The Polycomb complex PRC2 and its mark in life. Nature. 2011;469:343-349. [PMID: 21248841 DOI: 10.1038/nature09784]

Histone modifiers in cancer: friends or foes?

Covalent modifications of histones can regulate all DNA-dependent processes. In the last few years, it has become more and more evident that histone modifications are key players in the regulation of chromatin states and dynamics as well as in gene expression. Therefore, histone modifications and the enzymatic machineries that set them are crucial regulators that can control cellular proliferation, differentiation, plasticity, and malignancy processes. This review discusses the biology and biochemistry of covalent histone posttranslational modifications (PTMs) and evaluates the dual role of their modifiers in cancer: as oncogenes that can initiate and amplify tumorigenesis or as tumor suppressors.

The noncoding RNA Mistral activates Hoxa6 and Hoxa7 expression and stem cell differentiation by recruiting MLL1 to chromatin

The epigenetic activator Mixed lineage leukemia 1 (MLL1) is paramount for embryonic development and hematopoiesis. Here, we demonstrate that the long, noncoding RNA (lncRNA) Mistral (Mira) activates transcription of the homeotic genes Hoxa6 and Hoxa7 in mouse embryonic stem cells (mESC) by recruiting MLL1 to chromatin. The Mira gene is located in the spacer DNA region (SDR) separating Hoxa6 and Hoxa7, transcriptionally silent in mESCs, and activated by retinoic acid. Mira-mediated recruitment of MLL1 to the Mira gene triggers dynamic changes in chromosome conformation, culminating in activation of Hoxa6 and Hoxa7 transcription. Hoxa6 and Hoxa7 activate the expression of genes involved in germ layer specification during mESC differentiation in a cooperative and redundant fashion. Our results connect the lncRNA Mira with the recruitment of MLL1 to target genes and implicate lncRNAs in epigenetic activation of gene expression during vertebrate cell-fate determination.

An interspecies analysis reveals a key role for unmethylated CpG dinucleotides in vertebrate Polycomb complex recruitment

The role of DNA sequence in determining chromatin state is incompletely understood. We have previously demonstrated that large chromosomal segments from human cells recapitulate their native chromatin state in mouse cells, but the relative contribution of local sequences versus their genomic context remains unknown. In this study, we compare orthologous chromosomal regions for which the human locus establishes prominent sites of Polycomb complex recruitment in pluripotent stem cells, whereas the corresponding mouse locus does not. Using recombination-mediated cassette exchange at the mouse locus, we establish the primacy of local sequences in the encoding of chromatin state. We show that the signal for chromatin bivalency is redundantly encoded across a bivalent domain and that this reflects competition between Polycomb complex recruitment and transcriptional activation. Furthermore, our results suggest that a high density of unmethylated CpG dinucleotides is sufficient for vertebrate Polycomb recruitment. This model is supported by analysis of DNA methyltransferase-deficient embryonic stem cells.

Transcriptional regulation of Arabidopsis LEAFY COTYLEDON2 involves RLE, a cis-element that regulates trimethylation of histone H3 at lysine-27

LEAFY COTYLEDON2 (LEC2) is a master regulator of seed development in Arabidopsis thaliana. In vegetative organs, LEC2 expression is negatively regulated by Polycomb Repressive Complex2 (PRC2) that catalyzes histone H3 Lys 27 trimethylation (H3K27me3) and plays a crucial role in developmental phase transitions. To characterize the cis-regulatory elements involved in the transcriptional regulation of LEC2, molecular dissections and functional analyses of the promoter region were performed in vitro, both in yeast and in planta. Two cis-activating elements and a cis-repressing element (RLE) that is required for H3K27me3 marking were characterized. Remarkably, insertion of the RLE cis-element into pF3H, an unrelated promoter, is sufficient for repressing its transcriptional activity in different tissues. Besides improving our understanding of LEC2 regulation, this study provides important new insights into the mechanisms underlying H3K27me3 deposition and PRC2 recruitment at a specific locus in plants.

Computational analysis of expression of human embryonic stem cell-associated signatures in tumors

Background

The cancer stem cell model has been proposed based on the linkage between human embryonic stem cells and human cancer cells. However, the evidences supporting the cancer stem cell model remain to be collected. In this study, we extensively examined the expression of human embryonic stem cell-associated signatures including core genes, transcription factors, pathways and microRNAs in various cancers using the computational biology approach.

Results

We used the class comparison analysis and survival analysis algorithms to identify differentially expressed genes and their associated transcription factors, pathways and microRNAs among normal vs. tumor or good prognosis vs. poor prognosis phenotypes classes based on numerous human cancer gene expression data. We found that most of the human embryonic stem cell- associated signatures were frequently identified in the analysis, suggesting a strong linkage between human embryonic stem cells and cancer cells.

Conclusions

The present study revealed the close linkage between the human embryonic stem cell associated gene expression profiles and cancer-associated gene expression profiles, and therefore offered an indirect support for the cancer stem cell theory. However, many interest issues remain to be addressed further.

Epigenetic repression of the Igk locus by STAT5-mediated recruitment of the histone methyltransferase Ezh2

During B lymphopoiesis, recombination of the locus encoding the immunoglobulin κ-chain complex (Igk) requires expression of the precursor to the B cell antigen receptor (pre-BCR) and escape from signaling via the interleukin 7 receptor (IL-7R). By activating the transcription factor STAT5, IL-7R signaling maintains proliferation and represses Igk germline transcription by unknown mechanisms. We demonstrate that a STAT5 tetramer bound the Igk intronic enhancer (E(κi)), which led to recruitment of the histone methyltransferase Ezh2. Ezh2 marked trimethylation of histone H3 at Lys27 (H3K27me3) throughout the κ-chain joining region (J(κ)) to the κ-chain constant region (C(κ)). In the absence of Ezh2, IL-7 failed to repress Igk germline transcription. H3K27me3 modifications were lost after termination of IL-7R-STAT5 signaling, and the transcription factor E2A bound E(κi), which resulted in acquisition of H3K4me1 and acetylated histone H4 (H4Ac). Genome-wide analyses showed a STAT5 tetrameric binding motif associated with transcriptional repression. Our data demonstrate how IL-7R signaling represses Igk germline transcription and provide a general model for STAT5-mediated epigenetic transcriptional repression.

Epigenetics and cardiovascular development

The cardiovascular system is broadly composed of the heart, which pumps blood, and the blood vessels, which carry blood to and from tissues of the body. Heart malformations are the most serious common birth defect, affecting at least 2% of newborns and leading to significant morbidity and mortality. Severe heart malformations cause heart failure in fetuses, infants, and children, whereas milder heart defects may not trigger significant heart dysfunction until early or midadulthood. Severe vasculogenesis or angiogenesis defects in embryos are incompatible with life, and anomalous arterial patterning may cause vascular aberrancies that often require surgical treatment. It is therefore important to understand the underlying mechanisms that control cardiovascular development. Understanding developmental mechanisms will also help us design better strategies to regenerate cardiovascular tissues for therapeutic purposes. An important mechanism regulating genes involves the modification of chromatin, the higher-order structure in which DNA is packaged. Recent studies have greatly expanded our understanding of the regulation of cardiovascular development at the chromatin level, including the remodeling of chromatin and the modification of histones. Chromatin-level regulation integrates multiple inputs and coordinates broad gene expression programs. Thus, understanding chromatin-level regulation will allow for a better appreciation of gene regulation as a whole and may set a fundamental basis for cardiovascular disease. This review focuses on how chromatin-remodeling and histone-modifying factors regulate gene expression to control cardiovascular development.

Corepressor protein CDYL functions as a molecular bridge between polycomb repressor complex 2 and repressive chromatin mark trimethylated histone lysine 27

Polycomb group proteins play essential roles in transcriptional regulation of multiple gene families involved in various pathophysiological processes. It is believed that Polycomb Repressive Complex 2 (PRC2) is targeted to chromatin by the EED subunit to methylate histone H3 lysine 27 (H3K27), leading to a repressive chromatin state that inhibits gene expression. Here we report that the chromodomain-containing protein CDYL specifically recognizes di- and tri-methylated H3K27 (H3K27me2 and H3K27me3) and directly interacts with EZH2, the catalytic subunit of PRC2. We show that CDYL dramatically enhances the methyltransferase activity of PRC2 toward oligonucleosome substrates in vitro. Genome-wide analysis of CDYL targets by ChIP sequencing revealed that CDYL and PRC2 share a number of genomic targets. CDYL is required for chromatin targeting and maximal enzymatic activity of PRC2 at their common target sites. Our experiments indicate that CDYL functions as a molecular bridge between PRC2 and the repressive chromatin mark H3K27me3, forming a positive feedback loop to facilitate the establishment and propagation of H3K27me3 modifications along the chromatin.

Bmi1 marks intermediate precursors during differentiation of human brain tumor initiating cells

The master regulatory gene Bmi1 modulates key stem cell properties in neural precursor cells (NPCs), and has been implicated in brain tumorigenesis. We previously identified a population of CD133+ brain tumor cells possessing stem cell properties, known as brain tumor initiating cells (BTICs). Here, we characterize the expression and role of Bmi1 in primary minimally cultured human glioblastoma (GBM) patient isolates in CD133+ and CD133- sorted populations. We find that Bmi1 expression is increased in CD133- cells, and Bmi1 protein and transcript expression are highest during intermediate stages of differentiation as CD133+ BTICs lose their CD133 expression. Furthermore, in vitro stem cell assays and Bmi1 knockdown show that Bmi1 contributes to self-renewal in CD133+ populations, but regulates proliferation and cell fate determination in CD133- populations. Finally, we test if our in vitro stem cell assays and Bmi1 expression in BTIC patient isolates are predictive of clinical outcome for GBM patients. Bmi1 expression profiles show a marked elevation in the proneural GBM subtype, and stem cell frequency as assessed by tumor sphere assays correlates with patient outcome.

Epigenetic drug discovery: targeting DNA methyltransferases

Epigenetic modification of DNA leads to changes in gene expression. DNA methyltransferases (DNMTs) comprise a family of nuclear enzymes that catalyze the methylation of CpG dinucleotides, resulting in an epigenetic methylome distinguished between normal cells and those in disease states such as cancer. Disrupting gene expression patterns through promoter methylation has been implicated in many malignancies and supports DNMTs as attractive therapeutic targets. This review focuses on the rationale of targeting DNMTs in cancer, the historical approach to DNMT inhibition, and current marketed hypomethylating therapeutics azacytidine and decitabine. In addition, we address novel DNMT inhibitory agents emerging in development, including CP-4200 and SGI-110, analogs of azacytidine and decitabine, respectively; the oligonucleotides MG98 and miR29a; and a number of reversible inhibitors, some of which appear to be selective against particular DNMT isoforms. Finally, we discuss future opportunities and challenges for next-generation therapeutics.

Aebp2 as an epigenetic regulator for neural crest cells

Aebp2 is a potential targeting protein for the mammalian Polycomb Repression Complex 2 (PRC2). We generated a mutant mouse line disrupting the transcription of Aebp2 to investigate its in vivo roles. Aebp2-mutant homozygotes were embryonic lethal while heterozygotes survived to adulthood with fertility. In developing mouse embryos, Aebp2 is expressed mainly within cells of neural crest origin. In addition, many heterozygotes display a set of phenotypes, enlarged colon and hypopigmentation, similar to those observed in human patients with Hirschsprung's disease and Waardenburg syndrome. These phenotypes are usually caused by the absence of the neural crest-derived ganglia in hindguts and melanocytes. ChIP analyses demonstrated that the majority of the genes involved in the migration and development process of neural crest cells are downstream target genes of AEBP2 and PRC2. Furthermore, expression analyses confirmed that some of these genes are indeed affected in the Aebp2 heterozygotes. Taken together, these results suggest that Aebp2 may regulate the migration and development of the neural crest cells through the PRC2-mediated epigenetic mechanism.

Roles of the Polycomb group proteins in stem cells and cancer

Polycomb group proteins have long been linked to the occurrence of different forms of cancer. Polycomb proteins form at least two distinct complexes, the Polycomb-repressive complexes 1 and 2 (PRC1 and PRC2). Some of the PRC complex subunits have been found to be overexpressed in a variety of different tumors. Epigenetic perturbations are likely to be the cause for transcriptional misregulation of tumor suppressor genes and of certain cell fates. It is especially critical for stem cells that their potential to self-renewal and to differentiate is tightly controlled and properly orchestrated. Misregulation of Polycomb protein levels often leads to either a block or unscheduled activation of developmental pathways, thereby enhancing the proliferation capability of a cell. The consequences of this misregulation have been linked to the establishment of cancer stem cells, which can produce tumors through a combination of increased self-renewal and the lack of complete cellular differentiation. Cancer stem cells are believed to persist within tumors and to elicit relapse and metastasis. In this review, we recapitulate the roles of Polycomb proteins in stem cell biology, and the impact their misregulation can have on cancer.

Application of ChIP-Seq and related techniques to the study of immune function

Behaviors observed at the cellular level such as development and acquisition of effector functions by immune cells result from transcriptional changes. The biochemical mediators of transcription are sequence-specific transcription factors (TFs), chromatin modifying enzymes, and chromatin, the complex of DNA and histone proteins. Covalent modification of DNA and histones, also termed epigenetic modification, influences the accessibility of target sequences for transcription factors on chromatin and the expression of linked genes required for immune functions. Genome-wide techniques such as ChIP-Seq have described the entire "cistrome" of transcription factors involved in specific developmental steps of B and T cells and started to define specific immune responses in terms of the binding profiles of critical effectors and epigenetic modification patterns. Current data suggest that both promoters and enhancers are prepared for action at different stages of activation by epigenetic modification through distinct transcription factors in different cells.

Chromatin connections to pluripotency and cellular reprogramming

The pluripotent state of embryonic stem cells (ESCs) provides a unique perspective on regulatory programs that govern self-renewal and differentiation and somatic cell reprogramming. Here, we review the highly connected protein and transcriptional networks that maintain pluripotency and how they are intertwined with factors that affect chromatin structure and function. The complex interrelationships between pluripotency and chromatin factors are illustrated by X chromosome inactivation, regulatory control by noncoding RNAs, and environmental influences on cell states. Manipulation of cell state through the process of transdifferentiation suggests that environmental cues may direct transcriptional programs as cells enter a transiently "plastic" state during reprogramming.

Coordinated regulation of polycomb group complexes through microRNAs in cancer

Polycomb Repressive Complexes (PRC1 and PRC2)-mediated epigenetic regulation is critical for maintaining cellular homeostasis. Members of Polycomb Group (PcG) proteins including EZH2, a PRC2 component, are upregulated in various cancer types, implicating their role in tumorigenesis. Here, we have identified several microRNAs (miRNAs) that are repressed by EZH2. These miRNAs, in turn, regulate the expression of PRC1 proteins BMI1 and RING2. We found that ectopic overexpression of EZH2-regulated miRNAs attenuated cancer cell growth and invasiveness, and abrogated cancer stem cell properties. Importantly, expression analysis revealed an inverse correlation between miRNA and PRC protein levels in cell culture and prostate cancer tissues. Taken together, our data have uncovered a coordinate regulation of PRC1 and PRC2 activities that is mediated by miRNAs.

Epigenetic silencing of HIV-1 by the histone H3 lysine 27 methyltransferase enhancer of Zeste 2

Latent HIV proviruses are silenced as the result of deacetylation and methylation of histones located at the viral long terminal repeat (LTR). Inhibition of histone deacetylases (HDACs) leads to the reemergence of HIV-1 from latency, but the contribution of histone lysine methyltransferases (HKMTs) to maintaining HIV latency remains uncertain. Chromatin immunoprecipitation experiments using latently infected Jurkat T-cell lines demonstrated that the HKMT enhancer of Zeste 2 (EZH2) was present at high levels at the LTR of silenced HIV proviruses and was rapidly displaced following proviral reactivation. Knockdown of EZH2, a key component of the Polycomb repressive complex 2 (PRC2) silencing machinery, and the enzyme which is required for trimethyl histone lysine 27 (H3K27me3) synthesis induced up to 40% of the latent HIV proviruses. In contrast, there was less than 5% induction of latent proviruses following knockdown of SUV39H1, which is required for H3K9me3 synthesis. Knockdown of EZH2 also sensitized latent proviruses to external stimuli, such as T-cell receptor stimulation, and slowed the reversion of reactivated proviruses to latency. Similarly, cell populations that responded poorly to external stimuli carried HIV proviruses that were enriched in H3K27me3 and relatively depleted in H3K9me3. Treating latently infected cells with the HKMT inhibitor 3-deazaneplanocin A, which targets EZH2, led to the reactivation of silenced proviruses, whereas chaetocin and BIX01294 showed only minimal reactivation activities. These findings suggest that PRC2-mediated silencing is an important feature of HIV latency and that inhibitors of histone methylation may play a useful role in induction strategies designed to eradicate latent HIV pools.

Epigenetic regulation of cellular adhesion in cancer

Epigenetics describes the development and maintenance of stable heritable gene expression patterns, which allow cells to show different phenotypes despite of a commonly shared genetic code. The increasing knowledge in this field during the last decades reveals its importance for many physiological processes like differentiation, embryogenesis and parental imprinting, but also for some diseases such as cancer. Recent data have shown that the complexity of carcinogenesis can no longer be explained solely on the basis of genetic changes, but epigenomic alterations such as changes of the DNA methylation pattern and/or post-translational histone modifications and changes of microRNA expression need to be equally considered. Such epigenetic alterations may cause permanent changes in gene expression patterns and may therefore essentially contribute to some of the known phenotypic characteristics of cancer cells like the loss of growth control, altered intercellular communication and enhanced motility. The two latter may essentially be associated with the downregulation of cellular adhesion molecules, which may therefore be relevant in the context of cancer invasiveness and prognosis. The targeted modification of the epigenome may therefore open new horizons within the increasingly important field of epigenetic therapeutics-particularly in view of the regulation of cellular adhesion with particular attention to tumor cell invasion and metastasis.

Evolutionary diversity and developmental regulation of X-chromosome inactivation

X-chromosome inactivation (XCI) results in the transcriptional silencing of one X-chromosome in females to attain gene dosage parity between XX female and XY male mammals. Mammals appear to have developed rather diverse strategies to initiate XCI in early development. In placental mammals XCI depends on the regulatory noncoding RNA X-inactive specific transcript (Xist), which is absent in marsupials and monotremes. Surprisingly, even placental mammals show differences in the initiation of XCI in terms of Xist regulation and the timing to acquire dosage compensation. Despite this, all placental mammals achieve chromosome-wide gene silencing at some point in development, and this is maintained by epigenetic marks such as chromatin modifications and DNA methylation. In this review, we will summarise recent findings concerning the events that occur downstream of Xist RNA coating of the inactive X-chromosome (Xi) to ensure its heterochromatinization and the maintenance of the inactive state in the mouse and highlight similarities and differences between mammals.

Removing the brakes on cell identity

In this issue of Developmental Cell, Dhawan et al. (2011) show that deletion of the Dnmt1 DNA methyltransferase gene in pancreatic insulin-producing cells makes these cells convert into glucagon-producing cells. This suggests that manipulation of a general epigenetic mechanism may be used to redirect cell fates.

The time of timing: how Polycomb proteins regulate neurogenesis

The study of mammalian corticogenesis has revealed a critical role for Polycomb group (PcG) factors in timing the execution of developmental choices. Meanwhile, the study of post-translational modifications of PcG factors marks a symmetrical point, namely that the activity of PcG proteins is itself timed in a manner that links progression through the cell cycle to targeting of downstream genes. Finally, in a third symmetrical twist, the studies that dissect the timing of neural fate by Polycomb are also uncovering the importance of timing in the experimental mutation, since ablation of the same PcG member at different developmental stages yields dramatically different results. Here, I weave together these three lines of evidence and develop a unifying model that clarifies the dynamics of Polycomb function in neural development and defines the salient challenges ahead.

Sweet memories: epigenetic control in flowering

Many plants respond to winter with epigenetic factors that gradually dampen repression of flowering so that they can flower in spring. The study of this process was important for the identification of the plant Polycomb group (PcG) of proteins and their role in the epigenetic control of plant gene expression. Fittingly, these studies continue to illuminate our understanding of PcG function. We discuss recent advances, particularly the role of noncoding RNA in the recruitment of PcG to target genes, and the role of the PcG in regulating the stem cell pool in flowers.

SET domain proteins in plant development

Post-translational methylation of lysine residues on histone tails is an epigenetic modification crucial for regulation of chromatin structure and gene expression in eukaryotes. The majority of the histone lysine methyltransferases (HKMTases) conferring such modifications are proteins with a conserved SET domain responsible for the enzymatic activity. The SET domain proteins in the model plant Arabidopsis thaliana can be assigned to evolutionarily conserved classes with different specificities allowing for different outcomes on chromatin structure. Here we review the present knowledge of the biochemical and biological functions of plant SET domain proteins in developmental processes. This article is part of a Special Issue entitled: Epigenetic control of cellular and developmental processes in plants.

Polycomb proteins in mammalian cell differentiation and plasticity

During development cell differentiation is accompanied by progressive restriction of the developmental potential and increased structural and functional specialization of cells. In this context, mechanisms of cell memory guarantee that cells maintain different identities previously determined by the integrated action of signalling and specific sets of transcription factors. Unraveling the molecular basis by which cells build and maintain their memory represents one of the most fascinating problems in biology. PcG proteins were originally identified as part of an epigenetic cellular memory system that controls gene silencing via chromatin structure. However, recent reports suggest that they are also involved in controlling dynamics and plasticity of gene regulation, particularly during differentiation, by interacting with other components of the transcriptional apparatus. In this review, we discuss the role of PcG proteins in pluripotent ES cells and in well known mammalian cell differentiation systems including skeletal muscle, epidermal, neuronal differentiation. The emerging picture suggests that indeed, plasticity and not rigidity is a fundamental aspect of PcG physiology and cell memory function.

An E3 ligase complex regulates SET-domain polycomb group protein activity in Arabidopsis thaliana

Transcriptional repression via methylation of histone H3 lysine 27 (H3K27) by the polycomb repressive complex 2 (PRC2) is conserved in higher eukaryotes. The Arabidopsis PRC2 controls homeotic gene expression, flowering time, and gene imprinting. Although downstream target genes and the regulatory mechanism of PRC2 are well understood, much less is known about the significance of posttranslational regulation of PRC2 protein activity. Here, we show the posttranslational regulation of CURLY LEAF (CLF) SET-domain polycomb group (PcG) protein by the F-box protein, UPWARD CURLY LEAF1 (UCL1). Overexpression of UCL1 generates mutant phenotypes similar to those observed in plants with a loss-of-function mutation in the CLF gene. Leaf curling and early flowering phenotypes of UCL1 overexpression mutants, like clf mutants, are rescued by mutations in the AGAMOUS and FLOWERING LOCUS T genes, which is consistent with UCL1 and CLF functioning in the same genetic pathway. Overexpression of UCL1 reduces the level of CLF protein and alters expression and H3K27 methylation of CLF-target genes in transgenic plants, suggesting that UCL1 negatively regulates CLF. Interaction of UCL1 with CLF was detected in plant nuclei and in the yeast two-hybrid system. The UCL1 F-box binds in vivo to components of the E3 ligase complex, which ubiquitylate proteins that are subsequently degraded via the ubiquitin-26S proteasome pathway. Taken together, these results demonstrate the posttranslational regulation of the CLF SET-domain PcG activity by the UCL1 F-box protein in the E3 ligase complex.

Epigenetics in plants-vernalisation and hybrid vigour

In this review we have analysed two major biological systems involving epigenetic control of gene activity. In the first system we demonstrate the interplay between genetic and epigenetic controls over the transcriptional activity of FLC, a major repressor of flowering in Arabidopsis. FLC is down-regulated by low temperature treatment (vernalisation) releasing the repressor effect on flowering. We discuss the mechanisms of the reduced transcription and the memory of the vernalisation treatment through vegetative development. We also discuss the resetting of the repressed activity level of the FLC gene, following vernalisation, to the default high activity level and show it occurs during both male and female gametogenesis but with different timing in each. In the second part of the review discussed the complex multigenic system which is responsible for the patterns of gene activity which bring about hybrid vigour in crosses between genetically similar but epigenetically distinct parents. The epigenetic systems that we have identified as contributing to the heterotic phenotype are the 24nt siRNAs and their effects on RNA dependent DNA methylation (RdDM) at the target loci leading to changed expression levels. We conclude that it is likely that epigenetic controls are involved in expression systems in many aspects of plant development and plant function.

Down-regulation of the tumor suppressor C-terminal Src kinase (Csk)-binding protein (Cbp)/PAG1 is mediated by epigenetic histone modifications via the mitogen-activated protein kinase (MAPK)/phosphatidylinositol 3-kinase (PI3K) pathway

The transmembrane adaptor protein Cbp (or PAG1) functions as a suppressor of Src-mediated tumor progression by promoting the inactivation of Src. The expression of Cbp is down-regulated in Src-transformed cells and in various human cancer cells, suggesting a potential role for Cbp as a tumor suppressor. However, the mechanisms underlying the down-regulation of Cbp remain unknown. The present study shows that Cbp expression is down-regulated by epigenetic histone modifications via the MAPK/PI3K pathway. In mouse embryonic fibroblasts, transformation by oncogenic Src and Ras induced a marked down-regulation of Cbp expression. The levels of Cbp expression were inversely correlated with the activity of MEK and Akt, and Cbp down-regulation was suppressed by inhibiting MEK and PI3K. Src transformation did not affect the stability of Cbp mRNA, the transcriptional activity of the cbp promoter, or the DNA methylation status of the cbp promoter CpG islands. However, Cbp expression was restored by treatment with histone deacetylase (HDAC) inhibitors and by siRNA-mediated knockdown of HDAC1/2. Src transformation significantly decreased the acetylation levels of histone H4 and increased the trimethylation levels of histone H3 lysine 27 in the cbp promoter. EGF-induced Cbp down-regulation was also suppressed by inhibiting MEK and HDAC. Furthermore, the inhibition of MEK or HDAC restored Cbp expression in human cancer cells harboring Cbp down-regulation through promoter hypomethylation. These findings suggest that Cbp down-regulation is primarily mediated by epigenetic histone modifications via oncogenic MAPK/PI3K pathways in a subset of cancer cells.

Phosphoryl-EZH-ion

Polycomb group (PcG) proteins regulate gene expression in embryonic and adult stem cells, but the mechanisms responsible for PcG gene targeting and regulation remain largely unknown. Recent evidence shows that EZH2, the enzymatic subunit of Polycomb Repressive Complex 2 (PRC2), is a nuclear phosphoprotein linking cell-cycle-intrinsic or extracellular signals to specific epigenetic signatures.

Kcnq1ot1: a chromatin regulatory RNA

There is a growing interest for noncoding RNA (ncRNA)-mediated epigenetic regulation of transcription in diverse biological functions. Recent evidence suggests that a subset of long ncRNA epigenetically regulate the transcription of multiple genes in chromosomal domains via interaction with chromatin. Kcnq1ot1 is one such long chromatin-interacting ncRNA that silences multiple genes in the Kcnq1 domain by establishing a repressive higher order chromatin structure. This is done by the recruitment of chromatin and DNA-modifying proteins. This review looks at recent evidence supporting the notion that Kcnq1ot1-mediated silencing is a multilayered pathway. Comparing the mode of action of Kcnq1ot1 with other well-investigated chromatin regulatory long ncRNAs, such as Xist, HOTAIR and Airn, revealed that chromatin regulatory ncRNAs share common epigenetic pathways in the silencing of multiple genes.