Mll2 is required for H3K4 trimethylation on bivalent promoters in embryonic stem cells, whereas Mll1 is redundant.

Trimethylation of histone H3 lysine 4 (H3K4me3) at the promoters of actively transcribed genes is a universal epigenetic mark and a key product of Trithorax group action. Here, we show that Mll2, one of the six Set1/Trithorax-type H3K4 methyltransferases in mammals, is required for trimethylation of bivalent promoters in mouse embryonic stem cells. Mll2 is bound to bivalent promoters but also to most active promoters, which do not require Mll2 for H3K4me3 or mRNA expression. By contrast, the Set1 complex (Set1C) subunit Cxxc1 is primarily bound to active but not bivalent promoters. This indicates that bivalent promoters rely on Mll2 for H3K4me3 whereas active promoters have more than one bound H3K4 methyltransferase, including Set1C. Removal of Mll1, sister to Mll2, had almost no effect on any promoter unless Mll2 was also removed, indicating functional backup between these enzymes. Except for a subset, loss of H3K4me3 on bivalent promoters did not prevent responsiveness to retinoic acid, thereby arguing against a priming model for bivalency. In contrast, we propose that Mll2 is the pioneer trimethyltransferase for promoter definition in the naïve epigenome and that Polycomb group action on bivalent promoters blocks the premature establishment of active, Set1C-bound, promoters.

Quantitative interaction proteomics and genome-wide profiling of epigenetic histone marks and their readers.

Trimethyl-lysine (me3) modifications on histones are the most stable epigenetic marks and they control chromatin-mediated regulation of gene expression. Here, we determine proteins that bind these marks by high-accuracy, quantitative mass spectrometry. These chromatin "readers" are assigned to complexes by interaction proteomics of full-length BAC-GFP-tagged proteins. ChIP-Seq profiling identifies their genomic binding sites, revealing functional properties. Among the main findings, the human SAGA complex binds to H3K4me3 via a double Tudor-domain in the C terminus of Sgf29, and the PWWP domain is identified as a putative H3K36me3 binding motif. The ORC complex, including LRWD1, binds to the three most prominent transcriptional repressive lysine methylation sites. Our data reveal a highly adapted interplay between chromatin marks and their associated protein complexes. Reading specific trimethyl-lysine sites by specialized complexes appears to be a widespread mechanism to mediate gene expression.

High-resolution analysis of epigenetic changes associated with X inactivation.

Differentiation of female murine ES cells triggers silencing of one X chromosome through X-chromosome inactivation (XCI). Immunofluorescence studies showed that soon after Xist RNA coating the inactive X (Xi) undergoes many heterochromatic changes, including the acquisition of H3K27me3. However, the mechanisms that lead to the establishment of heterochromatin remain unclear. We first analyze chromatin changes by ChIP-chip, as well as RNA expression, around the X-inactivation center (Xic) in female and male ES cells, and their day 4 and 10 differentiated derivatives. A dynamic epigenetic landscape is observed within the Xic locus. Tsix repression is accompanied by deposition of H3K27me3 at its promoter during differentiation of both female and male cells. However, only in female cells does an active epigenetic landscape emerge at the Xist locus, concomitant with high Xist expression. Several regions within and around the Xic show unsuspected chromatin changes, and we define a series of unusual loci containing highly enriched H3K27me3. Genome-wide ChIP-seq analyses show a female-specific quantitative increase of H3K27me3 across the X chromosome as XCI proceeds in differentiating female ES cells. Using female ES cells with nonrandom XCI and polymorphic X chromosomes, we demonstrate that this increase is specific to the Xi by allele-specific SNP mapping of the ChIP-seq tags. H3K27me3 becomes evenly associated with the Xi in a chromosome-wide fashion. A selective and robust increase of H3K27me3 and concomitant decrease in H3K4me3 is observed over active genes. This indicates that deposition of H3K27me3 during XCI is tightly associated with the act of silencing of individual genes across the Xi.

Genome-wide profiling of PPARgamma:RXR and RNA polymerase II occupancy reveals temporal activation of distinct metabolic pathways and changes in RXR dimer composition during adipogenesis.

The nuclear receptor peroxisome proliferator-activated receptor gamma (PPARgamma) is a key regulator of adipocyte differentiation in vivo and ex vivo and has been shown to control the expression of several adipocyte-specific genes. In this study, we used chromatin immunoprecipitation combined with deep sequencing to generate genome-wide maps of PPARgamma and retinoid X receptor (RXR)-binding sites, and RNA polymerase II (RNAPII) occupancy at very high resolution throughout adipocyte differentiation of 3T3-L1 cells. We identify >5000 high-confidence shared PPARgamma:RXR-binding sites in adipocytes and show that during early stages of differentiation, many of these are preoccupied by non-PPARgamma RXR-heterodimers. Different temporal and compositional patterns of occupancy are observed. In addition, we detect co-occupancy with members of the C/EBP family. Analysis of RNAPII occupancy uncovers distinct clusters of similarly regulated genes of different biological processes. PPARgamma:RXR binding is associated with the majority of induced genes, and sites are particularly abundant in the vicinity of genes involved in lipid and glucose metabolism. Our analyses represent the first genome-wide map of PPARgamma:RXR target sites and changes in RNAPII occupancy throughout adipocyte differentiation and indicate that a hitherto unrecognized high number of adipocyte genes of distinctly regulated pathways are directly activated by PPARgamma:RXR.

Characterization of genome-wide p53-binding sites upon stress response.

The tumor suppressor p53 is a sequence-specific transcription factor, which regulates the expression of target genes involved in different stress responses. To understand p53's essential transcriptional functions, unbiased analysis of its DNA-binding repertoire is pivotal. In a genome-wide tiling ChIP-on-chip approach, we have identified and characterized 1546 binding sites of p53 upon Actinomycin D treatment. Among those binding sites were known as well as novel p53 target sites, which included regulatory regions of potentially novel transcripts. Using this collection of genome-wide binding sites, a new high-confidence algorithm was developed, p53scan, to identify the p53 consensus-binding motif. Strikingly, this motif was present in the majority of all bound sequences with 83% of all binding sites containing the motif. In the surrounding sequences of the binding sites, several motifs for potential regulatory cobinders were identified. Finally, we show that the majority of the genome-wide p53 target sites can also be bound by overexpressed p63 and p73 in vivo, suggesting that they can possibly play an important role at p53 binding sites. This emphasizes the possible interplay of p53 and its family members in the context of target gene binding. Our study greatly expands the known, experimentally validated p53 binding site repertoire and serves as a valuable knowledgebase for future research.

Genome-wide pattern of TCF7L2/TCF4 chromatin occupancy in colorectal cancer cells.

Wnt signaling activates gene expression through the induced formation of complexes between DNA-binding T-cell factors (TCFs) and the transcriptional coactivator beta-catenin. In colorectal cancer, activating Wnt pathway mutations transform epithelial cells through the inappropriate activation of a TCF7L2/TCF4 target gene program. Through a DNA array-based genome-wide analysis of TCF4 chromatin occupancy, we have identified 6,868 high-confidence TCF4-binding sites in the LS174T colorectal cancer cell line. Most TCF4-binding sites are located at large distances from transcription start sites, while target genes are frequently "decorated" by multiple binding sites. Motif discovery algorithms define the in vivo-occupied TCF4-binding site as evolutionarily conserved A-C/G-A/T-T-C-A-A-A-G motifs. The TCF4-binding regions significantly correlate with Wnt-responsive gene expression profiles derived from primary human adenomas and often behave as beta-catenin/TCF4-dependent enhancers in transient reporter assays.

Selective anchoring of TFIID to nucleosomes by trimethylation of histone H3 lysine 4.

Trimethylation of histone H3 at lysine 4 (H3K4me3) is regarded as a hallmark of active human promoters, but it remains unclear how this posttranslational modification links to transcriptional activation. Using a stable isotope labeling by amino acids in cell culture (SILAC)-based proteomic screening we show that the basal transcription factor TFIID directly binds to the H3K4me3 mark via the plant homeodomain (PHD) finger of TAF3. Selective loss of H3K4me3 reduces transcription from and TFIID binding to a subset of promoters in vivo. Equilibrium binding assays and competition experiments show that the TAF3 PHD finger is highly selective for H3K4me3. In transient assays, TAF3 can act as a transcriptional coactivator in a PHD finger-dependent manner. Interestingly, asymmetric dimethylation of H3R2 selectively inhibits TFIID binding to H3K4me3, whereas acetylation of H3K9 and H3K14 potentiates TFIID interaction. Our experiments reveal crosstalk between histone modifications and the transcription factor TFIID. This has important implications for regulation of RNA polymerase II-mediated transcription in higher eukaryotes.

Targeted discovery tools: proteomics and chromatin immunoprecipitation-on-chip.

Despite the availability of several completely sequenced genomes, we are still, for the most part, ignorant about how genes interact and regulate each other within a given cell type to specify identity, function and cellular memory. A realistic model of cellular regulation based on current knowledge indicates that many interacting networks operate at the epigenetic, transcriptional, translational and post-translational levels, with feedback between the various levels. Protein-protein and protein-DNA interactions help to define which genes may be activated in a particular cell, and determine whether external cues cause activation or repression. New technologies, e.g. proteomics using mass spectrometry, high-density DNA or oligonucleotide microarrays (chips), and chromatin immunoprecipitation (ChIP), provide new and exciting tools for deciphering the pathways and proteins controlling gene expression. Analysis of these pathways offers new insight that aids targeted drug development.