PHD fingers: epigenetic effectors and potential drug targets

Kdm2a/b Lysine Demethylases Regulate Canonical Wnt Signaling by Modulating the Stability of Nuclear β-Catenin

In the absence of Wnt activation, cytosolic β-catenin is degraded through GSK3/CK1-mediated phosphorylation at the N terminus. Here, we show that, upon Wnt activation, the stability of nuclear β-catenin is regulated via methylation/demethylation. The protein lysine demethylases Kdm2a and Kdm2b regulate the turnover of non-phosphorylated β-catenin specifically within the nucleus via direct interaction with the fourth and fifth armadillo repeats. The lysine residues within this region are required for the methylation of non-phosphorylated β-catenin, which is demethylated by Kdm2a/b and subsequently ubiquitylated. During Xenopus embryogenesis, kdm2a/b genes are transcribed during early embryogenesis and are required for the specification of the body axis. Kdm2a/b knockdown in Xenopus embryos leads to increases in non-phosphorylated and methylated β-catenin, concurrent with the upregulation of β-catenin target genes. This mechanism is required for controlling the output of the Wnt/β-catenin signaling pathway to maintain normal cellular functions.

The basal transcription complex component TAF3 transduces changes in nuclear phosphoinositides into transcriptional output

Phosphoinositides (PI) are important signaling molecules in the nucleus that influence gene expression. However, if and how nuclear PI directly affects the transcriptional machinery is not known. We report that the lipid kinase PIP4K2B regulates nuclear PI5P and the expression of myogenic genes during myoblast differentiation. A targeted screen for PI interactors identified the PHD finger of TAF3, a TATA box binding protein-associated factor with important roles in transcription regulation, pluripotency, and differentiation. We show that the PI interaction site is distinct from the known H3K4me3 binding region of TAF3 and that PI binding modulates association of TAF3 with H3K4me3 in vitro and with chromatin in vivo. Analysis of TAF3 mutants indicates that TAF3 transduces PIP4K2B-mediated alterations in PI into changes in specific gene transcription. Our study reveals TAF3 as a direct target of nuclear PI and further illustrates the importance of basal transcription components as signal transducers.

Immediate chromatin immunoprecipitation and on-bead quantitative PCR analysis: a versatile and rapid ChIP procedure

Genome-wide chromatin immunoprecipitation (ChIP) studies have brought significant insight into the genomic localization of chromatin-associated proteins and histone modifications. The large amount of data generated by these analyses, however, require approaches that enable rapid validation and analysis of biological relevance. Furthermore, there are still protein and modification targets that are difficult to detect using standard ChIP methods. To address these issues, we developed an immediate chromatin immunoprecipitation procedure which we call ZipChip. ZipChip significantly reduces the time and increases sensitivity allowing for rapid screening of multiple loci. Here we describe how ZipChIP enables detection of histone modifications (H3K4 mono- and trimethylation) and two yeast histone demethylases, Jhd2 and Rph1, which were previously difficult to detect using standard methods. Furthermore, we demonstrate the versatility of ZipChIP by analyzing the enrichment of the histone deacetylase Sir2 at heterochromatin in yeast and enrichment of the chromatin remodeler, PICKLE, at euchromatin in Arabidopsis thaliana.

A comprehensive view of the epigenetic landscape. Part II: Histone post-translational modification, nucleosome level, and chromatin regulation by ncRNAs

The complexity of the genome is regulated by epigenetic mechanisms, which act on the level of DNA, histones, and nucleosomes. Epigenetic machinery is involved in various biological processes, including embryonic development, cell differentiation, neurogenesis, and adult cell renewal. In the last few years, it has become clear that the number of players identified in the regulation of chromatin structure and function is still increasing. In addition to well-known phenomena, including DNA methylation and histone modification, new, important elements, including nucleosome mobility, histone tail clipping, and regulatory ncRNA molecules, are being discovered. The present paper provides the current state of knowledge about the role of 16 different histone post-translational modifications, nucleosome positioning, and histone tail clipping in the structure and function of chromatin. We also emphasize the significance of cross-talk among chromatin marks and ncRNAs in epigenetic control.

Staufen1-mediated mRNA decay induces Requiem mRNA decay through binding of Staufen1 to the Requiem 3'UTR

Requiem (REQ/DPF2) was originally identified as an apoptosis-inducing protein in mouse myeloid cells and belongs to the novel Krüppel-type zinc finger d4-protein family of proteins, which includes neuro-d4 (DPF1) and cer-d4 (DPF3). Interestingly, when a portion of the REQ messenger ribonucleic acid (mRNA) 3' untranslated region (3'UTR), referred to as G8, was overexpressed in K562 cells, β-globin expression was induced, suggesting that the 3'UTR of REQ mRNA plays a physiological role. Here, we present evidence that the REQ mRNA 3'UTR, along with its trans-acting factor, Staufen1 (STAU1), is able to reduce the level of REQ mRNA via STAU1-mediated mRNA decay (SMD). By screening a complementary deoxyribonucleic acid (cDNA) expression library with an RNA-ligand binding assay, we identified STAU1 as an interactor of the REQ mRNA 3'UTR. Specifically, we provide evidence that STAU1 binds to putative 30-nucleotide stem-loop-structured RNA sequences within the G8 region, which we term the protein binding site core; this binding triggers the degradation of REQ mRNA and thus regulates translation. Furthermore, we demonstrate that siRNA-mediated silencing of either STAU1 or UPF1 increases the abundance of cellular REQ mRNA and, consequently, the REQ protein, indicating that REQ mRNA is a target of SMD.

Retinoic acid induced-1 (Rai1) regulates craniofacial and brain development in Xenopus

Retinoic acid induced-1 (RAI1) is an important yet understudied histone code reader that when mutated in humans results in Smith-Magenis syndrome (SMS), a neurobehavioral disorder accompanied by signature craniofacial abnormalities. Despite previous studies in mouse and human cell models, very little is known about the function of RAI1 during embryonic development. In the present study, we have turned to the model vertebrates Xenopus laevis and Xenopus tropicalis to better understand the developmental roles of Rai1. First we demonstrate that the Rai1 protein sequence is conserved in frogs, especially in known functional domains. By in situ hybridization we revealed expression of rai1 in the developing craniofacial tissues and the nervous system. Knockdown of Rai1 using antisense morpholinos resulted in defects in the developing brain and face. In particular, Rai1 morphants display midface hypoplasia and malformed mouth shape analogous to defects in humans with SMS. These craniofacial defects were accompanied with aberrant neural crest migration and reduction in the size of facial cartilage elements. Rai1 morphants also had defects in axon patterns and decreased forebrain ventricle size. Such brain defects correlated with a decrease in the neurotrophic factor, bdnf, and increased forebrain apoptosis. Our results emphasize a critical role of Rai1 for normal neural and craniofacial development, and further the current understanding of potential mechanisms that cause SMS.

The biophysical and biochemical properties of the autoimmune regulator (AIRE) protein

AIRE (for autoimmune regulator) is a multidomain protein that performs a fundamental function in the thymus and possibly in the secondary lymphoid organs: the regulation, especially in the sense of activation, of the process of gene transcription in cell lines deputed to the presentation of self-antigens to the maturing T lymphocytes. The apoptosis of the elements bearing T-cell receptors with critical affinity for the exhibited self-antigens prevents the escape of autoreactive clones and represents a simple and efficient mechanism of deletional self-tolerance. However, AIRE action relies on an articulated complex of biophysical and biochemical properties, in most cases attributable to single subspecialized domains. Here a thorough review of the matter is presented, with a privileged look at the pathogenic changes of AIRE that interfere with such properties and lead to the impairment in its chief function.

Histone lysine demethylase (KDM) subfamily 4: structures, functions and therapeutic potential

KDM4 histone demethylases catalyze the removal of methyl marks from histone lysine residues to epigenetically regulate chromatin structure and gene expression. KDM4 expression is tightly regulated to insure proper function in diverse biological processes, such as cellular differentiation. Mounting evidence has shown that disrupting KDM4 expression is implicated in the establishment and progression of multiple diseases including cancer. In particular, genomic regions encoding the KDM4A, B and C genes are often amplified, disrupting normal cellular proliferation. Furthermore, KDM4 demethylases are promising druggable targets. In this review, we highlight the latest advances in characterizing the structures and regulatory mechanisms of KDM4 proteins, as well as our current understanding of their alterations and roles in tumorigenesis. We also review the reported KDM4 inhibitors and discuss their potential as therapeutic agents.

Diverse functions of PHD fingers of the MLL/KMT2 subfamily

Five members of the KMT2 family of lysine methyltransferases, originally named the mixed lineage leukemia (MLL1-5) proteins, regulate gene expression during embryogenesis and development. Each KMT2A-E contains a catalytic SET domain that methylates lysine 4 of histone H3, and one or several PHD fingers. Over the past few years a growing number of studies have uncovered diverse biological roles of the KMT2A-E PHD fingers, implicating them in binding to methylated histones and other nuclear proteins, and in mediating the E3 ligase activity and dimerization. Mutations in the PHD fingers or deletion of these modules are linked to human diseases including cancer and Kabuki syndrome. In this work, we summarize recently identified biological functions of the KMT2A-E PHD fingers, discuss mechanisms of their action, and examine preference of these domains for histone and non-histone ligands.

Structure of human Sp140 PHD finger: an atypical fold interacting with Pin1

Sp140 is a nuclear leukocyte-specific protein involved in primary biliary cirrhosis and a risk factor in chronic lymphocytic leukemia. The presence of several chromatin related modules such as plant homeodomain (PHD), bromodomain and SAND domain suggests a role in chromatin-mediated regulation of gene expression; however, its real function is still elusive. Herein we present the solution structure of Sp140-PHD finger and investigate its role as epigenetic reader in vitro. Sp140-PHD presents an atypical PHD finger fold which does not bind to histone H3 tails but is recognized by peptidylprolyl isomerase Pin1. Pin1 specifically binds to a phosphopeptide corresponding to the L3 loop of Sp140-PHD and catalyzes cis-trans isomerization of a pThr-Pro bond. Moreover co-immunoprecipitation experiments demonstrate FLAG-Sp140 interaction with endogenous Pin1 in vivo. Overall these data include Sp140 in the list of the increasing number of Pin1 binders and expand the regulatory potential of PHD fingers as versatile structural platforms for diversified interactions.

Nuclear phosphoinositides and their impact on nuclear functions

Polyphosphoinositides (PPIn) are important lipid molecules whose levels are de-regulated in human diseases such as cancer, neurodegenerative disorders and metabolic syndromes. PPIn are synthesized and degraded by an array of kinases, phosphatases and lipases which are localized to various subcellular compartments and are subject to regulation in response to both extra- and intracellular cues. Changes in the activities of enzymes that metabolize PPIn lead to changes in the profiles of PPIn in various subcellular compartments. Understanding how subcellular PPIn are regulated and how they affect downstream signaling is critical to understanding their roles in human diseases. PPIn are present in the nucleus, and their levels are changed in response to various stimuli, suggesting that they may serve to regulate specific nuclear functions. However, the lack of nuclear downstream targets has hindered the definition of which pathways nuclear PPIn affect. Over recent years, targeted and global proteomic studies have identified a plethora of potential PPIn-interacting proteins involved in many aspects of transcription, chromatin remodelling and mRNA maturation, suggesting that PPIn signalling within the nucleus represents a largely unexplored novel layer of complexity in the regulation of nuclear functions.

Solution NMR structure and histone binding of the PHD domain of human MLL5

Mixed Lineage Leukemia 5 (MLL5) is a histone methyltransferase that plays a key role in hematopoiesis, spermatogenesis and cell cycle progression. In addition to its catalytic domain, MLL5 contains a PHD finger domain, a protein module that is often involved in binding to the N-terminus of histone H3. Here we report the NMR solution structure of the MLL5 PHD domain showing a variant of the canonical PHD fold that combines conserved H3 binding features from several classes of other PHD domains (including an aromatic cage) along with a novel C-terminal α-helix, not previously seen. We further demonstrate that the PHD domain binds with similar affinity to histone H3 tail peptides di- and tri-methylated at lysine 4 (H3K4me2 and H3K4me3), the former being the putative product of the MLL5 catalytic reaction. This work establishes the PHD domain of MLL5 as a bone fide ‘reader’ domain of H3K4 methyl marks suggesting that it may guide the spreading or further methylation of this site on chromatin.

PtdIns5P: news and views of its appearance, disappearance and deeds

Accumulated evidence indicates that PtdIns5P, one of the seven phosphoinositides, found now to be constitutively present in yeast, plants and metazoa, serves as a signaling molecule to modulate pleiotropic cellular functions in both the nucleus and the cytoplasm. The enzymatic routes in biogenesis of basal PtdIns5P have remained incompletely understood. The role for candidate kinase PIKfyve that is principally involved in PtdIns(3,5)P2 production, has been questioned. In this review article we scrutinize the past obstacles that prevented the definitive implication of PIKfyve in PtdIns5P biosynthesis from PtdIns and focus on the recent pharmacological and genetic advancements that now make this conclusion well supported. We further summarize our current knowledge of the diverse stimuli modulating PtdIns5P levels, binding partners and regulated cellular process, with particular reference to the available mechanistic insights for the relevant signaling pathways.

Genome-wide RNAi screens in human brain tumor isolates reveal a novel viability requirement for PHF5A

To identify key regulators of human brain tumor maintenance and initiation, we performed multiple genome-wide RNAi screens in patient-derived glioblastoma multiforme (GBM) stem cells (GSCs). These screens identified the plant homeodomain (PHD)-finger domain protein PHF5A as differentially required for GSC expansion, as compared with untransformed neural stem cells (NSCs) and fibroblasts. Given PHF5A's known involvement in facilitating interactions between the U2 snRNP complex and ATP-dependent helicases, we examined cancer-specific roles in RNA splicing. We found that in GSCs, but not untransformed controls, PHF5A facilitates recognition of exons with unusual C-rich 3' splice sites in thousands of essential genes. PHF5A knockdown in GSCs, but not untransformed NSCs, astrocytes, or fibroblasts, inhibited splicing of these genes, leading to cell cycle arrest and loss of viability. Notably, pharmacologic inhibition of U2 snRNP activity phenocopied PHF5A knockdown in GSCs and also in NSCs or fibroblasts overexpressing MYC. Furthermore, PHF5A inhibition compromised GSC tumor formation in vivo and inhibited growth of established GBM patient-derived xenograft tumors. Our results demonstrate a novel viability requirement for PHF5A to maintain proper exon recognition in brain tumor-initiating cells and may provide new inroads for novel anti-GBM therapeutic strategies.

On your histone mark, SET, methylate!

Lysine methylation of histones and non-histone proteins has emerged in recent years as a posttranslational modification with wide-ranging cellular implications beyond epigenetic regulation. The molecular interactions between lysine methyltransferases and their substrates appear to be regulated by posttranslational modifications surrounding the lysine methyl acceptor. Two very interesting examples of this cross-talk between methyl-lysine sites are found in the SET (Su(var)3–9, Enhancer-of-zeste, Trithorax) domain-containing lysine methyltransferases SET7 and SETDB1, whereby the histone H3 trimethylated on lysine 4 (H3K4^me3) modification prevents methylation by SETDB1 on H3 lysine 9 (H3K9) and the histone H3 trimethylated on lysine 9 (H3K9^me3) modification prevents methylation by SET7 on H3K4. A similar cross-talk between posttranslational modifications regulates the functions of non-histone proteins such as the tumor suppressor p53 and the DNA methyltransferase DNMT1. Herein, in cis effects of acetylation, phosphorylation, as well as arginine and lysine methylation on lysine methylation events will be discussed.

The oncogenic potential of Jumonji D2 (JMJD2/KDM4) histone demethylase overexpression

The Jumonji D2 proteins (JMJD2/KDM4) function to demethylate di- and trimethylated (me2/3) histone 3 lysine 9 (H3K9me2/3) and H3K36me3. Knockout mouse models for Kdm4b and Kdm4d have not resulted in gross abnormalities, while mouse models for Kdm4a and Kdm4c have not been reported. However, the KDM4 subfamily of demethylases are overexpressed in several tumor types. Overexpression of KDM4 proteins alters transcription and chromatin remodeling, driving cellular proliferation, anchorage-independent growth, invasion, and migration. Increased proliferation occurs through KDM4-mediated modification of cell cycle timing, as well as through increased numbers of replication forks. Recent evidence also suggests that KDM4C overexpression contributes to the maintenance of a pluripotent state. Together these data suggest that overexpression of KDM4 proteins induces numerous oncogenic effects.

Exploring PHD fingers and H3K4me0 interactions with molecular dynamics simulations and binding free energy calculations: AIRE-PHD1, a comparative study

PHD fingers represent one of the largest families of epigenetic readers capable of decoding post-translationally modified or unmodified histone H3 tails. Because of their direct involvement in human pathologies they are increasingly considered as a potential therapeutic target. Several PHD/histone-peptide structures have been determined, however relatively little information is available on their dynamics. Studies aiming to characterize the dynamic and energetic determinants driving histone peptide recognition by epigenetic readers would strongly benefit from computational studies. Herein we focus on the dynamic and energetic characterization of the PHD finger subclass specialized in the recognition of histone H3 peptides unmodified in position K4 (H3K4me0). As a case study we focused on the first PHD finger of autoimmune regulator protein (AIRE-PHD1) in complex with H3K4me0. PCA analysis of the covariance matrix of free AIRE-PHD1 highlights the presence of a "flapping" movement, which is blocked in an open conformation upon binding to H3K4me0. Moreover, binding free energy calculations obtained through Molecular Mechanics/Poisson-Boltzmann Surface Area (MM/PBSA) methodology are in good qualitative agreement with experiments and allow dissection of the energetic terms associated with native and alanine mutants of AIRE-PHD1/H3K4me0 complexes. MM/PBSA calculations have also been applied to the energetic analysis of other PHD fingers recognizing H3K4me0. In this case we observe excellent correlation between computed and experimental binding free energies. Overall calculations show that H3K4me0 recognition by PHD fingers relies on compensation of the electrostatic and polar solvation energy terms and is stabilized by non-polar interactions.

Protein acetylation mechanisms in the regulation of insulin and insulin-like growth factor 1 signalling

Lysine acetylation is a protein post-translational modification (PTM) initially discovered in abundant proteins such as tubulin, whose acetylated form confers microtubule stability, and histones, where it promotes the transcriptionally active chromatin state. Other individual reports identified lysine acetylation as a PTM regulating transcription factors and co-activators including p53, c-Myc, PGC1α and Ku70. The subsequent employment of proteomics-based approaches revealed that lysine acetylation is a widespread PTM, contributing to cellular regulation as much as protein-phosphorylation based mechanisms. In particular, most of the enzymes of central metabolic processes - glycolysis, tricarboxylic acid and urea cycles, fatty acid and glycogen metabolism - have been shown to be regulated by lysine acetylation, through the opposite actions of protein acetyltransferases and deacetylases, making protein acetylation a PTM that connects the cell's energetic state and its consequent metabolic response. In multicellular organisms, insulin/insulin-like signalling (IIS) is a major hormonal regulator of metabolism and cell growth, and very recent research indicates that most of the enzymes participating in IIS are likewise subjected to acetylation-based regulatory mechanisms, that integrate the classical phosphorylation mechanisms. Here, we review the current knowledge on acetylation/deacetylation regulatory phenomena within the IIS cascade, with emphasis on the enzymatic machinery linking the acetylation/deacetylation switch to the metabolic state. We cover this recent area of investigation because pharmacological modulation of protein acetylation/deacetylation has been shown to be a promising target for the amelioration of the metabolic abnormalities occurring in the metabolic syndrome.

Tandem PHD fingers of MORF/MOZ acetyltransferases display selectivity for acetylated histone H3 and are required for the association with chromatin

MORF [MOZ (monocytic leukemia zinc-finger protein)-related factor] and MOZ are catalytic subunits of histone acetyltransferase (HAT) complexes essential in hematopoiesis, neurogenesis, skeletogenesis and other developmental programs and implicated in human leukemias. The canonical HAT domain of MORF/MOZ is preceded by a tandem of plant homeodomain (PHD) fingers whose biological roles and requirements for MORF/MOZ activity are unknown. Here, we demonstrate that the tandem PHD1/2 fingers of MORF recognize the N-terminal tail of histone H3. Acetylation of Lys9 (H3K9ac) or Lys14 (H3K14ac) enhances binding of MORF PHD1/2 to unmodified H3 peptides twofold to threefold. The selectivity for acetylated H3 tail is conserved in the double PHD1/2 fingers of MOZ. This interaction requires the intact N-terminus of histone H3 and is inhibited by trimethylation of Lys4. Biochemical analysis using NMR, fluorescence spectroscopy and mutagenesis identified key amino acids of MORF PHD1/2 necessary for the interaction with histones. Fluorescence microscopy and immunoprecipitation experiments reveal that both PHD fingers are required for binding to H3K14ac in vivo and localization to chromatin. The HAT assays indicate that the interaction with H3K14ac may promote enzymatic activity in trans. Together, our data suggest that the PHD1/2 fingers play a role in MOZ/MORF HATs association with the chromatic regions enriched in acetylated marks.

Multivalent recognition of histone tails by the PHD fingers of CHD5

The chromodomain, helicase, DNA-binding protein 5 (CHD5) is a chromatin remodeling enzyme which is implicated in tumor suppression. In this study, we demonstrate the ability of the CHD5 PHD fingers to specifically recognize the unmodified N-terminus of histone H3. We use two distinct modified peptide-library platforms (beads and glass slides) to determine the detailed histone binding preferences of PHD(1) and PHD(2) alone and the tandem PHD(1-2) construct. Both domains displayed similar binding preferences for histone H3, where modification (e.g., methylation, acetylation, and phosphorylation) at H3R2, H3K4, H3T3, H3T6, and H3S10 disrupts high-affinity binding, and the three most N-terminal amino acids (ART) are crucial for binding. The tandem CHD5-PHD(1-2) displayed similar preferences to those displayed by each PHD finger alone. Using NMR, surface plasmon resonance, and two novel biochemical assays, we demonstrate that CHD5-PHD(1-2) simultaneously engages two H3 N-termini and results in a 4-11-fold increase in affinity compared with either PHD finger alone. These studies provide biochemical evidence for the utility of tandem PHD fingers to recruit protein complexes at targeted genomic loci and provide the framework for understanding how multiple chromatin-binding modules function to interpret the combinatorial PTM capacity written in chromatin.

The H3K4me3 histone demethylase Fbxl10 is a regulator of chemokine expression, cellular morphology, and the metabolome of fibroblasts

Fbxl10 (Jhdm1b/Kdm2b) is a conserved and ubiquitously expressed member of the JHDM (JmjC domain-containing histone demethylase) family. Fbxl10 was implicated in the demethylation of H3K4me3 or H3K36me2 thereby removing active chromatin marks and inhibiting gene transcription. Apart from the JmjC domain, Fbxl10 consists of a CxxC domain, a PHD domain, and an Fbox domain. By purifying the JmjC and the PHD domain of Fbxl10 and using different approaches we were able to characterize the properties of these domains in vitro. Our results suggest that Fbxl10 is rather a H3K4me3 than a H3K36me2 histone demethylase. The PHD domain exerts a dual function in binding H3K4me3 and H3K36me2 and exhibiting E3 ubiquitin ligase activity. We generated mouse embryonic fibroblasts stably overexpressing Fbxl10. These cells reveal an increase in cell size but no changes in proliferation, mitosis, or apoptosis. Using a microarray approach we were able to identify potentially new target genes for Fbxl10 including chemokines, the noncoding RNA Xist, and proteins involved in metabolic processes. Additionally, we found that Fbxl10 is recruited to the promoters of Ccl7, Xist, Crabp2, and RipK3. Promoter occupancy by Fbxl10 was accompanied by reduced levels of H3K4me3 but unchanged levels of H3K36me2. Furthermore, knockdown of Fbxl10 using small interfering RNA approaches showed inverse regulation of Fbxl10 target genes. In summary, our data reveal a regulatory role of Fbxl10 in cell morphology, chemokine expression, and the metabolic control of fibroblasts.

SS18 together with animal-specific factors defines human BAF-type SWI/SNF complexes

BACKGROUND

Nucleosome translocation along DNA is catalyzed by eukaryotic SNF2-type ATPases. One class of SNF2-ATPases is distinguished by the presence of a C-terminal bromodomain and is conserved from yeast to man and plants. This class of SNF2 enzymes forms rather large protein complexes that are collectively called SWI/SNF complexes. They are involved in transcription and DNA repair. Two broad types of SWI/SNF complexes have been reported in the literature; PBAF and BAF. These are distinguished by the inclusion or not of polybromo and several ARID subunits. Here we investigated human SS18, a protein that is conserved in plants and animals. SS18 is a putative SWI/SNF subunit which has been implicated in the etiology of synovial sarcomas by virtue of being a target for oncogenic chromosomal translocations that underlie synovial sarcomas.

METHODOLOGY/PRINCIPAL FINDINGS

We pursued a proteomic approach whereby the SS18 open reading frame was fused to a tandem affinity purification tag and expressed in amenable human cells. The fusion permitted efficient and exclusive purification of so-called BAF-type SWI/SNF complexes which bear ARID1A/BAF250a or ARID1B/BAF250b subunits. This demonstrates that SS18 is a BAF subtype-specific SWI/SNF complex subunit. The same result was obtained when using the SS18-SSX1 oncogenic translocation product. Furthermore, SS18L1, DPF1, DPF2, DPF3, BRD9, BCL7A, BCL7B and BCL7C were identified. 'Complex walking' showed that they all co-purify with each other, defining human BAF-type complexes. By contrast,we demonstrate that human PHF10 is part of the PBAF complex, which harbors both ARID2/BAF200 and polybromo/BAF180 subunits, but not SS18 and nor the above BAF-specific subunits.

CONCLUSIONS/SIGNIFICANCE

SWI/SNF complexes are found in most eukaryotes and in the course of evolution new SWI/SNF subunits appeared. SS18 is found in plants as well as animals. Our results suggest that in both protostome and deuterostome animals, a class of BAF-type SWI/SNF complexes will be found that harbor SS18 or its paralogs, along with ARID1, DPF and BCL7 paralogs. Those BAF complexes are proteomically distinct from the eukaryote-wide PBAF-type SWI/SNF complexes. Finally, our results suggests that the human bromodomain factors BRD7 and BRD9 associate with PBAF and BAF, respectively.

Bivalent recognition of nucleosomes by the tandem PHD fingers of the CHD4 ATPase is required for CHD4-mediated repression

CHD4 is a catalytic subunit of the NuRD (nucleosome remodeling and deacetylase) complex essential in transcriptional regulation, chromatin assembly and DNA damage repair. CHD4 contains tandem plant homeodomain (PHD) fingers connected by a short linker, the biological function of which remains unclear. Here we explore the combinatorial action of the CHD4 PHD1/2 fingers and detail the molecular basis for their association with chromatin. We found that PHD1/2 targets nucleosomes in a multivalent manner, concomitantly engaging two histone H3 tails. This robust synergistic interaction displaces HP1γ from pericentric sites, inducing changes in chromatin structure and leading to the dispersion of the heterochromatic mark H3K9me3. We demonstrate that recognition of the histone H3 tails by the PHD fingers is required for repressive activity of the CHD4/NuRD complex. Together, our data elucidate the molecular mechanism of multivalent association of the PHD fingers with chromatin and reveal their critical role in the regulation of CHD4 functions.

Nuclear phosphoinositides: location, regulation and function

Lipid signalling in human disease is an important field of investigation and stems from the fact that phosphoinositide signalling has been implicated in the control of nearly all the important cellular pathways including metabolism, cell cycle control, membrane trafficking, apoptosis and neuronal conduction. A distinct nuclear inositide signalling metabolism has been identified, thus defining a new role for inositides in the nucleus, which are now considered essential co-factors for several nuclear processes, including DNA repair, transcription regulation, and RNA dynamics. Deregulation of phoshoinositide metabolism within the nuclear compartment may contribute to disease progression in several disorders, such as chronic inflammation, cancer, metabolic, and degenerative syndromes. In order to utilize these very druggable pathways for human benefit there is a need to identify how nuclear inositides are regulated specifically within this compartment and what downstream nuclear effectors process and integrate inositide signalling cascades in order to specifically control nuclear function. Here we describe some of the facets of nuclear inositide metabolism with a focus on their relationship to cell cycle control and differentiation.

Handpicking epigenetic marks with PHD fingers

Plant homeodomain (PHD) fingers have emerged as one of the largest families of epigenetic effectors capable of recognizing or ‘reading’ post-translational histone modifications and unmodified histone tails. These interactions are highly specific and can be modulated by the neighboring epigenetic marks and adjacent effectors. A few PHD fingers have recently been found to also associate with non-histone proteins. In this review, we detail the molecular mechanisms and biological outcomes of the histone and non-histone targeting by PHD fingers. We discuss the significance of crosstalk between the histone modifications and consequences of combinatorial readout for selective recruitment of the PHD finger-containing components of chromatin remodeling and transcriptional complexes.

The involvement of epigenetic defects in mental retardation

Mental retardation is a group of cognitive disorders with a significant worldwide prevalence rate. This high rate, together with the considerable familial and societal burden resulting from these disorders, makes it an important focus for prevention and intervention. While the diseases associated with mental retardation are diverse, a significant number are linked with disruptions in epigenetic mechanisms, mainly due to loss-of-function mutations in genes that are key components of the epigenetic machinery. Additionally, several disorders classed as imprinting syndromes are associated with mental retardation. This review will discuss the epigenetic abnormalities associated with mental retardation, and will highlight their importance for diagnosis, treatment, and prevention of these disorders.

Widely expressed Af17 is likely not required for embryogenesis, hematopoiesis, and animal survival

As a putative transcription factor, Af17 may play a role in multiple signaling pathways. However, the Af17 expression profile during development and in adult tissues remains largely uncharacterized. The importance of Af17 function in embryogenesis, hematopoiesis, and animal survival has never been addressed before. Here we report the generation of the first Af17 mutant mouse model and characterization of the Af17 temporal and spatial expression profile in various embryonic stages and adult tissues by X-gal staining, in situ hybridization, and RT-PCR. Af17 expression is detected in specific cell populations in all stages and in multiple tissues examined. In situ hybridization yielded a consistent Af17 expression pattern by X-gal staining. Homozygous mutant mice are viable, fertile, normal in size, and do not display any gross physical, behavioral, or hematopoietic abnormalities. Thus, our studies describe the generation of the first Af17 mutant mouse model, provide the first developmental profile of Af17 expression, and reveal that Af17 may be dispensable for normal embryogenesis, hematopoiesis, and animal survival.

Regulation of heterochromatic DNA replication by histone H3 lysine 27 methyltransferases

Multiple pathways prevent DNA replication from occurring more than once per cell cycle. These pathways block re-replication by strictly controlling the activity of pre-replication complexes, which assemble at specific sites in the genome called origins. Here we show that mutations in the homologous histone 3 lysine 27 (H3K27) monomethyltransferases, ARABIDOPSIS TRITHORAX-RELATED PROTEIN5 (ATXR5) and ATXR6, lead to re-replication of specific genomic locations. Most of these locations correspond to transposons and other repetitive and silent elements of the Arabidopsis genome. These sites also correspond to high levels of H3K27 monomethylation, and mutation of the catalytic SET domain is sufficient to cause the re-replication defect. Mutation of ATXR5 and ATXR6 also causes upregulation of transposon expression and has pleiotropic effects on plant development. These results uncover a novel pathway that prevents over-replication of heterochromatin in Arabidopsis.

Binding of the MLL PHD3 finger to histone H3K4me3 is required for MLL-dependent gene transcription

The MLL (mixed-lineage leukemia) proto-oncogene encodes a histone methyltransferase that creates the methylated histone H3K4 epigenetic marks, commonly associated with actively transcribed genes. In addition to its canonical histone methyltransferase SET domain, the MLL protein contains three plant homeodomain (PHD) fingers that are well conserved between species but whose potential roles and requirements for MLL function are unknown. Here, we demonstrate that the third PHD domain of MLL (PHD3) binds histone H3 trimethylated at lysine 4 (H3K4me3) with high affinity and specificity and H3K4me2 with 8-fold lower affinity. Biochemical and structural analyses using NMR and fluorescence spectroscopy identified key amino acids essential for the interaction with H3K4me3. Site-directed mutations of the residues involved in recognition of H3K4me3 compromised in vitro H3K4me3 binding but not in vivo localization of full-length MLL to chromatin sites in target promoters of MEIS1 and HOXA genes. Whereas intact PHD3 finger was necessary for MLL occupancy at these promoters, H3K4me3 binding was critical for MLL transcriptional activity. These results demonstrate that MLL occupancy and target gene activation can be functionally separated. Furthermore, these findings reveal that MLL not only "writes" the H3K4me3 mark but also binds the mark, and this binding is required for the transcriptional maintenance functions of MLL.