Identification of new regulators of the three dimensional Polycomb organization by a microscopy-based genome-wide RNAi screen
ABSTRACT: Polycomb group (PcG) proteins dynamically define cellular identities through epigenetic repression of key developmental genes. PcG target gene repression can be stabilized through the interaction in the nucleus at PcG foci. Here, we report the results of a high-resolution microscopy genome-wide RNAi screen that identifies 129 genes that regulate the nuclear organization of Pc foci. Candidate genes include PcG components and chromatin factors, as well as many novel protein-modifying enzymes, including components of the SUMOylation pathway. In the absence of SUMO Pc foci coagulate into larger aggregates. Conversely, loss of function of the SUMO peptidase velo disperses Pc foci. Moreover, SUMO and velo colocalize with PcG proteins at PREs and Pc SUMOylation affects its chromatin targeting, suggesting that the dynamic regulation of Pc SUMOylation regulates PcG-mediated silencing by modulating the kinetics of Pc binding to chromatin as well as its ability to form Polycomb foci. ChIP-Seq mapping of Polycomb (PC), SUMO and Velo on Drosophila Melanogaster
Project description:Leukemias are characterized by bone marrow failure due to oncogenic mutations of hematopoietic stem cells (HSC) or blood precursor cells. HSC differentiation and self-renewal properties are tightly regulated by Polycomb group (PcG) proteins, a well-characterized family of transcriptional epigenetic regulators. PcG proteins form two canonical complexes: Polycomb repressive complex 1 (PRC1), and Polycomb repressive complex 2 (PRC2).CBX proteins link the activity of PRC1 with PRC2, serving as critical regulators of PcG-mediating activity. While the functional role of some CBX proteins in cancer has been largely explored, recent reports support the specific role of CBX2 in human tumors, thus it represent a promising new target for anti-cancer strategies. To date, chromodomain inhibitors have been identified for CBX7 , but no molecules inhibiting CBX2 have been described. Nevertheless, different chromatin-modulating drugs such as histone deacetylase inhibitors (HDACi) are reported to regulate CBX2 targets on chromatin, suggesting that HDACi might be used to indirectly modulate aberrant effects of CBX2 in cancer. We describe a novel SAHA-mediated mechanism of CBX2 post-translational regulation. We found that CBX2 undergoes SAHA-induced SUMO2/3 modification and that CBX2 SUMOylation promotes its ubiquitination and proteasome-dependent degradation. We also identified the specific molecular pathway and key players regulating CBX2 stability, demonstrating that CBX4 and RNF4 act as the E3 SUMO and E3 ubiquitin ligase, respectively. Additionally, CBX2-depleted leukemic cells display impaired proliferation, showing that CBX2 is required for leukemia cell clonogenicity. Our study provides the first evidence of a non-canonical SAHA-mediated anti-tumorigenic activity via CBX2 SUMOylation and degradation
Project description:The Polycomb group (PcG) and Trithorax group (TrxG) of proteins are required for stable and heritable maintenance of repressed and active gene expression states. Their antagonistic function on gene control, repression for PcG and activity for TrxG, is mediated by binding to chromatin and subsequent epigenetic modification of target loci. Despite our broad knowledge about composition and enzymatic activities of the protein complexes involved, our understanding still lacks important mechanistic detail and a comprehensive view on target genes. In this study, we use an extensive data set of ChIP-seq, RNA-seq, and genome-wide detection of transcription start sites (TSSs) to identify and analyze thousands of binding sites for the PcG proteins and Trithorax from a Drosophila S2 cell line. In addition to finding a preference for stalled promoter regions of annotated genes, we uncover many intergenic PcG-binding sites coinciding with non-annotated transcription start sites. Interestingly, this set includes previously unknown promoters for primary transcripts of microRNA genes, thereby expanding the scope of Polycomb control to non-coding RNAs essential for development, apoptosis and growth. Chromatin from S2 cells was immunoprecipitated using antibodies against Pc, Ph, Psc, Trx-C or H3K4me3. In parallel, we isolated RNA from S2 cells and generated global gene expression profiles by RNA-seq. We also surveyed the Drosophila genome for yet non-annotated transcription start sites (TSSs) using a newly adapted protocol for Illumina sequencing (termed 5’-MACE) with RNA isolated from S2 cells and embryos.
Project description:Polycomb group (PcG) proteins are critical epigenetic regulators of development. Here we compared genomic distribution of Polycomb (Pc) protein in cultured Drosophila cells lacking Su(z)12 protein to that of the wild type. Overall design: ChIP of Pc protein coupled to next-generation sequencing from wild-type and Su(z)12 - null cultured cells. Done in two replicates, with DNA from corresponding chromatin inputs sequenced as a control.
Project description:Polycomb group (PcG) proteins maintain transcriptional repression of developmentally important genes and have been implicated in cell proliferation and stem-cell self-renewal. We used a genome-wide approach to map binding patterns of PcG proteins (Pc, esc and Sce) in Drosophila Kc cells. We found that Pc associates with large genomic regions of up to ~150kb in size, hereafter referred to as “Pc-domains”. Sce and esc accompany Pc in most of these domains. PcG-bound chromatin is trimethylated at histone H3 lysine 27 and in general transcriptionally silent. Furthermore, PcG proteins preferentially bind to developmental genes. Many of these encode transcriptional regulators and key components of signal transduction pathways, including Wingless, Hedgehog, Notch and Delta. We also identify several new putative functions of PcG proteins, such as in steroid hormone biosynthesis. These results highlight the extensive involvement of PcG proteins in the coordination of development through the formation of large repressive chromatin domains. Keywords: DamID, Chromatin immunoprecipitation, ChIP-chip To study PcG binding profiles we used DamID, which is based on the ability of a chromatin protein fused to E.coli DNA adenine methyltransferase (Dam) to methylate the native binding site of the chromatin protein. Dam-fusion proteins are expressed at very low levels to avoid mistargeting. Subsequently, methylated DNA fragments are isolated, labeled and hybridized to a microarray. Methylated DNA fragments from cells transfected with Dam alone served as reference. Genomic binding sites of the protein can then be identified based on the targeted methylation pattern. For detailed background information on DamID, see: van Steensel, B., Delrow, J. & Henikoff, S. Chromatin profiling using targeted DNA adenine methyltransferase. Nat Genet 27, 304-8 (2001); van Steensel, B. & Henikoff, S. Identification of in vivo DNA targets of chromatin proteins using tethered dam methyltransferase. Nat Biotechnol 18, 424-8 (2000).
Project description:We made Polycomb (PC) and histone H3 lysine 27 trimethylation (H3K27me3) chromatin binding maps in central brain tissue from 3rd instar larvae, allowing us to make a direct comparison to our 4C data (http://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE23166). Our results demonstrate that our PC and H3K27me3 maps are highly similar, and fit with previously identified hallmarks of PcG-bound chromatin, namely: PC and H3K27me3 co-occur in the genome as large contiguous domains that largely repress transcription of the underlying genes, which encode important regulators of development. Keywords: Genome binding/occupancy profiling by genome tiling array DamID experiments for Polycomb were performed in Drosophila larval brain tissue. Samples were hybridized to 380k NimbleGen arrays with 300 bp probe spacing.
Project description:We have studied the linkage between the binding of Polycomb Group (PcG) proteins and the developmental regulation of gene expression using whole-genome mapping to identify sites bound by the PcG proteins, Polycomb (Pc) and Pleiohomeotic (Pho), in the Drosophila embryo and in a more restricted tissue, the imaginal discs of the third thoracic segment. Our data provide considerable support for the idea that Pho is a general component of the maintenance machinery since the majority of Pc targets are also associated with Pho binding. We find, in general, considerable developmental stability of Pc and Pho binding at target genes and observe that Pc/Pho binding can be associated with both expressed and inactive genes. In particular, at the Hox complexes both active and inactive genes have significant Pc and Pho binding; however, in comparison to inactive genes the active Hox genes show reduced and altered binding profiles. Keywords: ChIP-on-chip, tissue-specific binding Overall design: We used chromatin from whole embryos and larval T3 imaginal discs (haltere and third leg) to compare genomic binding of Pc and Pho between these tissues by ChIP-on-chip. Each chromatin source was represented with at least three biological replicates (in addition, Pc embryonic material and controls were hybridised to up to three different arrays), and enrichment profiles were generated by comparison of specific and control ChIP DNA samples. For Pc target analysis the specific reaction used chromatin from the Pc-GFP fly line immunopurified using anti-GFP, and the control reaction used wild type chromatin immunopurified using anti-GFP. For Pho analysis wild type chromatin was used with anti-Pho for the specific reaction and pre-immune antiserum for the control. Purified DNAs were then fragmented, TdT labeled and hybridized to the Affymetrix Drosophila genome Tiling Array 1.0 (reverse part number 520,054), as previously described (Manak et al. 2006).
Project description:Transient brain ischemia massively activates global SUMOylation. A large fraction of SUMO targets are nuclear proteins involved in gene expression. However, gene expression profile modulated by ischemia-induced SUMOylation has not been studied. Using a SUMO knockdown (SUMO-KD) mouse model and microarray technique, we investigated how SUMOylation modulated gene expression after brain ischemia. Overall design: Wild-type and SUMO-KD mice were subjected to transient forebrain ischemia or sham surgery. The hippocampal CA1 subfield samples collected at 3 hours reperfusion were analyzed using Affymetrix microarrays.
Project description:The locations of chromatin loops in Drosophila were determined by Hi-C (chemical cross-linking, restriction digestion, ligation, and high-throughput DNA sequencing). Whereas most loop boundaries or “anchors” are associated with CTFC protein in mammals, loop anchors in Drosophila were found most often in association with the polycomb group (PcG) protein Polycomb (Pc), a subunit of Polycomb Repressive Complex 1 (PRC1). Loops were frequently located within domains of PcG-repressed chromatin. Promoters located at PRC1 loop anchors regulate some of the most important developmental genes and are less likely to be expressed than those not at PRC1 loop anchors. Although DNA looping has most commonly been associated with enhancer-promoter communication, our results indicate that loops are also associated with gene repression. Overall design: Hi-C experiments where ligation is performed on beads (tethered) on embryonic Kc167 Drosophila cultured cells from two independent biological replicates.
Project description:Transient brain ischemia massively activates global SUMOylation. A large fraction of SUMO targets are nuclear proteins involved in gene expression. However, gene expression profile modulated by ischemia-induced SUMOylation has not been studied. Using a SUMO knockdown (SUMO-KD) mouse model and microarray technique, we investigated how SUMOylation modulated gene expression after brain ischemia. Wild-type and SUMO-KD mice were subjected to transient forebrain ischemia or sham surgery. The hippocampal CA1 subfield samples collected at 3 hours reperfusion were analyzed using Affymetrix microarrays.
Project description:Chromatin insulators shield promoters and chromatin domains from neighbouring enhancers or chromatin regions with opposing activities. Insulator binding proteins and their cofactors mediate the boundary function. In general, covalent modification of proteins by the Small Ubiqutin-like Modifier (SUMO) is an important mechanism to control the interaction of proteins within complexes. Here we addressed the impact of SUMO in respect to insulator function, chromatin binding of insulator factors and formation of insulator speckles. SUMOylation augments the enhancer blocking function of four different insulator sequences and increases the genome-wide binding of the insulator cofactor CP190. A model is discussed with enhanced chromatin binding of SUMOylated CP190 causing fusion of insulator speckles, which may allow for more efficient insulation. Overall design: ChIP-seq of FLAG-tagged SUMO, CTCF and CP190 before and after induction of FLAG-SUMO