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Silencing of imprinted CDKN1C gene expression is associated with loss of CpG and histone H3 lysine 9 methylation at DMR-LIT1 in esophageal cancer.

The putative tumor suppressor CDKN1C is an imprinted gene at 11p15.5, a well-known imprinted region often deleted in tumors. The absence of somatic mutations and the frequent diminished expression in tumors would suggest that CDKN1C expression is regulated epigenetically. It has been, however, controversial whether the diminution is caused by imprinting disruption of the CDKN1C/LIT1 domain or by promoter hypermethylation of CDKN1C itself. To clarify this, we investigated the CpG methylation index of the CDKN1C promoter and the differentially methylated region of the LIT1 CpG island (differentially methylated region (DMR)-LIT1), an imprinting control region of the domain, and CDKN1C expression in esophageal cancer cell lines. CDKN1C expression was diminished in 10 of 17 lines and statistically correlated with the loss of methylation at DMR-LIT1 in all but three. However, there was no statistical correlation between CDKN1C promoter MI and CDKN1C expression. Furthermore, loss of CpG methylation was associated with loss of histone H3 lysine 9 (H3K9) methylation at DMR-LIT1. Histone modifications at CDKN1C promoter were not correlated with CDKN1C expression. The data suggested that the diminished CDKN1C expression is associated with the loss of methylation of CpG and H3K9 at DMR-LIT1, not by its own promoter CpG methylation, and is involved in esophageal cancer, implying that DMR-LIT1 epigenetically regulates CDKN1C expression not through histone modifications at CDKN1C promoter, but through that of DMR-LIT1.

Cell Line, Tumor↗

Cdx4 and menin co-regulate Hoxa9 expression in hematopoietic cells.

BACKGROUND: Transcription factor Cdx4 and transcriptional coregulator menin are essential for Hoxa9 expression and normal hematopoiesis. However, the precise mechanism underlying Hoxa9 regulation is not clear. METHODS AND FINDINGS: Here, we show that the expression level of Hoxa9 is correlated with the location of increased trimethylated histone 3 lysine 4 (H3K4M3). The active and repressive histone modifications co-exist along the Hoxa9 regulatory region. We further demonstrate that both Cdx4 and menin bind to the same regulatory region at the Hoxa9 locus in vivo, and co-activate the reporter gene driven by the Hoxa9 cis-elements that contain Cdx4 binding sites. Ablation of menin abrogates Cdx4 access to the chromatin target and significantly reduces both active and repressive histone H3 modifications in the Hoxa9 locus. CONCLUSION: These results suggest a functional link among Cdx4, menin and histone modifications in Hoxa9 regulation in hematopoietic cells.

Animals↗

H2B ubiquitylation and de-ubiquitylation in gene activation.

Previous models for the role of histone modifications suggest that adding and removing modifications, such as acetylation/deacetylation in gene regulation, are functionally antagonistic. We have investigated a transcriptional role of H2B C-terminal ubiquitylation and de-ubiquitylation in Saccharomyces cerevisiae. H2B ubiquitylation is required for optimal transcription of SUC2 and GAL1 genes. The ubiquitin hydrolase Ubp8 is a stable component of SAGA but not ADA complexes, and is not required for overall integrity of SAGA. Biochemical and genetic evidence indicates that Ubp8 targets H2B for deubiquitylation. The dynamic balance of H2B ubiquitylation/deubiquitylation is important for GAL1 transcription since either substitution of the ubiquitylation site in H2B (Lys123), or loss of Ubp8, lowers GAL1 expression. Further, this balance of ubiquitylation appears to set the balance of histone H3 methylation at Lys4 relative to Lys36. Thus, unlike acetylation/deacetylation whose functions are mutually opposing, both ubiquitylation and de-ubiquitylation are required for gene activation. These results suggest that ubiquitylation of histones has a unique role among histone modifications, possibly to orchestrate an ordered pathway of chromatin alterations.

DNA Methylation↗

A novel lipid hydroperoxide-derived cyclic covalent modification to histone H4.

Previous studies have established that 4-hydroxy-2-nonenal is a lipid hydroperoxide-derived aldehydic bifunctional electrophile that reacts with DNA and proteins. However, it has now been recognized that 4-oxo-2-nonenal is also a major product of lipid hydroperoxide decomposition. Furthermore, 4-oxo-2-nonenal is more reactive than 4-hydroxy-2-nonenal toward the DNA-bases 2'-deoxyguanosine, 2'-deoxyadenosine, and 2'-deoxycytidine and proteins. The formation of 4-oxo-2-nonenal can be induced through vitamin C-mediated or transition metal ion-mediated homolytic decomposition of polyunsaturated omega-3 lipid hydroperoxides such as 13(S)-hydroperoxyoctadecadienoic acid. We have discovered that synthetic 4-oxo-nonenal or 4-oxo-2-nonenal-generated from 13(S)-hydroperoxyoctadecadienoic acid recognizes the specific amino acid motifs of His75, Ala76, and Lys77 in bovine histone H4. Reaction of the histidine and lysine residues with 4-oxo-2-nonenal results in the formation of a novel cyclic structure within the protein. The cyclic structure incorporates the histidine imidazole ring and a newly formed pyrrole derived from the lysine. The cyclic imidazole-pyrrole derivative that is formed from the small Nalpha-acetyl-His-Ala-Lys peptide exists as a mixture of two atropisomers that inter-convert upon heating. Such lipid hydroperoxide-derived modifications could potentially modulate transcriptional activation in vivo. Furthermore, the ability to synthesize cyclic peptides using 4-oxo-2-nonenal will facilitate the preparation of novel structural analogs with potential biological activity.

Aldehydes↗

Silencing of human T-cell leukemia virus type I gene transcription by epigenetic mechanisms.

BACKGROUND: Human T-cell leukemia virus type I (HTLV-I) causes adult T-cell leukemia (ATL) after a long latent period. Among accessory genes encoded by HTLV-I, the tax gene is thought to play a central role in oncogenesis. However, Tax expression is disrupted by several mechanims including genetic changes of the tax gene, deletion/hypermethylation of 5'-LTR. To clarify the role of epigenetic changes, we analyzed DNA methylation and histone modification in the whole HTLV-I provirus genome. RESULTS: The gag, pol and env genes of HTLV-I provirus were more methylated than pX region, whereas methylation of 5'-LTR was variable and 3'-LTR was not methylated at all. In ATL cell lines, complete DNA methylation of 5'-LTR was associated with transcriptional silencing of viral genes. HTLV-I provirus was more methylated in primary ATL cells than in carrier state, indicating the association with disease progression. In seroconvertors, DNA methylation was already observed in internal sequences of provirus just after seroconversion. Taken together, it is speculated that DNA methylation first occurs in the gag, pol and env regions and then extends in the 5' and 3' directions in vivo, and when 5'-LTR becomes methylated, viral transcription is silenced. Analysis of histone modification in the HTLV-I provirus showed that the methylated provirus was associated with hypoacetylation. However, the tax gene transcript could not be detected in fresh ATL cells regardless of hyperacetylated histone H3 in 5'-LTR. The transcription rapidly recovered after in vitro culture in such ATL cells. CONCLUSION: These results showed that epigenetic changes of provirus facilitated ATL cells to evade host immune system by suppressing viral gene transcription. In addition, this study shows the presence of another reversible mechanism that suppresses the tax gene transcription without DNA methylation and hypoacetylated histone.

Carrier State↗

The temporal program of DNA replication: new insights into old questions.

During the last decades it has been shown that the replication timing program in metazoans is related to chromosome structure, the nuclear positioning and AT/GC content of chromosomal loci, their patterns of histone modifications, and their transcriptional regulation. Here, the current state of knowledge concerning these relationships is reviewed. An integrated view on structure-function relationships in the nucleus is provided and the determination and functional role of the replication timing program is discussed in this context. A corresponding comprehensive model is developed and a key aspect of this model is the suggestion that mammalian chromosomes are organized into stable units equivalent to replicon clusters. It is proposed that the nuclear positions of these units would depend on their histone modifications and determine the replication timing of the whole unit. It is furthermore predicted that replication timing is only indirectly linked to transcriptional regulation and contributes to the maintenance of gene expression patterns. These clear predictions, and the fact that the tools are at hand now to further test them, open an avenue towards solving the long standing problem on how replication timing is determined in metazoan cells.

Animals↗

Overview of cancer epigenetics.

Epigenetic mechanisms including DNA and histone modifications result in silencing of genes without changing the coding sequence of the gene. Even though these events are heritable, they are potentially reversible, thus opening up opportunities for therapeutic intervention. The importance of epigenetic changes in human cancer is only now being recognized in the medical community. A series of discoveries over the last four decades has thrust epigenetics into the forefront of new drug discoveries. Three systems--DNA methylation, RNA-associated silencing, and histone modification--are used to initiate and sustain epigenetic silencing. Current knowledge suggests that agents that intervene in this process by "turning back on" silenced genes may represent a significant advancement in treating many forms of cancer. In addition, changed patterns of methylation can be detected with a high degree of sensitivity thus providing clinicians with prognostic information.

DNA Methylation↗

Histone H2A phosphorylation and H3 methylation are required for a novel Rad9 DSB repair function following checkpoint activation.

In budding yeast, the Rad9 protein is an important player in the maintenance of genomic integrity and has a well-characterised role in DNA damage checkpoint activation. Recently, roles for different post-translational histone modifications in the DNA damage response, including H2A serine 129 phosphorylation and H3 lysine 79 methylation, have also been demonstrated. Here, we show that Rad9 recruitment to foci and bulk chromatin occurs specifically after ionising radiation treatment in G2 cells. This stable recruitment correlates with late stages of double strand break (DSB) repair and, surprisingly, it is the hypophosphorylated form of Rad9 that is retained on chromatin rather than the hyperphosphorylated, checkpoint-associated, form. Stable Rad9 accumulation in foci requires the Mec1 kinase and two independently regulated histone modifications, H2A phosphorylation and Dot1-dependent H3 methylation. In addition, Rad9 is selectively recruited to a subset of Rad52 repair foci. These results, together with the observation that rad9Delta cells are defective in repair of IR breaks in G2, strongly indicate a novel post checkpoint activation role for Rad9 in promoting efficient repair of DNA DSBs by homologous recombination.

Cell Cycle Proteins↗

Role of the p53 homologue from Drosophila melanogaster in the maintenance of histone H3 acetylation and response to UV-light irradiation.

It has been demonstrated that the human tumor suppressor p53 has an important role in modulating histone modifications after UV light irradiation. In this work we explored if the p53 Drosophila homologue has a similar role. Taking advantage of the existence of polytene chromosomes in the salivary glands of third instar larvae, we analyzed K9 and K14 H3 acetylation patterns in situ after UV irradiation of wild-type and Dmp53 null flies. As in human cells, after UV damage there is an increase in H3 acetylation in wild-type organisms. In Dmp53 mutant flies, this response is significantly affected at the K9 position. These results are similar to those found in human p53 mutant tumor cells with one interesting difference, only the basal H3 acetylation of K14 is reduced in Dmp53 mutant flies, while the basal H3-K9 acetylation is not affected. This work shows, that the presence of Dmp53 is necessary to maintain normal H3-K14 acetylation levels in Drosophila chromatin and that the function of p53 to maintaining histone modifications, is conserved in Drosophila and humans.

Acetylation↗

Taking it off: regulation of H3 K56 acetylation by Hst3 and Hst4.

Histone modifications have been implicated in both DNA repair and checkpoint-mediated responses to DNA damage. Recently much attention has focused on the acetylation of H3 K56. Indeed, this modification is cell cycle-regulated, maintained upon replicative damage in a checkpoint-dependent manner, and is essential for surviving DNA damage. We and others have discovered that two members of the HDAC Sirtuin family, Hst3 and Hst4, negatively regulate H3 K56 acetylation in budding yeast. Additionally, we have shown that these two HDACs are targeted for repression by the DNA damage checkpoint, which is vital for DNA damage tolerance. Discovery that two HDACs are negative regulators of the cellular response to DNA damage and that they target the acetylation of H3 K56 reveals a complex relationship between histone modifications, HDACs, and the DNA damage response. Here, we discuss the recent reports of the regulation of H3 K56-Ac by Hst3 and Hst4 and put forth the critical questions that remain for understanding the intimate, though poorly characterized, connection between chromatin states and genomic maintenance.

Acetylation↗

Chromatin decondensation and nuclear reorganization of the HoxB locus upon induction of transcription.

The colinearity of genes in Hox clusters suggests a role for chromosome structure in gene regulation. We reveal programmed changes in chromatin structure and nuclear organization upon induction of Hoxb expression by retinoic acid. There is an early increase in the histone modifications that are marks of active chromatin at both the early expressed gene Hoxb1, and also at Hoxb9 that is not expressed until much later. There is also a visible decondensation of the chromatin between Hoxb1 and Hoxb9 at this early stage. However, a further change in higher-order chromatin structure, looping out of genes from the chromosome territory, occurs in synchrony with the execution of the gene expression program. We suggest that higher-order chromatin structure regulates the expression of the HoxB cluster at several levels. Locus-wide changes in chromatin structure (histone modification and chromatin decondensation) may establish a transcriptionally poised state but are not sufficient for the temporal program of gene expression. The choreographed looping out of decondensed chromatin from chromosome territories may then allow for activation of high levels of transcription from the sequence of genes along the cluster.

Animals↗

Cooperative control of Crb2 by ATM family and Cdc2 kinases is essential for the DNA damage checkpoint in fission yeast.

The cellular responses to double-stranded breaks (DSBs) typically involve the extensive accumulation of checkpoint proteins in chromatin surrounding the damaged DNA. One well-characterized example involves the checkpoint protein Crb2 in the fission yeast Schizosaccharomyces pombe. The accumulation of Crb2 at DSBs requires the C-terminal phosphorylation of histone H2A (known as gamma-H2A) by ATM family kinases in chromatin surrounding the break. It also requires the constitutive methylation of histone H4 on lysine-20 (K20). Interestingly, neither type of histone modification is essential for the Crb2-dependent checkpoint response. However, H4-K20 methylation is essential in a crb2-T215A strain that lacks a cyclin-dependent kinase phosphorylation site in Crb2. Here we explain this genetic interaction by describing a previously overlooked effect of the crb2-T215A mutation. We show that crb2-T215A cells are able to initiate but not sustain a checkpoint response. We also report that gamma-H2A is essential for the DNA damage checkpoint in crb2-T215A cells. Importantly, we show that inactivation of Cdc2 in gamma-H2A-defective cells impairs Crb2-dependent signaling to the checkpoint kinase Chk1. These findings demonstrate that full Crb2 activity requires phosphorylation of threonine-215 by Cdc2. This regulation of Crb2 is independent of the histone modifications that are required for the hyperaccumulation of Crb2 at DSBs.

Amino Acid Substitution↗

Beyond Bulk: Cell-Type-Resolved Epigenomics as the Path Forward in Alzheimer's Disease Research.

Alzheimer's disease (AD) is a complex neurodegenerative disorder in which most risk variants are noncoding and are enriched at gene regulatory regions, implicating epigenetic mechanisms as central mediators of disease pathogenesis. For most of the history of AD epigenetics research, bulk tissue analysis has dominated, obscuring the fundamentally distinct epigenomic landscapes of individual brain cell types and masking cell-type-specific contributions to disease. Advances in single-cell and single-nucleus sequencing, fluorescence-activated nuclei sorting and multiplexed epigenomic platforms have transformed this landscape, enabling cell-type-resolved profiling of chromatin accessibility, DNA methylation, histone modifications and transcription across the major neuronal, glial and neurovascular populations of the human brain. Here, we review these advances, structured around the argument that cell-type resolution is not a methodological refinement but a conceptual necessity. We describe the distinct epigenomic programs disrupted in neurons, microglia, astrocytes, oligodendrocytes and neurovascular cells in AD, highlighting how each cell type responds to pathology. We discuss the discovery of epigenomic erosion, the progressive loss of cell-type-specific epigenomic identity across virtually all brain cell populations as AD advances, as a unifying disease mechanism linking chromatin dysregulation to cognitive decline. Finally, we identify critical gaps in current knowledge, including the near-complete absence of cell-type-resolved histone modification and DNA methylation data for most brain cell types, the underrepresentation of rare populations in standard preparations and the untapped potential of metabolic acylation marks as indicators of the epigenome-metabolism interface in neurodegeneration.

Humans↗

Cross-species clues of an epigenetic imprinting regulatory code for the IGF2R gene.

The epigenetic marks on the IGF2R gene that encodes a receptor responsible for IGF-II degradation consist of differentially methylated DNA in association with multiple modifications on the associated histones. We review these epigenetic marks across various species during the evolution of IGF2R imprinting. Both IGF2 and IGF2R genesare imprinted in the mammal lineage that diverged from Monotremata approximately 150 million years ago. While IGF2 is consistently imprinted in all mammals following its divergence, IGF2R imprinting disappears in the Euarchonta lineage, including human species, approximately 75 million years ago. Differential DNA methylation marks on the two parental alleles correlate with imprinting in all imprinted genes including IGF2R. While the DNA methylation marks in the IGF2R promoter region 1 (DMR1) correlate with IGF2R allelic expression, the DNA methylation marks in the intron region 2 (DMR2) fail to correlate with IGF2R imprinting status in a number of species. Human IGF2R and mouse neuronal Igf2r are not imprinted despite the presence of DMR2. We have noted that human IGF2R is not imprinted in more than 100 informative samples including various tumor tissues. Furthermore, opossum (Marsupialia) IGF2R is consistently imprinted despite the absence of DMR2. These lines of evidence indicate that DNA methylation marks in DMR2 are neither necessary nor sufficient for consistent imprinting of IGF2R across species. Histone modification marks, however, correlate more consistently with the tissue-specific and species-specific imprinting status of IGF2R in human and mouse. Acetylated histone H3 and H4 and methylated lysine 4 of H3 (H3-K4Me) associate with transcriptionally active alleles while tri-methylated lysine 9 of H3 (H3-K9Me3) marks the silenced alleles. In the mouse, an antisense non-coding transcript called Air is transcribed from DMR2 on the paternal allele, and this imprinted transcript plays a central role in Igf2r imprinting. Mouse Igf2r imprinting depends on an Air RNA while the existence of AIR in other species is unknown. Overall, DNA methylation, histone acetylation, and histone methylation play a vital role in coordinating IGF2R allelic expression across all species. Rare monoallelic or skewed allelic expression of human IGF2R and their biological importance warrants further rigorous study.

Animals↗

Histone post-translational modifications regulate transcription and silent chromatin in Saccharomyces cerevisiae.

Regulation of chromatin structure is important for the control of DNA-templated processes such as gene expression and silencing, and its dysregulation is implicated in diverse developmental and cell proliferative defects such as tumorigenesis. Covalent post-translational modifications of histones are one of the prominent means to regulate the chromatin structure. Here, we summarize findings from our lab and others regarding the interactions between different covalent modifications of histones in the budding yeast Saccharomyces cerevisiae. First, we describe the effect of histone H3 phosphorylation at residue serine 10 in transcriptional gene activation, and its histone H3 acetylation dependent and independent modes of action and downstream effects on TATA-binding protein (TBP) recruitment. Further, we review how ubiquitylation of histone H2B and its deubiquitylation by ubiquitin proteases Ubp8 and Ubp10 regulate histone H3 methylations, and consequently affect co-activator-dependent gene transcription and silent chromatin, respectively.

Acetylation↗

LncRNA HOTAIR contributes to cigarette smoke-induced pro-inflammatory responses in human airway epithelial cells.

Inhalation of cigarette smoke (CS) is the primary risk factor for chronic obstructive pulmonary disease (COPD), inducing epigenetic changes in the airway epithelium, including dysregulation of long-noncoding RNAs (lncRNAs). LncRNA homeobox gene transcript antisense RNA (HOTAIR) regulates chromatin remodeling and has been implicated in CS-induced malignant transformation. We hypothesized that HOTAIR expression is altered in COPD, leading to airway epithelial abnormalities. HOTAIR expression and overall survival were studied in The Cancer Genome Atlas (TCGA) database. Airway epithelial cells (AECs) were isolated from transplanted lungs of 11 patients with COPD, tracheobronchial tissue of 9 non-COPD donors, and bronchial brushings of ex-smokers with/without COPD (n = 6/group). HOTAIR expression, histone modifications, and production of proinflammatory cytokines (CXCL8 and GM-CSF) were assessed in the absence/presence of CS extract (CSE) and HOTAIR-polycomb inhibitor AQB. High HOTAIR expression correlated with poor overall survival in cancer patients with COPD, but not those without. Although HOTAIR expression was not significantly different between AECs from controls and subjects with COPD at baseline, it was significantly increased by 20% CSE only in COPD-derived AECs. CSE significantly decreased H3K4me3 levels in COPD-derived AECs, but not those from controls. AQB reduced baseline H3K27me3 levels in both groups, with a stronger effect in control-derived AECs. In addition, it reduced H3K4me3 levels in the presence of CSE in both groups. Finally, although AQB significantly suppressed CSE-induced production of GM-CSF and CXCL8 in control AECs, it failed to do so in COPD. Together, these findings suggest that COPD-derived AECs are more susceptible to CSE-induced HOTAIR upregulation, which may have a proinflammatory effect that cannot be inhibited by AQB.NEW & NOTEWORTHY COPD-derived AECs exhibit higher susceptibility to CSE-induced HOTAIR upregulation. CSE induces distinct histone modification patterns (H3K4me3) specifically in COPD-derived AECs. HOTAIR is essential for mediating CSE-induced proinflammatory responses in AECs.

Humans↗

[Epigenetic inheritance and its possible role in the evolution of plant species].

As it is clear now, the level of gene expression in eukariotes is determined mainly by chromatin composition. Chromatin structure of a particular gene (it is a complex item, which includes nucleosome positioning, histone modifications and non-histone chromatin proteins) can be modified externally and is able to be inherited mitotically and meiotically. Changes in chromatine structure are the basis of so called epigenetic inheritance that occurs without modification of DNA sequence. One of the most striking examples of epigenetic inheritance in plants is epimutations--stable for many generation's alleles of some genes that do not differ in primary DNA structure. Molecular basis of epimutations seems to be DNA metylation. Epimutations may be widely distributed in nature and affect some basis morphological features that have a systematic significance. Possibility of inheritance of acquired epigenetic modifications lead us to reconsider an idea of multipLe independent origins of some plant forms (or ecotypes) under action of similar external conditions. Different populations of the same species may in this case be unrelated and has no common ancestor. Species should be considered as invariant of multiple ways of origin. Wide distribution of polyploids amongst higher plants suggests effective mechanism of repression of multicopy genes. Each allopolyploidisation event is followed by repression of random set of parent genes via changes in its chromatin structure. As a result, in the limits of the same hybrid formula may arise different stable combinations of epigenetically controlled features of parent species. These combinations may be classified as different species of other taxa.

Adaptation, Physiological↗

Gene expression analysis in human gastric cancer cell line treated with trichostatin A and S-adenosyl-L-homocysteine using cDNA microarray.

Trichostatin A (TSA) and S-adenosyl-L-homocysteine (AdoHcy) have been reported to affect histone modifications. To investigate the effects of two drugs that can reportedly affect chromatin remodeling, we analyzed the gene expression profiles of TSA and AdoHcy in a gastric cancer cell line using 14 K cDNA microarray. The significant analysis of microarray (SAM) identified 98 and 43 differentially expressed genes in TSA and AdoHcy treated sets, respectively, and selected genes were functionally classified. In the gastric cancer cell line, genes related to cell communication, cell growth/maintenance, and morphogenesis were highly expressed with TSA, and genes with cell growth/maintenance, metabolism, oxidoreductase activity were upregulated with AdoHcy. Genes downregulated with TSA included those controlling the cell cycle, cell growth/proliferation, DNA binding, and metabolism, whereas genes involved in calcium signaling, cell growth/proliferation, and metabolism were downregulated with AdoHcy. Furthermore, we identified the genes commonly expressed in both drug treatments. Compared to TSA, AdoHcy did not induce apoptosis in the SNU-16 gastric cancer cell line, and RT-PCR was performed for selective genes to confirm the microarray data. This gene expression profile analysis with TSA and AdoHcy should contribute to a greater understanding of the molecular mechanism of chromatin remodeling and cancer, and provide candidate genes for further studies involving the roles of histone modifications in gastric cancer.

Acetylation↗