Regulation of the nucleosome repeat length in vivo by the DNA sequence, protein concentrations and long-range interactions

Nucleosome positioning: resources and tools online

Nucleosome positioning is an important process required for proper genome packing and its accessibility to execute the genetic program in a cell-specific, timely manner. In the recent years hundreds of papers have been devoted to the bioinformatics, physics and biology of nucleosome positioning. The purpose of this review is to cover a practical aspect of this field, namely, to provide a guide to the multitude of nucleosome positioning resources available online. These include almost 300 experimental datasets of genome-wide nucleosome occupancy profiles determined in different cell types and more than 40 computational tools for the analysis of experimental nucleosome positioning data and prediction of intrinsic nucleosome formation probabilities from the DNA sequence. A manually curated, up to date list of these resources will be maintained at http://generegulation.info.

Identification of peculiar and common effects of histone modifications on transcription

Histone modifications (HMs) play an important role in controlling eukaryotic gene expression and next generation sequencing (NGS) has greatly advanced the research on this topic with generating many high-resolution maps for HMs. Here, we use these maps to analyze the relationship between HMs and transcription. By incorporating various segments of genes into analysis without restricting the scope only in the promoter region, we have collected more comprehensive data and captured some details of this process. A position effect of gene regions has been revealed and it can even inverse the property of some HMs from activating to repressing genes such as the cases of H3K4me3, H3K36me3 and H3K14ac. Especially H3K36me3, its dual character on gene transcription makes it able to serve as a criterion to distinguish high and low expressed genes. We also study the general property of different HMs based on the comprehensive data. Using exploratory factor analysis (EFA), we have extracted 4 latent structures underlying the HMs, which are able to represent their activating and repressing effects concisely. These 4 factors have fine properties in the aspects of distinguishing high and low expressed genes, predicting transcription level and identifying genes with unique attributes such as stable RNA generating genes found to have a close relationship with lifespan of organisms here. In summary, while the position effect associated HM peculiarities demonstrate some details of the complex HM regulation network divergently, the common factors catch the nature of the network convergently. This deepens our understanding on the HM-transcription relationship.

Transgenic DNA modules with pre-programmed self-destruction: Universal molecular devices to escape 'genetic litter' in gene and cell therapy

Gene delivery to human somatic cells is a well-established therapeutic strategy to treat a variety of diseases. In addition, gene transfer to human cells is required to generate human induced pluripotent cells and also to eliminate tumorigenic undifferentiated cells in many types of stem-cell derived transplantation material. The expression of transgenes in these medical technologies is often required only in some of the recipient cells and only in specific limited time-windows, with inappropriately located or untimely expressed transgenes presenting a risk of undesired collateral effects. Unfortunately, current gene transfer procedures commonly result in a number of cells in the patient's body containing fragments of transferred genetic material which are either not therapeutically necessary at all, are no longer necessary or are necessary but in some other cells. Such transgenic material in the patient, created as a by-product of the chosen therapeutic procedure, constitutes, in fact, 'genetic litter', that is, persisting potentially-hazardous foreign genetic material which is neither required therapeutically nor explicitly chosen by an informed and free-willing person as an artificial body element. Wider use and more frequent administration of gene and cell therapy in the future are likely to give greater prominence to the issue of misdelivered genetic medicines and of their unwanted remainders accumulating in human bodies. Thus, novel DNA templates, which, on the one hand, are capable of providing transgene expression over broad time-windows, and, on the other hand, do not leave unwanted permanent 'genetic traces', are required. I propose that the problem of 'genetic litter' in patients' bodies can be addressed through the employment of a new type of gene vectors delivering DNA-based transgenic modules with pre-programmed self-destruction. Such vectors could deliver therapeutic DNA cargo and then execute self-liquidation through pre-scheduled activation of co-delivered genome editing tools, such as CRISPR/Cas9 nucleases, specific for the DNA to be eliminated. In this model, all unnecessary transgenic DNA is edited away precisely at a desired time point. Activity of the gene correction apparatus for the specific and effective destruction of transgenic DNA could be turned on by well-timed external signals or could be triggered through intracellular sensors of particular epigenetic signatures. It is expected that the employment of the proposed DNA-based gene vectors equipped with a transgene self-destruct mechanism can extend the safe and ethical application of gene and cell therapy to a broader range of curative and lifestyle-choice medical treatments, e.g., full body prophylactic gene therapy of cancer.

Partial Unwrapping and Histone Tail Dynamics in Nucleosome Revealed by Coarse-Grained Molecular Simulations

Nucleosomes, basic units of chromatin, are known to show spontaneous DNA unwrapping dynamics that are crucial for transcriptional activation, but its structural details are yet to be elucidated. Here, employing a coarse-grained molecular model that captures residue-level structural details up to histone tails, we simulated equilibrium fluctuations and forced unwrapping of single nucleosomes at various conditions. The equilibrium simulations showed spontaneous unwrapping from outer DNA and subsequent rewrapping dynamics, which are in good agreement with experiments. We found several distinct partially unwrapped states of nucleosomes, as well as reversible transitions among these states. At a low salt concentration, histone tails tend to sit in the concave cleft between the histone octamer and DNA, tightening the nucleosome. At a higher salt concentration, the tails tend to bound to the outer side of DNA or be expanded outwards, which led to higher degree of unwrapping. Of the four types of histone tails, H3 and H2B tail dynamics are markedly correlated with partial unwrapping of DNA, and, moreover, their contributions were distinct. Acetylation in histone tails was simply mimicked by changing their charges, which enhanced the unwrapping, especially markedly for H3 and H2B tails.

Nucleosomes, folding units of chromatin, wrap DNA about 1.75 turns and provide bottlenecks for transcription. Recent experiments showed that nucleosomes are not rigid but dynamic, showing spontaneous and partial unwrapping which is thus important for transcriptional activation. Experimentally, however, one cannot directly watch DNA unwrapping at high resolution. On the other hand, molecular dynamics simulations have high spatio-temporal resolution and thus can be powerful and complementary to experiments. Here, we put forward coarse-grained modeling of protein-DNA interactions at residue-level resolution, which is rather generic and thus can be applied to any protein-DNA complexes. By this method, we could reveal spontaneous and salt-concentration dependent partial unwrapping of DNA from nucleosomes. In addition to consistency with single molecule experiments, the simulation showed multiple and distinct intermediate states of unwrapping. Interestingly, partial unwrapping of DNA is correlated with certain parts of histone tail dynamics. Deleting positive charges in histone tails that mimics histone acetylation facilitated partial unwrapping, most significantly for H3 and H2B.

Conditions for positioning of nucleosomes on DNA

Positioning of nucleosomes along a eukaryotic genome plays an important role in its organization and regulation. There are many different factors affecting the location of nucleosomes. Some can be viewed as preferential binding of a single nucleosome to different locations along the DNA and some as interactions between neighboring nucleosomes. In this study, we analyze positioning of nucleosomes and derive conditions for their good positioning. Using analytic and numerical approaches we find that, if the binding preferences are very weak, an interplay between the interactions and the binding preferences is essential for a good positioning of nucleosomes, especially on correlated energy landscapes. Analyzing the empirical energy landscape, we conclude that good positioning of nucleosomes in vivo is possible only if they strongly interact. In this case, our model, predicting long-length-scale fluctuations of nucleosomes' occupancy along the DNA, accounts well for the empirical observations.

Concentration- and chromosome-organization-dependent regulator unbinding from DNA for transcription regulation in living cells

Binding and unbinding of transcription regulators at operator sites constitute a primary mechanism for gene regulation. While many cellular factors are known to regulate their binding, little is known on how cells can modulate their unbinding for regulation. Using nanometer-precision single-molecule tracking, we study the unbinding kinetics from DNA of two metal-sensing transcription regulators in living Escherichia coli cells. We find that they show unusual concentration-dependent unbinding kinetics from chromosomal recognition sites in both their apo and holo forms. Unexpectedly, their unbinding kinetics further varies with the extent of chromosome condensation, and more surprisingly, varies in opposite ways for their apo-repressor versus holo-activator forms. These findings suggest likely broadly relevant mechanisms for facile switching between transcription activation and deactivation in vivo and in coordinating transcription regulation of resistance genes with the cell cycle.

Binding and unbinding of transcription regulators at operator sites regulates gene expression. By single-molecule tracking of metal-sensing regulators, here the authors show that the unbinding kinetics depends on regulator concentration and chromosome condensation, and varies with their metal-binding states.

Clinical applications of next generation sequencing in cancer: from panels, to exomes, to genomes

This article will review recent impact of massively parallel next-generation sequencing (NGS) in our understanding and treatment of cancer. While whole exome sequencing (WES) remains popular and effective as a method of genetically profiling different cancers, advances in sequencing technology has enabled an increasing number of whole-genome based studies. Clinically, NGS has been used or is being developed for genetic screening, diagnostics, and clinical assessment. Though challenges remain, clinicians are in the early stages of using genetic data to make treatment decisions for cancer patients. As the integration of NGS in the study and treatment of cancer continues to mature, we believe that the field of cancer genomics will need to move toward more complete 100% genome sequencing. Current technologies and methods are largely limited to coding regions of the genome. A number of recent studies have demonstrated that mutations in non-coding regions may have direct tumorigenic effects or lead to genetic instability. Non-coding regions represent an important frontier in cancer genomics.

Genome-wide identification, evolutionary, and expression analyses of histone H3 variants in plants

Histone variants alter the nucleosome structure and play important roles in chromosome segregation, transcription, DNA repair, and sperm compaction. Histone H3 is encoded by many genes in most eukaryotic species and is the histone that contains the largest variety of posttranslational modifications. Compared with the metazoan H3 variants, little is known about the complex evolutionary history of H3 variants proteins in plants. Here, we study the identification, evolutionary, and expression analyses of histone H3 variants from genomes in major branches in the plant tree of life. Firstly we identified all the histone three related (HTR) genes from the examined genomes, then we classified the four groups variants: centromeric H3, H3.1, H3.3 and H3-like, by phylogenetic analysis, intron information, and alignment. We further demonstrated that the H3 variants have evolved under strong purifying selection, indicating the conservation of HTR proteins. Expression analysis revealed that the HTR has a wide expression profile in maize and rice development and plays important roles in development.

Affinity, stoichiometry and cooperativity of heterochromatin protein 1 (HP1) binding to nucleosomal arrays

Heterochromatin protein 1 (HP1) participates in establishing and maintaining heterochromatin via its histone-modification-dependent chromatin interactions. In recent papers HP1 binding to nucleosomal arrays was measured in vitro and interpreted in terms of nearest-neighbour cooperative binding. This mode of chromatin interaction could lead to the spreading of HP1 along the nucleosome chain. Here, we reanalysed previous data by representing the nucleosome chain as a 1D binding lattice and showed how the experimental HP1 binding isotherms can be explained by a simpler model without cooperative interactions between neighboring HP1 dimers. Based on these calculations and spatial models of dinucleosomes and nucleosome chains, we propose that binding stoichiometry depends on the nucleosome repeat length (NRL) rather than protein interactions between HP1 dimers. According to our calculations, more open nucleosome arrays with long DNA linkers are characterized by a larger number of binding sites in comparison to chains with a short NRL. Furthermore, we demonstrate by Monte Carlo simulations that the NRL dependent folding of the nucleosome chain can induce allosteric changes of HP1 binding sites. Thus, HP1 chromatin interactions can be modulated by the change of binding stoichiometry and the type of binding to condensed (methylated) and non-condensed (unmethylated) nucleosome arrays in the absence of direct interactions between HP1 dimers.

Thermodynamic model of heterochromatin formation through epigenetic regulation

Gene regulation in eukaryotes requires the segregation of silenced genomic regions into densely packed heterochromatin, leaving the active genes in euchromatin regions more accessible. We introduce a model that connects the presence of epigenetically inherited histone marks, methylation at histone 3 lysine-9, to the physical compaction of chromatin fibers via the binding of heterochromatin protein 1 (HP1). Our model demonstrates some of the key physical features that are necessary to explain experimental observations. In particular, we demonstrate that strong cooperative interactions among the HP1 proteins are necessary to see the phase segregation of heterochromatin and euchromatin regions. We also explore how the cell can use the concentration of HP1 to control condensation and under what circumstances there is a threshold of methylation over which the fibers will compact. Finally, we consider how different potential in vivo fiber structures as well as the flexibility of the histone 3 tail can affect the bridging of HP1. Many of the observations that we make about the HP1 system are guided by general thermodynamics principles and thus could play a role in other DNA organizational processes such as the binding of linker histones.

Ubiquitous human 'master' origins of replication are encoded in the DNA sequence via a local enrichment in nucleosome excluding energy barriers

As the elementary building block of eukaryotic chromatin, the nucleosome is at the heart of the compromise between the necessity of compacting DNA in the cell nucleus and the required accessibility to regulatory proteins. The recent availability of genome-wide experimental maps of nucleosome positions for many different organisms and cell types has provided an unprecedented opportunity to elucidate to what extent the DNA sequence conditions the primary structure of chromatin and in turn participates in the chromatin-mediated regulation of nuclear functions, such as gene expression and DNA replication. In this study, we use in vivo and in vitro genome-wide nucleosome occupancy data together with the set of nucleosome-free regions (NFRs) predicted by a physical model of nucleosome formation based on sequence-dependent bending properties of the DNA double-helix, to investigate the role of intrinsic nucleosome occupancy in the regulation of the replication spatio-temporal programme in human. We focus our analysis on the so-called replication U/N-domains that were shown to cover about half of the human genome in the germline (skew-N domains) as well as in embryonic stem cells, somatic and HeLa cells (mean replication timing U-domains). The 'master' origins of replication (MaOris) that border these megabase-sized U/N-domains were found to be specified by a few hundred kb wide regions that are hyper-sensitive to DNase I cleavage, hypomethylated, and enriched in epigenetic marks involved in transcription regulation, the hallmarks of localized open chromatin structures. Here we show that replication U/N-domain borders that are conserved in all considered cell lines have an environment highly enriched in nucleosome-excluding-energy barriers, suggesting that these ubiquitous MaOris have been selected during evolution. In contrast, MaOris that are cell-type-specific are mainly regulated epigenetically and are no longer favoured by a local abundance of intrinsic NFRs encoded in the DNA sequence. At the smaller few hundred bp scale of gene promoters, CpG-rich promoters of housekeeping genes found nearby ubiquitous MaOris as well as CpG-poor promoters of tissue-specific genes found nearby cell-type-specific MaOris, both correspond to in vivo NFRs that are not coded as nucleosome-excluding-energy barriers. Whereas the former promoters are likely to correspond to high occupancy transcription factor binding regions, the latter are an illustration that gene regulation in human is typically cell-type-specific.

The polymorphisms of the chromatin fiber

In eukaryotes, the genome is packed into chromosomes, each consisting of large polymeric fibers made of DNA bound with proteins (mainly histones) and RNA molecules. The nature and precise 3D organization of this fiber has been a matter of intense speculations and debates. In the emerging picture, the local chromatin state plays a critical role in all fundamental DNA transactions, such as transcriptional control, DNA replication or repair. However, the molecular and structural mechanisms involved remain elusive. The purpose of this review is to give an overview of the tremendous efforts that have been made for almost 40 years to build physiologically relevant models of chromatin structure. The motivation behind building such models was to shift our representation and understanding of DNA transactions from a too simplistic 'naked DNA' view to a more realistic 'coated DNA' view, as a step towards a better framework in which to interpret mechanistically the control of genetic expression and other DNA metabolic processes. The field has evolved from a speculative point of view towards in vitro biochemistry and in silico modeling, but is still longing for experimental in vivo validations of the proposed structures or even proof of concept experiments demonstrating a clear role of a given structure in a metabolic transaction. The mere existence of a chromatin fiber as a relevant biological entity in vivo has been put into serious questioning. Current research is suggesting a possible reconciliation between theoretical studies and experiments, pointing towards a view where the polymorphic and dynamic nature of the chromatin fiber is essential to support its function in genome metabolism.

Electrostatic effect of H1-histone protein binding on nucleosome repeat length

Within a simple biophysical model we describe the effect of electrostatic binding of H1 histone proteins on the nucleosome repeat length in chromatin. The length of wrapped DNA optimizes its binding energy to the histone core and the elastic energy penalty of DNA wrapping. The magnitude of the effect predicted from our model is in agreement with the systematic experimental data on the linear variation of nucleosome repeat lengths with H1/nucleosome ratio (Woodcock C L et al 2006 Chromos. Res. 14 17-25). We compare our model to the data for different cell types and organisms, with a widely varying ratio of bound H1 histones per nucleosome. We underline the importance of this non-specific histone-DNA charge-balance mechanism in regulating the positioning of nucleosomes and the degree of compaction of chromatin fibers in eukaryotic cells.

Strong nucleosomes of mouse genome including recovered centromeric sequences

Recently discovered strong nucleosomes (SNs) characterized by visibly periodical DNA sequences have been found to concentrate in centromeres of Arabidopsis thaliana and in transient meiotic centromeres of Caenorhabditis elegans. To find out whether such affiliation of SNs to centromeres is a more general phenomenon, we studied SNs of the Mus musculus. The publicly available genome sequences of mouse, as well as of practically all other eukaryotes do not include the centromere regions which are difficult to assemble because of a large amount of repeat sequences in the centromeres and pericentromeric regions. We recovered those missing sequences using the data from MNase-seq experiments in mouse embryonic stem cells, where the sequence of DNA inside nucleosomes, including missing regions, was determined by 100-bp paired-end sequencing. Those nucleosome sequences, which are not matching to the published genome sequence, would largely belong to the centromeres. By evaluating SN densities in centromeres and in non-centromeric regions, we conclude that mouse SNs concentrate in the centromeres of telocentric mouse chromosomes, with ~3.9 times excess compared to their density in the rest of the genome. The remaining non-centromeric SNs are harbored mainly by introns and intergenic regions, by retro-transposons, in particular. The centromeric involvement of the SNs opens new horizons for the chromosome and centromere structure studies.