Chromatin-associated periodicity in genetic variation downstream of transcriptional start sites

[Translated article] Application of pharmacogenetic/pharmacogenomic data to personalise treatment in routine clinical practice. A narrative review

OBJECTIVE

The aim of this article was to perform a narrative review of how pharmacogenetics and pharmacogenomics is being applied in the clinics, especially in Spain.

METHOD

Publications and websites of major interest have been reviewed.

RESULTS

Pharmacogenes and variants used in several hospitals, available methodologies, and the implementation process are discussed.

Application of pharmacogenetic/pharmacogenomic data to personalize treatment in routine clinical practice. A narrative review

OBJECTIVE

The aim of this article is to perform a narrative review of how pharmacogenetics and pharmacogenomics is being applied in the clinic, especially in Spain.

METHOD

Publications and websites of major interest have been reviewed.

RESULTS

Pharmacogenes and variants used in several hospitals, available methodologies, and the implementation process are discussed.

Strand-resolved mutagenicity of DNA damage and repair

DNA base damage is a major source of oncogenic mutations1. Such damage can produce strand-phased mutation patterns and multiallelic variation through the process of lesion segregation2. Here we exploited these properties to reveal how strand-asymmetric processes, such as replication and transcription, shape DNA damage and repair. Despite distinct mechanisms of leading and lagging strand replication3,4, we observe identical fidelity and damage tolerance for both strands. For small alkylation adducts of DNA, our results support a model in which the same translesion polymerase is recruited on-the-fly to both replication strands, starkly contrasting the strand asymmetric tolerance of bulky UV-induced adducts5. The accumulation of multiple distinct mutations at the site of persistent lesions provides the means to quantify the relative efficiency of repair processes genome wide and at single-base resolution. At multiple scales, we show DNA damage-induced mutations are largely shaped by the influence of DNA accessibility on repair efficiency, rather than gradients of DNA damage. Finally, we reveal specific genomic conditions that can actively drive oncogenic mutagenesis by corrupting the fidelity of nucleotide excision repair. These results provide insight into how strand-asymmetric mechanisms underlie the formation, tolerance and repair of DNA damage, thereby shaping cancer genome evolution.

Dependence of nucleosome mechanical stability on DNA mismatches

The organization of nucleosomes into chromatin and their accessibility are shaped by local DNA mechanics. Conversely, nucleosome positions shape genetic variations, which may originate from mismatches during replication and chemical modification of DNA. To investigate how DNA mismatches affect the mechanical stability and the exposure of nucleosomal DNA, we used an optical trap combined with single-molecule FRET and a single-molecule FRET cyclization assay. We found that a single base-pair C-C mismatch enhances DNA bendability and nucleosome mechanical stability for the 601-nucleosome positioning sequence. An increase in force required for DNA unwrapping from the histone core is observed for single base-pair C-C mismatches placed at three tested positions: at the inner turn, at the outer turn, or at the junction of the inner and outer turn of the nucleosome. The results support a model where nucleosomal DNA accessibility is reduced by mismatches, potentially explaining the preferred accumulation of single-nucleotide substitutions in the nucleosome core and serving as the source of genetic variation during evolution and cancer progression. Mechanical stability of an intact nucleosome, that is mismatch-free, is also dependent on the species as we find that yeast nucleosomes are mechanically less stable and more symmetrical in the outer turn unwrapping compared to Xenopus nucleosomes.

Predicting regional somatic mutation rates using DNA motifs

How the locus-specificity of epigenetic modifications is regulated remains an unanswered question. A contributing mechanism is that epigenetic enzymes are recruited to specific loci by DNA binding factors recognizing particular sequence motifs (referred to as epi-motifs). Using these motifs to predict biological outputs depending on local epigenetic state such as somatic mutation rates would confirm their functionality. Here, we used DNA motifs including known TF motifs and epi-motifs as a surrogate of epigenetic signals to predict somatic mutation rates in 13 cancers at an average 23kbp resolution. We implemented an interpretable neural network model, called contextual regression, to successfully learn the universal relationship between mutations and DNA motifs, and uncovered motifs that are most impactful on the regional mutation rates such as TP53 and epi-motifs associated with H3K9me3. Furthermore, we identified genomic regions with significantly higher mutation rates than the expected values in each individual tumor and demonstrated that such cancer-related regions can accurately predict cancer types. Interestingly, we found that the same mutation signatures often have different contributions to cancer-related and cancer-independent regions, and we also identified the motifs with the most contribution to each mutation signature.

The Adaptability of Chromosomal Instability in Cancer Therapy and Resistance

Variation in chromosome structure is a central source of DNA damage and DNA damage response, together representinga major hallmark of chromosomal instability. Cancer cells under selective pressure of therapy use DNA damage and DNA damage response to produce newfunctional assets as an evolutionary mechanism. Recent efforts to understand DNA damage/chromosomal instability and elucidate its role in initiation or progression of cancer have also disclosed its vulnerabilities represented by inappropriate DNA damage response, chromatin changes, andinflammation. Understanding these vulnerabilities can provide important clues for predicting treatment response and for the development of novel strategies that prevent the emergence of therapy resistant tumors.

Nucleosomes effectively shield DNA from radiation damage in living cells

Eukaryotic DNA is organized in nucleosomes, which package DNA and regulate its accessibility to transcription, replication, recombination and repair. Here, we show that in living cells nucleosomes protect DNA from high-energy radiation and reactive oxygen species. We combined sequence-based methods (ATAC-seq and BLISS) to determine the position of both nucleosomes and double strand breaks (DSBs) in the genome of nucleosome-rich malignant mesothelioma cells, and of the same cells partially depleted of nucleosomes. The results were replicated in the human MCF-7 breast carcinoma cell line. We found that, for each genomic sequence, the probability of DSB formation is directly proportional to the fraction of time it is nucleosome-free; DSBs accumulate distal from the nucleosome dyad axis. Nucleosome free regions and promoters of actively transcribed genes are more sensitive to DSB formation, and consequently to mutation. We argue that this may be true for a variety of chemical and physical DNA damaging agents.

A Light-Inducible Strain for Genome-Wide Histone Turnover Profiling in Neurospora crassa

In chromatin, nucleosomes are composed of ∼146 bp of DNA wrapped around a histone octamer, and are highly dynamic structures subject to remodeling and exchange. Histone turnover has previously been implicated in various processes including the regulation of chromatin accessibility, segregation of chromatin domains, and dilution of histone marks. Histones in different chromatin environments may turnover at different rates, possibly with functional consequences. Neurospora crassa sports a chromatin environment that is more similar to that of higher eukaryotes than yeasts, which have been utilized in the past to explore histone exchange. We constructed a simple light-inducible system to profile histone exchange in N. crassa on a 3xFLAG-tagged histone H3 under the control of the rapidly inducible vvd promoter. After induction with blue light, incorporation of tagged H3 into chromatin occurred within 20 min. Previous studies of histone turnover involved considerably longer incubation periods and relied on a potentially disruptive change of medium for induction. We used this reporter to explore replication-independent histone turnover at genes and examine changes in histone turnover at heterochromatin domains in different heterochromatin mutant strains. In euchromatin, H3-3xFLAG patterns were almost indistinguishable from that observed in wild-type in all mutant backgrounds tested, suggesting that loss of heterochromatin machinery has little effect on histone turnover in euchromatin. However, turnover at heterochromatin domains increased with loss of trimethylation of lysine 9 of histone H3 or HP1, but did not depend on DNA methylation. Our reporter strain provides a simple yet powerful tool to assess histone exchange across multiple chromatin contexts.

Nucleosome positioning stability is a modulator of germline mutation rate variation across the human genome

Nucleosome organization has been suggested to affect local mutation rates in the genome. However, the lack of de novo mutation and high-resolution nucleosome data has limited the investigation of this hypothesis. Additionally, analyses using indirect mutation rate measurements have yielded contradictory and potentially confounding results. Here, we combine data on >300,000 human de novo mutations with high-resolution nucleosome maps and find substantially elevated mutation rates around translationally stable (‘strong’) nucleosomes. We show that the mutational mechanisms affected by strong nucleosomes are low-fidelity replication, insufficient mismatch repair and increased double-strand breaks. Strong nucleosomes preferentially locate within young SINE/LINE transposons, suggesting that when subject to increased mutation rates, transposons are then more rapidly inactivated. Depletion of strong nucleosomes in older transposons suggests frequent positioning changes during evolution. The findings have important implications for human genetics and genome evolution.

Nucleosome organization has been suggested to affect local mutation rates in the genome. Here, the authors analyse data on >300,000 human de novo mutations and high-resolution nucleosome maps and provide evidence that nucleosome positioning stability modulates germline mutation rate variation across the human genome.

Scales and mechanisms of somatic mutation rate variation across the human genome

Cancer genome sequencing has revealed that somatic mutation rates vary substantially across the human genome and at scales from megabase-sized domains to individual nucleotides. Here we review recent work that has both revealed the major mutation biases that operate across the genome and the molecular mechanisms that cause them. The default mutation rate landscape in mammalian genomes results in active genes having low mutation rates because of a combination of factors that increase DNA repair: early DNA replication, transcription, active chromatin modifications and accessible chromatin. Therefore, either an increase in the global mutation rate or a redistribution of mutations from inactive to active DNA can increase the rate at which consequential mutations are acquired in active genes. Several environmental carcinogens and intrinsic mechanisms operating in tumor cells likely cause cancer by this second mechanism: by specifically increasing the mutation rate in active regions of the genome.

A likelihood approach to testing hypotheses on the co-evolution of epigenome and genome

Central questions to epigenome evolution include whether interspecies changes of histone modifications are independent of evolutionary changes of DNA, and if there is dependence whether they depend on any specific types of DNA sequence changes. Here, we present a likelihood approach for testing hypotheses on the co-evolution of genome and histone modifications. The gist of this approach is to convert evolutionary biology hypotheses into probabilistic forms, by explicitly expressing the joint probability of multispecies DNA sequences and histone modifications, which we refer to as a class of Joint Evolutionary Model for the Genome and the Epigenome (JEMGE). JEMGE can be summarized as a mixture model of four components representing four evolutionary hypotheses, namely dependence and independence of interspecies epigenomic variations to underlying sequence substitutions and to underlying sequence insertions and deletions (indels). We implemented a maximum likelihood method to fit the models to the data. Based on comparison of likelihoods, we inferred whether interspecies epigenomic variations depended on substitution or indels in local genomic sequences based on DNase hypersensitivity and spermatid H3K4me3 ChIP-seq data from human and rhesus macaque. Approximately 5.5% of homologous regions in the genomes exhibited H3K4me3 modification in either species, among which approximately 67% homologous regions exhibited local-sequence-dependent interspecies H3K4me3 variations. Substitutions accounted for less local-sequence-dependent H3K4me3 variations than indels. Among transposon-mediated indels, ERV1 insertions and L1 insertions were most strongly associated with H3K4me3 gains and losses, respectively. By initiating probabilistic formulation on the co-evolution of genomes and epigenomes, JEMGE helps to bring evolutionary biology principles to comparative epigenomic studies.

Epigenetic modifications play a significant role in gene regulations and thus heavily influence phenotypic outcomes. Whereas cross-species epigenomic comparisons have been fruitful in revealing the function of epigenetic modifications, it still remains unclear how the epigenome changes across species. A central question in epigenome evolution studies is whether interspecies epigenomic variations rely on genomic changes in cis and, if partially yes, whether different genomic changes have distinct impacts. To tackle this question, we initiated a likelihood-based approach, in which different hypotheses related to the co-evolution of the genome and the epigenome could be converted into probabilistic models. By fitting the models to actual data, each model yielded a likelihood, and the hypothesis corresponded to the largest likelihood was selected as most supported by observed data. In this work, we focused on the influence of two types of underlying sequence changes: substitutions, and insertions and deletions (indels). We quantitatively assessed the dependence of H3K4me3 variations on substitutions and indels between human and rhesus, and separated their relative impacts within each genomic region with H3K4me3. The methodology presented here provides a framework for modeling the epigenome together with the genome and a quantitative approach to test different evolutionary hypotheses.

Human exonization through differential nucleosome occupancy

Nucleosomal modifications have been implicated in fundamental epigenetic regulation, whereas the roles of nucleosome binding in shaping changes through evolution remain to be addressed. Here we performed a comparative study to clarify the roles of nucleosome occupancy in exon origination. By profiling a high-resolution, cross-species mononucleosome landscape for mammalian tissues, we found nucleosome occupancy profiles are conserved across tissues and species. Further, through a phylogenetic approach, we found that the feature of differential nucleosome occupancy appears prior to the origination of new exons and, presumably, facilitates the origin of new exons by increasing the splice strength of the ancestral nonexonic regions through driving a local difference in GC content, which suggests the function of nucleosome binding in exonization.

Nucleosomal modifications have been implicated in fundamental epigenetic regulation, but the roles of nucleosome occupancy in shaping changes through evolution remain to be addressed. Here we present high-resolution nucleosome occupancy profiles for multiple tissues derived from human, macaque, tree shrew, mouse, and pig. Genome-wide comparison reveals conserved nucleosome occupancy profiles across both different species and tissue types. Notably, we found significantly higher levels of nucleosome occupancy in exons than in introns, a pattern correlated with the different exon–intron GC content. We then determined whether this biased occupancy may play roles in the origination of new exons through evolution, rather than being a downstream effect of exonization, through a comparative approach to sequentially trace the order of the exonization and biased nucleosome binding. By identifying recently evolved exons in human but not in macaque using matched RNA sequencing, we found that higher exonic nucleosome occupancy also existed in macaque regions orthologous to these exons. Presumably, such biased nucleosome occupancy facilitates the origination of new exons by increasing the splice strength of the ancestral nonexonic regions through driving a local difference in GC content. These data thus support a model that sites bound by nucleosomes are more likely to evolve into exons, which we term the “nucleosome-first” model.

Identification and analysis of adenine N6-methylation sites in the rice genome

DNA N⁶-methyladenine (6mA) is a non-canonical DNA modification that is present at low levels in different eukaryotes^1-8, but its prevalence and genomic function in higher plants are unclear. Using mass spectrometry, immunoprecipitation and validation with analysis of single-molecule real-time sequencing, we observed that about 0.2% of all adenines are 6mA methylated in the rice genome. 6mA occurs most frequently at GAGG motifs and is mapped to about 20% of genes and 14% of transposable elements. In promoters, 6mA marks silent genes, but in bodies correlates with gene activity. 6mA overlaps with 5-methylcytosine (5mC) at CG sites in gene bodies and is complementary to 5mC at CHH sites in transposable elements. We show that OsALKBH1 may be potentially involved in 6mA demethylation in rice. The results suggest that 6mA is complementary to 5mC as an epigenomic mark in rice and reinforce a distinct role for 6mA as a gene expression-associated epigenomic mark in eukaryotes.

The Sorghum bicolor reference genome: improved assembly, gene annotations, a transcriptome atlas, and signatures of genome organization

Sorghum bicolor is a drought tolerant C4 grass used for the production of grain, forage, sugar, and lignocellulosic biomass and a genetic model for C4 grasses due to its relatively small genome (approximately 800 Mbp), diploid genetics, diverse germplasm, and colinearity with other C4 grass genomes. In this study, deep sequencing, genetic linkage analysis, and transcriptome data were used to produce and annotate a high-quality reference genome sequence. Reference genome sequence order was improved, 29.6 Mbp of additional sequence was incorporated, the number of genes annotated increased 24% to 34 211, average gene length and N50 increased, and error frequency was reduced 10-fold to 1 per 100 kbp. Subtelomeric repeats with characteristics of Tandem Repeats in Miniature (TRIM) elements were identified at the termini of most chromosomes. Nucleosome occupancy predictions identified nucleosomes positioned immediately downstream of transcription start sites and at different densities across chromosomes. Alignment of more than 50 resequenced genomes from diverse sorghum genotypes to the reference genome identified approximately 7.4 M single nucleotide polymorphisms (SNPs) and 1.9 M indels. Large-scale variant features in euchromatin were identified with periodicities of approximately 25 kbp. A transcriptome atlas of gene expression was constructed from 47 RNA-seq profiles of growing and developed tissues of the major plant organs (roots, leaves, stems, panicles, and seed) collected during the juvenile, vegetative and reproductive phases. Analysis of the transcriptome data indicated that tissue type and protein kinase expression had large influences on transcriptional profile clustering. The updated assembly, annotation, and transcriptome data represent a resource for C4 grass research and crop improvement.

Hypomethylated domain-enriched DNA motifs prepattern the accessible nucleosome organization in teleosts

Background

Gene promoters in vertebrate genomes show distinct chromatin features such as stably positioned nucleosome array and DNA hypomethylation. The nucleosomes are known to have certain sequence preferences, and the prediction of nucleosome positioning from DNA sequence has been successful in some organisms such as yeast. However, at gene promoters where nucleosomes are much more stably positioned than in other regions, the sequence-based model has failed to work well, and sequence-independent mechanisms have been proposed.

Results

Using DNase I-seq in medaka embryos, we demonstrated that hypomethylated domains (HMDs) specifically possess accessible nucleosome organization with longer linkers, and we reassessed the DNA sequence preference for nucleosome positioning in these specific regions. Remarkably, we found with a supervised machine learning algorithm, k-mer SVM, that nucleosome positioning in HMDs is accurately predictable from DNA sequence alone. Specific short sequences (6-mers) that contribute to the prediction are specifically enriched in HMDs and distribute periodically with approximately 200-bp intervals which prepattern the position of accessible linkers. Surprisingly, the sequence preference of the nucleosome and linker in HMDs is opposite from that reported previously. Furthermore, the periodicity of specific motifs at hypomethylated promoters was conserved in zebrafish.

Conclusion

This study reveals strong link between nucleosome positioning and DNA sequence at vertebrate promoters, and we propose hypomethylated DNA-specific regulation of nucleosome positioning.

Electronic supplementary material

The online version of this article (doi:10.1186/s13072-017-0152-2) contains supplementary material, which is available to authorized users.

Intrinsic Molecular Processes: Impact on Mutagenesis

Mutations provide resources for genome evolution by generating genetic variability. In addition, mutations act as a driving force leading to disease pathogenesis, and thus have important implications for disease diagnosis, prognosis, and treatment. Understanding the mechanisms underlying how mutations occur is therefore of prime importance for elucidating evolutionary and pathogenic processes. Recent genomics studies have revealed that mutations occur non-randomly across the human genome. In particular, the distribution of mutations is highly associated with intrinsic molecular processes including transcription, chromatin organization, DNA replication timing, and DNA repair. Interplay between intrinsic processes and extrinsic mutagenic exposure may thus imprint a characteristic mutational landscape on tumors. We discuss the impact of intrinsic molecular processes on mutation acquisition in cancer.

Comparative Analysis of Genome and Epigenome in Closely Related Medaka Species Identifies Conserved Sequence Preferences for DNA Hypomethylated Domains

The genomes of vertebrates are globally methylated, but a small portion of genomic regions are known to be hypomethylated. Although hypomethylated domains (HMDs) have been implicated in transcriptional regulation in various ways, how a HMD is determined in a particular genomic region remains elusive. To search for DNA motifs essential for the formation of HMDs, we performed the genome-wide comparative analysis of genome and DNA methylation patterns of the two medaka inbred lines, Hd-rRII1 and HNI-II, which are derived from northern and southern subpopulations of Japan and exhibit high levels of genetic variations (SNP, ∼ 3%). We successfully mapped > 70% of HMDs in both genomes and found that the majority of those mapped HMDs are conserved between the two lines (common HMDs). Unexpectedly, the average genetic variations are similar in the common HMD and other genome regions. However, we identified short well-conserved motifs that are specifically enriched in HMDs, suggesting that they may play roles in the establishment of HMDs in the medaka genome.

Extremely Rare Polymorphisms in Saccharomyces cerevisiae Allow Inference of the Mutational Spectrum

The characterization of mutational spectra is usually carried out in one of three ways-by direct observation through mutation accumulation (MA) experiments, through parent-offspring sequencing, or by indirect inference from sequence data. Direct observations of spontaneous mutations with MA experiments are limited, given (i) the rarity of spontaneous mutations, (ii) applicability only to laboratory model species with short generation times, and (iii) the possibility that mutational spectra under lab conditions might be different from those observed in nature. Trio sequencing is an elegant solution, but it is not applicable in all organisms. Indirect inference, usually from divergence data, faces no such technical limitations, but rely upon critical assumptions regarding the strength of natural selection that are likely to be violated. Ideally, new mutational events would be directly observed before the biased filter of selection, and without the technical limitations common to lab experiments. One approach is to identify very young mutations from population sequencing data. Here we do so by leveraging two characteristics common to all new mutations-new mutations are necessarily rare in the population, and absent in the genomes of immediate relatives. From 132 clinical yeast strains, we were able to identify 1,425 putatively new mutations and show that they exhibit extremely low signatures of selection, as well as display a mutational spectrum that is similar to that identified by a large scale MA experiment. We verify that population sequencing data are a potential wealth of information for inferring mutational spectra, and should be considered for analysis where MA experiments are infeasible or especially tedious.

Dissecting relative contributions of cis- and trans-determinants to nucleosome distribution by comparing Tetrahymena macronuclear and micronuclear chromatin

The ciliate protozoan Tetrahymena thermophila contains two types of structurally and functionally differentiated nuclei: the transcriptionally active somatic macronucleus (MAC) and the transcriptionally silent germ-line micronucleus (MIC). Here, we demonstrate that MAC features well-positioned nucleosomes downstream of transcription start sites and flanking splice sites. Transcription-associated trans-determinants promote nucleosome positioning in MAC. By contrast, nucleosomes in MIC are dramatically delocalized. Nucleosome occupancy in MAC and MIC are nonetheless highly correlated with each other, as well as with in vitro reconstitution and predictions based upon DNA sequence features, revealing unexpectedly strong contributions from cis-determinants. In particular, well-positioned nucleosomes are often matched with GC content oscillations. As many nucleosomes are coordinately accommodated by both cis- and trans-determinants, we propose that their distribution is shaped by the impact of these nucleosomes on the mutational and transcriptional landscape, and driven by evolutionary selection.

Nucleosomal signatures impose nucleosome positioning in coding and noncoding sequences in the genome

In the yeast genome, a large proportion of nucleosomes occupy well-defined and stable positions. While the contribution of chromatin remodelers and DNA binding proteins to maintain this organization is well established, the relevance of the DNA sequence to nucleosome positioning in the genome remains controversial. Through quantitative analysis of nucleosome positioning, we show that sequence changes distort the nucleosomal pattern at the level of individual nucleosomes in three species of Schizosaccharomyces and in Saccharomyces cerevisiae. This effect is equally detected in transcribed and nontranscribed regions, suggesting the existence of sequence elements that contribute to positioning. To identify such elements, we incorporated information from nucleosomal signatures into artificial synthetic DNA molecules and found that they generated regular nucleosomal arrays indistinguishable from those of endogenous sequences. Strikingly, this information is species-specific and can be combined with coding information through the use of synonymous codons such that genes from one species can be engineered to adopt the nucleosomal organization of another. These findings open the possibility of designing coding and noncoding DNA molecules capable of directing their own nucleosomal organization.

Evidence of selection for an accessible nucleosomal array in human

BACKGROUND

Recently, a physical model of nucleosome formation based on sequence-dependent bending properties of the DNA double-helix has been used to reveal some enrichment of nucleosome-inhibiting energy barriers (NIEBs) nearby ubiquitous human "master" replication origins. Here we use this model to predict the existence of about 1.6 millions NIEBs over the 22 human autosomes.

RESULTS

We show that these high energy barriers of mean size 153 bp correspond to nucleosome-depleted regions (NDRs) in vitro, as expected, but also in vivo. On either side of these NIEBs, we observe, in vivo and in vitro, a similar compacted nucleosome ordering, suggesting an absence of chromatin remodeling. This nucleosomal ordering strongly correlates with oscillations of the GC content as well as with the interspecies and intraspecies mutation profiles along these regions. Comparison of these divergence rates reveals the existence of both positive and negative selections linked to nucleosome positioning around these intrinsic NDRs. Overall, these NIEBs and neighboring nucleosomes cover 37.5 % of the human genome where nucleosome occupancy is stably encoded in the DNA sequence. These 1 kb-sized regions of intrinsic nucleosome positioning are equally found in GC-rich and GC-poor isochores, in early and late replicating regions, in intergenic and genic regions but not at gene promoters.

CONCLUSION

The source of selection pressure on the NIEBs has yet to be resolved in future work. One possible scenario is that these widely distributed chromatin patterns have been selected in human to impair the condensation of the nucleosomal array into the 30 nm chromatin fiber, so as to facilitate the epigenetic regulation of nuclear functions in a cell-type-specific manner.

Preferential Protection of Genetic Fidelity within Open Chromatin by the Mismatch Repair Machinery

Epigenetic systems are well known for the roles they play in regulating the differential expression of the same genome in different cell types. However, epigenetic systems can also directly impact genomic integrity by protecting genetic sequences. Using an experimental evolutionary approach, we studied rates of mutation in the fission yeast Schizosaccharomyces pombe strains that lacked genes encoding several epigenetic regulators or mismatch repair components. We report that loss of a functional mismatch repair pathway in S. pombe resulted in the preferential enrichment of mutations in euchromatin, indicating that the mismatch repair machinery preferentially protected genetic fidelity in euchromatin. This preference is probably determined by differences in the accessibility of chromatin at distinct chromatin regions, which is supported by our observations that chromatin accessibility positively correlated with mutation rates in S. pombe or human cancer samples with deficiencies in mismatch repair. Importantly, such positive correlation was not observed in S. pombe strains or human cancer samples with functional mismatch repair machinery.

Transcriptional Regulation During Zygotic Genome Activation in Zebrafish and Other Anamniote Embryos

Embryo development commences with the fusion of two terminally differentiated haploid gametes into the totipotent fertilized egg, which through a series of major cellular and molecular transitions generate a pluripotent cell mass. The activation of the zygotic genome occurs during the so-called maternal to zygotic transition and prepares the embryo for zygotic takeover from maternal factors, in the control of the development of cellular lineages during differentiation. Recent advances in next generation sequencing technologies have allowed the dissection of the genomic and epigenomic processes mediating this transition. These processes include reorganization of the chromatin structure to a transcriptionally permissive state, changes in composition and function of structural and regulatory DNA-binding proteins, and changeover of the transcriptome as it is overhauled from that deposited by the mother in the oocyte to a zygotically transcribed complement. Zygotic genome activation in zebrafish occurs 10 cell cycles after fertilization and provides an ideal experimental platform for elucidating the temporal sequence and dynamics of establishment of a transcriptionally active chromatin state and helps in identifying the determinants of transcription activation at polymerase II transcribed gene promoters. The relatively large number of pluripotent cells generated by the fast cell divisions before zygotic transcription provides sufficient biomass for next generation sequencing technology approaches to establish the temporal dynamics of events and suggest causative relationship between them. However, genomic and genetic technologies need to be improved further to capture the earliest events in development, where cell number is a limiting factor. These technologies need to be complemented with precise, inducible genetic interference studies using the latest genome editing tools to reveal the function of candidate determinants and to confirm the predictions made by classic embryological tools and genome-wide assays. In this review we summarize recent advances in the characterization of epigenetic regulation, transcription control, and gene promoter function during zygotic genome activation and how they fit with old models for the mechanisms of the maternal to zygotic transition. This review will focus on the zebrafish embryo but draw comparisons with other vertebrate model systems and refer to invertebrate models where informative.

The topography of mutational processes in breast cancer genomes

Somatic mutations in human cancers show unevenness in genomic distribution that correlate with aspects of genome structure and function. These mutations are, however, generated by multiple mutational processes operating through the cellular lineage between the fertilized egg and the cancer cell, each composed of specific DNA damage and repair components and leaving its own characteristic mutational signature on the genome. Using somatic mutation catalogues from 560 breast cancer whole-genome sequences, here we show that each of 12 base substitution, 2 insertion/deletion (indel) and 6 rearrangement mutational signatures present in breast tissue, exhibit distinct relationships with genomic features relating to transcription, DNA replication and chromatin organization. This signature-based approach permits visualization of the genomic distribution of mutational processes associated with APOBEC enzymes, mismatch repair deficiency and homologous recombinational repair deficiency, as well as mutational processes of unknown aetiology. Furthermore, it highlights mechanistic insights including a putative replication-dependent mechanism of APOBEC-related mutagenesis.

Distribution of human single-nucleotide polymorphisms is approximated by the power law and represents a fractal structure

Single-nucleotide polymorphisms (SNPs) are one of the main causes of evolution. The distribution of human SNPs, which were examined in detail genomewide, was analyzed. Three discrete databases of human SNPs were used for this analysis, and similar results were obtained from these databases. It was found that the distribution of the distance between SNPs was approximated by the power law, and the shape of the regions including SNPs had the so-called fractal structure. Although the reason why the distribution of SNPs obeys such a certain law of physics is unclear, a speculation was attempted in connection with the three-dimensional structure of human chromatin which has a fractal structure.

Altered primary chromatin structures and their implications in cancer development

BACKGROUND

Cancer development is a complex process involving both genetic and epigenetic changes. Genetic changes in oncogenes and tumor-suppressor genes are generally considered as primary causes, since these genes may directly regulate cellular growth. In addition, it has been found that changes in epigenetic factors, through mutation or altered gene expression, may contribute to cancer development. In the nucleus of eukaryotic cells DNA and histone proteins form a structure called chromatin which consists of nucleosomes that, like beads on a string, are aligned along the DNA strand. Modifications in chromatin structure are essential for cell type-specific activation or repression of gene transcription, as well as other processes such as DNA repair, DNA replication and chromosome segregation. Alterations in epigenetic factors involved in chromatin dynamics may accelerate cell cycle progression and, ultimately, result in malignant transformation. Abnormal expression of remodeler and modifier enzymes, as well as histone variants, may confer to cancer cells the ability to reprogram their genomes and to yield, maintain or exacerbate malignant hallmarks. At the end, genetic and epigenetic alterations that are encountered in cancer cells may culminate in chromatin changes that may, by altering the quantity and quality of gene expression, promote cancer development.

METHODS

During the last decade a vast number of studies has uncovered epigenetic abnormalities that are associated with the (anomalous) packaging and remodeling of chromatin in cancer genomes. In this review I will focus on recently published work dealing with alterations in the primary structure of chromatin resulting from imprecise arrangements of nucleosomes along DNA, and its functional implications for cancer development.

CONCLUSIONS

The primary chromatin structure is regulated by a variety of epigenetic mechanisms that may be deregulated through gene mutations and/or gene expression alterations. In recent years, it has become evident that changes in chromatin structure may coincide with the occurrence of cancer hallmarks. The functional interrelationships between such epigenetic alterations and cancer development are just becoming manifest and, therefore, the oncology community should continue to explore the molecular mechanisms governing the primary chromatin structure, both in normal and in cancer cells, in order to improve future approaches for cancer detection, prevention and therapy, as also for circumventing drug resistance.

Genome-wide analysis of the specificity and mechanisms of replication infidelity driven by imbalanced dNTP pools

The absolute and relative concentrations of the four dNTPs are key determinants of DNA replication fidelity, yet the consequences of altered dNTP pools on replication fidelity have not previously been investigated on a genome-wide scale. Here, we use deep sequencing to determine the types, rates and locations of uncorrected replication errors that accumulate in the nuclear genome of a mismatch repair-deficient diploid yeast strain with elevated dCTP and dTTP concentrations. These imbalanced dNTP pools promote replication errors in specific DNA sequence motifs suggesting increased misinsertion and increased mismatch extension at the expense of proofreading. Interestingly, substitution rates are similar for leading and lagging strand replication, but are higher in regions replicated late in S phase. Remarkably, the rate of single base deletions is preferentially increased in coding sequences and in short rather than long mononucleotides runs. Based on DNA sequence motifs, we propose two distinct mechanisms for generating single base deletions in vivo. Collectively, the results indicate that elevated dCTP and dTTP pools increase mismatch formation and decrease error correction across the nuclear genome, and most strongly increases mutation rates in coding and late replicating sequences.

Organization of DNA in a bacterial nucleoid

BACKGROUND

It is unclear how DNA is packaged in a bacterial cell in the absence of nucleosomes. To investigate the initial level of DNA condensation in bacterial nucleoid we used in vivo DNA digestion coupled with high-throughput sequencing of the digestion-resistant fragments. To this end, we transformed E. coli cells with a plasmid expressing micrococcal nuclease. The nuclease expression was under the control of AraC repressor, which enabled us to perform an inducible digestion of bacterial nucleoid inside a living cell.

RESULTS

Analysis of the genomic localization of the digestion-resistant fragments revealed their non-random distribution. The patterns observed in the distribution of the sequenced fragments indicate the presence of short DNA segments protected from the enzyme digestion, possibly because of interaction with DNA-binding proteins. The average length of such digestion-resistant segments is about 50 bp and the characteristic repeat in their distribution is about 90 bp. The gene starts are depleted of the digestion-resistant fragments, suggesting that these genomic regions are more exposed than genomic sequences on average. Sequence analysis of the digestion-resistant segments showed that while the GC-content of such sequences is close to the genome-wide value, they are depleted of A-tracts as compared to the bulk genomic DNA or to the randomized sequence of the same nucleotide composition.

CONCLUSIONS

Our results suggest that DNA is packaged in the bacterial nucleoid in a non-random way that facilitates interaction of the DNA binding factors with regulatory regions of the genome.

Assessing Cell-to-Cell DNA Methylation Variability on Individual Long Reads

Understanding cell-to-cell variability in cytosine methylation is essential for understanding cellular perturbation and its molecular machinery. However, conventional methylation studies have focused on the differences in the average levels between cell types while overlooking methylation heterogeneity within cell types. Little information has been uncovered using recent single-cell methods because of either technical limitations or the great labor required to process many single cells. Here, we report the highly efficient detection of cell-to-cell DNA methylation variability in liver tissue, based on comparing the methylation status of adjacent CpG sites on long sequencing reads. This method provides abundant methylation linkage information and enables genome-wide estimation of cell-to-cell variability. We observed repressed methylation variability in hypomethylated regions compared with the variability in hypomethylated regions across the genome, which we confirmed using public human sperm data. A gradual change in methylation status at the boundaries of hypomethylated regions was observed for the first time. This approach allows the concise, comprehensive assessment of cell-to-cell DNA methylation variability.

A species-specific nucleosomal signature defines a periodic distribution of amino acids in proteins

Nucleosomes are the basic structural units of chromatin. Most of the yeast genome is organized in a pattern of positioned nucleosomes that is stably maintained under a wide range of physiological conditions. In this work, we have searched for sequence determinants associated with positioned nucleosomes in four species of fission and budding yeasts. We show that mononucleosomal DNA follows a highly structured base composition pattern, which differs among species despite the high degree of histone conservation. These nucleosomal signatures are present in transcribed and non-transcribed regions across the genome. In the case of open reading frames, they correctly predict the relative distribution of codons on mononucleosomal DNA, and they also determine a periodicity in the average distribution of amino acids along the proteins. These results establish a direct and species-specific connection between the position of each codon around the histone octamer and protein composition.

Medaka as a model for studying environmentally induced epigenetic transgenerational inheritance of phenotypes

Ability of environmental stressors to induce transgenerational diseases has been experimentally demonstrated in plants, worms, fish, and mammals, indicating that exposures affect not only human health but also fish and ecosystem health. Small aquarium fish have been reliable model to study genetic and epigenetic basis of development and disease. Additionally, fish can also provide better, economic opportunity to study transgenerational inheritance of adverse health and epigenetic mechanisms. Molecular mechanisms underlying germ cell development in fish are comparable to those in mammals and humans. This review will provide a short overview of long-term effects of environmental chemical contaminant exposure in various models, associated epigenetic mechanisms, and a perspective on fish as model to study environmentally induced transgenerational inheritance of altered phenotypes.

Facilitation of base excision repair by chromatin remodeling

Base Excision Repair (BER) is a conserved, intracellular DNA repair system that recognizes and removes chemically modified bases to insure genomic integrity and prevent mutagenesis. Aberrant BER has been tightly linked with a broad spectrum of human pathologies, such as several types of cancer, neurological degeneration, developmental abnormalities, immune dysfunction and aging. In the cell, BER must recognize and remove DNA lesions from the tightly condensed, protein-coated chromatin. Because chromatin is necessarily refractory to DNA metabolic processes, like transcription and replication, the compaction of the genomic material is also inhibitory to the repair systems necessary for its upkeep. Multiple ATP-dependent chromatin remodelling (ACR) complexes play essential roles in modulating the protein-DNA interactions within chromatin, regulating transcription and promoting activities of some DNA repair systems, including double-strand break repair and nucleotide excision repair. However, it remains unclear how BER operates in the context of chromatin, and if the chromatin remodelling processes that govern transcription and replication also actively regulate the efficiency of BER. In this review we highlight the emerging role of ACR in regulation of BER.

Associations between nucleosome phasing, sequence asymmetry, and tissue-specific expression in a set of inbred Medaka species

BACKGROUND

Transcription start sites (TSSs) with pronounced and phased nucleosome arrays downstream and nucleosome-depleted regions upstream of TSSs are observed in various species.

RESULTS

We have characterized sequence variation and expression properties of this set of TSSs (which we call "Nucleocyclic TSSs") using germline and somatic cells of three medaka (Oryzias latipes) inbred isolates from different locations. We found nucleocyclic TSSs in medaka to be associated with higher gene expression and characterized by a clear boundary in sequence composition with potentially-nucleosome-destabilizing A/T-enrichment upstream (p < 10(-60)) and nucleosome- accommodating C/G-enrichment downstream (p < 10(-40)) that was highly conserved from an ancestor. A substantial genetic distance between the strains facilitated the in-depth analysis of patterns of fixed mutations, revealing a localization-specific equilibrium between the rates of distinct mutation categories that would serve to maintain the conserved sequence anisotropy around TSSs. Downstream of nucleocyclic TSSs, C to T, T to C, and other mutation rates on the sense strand increased around first nucleosome dyads and decreased around first linkers, which contrasted with genomewide mutational patterns around nucleosomes (p < 5 %). C to T rates are higher than G to A rates around nucleosome associated with germline nucleocyclic TSS sites (p < 5 %), potentially due to the asymmetric effect of transcription-coupled repair.

CONCLUSIONS

Our results demonstrate an atypical evolutionary process surrounding nucleocyclic TSSs.

Genomic approaches to DNA repair and mutagenesis

DNA damage is a constant threat to cells, causing cytotoxicity as well as inducing genetic alterations. The steady-state abundance of DNA lesions in a cell is minimized by a variety of DNA repair mechanisms, including DNA strand break repair, mismatch repair, nucleotide excision repair, base excision repair, and ribonucleotide excision repair. The efficiencies and mechanisms by which these pathways remove damage from chromosomes have been primarily characterized by investigating the processing of lesions at defined genomic loci, among bulk genomic DNA, on episomal DNA constructs, or using in vitro substrates. However, the structure of a chromosome is heterogeneous, consisting of heavily protein-bound heterochromatic regions, open regulatory regions, actively transcribed genes, and even areas of transient single stranded DNA. Consequently, DNA repair pathways function in a much more diverse set of chromosomal contexts than can be readily assessed using previous methods. Recent efforts to develop whole genome maps of DNA damage, repair processes, and even mutations promise to greatly expand our understanding of DNA repair and mutagenesis. Here we review the current efforts to utilize whole genome maps of DNA damage and mutation to understand how different chromosomal contexts affect DNA excision repair pathways.

Replication dynamics in fission and budding yeasts through DNA polymerase tracking

The dynamics of eukaryotic DNA polymerases has been difficult to establish because of the difficulty of tracking them along the chromosomes during DNA replication. Recent work has addressed this problem in the yeasts Schizosaccharomyces pombe and Saccharomyces cerevisiae through the engineering of replicative polymerases to render them prone to incorporating ribonucleotides at high rates. Their use as tracers of the passage of each polymerase has provided a picture of unprecedented resolution of the organization of replicons and replication origins in the two yeasts and has uncovered important differences between them. Additional studies have found an overlapping distribution of DNA polymorphisms and the junctions of Okazaki fragments along mononucleosomal DNA. This sequence instability is caused by the premature release of polymerase δ and the retention of non proof‐read DNA tracts replicated by polymerase α. The possible implementation of these new experimental approaches in multicellular organisms opens the door to the analysis of replication dynamics under a broad range of genetic backgrounds and physiological or pathological conditions.

Genome-Wide Nucleosome Occupancy and Positioning and Their Impact on Gene Expression and Evolution in Plants

The fundamental unit of chromatin is the nucleosome that consists of a protein octamer composed of the four core histones (Hs; H3, H4, H2A, and H2B) wrapped by 147 bp of DNA. Nucleosome occupancy and positioning have proven to be dynamic and have a critical impact on expression, regulation, and evolution of eukaryotic genes. We developed nucleosome occupancy and positioning data sets using leaf tissue of rice (Oryza sativa) and both leaf and flower tissues of Arabidopsis (Arabidopsis thaliana). We show that model plant and animal species share the fundamental characteristics associated with nucleosome dynamics. Only 12% and 16% of the Arabidopsis and rice genomes, respectively, were occupied by well-positioned nucleosomes. The cores of positioned nucleosomes were enriched with G/C dinucleotides and showed a lower C→T mutation rate than the linker sequences. We discovered that nucleosomes associated with heterochromatic regions were more spaced with longer linkers than those in euchromatic regions in both plant species. Surprisingly, different nucleosome densities were found to be associated with chromatin in leaf and flower tissues in Arabidopsis. We show that deep MNase-seq data sets can be used to map nucleosome occupancy of specific genomic loci and reveal gene expression patterns correlated with chromatin dynamics in plant genomes.

The effects of chromatin organization on variation in mutation rates in the genome

The variation in local rates of mutations can affect both the evolution of genes and their function in normal and cancer cells. Deciphering the molecular determinants of this variation will be aided by the elucidation of distinct types of mutations, as they differ in regional preferences and in associations with genomic features. Chromatin organization contributes to regional variation in mutation rates, but its contribution differs among mutation types. In both germline and somatic mutations, base substitutions are more abundant in regions of closed chromatin, perhaps reflecting error accumulation late in replication. By contrast, a distinctive mutational state with very high levels of insertions and deletions (indels) and substitutions is enriched in regions of open chromatin. These associations indicate an intricate interplay between the nucleotide sequence of DNA and its dynamic packaging into chromatin, and have important implications for current biomedical research. This Review focuses on recent studies showing associations between chromatin state and mutation rates, including pairwise and multivariate investigations of germline and somatic (particularly cancer) mutations.

Lagging-strand replication shapes the mutational landscape of the genome

The origin of mutations is central to understanding evolution and of key relevance to health. Variation occurs non-randomly across the genome, and mechanisms for this remain to be defined. Here, we report that the 5′-ends of Okazaki fragments have significantly elevated levels of nucleotide substitution, indicating a replicative origin for such mutations. With a novel method, emRiboSeq, we map the genome-wide contribution of polymerases, and show that despite Okazaki fragment processing, DNA synthesised by error-prone Pol-α is retained in vivo, comprising ~1.5% of the mature genome. We propose that DNA-binding proteins that rapidly re-associate post-replication act as partial barriers to Pol-δ mediated displacement of Pol-α synthesised DNA, resulting in incorporation of such Pol-α tracts and elevated mutation rates at specific sites. We observe a mutational cost to chromatin and regulatory protein binding, resulting in mutation hotspots at regulatory elements, with signatures of this process detectable in both yeast and humans.

Chromatin structure is distinct between coding and non-coding single nucleotide polymorphisms

Background

Previous studies suggested that nucleosomes are enriched with single nucleotide polymorphisms (SNPs) in humans and that the occurrence of mutations is closely associated with CpG dinucleotides. We aimed to determine if the chromatin organization is genomic locus specific around SNPs, and if newly occurring mutations are associated with SNPs.

Results

Here, we classified SNPs according their loci and investigated chromatin organization in both CD4⁺ T cell and lymphoblastoid cell in humans. We calculated the SNP frequency around somatic mutations. The results indicated that nucleosome occupancy is different around SNPs sites in different genomic loci. Coding SNPs are mainly enriched at nucleosomes and associated with repressed histone modifications (HMs) and DNA methylation. Contrastingly, intron SNPs occur in nucleosome-depleted regions and lack HMs. Interestingly, risk-associated non-coding SNPs are also enriched at nucleosomes with HMs but associated with low GC-content and low DNA methylation level. The base-transversion allele frequency is significantly low in coding-synonymous SNPs (P < 10^-11). Another finding is that at the -1 and +1 positions relative to the somatic mutation sites, the SNP frequency was significantly higher (P < 3.2 × 10^-5).

Conclusions

The results suggested chromatin structure is different around coding SNPs and non-coding SNPs. New mutations tend to occur at the -1 and +1 position immediately near the SNPs.

Heterogeneous polymerase fidelity and mismatch repair bias genome variation and composition

Mutational heterogeneity must be taken into account when reconstructing evolutionary histories, calibrating molecular clocks, and predicting links between genes and disease. Selective pressures and various DNA transactions have been invoked to explain the heterogeneous distribution of genetic variation between species, within populations, and in tissue-specific tumors. To examine relationships between such heterogeneity and variations in leading- and lagging-strand replication fidelity and mismatch repair, we accumulated 40,000 spontaneous mutations in eight diploid yeast strains in the absence of selective pressure. We found that replicase error rates vary by fork direction, coding state, nucleosome proximity, and sequence context. Further, error rates and DNA mismatch repair efficiency both vary by mismatch type, responsible polymerase, replication time, and replication origin proximity. Mutation patterns implicate replication infidelity as one driver of variation in somatic and germline evolution, suggest mechanisms of mutual modulation of genome stability and composition, and predict future observations in specific cancers.

Nucleosomes shape DNA polymorphism and divergence

An estimated 80% of genomic DNA in eukaryotes is packaged as nucleosomes, which, together with the remaining interstitial linker regions, generate higher order chromatin structures [1]. Nucleosome sequences isolated from diverse organisms exhibit ∼10 bp periodic variations in AA, TT and GC dinucleotide frequencies. These sequence elements generate intrinsically curved DNA and help establish the histone-DNA interface. We investigated an important unanswered question concerning the interplay between chromatin organization and genome evolution: do the DNA sequence preferences inherent to the highly conserved histone core exert detectable natural selection on genomic divergence and polymorphism? To address this hypothesis, we isolated nucleosomal DNA sequences from Drosophila melanogaster embryos and examined the underlying genomic variation within and between species. We found that divergence along the D. melanogaster lineage is periodic across nucleosome regions with base changes following preferred nucleotides, providing new evidence for systematic evolutionary forces in the generation and maintenance of nucleosome-associated dinucleotide periodicities. Further, Single Nucleotide Polymorphism (SNP) frequency spectra show striking periodicities across nucleosomal regions, paralleling divergence patterns. Preferred alleles occur at higher frequencies in natural populations, consistent with a central role for natural selection. These patterns are stronger for nucleosomes in introns than in intergenic regions, suggesting selection is stronger in transcribed regions where nucleosomes undergo more displacement, remodeling and functional modification. In addition, we observe a large-scale (∼180 bp) periodic enrichment of AA/TT dinucleotides associated with nucleosome occupancy, while GC dinucleotide frequency peaks in linker regions. Divergence and polymorphism data also support a role for natural selection in the generation and maintenance of these super-nucleosomal patterns. Our results demonstrate that nucleosome-associated sequence periodicities are under selective pressure, implying that structural interactions between nucleosomes and DNA sequence shape sequence evolution, particularly in introns.

In eukaryotic cells, the majority of DNA is packaged in nucleosomes comprised of ∼147 bp of DNA wound tightly around the highly conserved histone octamer. Nucleosomal DNA from diverse organisms shows an anti-correlated ∼10 bp periodicity of AT-rich and GC-rich dinucleotides. These sequence features influence DNA bending and shape, facilitating structural interactions. We asked whether natural selection mediated through the periodic sequence preferences of nucleosomes shapes the evolution of non-protein-coding regions of D. melanogaster by examining the inter- and intra-species genomic variation relative to these fundamental chromatin building blocks. The sequence changes across nucleosome-bound regions on the melanogaster lineage mirror the observed nucleosome dinucleotide periodicities. Importantly, we show that the frequencies of polymorphisms in natural populations vary across these regions, paralleling divergence, with higher frequencies of preferred alleles. These patterns are most evident for intronic regions and indicate that non-protein coding regions are evolving toward sequences that facilitate the canonical association with the histone core. This result is consistent with the hypothesis that interactions between DNA and the core have systematic impacts on function that are subject to natural selection and are not solely due to mutational bias. These ubiquitous interactions with the histone core partially account for the evolutionary constraint observed in unannotated genomic regions, and may drive broad changes in base composition.

Large hypomethylated domains serve as strong repressive machinery for key developmental genes in vertebrates

DNA methylation is a fundamental epigenetic modification in vertebrate genomes and a small fraction of genomic regions is hypomethylated. Previous studies have implicated hypomethylated regions in gene regulation, but their functions in vertebrate development remain elusive. To address this issue, we generated epigenomic profiles that include base-resolution DNA methylomes and histone modification maps from both pluripotent cells and mature organs of medaka fish and compared the profiles with those of human ES cells. We found that a subset of hypomethylated domains harbor H3K27me3 (K27HMDs) and their size positively correlates with the accumulation of H3K27me3. Large K27HMDs are conserved between medaka and human pluripotent cells and predominantly contain promoters of developmental transcription factor genes. These key genes were found to be under strong transcriptional repression, when compared with other developmental genes with smaller K27HMDs. Furthermore, human-specific K27HMDs show an enrichment of neuronal activity-related genes, which suggests a distinct regulation of these genes in medaka and human. In mature organs, some of the large HMDs become shortened by elevated DNA methylation and associate with sustained gene expression. This study highlights the significance of domain size in epigenetic gene regulation. We propose that large K27HMDs play a crucial role in pluripotent cells by strictly repressing key developmental genes, whereas their shortening consolidates long-term gene expression in adult differentiated cells.

Genome-wide nucleosome positioning is orchestrated by genomic regions associated with DNase I hypersensitivity in rice

Nucleosome positioning dictates the DNA accessibility for regulatory proteins, and thus is critical for gene expression and regulation. It has been well documented that only a subset of nucleosomes are reproducibly positioned in eukaryotic genomes. The most prominent example of phased nucleosomes is the context of genes, where phased nucleosomes flank the transcriptional starts sites (TSSs). It is unclear, however, what factors determine nucleosome positioning in regions that are not close to genes. We mapped both nucleosome positioning and DNase I hypersensitive site (DHS) datasets across the rice genome. We discovered that DHSs located in a variety of contexts, both genic and intergenic, were flanked by strongly phased nucleosome arrays. Phased nucleosomes were also found to flank DHSs in the human genome. Our results suggest the barrier model may represent a general feature of nucleosome organization in eukaryote genomes. Specifically, regions bound with regulatory proteins, including intergenic regions, can serve as barriers that organize phased nucleosome arrays on both sides. Our results also suggest that rice DHSs often span a single, phased nucleosome, similar to the H2A.Z-containing nucleosomes observed in DHSs in the human genome.

The fundamental unit of chromatin is the nucleosome, which consists of 147 bp of DNA wrapped around a histone octamer containing four core histones (H3, H4, H2A, and H2B). Nucleosome positioning in the genome affects the DNA accessibility for regulatory proteins, and thus is critical for gene expression and regulation. Genomic regions associated with regulatory proteins are associated with a pronounced sensitivity to DNase I digestion, and are thus called DNase I hypersensitive sites (DHSs). It is well known that only a subset of nucleosomes are reproducibly positioned in eukaryotic genomes. However, it is less clear what factors determine genome-wide nucleosome positioning, especially in intergenic regions. We mapped both nucleosome positioning and DHS datasets across the rice genome. We discovered that DHSs located in a variety of contexts, both genic and intergenic, were flanked by strongly phased nucleosome arrays. We confirmed the same association of DHSs with phased nucleosomes in the human genome. We conclude that genomic loci associated with a diverse set of regulatory proteins are major determinants of nucleosome phasing, and this is true in both genic and intergenic regions.

Genomic and phenotypic characterization of a wild medaka population: towards the establishment of an isogenic population genetic resource in fish

Oryzias latipes (medaka) has been established as a vertebrate genetic model for more than a century and recently has been rediscovered outside its native Japan. The power of new sequencing methods now makes it possible to reinvigorate medaka genetics, in particular by establishing a near-isogenic panel derived from a single wild population. Here we characterize the genomes of wild medaka catches obtained from a single Southern Japanese population in Kiyosu as a precursor for the establishment of a near-isogenic panel of wild lines. The population is free of significant detrimental population structure and has advantageous linkage disequilibrium properties suitable for the establishment of the proposed panel. Analysis of morphometric traits in five representative inbred strains suggests phenotypic mapping will be feasible in the panel. In addition, high-throughput genome sequencing of these medaka strains confirms their evolutionary relationships on lines of geographic separation and provides further evidence that there has been little significant interbreeding between the Southern and Northern medaka population since the Southern/Northern population split. The sequence data suggest that the Southern Japanese medaka existed as a larger older population that went through a relatively recent bottleneck approximately 10,000 years ago. In addition, we detect patterns of recent positive selection in the Southern population. These data indicate that the genetic structure of the Kiyosu medaka samples is suitable for the establishment of a vertebrate near-isogenic panel and therefore inbreeding of 200 lines based on this population has commenced. Progress of this project can be tracked at http://www.ebi.ac.uk/birney-srv/medaka-ref-panel.

Reduced local mutation density in regulatory DNA of cancer genomes is linked to DNA repair

Carcinogenesis and neoplastic progression are mediated by the accumulation of somatic mutations. Here we report that the local density of somatic mutations in cancer genomes is highly reduced specifically in accessible regulatory DNA defined by DNase I hypersensitive sites. This reduction is independent of any known factors influencing somatic mutation density and is observed in diverse cancer types, suggesting a general mechanism. By analyzing individual cancer genomes¹, we show that the reduced local mutation density within regulatory DNA is linked to intact global genome repair machinery, with nearly complete abrogation of the hypomutation phenomenon in individual cancers that possess mutations in multiple nucleotide excision repair components. Together, our results connect chromatin structure, gene regulation and cancer-associated somatic mutation.

The transcription start site landscape of C. elegans

More than half of Caenorhabditis elegans pre-mRNAs lose their original 5' ends in a process termed "trans-splicing" in which the RNA extending from the transcription start site (TSS) to the site of trans-splicing of the primary transcript, termed the "outron," is replaced with a 22-nt spliced leader. This complicates the mapping of TSSs, leading to a lack of available TSS mapping data for these genes. We used growth at low temperature and nuclear isolation to enrich for transcripts still containing outrons, applying a modified SAGE capture procedure and high-throughput sequencing to characterize 5' termini in this transcript population. We report from this data both a landscape of 5'-end utilization for C. elegans and a representative collection of TSSs for 7351 trans-spliced genes. TSS distributions for individual genes were often dispersed, with a greater average number of TSSs for trans-spliced genes, suggesting that trans-splicing may remove selective pressure for a single TSS. Upstream of newly defined TSSs, we observed well-known motifs (including TATAA-box and SP1) as well as novel motifs. Several of these motifs showed association with tissue-specific expression and/or conservation among six worm species. Comparing TSS features between trans-spliced and non-trans-spliced genes, we found stronger signals among outron TSSs for preferentially positioning of flanking nucleosomes and for downstream Pol II enrichment. Our data provide an enabling resource for both experimental and theoretical analysis of gene structure and function in C. elegans.

Periodic distribution of a putative nucleosome positioning motif in human, nonhuman primates, and archaea: mutual information analysis

Recently, Trifonov's group proposed a 10-mer DNA motif YYYYYRRRRR as a solution of the long-standing problem of sequence-based nucleosome positioning. To test whether this generic decamer represents a biological meaningful signal, we compare the distribution of this motif in primates and Archaea, which are known to contain nucleosomes, and in Eubacteria, which do not possess nucleosomes. The distribution of the motif is analyzed by the mutual information function (MIF) with a shifted version of itself (MIF profile). We found common features in the patterns of this generic decamer on MIF profiles among primate species, and interestingly we found conspicuous but dissimilar MIF profiles for each Archaea tested. The overall MIF profiles for each chromosome in each primate species also follow a similar pattern. Trifonov's generic decamer may be a highly conserved motif for the nucleosome positioning, but we argue that this is not the only motif. The distribution of this generic decamer exhibits previously unidentified periodicities, which are associated to highly repetitive sequences in the genome. Alu repetitive elements contribute to the most fundamental structure of nucleosome positioning in higher Eukaryotes. In some regions of primate chromosomes, the distribution of the decamer shows symmetrical patterns including inverted repeats.

Dynamic nucleosome organization at hox promoters during zebrafish embryogenesis

Nucleosome organization at promoter regions plays an important role in regulating gene activity. Genome-wide studies in yeast, flies, worms, mammalian embryonic stem cells and transformed cell lines have found well-positioned nucleosomes flanking a nucleosome depleted region (NDR) at transcription start sites. This nucleosome arrangement depends on DNA sequence (cis-elements) as well as DNA binding factors and ATP-dependent chromatin modifiers (trans-factors). However, little is understood about how the nascent embryonic genome positions nucleosomes during development. This is particularly intriguing since the embryonic genome must undergo a broad reprogramming event upon fusion of sperm and oocyte. Using four stages of early embryonic zebrafish development, we map nucleosome positions at the promoter region of 37 zebrafish hox genes. We find that nucleosome arrangement at the hox promoters is a progressive process that takes place over several stages. At stages immediately after fertilization, nucleosomes appear to be largely disordered at hox promoter regions. At stages after activation of the embryonic genome, nucleosomes are detectable at hox promoters, with positions becoming more uniform and more highly occupied. Since the genomic sequence is invariant during embryogenesis, this progressive change in nucleosome arrangement suggests that trans-factors play an important role in organizing nucleosomes during embryogenesis. Separating hox genes into expressed and non-expressed groups shows that expressed promoters have better positioned and occupied nucleosomes, as well as distinct NDRs, than non-expressed promoters. Finally, by blocking the retinoic acid-signaling pathway, we disrupt early hox gene transcription, but observe no effect on nucleosome positions, suggesting that active hox transcription is not a driving force behind the arrangement of nucleosomes at the promoters of hox genes during early development.

Chromatin remodeling at DNA double-strand breaks

DNA double-strand breaks (DSBs) can arise from multiple sources, including exposure to ionizing radiation. The repair of DSBs involves both posttranslational modification of nucleosomes and concentration of DNA-repair proteins at the site of damage. Consequently, nucleosome packing and chromatin architecture surrounding the DSB may limit the ability of the DNA-damage response to access and repair the break. Here, we review early chromatin-based events that promote the formation of open, relaxed chromatin structures at DSBs and that allow the DNA-repair machinery to access the spatially confined region surrounding the DSB, thereby facilitating mammalian DSB repair.