Localized histone acetylation and deacetylation triggered by the homologous recombination pathway of double-strand DNA repair

HDAC Inhibitors Can Enhance Radiosensitivity of Head and Neck Cancer Cells Through Suppressing DNA Repair

Background/Objectives: The incidence of head and neck squamous cell carcinoma (HNSCC), currently ~800,000 cases per year worldwide, is rising. Radiotherapy remains a mainstay for the treatment of HNSCC, although inherent radioresistance, particularly in human papillomavirus (HPV)-negative disease subtypes, remains a significant barrier to effective treatment. Therefore, combinatorial strategies using drugs or inhibitors against specific cellular targets are necessary to enhance HNSCC radiosensitivity to lead to an improvement in patient outcomes. Given that radiotherapy acts through targeting and damaging DNA, a common strategy is to focus on enzymes within DNA-dependent cellular pathways, such as DNA damage repair. Methods: Here, we have employed a 3D spheroid model of HNSCC (FaDu) in combination with a targeted drug screen to identify novel radiosensitisers that suppress tumour growth. Results: We identified that histone deacetylases (HDACs) were prominent candidates, and subsequently identified that the HDAC inhibitors mocetinostat and pracinostat, as well as the combined HDAC-epidermal growth factor receptor inhibitor CUDC-101, were effective at radiosensitising cell models of HNSCC (FaDu, A253, UMSCC11b) through their impact on both spheroid growth and clonogenic survival assays. We also demonstrated that this combinatorial strategy leads to inhibition of the repair of DNA double-strand breaks through the neutral comet assay and γH2AX foci analysis using immunofluorescence microscopy, providing a mechanism of action through which HDAC inhibition functions in HNSCC radiosensitisation. Conclusions: We believe that this approach should be further investigated in preclinical models, in order to realise the full therapeutic potential of HDAC inhibition for the radiosensitisation of HNSCC, eventually leading to improved patient treatment efficacy and outcomes.

DNA double-strand break movement in heterochromatin depends on the histone acetyltransferase dGcn5

Cells employ diverse strategies to repair double-strand breaks (DSBs), a dangerous form of DNA damage that threatens genome integrity. Eukaryotic nuclei consist of different chromatin environments, each displaying distinct molecular and biophysical properties that can significantly influence the DSB-repair process. DSBs arising in the compact and silenced heterochromatin domains have been found to move to the heterochromatin periphery in mouse and Drosophila to prevent aberrant recombination events. However, it is poorly understood how chromatin components, such as histone post-translational modifications, contribute to these DSB movements within heterochromatin. Using irradiation as well as locus-specific DSB induction in Drosophila tissues and cultured cells, we find enrichment of histone H3 lysine 9 acetylation (H3K9ac) at DSBs in heterochromatin but not euchromatin. We find this increase is mediated by the histone acetyltransferase dGcn5, which rapidly localizes to heterochromatic DSBs. Moreover, we demonstrate that in the absence of dGcn5, heterochromatic DSBs display impaired recruitment of the SUMO E3 ligase Nse2/Qjt and fail to relocate to the heterochromatin periphery to complete repair. In summary, our results reveal a previously unidentified role for dGcn5 and H3K9ac in heterochromatic DSB repair and underscore the importance of differential chromatin responses at heterochromatic and euchromatic DSBs to promote safe repair.

Epigenetic dynamics of aging and cancer development: current concepts from studies mapping aging and cancer epigenomes

PURPOSE OF REVIEW

This review emphasizes the role of epigenetic processes as incidental changes occurring during aging, which, in turn, promote the development of cancer.

RECENT FINDINGS

Aging is a complex biological process associated with the progressive deterioration of normal physiological functions, making age a significant risk factor for various disorders, including cancer. The increasing longevity of the population has made cancer a global burden, as the risk of developing most cancers increases with age due to the cumulative effect of exposure to environmental carcinogens and DNA replication errors. The classical 'somatic mutation theory' of cancer cause is being challenged by the observation that multiple normal cells harbor cancer driver mutations without resulting in cancer. In this review, we discuss the role of age-associated epigenetic alterations, including DNA methylation, which occur across all cell types and tissues with advancing age. There is an increasing body of evidence linking these changes with cancer risk and prognosis.

SUMMARY

A better understanding about the epigenetic changes acquired during aging is critical for comprehending the mechanisms leading to the age-associated increase in cancer and for developing novel therapeutic strategies for cancer treatment and prevention.

Targeting BRPF3 moderately reverses olaparib resistance in high grade serous ovarian carcinoma

PARP inhibitors (PARPi) kill cancer cells by stalling DNA replication and preventing DNA repair, resulting in a critical accumulation of DNA damage. Resistance to PARPi is a growing clinical problem in the treatment of high grade serous ovarian carcinoma (HGSOC). Acetylation of histone H3 lysine 14 (H3K14ac) and associated histone acetyltransferases (HATs) and epigenetic readers have known functions in DNA repair and replication. Our objectives are to examine their expression and activities in the context of PARPi-resistant HGSOC, and to determine if targeting H3K14ac or associated proteins has therapeutic potential. Using mass spectrometry profiling of histone modifications, we observed increased H3K14ac enrichment in PARPi-resistant HGSOC cells relative to isogenic PARPi-sensitive lines. By reverse-transcriptase quantitative PCR and RNA-seq, we also observed altered expression of numerous HATs in PARPi-resistant HGSOC cells and a PARPi-resistant PDX model. Knockdown of HATs only modestly altered PARPi response, although knockdown and inhibition of PCAF significantly increased resistance. Pharmacologic inhibition of HBO1 depleted H3K14ac but did not affect PARPi response. However, knockdown and inhibition of BRPF3, a bromodomain and PHD-finger containing protein that is known to interact in a complex with HBO1, did reduce PARPi resistance. This study demonstrates that depletion of H3K14ac does not affect PARPi response in HGSOC. Our data suggest that the bromodomain function of HAT proteins, such as PCAF, or accessory proteins, such as BRPF3, may play a more direct role compared to direct HATs function in PARPi response.

The increase in cell death rates in caloric restricted cells of the yeast helicase mutant rrm3 is Sir complex dependent

Calorie restriction (CR), which is a reduction in calorie intake without malnutrition, usually extends lifespan and improves tissue integrity. This report focuses on the relationship between nuclear genomic instability and dietary-restriction and its effect on cell survival. We demonstrate that the cell survival rates of the genomic instability yeast mutant rrm3 change under metabolic restricted conditions. Rrm3 is a DNA helicase, chromosomal replication slows (and potentially stalls) in its absence with increased rates at over 1400 natural pause sites including sites within ribosomal DNA and tRNA genes. Whereas rrm3 mutant cells have lower cell death rates compared to wild type (WT) in growth medium containing normal glucose levels (i.e., 2%), under CR growth conditions cell death rates increase in the rrm3 mutant to levels, which are higher than WT. The silent-information-regulatory (Sir) protein complex and mitochondrial oxidative stress are required for the increase in cell death rates in the rrm3 mutant when cells are transferred from growth medium containing 2% glucose to CR-medium. The Rad53 checkpoint protein is highly phosphorylated in the rrm3 mutant in response to genomic instability in growth medium containing 2% glucose. Under CR, Rad53 phosphorylation is largely reduced in the rrm3 mutant in a Sir-complex dependent manner. Since CR is an adjuvant treatment during chemotherapy, which may target genomic instability in cancer cells, our studies may gain further insight into how these therapy strategies can be improved.

RNA methylation, homologous recombination repair and therapeutic resistance

Homologous recombination (HR) repair of DNA double-strand breaks (DSBs) is critical for maintaining genomic integrity and stability. Defects in HR increase the risk of tumorigenesis. However, many human tumors exhibit enhanced HR repair capabilities, consequently endowing tumor cells with resistance to DNA-damaging chemotherapy and radiotherapy. This review summarizes the role of RNA methylation in HR repair and therapeutic resistance in human tumors. We also analyzed the interactions between RNA methylation and other HR-modulating modifications including histone acetylation, histone deacetylation, ubiquitination, deubiquitination, protein arginine methylation, and gene transcription. This review proposes that targeting RNA methylation is a promising approach to overcoming HR-mediated therapeutic resistance.

Ubiquitination of acetyltransferase Gcn5 contributes to fungal virulence in Fusarium graminearum

The histone acetyltransferase general control non-depressible 5 (Gcn5) plays a critical role in the epigenetic landscape and chromatin modification for regulating a wide variety of biological events. However, the post-translational regulation of Gcn5 itself is poorly understood. Here, we found that Gcn5 was ubiquitinated and deubiquitinated by E3 ligase Tom1 and deubiquitinating enzyme Ubp14, respectively, in the important plant pathogenic fungus Fusarium graminearum. Tom1 interacted with Gcn5 in the nucleus and subsequently ubiquitinated Gcn5 mainly at K252 to accelerate protein degradation. Conversely, Ubp14 deubiquitinated Gcn5 and enhanced its stability. In the deletion mutant Δubp14, protein level of Gcn5 was significantly reduced and resulted in attenuated virulence in the fungus by affecting the mycotoxin production, autophagy process, and the penetration ability. Our findings indicate that Tom1 and Ubp14 show antagonistic functions in the control of the protein stability of Gcn5 via post-translational modification and highlight the importance of Tom1-Gcn5-Ubp14 circuit in the fungal virulence. IMPORTANCE Post-translational modification (PTM) enzymes have been reported to be involved in regulating numerous cellular processes. However, the modification of these PTM enzymes themselves is largely unknown. In this study, we found that the E3 ligase Tom1 and deubiquitinating enzyme Ubp14 contributed to the regulation of ubiquitination and deubiquitination of acetyltransferase Gcn5, respectively, in Fusarium graminearum, the causal agent of Fusarium head blight of cereals. Our findings provide deep insights into the modification of acetyltransferase Gcn5 and its dynamic regulation via ubiquitination and deubiquitination. To our knowledge, this work is the most comprehensive analysis of a regulatory network of ubiquitination that impinges on acetyltransferase in filamentous pathogens. Moreover, our findings are important because we present the novel roles of the Tom1-Gcn5-Ubp14 circuit in fungal virulence, providing novel possibilities and targets to control fungal diseases.

Exogenous proline activated an integrated response of NER and HR pathways to reduce DNA damage in rice seedlings under chromium stress

The DNA damage induced by hexavalent chromium [Cr(VI)] pollutant causes a genotoxic effect on rice seedlings. Hereby, we examined the effects of exogenous proline (Pro) on the alleviation of DNA damage in rice seedlings under different effective concentrations of Cr(VI). Our results revealed that Cr(VI) stress induced reactive oxygen species (ROS), i.e., H₂O₂ and O₂^·- accumulation in rice seedlings, repressed genes expression activated in the homologous recombination (HR) and nucleotide excision repair (NER) pathways, and caused DNA damage. Exogenous application of Pro increased Cr accumulation in rice roots, but decreased Cr accumulation in rice shoots, wherein Pro application decreased ROS accumulation in both tissues of rice seedlings. The comet assays suggested that exogenous application of Pro significantly alleviated the DNA damage in rice seedlings during Cr(VI) treatments, judged by the Olive tail moment and tail DNA. Transcriptional assays revealed that exogenous Pro upregulated the expression level of genes associated with the HR and NER pathways and triggered coordinated actions of both repairing pathways to modulate DNA lesion in rice plants during exposure to Cr(VI). Calculations from gene expression variation factors showed that regulative effect of exogenous application of Pro on DNA repair pathways was highly activated at 2.0 mg Cr/L. The current study revealed that Cr(VI) affect rice plants and exogenous Pro rescue these effects by the activation of HR and NER pathways.

PBX1-SIRT1 Positive Feedback Loop Attenuates ROS-Mediated HF-MSC Senescence and Apoptosis

Stem cell senescence and depletion are major causes of aging and aging-related diseases. The NAD (Nicotinamide adenine dinucleotide) - SIRT1 (Silent Information Regulator 1) - PARP1 (Poly (ADP-ribose) polymerase-1) axis has gained interest owing to its significant role in regulating stem cell senescence and organismal aging. A recent study from our lab showed that pre-B-cell leukemia transcription factor1 (PBX1) overexpression attenuates hair follicle-derived mesenchymal stem cells (HF-MSCs) senescence and apoptosis by regulating ROS-mediated DNA damage via PARP1 downregulation; thus, suggesting that PARP1 downregulation is a common manifestation of the roles of both PBX1 and SIRT1 in HF-MSCs senescence attenuation, and implying a potential link between PBX1 and SIRT1. To this end, HF-MSCs overexpressing PBX1, overexpressing both PBX1 and PARP1, downregulating SIRT1, and overexpressing PBX1 as well as downregulating SIRT1 were generated, and senescence, apoptosis, DNA damage, and repair biomarkers were analyzed. Our results showed that (1) PBX1 overexpression alleviated HF-MSCs senescence and apoptosis accompanied by SIRT1 upregulation, PARP1 downregulation, and increased intracellular NAD and ATP levels. (2) SIRT1 knockdown enhanced cellular senescence and apoptosis, accompanied by increased ROS accumulation, DNA damage aggravation, and decreased intracellular NAD and ATP levels. (3) PBX1 overexpression rescued HF-MSCs senescence and apoptosis induced by SIRT1 knockdown. (4) PBX1 rescued PARP1 overexpression-mediated ATP and NAD depletion, accompanied by increased SIRT1 expression. Collectively, our results revealed that a positive interaction feedback loop exists between PBX1 and SIRT1. To the best of our knowledge we are the first to report that there is a PBX1-SIRT1-PARP1 axis that plays a critical role in alleviating HF-MSCs senescence and apoptosis. We provide a new perspective on the mechanisms underlying stem cell senescence as well as age-related disease prevention and treatment.

Loss of epigenetic information as a cause of mammalian aging

All living things experience an increase in entropy, manifested as a loss of genetic and epigenetic information. In yeast, epigenetic information is lost over time due to the relocalization of chromatin-modifying proteins to DNA breaks, causing cells to lose their identity, a hallmark of yeast aging. Using a system called "ICE" (inducible changes to the epigenome), we find that the act of faithful DNA repair advances aging at physiological, cognitive, and molecular levels, including erosion of the epigenetic landscape, cellular exdifferentiation, senescence, and advancement of the DNA methylation clock, which can be reversed by OSK-mediated rejuvenation. These data are consistent with the information theory of aging, which states that a loss of epigenetic information is a reversible cause of aging.

Histone acetylation dynamics in repair of DNA double-strand breaks

Packaging of eukaryotic genome into chromatin is a major obstacle to cells encountering DNA damage caused by external or internal agents. For maintaining genomic integrity, the double-strand breaks (DSB) must be efficiently repaired, as these are the most deleterious type of DNA damage. The DNA breaks have to be detected in chromatin context, the DNA damage response (DDR) pathways have to be activated to repair breaks either by non‐ homologous end joining and homologous recombination repair. It is becoming clearer now that chromatin is not a mere hindrance to DDR, it plays active role in sensing, detection and repair of DNA damage. The repair of DSB is governed by the reorganization of the pre-existing chromatin, leading to recruitment of specific machineries, chromatin remodelling complexes, histone modifiers to bring about dynamic alterations in histone composition, nucleosome positioning, histone modifications. In response to DNA break, modulation of chromatin occurs via various mechanisms including post-translational modification of histones. DNA breaks induce many types of histone modifications, such as phosphorylation, acetylation, methylation and ubiquitylation on specific histone residues which are signal and context dependent. DNA break induced histone modifications have been reported to function in sensing the breaks, activating processing of breaks by specific pathways, and repairing damaged DNA to ensure integrity of the genome. Favourable environment for DSB repair is created by generating open and relaxed chromatin structure. Histone acetylation mediate de-condensation of chromatin and recruitment of DSB repair proteins to their site of action at the DSB to facilitate repair. In this review, we will discuss the current understanding on the critical role of histone acetylation in inducing changes both in chromatin organization and promoting recruitment of DSB repair proteins to sites of DNA damage. It consists of an overview of function and regulation of the deacetylase enzymes which remove these marks and the function of histone acetylation and regulators of acetylation in genome surveillance.

New insights into the DNA repair pathway choice with NuA4/TIP60

In eukaryotic cells, DNA double-strand breaks (DSBs) can be repaired through two main pathways, non-homologous end-joining (NHEJ) or homologous recombination (HR). The selection of the repair pathway choice is governed by an antagonistic relationship between repair factors specific to each pathway, in a cell cycle-dependent manner. The molecular mechanisms of this decision implicate post-translational modifications of chromatin surrounding the break. Here, we discuss the recent advances regarding the function of the NuA4/TIP60 histone acetyltransferase/chromatin remodeling complex during DSBs repair. In particular, we emphasise the contribution of NuA4/TIP60 in repair pathway choice, in collaboration with the SAGA acetyltransferase complex, and how they regulate chromatin dynamics, modify non-histone substrates to allow DNA end resection and recombination.

Histone Deacetylases and Their Potential as Targets to Enhance Tumour Radiosensitisation

In mammalian cells, genomic DNA is packaged with histone proteins and condensed into chromatin. To gain access to the DNA, chromatin remodelling is required that is enhanced through histone post-translational modifications, which subsequently stimulate processes including DNA repair and transcription. Histone acetylation is one of the most well understood modifications and is controlled by histone acetyltransferases (HATs) and histone deacetylases (HDACs). These enzymes play critical roles in normal cellular functioning, and the dysregulation of HDAC expression in particular has been linked with the development of a number of different cancer types. Conversely, tumour cell killing following radiotherapy is triggered through DNA damage and HDACs can help co-ordinate the cellular DNA damage response which promotes radioresistance. Consequently, HDAC inhibitors have been investigated as potential radiosensitizers in vitro and in vivo to improve the efficacy or radiotherapy in specific tumour types. In this review, we provide an up-to-date summary of HDACs and their cellular functions, including in DNA damage repair. We also review evidence demonstrating that HDAC inhibitors can effectively enhance tumour radiosensitisation, and which therefore show potential for translation into the clinic for cancer patient benefit.

The Role of Oxidative Stress in Epigenetic Changes Underlying Autoimmunity

Significance: Epigenetic dysregulation plays an important role in the pathogenesis and development of autoimmune diseases. Oxidative stress is associated with autoimmunity and is also known to alter epigenetic mechanisms. Understanding the interplay between oxidative stress and epigenetics will provide insights into the role of environmental triggers in the development of autoimmunity in genetically susceptible individuals. Recent Advances: Abnormal DNA and histone methylation patterns in genes and pathways involved in interferon and tumor necrosis factor signaling, cellular survival, proliferation, metabolism, organ development, and autoantibody production have been described in autoimmunity. Inhibitors of DNA and histone methyltransferases showed potential therapeutic effects in animal models of autoimmune diseases. Oxidative stress can regulate epigenetic mechanisms via effects on DNA damage repair mechanisms, cellular metabolism and the local redox environment, and redox-sensitive transcription factors and pathways. Critical Issues: Studies looking into oxidative stress and epigenetics in autoimmunity are relatively limited. The number of available longitudinal studies to explore the role of DNA methylation in the development of autoimmune diseases is small. Future Directions: Exploring the relationship between oxidative stress and epigenetics in autoimmunity will provide clues for potential preventative measures and treatment strategies. Inception cohorts with longitudinal follow-up would help to evaluate epigenetic marks as potential biomarkers for disease development, progression, and treatment response in autoimmunity. Antioxid. Redox Signal. 36, 423-440.

XRCC4 and MRE11 Roles and Transcriptional Response to Repair of TALEN-Induced Double-Strand DNA Breaks

Double-strand breaks (DSB) are one of the most lethal forms of DNA damage that, if left unrepaired, can lead to genomic instability, cellular transformation, and cell death. In this work, we examined how repair of transcription activator-like effector nuclease (TALEN)-induced DNA damage was altered when knocking out, or inhibiting a function of, two DNA repair proteins, XRCC4 and MRE11, respectively. We developed a fluorescent reporter assay that uses TALENs to introduce DSB and detected repair by the presence of GFP fluorescence. We observed repair of TALEN-induced breaks in the XRCC4 knockout cells treated with mirin (a pharmacological inhibitor of MRE11 exonuclease activity), albeit with ~40% reduced efficiency compared to normal cells. Editing in the absence of XRCC4 or MRE11 exonuclease was robust, with little difference between the indel profiles amongst any of the groups. Reviewing the transcriptional profiles of the mirin-treated XRCC4 knockout cells showed 307 uniquely differentially expressed genes, a number far greater than for either of the other cell lines (the HeLa XRCC4 knockout sample had 83 genes, and the mirin-treated HeLa cells had 30 genes uniquely differentially expressed). Pathways unique to the XRCC4 knockout+mirin group included differential expression of p53 downstream pathways, and metabolic pathways indicating cell adaptation for energy regulation and stress response. In conclusion, our study showed that TALEN-induced DSBs are repaired, even when a key DSB repair protein or protein function is not operational, without a change in indel profiles. However, transcriptional profiles indicate the induction of unique cellular responses dependent upon the DNA repair protein(s) hampered.

Role of the Histone Acetyl Transferase MOF and the Histone Deacetylase Sirtuins in Regulation of H4K16ac During DNA Damage Repair and Metabolic Programming: Implications in Cancer and Aging

The accurate repair of genomic damage mediated by ionizing radiation (IR), chemo- or radiomimetic drugs, or other exogenous agents, is necessary for maintenance of genome integrity, preservation of cellular viability and prevention of oncogenic transformation. Eukaryotes have conserved mechanisms designed to perceive and repair the damaged DNA quite efficiently. Among the different types of DNA damage, double strand breaks (DSB) are the most detrimental. The cellular DNA DSB response is a hierarchical signaling network that integrates damage sensing and repair with chromatin structural changes that involve a range of pre-existing and induced covalent modifications. Recent studies have revealed that pre-existing histone modifications are important contributors within this signaling/repair network. This chapter discusses the role of a critical histone acetyl transferase (HAT) known as MOF (males absent on the first) and the histone deacetylases (HDACs) Sirtuins on histone H4K16 acetylation (H4K16ac) and DNA damage repair. We also discuss the role of this important histone modification in light of metabolic rewiring and its role in regulating human pathophysiologic states.

Heterogeneity of DNA damage incidence and repair in different chromatin contexts

It has been long known that some regions of the genome are more susceptible to damage and mutagenicity than others. Recent advances have determined a critical role of chromatin both in the incidence of damage and in its repair. Thus, chromatin arises as a guardian of the stability of the genome, which is altered in cancer cells. In this review, we focus into the mechanisms by which chromatin influences the occurrence and repair of the most cytotoxic DNA lesions, double-strand breaks, in particular at actively transcribed chromatin or related to DNA replication.

Mitochondrial cytochrome c shot towards histone chaperone condensates in the nucleus

Despite mitochondria being key for the control of cell homeostasis and fate, their role in DNA damage response is usually just regarded as an apoptotic trigger. However, growing evidence points to mitochondrial factors modulating nuclear functions. Remarkably, after DNA damage, cytochrome c (Cc) interacts in the cell nucleus with a variety of well-known histone chaperones, whose activity is competitively inhibited by the haem protein. As nuclear Cc inhibits the nucleosome assembly/disassembly activity of histone chaperones, it might indeed affect chromatin dynamics and histone deposition on DNA. Several histone chaperones actually interact with Cc Lys residues through their acidic regions, which are also involved in heterotypic interactions leading to liquid-liquid phase transitions responsible for the assembly of nuclear condensates, including heterochromatin. This relies on dynamic histone-DNA interactions that can be modulated by acetylation of specific histone Lys residues. Thus, Cc may have a major regulatory role in DNA repair by fine-tuning nucleosome assembly activity and likely nuclear condensate formation.

Sp1-dependent recruitment of the histone acetylase p300 to DSBs facilitates chromatin remodeling and recruitment of the NHEJ repair factor Ku70

In response to DNA damage, most factors involved in damage recognition and repair are tightly regulated to ensure proper repair pathway choice. Histone acetylation at DNA double strand breaks (DSBs) by p300 histone acetyltransferase (HAT) is critical for the recruitment of DSB repair proteins to chromatin. Here, we show that phosphorylation of Sp1 by ATM increases its interaction with p300 and that Sp1-dependent recruitment of p300 to DSBs is necessary to modify the histones associated with p300 activity and NHEJ repair factor recruitment and repair. p300 is known to acetylate multiple residues on histones H3 and H4 necessary for NHEJ. Acetylation of H3K18 by p300 is associated with the recruitment of the SWI/SNF chromatin remodeling complex and Ku70 to DSBs for NHEJ repair. Depletion of Sp1 results in decreased acetylation of lysines on histones H3 and H4. Specifically, cells depleted of Sp1 display defects in the acetylation of H3K18, resulting in defective SWI/SNF and Ku70 recruitment to DSBs. These results shed light on mechanisms by which chromatin remodelers are regulated to ensure activation of the appropriate DSB repair pathway.

Transcription-coupled DNA double-strand break repair

The genomic DNA is constantly under attack by cellular and/or environmental factors. Fortunately, the cell is armed to safeguard its genome by various mechanisms such as nucleotide excision, base excision, mismatch and DNA double-strand break repairs. While these processes maintain the integrity of the genome throughout, DNA repair occurs preferentially faster at the transcriptionally active genes. Such transcription-coupled repair phenomenon plays important roles to maintain active genome integrity, failure of which would interfere with transcription, leading to an altered gene expression (and hence cellular pathologies/diseases). Among the various DNA damages, DNA double-strand breaks are quite toxic to the cells. If DNA double-strand break occurs at the active gene, it would interfere with transcription/gene expression, thus threatening cellular viability. Such DNA double-strand breaks are found to be repaired faster at the active gene in comparison to its inactive state or the inactive gene, thus supporting the existence of a new phenomenon of transcription-coupled DNA double-strand break repair. Here, we describe the advances of this repair process.

Single-cell damagenome profiling unveils vulnerable genes and functional pathways in human genome toward DNA damage

We report a novel single-cell whole-genome amplification method (LCS-WGA) that can efficiently capture spontaneous DNA damage existing in single cells. We refer to these damage-associated single-nucleotide variants as "damSNVs," and the whole-genome distribution of damSNVs as the damagenome. We observed that in single human neurons, the damagenome distribution was significantly correlated with three-dimensional genome structures. This nonuniform distribution indicates different degrees of DNA damage effects on different genes. Next, we identified the functionals that were significantly enriched in the high-damage genes. Similar functionals were also enriched in the differentially expressed genes (DEGs) detected by single-cell transcriptome of both Alzheimer's disease (AD) and autism spectrum disorder (ASD). This result can be explained by the significant enrichment of high-damage genes in the DEGs of neurons for both AD and ASD. The discovery of high-damage genes sheds new lights on the important roles of DNA damage in human diseases and disorders.

Inactivating Mutations of the IK Gene Weaken Ku80/Ku70-Mediated DNA Repair and Sensitize Endometrial Cancer to Chemotherapy

IK is a mitotic factor that promotes cell cycle progression. Our previous investigation of 271 endometrial cancer (EC) samples from the Cancer Genome Atlas (TCGA) dataset showed IK somatic mutations were enriched in a cluster of patients with high-grade and high-stage cancers, and this group had longer survival. This study provides insight into how IK somatic mutations contribute to EC pathophysiology. We analyzed the somatic mutational landscape of IK gene in 547 EC patients using expanded TCGA dataset. Co-immunoprecipitation and mass spectrometry were used to identify protein interactions. In vitro and in vivo experiments were used to evaluate IK's role in EC. The patients with IK-inactivating mutations had longer survival during 10-year follow-up. Frameshift and stop-gain were common mutations and were associated with decreased IK expression. IK knockdown led to enrichment of G2/M phase cells, inactivation of DNA repair signaling mediated by heterodimerization of Ku80 and Ku70, and sensitization of EC cells to cisplatin treatment. IK/Ku80 mutations were accompanied by higher mutation rates and associated with significantly better overall survival. Inactivating mutations of IK gene and loss of IK protein expression were associated with weakened Ku80/Ku70-mediated DNA repair, increased mutation burden, and better response to chemotherapy in patients with EC.

ING Proteins: Tumour Suppressors or Oncoproteins

The INhibitor of Growth family was defined in the mid-1990s by the identification of a tumour suppressor, ING1, and subsequent expansion of the family based essentially on sequence similarities. However, later work and more recent investigations demonstrate that at least a few ING proteins are actually required for normal proliferation of eukaryotic cells, from yeast to human. ING proteins are also part of a larger family of chromatin-associated factors marked by a plant homeodomain (PHD), which mediates interactions with methylated lysine residues. Herein, we discuss the role of ING proteins and their various roles in chromatin signalling in the context of cancer development and progression.

Cell Metabolism and DNA Repair Pathways: Implications for Cancer Therapy

DNA repair and metabolic pathways are vital to maintain cellular homeostasis in normal human cells. Both of these pathways, however, undergo extensive changes during tumorigenesis, including modifications that promote rapid growth, genetic heterogeneity, and survival. While these two areas of research have remained relatively distinct, there is growing evidence that the pathways are interdependent and intrinsically linked. Therapeutic interventions that target metabolism or DNA repair systems have entered clinical practice in recent years, highlighting the potential of targeting these pathways in cancer. Further exploration of the links between metabolic and DNA repair pathways may open new therapeutic avenues in the future. Here, we discuss the dependence of DNA repair processes upon cellular metabolism; including the production of nucleotides required for repair, the necessity of metabolic pathways for the chromatin remodeling required for DNA repair, and the ways in which metabolism itself can induce and prevent DNA damage. We will also discuss the roles of metabolic proteins in DNA repair and, conversely, how DNA repair proteins can impact upon cell metabolism. Finally, we will discuss how further research may open therapeutic avenues in the treatment of cancer.

Histone Modifications in Papillomavirus Virion Minichromosomes

An unusual feature of papillomaviruses is that their genomes are packaged into virions along with host histones. Viral minichromosomes were visualized as “beads on a string” by electron microscopy in the 1970s but, to date, little is known about the posttranslational modifications of these histones. To investigate this, we analyzed the histone modifications in HPV16/18 quasivirions, wart-derived bovine papillomavirus (BPV1), and wart-derived human papillomavirus type 1 (HPV1) using quantitative mass spectrometry. The chromatin from all three virion samples had abundant posttranslational modifications (acetylation, methylation, and phosphorylation). These histone modifications were verified by acid urea polyacrylamide electrophoresis and immunoblot analysis. Compared to matched host cell controls, the virion minichromosome was enriched in histone modifications associated with active chromatin and depleted for those commonly found in repressed chromatin. We propose that the viral minichromosome acquires specific histone modifications late in infection that are coupled to the mechanisms of viral replication, late gene expression, and encapsidation. We predict that, in turn, these same modifications benefit early stages of infection by helping to evade detection, promoting localization of the viral chromosome to beneficial regions of the nucleus, and promoting early transcription and replication.

Altered transcriptome in cumulus cells of infertile women with advanced endometriosis with and without endometrioma

RESEARCH QUESTION

Is the transcriptome of cumulus cells of infertile women with advanced endometriosis (EIII/IV), with and without endometrioma, altered?

DESIGN

In this prospective case-control study, next-generation RNA sequencing was used to compare the transcript profile of cumulus cells among infertile patients undergoing ovarian stimulation for intracytoplasmic sperm injection with EIII/IV, with (n = 9) and without endometrioma (n = 9), and controls (n = 9). An in-silico enrichment analysis was conducted to establish the possibly altered pathways in cumulus cells of patients with endometriosis.

RESULTS

Most of the differentially expressed genes (DEG) were found when cumulus cells from women with EIII/IV with endometrioma were compared with controls (DEG, n = 461). In women with EIII/IV without endometrioma, only 66 DEG were verified compared with controls. The enrichment analysis showed that some DEG in cumulus cells of endometriosis are involved in important pathways for the oocyte competence acquisition, such as oxidative phosphorylation, metabolism, mitochondrial function, acetylation and steroid biosynthesis. No DEG were found when cumulus cells from women with EIII/IV with and without endometrioma were compared.

CONCLUSION

RNA sequencing results suggest that cumulus cells of infertile women with EIII/IV have an altered transcriptome, regardless of endometrioma. The present findings offer a better understanding of the genes and molecular mechanisms that may be involved in endometriosis-related infertility, mostly in the oocyte competence acquisition process.

Collaborations between chromatin and nuclear architecture to optimize DNA repair fidelity

Homologous recombination (HR), considered the highest fidelity DNA double-strand break (DSB) repair pathway that a cell possesses, is capable of repairing multiple DSBs without altering genetic information. However, in "last resort" scenarios, HR can be directed to low fidelity subpathways which often use non-allelic donor templates. Such repair mechanisms are often highly mutagenic and can also yield chromosomal rearrangements and/or deletions. While the choice between HR and its less precise counterpart, non-homologous end joining (NHEJ), has received much attention, less is known about how cells manage and prioritize HR subpathways. In this review, we describe work focused on how chromatin and nuclear architecture orchestrate subpathway choice and repair template usage to maintain genome integrity without sacrificing cell survival. Understanding the relationships between nuclear architecture and recombination mechanics will be critical to understand these cellular repair decisions.

The role of the stem cell epigenome in normal aging and rejuvenative therapy

Adult stem cells are ultimately responsible for the lifelong maintenance of regenerating of tissues during both homeostasis and following injury. Hence, the functional attrition of adult stem cells is thought to be an important driving factor behind the progressive functional decline of tissues and organs that is observed during aging. The mechanistic cause underlying this age-associated exhaustion of functional stem cells is likely to be complex and multifactorial. However, it is clear that progressive remodeling of the epigenome and the resulting deregulation of gene expression programs can be considered a hallmark of aging, and is likely a key factor in mediating altered biological function of aged stem cells. In this review, we outline cell intrinsic and extrinsic mediators of epigenome remodeling during aging; discuss how such changes can impact on stem cell function; and describe how resetting the aged epigenome may rejuvenate some of the biological characteristics of stem cells.

Gene repression in S. cerevisiae-looking beyond Sir-dependent gene silencing

Gene silencing by the SIR (Silent Information Region) family of proteins in S. cerevisiae has been extensively studied and has served as a founding paradigm for our general understanding of gene repression and its links to histone deacetylation and chromatin structure. In recent years, our understanding of other mechanisms of gene repression in S.cerevisiae was significantly advanced. In this review, we focus on such Sir-independent mechanisms of gene repression executed by various Histone Deacetylases (HDACs) and Histone Methyl Transferases (HMTs). We focus on the genes regulated by these enzymes and their known mechanisms of action. We describe the cooperation and redundancy between HDACs and HMTs, and their involvement in gene repression by non-coding RNAs or by their non-histone substrates. We also propose models of epigenetic transmission of the chromatin structures produced by these enzymes and discuss these in the context of gene repression phenomena in other organisms. These include the recycling of the epigenetic marks imposed by HMTs or the recycling of the complexes harboring HDACs.

The E2F1 transcription factor and RB tumor suppressor moonlight as DNA repair factors

The E2F1 transcription factor and RB tumor suppressor are best known for their roles in regulating the expression of genes important for cell cycle progression but, they also have transcription-independent functions that facilitate DNA repair at sites of damage. Depending on the type of DNA damage, E2F1 can recruit either the GCN5 or p300/CBP histone acetyltransferases to deposit different histone acetylation marks in flanking chromatin. At DNA double-strand breaks, E2F1 also recruits RB and the BRG1 ATPase to remodel chromatin and promote loading of the MRE11-RAD50-NBS1 complex. Knock-in mouse models demonstrate important roles for E2F1 post-translational modifications in regulating DNA repair and physiological responses to DNA damage. This review highlights how E2F1 moonlights in DNA repair, thus revealing E2F1 as a versatile protein that recruits many of the same chromatin-modifying enzymes to sites of DNA damage to promote repair that it recruits to gene promoters to regulate transcription.

Roles for MDC1 in cancer development and treatment

The DNA damage response (DDR) is necessary to maintain genome integrity and prevent the accumulation of oncogenic mutations. Consequently, proteins involved in the DDR often serve as tumor suppressors, carrying out the crucial task of keeping DNA fidelity intact. Mediator of DNA damage checkpoint 1 (MDC1) is a scaffold protein involved in the early steps of the DDR. MDC1 interacts directly with γ-H2AX, the phosphorylated form of H2AX, a commonly used marker for DNA damage. It then propagates the phosphorylation of H2AX by recruiting ATM kinase. While the function of MDC1 in the DDR has been reviewed previously, its role in cancer has not been reviewed, and numerous studies have recently identified a link between MDC1 and carcinogenesis. This includes MDC1 functioning as a tumor suppressor, with its loss serving as a biomarker for cancer and contributor to drug sensitivity. Studies also indicate that MDC1 operates outside of its traditional role in DDR, and functions as a co-regulator of nuclear receptor transcriptional activity, and that mutations in MDC1 are present in tumors and can also cause germline predisposition to cancer. This review will discuss reports that link MDC1 to cancer and identify MDC1 as an important player in tumor formation, progression, and treatment. We also discuss mechanisms by which MDC1 levels are regulated and how this contributes to tumor formation.

The Chromatin Response to Double-Strand DNA Breaks and Their Repair

Cellular DNA is constantly being damaged by numerous internal and external mutagenic factors. Probably the most severe type of insults DNA could suffer are the double-strand DNA breaks (DSBs). They sever both DNA strands and compromise genomic stability, causing deleterious chromosomal aberrations that are implicated in numerous maladies, including cancer. Not surprisingly, cells have evolved several DSB repair pathways encompassing hundreds of different DNA repair proteins to cope with this challenge. In eukaryotic cells, DSB repair is fulfilled in the immensely complex environment of the chromatin. The chromatin is not just a passive background that accommodates the multitude of DNA repair proteins, but it is a highly dynamic and active participant in the repair process. Chromatin alterations, such as changing patterns of histone modifications shaped by numerous histone-modifying enzymes and chromatin remodeling, are pivotal for proficient DSB repair. Dynamic chromatin changes ensure accessibility to the damaged region, recruit DNA repair proteins, and regulate their association and activity, contributing to DSB repair pathway choice and coordination. Given the paramount importance of DSB repair in tumorigenesis and cancer progression, DSB repair has turned into an attractive target for the development of novel anticancer therapies, some of which have already entered the clinic.

Epigenomic landscape and epigenetic regulation in maize

Epigenetic regulation has been implicated in the control of multiple agronomic traits in maize. Here, we review current advances in our understanding of epigenetic regulation, which has great potential for improving agronomic traits and the environmental adaptability of crops. Epigenetic regulation plays vital role in the control of complex agronomic traits. Epigenetic variation could contribute to phenotypic diversity and can be used to improve the quality and productivity of crops. Maize (Zea mays L.), one of the most widely cultivated crops for human food, animal feed, and ethanol biofuel, is a model plant for genetic studies. Recent advances in high-throughput sequencing technology have made possible the study of epigenetic regulation in maize on a genome-wide scale. In this review, we discuss recent epigenetic studies in maize many achieved by Chinese research groups. These studies have explored the roles of DNA methylation, posttranslational modifications of histones, chromatin remodeling, and noncoding RNAs in the regulation of gene expression in plant development and environment response. We also provide our future prospects for manipulating epigenetic regulation to improve crops.

Histone Post-Translational Modifications and CircRNAs in Mouse and Human Spermatozoa: Potential Epigenetic Marks to Assess Human Sperm Quality

Spermatozoa (SPZ) are motile cells, characterized by a cargo of epigenetic information including histone post-translational modifications (histone PTMs) and non-coding RNAs. Specific histone PTMs are present in developing germ cells, with a key role in spermatogenic events such as self-renewal and commitment of spermatogonia (SPG), meiotic recombination, nuclear condensation in spermatids (SPT). Nuclear condensation is related to chromatin remodeling events and requires a massive histone-to-protamine exchange. After this event a small percentage of chromatin is condensed by histones and SPZ contain nucleoprotamines and a small fraction of nucleohistone chromatin carrying a landascape of histone PTMs. Circular RNAs (circRNAs), a new class of non-coding RNAs, characterized by a nonlinear back-spliced junction, able to play as microRNA (miRNA) sponges, protein scaffolds and translation templates, have been recently characterized in both human and mouse SPZ. Since their abundance in eukaryote tissues, it is challenging to deepen their biological function, especially in the field of reproduction. Here we review the critical role of histone PTMs in male germ cells and the profile of circRNAs in mouse and human SPZ. Furthermore, we discuss their suggested role as novel epigenetic biomarkers to assess sperm quality and improve artificial insemination procedure.

Transcriptional Regulation at DSBs: Mechanisms and Consequences

Defective double-strand break (DSB) repair leads to genomic instabilities that may augment carcinogenesis. DSBs trigger transient transcriptional silencing in the vicinity of transcriptionally active genes through multilayered processes instigated by Ataxia telangiectasia mutated (ATM), DNA-dependent protein kinase (DNA-PK), and poly-(ADP-ribose) polymerase 1 (PARP1). Novel factors have been identified that ensure DSB-induced silencing via two distinct pathways: direct inhibition of RNA Polymerase II (Pol II) mediated by negative elongation factor (NELF), and histone code editing by CDYL1 and histone deacetylases (HDACs) that catalyze H3K27me3 and erase lysine crotonylation, respectively. Here, we highlight major advances in understanding the mechanisms underlying transcriptional silencing at DSBs, and discuss its functional implications on repair. Furthermore, we discuss consequential links between DSB-silencing factors and carcinogenesis and discuss the potential of exploiting them for targeted cancer therapy.

MRX Increases Chromatin Accessibility at Stalled Replication Forks to Promote Nascent DNA Resection and Cohesin Loading

The recovery of stalled replication forks depends on the controlled resection of nascent DNA and on the loading of cohesin. These processes operate in the context of nascent chromatin, but the impact of nucleosome structure on a fork restart remains poorly understood. Here, we show that the Mre11-Rad50-Xrs2 (MRX) complex acts together with the chromatin modifiers Gcn5 and Set1 and the histone remodelers RSC, Chd1, and Isw1 to promote chromatin remodeling at stalled forks. Increased chromatin accessibility facilitates the resection of nascent DNA by the Exo1 nuclease and the Sgs1 and Chl1 DNA helicases. Importantly, increased ssDNA promotes the recruitment of cohesin to arrested forks in a Scc2-Scc4-dependent manner. Altogether, these results indicate that MRX cooperates with chromatin modifiers to orchestrate the action of remodelers, nucleases, and DNA helicases, promoting the resection of nascent DNA and the loading of cohesin, two key processes involved in the recovery of arrested forks.

Does genome surveillance explain the global discrepancy between binding and effect of chromatin factors?

Knocking out a chromatin factor often does not alter the transcription of its binding targets. What explains the observed disconnect between binding and effect? We hypothesize that this discrepancy could be associated with the role of chromatin factors in maintaining genetic and epigenetic integrity at promoters, and not necessarily with transcription. Through re-analysis of published datasets, we present several lines of evidence that support our hypothesis and deflate the popular assumptions. We also tested the hypothesis through mutation accumulation assays on yeast knockouts of chromatin factors. Altogether, the proposed hypothesis presents a simple explanation for the global discord between chromatin factor binding and effect. Future work in this direction might fortify the hypothesis and elucidate the underlying mechanisms.

Dietary Restriction and Epigenetics: Part I

Biological aging occurs concomitantly with chronological aging and is commonly burdened by the development of age-related conditions, such as neurodegenerative, cardiovascular, and a myriad of metabolic diseases. With a current global shift in disease epidemiology associated with aging and the resultant social, economic, and healthcare burdens faced by many countries, the need to achieve successful aging has fueled efforts to address this problem. Aging is a complex biological phenomenon that has confounded much of the historical research effort to understand it, with still limited knowledge of the underlying molecular mechanisms. Interestingly, dietary restriction (DR) is one intervention that produces anti-aging effects from simple organisms to mammals. Research into DR has revealed robust systemic effects that can result in attenuation of age-related diseases via a myriad of molecular mechanisms. Given that numerous age-associated diseases are often polygenic and affect individuals differently, it is possible that they are confounded by interactions between environmental influences and the genome, a process termed 'epigenetics'. In part one of the review, we summarize the different variants of DR regimens and their corresponding mechanism(s) and resultant effects, as well as in-depth analysis of current knowledge of the epigenetic landscape.

Rpd3L and Hda1 histone deacetylases facilitate repair of broken forks by promoting sister chromatid cohesion

Genome stability involves accurate replication and DNA repair. Broken replication forks, such as those encountering a nick, lead to double strand breaks (DSBs), which are preferentially repaired by sister-chromatid recombination (SCR). To decipher the role of chromatin in eukaryotic DSB repair, here we analyze a collection of yeast chromatin-modifying mutants using a previously developed system for the molecular analysis of repair of replication-born DSBs by SCR based on a mini-HO site. We confirm the candidates through FLP-based systems based on a mutated version of the FLP flipase that causes nicks on either the leading or lagging DNA strands. We demonstrate that Rpd3L and Hda1 histone deacetylase (HDAC) complexes contribute to the repair of replication-born DSBs by facilitating cohesin loading, with no effect on other types of homology-dependent repair, thus preventing genome instability. We conclude that histone deacetylation favors general sister chromatid cohesion as a necessary step in SCR.

Escaping the immune system by DNA repair and recombination in African trypanosomes

African trypanosomes escape the mammalian immune response by antigenic variation-the periodic exchange of one surface coat protein, in Trypanosoma brucei the variant surface glycoprotein (VSG), for an immunologically distinct one. VSG transcription is monoallelic, with only one VSG being expressed at a time from a specialized locus, known as an expression site. VSG switching is a predominantly recombination-driven process that allows VSG sequences to be recombined into the active expression site either replacing the currently active VSG or generating a 'new' VSG by segmental gene conversion. In this review, we describe what is known about the factors that influence this process, focusing specifically on DNA repair and recombination.

TRIM66 reads unmodified H3R2K4 and H3K56ac to respond to DNA damage in embryonic stem cells

Recognition of specific chromatin modifications by distinct structural domains within “reader” proteins plays a critical role in the maintenance of genomic stability. However, the specific mechanisms involved in this process remain unclear. Here we report that the PHD-Bromo tandem domain of tripartite motif-containing 66 (TRIM66) recognizes the unmodified H3R2-H3K4 and acetylated H3K56. The aberrant deletion of Trim66 results in severe DNA damage and genomic instability in embryonic stem cells (ESCs). Moreover, we find that the recognition of histone modification by TRIM66 is critical for DNA damage repair (DDR) in ESCs. TRIM66 recruits Sirt6 to deacetylate H3K56ac, negatively regulating the level of H3K56ac and facilitating the initiation of DDR. Importantly, Trim66-deficient blastocysts also exhibit higher levels of H3K56ac and DNA damage. Collectively, the present findings indicate the vital role of TRIM66 in DDR in ESCs, establishing the relationship between histone readers and maintenance of genomic stability.

TRIM66 protein has an N-terminal tripartite motif and a C-terminal PHD Bromodomain. Here the authors show the specific histone modification recognition of TRIM66-PHD-Bromodomain through crystallography and biochemistry assay, and further reveal that TRIM66 recognition of certain histone modification is important for DNA damage repair in ESCs.

A Sir2-regulated locus control region in the recombination enhancer of Saccharomyces cerevisiae specifies chromosome III structure

The NAD+-dependent histone deacetylase Sir2 was originally identified in Saccharomyces cerevisiae as a silencing factor for HML and HMR, the heterochromatic cassettes utilized as donor templates during mating-type switching. MATa cells preferentially switch to MATα using HML as the donor, which is driven by an adjacent cis-acting element called the recombination enhancer (RE). In this study we demonstrate that Sir2 and the condensin complex are recruited to the RE exclusively in MATa cells, specifically to the promoter of a small gene within the right half of the RE known as RDT1. We also provide evidence that the RDT1 promoter functions as a locus control region (LCR) that regulates both transcription and long-range chromatin interactions. Sir2 represses RDT1 transcription until it is removed from the promoter in response to a dsDNA break at the MAT locus induced by HO endonuclease during mating-type switching. Condensin is also recruited to the RDT1 promoter and is displaced upon HO induction, but does not significantly repress RDT1 transcription. Instead condensin appears to promote mating-type donor preference by maintaining proper chromosome III architecture, which is defined by the interaction of HML with the right arm of chromosome III, including MATa and HMR. Remarkably, eliminating Sir2 and condensin recruitment to the RDT1 promoter disrupts this structure and reveals an aberrant interaction between MATa and HMR, consistent with the partially defective donor preference for this mutant. Global condensin subunit depletion also impairs mating-type switching efficiency and donor preference, suggesting that modulation of chromosome architecture plays a significant role in controlling mating-type switching, thus providing a novel model for dissecting condensin function in vivo.

Chromatin Modifiers Alter Recombination Between Divergent DNA Sequences

Recombination between divergent DNA sequences is actively prevented by heteroduplex rejection mechanisms. In baker's yeast, such antirecombination mechanisms can be initiated by the recognition of DNA mismatches in heteroduplex DNA by MSH proteins, followed by recruitment of the Sgs1-Top3-Rmi1 helicase-topoisomerase complex to unwind the recombination intermediate. We previously showed that the repair/rejection decision during single-strand annealing recombination is temporally regulated by MSH (MutShomolog) protein levels and by factors that excise nonhomologous single-stranded tails. These observations, coupled with recent studies indicating that mismatch repair (MMR) factors interact with components of the histone chaperone machinery, encouraged us to explore roles for epigenetic factors and chromatin conformation in regulating the decision to reject vs. repair recombination between divergent DNA substrates. This work involved the use of an inverted repeat recombination assay thought to measure sister chromatid repair during DNA replication. Our observations are consistent with the histone chaperones CAF-1 and Rtt106, and the histone deacetylase Sir2, acting to suppress heteroduplex rejection and the Rpd3, Hst3, and Hst4 deacetylases acting to promote heteroduplex rejection. These observations, and double-mutant analysis, have led to a model in which nucleosomes located at DNA lesions stabilize recombination intermediates and compete with MMR factors that mediate heteroduplex rejection.

The Role of SIRT1 on DNA Damage Response and Epigenetic Alterations in Cancer

Sirtuin-1 (SIRT1) is a class-III histone deacetylase (HDAC), an NAD+-dependent enzyme deeply involved in gene regulation, genome stability maintenance, apoptosis, autophagy, senescence, proliferation, aging, and tumorigenesis. It also has a key role in the epigenetic regulation of tissue homeostasis and many diseases by deacetylating both histone and non-histone targets. Different studies have shown ambiguous implications of SIRT1 as both a tumor suppressor and tumor promoter. However, this contradictory role seems to be determined by the cell type and SIRT1 localization. SIRT1 upregulation has already been demonstrated in some cancer cells, such as acute myeloid leukemia (AML) and primary colon, prostate, melanoma, and non-melanoma skin cancers, while SIRT1 downregulation was described in breast cancer and hepatic cell carcinomas. Even though new functions of SIRT1 have been characterized, the underlying mechanisms that define its precise role on DNA damage and repair and their contribution to cancer development remains underexplored. Here, we discuss the recent findings on the interplay among SIRT1, oxidative stress, and DNA repair machinery and its impact on normal and cancer cells.

The emerging role of epigenetic modifiers in repair of DNA damage associated with chronic inflammatory diseases

At sites of chronic inflammation epithelial cells are exposed to high levels of reactive oxygen species (ROS), which can contribute to the initiation and development of many different human cancers. Aberrant epigenetic alterations that cause transcriptional silencing of tumor suppressor genes are also implicated in many diseases associated with inflammation, including cancer. However, it is not clear how altered epigenetic gene silencing is initiated during chronic inflammation. The high level of ROS at sites of inflammation is known to induce oxidative DNA damage in surrounding epithelial cells. Furthermore, DNA damage is known to trigger several responses, including recruitment of DNA repair proteins, transcriptional repression, chromatin modifications and other cell signaling events. Recruitment of epigenetic modifiers to chromatin in response to DNA damage results in transient covalent modifications to chromatin such as histone ubiquitination, acetylation and methylation and DNA methylation. DNA damage also alters non-coding RNA expression. All of these alterations have the potential to alter gene expression at sites of damage. Typically, these modifications and gene transcription are restored back to normal once the repair of the DNA damage is completed. However, chronic inflammation may induce sustained DNA damage and DNA damage responses that result in these transient covalent chromatin modifications becoming mitotically stable epigenetic alterations. Understanding how epigenetic alterations are initiated during chronic inflammation will allow us to develop pharmaceutical strategies to prevent or treat chronic inflammation-induced cancer. This review will focus on types of DNA damage and epigenetic alterations associated with chronic inflammatory diseases, the types of DNA damage and transient covalent chromatin modifications induced by inflammation and oxidative DNA damage and how these modifications may result in epigenetic alterations.

Chromatin control in double strand break repair

DNA double strand breaks (DSB) are the most deleterious type of damage inflicted on DNA by various environmental factors and as consequences of normal cellular metabolism. The multistep nature of DSB repair and the need to assemble large protein complexes at repair sites necessitate multiple chromatin changes there. This review focuses on the key findings of how chromatin regulators exert temporal and spatial control on DSB repair. These mechanisms coordinate repair with cell cycle progression, lead to DSB repair pathway choice, provide accessibility of repair machinery to damaged sites and move the lesions to nuclear environments permissive for repair.

Phosphorylation of TIP60 Suppresses 53BP1 Localization at DNA Damage Sites

A proper balance between the repair of DNA double-strand breaks (DSBs) by homologous recombination and nonhomologous end joining is critical for maintaining genome integrity and preventing tumorigenesis. This balance is regulated and fine-tuned by a variety of factors, including cell cycle and the chromatin environment. The histone acetyltransferase TIP60 was previously shown to suppress pathological end joining and promote homologous recombination. However, it is unknown how regulatory posttranslational modifications impact TIP60 acetyltransferase activity to influence the outcome of DSB responses. In this study, we report that phosphorylation of TIP60 on serines 90 and 86 is important for limiting the accumulation of the pro-end joining factor 53BP1 at DSBs in S and G₂ cell cycle phases. Mutation of these sites disrupts histone acetylation changes in response to DNA damage, BRCA1 localization to DSBs, and poly(ADP-ribose) polymerase (PARP) inhibitor resistance. These findings reveal that phosphorylation directs TIP60-dependent acetylation to promote homologous recombination and maintain genome stability.

Nonmutational mechanism of inheritance in the Archaeon Sulfolobus solfataricus

Epigenetic phenomena have not yet been reported in archaea, which are presumed to use a classical genetic process of heritability. Here, analysis of independent lineages of Sulfolobus solfataricus evolved for enhanced fitness implicated a non-Mendelian basis for trait inheritance. The evolved strains, called super acid-resistant Crenarchaeota (SARC), acquired traits of extreme acid resistance and genome stability relative to their wild-type parental lines. Acid resistance was heritable because it was retained regardless of extensive passage without selection. Despite the hereditary pattern, in one strain, it was impossible for these SARC traits to result from mutation because its resequenced genome had no mutation. All strains also had conserved, heritable transcriptomes implicated in acid resistance. In addition, they had improved genome stability with absent or greatly decreased mutation and transposition relative to a passaged control. A mechanism that would confer these traits without DNA sequence alteration could involve posttranslationally modified archaeal chromatin proteins. To test this idea, homologous recombination with isogenic DNA was used to perturb native chromatin structure. Recombination at up-regulated loci from the heritable SARC transcriptome reduced acid resistance and gene expression in the majority of recombinants. In contrast, recombination at a control locus that was not part of the heritable transcriptome changed neither acid resistance nor gene expression. Variation in the amount of phenotypic and expression changes across individuals was consistent with Rad54-dependent chromatin remodeling that dictated crossover location and branch migration. These data support an epigenetic model implicating chromatin structure as a contributor to heritable traits.