Project description:Using the estrogen receptor alpha (ERalpha) as a model ligand inducible transcription factor, we sought to explicitly define parameters that determine transcription factor binding site selection on a genomic scale in an inducible system that minimizes confounding chromatin effects by the transcription factor itself. By examining several genetic and epigenetic parameters, we find that an energetically favorable estrogen response element (ERE) motif sequence, evidence of occupancy of a "pioneering" transcription factor FOXA1, the presence of the enhancer mark, H3K4me1, and an open chromatin configuration (FAIRE) at the pre-ligand state provide specificity for ER binding. Genome-wide ChIP-sequencing was done in MCF-7 cancer cell line for the following histone H3 modifications: monomethylation H3K4me1, trimethylation H3K4me3, H3K9me3, H3K27me3, acetylation H3K9ac, H3K14ac. In addition sequencing of RNA Pol II was done at same treatment conditions (E2 and DMSO). In addition, we assessed the chromatin configuration of ERα binding sites by deeply sequencing fragments isolated by Formaldehyde-Assisted Isolation of Regulatory Elements (FAIRE) (Giresi et al, 2007) which enriches for nucleosome free genomic DNA in the aqueous phase of a phenol extraction. The analysis histone modifications in MCF-7 cancer cells was done by ChIP-seq data obtained either with E2 stimulation or without stimulation using vehicle as a control. Using the ERα binding sites defined by ChIP-seq (separate submission), we analyzed the population characteristics of the chromatin configuration of the ERα binding sites. To this end, we performed ChIP-seq analysis for the occupancy configuration of each of the following marks before and after E2 exposure: RNA Pol II, the activation marks H3K4me1, H3K4me3, H3K9ac and H3K14ac, and the repression marks H3K9me3 and H3K27me3. We assessed the chromatin configuration of ERα binding sites by deeply sequencing fragments isolated by Formaldehyde-Assisted Isolation of Regulatory Elements (FAIRE) (Giresi et al, 2007) which enriches for nucleosome free genomic DNA in the aqueous phase of a phenol extraction. The tag count of FAIRE fragments reflects the nucleosome depletion at any given site. RNA Pol II - Cat# ab5408, Abcam; H3K9me3 - Cat# ab8898, Abcam; H3K27me3 - Cat# 07-449, Upstate Biotechnology Inc.; H3K4me1 - Cat# ab8895, Abcam; H3K4me3 - Cat# ab8580, Abcam; H3K9ac - Cat# 07-352, Upstate Biotechnology Inc.; H3K14ac - Cat# 07-353, Upstate Biotechnology Inc.

Project description:FOXA1 is a pioneer factor that is important in hormone dependent cancer cells to stabilise nuclear receptors, such as estrogen receptor (ER) to chromatin. FOXA1 binds to enhancers regions that are enriched in H3K4mono- and dimethylation (H3K4me1, H3K4me2) histone marks and evidence suggests that these marks are requisite events for FOXA1 to associate with enhancers to initate subsequent gene expression events. However, exogenous expression of FOXA1 has been shown to induce H3K4me1 and H3K4me2 signal at enhancer elements and the order of events and the functional importance of these events is not clear. We performed a FOXA1 Rapid Immunoprecipitation Mass Spectrometry of Endogenous Proteins (RIME) screen in ERα-positive MCF-7 breast cancer cells in order to identify FOXA1 interacting partners and we found histone-lysine N-methyltransferase (MLL3) as the top FOXA1 interacting protein. MLL3 is typically thought to induce H3K4me3 at promoter regions, but recent findings suggest it may contribute to H3K4me1 deposition, in line with our observation that MLL3 associates with an enhancer specific protein. We performed MLL3 ChIP-seq in breast cancer cells and unexpectedly found that MLL3 binds mostly at non-promoter regions enhancers, in contrast to the prevailing hypothesis. MLL3 was shown to occupy regions marked by FOXA1 occupancy and as expected, H3K4me1 and H3K4me2. MLL3 binding was dependent on FOXA1, indicating that FOXA1 recruits MLL3 to chromatin. Motif analysis and subsequent genomic mapping revealed a role for Grainy head like protein-2 (GRHL2) which was shown to co-occupy regions of the chromatin with MLL3. Regions occupied by all three factors, namely FOXA1, MLL3 and GRHL2, were most enriched in H3K4me1. MLL3 silencing decreased H3K4me1 at enhancer elements, but had no appreciable impact on H3K4me3 at enhancer elements. We identify a complex relationship between FOXA1, MLL3 and H3K4me1 at enhancers in breast cancer and propose a mechanism whereby the pioneer factor FOXA1 can interact with a chromatin modifier MLL3, recruiting it to chromatin to facilitate the deposition of H3K4me1 histone marks, subsequently demarcating active enhancer elements.

Project description:These tracks display a synthesis of evidence from different assays as part of the four Open Chromatin track sets. This track displays open chromatin regions and/or transcription factor binding sites identified in multiple cell types by one or more complementary methodologies, DNaseI hypersensitivity (HS) (Duke DNaseI HS), Formaldehyde-Assisted Isolation of Regulatory Elements (FAIRE) (UNC FAIRE), and chromatin immunoprecipitation (ChIP) for select regulatory factors (UTA TFBS). Each methodology was performed on the same cell type using identical growth conditions. (Note: Data for some or all ChIP experiments may not be available for all cell types). Regions that overlap between methodologies identify regulatory elements that are cross-validated indicating high confidence regions. In addition, multiple lines of evidence suggest that regions detected by a single assay (e.g., DNase-only or FAIRE-only) are also biologically relevant (Song et al., submitted). DNaseI HS data: DNaseI is an enzyme that has long been used to map general chromatin accessibility, and DNaseI "hypersensitivity" is a feature of active cis-regulatory sequences.The use of this method has led to the discovery of functional regulatory elements that include promoters, enhancers, silencers, insulators, locus control regions, and novel elements. DNaseI hypersensitivity signifies chromatin accessibility following binding of trans-acting factors in place of a canonical nucleosome. FAIRE data: FAIRE (Formaldehyde Assisted Isolation of Regulatory Elements) is a method to isolate and identify nucleosome-depleted regions of the genome. FAIRE was initially discovered in yeast and subsequently shown to identify active regulatory elements in human cells (Giresi et al., 2007). Similar to DNaseI HS, FAIRE appears to identify functional regulatory elements that include promoters, enhancers, silencers, insulators, locus control regions and novel elements. ChIP data: ChIP (Chromatin Immunoprecipitation) is a method to identify the specific location of proteins that are directly or indirectly bound to genomic DNA. By identifying the binding location of sequence-specific transcription factors, general transcription machinery components, and chromatin factors, ChIP can help in the functional annotation of the open chromatin regions identified by DNaseI HS mapping and FAIRE. Input data: As a background control experiment, we sequenced the input genomic DNA sample that was used for ChIP. Crosslinked chromatin is sheared and the crosslinks are reversed without carrying out the immunoprecipitation step. This sample is otherwise processed in a manner identical to the ChIP sample as described below. The input track is useful in revealing potential artifacts arising from the sequence alignment process such as copy number differences between the reference genome and the sequenced samples, as well as regions of poor sequence alignability. For data usage terms and conditions, please refer to http://www.genome.gov/27528022 and http://www.genome.gov/Pages/Research/ENCODE/ENCODEDataReleasePolicyFinal2008.pdf For each site, the maximum F-Seq Density Signal value has been calculated for each assay that was performed in that cell type. F-Seq employs Parzen kernel density estimation to create base pair scores (Boyle et al., 2008b). Significant regions, or peaks, were determined by fitting the data to a gamma distribution to calculate p-values. Contiguous regions where p-values were below a 0.05 (DNaseI HS, ChIP) or 0.1 (FAIRE) threshold were considered significant. See assay specific description pages ( Duke DNaseI HS, UNC FAIRE and UTA TFBS) for more details. A Fisher's Combined P-value for DNaseI HS and FAIRE was calculated using Fisher's combined probability test. First, a test statistic is calculated using the formula x^2 = -2*sum(ln(pi)) where pi are the p-values calculated for DNaseI HS and FAIRE. X2 follows a chi-squared distribution, thus a combined p-value can be assigned to this test statistic.

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Integrative model of genomic factors for determining binding site selection by estrogen receptor alpha in MCF-7 cancer cells

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