Histone variants: deviants?

Unveiling the distinctive traits of functional rye centromeres: minisatellites, retrotransposons, and R-loop formation

Centromeres play a vital role in cellular division by facilitating kinetochore assembly and spindle attachments. Despite their conserved functionality, centromeric DNA sequences exhibit rapid evolution, presenting diverse sizes and compositions across species. The functional significance of rye centromeric DNA sequences, particularly in centromere identity, remains unclear. In this study, we comprehensively characterized the sequence composition and organization of rye centromeres. Our findings revealed that these centromeres are primarily composed of long terminal repeat retrotransposons (LTR-RTs) and interspersed minisatellites. We systematically classified LTR-RTs into five categories, highlighting the prevalence of younger CRS1, CRS2, and CRS3 of CRSs (centromeric retrotransposons of Secale cereale) were primarily located in the core centromeres and exhibited a higher association with CENH3 nucleosomes. The minisatellites, mainly derived from retrotransposons, along with CRSs, played a pivotal role in establishing functional centromeres in rye. Additionally, we observed the formation of R-loops at specific regions of CRS1, CRS2, and CRS3, with both rye pericentromeres and centromeres exhibiting enrichment in R-loops. Notably, these R-loops selectively formed at binding regions of the CENH3 nucleosome in rye centromeres, suggesting a potential role in mediating the precise loading of CENH3 to centromeres and contributing to centromere specification. Our work provides insights into the DNA sequence composition, distribution, and potential function of R-loops in rye centromeres. This knowledge contributes valuable information to understanding the genetics and epigenetics of rye centromeres, offering implications for the development of synthetic centromeres in future plant modifications and beyond.

Fusion of histone variants to Cas9 suppresses non-homologous end joining

As a versatile genome editing tool, the CRISPR-Cas9 system induces DNA double-strand breaks at targeted sites to activate mainly two DNA repair pathways: HDR which allows precise editing via recombination with a homologous template DNA, and NHEJ which connects two ends of the broken DNA, which is often accompanied by random insertions and deletions. Therefore, how to enhance HDR while suppressing NHEJ is a key to successful applications that require precise genome editing. Histones are small proteins with a lot of basic amino acids that generate electrostatic affinity to DNA. Since H2A.X is involved in DNA repair processes, we fused H2A.X to Cas9 and found that this fusion protein could improve the HDR/NHEJ ratio by suppressing NHEJ. As various post-translational modifications of H2A.X play roles in the regulation of DNA repair, we also fused H2A.X mimicry variants to replicate these post-translational modifications including phosphorylation, methylation, and acetylation. However, none of them were effective to improve the HDR/NHEJ ratio. We further fused other histone variants to Cas9 and found that H2A.1 suppressed NHEJ better than H2A.X. Thus, the fusion of histone variants to Cas9 is a promising option to enhance precise genome editing.

Young retrotransposons and non-B DNA structures promote the establishment of dominant rye centromere in the 1RS.1BL fused centromere

The fine centromere structure in Robertsonian wheat-rye translocation chromosomes exhibits variation among different translocation genotypes. Within extensively employed wheat-rye 1RS.1BL translocation lines in wheat breeding, their translocated chromosomes frequently display fused centromere. Nevertheless, the mechanism governing the functionality of the fused centromere in 1RS.1BL translocated chromosomes remains to be clarified. In this study, we investigated the fine centromere structure of the 1RS.1BL translocated chromosome through a combination of cytological and genomics methods. We found that only the rye-derived centromere exhibits functional activity, whether in breeding applications or artificially synthesized translocation chromosomes. The active rye-derived centromere had higher proportion of young full-length long terminal repeat retrotransposons (flLTR-RTs) and more stable non-B DNA structures, which may be beneficial toward transcription of centromeric repeats and CENH3 loading to maintain the activity of rye centromeres. High levels of DNA methylation and H3K9me2 were found in the inactive wheat-derived centromeres, suggesting that it may play a crucial role in maintaining the inactive status of the wheat centromere. Our works elucidate the fine structure of 1RS.1BL translocations and the potential mechanism of centromere inactivation in the fused centromere, contributing knowledge to the application of fused centromere in wheat breeding formation of new wheat-rye translocation lines.

Epigenetic Control of Plant Response to Heavy Metals

Plants are sessile organisms that must adapt to environmental conditions, such as soil characteristics, by adjusting their development during their entire life cycle. In case of low-distance seed dispersal, the new generations are challenged with the same abiotic stress encountered by the parents. Epigenetic modification is an effective option that allows plants to face an environmental constraint and to share the same adaptative strategy with their progeny through transgenerational inheritance. This is the topic of the presented review that reports the scientific progress, up to date, gained in unravelling the epigenetic response of plants to soil contamination by heavy metals and metalloids, collectively known as potentially toxic elements. The effect of the microbial community inhabiting the rhizosphere is also considered, as the evidence of a transgenerational transfer of the epigenetic status that contributes to the activation in plants of response mechanisms to soil pollution.

Patient-facing digital tools for delivering genetic services: a systematic review

This study systematically reviewed the literature on the impact of digital genetics tools on patient care and system efficiencies. MEDLINE and Embase were searched for articles published between January 2010 and March 2021. Studies evaluating the use of patient-facing digital tools in the context of genetic service delivery were included. Two reviewers screened and extracted patient-reported and system-focused outcomes from each study. Data were synthesised using a descriptive approach. Of 3226 unique studies identified, 87 were included. A total of 70 unique digital tools were identified. As a result of using digital tools, 84% of studies reported a positive outcome in at least one of the following patient outcomes: knowledge, psychosocial well-being, behavioural/management changes, family communication, decision-making or level of engagement. Digital tools improved workflow and efficiency for providers and reduced the amount of time they needed to spend with patients. However, we identified a misalignment between study purpose and patient-reported outcomes measured and a lack of tools that encompass the entire genetic counselling and testing trajectory. Given increased demand for genetic services and the shift towards virtual care, this review provides evidence that digital tools can be used to efficiently deliver patient-centred care. Future research should prioritise development, evaluation and implementation of digital tools that can support the entire patient trajectory across a range of clinical settings. PROSPERO registration numberCRD42020202862.

Cucurbitaceae phloem exudate lectins: Purification, molecular characterization and carbohydrate binding characteristics

Much of the plant lectin research was focused on these proteins from seeds, whereas lectins from other plant tissues have been less investigated. Although presence of lectins in the phloem exudate of Cucurbitaceae species was reported over 40 years ago, only a few proteins from this family have been purified and characterized with respect to ligand binding properties, primary and secondary structures, while no 3D structure of a member of this family is known so far. Unlike lectins from other plant families and sources (e.g., seeds and tubers), which exhibit specificity towards different carbohydrate structures, all the Cucurbitaceae phloem exudate lectins characterized so far have been shown to recognize only chitooligosaccharides or glycans containing chitooligosaccharides. Interestingly, some of these proteins also bind various types of RNAs, suggesting that they may also play a role in the transport of RNA information molecules in the phloem. The present review gives an overview of the current knowledge of Cucurbitaceae phloem exudate lectins with regard to their purification, determination of primary and secondary structures, elucidation of thermodynamics and kinetics of carbohydrate binding and computational modeling to get information on their 3D structures. Finally, future perspectives of research on this important class of proteins are considered.

Breaking the aging epigenetic barrier

Aging is an inexorable event occurring universally for all organisms characterized by the progressive loss of cell function. However, less is known about the key events occurring inside the nucleus in the process of aging. The advent of chromosome capture techniques and extensive modern sequencing technologies have illuminated a rather dynamic structure of chromatin inside the nucleus. As cells advance along their life cycle, chromatin condensation states alter which leads to a different epigenetic landscape, correlated with modified gene expression. The exact factors mediating these changes in the chromatin structure and function remain elusive in the context of aging cells. The accumulation of DNA damage, reactive oxygen species and loss of genomic integrity as cells cease to divide can contribute to a tumor stimulating environment. In this review, we focus on genomic and epigenomic changes occurring in an aged cell which can contribute to age-related tumor formation.

Histone macroH2A1 is a stronger regulator of hippocampal transcription and memory than macroH2A2 in mice

Histone variants H2A.Z and H3.3 are epigenetic regulators of memory, but roles of other variants are not well characterized. macroH2A (mH2A) is a structurally unique histone that contains a globular macrodomain connected to the histone region by an unstructured linker. Here we assessed if mH2A regulates memory and if this role varies for the two mH2A-encoding genes, H2afy (mH2A1) and H2afy2 (mH2A2). We show that fear memory is impaired in mH2A1, but not in mH2A2-deficient mice, whereas both groups were impaired in a non-aversive spatial memory task. However, impairment was larger for mH2A1- deficient mice, indicating a preferential role for mH2A1 over mH2A2 in memory. Accordingly, mH2A1 depletion in the mouse hippocampus resulted in more extensive transcriptional de-repression compared to mH2A2 depletion. mH2A1-depleted mice failed to induce a normal transcriptional response to fear conditioning, suggesting that mH2A1 depletion impairs memory by altering transcription. Using chromatin immunoprecipitation (ChIP) sequencing, we found that both mH2A proteins are enriched on transcriptionally repressed genes, but only mH2A1 occupancy was dynamically modified during learning, displaying reduced occupancy on upregulated genes after training. These data identify mH2A as a regulator of memory and suggest that mH2A1 supports memory by repressing spurious transcription and promoting learning-induced transcriptional activation.

Differential condensation of sister chromatids acts with Cdc6 to ensure asynchronous S-phase entry in Drosophila male germline stem cell lineage

During Drosophila melanogaster male germline stem cell (GSC) asymmetric division, preexisting old versus newly synthesized histones H3 and H4 are asymmetrically inherited. However, the biological outcomes of this phenomenon have remained unclear. Here, we tracked old and new histones throughout the GSC cell cycle through the use of high spatial and temporal resolution microscopy. We found unique features that differ between old and new histone-enriched sister chromatids, including differences in nucleosome density, chromosomal condensation, and H3 Ser10 phosphorylation. These distinct chromosomal features lead to their differential association with Cdc6, a pre-replication complex component, and subsequent asynchronous DNA replication initiation in the resulting daughter cells. Disruption of asymmetric histone inheritance abolishes differential Cdc6 association and asynchronous S-phase entry, demonstrating that histone asymmetry acts upstream of these critical cell-cycle progression events. Furthermore, disruption of these GSC-specific chromatin features leads to GSC defects, indicating a connection between histone inheritance, cell-cycle progression, and cell fate determination.

Epigenetic regulation of thermomorphogenesis and heat stress tolerance

Many environmental conditions fluctuate and organisms need to respond effectively. This is especially true for temperature cues that can change in minutes to seasons and often follow a diurnal rhythm. Plants cannot migrate and most cannot regulate their temperature. Therefore, a broad array of responses have evolved to deal with temperature cues from freezing to heat stress. A particular response to mildly elevated temperatures is called thermomorphogenesis, a suite of morphological adaptations that includes thermonasty, formation of thin leaves and elongation growth of petioles and hypocotyl. Thermomorphogenesis allows for optimal performance in suboptimal temperature conditions by enhancing the cooling capacity. When temperatures rise further, heat stress tolerance mechanisms can be induced that enable the plant to survive the stressful temperature, which typically comprises cellular protection mechanisms and memory thereof. Induction of thermomorphogenesis, heat stress tolerance and stress memory depend on gene expression regulation, governed by diverse epigenetic processes. In this Tansley review we update on the current knowledge of epigenetic regulation of heat stress tolerance and elevated temperature signalling and response, with a focus on thermomorphogenesis regulation and heat stress memory. In particular we highlight the emerging role of H3K4 methylation marks in diverse temperature signalling pathways.

Recurrent de novo missense variants across multiple histone H4 genes underlie a neurodevelopmental syndrome

Chromatin is essentially an array of nucleosomes, each of which consists of the DNA double-stranded fiber wrapped around a histone octamer. This organization supports cellular processes such as DNA replication, DNA transcription, and DNA repair in all eukaryotes. Human histone H4 is encoded by fourteen canonical histone H4 genes, all differing at the nucleotide level but encoding an invariant protein. Here, we present a cohort of 29 subjects with de novo missense variants in six H4 genes (H4C3, H4C4, H4C5, H4C6, H4C9, and H4C11) identified by whole-exome sequencing and matchmaking. All individuals present with neurodevelopmental features of intellectual disability and motor and/or gross developmental delay, while non-neurological features are more variable. Ten amino acids are affected, six recurrently, and are all located within the H4 core or C-terminal tail. These variants cluster to specific regions of the core H4 globular domain, where protein-protein interactions occur with either other histone subunits or histone chaperones. Functional consequences of the identified variants were evaluated in zebrafish embryos, which displayed abnormal general development, defective head organs, and reduced body axis length, providing compelling evidence for the causality of the reported disorder(s). While multiple developmental syndromes have been linked to chromatin-associated factors, missense-bearing histone variants (e.g., H3 oncohistones) are only recently emerging as a major cause of pathogenicity. Our findings establish a broader involvement of H4 variants in developmental syndromes.

Extra-nuclear histones: origin, significance and perspectives

Histones are classically known to organize the eukaryotic DNA into chromatin. They are one of the key players in regulating transcriptionally permissive and non-permissive states of the chromatin. Nevertheless, their context-dependent appearance within the cytoplasm and systemic circulation has also been observed. The past decade has also witnessed few scientific communications on the existence of vesicle-associated histones. Diverse groups have attempted to determine the significance of these extra-nuclear histones so far, with many of those studies still underway. Of note amongst these are interactions of extra-nuclear or free histones with cellular membranes, mediated by mutual cationic and anionic natures, respectively. It is here aimed to consolidate the mechanism of formation of extra-nuclear histones; implications of histone-induced membrane destabilization and explore the mechanisms of their association/release with extracellular vesicles, along with the functional aspects of these extra-nuclear histones in cell and systemic physiology.

DNA methylation mediated downregulation of histone H3 variant H3.3 affects cell proliferation contributing to the development of HCC

Chromatin alterations brought by histone variants and modifications potentially regulate gene transcription from tumor initiation to progression. Histone H3.3 variant is one such epigenetic player important for disease progression and development. Though many studies have implicated H3.3 role in cancer progression and metastasis, its regulation, importance of specific modifications and chaperones have been not understood yet. We report DNA methylation mediated downregulation of histone H3 variant H3.3 in HCC and a concomitant increase in the level of the H3.2 variant. The loss of H3.3 in cancer tissues correlates with a decrease in the histone modifications associated with active transcription like H3K9/K14/K27Ac and H3K4Me3. The ectopic overexpression of H3.3 and H3.2 did not affect global PTMs and cell physiology, probably owing to the deregulation of specific histone chaperones CAF-1 (for H3.2) and HIRA (for H3.3) as observed in HCC tissues. Notably, knockdown of P150, a subunit of CAF-1 leads to a cell cycle arrest in S-phase in a neoplastic rat liver cell line, possibly due to the decrease in the histone levels necessary for DNA packaging. Remarkably, modulation of H3.3 in pre-neoplastic rat liver cells lead to an increase in cell proliferation and a decreased transcription of tumor suppressor genes, recapitulating the tumor cell phenotype. Our data suggests, inhibition of DNA methylation and histone deacetylation leads to the restoration of histone H3 variant expression in tumor cells.

Mechanism and Therapeutic Opportunities of Histone Modifications in Chronic Liver Disease

Chronic liver disease (CLD) represents a global health problem, accounting for the heavy burden of disability and increased health care utilization. Epigenome alterations play an important role in the occurrence and progression of CLD. Histone modifications, which include acetylation, methylation, and phosphorylation, represent an essential part of epigenetic modifications that affect the transcriptional activity of genes. Different from genetic mutations, histone modifications are plastic and reversible. They can be modulated pharmacologically without changing the DNA sequence. Thus, there might be chances to establish interventional solutions by targeting histone modifications to reverse CLD. Here we summarized the roles of histone modifications in the context of alcoholic liver disease (ALD), metabolic associated fatty liver disease (MAFLD), viral hepatitis, autoimmune liver disease, drug-induced liver injury (DILI), and liver fibrosis or cirrhosis. The potential targets of histone modifications for translation into therapeutics were also investigated. In prospect, high efficacy and low toxicity drugs that are selectively targeting histone modifications are required to completely reverse CLD and prevent the development of liver cirrhosis and malignancy.

H2A.B is a cancer/testis factor involved in the activation of ribosome biogenesis in Hodgkin lymphoma

Testis-specific regulators of chromatin function are commonly ectopically expressed in human cancers, but their roles are poorly understood. Examination of 81 primary Hodgkin lymphoma (HL) samples showed that the ectopic expression of the eutherian testis-specific histone variant H2A.B is an inherent feature of HL. In experiments using two HL cell lines derived from different subtypes of HL, H2A.B knockdown inhibited cell proliferation. H2A.B was enriched in both nucleoli of these HL cell lines and primary HL samples. We found that H2A.B enhanced ribosomal DNA (rDNA) transcription, was enriched at the rDNA promoter and transcribed regions, and interacted with RNA Pol I. Depletion of H2A.B caused the loss of RNA Pol I from rDNA chromatin. Remarkably, H2A.B was also required for high levels of ribosomal protein gene expression being located at the transcriptional start site and within the gene body. H2A.B knockdown reduced gene body chromatin accessibility of active RNA Pol II genes concurrent with a decrease in transcription. Taken together, our data show that in HL H2A.B has acquired a new function, the ability to increase ribosome biogenesis.

Peculiarities of Plasmodium falciparum Gene Regulation and Chromatin Structure

The highly complex life cycle of the human malaria parasite, Plasmodium falciparum, is based on an orchestrated and tightly regulated gene expression program. In general, eukaryotic transcription regulation is determined by a combination of sequence-specific transcription factors binding to regulatory DNA elements and the packaging of DNA into chromatin as an additional layer. The accessibility of regulatory DNA elements is controlled by the nucleosome occupancy and changes of their positions by an active process called nucleosome remodeling. These epigenetic mechanisms are poorly explored in P. falciparum. The parasite genome is characterized by an extraordinarily high AT-content and the distinct architecture of functional elements, and chromatin-related proteins also exhibit high sequence divergence compared to other eukaryotes. Together with the distinct biochemical properties of nucleosomes, these features suggest substantial differences in chromatin-dependent regulation. Here, we highlight the peculiarities of epigenetic mechanisms in P. falciparum, addressing chromatin structure and dynamics with respect to their impact on transcriptional control. We focus on the specialized chromatin remodeling enzymes and discuss their essential function in P. falciparum gene regulation.

A ride through the epigenetic landscape: aging reversal by reprogramming

Aging has become one of the fastest-growing research topics in biology. However, exactly how the aging process occurs remains unknown. Epigenetics plays a significant role, and several epigenetic interventions can modulate lifespan. This review will explore the interplay between epigenetics and aging, and how epigenetic reprogramming can be harnessed for age reversal. In vivo partial reprogramming holds great promise as a possible therapy, but several limitations remain. Rejuvenation by reprogramming is a young but rapidly expanding subfield in the biology of aging.

Epigenetic alterations in the gastrointestinal tract: Current and emerging use for biomarkers of cancer

Colorectal cancer is a leading cause of cancer related deaths worldwide. One of the hallmarks of cancer and a fundamental trait of virtually all gastrointestinal cancers is genomic and epigenomic DNA alterations. Cancer cells acquire genetic and epigenetic alterations that drive the initiation and progression of the cancers by altering the molecular and cell biological process of the cells. These alterations, as well as other host and microenvironment factors, ultimately mediate the initiation and progression of cancers, including colorectal cancer. Epigenetic alterations, which include changes affecting DNA methylation, histone modifications, chromatin structure, and noncoding RNA expression, have emerged as a major class of molecular alteration in colon polyps and colorectal cancer. The classes of epigenetic alterations, their status in colorectal polyps and cancer, their effects on neoplasm biology, and their application to clinical care will be discussed.

Tumor-specific overexpression of histone gene, H3C14 in gastric cancer is mediated through EGFR-FOXC1 axis

Incorporation of different H3 histone isoforms/variants have been reported to differentially regulate gene expression via alteration in chromatin organization during diverse cellular processes. However, the differential expression of highly conserved histone H3.2 genes, H3C14 and H3C13 in human cancer has not been delineated. In this study, we investigated the expression of H3.2 genes in primary human gastric, brain, breast, colon, liver, and head and neck cancer tissues and tumor cell lines. The data showed overexpression of H3.2 transcripts in tumor samples and cell lines with respect to normal counterparts. Furthermore, TCGA data of individual and TCGA PANCAN cohort also showed significant up-regulation of H3.2 genes. Further, overexpressed H3C14 gene coding for H3.2 protein was regulated by FOXC1 transcription factor and G4-cassette in gastric cancer cell lines. Elevated expression of FOXC1 protein and transcripts were also observed in human gastric cancer samples and cell lines. Further, FOXC1 protein was predominantly localized in the nuclei of neoplastic gastric cells compared to normal counterpart. In continuation, studies with EGF induction, FOXC1 knockdown, and ChIP-qPCR for the first time identified a novel axis, EGFR-FOXC1-H3C14 for regulation of H3C14 gene overexpression in gastric cancer. Therefore, the changes the epigenomic landscape due to incorporation of differential expression H3 variant contributes to change in gene expression pattern and thereby contributing to pathogenesis of cancer.

Effects of H2A.B incorporation on nucleosome structures and dynamics

The H2A.B histone variant is an epigenetic regulator involved in transcriptional upregulation, DNA synthesis, and splicing that functions by replacing the canonical H2A histone in the nucleosome core particle. Introduction of H2A.B results in less compact nucleosome states with increased DNA unwinding and accessibility at the nucleosomal entry and exit sites. Despite being well characterized experimentally, the molecular mechanisms by which H2A.B incorporation alters nucleosome stability and dynamics remain poorly understood. To study the molecular mechanisms of H2A.B, we have performed a series of conventional and enhanced sampling molecular dynamics simulation of H2A.B- and canonical H2A-containing nucleosomes. Results of conventional simulations show that H2A.B weakens protein-protein and protein-DNA interactions at specific locations throughout the nucleosome. These weakened interactions result in significantly more DNA opening from both the entry and exit sites in enhanced sampling simulations. Furthermore, free energy profiles show that H2A.B-containing nucleosomes have significantly broader free wells and that H2A.B allows for sampling of states with increased DNA breathing, which are shown to be stable on the hundreds of nanoseconds timescale with further conventional simulations. Together, our results show the molecular mechanisms by which H2A.B creates less compacted nucleosome states as a means of increasing genetic accessibility and gene transcription.

Epigenetic Alterations in the Gastrointestinal Tract: Current and Emerging Use for Biomarkers of Cancer

Colorectal cancer, liver cancer, stomach cancer, pancreatic cancer, and esophageal cancer are leading causes of cancer-related deaths worldwide. A fundamental trait of virtually all gastrointestinal cancers is genomic and epigenomic DNA alterations. Cancer cells acquire genetic and epigenetic alterations that drive the initiation and progression of the cancers by altering the molecular and cell biological processes of the cells. These alterations, as well as other host and microenvironment factors, ultimately mediate the clinical behavior of the precancers and cancers and can be used as biomarkers for cancer risk determination, early detection of cancer and precancer, determination of the prognosis of cancer and prediction of the response to therapy. Epigenetic alterations have emerged as one of most robust classes of biomarkers and are the basis for a growing number of clinical tests for cancer screening and surveillance.

Improving the Level of the Tyrosine Biosynthesis Pathway in Saccharomyces cerevisiae through HTZ1 Knockout and Atmospheric and Room Temperature Plasma (ARTP) Mutagenesis

In recent years, many studies have been conducted on the expression of multiple aromatic compounds by Saccharomyces cerevisiae. The concentration of l-tyrosine, as a precursor of such valuable compounds, is crucial for the biosynthesis of aromatic metabolites. In this study, a novel function of HTZ1 was found to be related to tyrosine biosynthesis, which has not yet been reported. Knockout of this gene could significantly improve the ability of yeast cells to synthesize tyrosine, and its p-coumaric acid (p-CA) titer was approximately 3.9-fold higher than that of the wild-type strain BY4742. Subsequently, this strain was selected for random mutagenesis through an emerging mutagenesis technique, namely, atmospheric and room temperature plasma (ARTP). After two rounds of mutagenesis, five tyrosine high-producing mutants were obtained. The highest production of p-CA was 7.6-fold higher than that of the wild-type strain. Finally, transcriptome data of the htz1Δ strain and the five mutants were analyzed. The genome of mutagenic strains was also resequenced to reveal the mechanism underlying the high titer of tyrosine. This system of target engineering combined with random mutagenesis to screen excellent mutants provides a new basis for synthetic biology.

Histone Variant H3.3 Mutations in Defining the Chromatin Function in Mammals

The systematic mutation of histone 3 (H3) genes in model organisms has proven to be a valuable tool to distinguish the functional role of histone residues. No system exists in mammalian cells to directly manipulate canonical histone H3 due to a large number of clustered and multi-loci histone genes. Over the years, oncogenic histone mutations in a subset of H3 have been identified in humans, and have advanced our understanding of the function of histone residues in health and disease. The oncogenic mutations are often found in one allele of the histone variant H3.3 genes, but they prompt severe changes in the epigenetic landscape of cells, and contribute to cancer development. Therefore, mutation approaches using H3.3 genes could be relevant to the determination of the functional role of histone residues in mammalian development without the replacement of canonical H3 genes. In this review, we describe the key findings from the H3 mutation studies in model organisms wherein the genetic replacement of canonical H3 is possible. We then turn our attention to H3.3 mutations in human cancers, and discuss H3.3 substitutions in the N-terminus, which were generated in order to explore the specific residue or associated post-translational modification.

Primate-specific histone variants

Canonical histones (H2A, H2B, H3, and H4) are present in all eukaryotes where they package genomic DNA and participate in numerous cellular processes, such as transcription regulation and DNA repair. In addition to the canonical histones, there are many histone variants, which have different amino acid sequences, possess tissue-specific expression profiles, and function distinctly from the canonical counterparts. A number of histone variants, including both core histones (H2A/H2B/H3/H4) and linker histones (H1/H5), have been identified to date. Htz1 (H2A.Z) and CENP-A (CenH3) are present from yeasts to mammals, and H3.3 is present from Tetrahymena to humans. In addition to the prevalent variants, others like H3.4 (H3t), H2A.Bbd, and TH2B, as well as several H1 variants, are found to be specific to mammals. Among them, H2BFWT, H3.5, H3.X, H3.Y, and H4G are unique to primates (or Hominidae). In this review, we focus on localization and function of primate- or hominidae-specific histone variants.

Short Histone H2A Variants: Small in Stature but not in Function

The dynamic packaging of DNA into chromatin regulates all aspects of genome function by altering the accessibility of DNA and by providing docking pads to proteins that copy, repair and express the genome. Different epigenetic-based mechanisms have been described that alter the way DNA is organised into chromatin, but one fundamental mechanism alters the biochemical composition of a nucleosome by substituting one or more of the core histones with their variant forms. Of the core histones, the largest number of histone variants belong to the H2A class. The most divergent class is the designated “short H2A variants” (H2A.B, H2A.L, H2A.P and H2A.Q), so termed because they lack a H2A C-terminal tail. These histone variants appeared late in evolution in eutherian mammals and are lineage-specific, being expressed in the testis (and, in the case of H2A.B, also in the brain). To date, most information about the function of these peculiar histone variants has come from studies on the H2A.B and H2A.L family in mice. In this review, we describe their unique protein characteristics, their impact on chromatin structure, and their known functions plus other possible, even non-chromatin, roles in an attempt to understand why these peculiar histone variants evolved in the first place.

Epigenetic regulators of the revascularization response to chronic arterial occlusion

Peripheral arterial disease (PAD) is the leading cause of lower limb amputation and estimated to affect over 202 million people worldwide. PAD is caused by atherosclerotic lesions that occlude large arteries in the lower limbs, leading to insufficient blood perfusion of distal tissues. Given the severity of this clinical problem, there has been long-standing interest in both understanding how chronic arterial occlusions affect muscle tissue and vasculature and identifying therapeutic approaches capable of restoring tissue composition and vascular function to a healthy state. To date, the most widely utilized animal model for performing such studies has been the ischaemic mouse hindlimb. Despite not being a model of PAD per se, the ischaemic hindlimb model does recapitulate several key aspects of PAD. Further, it has served as a valuable platform upon which we have built much of our understanding of how chronic arterial occlusions affect muscle tissue composition, muscle regeneration and angiogenesis, and collateral arteriogenesis. Recently, there has been a global surge in research aimed at understanding how gene expression is regulated by epigenetic factors (i.e. non-coding RNAs, histone post-translational modifications, and DNA methylation). Thus, perhaps not unexpectedly, many recent studies have identified essential roles for epigenetic factors in regulating key responses to chronic arterial occlusion(s). In this review, we summarize the mechanisms of action of these epigenetic regulators and highlight several recent studies investigating the role of said regulators in the context of hindlimb ischaemia. In addition, we focus on how these recent advances in our understanding of the role of epigenetics in regulating responses to chronic arterial occlusion(s) can inform future therapeutic applications to promote revascularization and perfusion recovery in the setting of PAD.

SWR1 Chromatin Remodeling Complex: A Key Transcriptional Regulator in Plants

The nucleosome is the structural and fundamental unit of eukaryotic chromatin. The chromatin remodeling complexes change nucleosome composition, packaging and positioning to regulate DNA accessibility for cellular machinery. SWI2/SNF2-Related 1 Chromatin Remodeling Complex (SWR1-C) belongs to the INO80 chromatin remodeling family and mainly catalyzes the exchange of H2A-H2B with the H2A.Z-H2B dimer. The replacement of H2A.Z into nucleosomes affects nucleosome stability and chromatin structure. Incorporation of H2A.Z into the chromatin and its physiochemical properties play a key role in transcriptional regulation during developmental and environmental responses. In Arabidopsis, various studies have uncovered several pivotal roles of SWR1-C. Recently, notable progress has been achieved in understanding the role of SWR1-C in plant developmental and physiological processes such as DNA damage repair, stress tolerance, and flowering time. The present article introduces the SWR1-C and comprehensively reviews recent discoveries made in understanding the function of the SWR1 complex in plants.

A Read/Write Mechanism Connects p300 Bromodomain Function to H2A.Z Acetylation

Acetylation of the histone variant H2A.Z (H2A.Zac) occurs at active regulatory regions associated with gene expression. Although the Tip60 complex is proposed to acetylate H2A.Z, functional studies suggest additional enzymes are involved. Here, we show that p300 acetylates H2A.Z at multiple lysines. In contrast, we found that although Tip60 does not efficiently acetylate H2A.Z in vitro, genetic inhibition of Tip60 reduces H2A.Zac in cells. Importantly, we found that interaction between the p300-bromodomain and H4 acetylation (H4ac) enhances p300-driven H2A.Zac. Indeed, H2A.Zac and H4ac show high genomic overlap, especially at active promoters. We also reveal unique chromatin features and transcriptional states at enhancers correlating with co-occurrence or exclusivity of H4ac and H2A.Zac. We propose that differential H4 and H2A.Z acetylation signatures can also define the enhancer state. In conclusion, we show both Tip60 and p300 contribute to H2A.Zac and reveal molecular mechanisms of writer/reader crosstalk between H2A.Z and H4 acetylation through p300.

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p300 acetylates H2A.Z at multiple N-terminal lysine residues

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Interaction of p300 bromodomain with H4ac enhances H2A.Zac

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H2A.Zac and H4ac co-localize at active regulatory regions

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H4ac and H2A.Zac differential signature can define the enhancer state

A novel histone H4 variant H4G regulates rDNA transcription in breast cancer

Histone variants, present in various cell types and tissues, are known to exhibit different functions. For example, histone H3.3 and H2A.Z are both involved in gene expression regulation, whereas H2A.X is a specific variant that responds to DNA double-strand breaks. In this study, we characterized H4G, a novel hominidae-specific histone H4 variant. We found that H4G is expressed in a variety of human cell lines and exhibit tumor-stage dependent overexpression in tissues from breast cancer patients. We found that H4G localized primarily to the nucleoli of the cell nucleus. This localization was controlled by the interaction of the alpha-helix 3 of the histone fold motif with a histone chaperone, nucleophosmin 1. In addition, we found that modulating H4G expression affects rRNA expression levels, protein synthesis rates and cell-cycle progression. Our data suggest that H4G expression alters nucleolar chromatin in a way that enhances rDNA transcription in breast cancer tissues.

Dissecting the role of the germinal vesicle nuclear envelope and soluble content in the process of somatic cell remodelling and reprogramming

Differentiated nuclei can be reprogrammed/remodelled to totipotency after their transfer to enucleated metaphase II (MII) oocytes. The process of reprogramming/remodelling is, however, only partially characterized. It has been shown that the oocyte nucleus (germinal vesicle – GV) components are essential for a successful remodelling of the transferred nucleus by providing the materials for pseudo-nucleus formation. However, the nucleus is a complex structure and exactly what nuclear components are required for a successful nucleus remodelling and reprogramming is unknown. Till date, the only nuclear sub-structure experimentally demonstrated to be essential is the oocyte nucleolus (nucleolus-like body, NLB). In this study, we investigated what other GV components might be necessary for the formation of normal-sized pseudo-pronuclei (PNs). Our results showed that the removal of the GV nuclear envelope with attached chromatin and chromatin-bound factors does not substantially influence the size of the remodelled nuclei in reconstructed cells and that their nuclear envelopes seem to have normal parameters. Rather than the insoluble nuclear lamina, the GV content, which is dissolved in the cytoplasm with the onset of oocyte maturation, influences the characteristics and size of transferred nuclei.

Replication-dependent histone isoforms: a new source of complexity in chromatin structure and function

Replication-dependent histones are expressed in a cell cycle regulated manner and supply the histones necessary to support DNA replication. In mammals, the replication-dependent histones are encoded by a family of genes that are located in several clusters. In humans, these include 16 genes for histone H2A, 22 genes for histone H2B, 14 genes for histone H3, 14 genes for histone H4 and 6 genes for histone H1. While the proteins encoded by these genes are highly similar, they are not identical. For many years, these genes were thought to encode functionally equivalent histone proteins. However, several lines of evidence have emerged that suggest that the replication-dependent histone genes can have specific functions and may constitute a novel layer of chromatin regulation. This Survey and Summary reviews the literature on replication-dependent histone isoforms and discusses potential mechanisms by which the small variations in primary sequence between the isoforms can alter chromatin function. In addition, we summarize the wealth of data implicating altered regulation of histone isoform expression in cancer.

Centromeric non-coding RNA as a hidden epigenetic factor of the point centromere

To ensure proper chromosome segregation during cell division, the centromere in many organisms is transcribed to produce a low level of long non-coding RNA to regulate the activity of the kinetochore. In the budding yeast point centromere, our recent work has shown that the level of centromeric RNAs (cenRNAs) is tightly regulated and repressed by the kinetochore protein Cbf1 and histone H2A variant H2A.Z^Htz1, and de-repressed during S phase of the cell cycle. Too little or too much cenRNAs will disrupt centromere activity. Here, we discuss the current advance in the understanding of the action and regulation of cenRNAs at the point centromere of Saccharomyces cerevisiae. We further show that budding yeast cenRNAs are cryptic unstable transcripts (CUTs) that can be degraded by the nuclear RNA decay pathway. CenRNA provides an example that even CUTs, when present at the right time with the right level, can serve important cellular functions.

Histone variant MacroH2A1 is downregulated in prostate cancer and influences malignant cell phenotype

Background

Prostate cancer (PCa), a major cause of cancer-related morbidity and mortality worldwide and mostly asymptomatic at earliest stages, is characterized by disruption of genetic and epigenetic balance. A better understanding of how those mechanisms orchestrate disease might improve diagnostic and prognostic tools, allowing for improvements in treatment efficacy. Replacement of canonical histones, an epigenetic mechanism, is highly conserved among species and altered expression of histones variants (e.g., MacroH2A1) has been associated with tumorigenesis. H2AFY gene encodes two isoforms of H2A histone variant MacroH2A1: MacroH2A1.1 and MacroH2A1.2. Specifically, MacroH2A1.1 isoform inhibits cell proliferation and promotes cellular differentiation. Because the contribution of this histone variant to carcinogenesis has been reported in several cancer types, but not for PCa, we aimed to investigate the contribution of MacroH2A1 for prostate carcinogenesis.

Methods

MacroH2A1, MacroH2A1.1 and MacroH2A1.2 isoforms and the corresponding splicing regulators transcript levels were evaluated by RT-qPCR, in a tissue cohort composed by PCa, prostatic intraepithelial neoplasia (PIN) and normal prostate cases. Knockdown for MacroH2A1 and MacroH2A1.1 was performed through lentiviral transduction in DU145 cells, and MacroH2A1.1 overexpression was achieved in LNCaP cells by plasmid transfection, followed by functional assays. Biological and/or experimental replicates were performed when necessary, and specific statistical tests were applied to perform data analysis.

Results

MacroH2A1.1 transcript levels were downregulated in PIN and primary PCa compared to normal prostate tissues. The same was found for QKI, a MacroH2A1.1’s splicing regulator. Moreover, lower MacroH2A1.1 and QKI expression levels associated with less differentiated tumors (Gleason score ≥ 7). Interestingly, MacroH2A1.1, but more impressively DDX17 (AUC = 0.93; p < 0.0001) and QKI (AUC = 0.94; p < 0.0001), accurately discriminated cancerous from noncancerous prostate tissues. Furthermore, in PCa cell lines, total MacroH2A1 knockdown augmented malignant features, whereas MacroH2A1.1 overexpression impressively attenuated the malignant phenotype.

Conclusions

Overall, our data, derived from primary PCa tissues and cell lines, anticipate a tumor suppressive role for MacroH2A1, particularly for the MacroH2A1.1 isoform, in prostate carcinogenesis.

Electronic supplementary material

The online version of this article (10.1186/s12935-019-0835-9) contains supplementary material, which is available to authorized users.

Point centromere activity requires an optimal level of centromeric noncoding RNA

Budding yeast harbors a simple point centromere, which is originally believed to be sequence dependent without much epigenetic regulation and is transcription incompatible, as inserting a strong promoter upstream inactivates the centromere completely. Here, we demonstrate that an optimal level centromeric noncoding RNA is required for budding yeast centromere activity. Centromeric transcription is induced in S phase, coinciding with the assembly of new centromeric proteins. Too much or too little centromeric noncoding RNA leads to centromere malfunction. Overexpression of centromeric noncoding RNA reduces the protein levels and chromatin localization of inner centromere and kinetochore proteins, such as CENP-A, CENP-C, and the chromosome passenger complex. This work shows that point centromere is epigenetically regulated by noncoding RNA.

In budding yeast, which possesses simple point centromeres, we discovered that all of its centromeres express long noncoding RNAs (cenRNAs), especially in S phase. Induction of cenRNAs coincides with CENP-A^Cse4 loading time and is dependent on DNA replication. Centromeric transcription is repressed by centromere-binding factor Cbf1 and histone H2A variant H2A.Z^Htz1. Deletion of CBF1 and H2A.Z^HTZ1 results in an up-regulation of cenRNAs; an increased loss of a minichromosome; elevated aneuploidy; a down-regulation of the protein levels of centromeric proteins CENP-A^Cse4, CENP-A chaperone HJURP^Scm3, CENP-C^Mif2, Survivin^Bir1, and INCENP^Sli15; and a reduced chromatin localization of CENP-A^Cse4, CENP-C^Mif2, and Aurora B^Ipl1. When the RNA interference system was introduced to knock down all cenRNAs from the endogenous chromosomes, but not the cenRNA from the circular minichromosome, an increase in minichromosome loss was still observed, suggesting that cenRNA functions in trans to regulate centromere activity. CenRNA knockdown partially alleviates minichromosome loss in cbf1Δ, htz1Δ, and cbf1Δ htz1Δ in a dose-dependent manner, demonstrating that cenRNA level is tightly regulated to epigenetically control point centromere function.

Pioneer Factors and Architectural Proteins Mediating Embryonic Expression Signatures in Cancer

Accumulation of mutations causing aberrant changes in the genome promotes cancer. However, mutations do not occur in every cancer subtype, suggesting additional events that trigger cancer. Chromatin rearrangements initiated by pioneer factors and architectural proteins are key events occurring before cancer-related genes are expressed. Both protein groups are also master regulators of important processes during embryogenesis. Several publications demonstrated that embryonic gene expression signatures are reactivated during cancer. This review article highlights current knowledge on pioneer factors and architectural proteins mediating chromatin rearrangements, which are the backbone of embryonic expression signatures promoting malignant transformation. Understanding chromatin rearrangements inducing embryonic expression signatures in adult cells might be the key to novel therapeutic approaches against cancers subtypes that arise without genomic mutations.

Epigenetic changes during aging and their reprogramming potential

The aging process results in significant epigenetic changes at all levels of chromatin and DNA organization. These include reduced global heterochromatin, nucleosome remodeling and loss, changes in histone marks, global DNA hypomethylation with CpG island hypermethylation, and the relocalization of chromatin modifying factors. Exactly how and why these changes occur is not fully understood, but evidence that these epigenetic changes affect longevity and may cause aging, is growing. Excitingly, new studies show that age-related epigenetic changes can be reversed with interventions such as cyclic expression of the Yamanaka reprogramming factors. This review presents a summary of epigenetic changes that occur in aging, highlights studies indicating that epigenetic changes may contribute to the aging process and outlines the current state of research into interventions to reprogram age-related epigenetic changes.

The senescent cell epigenome

A critical hallmark of aging is cellular senescence, a state of growth arrest and inflammatory cytokine release in cells, caused by a variety of stresses. Recent work has convincingly linked the accumulation of senescent cells in aged tissues to a decline in health and a limit of lifespan, primarily through "inflammaging". Importantly, interventions that clear senescent cells have achieved marked improvements in healthspan and lifespan in mice. A growing list of studies show that environmental stimuli can affect aging and longevity through conserved pathways which, in turn, modulate chromatin states. This review consolidates key findings of chromatin state changes in senescence including histone modifications, histone variants, DNA methylation and changes in three-dimensional genome organization. This information will facilitate the identification of mechanisms and discovery of potential epigenetic targets for therapeutic interventions in aging and age-related disease.

Endothelial cell regulation through epigenetic mechanisms: Depicting parallels and its clinical application within an intra-islet microenvironment

The intra-islet endothelial cells (ECs), the building blocks of islet microvasculature, govern a number of cellular and pathophysiological processes associated with the pancreatic tissue. These cells are key to the angiogenic process and essential for islet revascularization after transplantation. Understanding fundamental mechanisms by which ECs regulate the angiogenic process is important as these cells maintain and regulate the intra-islet environment facilitated by a complex signaling crosstalk with the surrounding endocrine cells. In recent years, many studies have demonstrated the impact of epigenetic regulation on islet cell development and function. This review will present an overview of the reports involving endothelial epigenetic mechanisms particularly focusing on histone modifications which have been identified to play a critical role in governing EC functions by modifying the chromatin structure. A better understanding of epigenetic mechanisms by which these cells regulate gene expression and function to orchestrate cellular physiology and pathology is likely to offer improved insights on the functioning and regulation of an intra-islet endothelial microvascular environment.

Studies of Impending Oligonucleotide Therapeutics in Simulated Biofluids

Synthetic oligonucleotides, their complexes and conjugates with other biomolecules represent valuable research tools and therapeutic agents. In spite of growing applications in basic research and clinical science, only few studies have addressed the issue of such compounds' stability in biological media. Herein, we studied the stability of two therapeutically relevant oligonucleotide probes in simulated biofluids; the 21 nucleotide-long DNA/locked nucleic acid oligonucleotide ON targeted toward cancer-associated BRAF V600E mutation, and a longer DNA analog (TTC) originating from BRAF gene. We found that stability of peptide-oligonucleotide conjugates (POCs) in human serum (HS) was superior compared with the naked or complexed 21mer oligonucleotide, whereas stability of POCs in simulated gastric juice (GJ) was dependent on the peptide sequence. Addition of pepstatin A in general increased the stability of oligonucleotides after 24 h digestion in HS and simulated GJ. Similarly, complexation with optimal amounts of histone proteins was found to rescue oligonucleotide stability after 24 h digestion in hydrochloric acid.

Functional diversification of structurally alike NLR proteins in plants

In due course of evolution many pathogens alter their effector molecules to modulate the host plants' metabolism and immune responses triggered upon proper recognition by the intracellular nucleotide-binding oligomerization domain containing leucine-rich repeat (NLR) proteins. Likewise, host plants have also evolved with diversified NLR proteins as a survival strategy to win the battle against pathogen invasion. NLR protein indeed detects pathogen derived effector proteins leading to the activation of defense responses associated with programmed cell death (PCD). In this interactive process, genome structure and plasticity play pivotal role in the development of innate immunity. Despite being quite conserved with similar biological functions in all eukaryotes, the intracellular NLR immune receptor proteins happen to be structurally distinct. Recent studies have made progress in identifying transcriptional regulatory complexes activated by NLR proteins. In this review, we attempt to decipher the intracellular NLR proteins mediated surveillance across the evolutionarily diverse taxa, highlighting some of the recent updates on NLR protein compartmentalization, molecular interactions before and after activation along with insights into the finer role of these receptor proteins to combat invading pathogens upon their recognition. Latest information on NLR sensors, helpers and NLR proteins with integrated domains in the context of plant pathogen interactions are also discussed.

The plant-specific histone residue Phe41 is important for genome-wide H3.1 distribution

The dynamic incorporation of histone variants influences chromatin structure and many biological processes. In Arabidopsis, the canonical variant H3.1 differs from H3.3 in four residues, one of which (H3.1Phe41) is unique and conserved in plants. However, its evolutionary significance remains unclear. Here, we show that Phe41 first appeared in H3.1 in ferns and became stable during land plant evolution. Unlike H3.1, which is specifically enriched in silent regions, H3.1F41Y variants gain ectopic accumulation at actively transcribed regions. Reciprocal tail and core domain swap experiments between H3.1 and H3.3 show that the H3.1 core, while necessary, is insufficient to restrict H3.1 to silent regions. We conclude that the vascular-plant-specific Phe41 is critical for H3.1 genomic distribution and may act collaboratively with the H3.1 core to regulate deposition patterns. This study reveals that Phe41 may have evolved to provide additional regulation of histone deposition in plants.

The canonical histone variant H3.1 of vascular plants contains a conserved Phe residue at position 41 that is unique to the plant kingdom. Here, Lu et al. provide evidence that H3.1Phe41 acts collaboratively with the H3.1 core domain to restrict H3.1 deposition to silent regions of the genome.

High MYC Levels Favour Multifocal Carcinogenesis

The term "field cancerisation" describes the formation of tissue sub-areas highly susceptible to multifocal tumourigenesis. In the earlier stages of cancer, cells may indeed display a series of molecular alterations that allow them to proliferate faster, eventually occupying discrete tissue regions with irrelevant morphological anomalies. This behaviour recalls cell competition, a process based on a reciprocal fitness comparison: when cells with a growth advantage arise in a tissue, they are able to commit wild-type neighbours to death and to proliferate at their expense. It is known that cells expressing high MYC levels behave as super-competitors, able to kill and replace less performant adjacent cells; given MYC upregulation in most human cancers, MYC-mediated cell competition is likely to pioneer field cancerisation. Here we show that MYC overexpression in a sub-territory of the larval wing epithelium of Drosophila is sufficient to trigger a number of cellular responses specific to mammalian pre-malignant tissues. Moreover, following induction of different second mutations, high MYC-expressing epithelia were found to be susceptible to multifocal growth, a hallmark of mammalian pre-cancerous fields. In summary, our study identified an early molecular alteration implicated in field cancerisation and established a genetically amenable model which may help study the molecular basis of early carcinogenesis.

Sperm Nuclear Basic Proteins of Marine Invertebrates

In this chapter, a short evolutionary history and comparative analysis of sperm nuclear basic proteins (SNBPs) in marine invertebrates are presented based on some of the most recent publications in the field and building upon previously published reviews on the topic. Putative functions of SNBPs in sperm chromatin beyond DNA packaging will also be discussed with a primary focus on outstanding research questions.In somatic cells of all metazoans, DNA is packaged into tightly folded and dynamically accessible chromatin by canonical histones H2A, H2B, H3 and H4. Sperm chromatin of many animals, on the other hand, is organised by small yet structurally highly heterogeneous proteins called SNBPs, which can package sperm DNA on their own or in combination with each other. In extreme cases, sperm chromatin is condensed into a volume 6-10 times smaller than that of a somatic nucleus. SNBPs are classified into three major groups: H1 histone-type proteins (H-type SNBPs), protamines (P-type SNBPs) and protamine-like proteins (PL-type SNBPs). P-type SNBPs are mostly found in vertebrates, while PL-type SNBPs are ubiquitous in many invertebrate phyla. PL-type and P-type SNBPs evolved from histone H-type SNBP precursors through vertical evolution. Porifera, Ctenophora and Crustacea, Echinoidea (phylum Echinodermata) and Hydrozoa (phylum Hydrozoa) lack SNBPs. Echinoidea and Hydrozoa, however, evolved novel nucleosomal histone variants with specific roles during spermatogenesis. Seemingly, chromatin condensation plays a critical role in the silencing and tight packing of the genome within the sperm nucleus of most animals. However, the question of what necessitates the compaction of some sperm DNA beyond classical nucleosomal packaging while other sperm function using 'normal' histones remains unanswered to date.

Histone variants in environmental-stress-induced DNA damage repair

Environmental stress such as genotoxic agents can cause DNA damage either indirectly through the generation of reactive oxygen species or directly by interactions with the DNA molecule. Damage to the genetic material may cause mutations and ultimately cancer. Genotoxic mutation can be prevented either by apoptosis or DNA repair. In response to DNA damage, cells have evolved DNA damage responses (DDR) to detect, signal, and repair DNA lesions. Epigenetic mechanisms play critically important roles in DDR, which requires changes in chromatin structure and dynamics to modulate DNA accessibility. Incorporation of histone variants into chromatin is considered as an epigenetic mechanism. Canonical histones can be replaced with variant histones that change chromatin structure, stability, and dynamics. Recent studies have demonstrated involvement of nearly all histone variants in environmental-stress-induced DNA damage repair through various mechanisms, including affecting nucleosome dynamics, carrying variant-specific modification, promoting transcriptional competence or silencing, mediating rearrangement of chromosomes, attracting specific repair proteins, among others. In this review, we will focus on the role of histone variants in DNA damage repair after exposure to environmental genotoxic agents. Understanding the mechanisms regulating environmental exposure-induced epigenetic changes, including replacement of canonical histones with histone variants, will promote the development of strategies to prevent or reverse these changes.

Acetylated histone variant H2A.Z is involved in the activation of neo-enhancers in prostate cancer

Acetylation of the histone variant H2A.Z (H2A.Zac) occurs at active promoters and is associated with oncogene activation in prostate cancer, but its role in enhancer function is still poorly understood. Here we show that H2A.Zac containing nucleosomes are commonly redistributed to neo-enhancers in cancer resulting in a concomitant gain of chromatin accessibility and ectopic gene expression. Notably incorporation of acetylated H2A.Z nucleosomes is a pre-requisite for activation of Androgen receptor (AR) associated enhancers. H2A.Zac nucleosome occupancy is rapidly remodeled to flank the AR sites to initiate the formation of nucleosome-free regions and the production of AR-enhancer RNAs upon androgen treatment. Remarkably higher levels of global H2A.Zac correlate with poorer prognosis. Altogether these data demonstrate the novel contribution of H2A.Zac in activation of newly formed enhancers in prostate cancer.

Acetylation of the histone variant H2A.Z at gene promoters is associated with oncogene activation; however, it is unclear if such modification has a role in regulating the function of enhancers. Here the authors show that acetylated H2A.Z is redistributed at cancer neo-enhancers and regulates the activity of specific enhancers of cancer-related genes.

Molecular structures guide the engineering of chromatin

Chromatin is a system of proteins, RNA, and DNA that interact with each other to organize and regulate genetic information within eukaryotic nuclei. Chromatin proteins carry out essential functions: packing DNA during cell division, partitioning DNA into sub-regions within the nucleus, and controlling levels of gene expression. There is a growing interest in manipulating chromatin dynamics for applications in medicine and agriculture. Progress in this area requires the identification of design rules for the chromatin system. Here, we focus on the relationship between the physical structure and function of chromatin proteins. We discuss key research that has elucidated the intrinsic properties of chromatin proteins and how this information informs design rules for synthetic systems. Recent work demonstrates that chromatin-derived peptide motifs are portable and in some cases can be customized to alter their function. Finally, we present a workflow for fusion protein design and discuss best practices for engineering chromatin to assist scientists in advancing the field of synthetic epigenetics.