Barry M. Zee

Asymmetrically Modified Nucleosomes

Mononucleosomes, the basic building blocks of chromatin, contain two copies of each core histone. The associated posttranslational modifications regulate essential chromatin-dependent processes, yet whether each histone copy is identically modified in vivo is unclear. We demonstrate that nucleosomes in embryonic stem cells, fibroblasts, and cancer cells exist in both symmetrically and asymmetrically modified populations for histone H3 lysine 27 di/trimethylation (H3K27me2/3) and H4K20me1. Further, we obtained direct physical evidence for bivalent nucleosomes carrying H3K4me3 or H3K36me3 along with H3K27me3, albeit on opposite H3 tails. Bivalency at target genes was resolved upon differentiation of ES cells. Polycomb repressive complex 2-mediated methylation of H3K27 was inhibited when nucleosomes contain symmetrically, but not asymmetrically, placed H3K4me3 or H3K36me3. These findings uncover a potential mechanism for the incorporation of bivalent features into nucleosomes and demonstrate how asymmetry might set the stage to diversify functional nucleosome states.

In Vivo Residue-specific Histone Methylation Dynamics

Methylation of specific histone residues is capable of both gene activation and silencing. Despite vast work on the function of methylation, most studies either present a static snapshot of methylation or fail to assign kinetic information to specific residues. Using liquid chromatography-tandem mass spectrometry on a high-resolution mass spectrometer and heavy methyl-SILAC labeling, we studied site-specific histone lysine and arginine methylation dynamics. The detection of labeled intermediates within a methylation state revealed that mono-, di-, and trimethylated residues generally have progressively slower rates of formation. Furthermore, methylations associated with active genes have faster rates than methylations associated with silent genes. Finally, the presence of both an active and silencing mark on the same peptide results in a slower rate of methylation than the presence of either mark alone. Here we show that quantitative proteomic approaches such as this can determine the dynamics of multiple methylated residues, an understudied portion of histone biology.

NSD2 Links Dimethylation of Histone H3 at Lysine 36 to Oncogenic Programming

The histone lysine methyltransferase NSD2 (MMSET/WHSC1) is implicated in diverse diseases and commonly overexpressed in multiple myeloma due to a recurrent t(4;14) chromosomal translocation. However, the precise catalytic activity of NSD2 is obscure, preventing progress in understanding how this enzyme influences chromatin biology and myeloma pathogenesis. Here, we show that dimethylation of histone H3 at lysine 36 (H3K36me2) is the principal chromatin-regulatory activity of NSD2. Catalysis of H3K36me2 by NSD2 is sufficient for gene activation. In t(4;14)-positive myeloma cells, the normal genome-wide and gene-specific distribution of H3K36me2 is obliterated, creating a chromatin landscape that selects for a transcription profile favorable for myelomagenesis. Catalytically active NSD2 confers xenograft tumor formation upon t(4;14)-negative cells and promotes oncogenic transformation of primary cells in an H3K36me2-dependent manner. Together, our findings establish H3K36me2 as the primary product generated by NSD2 and demonstrate that genomic disorganization of this canonical chromatin mark by NSD2 initiates oncogenic programming.

Lysine methylation of the NF-κB subunit RelA by SETD6 couples activity of the histone methyltransferase GLP at chromatin to tonic repression of NF-κB signaling

Signaling via the methylation of lysine residues in proteins has been linked to diverse biological and disease processes, yet the catalytic activity and substrate specificity of many human protein lysine methyltransferases (PKMTs) are unknown. We screened over 40 candidate PKMTs and identified SETD6 as a methyltransferase that monomethylated chromatin-associated transcription factor NF-κB subunit RelA at Lys310 (RelAK310me1). SETD6-mediated methylation rendered RelA inert and attenuated RelA-driven transcriptional programs, including inflammatory responses in primary immune cells. RelAK310me1 was recognized by the ankryin repeat of the histone methyltransferase GLP, which under basal conditions promoted a repressed chromatin state at RelA target genes through GLP-mediated methylation of histone H3 Lys9 (H3K9). NF-κB-activation–linked phosphorylation of RelA at Ser311 by protein kinase C-ζ (PKC-ζ) blocked the binding of GLP to RelAK310me1 and relieved repression of the target gene. Our findings establish a previously uncharacterized mechanism by which chromatin signaling regulates inflammation programs.Protein lysine methylation signaling is implicated in diverse biological and disease processes. Yet the catalytic activity and substrate specificity are unknown for many human protein lysine methyltransferases (PKMTs). We screened over forty candidate PKMTs and identified SETD6 as a methyltransferase that monomethylates chromatin-associated NF-κB RelA at lysine 310 (RelAK310me1). SETD6-mediated methylation rendered RelA inert and attenuated RelA-driven transcriptional programs, including inflammatory responses in primary immune cells. RelAK310me1 was recognized by the ankryin repeat of GLP, which under basal conditions, promoted a repressed chromatin state at RelA target genes through GLP-mediated H3K9 methylation. NF-κB activation-linked phosphorylation of RelA by PKCζ at serine 311 blocked GLP binding to RelAK310me1 and relieved target gene repression. Our findings establish a new mechanism by which chromatin signaling regulates inflammation programs.

One-Pot Shotgun Quantitative Mass Spectrometry Characterization of Histones

Despite increasing applications of mass spectrometry (MS) to characterize post-translational modifications (PTMs) on histone proteins, most existing protocols are not properly suited to robustly measure them in a high-throughput quantitative manner. In this work, we expand on current protocols and describe improved methods for quantitative Bottom Up characterization of histones and their PTMs with comparable sensitivity but much higher throughput than standard MS approaches. This is accomplished by first bypassing off-line fractionation of histone proteins and working directly with total histones from a typical nuclei acid extraction. Next, using a chemical derivatization procedure that is combined with stable-isotope labeling in a two-step process, we can quantitatively compare samples using nanoLC-MS/MS. We show that our method can successfully detect 17 combined H2A/H2B variants and over 25 combined histone H3 and H4 PTMs in a single MS experiment. We test our method by quantifying differentially expressed histone PTMs from wild-type yeast and a methyltransferase knockout strain. This improved methodology establishes that time and sample consuming off-line HPLC or SDS-PAGE purification of individual histone variants prior to MS interrogation as commonly performed is not strictly required. Our protocol significantly streamlines the analysis of histone PTMs and will allow for studies of differentially expressed PTMs between multiple samples during biologically relevant processes in a rapid and quantitative fashion.

The RING finger protein MSL2 in the MOF complex is an E3 ubiquitin ligase for H2B K34 and is involved in crosstalk with H3 K4 and K79 methylation.

We demonstrate that RING finger protein MSL2 in the MOF-MSL complex is a histone ubiquitin E3 ligase. MSL2, together with MSL1, has robust histone ubiquitylation activity that mainly targets nucleosomal H2B on lysine 34 (H2B K34ub), a site within a conserved basic patch on H2B tail. H2B K34ub by MSL1/2 directly regulates H3 K4 and K79 methylation through trans-tail crosstalk both in vitro and in cells. The significance of MSL1/2-mediated histone H2B ubiquitylation is underscored by the facts that MSL1/2 activity is important for transcription activation at HOXA9 and MEIS1 loci and that this activity is evolutionarily conserved in the Drosophila dosage compensation complex. Altogether, these results indicate that the MOF-MSL complex possesses two distinct chromatin-modifying activities (i.e., H4 K16 acetylation and H2B K34 ubiquitylation) through MOF and MSL2 subunits. They also shed light on how an intricate network of chromatin-modifying enzymes functions coordinately in gene activation.

Breaking the histone code with quantitative mass spectrometry

Histone post-translational modifications (PTMs) comprise one of the most intricate nuclear signaling networks that govern gene expression in a long-term and dynamic fashion. These PTMs are considered to be ‘epigenetic’ or heritable from one cell generation to the next and help establish genomic expression patterns. While much of the analyses of histones have historically been performed using site-specific antibodies, these methods are replete with technical obstacles (i.e., cross-reactivity and epitope occlusion). Mass spectrometry-based proteomics has begun to play a significant role in the interrogation of histone PTMs, revealing many new aspects of these modifications that cannot be easily determined with standard biological approaches. Here, we review the accomplishments of mass spectrometry in the histone field, and outline the future roadblocks that must be overcome for mass spectrometry-based proteomics to become the method of choice for chromatin biologists.

Global turnover of histone post-translational modifications and variants in human cells

BackgroundPost-translational modifications (PTMs) on the N-terminal tails of histones and histone variants regulate distinct transcriptional states and nuclear events. Whereas the functional effects of specific PTMs are the current subject of intense investigation, most studies characterize histone PTMs/variants in a non-temporal fashion and very few studies have reported kinetic information about these histone forms. Previous studies have used radiolabeling, fluorescence microscopy and chromatin immunoprecipitation to determine rates of histone turnover, and have found interesting correlations between increased turnover and increased gene expression. Therefore, histone turnover is an understudied yet potentially important parameter that may contribute to epigenetic regulation. Understanding turnover in the context of histone modifications and sequence variants could provide valuable additional insight into the function of histone replacement.ResultsIn this study, we measured the metabolic rate of labeled isotope incorporation into the histone proteins of HeLa cells by combining stable isotope labeling of amino acids in cell culture (SILAC) pulse experiments with quantitative mass spectrometry-based proteomics. In general, we found that most core histones have similar turnover rates, with the exception of the H2A variants, which exhibit a wider range of rates, potentially consistent with their epigenetic function. In addition, acetylated histones have a significantly faster turnover compared with general histone protein and methylated histones, although these rates vary considerably, depending on the site and overall degree of methylation. Histones containing transcriptionally active marks have been consistently found to have faster turnover rates than histones containing silent marks. Interestingly, the presence of both active and silent marks on the same peptide resulted in a slower turnover rate than either mark alone on that same peptide. Lastly, we observed little difference in the turnover between nearly all modified forms of the H3.1, H3.2 and H3.3 variants, with the notable exception that H3.2K36me2 has a faster turnover than this mark on the other H3 variants.ConclusionsQuantitative proteomics provides complementary insight to previous work aimed at quantitatively measuring histone turnover, and our results suggest that turnover rates are dependent upon site-specific post-translational modifications and sequence variants.

Histone H1 variant-specific lysine methylation by G9a/KMT1C and Glp1/KMT1D

BackgroundThe linker histone H1 has a key role in establishing and maintaining higher order chromatin structure and in regulating gene expression. Mammals express up to 11 different H1 variants, with H1.2 and H1.4 being the predominant ones in most somatic cells. Like core histones, H1 has high levels of covalent modifications; however, the full set of modifications and their biological role are largely unknown.ResultsIn this study, we used a candidate screen to identify enzymes that methylate H1 and to map their corresponding methylation sites. We found that the histone lysine methyltransferases G9a/KMT1C and Glp1/KMT1D methylate H1.2 in vitro and in vivo, and we mapped this novel site to lysine 187 (H1.2K187) in the C-terminus of H1. This H1.2K187 methylation is variant-specific. The main target for methylation by G9a in H1.2, H1.3, H1.5 and H1.0 is in the C-terminus, whereas H1.4 is preferentially methylated at K26 (H1.4K26me) in the N-terminus. We found that the readout of these marks is different; H1.4K26me can recruit HP1, but H1.2K187me cannot. Likewise, JMJD2D/KDM4 only reverses H1.4K26 methylation, clearly distinguishing these two methylation sites. Further, in contrast to C-terminal H1 phosphorylation, H1.2K187 methylation level is steady throughout the cell cycle.ConclusionsWe have characterised a novel methylation site in the C-terminus of H1 that is the target of G9a/Glp1 both in vitro and in vivo. To our knowledge, this is the first demonstration of variant-specific histone methylation by the same methyltransferases, but with differing downstream readers, thereby supporting the hypothesis of H1 variants having specific functions.

A quantitative atlas of histone modification signatures from human cancer cells

BackgroundAn integral component of cancer biology is the understanding of molecular properties uniquely distinguishing one cancer type from another. One class of such properties is histone post-translational modifications (PTMs). Many histone PTMs are linked to the same diverse nuclear functions implicated in cancer development, including transcriptional activation and epigenetic regulation, which are often indirectly assayed with standard genomic technologies. Thus, there is a need for a comprehensive and quantitative profiling of cancer lines focused on their chromatin modification states.ResultsTo complement genomic expression profiles of cancer lines, we report the proteomic classification of 24 different lines, the majority of which are cancer cells, by quantifying the abundances of a large panel of single and combinatorial histone H3 and H4 PTMs, and histone variants. Concurrent to the proteomic analysis, we performed transcriptomic analysis on histone modifying enzyme abundances as a proxy for quantifying their activity levels. While the transcriptomic and proteomic results were generally consistent in terms of predicting histone PTM abundance from enzyme abundances, several PTMs were regulated independently of the modifying enzyme expression. In addition, combinatorial PTMs containing H3K27 methylation were especially enriched in breast cell lines. Knockdown of the predominant H3K27 methyltransferase, enhancer of zeste 2 (EZH2), in a mouse mammary xenograft model significantly reduced tumor burden in these animals and demonstrated the predictive utility of proteomic techniques.ConclusionsOur proteomic and genomic characterizations of the histone modification states provide a resource for future investigations of the epigenetic and non-epigenetic determinants for classifying and analyzing cancer cells.