activating H3K4me3 mark at the GP1BA gene promoter, thus contributing to transcriptional repression (Herglotz et al. 2013).
The RUNX1:CBFB complex can bind to the promoter of the GP1BA (CD42b) gene, encoding Platelet glycoprotein Ib alpha chain. Based on the analogy with the ITGA2B gene transcription (Zhao et al. 2008), the PRMT1-mediated arginine-methylation increases transcriptional activity of the RUNX1:CBFB complex at the GP1BA promoter (Herglotz et al. 2013). In addition to the RUNX1:CBFB complex, the complex of GATA1 and ZFPM1 (FOG1) (Freson et al. 2003) is also recruited to the GP1BA promoter (Herglotz et al. 2013), likely through the interaction between GATA1 and RUNX1 (Elagib et al. 2003). The zinc finger domain of GATA1 is involved in binding to RUNX1 (Xu et al. 2006). Along with RUNX1 and GATA1, histone acetyltransferases p300 (EP300) and PCAF (KAT2B), as well as the WDR5-containing histone methyltransferase MLL complex are also recruited to the GP1BA promoter. Dimethylation of histone H3 on lysine residue K4 (K5 when taking into account the initiator methionine), known as the H3K4me2 mark, is characteristic of nucleosomes associated with megakaryocyte specific promoters, including the GP1BA gene, prior to the onset of differentiation (Herglotz et al. 2013).
The WDR5-containing histone methyltransferase MLL complex, recruited to the GP1BA promoter via RUNX1 (and possibly GATA1), methylates histone H3 on dimethylated lysine residue K4 (K5 when taking into account the initiator methionine), producing the H3K4me3 mark. The H3K4me3 mark is characteristic of nucleosome associated with transcriptionally active promoters of megakaryocyte-specific genes (Herglotz et al. 2013).
The RUNX1:CBFB complex binds the promoter of the GP1BA (CD42b) gene, encoding Platelet glycoprotein Ib alpha chain, and stimulates GP1BA transcription. Based on analogy with the ITGA2B gene transcription, transcription of GP1BA is significantly upregulated by PRMT1-dependent arginine methylation of RUNX1, which interferes with the recruitment of the SIN3A (or, possibly, SIN3B) co-repressor (Zhao et al. 2008). The transcription activator complex at the GP1BA gene promoter includes the RUNX1:CBFB complex, PRMT1, the GATA1:ZFPM1 complex, histone acetyltransferases p300 (EP300) and PCAF (KAT2B), and the WDR5-containing histone methyltransferase MLL complex. The MLL complex produces the activating H3K4me3 mark on nucleosomes associated with the GP1BA gene promoter (Herglotz et al. 2013). The transcription repressor complex at the GP1BA promoter is formed when the SIN3A (or possibly SIN3B) co-repressor binds to the RUNX1:CBFB complex along with histone arginine methyltransferase PRMT6 and histone deacetylase HDAC1. Histone H3 arginine methylation by PRMT6 interferes with methylation of H3K4me2 to generate the activating H3K4me3 mark at the GP1BA gene promoter, thus contributing to transcriptional repression (Herglotz et al. 2013). Platelet glycoprotein Ib (GP-Ib) alpha chain, encoded by the GP1BA gene, is expressed at the cell surface membrane of platelets and participates in the formation of platelet plugs (Cauwenberghs et al. 2000, Jilma-Stohlawetz et al. 2003). Gp-Ib protein is first detected on the plasma membrane of maturing megakaryocytes (Debili et al. 1990).
The RUNX1:CBFB complex can bind to the promoter of the THBS1 (TSP-1) gene, encoding Thrombospondin-1. Based on the analogy with the ITGA2B gene transcription (Zhao et al. 2008), the PRMT1-mediated arginine-methylation increases transcriptional activity of the RUNX1:CBFB complex at the THBS1 promoter (Herglotz et al. 2013). In addition to the RUNX1:CBFB complex, the complex of GATA1 and ZFPM1 (FOG1) (Freson et al. 2003) is also recruited to the THBS1 promoter (Herglotz et al. 2013), likely through the interaction between GATA1 and RUNX1 (Elagib et al. 2003). The zinc finger domain of GATA1 is involved in binding to RUNX1 (Xu et al. 2006). Along with RUNX1 and GATA1, histone acetyltransferases p300 (EP300) and PCAF (KAT2B), as well as the WDR5-containing histone methyltransferase MLL complex are also recruited to the THBS1 promoter. Dimethylation of histone H3 on lysine residue K4 (K5 when taking into account the initiator methionine), known as the H3K4me2 mark, is characteristic of nucleosomes associated with megakaryocyte promoters prior to the onset of differentiation (Herglotz et al. 2013) and is assumed to be present at the THBS1 promoter.
The transcriptional co-repressor SIN3A (and possibly SIN3B) can bind to the RUNX1:CBFB complex at the promoter of the THBS1 (TSP-1) gene, encoding Thrombospondin-1. Binding of SIN3A (and probably SIN3B) to RUNX1 is inhibited by PRMT1-mediated arginine methylation of RUNX1 arginine residues R206 and R210 (Zhao et al. 2008). In addition to SIN3A, the RUNX1-containing transcriptional repressor complex at the THBS1 promoter also includes histone arginine methyltransferase PRMT6 and histone deacetylase HDAC1 (Herglotz et al. 2013). Dimethylation of histone H3 on lysine residue K4 (K5 when taking into account the initiator methionine), known as the H3K4me2 mark, is characteristic of nucleosomes associated with megakaryocyte promoters prior to the onset of differentiation (Herglotz et al. 2013), and based on epigenetic modifications that affect transactivation of the THBS1 gene (Michaud-Levesque and Richard 2009), the H3K4me2 mark is assumed to be present at the inactive THBS1 promoter.
The RUNX1:CBFB complex binds the promoter of the THBS1 (TSP-1) gene, encoding Thrombospondin-1, and stimulates THBS1 transcription. Based on the analogy with the ITGA2B gene transcription, transcription of THBS1 is significantly upregulated by PRMT1-dependent arginine methylation of RUNX1, which interferes with the recruitment of the SIN3A (or, possibly, SIN3B) co-repressor (Zhao et al. 2008). The transcription activator complex at the THBS1 gene promoter includes the RUNX1:CBFB complex, PRMT1, the GATA1:ZFPM1 complex, histone acetyltransferases p300 (EP300) and PCAF (KAT2B), and the WDR5-containing histone methyltransferase MLL complex. The MLL complex produces the activating H3K4me3 mark on nucleosomes associated with RUNX1-regulated megakaryocyte promoters (Herglotz et al. 2013). The presence of the H3K4me3 mark is characteristic of the activated THBS1 promoter (Michaud-Levesque and Richard 2009). The transcription repressor complex at the THBS1 promoter is formed when SIN3A (or possibly SIN3B) co-repressor binds to the RUNX1:CBFB complex along with histone arginine methyltransferase PRMT6 and histone deacetylase HDAC1. Histone H3 arginine methylation by PRMT6 interferes with methylation of H3K4me2 to generate the activating H3K4me3 mark at RUNX1-regulated megakaryocyte promoters (Herglotz et al. 2013), including THBS1 promoter (Michaud-Levesque and Richard 2009). Thrombospondin-1, encoded by the THBS1 gene, forms homotrimers which can be detected in many different cell types and are very abundant in platelet alpha granules. While THBS1 is not necessary for platelet aggregation, it contributes to stabilization of the platelet aggregate (Bonnefoy and Hoylaerts 2008).
The WDR5-containing histone methyltransferase MLL complex, recruited to the THBS1 (TSP-1) promoter via RUNX1 (and possibly GATA1), is assumed to methylate histone H3 on dimethylated lysine residue K4 (K5 when taking into account the initiator methionine), producing the H3K4me3 mark. The H3K4me3 mark is characteristic of nucleosome associated with transcriptionally active promoters of megakaryocyte-specific genes (Herglotz et al. 2013) and the appearance of the H3K4me3 mark at the THBS1 promoter coincides with THBS1 transactivation (Michaud-Levesque and Richard 2009).
The histone arginine methyltransferase PRMT6 asymmetrically dimethylates histone H3 on arginine residue R2 (R3 when taking into account the initiator methionine), thus creating the H3R2me2a mark on nucleosomes at the THBS1 gene promoter. Histone H3 arginine methylation by PRMT6 interferes with generation of the activating H3K4me3 mark at the THBS1 gene promoter, thus contributing to transcriptional repression (Michaud-Levesque and Richard 2009).
The RUNX1:CBFB complex can bind to the promoter of the MIR27A gene, encoding microRNA miR-27A (Ben-Ami et al. 2009). Based on analogy with the ITGA2B gene transcription (Zhao et al. 2008), the PRMT1-mediated arginine-methylation increases transcriptional activity of the RUNX1:CBFB complex at the MIR27A gene promoter (Herglotz et al. 2013). In addition to the RUNX1:CBFB complex, the complex of GATA1 and ZFPM1 (FOG1) (Freson et al. 2003) is also recruited to the MIR27A promoter (Herglotz et al. 2013), likely through the interaction between GATA1 and RUNX1 (Elagib et al. 2003). The zinc finger domain of GATA1 is involved in binding to RUNX1 (Xu et al. 2006). Along with RUNX1 and GATA1, histone acetyltransferases p300 (EP300) and PCAF (KAT2B), as well as the WDR5-containing histone methyltransferase MLL complex are also recruited to the MIR27A promoter. Dimethylation of histone H3 on lysine residue K4 (K5 when taking into account the initiator methionine), known as the H3K4me2 mark, is characteristic of nucleosomes associated with megakaryocyte specific promoters, including the MIR27A gene, prior to the onset of differentiation (Herglotz et al. 2013).
The WDR5-containing histone methyltransferase MLL complex, recruited to the MIR27A promoter via RUNX1 (and possibly GATA1), methylates histone H3 on dimethylated lysine residue K4 (K5 when taking into account the initiator methionine), producing the H3K4me3 mark. The H3K4me3 mark is characteristic of nucleosome associated with transcriptionally active promoters of megakaryocyte-specific genes (Herglotz et al. 2013).
The RUNX1:CBFB complex binds the promoter of the MIR27A gene, encoding microRNA miR-27a, and stimulates MIR27A transcription. Based on the analogy with the ITGA2B gene transcription, transcription of MIR27A is significantly upregulated by PRMT1-dependent arginine methylation of RUNX1, which interferes with the recruitment of the SIN3A (or, possibly, SIN3B) co-repressor (Zhao et al. 2008). The transcription activator complex at the MIR27A gene promoter includes the RUNX1:CBFB complex, PRMT1, the GATA1:ZFPM1 complex, histone acetyltransferases p300 (EP300) and PCAF (KAT2B), and the WDR5-containing histone methyltransferase MLL complex. The MLL complex produces the activating H3K4me3 mark on nucleosomes associated with the MIR27A gene promoter (Herglotz et al. 2013). The transcription repressor complex at the MIR27A promoter is formed when SIN3A (or possibly SIN3B) co-repressor binds to the RUNX1:CBFB complex along with histone arginine methyltransferase PRMT6 and histone deacetylase HDAC1. Histone H3 arginine methylation by PRMT6 interferes with methylation of H3K4me2 to generate the activating H3K4me3 mark at the MIR27A gene promoter, thus contributing to transcriptional repression (Herglotz et al. 2013). MicroRNA miR-27a binds the 3'UTR of RUNX1 mRNA and inhibits RUNX1 mRNA translation without affecting RUNX1 mRNA stability. RUNX1 and MIR27A thus constitute a negative feedback loop that regulates megakaryocytic differentiation and may be involved in erythroid/megakaryocytic lineage determination (Ben-Ami et al. 2009).
The histone arginine methyltransferase PRMT6 asymmetrically dimethylates histone H3 on arginine residue R2 (R3 when taking into account the initiator methionine), thus creating the H3R2me2a mark on nucleosomes at the MIR27A gene promoter. Histone H3 arginine methylation by PRMT6 interferes with generation of the activating H3K4me3 mark at the MIR27A gene promoter, thus contributing to transcriptional repression (Herglotz et al. 2013).
The transcriptional co-repressor SIN3A (and possibly SIN3B) can bind to the RUNX1:CBFB complex at the promoter of the MIR27A gene, encoding microRNA miR-27a (Ben-Ami et al. 2009, Herglotz et al. 2013). Binding of SIN3A (and probably SIN3B) to RUNX1 is inhibited by PRMT1-mediated arginine methylation of RUNX1 arginine residues R206 and R210 (Zhao et al. 2008). In addition to SIN3A, the RUNX1-containing transcriptional repressor complex at the MIR27A promoter also includes histone arginine methyltransferase PRMT6 and histone deacetylase HDAC1 (Herglotz et al. 2013). Dimethylation of histone H3 on lysine residue K4 (K5 when taking into account the initiator methionine), known as the H3K4me2 mark, is characteristic of nucleosomes associated with megakaryocyte specific promoters, including the MIR27A gene, prior to the onset of differentiation (Herglotz et al. 2013).
Binding of the RUNX1:CBFB complex to the NR4A3 gene promoter stimulates NR4A3 gene transcription, leading to reduction in the clonogenic potential of hematopoietic progenitors. RUNX1 mutants associated with familial platelet disorders (FPD) and acute myeloid leukemia (AML) are unable to transactivate the NR4A3 gene (Bluteau et al. 2011).
Binding of the RUNX1:CBFB complex to the promoter of the PRKCQ gene, encoding Protein kinase C theta type, stimulates PRKCQ transcription. RUNX1 mutants associated with inherited thrombocytopenia are unable to transactivate the PRKCQ gene. PRKCQ is important for the functioning of megakaryocytes and platelets, but is not megakaryocyte specific (Jalagadugula et al. 2011).
Binding of the RUNX1:CBFB complex to the promoter of the PF4 gene stimulates transcription of PF4. The PF4 gene encodes Platelet factor 4, a protein stored in platelet alpha granules. Deficiency of alpha granule proteins, including PF4, is the cause of gray platelet syndrome. PF4 deficiency can be caused by RUNX1 haploinsuficiency (Aneja et al. 2011).
The RUNX1:CBFB complex binds to four RUNX1 response elements in the promoter of the MYL9 gene, encoding Myosin regulatory light polypeptide 9, which functions as the regulatory subunit of the myosin complex (Jalagadugula et al. 2010).
Binding of the RUNX1:CBFB complex to the promoter of the MYL9 gene stimulates MYL9 transcription. All four RUNX1 response elements in the MYL9 promoter contribute to transactivation of the MYL9 gene. The MYL9 gene encodes Myosin regulatory light polypeptide, which functions as a regulatory subunit of the myosin complex. Myosin plays an important role in platelet activation and thrombopoiesis. RUNX1 haploinsuficiency is associated with decreased MYL9 expression and myosin light chain phosphorylation, which likely contributes to thrombocytopenia and platelet dysfunction (Jalagadugula et al. 2010).
The RUNX1:CBFB complex binds RUNX1 response elements in the promoter of the NFE2 gene, encoding Transcription factor NF-E2 45 kDa subunit (Wang et al. 2010).
Binding of the RUNX1:CBFB complex to the promoter of the NFE2 gene stimulates NFE2 transcription. The NFE2 gene encodes the Transcription factor NF-E2 45 kDa subunit. The NF-E2 transcription factor regulates erythroid and megakaryocytic maturation and differentiation and is overexpressed in myeloproliferative neoplasms (Wang et al. 2010).
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Expression and
ActivityRUNX1 mRNAs transcribed from alternative promoters differ in their 5'UTRs and splicing isoforms of RUNX1 have also been described. The function of alternative splice isoforms and alternative 5'UTRs has not been fully elucidated (Challen and Goodell 2010, Komeno et al. 2014).
During zebrafish hematopoiesis, RUNX1 expression increases in response to NOTCH signaling, but direct transcriptional regulation of RUNX1 by NOTCH has not been demonstrated (Burns et al. 2005). RUNX1 transcription also increases in response to WNT signaling. BothTCF7 and TCF4 bind the RUNX1 promoter (Wu et al. 2012, Hoverter et al. 2012), and RUNX1 transcription driven by the TCF binding element (TBE) in response to WNT3A treatment is inhibited by the dominant-negative mutant of TCF4 (Medina et al. 2016). In developing mouse ovary, Runx1 expression is positively regulated by Wnt4 signaling (Naillat et al. 2015).
Studies in mouse hematopoietic stem and progenitor cells imply that RUNX1 may be a direct transcriptional target of HOXB4 (Oshima et al. 2011).
Conserved cis-regulatory elements were recently identified in intron 5 of RUNX1. The RUNX1 breakpoints observed in acute myeloid leukemia (AML) with translocation (8;21), which result in expression of a fusion RUNX1-ETO protein, cluster in intron 5, in proximity to these not yet fully characterized cis regulatory elements (Rebolledo-Jaramillo et al. 2014).
At the level of translation, RUNX1 expression is regulated by various microRNAs which bind to the 3'UTR of RUNX1 mRNA and inhibit its translation through endonucleolytic and/or nonendonucleolytic mechanisms. MicroRNAs that target RUNX1 include miR-378 (Browne et al. 2016), miR-302b (Ge et al. 2014), miR-18a (Miao et al. 2015), miR-675 (Zhuang et al. 2014), miR-27a (Ben-Ami et al. 2009), miR-17, miR-20a, miR106 (Fontana et al. 2007) and miR-215 (Li et al. 2016).
At the posttranslational level, RUNX1 activity is regulated by postranslational modifications and binding to co-factors. SRC family kinases phosphorylate RUNX1 on multiple tyrosine residues in the negative regulatory domain, involved in autoinhibition of RUNX1. RUNX1 tyrosine phosphorylation correlates with reduced binding of RUNX1 to GATA1 and increased binding of RUNX1 to the SWI/SNF complex, leading to inhibition of RUNX1-mediated differentiation of T-cells and megakaryocytes. SHP2 (PTPN11) tyrosine phosphatase binds to RUNX1 and dephosphorylates it (Huang et al. 2012).
Formation of the complex with CBFB is necessary for the transcriptional activity of RUNX1 (Wang et al. 1996). Binding of CCND3 and probably other two cyclin D family members, CCND1 and CCND2, to RUNX1 inhibits its association with CBFB (Peterson et al. 2005), while binding to CDK6 interferes with binding of RUNX1 to DNA without affecting formation of the RUNX1:CBFB complex. Binding of RUNX1 to PML plays a role in subnuclear targeting of RUNX1 (Nguyen et al. 2005).
RUNX1 activity and protein levels vary during the cell cycle. RUNX1 protein levels increase from G1 to S and from S to G2 phases, with no increase in RUNX1 mRNA levels. CDK1-mediated phosphorylation of RUNX1 at the G2/M transition is implicated in reduction of RUNX1 transactivation potency and may promote RUNX1 protein degradation by the anaphase promoting complex (reviewed by Friedman 2009).
Nonendonucleolytic
RISCAnnotated Interactions
The transcription repressor complex at the ITGA2B promoter is formed when the SIN3A (or possibly SIN3B) co-repressor binds to the RUNX1:CBFB complex along with histone arginine methyltransferase PRMT6 and histone deacetylase HDAC1. Histone H3 arginine methylation by PRMT6 interferes with methylation of H3K4me2 to generate the activating H3K4me3 mark at the ITGA2B gene promoter, thus contributing to transcriptional repression (Herglotz et al. 2013).
ITGA2B, involved in platelet aggregation, is only expressed in maturing megakaryocytes and platelets and is a model gene for megakaryocyte specific expression (Block and Poncz 1995, Jackson 2007).
The transcription repressor complex at the GP1BA promoter is formed when the SIN3A (or possibly SIN3B) co-repressor binds to the RUNX1:CBFB complex along with histone arginine methyltransferase PRMT6 and histone deacetylase HDAC1. Histone H3 arginine methylation by PRMT6 interferes with methylation of H3K4me2 to generate the activating H3K4me3 mark at the GP1BA gene promoter, thus contributing to transcriptional repression (Herglotz et al. 2013).
Platelet glycoprotein Ib (GP-Ib) alpha chain, encoded by the GP1BA gene, is expressed at the cell surface membrane of platelets and participates in the formation of platelet plugs (Cauwenberghs et al. 2000, Jilma-Stohlawetz et al. 2003). Gp-Ib protein is first detected on the plasma membrane of maturing megakaryocytes (Debili et al. 1990).
The transcription repressor complex at the THBS1 promoter is formed when SIN3A (or possibly SIN3B) co-repressor binds to the RUNX1:CBFB complex along with histone arginine methyltransferase PRMT6 and histone deacetylase HDAC1. Histone H3 arginine methylation by PRMT6 interferes with methylation of H3K4me2 to generate the activating H3K4me3 mark at RUNX1-regulated megakaryocyte promoters (Herglotz et al. 2013), including THBS1 promoter (Michaud-Levesque and Richard 2009).
Thrombospondin-1, encoded by the THBS1 gene, forms homotrimers which can be detected in many different cell types and are very abundant in platelet alpha granules. While THBS1 is not necessary for platelet aggregation, it contributes to stabilization of the platelet aggregate (Bonnefoy and Hoylaerts 2008).
The transcription repressor complex at the MIR27A promoter is formed when SIN3A (or possibly SIN3B) co-repressor binds to the RUNX1:CBFB complex along with histone arginine methyltransferase PRMT6 and histone deacetylase HDAC1. Histone H3 arginine methylation by PRMT6 interferes with methylation of H3K4me2 to generate the activating H3K4me3 mark at the MIR27A gene promoter, thus contributing to transcriptional repression (Herglotz et al. 2013).
MicroRNA miR-27a binds the 3'UTR of RUNX1 mRNA and inhibits RUNX1 mRNA translation without affecting RUNX1 mRNA stability. RUNX1 and MIR27A thus constitute a negative feedback loop that regulates megakaryocytic differentiation and may be involved in erythroid/megakaryocytic lineage determination (Ben-Ami et al. 2009).
Nonendonucleolytic
RISC