Variations on the topic of the "histone code"

Histones are the fundamental structural proteins intimately associated with eukaryotic DNA to form a highly ordered and condensed nucleoproteic complex termed chromatin. They are the targets of various posttranslational modifications including acetylation, methylation, phosphorylation and ubiquitination that modulate the structure/function of chromatin. The combinatorial nature of histone modifications is hypothesized to define a "histone code" that considerably extends the information potential of the genetic code, giving rise to epigenetic information. Moreover, most core histones consist of several nonallelic variants that can mark specific loci and could play an important role in establishment and maintenance of epigenetic memory. Here we will briefly present our current knowledge about histone posttranslational modifications and their implications in the regulation of epigenetic information. We will next describe core histone variants, insisting on their mode of incorporation into chromatin to discuss their epigenetic function and inheritance.     

Development of live-cell imaging probes for monitoring histone modifications

The combination of histone posttranslational modifications occurring in nucleosomal histones determines the epigenetic code. Histone modifications such as acetylation are dynamically controlled in response to a variety of signals during the cell cycle and differentiation, but they are paradoxically maintained through cell division to impart tissue specific gene expression patterns to progeny. The dynamics of histone modifications in living cells are poorly understood, because of the lack of experimental tools to monitor them in a real-time fashion. Recently, FRET-based imaging probes for histone H4 acetylation have been developed, which enabled monitoring of changes in histone acetylation during the cell cycle and drug treatment. Further development of this type of fluorescent probes for other modifications will make it possible to visualize complicated epigenetic regulation in living cells.     

Distinct domains in high mobility group N variants modulate specific chromatin modifications

We have demonstrated that levels of specific modification in histone H3 are modulated by members of the nucleosome-binding high mobility group N (HMGN) protein family in a variant-specific manner. HMGN1 (but not HMGN2) inhibits the phosphorylation of both H3S10 and H3S28, whereas HMGN2 enhances H3K14 acetylation more robustly than HMGN1. Two HMGN domains are necessary for modulating chromatin modifications, a non-modification-specific domain necessary for chromatin binding and a modification-specific domain localized in the C terminus of the HMGNs. Thus, chromatin-binding structural proteins such as HMGNs affect the levels of specific chromatin modifications and therefore may play a role in epigenetic regulation.     

Relationship between DNA methylation, histone H4 acetylation and gene expression in the mouse imprinted Igf2-H19 domain

DNA methylation and histone H4 acetylation play a role in gene regulation by modulating the structure of the chromatin. Recently, these two epigenetic modifications have dynamically and physically been linked. Evidence suggests that both modifications are involved in regulating imprinted genes - a subset of genes whose expression depends on their parental origin. Using immunoprecipitation assays, we investigate the relationship between DNA methylation, histone H4 acetylation and gene expression in the well-characterised imprinted Igf2-H19 domain on mouse chromosome 7. A systematic regional analysis of the acetylation status of the domain shows that parental-specific differences in acetylation of the core histone H4 are present in the promoter regions of both Igf2 and H19 genes, with the expressed alleles being more acetylated than the silent alleles. A correlation between DNA methylation, histone hypoacetylation and gene repression is evident only at the promoter region of the H19 gene. Treatment with trichostatin A, a specific inhibitor of histone deacetylase, reduces the expression of the active maternal H19 allele and this can be correlated with regional changes in acetylation within the upstream regulatory domain. The data suggest that histone H4 acetylation and DNA methylation have distinct functions on the maternal and paternal Igf2-H19 domains.     

Histone modifications and nuclear architecture: a review.

Epigenetic modifications, such as acetylation, phosphorylation, methylation, ubiquitination, and ADP ribosylation, of the highly conserved core histones, H2A, H2B, H3, and H4, influence the genetic potential of DNA. The enormous regulatory potential of histone modification is illustrated in the vast array of epigenetic markers found throughout the genome. More than the other types of histone modification, acetylation and methylation of specific lysine residues on N-terminal histone tails are fundamental for the formation of chromatin domains, such as euchromatin, and facultative and constitutive heterochromatin. In addition, the modification of histones can cause a region of chromatin to undergo nuclear compartmentalization and, as such, specific epigenetic markers are non-randomly distributed within interphase nuclei. In this review, we summarize the principles behind epigenetic compartmentalization and the functional consequences of chromatin arrangement within interphase nuclei.     

Histone acetylation and chromatin pattern in cancer. A review

Chromatin phenotype is known to be significantly disrupted in cancer. This has been demonstrated in many morphologic studies on cancer and in recent years by the application of digital texture analysis for quantitative evaluation of chromatin phenotype in neoplasia. Studies have consistently demonstrated the role of chromatin phenotype as a biomarker of diagnosis and prognosis. The underlying molecular mechanisms for chromatin reorganization and its role as a biomarker are largely unknown, but epigenetic processes are likely to be a main factor that not only modify chromatin arrangement but in doing so alter gene expression profiles in a reversible fashion. Of the range of epigenetic modifications that might control chromatin phenotype, histone acetylation is a strong candidate because of its role in the direct modification of chromatin, both through local relaxation of nucleosomal structure and recruitment of chromatin remodeling complexes. The reversible nature of histone acetylation is therapeutically attractive for treatment of aberrant histone acetylation; however, it still remains to be seen whether histone deacetylase inhibitors are clinically applicable or for use primarily as valuable research tools. This review explores the role of histone acetylation in cancer development, as a potential therapeutic candidate and a potential biomarker in tissue pathology.     

Epigenetic regulation of nuclear lamina-associated heterochromatin by HAT1 and the acetylation of newly synthesized histones

A central component of the epigenome is the pattern of histone post-translational modifications that play a critical role in the formation of specific chromatin states. Following DNA replication, nascent chromatin is a 1:1 mixture of parental and newly synthesized histones and the transfer of modification patterns from parental histones to new histones is a fundamental step in epigenetic inheritance. Here we report that loss of HAT1, which acetylates lysines 5 and 12 of newly synthesized histone H4 during replication-coupled chromatin assembly, results in the loss of accessibility of large domains of heterochromatin, termed HAT1-dependent Accessibility Domains (HADs). HADs are mega base-scale domains that comprise ∼10% of the mouse genome. HAT1 globally represses H3 K9 me3 levels and HADs correspond to the regions of the genome that display HAT1-dependent increases in H3 K9me3 peak density. HADs display a high degree of overlap with a subset of Lamin-Associated Domains (LADs). HAT1 is required to maintain nuclear structure and integrity. These results indicate that HAT1 and the acetylation of newly synthesized histones may be critical regulators of the epigenetic inheritance of heterochromatin and suggest a new mechanism for the epigenetic regulation of nuclear lamina-heterochromatin interactions.     

Epigenetic regulatory elements associate with specific histone modifications to prevent silencing of telomeric genes

In eukaryotic cells, transgene expression levels may be limited by an unfavourable chromatin structure at the integration site. Epigenetic regulators are DNA sequences which may protect transgenes from such position effect. We evaluated different epigenetic regulators for their ability to protect transgene expression at telomeres, which are commonly associated to low or inconsistent expression because of their repressive chromatin environment. Although to variable extents, matrix attachment regions (MARs), ubiquitous chromatin opening element (UCOE) and the chicken cHS4 insulator acted as barrier elements, protecting a telomeric-distal transgene from silencing. MARs also increased the probability of silent gene reactivation in time-course experiments. Additionally, all MARs improved the level of expression in non-silenced cells, unlike other elements. MARs were associated to histone marks usually linked to actively expressed genes, especially acetylation of histone H3 and H4, suggesting that they may prevent the spread of silencing chromatin by imposing acetylation marks on nearby nucleosomes. Alternatively, an UCOE was found to act by preventing deposition of repressive chromatin marks. We conclude that epigenetic DNA elements used to enhance and stabilize transgene expression all have specific epigenetic signature that might be at the basis of their mode of action.     

Centromeric chromatin exhibits a histone modification pattern that is distinct from both euchromatin and heterochromatin

Post-translational histone modifications regulate epigenetic switching between different chromatin states. Distinct histone modifications, such as acetylation, methylation and phosphorylation, define different functional chromatin domains, and often do so in a combinatorial fashion. The centromere is a unique chromosomal locus that mediates multiple segregation functions, including kinetochore formation, spindle-mediated movements, sister cohesion and a mitotic checkpoint. Centromeric (CEN) chromatin is embedded in heterochromatin and contains blocks of histone H3 nucleosomes interspersed with blocks of CENP-A nucleosomes, the histone H3 variant that provides a structural and functional foundation for the kinetochore. Here, we demonstrate that the spectrum of histone modifications present in human and Drosophila melanogaster CEN chromatin is distinct from that of both euchromatin and flanking heterochromatin. We speculate that this distinct modification pattern contributes to the unique domain organization and three-dimensional structure of centromeric regions, and/or to the epigenetic information that determines centromere identity.     

Interaction of SET domains with histones and nucleic acid structures in active chromatin

Changes in the normal program of gene expression are the basis for a number of human diseases. Epigenetic control of gene expression is programmed by chromatin modifications-the inheritable "histone code"-the major component of which is histone methylation. This chromatin methylation code of gene activity is created upon cell differentiation and is further controlled by the "SET" (methyltransferase) domain proteins which maintain this histone methylation pattern and preserve it through rounds of cell division. The molecular principles of epigenetic gene maintenance are essential for proper treatment and prevention of disorders and their complications. However, the principles of epigenetic gene programming are not resolved. Here we discuss some evidence of how the SET proteins determine the required states of target genes and maintain the required levels of their activity. We suggest that, along with other recognition pathways, SET domains can directly recognize the nucleosome and nucleic acids intermediates that are specific for active chromatin regions.