Preventing transcriptional gene silencing by active DNA demethylation

Interplay of phytohormones and epigenetic regulation: A recipe for plant development and plasticity

Both phytohormone signaling and epigenetic mechanisms have long been known to play crucial roles in plant development and plasticity in response to ambient stimuli. Indeed, diverse signaling pathways mediated by phytohormones and epigenetic processes integrate multiple upstream signals to regulate various plant traits. Emerging evidence indicates that phytohormones and epigenetic processes interact at multiple levels. In this review, we summarize the current knowledge of the interplay between phytohormones and epigenetic processes from the perspective of phytohormone biology. We also review chemical regulators used in epigenetic studies and propose strategies for developing novel regulators using multidisciplinary approaches.

Structural Aspects of DNA Repair and Recombination in Crop Improvement

The adverse effects of global climate change combined with an exponentially increasing human population have put substantial constraints on agriculture, accelerating efforts towards ensuring food security for a sustainable future. Conventional plant breeding and modern technologies have led to the creation of plants with better traits and higher productivity. Most crop improvement approaches (conventional breeding, genome modification, and gene editing) primarily rely on DNA repair and recombination (DRR). Studying plant DRR can provide insights into designing new strategies or improvising the present techniques for crop improvement. Even though plants have evolved specialized DRR mechanisms compared to other eukaryotes, most of our insights about plant-DRRs remain rooted in studies conducted in animals. DRR mechanisms in plants include direct repair, nucleotide excision repair (NER), base excision repair (BER), mismatch repair (MMR), non-homologous end joining (NHEJ) and homologous recombination (HR). Although each DRR pathway acts on specific DNA damage, there is crosstalk between these. Considering the importance of DRR pathways as a tool in crop improvement, this review focuses on a general description of each DRR pathway, emphasizing on the structural aspects of key DRR proteins. The review highlights the gaps in our understanding and the importance of studying plant DRR in the context of crop improvement.

The assessment of epigenetic diversity, differentiation, and structure in the 'Fuji' mutation line implicates roles of epigenetic modification in the occurrence of different mutant groups as well as spontaneous mutants

The ‘Fuji’ line includes many varieties with a similar genetic background and consistent inducement factors with epigenetic occurrence, thus it may be considered an ideal candidate for epigenetic research. In this study, 91 bud mutations of ‘Fuji’ apple were used as the test materials. Using the genetic variation within ‘Fuji’ as the control, the characteristics of epigenetic variation at different levels in both varieties and mutant groups were examined. The results showed that: (1) the global genomic DNA methylation level of the 91 bud mutants of ‘Fuji’ ranged from 29.120%-45.084%, with an average of 35.910%. Internal cytosine methylation was the main DNA methylation pattern. Regarding the variation of methylation patterns of ‘Fuji’ mutants, the vast majority of loci maintained the original methylation pattern existed in ‘Fuji’. CHG methylation variation was the main type of variation; (2) the variation in methylation patterns between the mutant groups was greater than that of methylation levels. Among these patterns, the variation in CHG methylation patterns (including CHG hypermethylation and CHG demethylation) was expected to be dominant. The observed variation in methylation levels was more important in the Color mutant group; however, the variation in methylation patterns was more obvious in both the early maturation and Spur mutant groups. Moreover, the range of variation in the Early-maturation group was much wider than that in the Spur mutant group; (3) epigenetic diversity and genetic diversity were both low between the mutant groups. In the ‘Fuji’ mutant groups, there was few correlation between genetic and epigenetic variation, and epigenetic differentiation resulted in more loci with moderate or greater differentiation; (4) the purifying selection seemed to play a major role in the differentiation of different groups of ‘Fuji’ mutants (65.618%), but epigenetic diversity selection still occurred at nearly 35% of loci. Sixteen epigenetic outlier loci were detected.

A novel DNA methylation motif identified in Bacillus pumilus BA06 and possible roles in the regulation of gene expression

Single-molecule real-time (SMRT) sequencing can be used to identify a wide variety of chemical modifications of the genome, such as methylation. Here, we applied this approach to identify N6-methyl-adenine (m6A) and N4-methyl-cytosine (m4C) modification in the genome of Bacillus pumilus BA06. A typical methylation recognition motif of the type I restriction-modification system (R-M), 5'-TC^m6AN8TTGG-3'/3'-AGTN8^m6AACC-5', was identified. We confirmed that this motif was a new type I methylation site using REBASE analysis and that it was recognized by a type I R-M system, Bpu6ORFCP, according to methylation sensitivity assays in vivo and vitro. Furthermore, we found that deletion of the R-M system Bpu6ORFCP induced transcriptional changes in many genes and led to increased gene expression in pathways related to ABC transporters, sulfur metabolism, ribosomes, cysteine and methionine metabolism and starch and sucrose metabolism, suggesting that the R-M system in B. pumilus BA06 has other significant biological functions beyond protecting the B. pumilus BA06 genome from foreign DNA.

Genome-wide investigation and transcriptional analysis of cytosine-5 DNA methyltransferase and DNA demethylase gene families in tea plant (Camellia sinensis) under abiotic stress and withering processing

DNA methylation is a highly conserved epigenetic modification involved in many biological processes, including growth and development, stress response, and secondary metabolism. In the plant kingdom, cytosine-5 DNA methyltransferase (C5-MTase) and DNA demethylase (dMTase) genes have been identified in some plant species. However, to the best of our knowledge, no investigator has focused on the identification and analysis of C5-MTase and dMTase genes in tea plants (Camellia sinensis) based on genome-wide levels. In this study, eight CsC5-MTases and four dMTases were identified in tea plants. These CsC5-MTase genes were divided into four subfamilies, including CsMET, CsCMT, CsDRM and CsDNMT2. The CsdMTase genes can be classified into CsROS, CsDME and CsDML. Based on conserved domain analysis of these genes, the gene loss and duplication events occurred during the evolution of CsC5-MTase and CsdMTase. Furthermore, multiple cis-acting elements were observed in the CsC5-MTase and CsdMTase, including light responsiveness, phytohormone responsiveness, stress responsiveness, and plant growth and development-related elements. Then, we investigated the transcript abundance of CsC5-MTase and CsdMTase under abiotic stress (cold and drought) and withering processing (white tea and oolong tea). Notably, most CsC5-MTases, except for CsCMT1 and CsCMT2, were significantly downregulated under abiotic stress, while the transcript abundance of all four CsdMTase genes was significantly induced. Similarly, the same transcript abundance of CsC5-MTase and CsdMTase was found during withering processing of white tea and oolong tea, respectively. In total, our findings will provide a basis for the roles of CsC5-MTase and CsdMTase in response to abiotic stress and the potential functions of these two gene families in affecting tea flavor during tea withering processing.

Different Methylation of CpG-SNPs in Behcet's Disease

Purpose

We recently performed an Epigenome-Wide Association Studies (EWAS) study in Behcet's disease (BD) and identified various cytosine–phosphate–guanine (CpG) loci that were aberrantly methylated. In the current study, we wanted to investigate whether these sites contained genetic polymorphisms and whether the frequency of these polymorphisms was altered in BD.

Methods

A two-stage study was performed. The first stage involved 358 BD patients and 704 healthy controls to investigate genetic variants of 10 CpG-SNPs (rs10454134, rs176249, rs3808620, rs10176517, rs11247118, rs78016579, rs9461624, rs10492166, rs34929465, and rs6507921) using an iPLEX Gold genotyping assay and a Sequenom MassARRAY. In the second stage, an additional 172 independent BD patients and 330 healthy individuals are to confirm trends found in the first stage.

Results

A higher frequency of both the rs10454134 AG genotypes (p = 0.008, OR = 1.413, 95% CI = 1.094-1.826) and a lower GG genotype frequency (p = 0.003, OR = 0.630, 95% CI = 0.465-0.854) were found in BD patients compared to the controls in the first stage. However, after correcting for multiple comparisons, all associations identified in the first stage lost statistical significance. The frequencies of the other CpG-SNPs investigated were not different between BD patients and controls. The second stage was designed using an additional cohort to confirm the association with CpG-SNP, rs10454134. The data failed to confirm the association between this CpG-SNP and BD.

Conclusions

This study did not show an association between BD and CpG-SNPs in gene sites that were earlier shown to be aberrantly methylated.

Convergence of evidence from a methylome-wide CpG-SNP association study and GWAS of major depressive disorder

DNA methylation is an epigenetic modification that provides stability and diversity to the cellular phenotype. It is influenced by both genetic sequence variation and environmental factors, and can therefore potentially account for variation of heritable phenotypes and disorders. Therefore, methylome-wide association studies (MWAS) are promising complements to genome-wide association studies (GWAS) of genetic variants. Of particular interest are methylation sites (CpGs) that are created or destroyed by the alleles of single-nucleotide polymorphisms (SNPs), as these so-called CpG-SNPs may show variation in methylation levels on top of what can be explained by the sequence variation. Using sequencing-based data from 1132 major depressive disorder (MDD) cases and controls, we performed a MWAS of 970,414 common CpG-SNPs. The analysis identified 27 suggestively significant (P < 1.00 × 10^-5) CpG-SNPs associations. Furthermore, the MWAS results were over-represented (odds ratios ranging 1.36-5.00; P ranging 4.9 × 10^-3-8.1 × 10^-2) among findings from three recent GWAS for MDD-related phenotypes. Overlapping loci included, e.g., ROBO2, ASIC2, and DCC. As the CpG-SNP analysis accounts for the number of alleles that creates CpGs, the methylation differences could not be explained by differences in allele frequencies. Thus, the results show that the MWAS and GWASs provide independent lines of evidence for the involvement of these loci in MDD. In conclusion, our methylation study of MDD contributes novel information about loci of relevance that complements previous findings and generates new hypothesis about MDD etiology, such as that the functional effects of genetic association may be partly mediated and/or enhanced by the methylation status in these loci.

The genome-wide identification and transcriptional levels of DNA methyltransferases and demethylases in globe artichoke

Changes to the cytosine methylation status of DNA, driven by the activity of C5 methyltransferases (C5-MTases) and demethylases, exert an important influence over development, transposon movement, gene expression and imprinting. Three groups of C5-MTase enzymes have been identified in plants, namely MET (methyltransferase 1), CMT (chromomethyltransferases) and DRM (domains rearranged methyltransferases). Here the repertoire of genes encoding C5-MTase and demethylase by the globe artichoke (Cynara cardunculus var. scolymus) is described, based on sequence homology, a phylogenetic analysis and a characterization of their functional domains. A total of ten genes encoding C5-MTase (one MET, five CMTs and four DRMs) and five demethylases was identified. An analysis of their predicted product's protein structure suggested an extensive level of conservation has been retained by the C5-MTases. Transcriptional profiling based on quantitative real time PCR revealed a number of differences between the genes encoding maintenance and de novo methyltransferases, sometimes in a tissue- or development-dependent manner, which implied a degree of functional specialization.

The developmental regulator PKL is required to maintain correct DNA methylation patterns at RNA-directed DNA methylation loci

Background

The chromodomain helicase DNA-binding family of ATP-dependent chromatin remodeling factors play essential roles during eukaryote growth and development. They are recruited by specific transcription factors and regulate the expression of developmentally important genes. Here, we describe an unexpected role in non-coding RNA-directed DNA methylation in Arabidopsis thaliana.

Results

Through forward genetic screens we identified PKL, a gene required for developmental regulation in plants, as a factor promoting transcriptional silencing at the transgenic RD29A promoter. Mutation of PKL results in DNA methylation changes at more than half of the loci that are targeted by RNA-directed DNA methylation (RdDM). A small number of transposable elements and genes had reduced DNA methylation correlated with derepression in the pkl mutant, though for the majority, decreases in DNA methylation are not sufficient to cause release of silencing. The changes in DNA methylation in the pkl mutant are positively correlated with changes in 24-nt siRNA levels. In addition, PKL is required for the accumulation of Pol V-dependent transcripts and for the positioning of Pol V-stabilized nucleosomes at several tested loci, indicating that RNA polymerase V-related functions are impaired in the pkl mutant.

Conclusions

PKL is required for transcriptional silencing and has significant effects on RdDM in plants. The changes in DNA methylation in the pkl mutant are correlated with changes in the non-coding RNAs produced by Pol IV and Pol V. We propose that at RdDM target regions, PKL may be required to create a chromatin environment that influences non-coding RNA production, DNA methylation, and transcriptional silencing.

Electronic supplementary material

The online version of this article (doi:10.1186/s13059-017-1226-y) contains supplementary material, which is available to authorized users.

A Whole Methylome CpG-SNP Association Study of Psychosis in Blood and Brain Tissue

Mutated CpG sites (CpG-SNPs) are potential hotspots for human diseases because in addition to the sequence variation they may show individual differences in DNA methylation. We performed methylome-wide association studies (MWAS) to test whether methylation differences at those sites were associated with schizophrenia. We assayed all common CpG-SNPs with methyl-CpG binding domain protein-enriched genome sequencing (MBD-seq) using DNA extracted from 1408 blood samples and 66 postmortem brain samples (BA10) of schizophrenia cases and controls. Seven CpG-SNPs passed our FDR threshold of 0.1 in the blood MWAS. Of the CpG-SNPs methylated in brain, 94% were also methylated in blood. This significantly exceeded the 46.2% overlap expected by chance (P-value < 1.0×10(-8)) and justified replicating findings from blood in brain tissue. CpG-SNP rs3796293 in IL1RAP replicated (P-value = .003) with the same direction of effects. This site was further validated through targeted bisulfite pyrosequencing in 736 independent case-control blood samples (P-value < 9.5×10(-4)). Our top result in the brain MWAS (P-value = 8.8×10(-7)) was CpG-SNP rs16872141 located in the potential promoter of ENC1. Overall, our results suggested that CpG-SNP methylation may reflect effects of environmental insults and can provide biomarkers in blood that could potentially improve disease management.

AtROS1 overexpression provides evidence for epigenetic regulation of genes encoding enzymes of flavonoid biosynthesis and antioxidant pathways during salt stress in transgenic tobacco

In plants, epigenetic changes have been identified as regulators of developmental events during normal growth as well as environmental stress exposures. Flavonoid biosynthetic and antioxidant pathways play a significant role in plant defence during their exposure to environmental cues. The aim of this study was to unravel whether genes encoding enzymes of flavonoid biosynthetic and antioxidant pathways are under epigenetic regulation, particularly DNA methylation, during salt stress. For this, a repressor of silencing from Arabidopsis, AtROS1, was overexpressed in transgenic tobacco. Generated transgenics were evaluated to examine the influence of AtROS1 on methylation status of promoters as well as on coding regions of genes encoding enzymes of flavonoids biosynthesis and antioxidant pathways. Overexpression of AtROS1 increases the demethylation levels of both promoters as well as coding regions of genes encoding chalcone synthase, chalcone isomerase, flavanone 3-hydroxylase, flavonol synthase, dihydroflavonol 4-reductase, and anthocyanidin synthase of the flavonoid biosynthetic pathway, and glutathione S-transferase, ascorbate peroxidase, glutathione peroxidase, and glutathione reductase of the antioxidant pathway during control conditions. The level of demethylation was further increased at promoters as well as coding regions of these genes during salt-stress conditions. Transgenic tobacco overexpressing AtROS1 showed tolerance to salt stress that could have been due to the higher expression levels of the genes encoding enzymes of the flavonoid biosynthetic and antioxidant pathways. This is the first comprehensive study documenting the epigenetic regulation of flavonoid biosynthetic and antioxidant pathways during salt-stress exposure of plants.

Involvement of Multiple Gene-Silencing Pathways in a Paramutation-like Phenomenon in Arabidopsis

Paramutation is an epigenetic phenomenon that has been observed in a number of multicellular organisms. The epigenetically silenced state of paramutated alleles is not only meiotically stable but also "infectious" to active homologous alleles. The molecular mechanism of paramutation remains unclear, but components involved in RNA-directed DNA methylation (RdDM) are required. Here, we report a multi-copy pRD29A-LUC transgene in Arabidopsis thaliana that behaves like a paramutation locus. The silent state of LUC is induced by mutations in the DNA glycosylase gene ROS1. The silent alleles of LUC are not only meiotically stable but also able to transform active LUC alleles into silent ones, in the absence of ros1 mutations. Maintaining silencing at the LUC gene requires action of multiple pathways besides RdDM. Our study identified specific factors that are involved in the paramutation-like phenomenon and established a model system for the study of paramutation in Arabidopsis.

Epigenetic Mechanisms of Psychostimulant-Induced Addiction

Psychostimulant-induced addiction involves potentially life-long behavioral abnormalities that are caused by repeated exposure to a drug of abuse in vulnerable individuals. The persistence of these behavioral changes suggests that long-lasting alterations in gene expression, particularly within the brain's reward regions, may contribute significantly to the addiction phenotype. An increasing number of works over the past decade have demonstrated the important role of epigenetic regulatory events in mediating the lasting effects of drugs of abuse (including psychostimulants, such as cocaine and amphetamine) in animal models of drug addiction. In this review, we have introduced the importance of epigenetic processes in regulating gene expression and have described the role that dynamic epigenetic changes may play in psychostimulant-induced addiction via long-lasting transcriptional changes following repeated drug exposure. We overviewed the evidence showing that repeated exposure to psychostimulants induces three major modes of epigenetic regulation within the brain's reward regions-histone modification, DNA methylation, and noncoding RNAs. In several instances, it has been possible to demonstrate directly the contribution of these epigenetic changes to psychostimulant-related behavioral abnormalities. Studies of epigenetics may also help to determine the role environmental factors play in an individual's vulnerability to addiction. Further studies are required to validate these epigenetic changes in human addiction and to evaluate the possibility of developing new diagnostic tests and more effective treatments for addiction syndromes.

Uracil-DNA glycosylase is involved in DNA demethylation and required for embryonic development in the zebrafish embryo

Uracil-DNA glycosylase (Ung) is a component of the base excision repair process and has the ability to remove uracil from U:G mispairs in DNA. However, its implications in development of vertebrate embryos are poorly understood. In this study, we found that zebrafish uracil-DNA glycosylase a (Unga) is maternally expressed at high levels and accumulated in nuclei during cleavage and blastulation periods. Knockdown of unga in zebrafish embryos causes an increase of the global DNA methylation level concomitantly with a reduction of overall transcriptional activity in the nucleus, ultimately resulting in embryonic lethality during segmentation period. Conversely, unga overexpression is sufficient to reduce the global DNA methylation level, to increase H3K4me3 and H3K27me3 marks, and to activate genome transcription. Furthermore, overexpression of unga(D132A) mRNA, encoding a mutant Unga without DNA glycosylase activity, does not affect global DNA methylation level, indicating that its involvement in DNA demethylation is dependent on its glycosylase activity. These results together suggest that Unga is implicated in postfertilization genomic DNA demethylation, zygotic gene transcription, and normal embryonic development in zebrafish.

Tissue culture-induced genetic and epigenetic alterations in rice pure-lines, F1 hybrids and polyploids

BACKGROUND

Genetic and epigenetic alterations can be invoked by plant tissue culture, which may result in heritable changes in phenotypes, a phenomenon collectively termed somaclonal variation. Although extensive studies have been conducted on the molecular nature and spectrum of tissue culture-induced genomic alterations, the issue of whether and to what extent distinct plant genotypes, e.g., pure-lines, hybrids and polyploids, may respond differentially to the tissue culture condition remains poorly understood.

RESULTS

We investigated tissue culture-induced genetic and epigenetic alterations in a set of rice genotypes including two pure-lines (different subspecies), a pair of reciprocal F1 hybrids parented by the two pure-lines, and a pair of reciprocal tetraploids resulted from the hybrids. Using two molecular markers, amplified fragment length polymorphism (AFLP) and methylation-sensitive amplified polymorphism (MSAP), both genetic and DNA methylation alterations were detected in calli and regenerants from all six genotypes, but genetic alteration is more prominent than epigenetic alteration. While significant genotypic difference was observed in frequencies of both types of alterations, only genetic alteration showed distinctive features among the three types of genomes, with one hybrid (N/9) being exceptionally labile. Surprisingly, difference in genetic alteration frequencies between the pair of reciprocal F1 hybrids is much greater than that between the two pure-line subspecies. Difference also exists in the pair of reciprocal tetraploids, but is to a less extent than that between the hybrids. The steady-state transcript abundance of genes involved in DNA repair and DNA methylation was significantly altered in both calli and regenerants, and some of which were correlated with the genetic and/or epigenetic alterations.

CONCLUSIONS

Our results, based on molecular marker analysis of ca. 1,000 genomic loci, document that genetic alteration is the major cause of somaclonal variation in rice, which is concomitant with epigenetic alterations. Perturbed expression by tissue culture of a set of 41 genes encoding for enzymes involved in DNA repair and DNA methylation is associated with both genetic and epigenetic alterations. There exist fundamental differences among distinct genotypes, pure-lines, hybrids and tetraploids, in propensities of generating both genetic and epigenetic alterations under the tissue culture condition. Parent-of-origin has a conspicuous effect on the alteration frequencies.

Analysis method of epigenetic DNA methylation to dynamically investigate the functional activity of transcription factors in gene expression

Background

DNA methylation is a fundamental component of epigenetic modification, which is intimately involved in the regulation of gene expression. One important DNA methylation pathway reduces the abilities of transcription factors to bind to gene promoter regions. Although many experiments have been designed to measure genome-wide DNA methylation levels at high resolution, the meaning of these different DNA methylation levels on transcription factor binding abilities remains poorly understood. We have, therefore, developed a method to quantitatively explore the extent to which DNA methylation levels can significantly reduce or even abolish the binding of certain transcription factors, resulting in reduced or non-expression of flanking genes. This method allows transcription factors that are functionally active in gene expression to be investigated.

Results

The method is based on a general model that depicts the relationship between DNA methylation and transcription factor binding ability based on intrinsic component properties, and the model parameters can be optimized through relative analysis of recognized transcription factor binding status and gene expression profiling. With fixed models, transcription factors functionally active in the regulation of gene expression and affected by epigenetic DNA methylation can be identified and subsequently confirmed. The method identified eleven apparently functionally active transcriptional factors in SH-SY5Y neuroblastoma cells.

Conclusions

Compared with gene regulatory elements, epigenetic modifications are able to change to dynamically respond to signals from physical, biological and social environments. Our proposed method is therefore designed to provide a dynamic assessment to investigate functionally active transcription factors. With the information deduced from our method, we can predict transcription factor binding status in promoter regions to further investigate how a particular gene is regulated by a specific group of transcription factors organized in a particular pattern. This will be helpful in the diagnosis and development of treatment for numerous diseases, including cancer. Although the method only investigates DNA methylation, it has the potential to be applied to more epigenetic factors, such as histone modification.

Transgenerational maintenance of transgene body CG but not CHG and CHH methylation

In plants, RNA-directed DNA methylation (RdDM) can target both transgene promoters and coding regions/gene bodies. RdDM leads to methylation of cytosines in all sequence contexts: CG, CHG and CHH. Upon segregation of the RdDM trigger, at least CG methylation can be maintained at promoter regions in the progeny. So far, it is not clear whether coding region methylation can be also maintained. We showed that the body of Potato spindle tuber viroid (PSTVd) transgene constructs became densely de novo methylated at CG, CHG and CHH sites upon PSTVd infection. In this study, we demonstrate that in viroid-free progeny plants, asymmetric CHH and CHG methylation was completely lost. However, symmetric CG methylation was stably maintained for at least two generations. Importantly, the presence of transgene body methylation did not lead to an increase of dimethylation of histone H3 lysine 9 or a decrease of acetylation of H3. Our data supports the view that CG methylation can be maintained not only in promoters but also in the body of transgenes. They further suggest that maintenance of methylation may occur independently of tested chromatin modifications.

Epigenetic control on cell fate choice in neural stem cells

Derived from neural stem cells (NSCs) and progenitor cells originated from the neuroectoderm, the nervous system presents an unprecedented degree of cellular diversity, interwoven to ensure correct connections for propagating information and responding to environmental cues. NSCs and progenitor cells must integrate cell-intrinsic programs and environmental cues to achieve production of appropriate types of neurons and glia at appropriate times and places during development. These developmental dynamics are reflected in changes in gene expression, which is regulated by transcription factors and at the epigenetic level. From early commitment of neural lineage to functional plasticity in terminal differentiated neurons, epigenetic regulation is involved in every step of neural development. Here we focus on the recent advance in our understanding of epigenetic regulation on orderly generation of diverse neural cell types in the mammalian nervous system, an important aspect of neural development and regenerative medicine.

Active DNA demethylation by Gadd45 and DNA repair

How DNA methylation patterns are established, maintained and remodeled is incompletely understood, however, it has become clear that DNA methylation is reversible and dynamic as a result of enzymatic DNA demethylation. Several different mechanisms that may account for demethylation have recently been put forward and all seem to involve DNA repair. Here, we review DNA demethylation mediated by multifunctional growth arrest and DNA damage 45 (Gadd45) protein family members which mediate DNA demethylation during cell differentiation and stress response. Gadd45 recruits nucleotide and/or base excision repair factors to gene-specific loci and acts as an adapter between repair factors and chromatin, thereby creating a nexus between epigenetics and DNA repair.

IL-2 and GM-CSF are regulated by DNA demethylation during activation of T cells, B cells and macrophages

DNA demethylation has been found to occur at the promoters of a number of actively expressed cytokines and is believed to play a critical role in transcriptional regulation. While many DNA demethylation studies have focused on T cell activation, proliferation and differentiation, changes in DNA methylation in other types of immune cells are less well studied. We found that the expression of two cytokines (IL-2 and GM-CSF) responded differently to activation in three types of immune cells: EL4, A20 and RAW264.7 cells. Using the McrBC and MeDIP approaches, we observed decreases in DNA methylation at a genome-wide level and at the promoters of the genes of these cytokines. The expression of several potential enzymes/co-enzymes involved in the DNA demethylation pathways seemed to be associated with immune cell activation.

A 5-methylcytosine DNA glycosylase/lyase demethylates the retrotransposon Tos17 and promotes its transposition in rice

DNA 5-methylcytosine (5-meC) is an important epigenetic mark for transcriptional gene silencing in many eukaryotes. In Arabidopsis, 5-meC DNA glycosylase/lyases actively remove 5-meC to counteract transcriptional gene silencing in a locus-specific manner, and have been suggested to maintain the expression of transposons. However, it is unclear whether plant DNA demethylases can promote the transposition of transposons. Here we report the functional characterization of the DNA glycosylase/lyase DNG701 in rice. DNG701 encodes a large (1,812 amino acid residues) DNA glycosylase domain protein. Recombinant DNG701 protein showed 5-meC DNA glycosylase and lyase activities in vitro. Knockout or knockdown of DNG701 in rice plants led to DNA hypermethylation and reduced expression of the retrotransposon Tos17. Tos17 showed less transposition in calli derived from dng701 knockout mutant seeds compared with that in wild-type calli. Overexpression of DNG701 in both rice calli and transgenic plants substantially reduced DNA methylation levels of Tos17 and enhanced its expression. The overexpression also led to more frequent transposition of Tos17 in calli. Our results demonstrate that rice DNG701 is a 5-meC DNA glycosylase/lyase responsible for the demethylation of Tos17 and this DNA demethylase plays a critical role in promoting Tos17 transposition in rice calli.

Establishing, maintaining and modifying DNA methylation patterns in plants and animals

Cytosine DNA methylation is a stable epigenetic mark that is crucial for diverse biological processes, including gene and transposon silencing, imprinting and X chromosome inactivation. Recent findings in plants and animals have greatly increased our understanding of the pathways used to accurately target, maintain and modify patterns of DNA methylation and have revealed unanticipated mechanistic similarities between these organisms. Key roles have emerged for small RNAs, proteins with domains that bind methylated DNA and DNA glycosylases in these processes. Drawing on insights from both plants and animals should deepen our understanding of the regulation and biological significance of DNA methylation.

DNA methylation in cancer development, diagnosis and therapy--multiple opportunities for genotoxic agents to act as methylome disruptors or remediators

The role of DNA methylation and recently discovered hydroxymethylation in the function of the human epigenome is currently one of the hottest topics in the life sciences. Progress in this field of research has been further accelerated by the discovery that alterations in the methylome are not only associated with key functions of cells and organisms, such as development, differentiation and gene expression, but may underlie a number of human diseases, including cancer. This review describes both well established and more recent observations concerning alterations in the methylome, i.e. the global and local distribution of 5-methylcytosines, involved in its normal functions. Then, the changes in DNA methylation pattern seen in cancer cells are discussed in the context of their utilisation in cancer diagnostics and treatment. On this basis, comparisons are made between natural covalent DNA modification and that induced by genotoxic agents, chemical carcinogens and antitumour drugs as regards their impact on epigenetic mechanisms. The available data suggest that DNA damage by genotoxins can mimic epigenetic markers and in consequence disrupt the proper function of the epigenome. On the other hand, the same processes in cancer cells, e.g. DNA demethylation as a result of DNA methyltransferase blocking or the induction of DNA repair by DNA adducts, may restore the activity of hypermethylated anticancer genes. The observed multiple mechanisms by which genotoxic agents directly affect methylome function suggest that chemical carcinogens act primarily as epigenome disruptors, whereas mutations are secondary events that occur at later stages of cancer development when genome-protecting mechanisms have already been deregulated.

DNA replication factor C1 mediates genomic stability and transcriptional gene silencing in Arabidopsis

Genetic screening identified a suppressor of ros1-1, a mutant of REPRESSOR OF SILENCING1 (ROS1; encoding a DNA demethylation protein). The suppressor is a mutation in the gene encoding the largest subunit of replication factor C (RFC1). This mutation of RFC1 reactivates the unlinked 35S-NPTII transgene, which is silenced in ros1 and also increases expression of the pericentromeric Athila retrotransposons named transcriptional silent information in a DNA methylation-independent manner. rfc1 is more sensitive than the wild type to the DNA-damaging agent methylmethane sulphonate and to the DNA inter- and intra- cross-linking agent cisplatin. The rfc1 mutant constitutively expresses the G2/M-specific cyclin CycB1;1 and other DNA repair-related genes. Treatment with DNA-damaging agents mimics the rfc1 mutation in releasing the silenced 35S-NPTII, suggesting that spontaneously induced genomic instability caused by the rfc1 mutation might partially contribute to the released transcriptional gene silencing (TGS). The frequency of somatic homologous recombination is significantly increased in the rfc1 mutant. Interestingly, ros1 mutants show increased telomere length, but rfc1 mutants show decreased telomere length and reduced expression of telomerase. Our results suggest that RFC1 helps mediate genomic stability and TGS in Arabidopsis thaliana.

Active DNA demethylation mediated by DNA glycosylases

Active DNA demethylation is involved in many vital developmental and physiological processes of plants and animals. Recent genetic and biochemical studies in Arabidopsis have demonstrated that a subfamily of DNA glycosylases function to promote DNA demethylation through a base excision-repair pathway. These specialized bifunctional DNA glycosylases remove the 5-methylcytosine base and then cleave the DNA backbone at the abasic site, resulting in a gap that is then filled with an unmethylated cytosine nucleotide by as yet unknown DNA polymerase and ligase enzymes. Evidence suggests that active DNA demethylation in mammalian cells is also mediated at least in part by a base excision repair pathway where the AID/Apobec family of deaminases convert 5-methylcytosine to thymine followed by G/T mismatch repair by the DNA glycosylase MBD4 or TDG. This review also discusses other possible mechanisms of active DNA demethylation, how genome DNA methylation status might be sensed to regulate the expression of demethylase genes, and the targeting of demethylases by small RNAs.

Epigenetic DNA-(cytosine-5-carbon) modifications: 5-aza-2'-deoxycytidine and DNA-demethylation

DNA (cytosine-5-carbon) methylation is one of the hallmarks of mammalian chromatin modifications. Distinct methylation pattern can generate synergistic or antagonistic interaction affinities for CpG-islands associated with methylated or unmethylated cytosine binding proteins, which also may dictate histone modifications and dynamic transition between transcriptionally silent or transcriptionally active chromatin states. The enzymes and cofactors associated with DNA-methylation reactions are convincing in terms of chemistry and chemical thermodynamics. The mechanism of demethylation, the candidate enzyme(s) exhibiting direct demethylase activity, and associated cofactors are not firmly established. Use of azanucleosides, such as 5-azacytidine and 5-aza-2'-deoxycytidine (AzadC), in cell culture produces re-expression of certain genes, which otherwise were repressed in association with hypermethylated CpG-rich promoters. Hence the notion developed that AzadC is a demethylating agent. Here we discuss the broad global pictures with the following points: first, chemical definition and recent advances regarding the mechanism of DNA (cytosine-5-carbon) methylation ((Me)CpG-DNA or (Me)CpNpG-DNA formation) and (Me)CpG/(Me)CpNpG-DNA-demethylation, and then with the mechanistic basis of inactivation of DNA-methyltransferase 1 by AzadC. This will clarify that: (i) AzadC has nothing to do with DNA-demethylation; (ii) it cannot prevent even de novo methylation in non-replicating cells; (iii) it can only prevent replication coupled maintenance as well as de novo methylations. Finally, we would like to suggest that terming/designating AzadC as DNA-demethylating agent is a serious misuse of chemistry and chemical terminology.

Comparative analysis of the mammalian WNT4 promoter

Background

WNT4 is a critical signalling molecule in embryogenesis and homeostasis, but the elements that control its transcriptional regulation are largely unknown. This study uses comparative cross species sequence and functional analyses between humans and a marsupial (the tammar wallaby,Macropus eugenii) to refine the mammalian Wnt4 promoter.

Results

We have defined a highly conserved 89 bp minimal promoter region in human WNT4 by comparative analysis with the tammar wallaby. There are many conserved transcription factor binding sites in the proximal promoter region, including SP1, MyoD, NFκB and AP2, as well as highly conserved CpG islands within the human, mouse and marsupial promoters, suggesting that DNA methylation may play an important role in WNT4 transcriptional regulation.

Conclusion

Using a marsupial model, we have been able to provide new information on the transcriptional regulators in the promoter of this essential mammalian developmental gene, WNT4. These transcription factor binding sites and CpG islands are highly conserved in two disparate mammals, and are likely key controlling elements in the regulation of this essential developmental gene.

DNA excision repair proteins and Gadd45 as molecular players for active DNA demethylation

DNA cytosine methylation represents an intrinsic modification signal of the genome that plays important roles in heritable gene silencing, heterochromatin formation and certain transgenerational epigenetic inheritance. In contrast to the process of DNA methylation that is catalyzed by specific classes of methyltransferases, molecular players underlying active DNA demethylation have long been elusive. Emerging biochemical and functional evidence suggests that active DNA demethylation in vertebrates can be mediated through DNA excision repair enzymes, similar to the well-known repair-based DNA demethylation mechanism in Arabidopsis. As key regulators, non-enzymatic Gadd45 proteins function to recruit enzymatic machineries and promote coupling of deamination, base and nucleotide-excision repair in the process of DNA demethylation. In this article, we review recent findings and discuss functional and evolutionary implications of such mechanisms underlying active DNA demethylation.

RTA promoter demethylation and histone acetylation regulation of murine gammaherpesvirus 68 reactivation

Gammaherpesviruses have a common biological characteristic, latency and lytic replication. The balance between these two phases in murine gammaherpesvirus 68 (MHV-68) is controlled by the replication and transcription activator (RTA) gene. In this report, we investigated the effect of DNA demethylation and histone acetylation on MHV-68 replication. We showed that distinctive methylation patterns were associated with MHV-68 at the RTA promoter during latency or lytic replication. Treatment of MHV-68 latently-infected S11E cells with a DNA methyltransferases (DNMTs) inhibitor 5-azacytidine (5-AzaC), only weakly reactivated MHV-68, despite resulted in demethylation of the viral RTA promoter. In contrast, treatment with a histone deacetylase (HDAC) inhibitor trichostatin A (TSA) strongly reactivated MHV-68 from latency, and this was associated with significant change in histone H3 and H4 acetylation levels at the RTA promoter. We further showed that HDAC3 was recruited to the RTA promoter and inhibited RTA transcription during viral latency. However, TSA treatment caused rapid removal of HDAC3 and also induced passive demethylation at the RTA promoter. In vivo, we found that the RTA promoter was hypomethylated during lytic infection in the lung and that methylation level increased with virus latent infection in the spleen. Collectively, our data showed that histone acetylation, but not DNA demethylation, is sufficient for effective reactivation of MHV-68 from latency in S11E cells.

Interplay of RNA Pol IV and ROS1 during post-embryonic 5S rDNA chromatin remodeling

We have investigated the chromatin structure of 5S rDNA, a heterochromatic pericentromeric tandemly repeated family, at 2, 3, 4 and 5 days post-germination. Our results revealed a large-scale reorganization of 5S rDNA chromatin that occurs during the first days of development. Unexpectedly, there is a decondensation followed by a 're'condensation of 5S rDNA chromatin, to obtain almost mature nuclei 5 d post-germination. The reorganization of 5S rDNA chromatin is accompanied by a rapid and active demethylation of 5S rDNA mediated by the ROS1 (repressor of silencing 1) demethylase, whereas the plant-specific RNA polymerase IV (Pol IV) is essential to the 5S chromatin 're'condensation. In conclusion, Pol IV and ROS1 collaborate to unlock the 5S rDNA chromatin inherited from the seed, and establish adult features.

Active DNA demethylation and DNA repair

DNA methylation on cytosine is an epigenetic modification and is essential for gene regulation and genome stability in vertebrates. Traditionally DNA methylation was considered as the most stable of all heritable epigenetic marks. However, it has become clear that DNA methylation is reversible by enzymatic "active" DNA demethylation, with examples in plant cells, animal development and immune cells. It emerges that "pruning" of methylated cytosines by active DNA demethylation is an important determinant for the DNA methylation signature of a cell. Work in plants and animals shows that demethylation occurs by base excision and nucleotide excision repair. Far from merely protecting genomic integrity from environmental insult, DNA repair is therefore at the heart of an epigenetic activation process.

Base excision repair and its role in maintaining genome stability

For all living organisms, genome stability is important, but is also under constant threat because various environmental and endogenous damaging agents can modify the structural properties of DNA bases. As a defense, organisms have developed different DNA repair pathways. Base excision repair (BER) is the predominant pathway for coping with a broad range of small lesions resulting from oxidation, alkylation, and deamination, which modify individual bases without large effect on the double helix structure. As, in mammalian cells, this damage is estimated to account daily for 10(4) events per cell, the need for BER pathways is unquestionable. The damage-specific removal is carried out by a considerable group of enzymes, designated as DNA glycosylases. Each DNA glycosylase has its unique specificity and many of them are ubiquitous in microorganisms, mammals, and plants. Here, we review the importance of the BER pathway and we focus on the different roles of DNA glycosylases in various organisms.

DNA demethylation: a lesson from the garden

Gene silencing by DNA methylation is well documented and known to be essential for various biological phenomena in many organisms. In contrast, the processes that convert the silent state of a gene whose DNA is methylated and predicted to form facultative heterochromatin to the actively transcribed state remain elusive. In Arabidopsis, recent studies have shown that the DNA glycosylases DEMETER (DME) and REPRESSOR OF SILENCING1 (ROS1) participate in DNA demethylation. DME is necessary for genomic imprinting in the endosperm, while ROS1 is involved in pruning DNA methylation patterns in transposons and genic regions of vegetative tissues. These findings provide us with molecular clues for understanding the underlying mechanisms of DNA demethylation and gene activation. In this review, we will consider and discuss the processes of controlling gene activation through DNA demethylation, which are predicted to include the recognition of target sequences, DNA demethylation, the transformation of the chromatin to the active state, and transcription. Many of these processes remain poorly understood at this stage.

ROS3 is an RNA-binding protein required for DNA demethylation in Arabidopsis

DNA methylation is an important epigenetic mark for transcriptional gene silencing (TGS) in diverse organisms. Recent studies suggest that the methylation status of a number of genes is dynamically regulated by methylation and demethylation. In Arabidopsis, active DNA demethylation is mediated by the ROS1 (repressor of silencing 1) subfamily of 5-methylcytosine DNA glycosylases through a base excision repair pathway. These demethylases have critical roles in erasing DNA methylation and preventing TGS of target genes. However, it is not known how the demethylases are targeted to specific sequences. Here we report the identification of ROS3, an essential regulator of DNA demethylation that contains an RNA recognition motif. Analysis of ros3 mutants and ros1 ros3 double mutants suggests that ROS3 acts in the same genetic pathway as ROS1 to prevent DNA hypermethylation and TGS. Gel mobility shift assays and analysis of ROS3 immunoprecipitate from plant extracts shows that ROS3 binds to small RNAs in vitro and in vivo. Immunostaining shows that ROS3 and ROS1 proteins co-localize in discrete foci dispersed throughout the nucleus. These results demonstrate a critical role for ROS3 in preventing DNA hypermethylation and suggest that DNA demethylation by ROS1 may be guided by RNAs bound to ROS3.

Hypomethylation and hypermethylation of the tandem repetitive 5S rRNA genes in Arabidopsis

5S ribosomal DNA (5S rDNA) is organized in tandem repeats on chromosomes 3, 4 and 5 in Arabidopsis thaliana. One part of the 5S rDNA is located within the heterochromatic chromocenters, and the other fraction forms loops with euchromatic features that emanate from the chromocenters. We investigated whether the A. thaliana heterochromatin, and particularly the 5S rDNA, is modified when changing the culture conditions (cultivation in growth chamber versus greenhouse). Nuclei from challenged tissues displayed larger total, as well as 5S rDNA, heterochromatic fractions, and the DNA methyltransferase mutants met1 and cmt3 had different impacts in Arabidopsis. The enlarged fraction of heterochromatic 5S rDNA was observed, together with the reversal of the silencing of some 5S rRNA genes known as minor genes. We observed hypermethylation at CATG sites, and a concomitant DNA hypomethylation at CG/CXG sites in 5S rDNA. Our results show that the asymmetrical hypermethylation is correlated with the ageing of the plants, whereas hypomethylation results from the growth chamber/culture conditions. In spite of severely reduced DNA methylation, the met1 mutant revealed no increase in minor 5S rRNA transcripts in these conditions. The increasing proportion of cytosines in asymmetrical contexts during transition from the euchromatic to the heterochromatic state in the 5S rDNA array suggests that 5S rDNA units are differently affected by the (hypo and hyper)methylation patterns along the 5S rDNA locus. This might explain the different behaviour of 5S rDNA subpopulations inside a 5S array in terms of chromatin compaction and expression, i.e. some 5S rRNA genes would become derepressed, whereas others would join the heterochromatic fraction.

Epigenetic regulation of normal and malignant hematopoiesis

The molecular processes governing hematopoiesis involve the interplay between lineage-specific transcription factors and a series of epigenetic tags, including DNA methylation and covalent histone tail modifications, such as acetylation, methylation, phosphorylation, SUMOylation and ubiquitylation. These post-translational modifications, which collectively constitute the 'histone code', are capable of affecting chromatin structure and gene transcription and are catalysed by opposing families of enzymes, allowing the developmental potential of hematopoietic stem cells to be dynamically regulated. The essential role of these enzymes in regulating normal blood development is highlighted by the finding that members from all families of chromatin regulators are targets for dysregulation in many hematological malignancies, and that patterns of histone modification are globally affected in cancer as well as the regulatory regions of specific oncogenes and tumor suppressors. The discovery that these epigenetic marks can be reversed by compounds targeting aberrant transcription factor/co-activator/co-repressor interactions and histone-modifying activities, provides the basis for an exciting field in which the epigenome of cancer cells may be manipulated with potential therapeutic benefits.

An epigenetic perspective on the free radical theory of development

The development of organisms requires concerted changes in gene activity. The free radical theory of development proposes that oxygen serves as a morphogen to educe development by influencing the production of metabolic oxidants such as free radicals and reactive oxygen species. One of the central tenets of this theory is that these metabolic oxidants influence development by altering the antioxidant capacity of cells by changing their production of glutathione (GSH). Here we extend on these principles by linking GSH production and oxygen sensing in the control of gene expression to establish the epigenotype of cells during development. We prescribe this novel role to GSH and oxygen during development because these metabolites influence the activity of enzymes responsible for initiating and perpetuating epigenetic control of gene expression. Increased GSH production influences epigenetic processes including DNA and histone methylation by limiting the availability of S-adenosylmethionine, the cofactor utilized during epigenetic control of gene expression by DNA and histone methyltransferases. Moreover, the recent discovery of histone demethylases that require oxygen as a cofactor directly links epigenetic processes to oxygen gradients during development.

The plant genome's methylation status and response to stress: implications for plant improvement

Plant improvement depends on generating phenotypic variation and selecting for characteristics that are heritable. Classical genetics and early molecular genetics studies on single genes showed that differences in chromatin structure, especially cytosine methylation, can contribute to heritable phenotypic variation. Recent molecular genetic and genomic studies have revealed a new importance of cytosine methylation for gene regulation and have identified RNA interference (RNAi)-related proteins that are necessary for methylation. Methylation differences among plants can be caused by cis- or trans-acting DNA polymorphisms or by epigenetic phenomena. Although regulatory proteins might be important in creating this variation, recent examples highlight the central role of transposable elements and DNA repeats in generating both genetic and epigenetic methylation polymorphisms. The plant genome's response to environmental and genetic stress generates both novel genetic and epigenetic methylation polymorphisms. Novel, stress-induced genotypes may contribute to phenotypic diversity and plant improvement.

RNA-directed DNA methylation mediated by DRD1 and Pol IVb: A versatile pathway for transcriptional gene silencing in plants

RNA-directed DNA methylation, which is one of several RNAi-mediated pathways in the nucleus, has been highly elaborated in the plant kingdom. RNA-directed DNA methylation requires for the most part conventional DNA methyltransferases, histone modifying enzymes and RNAi proteins; however, several novel, plant-specific proteins that are essential for this process have been identified recently. DRD1 (defective in RNA-directed DNA methylation) is a putative SWI2/SNF2-like chromatin remodelling protein; DRD2 and DRD3 (renamed NRPD2a and NRPD1b, respectively) are subunits of Pol IVb, a putative RNA polymerase found only in plants. Interestingly, DRD1 and Pol IVb appear to be required not only for RNA-directed de novo methylation, but also for full erasure of methylation when the RNA trigger is withdrawn. These proteins thus have the potential to facilitate dynamic regulation of DNA methylation. Prominent targets of RNA-directed DNA methylation in the Arabidopsis thaliana genome include retrotransposon long terminal repeats (LTRs), which have bidirectional promoter/enhancer activities, and other types of intergenic transposons and repeats. Intergenic solitary LTRs that are targeted for reversible methylation by the DRD1/Pol IVb pathway can potentially act as switches or rheostats for neighboring plant genes. The resulting alterations in gene expression patterns may promote physiological flexibility and adaptation to the environment.

The DNA glycosylase/lyase ROS1 functions in pruning DNA methylation patterns in Arabidopsis

The Arabidopsis DNA glycosylase/lyase ROS1 participates in active DNA demethylation by a base-excision pathway. ROS1 has been shown to be required for demethylating a transgene promoter. To determine the function of ROS1 in demethylating endogenous loci, we carried out bisulfite-sequencing analysis of several transposons and other genes in the ros1 mutant. In the wild-type, although CpG sites at the majority of these loci are heavily methylated, many of the CpXpG and CpXpX sites have low levels of methylation or are not at all methylated. However, these CpXpG and CpXpX sites become heavily methylated in the ros1 mutant. Associated with this increased DNA methylation, these loci show decreased expression in the ros1 mutant. Our results suggest that active DNA demethylation is important in pruning the methylation patterns of the genome, and even the normally "silent" transposons are under dynamic control by both methylation and demethylation. This dynamic control may be important in keeping the plant epigenome plastic so that it can efficiently respond to developmental and environmental cues.

The protein kinase TOUSLED is required for maintenance of transcriptional gene silencing in Arabidopsis

TOUSLED-like kinases (TLKs) are highly conserved in plants and animals, but direct evidence linking TLKs and transcriptional gene silencing is lacking. We isolated two new alleles of TOUSLED (TSL). Mutations of TSL in ros1 reactivate the transcriptionally silent 35S-NPTII transgene and the transcriptionally silent endogenous loci TSI (TRANSCRIPTIONAL SILENCING INFORMATION). Chromatin immunoprecipitation (ChIP) analysis shows that histone H3Lys9 dimethylation is decreased in the reactivated transgene and endogenous TSI loci in the tsl ros1 mutant. However, there is no change in DNA methylation in the affected loci. Western blot and ChIP assay suggest that TSL might not be responsible for histone H3Ser10 phosphorylation. The tsl seedlings were more sensitive to DNA damage reagent methyl methanesulphonate and UV-B light. Our results provide direct evidence for a crucial role of the TOUSLED protein kinase in the maintenance of transcriptional gene silencing in some genomic regions in a DNA-methylation-independent manner in Arabidopsis.

Epigenetic gene regulation in the bacterial world

Like many eukaryotes, bacteria make widespread use of postreplicative DNA methylation for the epigenetic control of DNA-protein interactions. Unlike eukaryotes, however, bacteria use DNA adenine methylation (rather than DNA cytosine methylation) as an epigenetic signal. DNA adenine methylation plays roles in the virulence of diverse pathogens of humans and livestock animals, including pathogenic Escherichia coli, Salmonella, Vibrio, Yersinia, Haemophilus, and Brucella. In Alphaproteobacteria, methylation of adenine at GANTC sites by the CcrM methylase regulates the cell cycle and couples gene transcription to DNA replication. In Gammaproteobacteria, adenine methylation at GATC sites by the Dam methylase provides signals for DNA replication, chromosome segregation, mismatch repair, packaging of bacteriophage genomes, transposase activity, and regulation of gene expression. Transcriptional repression by Dam methylation appears to be more common than transcriptional activation. Certain promoters are active only during the hemimethylation interval that follows DNA replication; repression is restored when the newly synthesized DNA strand is methylated. In the E. coli genome, however, methylation of specific GATC sites can be blocked by cognate DNA binding proteins. Blockage of GATC methylation beyond cell division permits transmission of DNA methylation patterns to daughter cells and can give rise to distinct epigenetic states, each propagated by a positive feedback loop. Switching between alternative DNA methylation patterns can split clonal bacterial populations into epigenetic lineages in a manner reminiscent of eukaryotic cell differentiation. Inheritance of self-propagating DNA methylation patterns governs phase variation in the E. coli pap operon, the agn43 gene, and other loci encoding virulence-related cell surface functions.

Role of the Arabidopsis DNA glycosylase/lyase ROS1 in active DNA demethylation

DNA methylation is a stable epigenetic mark for transcriptional gene silencing in diverse organisms including plants and many animals. In contrast to the well characterized mechanism of DNA methylation by methyltransferases, the mechanisms and function of active DNA demethylation have been controversial. Genetic evidence suggested that the DNA glycosylase domain-containing protein ROS1 of Arabidopsis is a putative DNA demethylase, because loss-of-function ros1 mutations cause DNA hypermethylation and enhance transcriptional gene silencing. We report here the biochemical characterization of ROS1 and the effect of its overexpression on the DNA methylation of target genes. Our data suggest that the DNA glycosylase activity of ROS1 removes 5-methylcytosine from the DNA backbone and then its lyase activity cleaves the DNA backbone at the site of 5-methylcytosine removal by successive beta- and delta-elimination reactions. Overexpression of ROS1 in transgenic plants led to a reduced level of cytosine methylation and increased expression of a target gene. These results demonstrate that ROS1 is a 5-methylcytosine DNA glycosylase/lyase important for active DNA demethylation in Arabidopsis.

Endogenous targets of RNA-directed DNA methylation and Pol IV in Arabidopsis

DRD1 is a SWI/SNF-like protein that cooperates with a plant-specific RNA polymerase, Pol IVb, to facilitate RNA-directed de novo methylation and silencing of homologous DNA. Screens to identify endogenous targets of this pathway in Arabidopsis revealed intergenic regions and plant genes located primarily in euchromatin. Many putative targets are near retrotransposon LTRs or other intergenic sequences that encode short RNAs, which might epigenetically regulate adjacent genes. Consistent with this, derepression of a solo LTR in drd1 and pol IVb mutants was accompanied by reduced cytosine methylation and transcriptional upregulation of neighboring sequences. The solo LTR and several other LTRs that flank reactivated targets are associated with euchromatic histone modifications but little or no H3K9 dimethylation, a hallmark of constitutive heterochromatin. By contrast, LTRs of retrotransposons that remain silent in the mutants despite reduced cytosine methylation lack euchromatic marks and have H3K9 dimethylation. We propose that DRD1 and Pol IVb establish a basal level of silencing that can potentially be reversed in euchromatin, and further reinforced in heterochromatin by other proteins that induce more stable modifications.

RNA-directed DNA methylation and Pol IVb in Arabidopsis

Recent work in Arabidopsis has revealed a plant-specific RNA polymerase, pol IV, that is specialized for RNA interference (RNAi)-mediated, chromatin-based gene silencing. Two functionally diversified pol IV complexes have been identified: pol IVa is required to produce or amplify the small RNA trigger, whereas pol IVb, together with the plant-specific SWI/SNF-like chromatin remodeling factor DRD1, acts downstream from small RNA formation to induce de novo cytosine methylation of homologous DNA by an unknown mechanism. Retrotransposon long terminal repeats (LTRs) and other unannotated sequences that encode small RNAs are prime targets for DRD1/pol IVb-mediated cytosine methylation. In drd1 and pol IVb mutants, silent LTRs in euchromatin can be derepressed, resulting in enhanced transcription of adjacent genes or intergenic regions. In addition to mediating de novo methylation, some evidence suggests that DRD1 and pol IVb are also involved in a reciprocal process of active demethylation, perhaps in conjunction with DNA glycosylase domain-containing proteins such as ROS1. We speculate that DRD1/pol IV-dependent methylation/demethylation evolved in the plant kingdom as a means to facilitate rapid, reversible changes in gene expression, which might have adaptive significance for immobile plants growing in unpredictable environments.

Antiviral silencing in animals

RNA silencing or RNA interference (RNAi) refers to the small RNA-guided gene silencing mechanism conserved in a wide range of eukaryotic organisms from plants to mammals. As part of this special issue on the biology, mechanisms and applications of RNAi, here we review the recent advances on defining a role of RNAi in the responses of invertebrate and vertebrate animals to virus infection. Approximately 40 miRNAs and 10 RNAi suppressors encoded by diverse mammalian viruses have been identified. Assays used for the identification of viral suppressors and possible biological functions of both viral miRNAs and suppressors are discussed. We propose that herpes viral miRNAs may act as specificity factors to initiate heterochromatin assembly of the latent viral DNA genome in the nucleus.