Chromatin dynamics in DNA double-strand break repair

SOG1 and BRCA1 Interdependently Regulate RAD54 Expression for Repairing Salinity-Induced DNA Double-Strand Breaks in Arabidopsis

As sessile organisms, land plants experience various forms of environmental stresses throughout their life span. Therefore, plants have developed extensive and complicated defense mechanisms, including a robust DNA damage response (DDR) and DNA repair systems for maintaining genome integrity. In Arabidopsis, the NAC [NO APICAL MERISTEM (NAM), ARABIDOPSIS TRANSCRIPTION ACTIVATION FACTOR (ATAF), CUP-SHAPED COTYLEDON (CUC)] domain family transcription factor SUPPRESSOR OF GAMMA RESPONSE 1 (SOG1) plays an important role in regulating DDR. Here, we show that SOG1 plays a key role in regulating the repair of salinity-induced DNA double-strand breaks (DSBs) via the homologous recombination (HR) pathway in Arabidopsis. The sog1-1 mutant seedlings display a considerably slower rate of repair of salinity-induced DSBs. Accumulation of SOG1 protein increases in wild-type Arabidopsis under salinity stress, and it enhances the expression of HR pathway-related genes, including RAD51, RAD54 and BReast CAncer gene 1 (BRCA1), respectively, as found in SOG1 overexpression lines. SOG1 binds specifically to the AtRAD54 promoter at the 5'-(N)4GTCAA(N)3C-3' consensus sequence and positively regulates its expression under salinity stress. The phenotypic responses of sog1-1/atrad54 double mutants suggest that SOG1 functions upstream of RAD54, and both these genes are essential in regulating DDR under salinity stress. Furthermore, SOG1 interacts directly with BRCA1, an important component of the HR-mediated DSB repair pathway in plants, where BRCA1 appears to facilitate the binding of SOG1 to the RAD54 promoter. At the genetic level, SOG1 and BRCA1 function interdependently in modulating RAD54 expression under salinity-induced DNA damage. Together, our results suggest that SOG1 regulates the repair of salinity-induced DSBs via the HR-mediated pathway through genetic interactions with RAD54 and BRCA1 in Arabidopsis.

Inducible Expression of the Restriction Enzyme Uncovered Genome-Wide Distribution and Dynamic Behavior of Histones H4K16ac and H2A.Z at DNA Double-Strand Breaks in Arabidopsis

DNA double-strand breaks (DSBs) are among the most serious types of DNA damage, causing mutations and chromosomal rearrangements. In eukaryotes, DSBs are immediately repaired in coordination with chromatin remodeling for the deposition of DSB-related histone modifications and variants. To elucidate the details of DSB-dependent chromatin remodeling throughout the genome, artificial DSBs need to be reproducibly induced at various genomic loci. Recently, a comprehensive method for elucidating chromatin remodeling at multiple DSB loci via chemically induced expression of a restriction enzyme was developed in mammals. However, this DSB induction system is unsuitable for investigating chromatin remodeling during and after DSB repair, and such an approach has not been performed in plants. Here, we established a transgenic Arabidopsis plant harboring a restriction enzyme gene Sbf I driven by a heat-inducible promoter. Using this transgenic line, we performed chromatin immunoprecipitation followed by deep sequencing (ChIP-seq) of histones H4K16ac and H2A.Z and investigated the dynamics of these histone marks around the endogenous 623 Sbf I recognition sites. We also precisely quantified DSB efficiency at all cleavage sites using the DNA resequencing data obtained by the ChIP-seq procedure. From the results, Sbf I-induced DSBs were detected at 360 loci, which induced the transient deposition of H4K16ac and H2A.Z around these regions. Interestingly, we also observed the co-localization of H4K16ac and H2A.Z at some DSB loci. Overall, DSB-dependent chromatin remodeling was found to be highly conserved between plants and animals. These findings provide new insights into chromatin remodeling that occurs in response to DSBs in Arabidopsis.

Regulation of DNA damage response by trimeric G-proteins

Upon sensing DNA double-strand breaks (DSBs), eukaryotic cells either die or repair DSBs via one of the two competing pathways, i.e., non-homologous end-joining (NHEJ) or homologous recombination (HR). We show that cell fate after DSBs hinges on GIV/Girdin, a guanine nucleotide-exchange modulator of heterotrimeric Giα•βγ protein. GIV suppresses HR by binding and sequestering BRCA1, a key coordinator of multiple steps within the HR pathway, away from DSBs; it does so using a C-terminal motif that binds BRCA1's BRCT-modules via both phospho-dependent and -independent mechanisms. Using another non-overlapping C-terminal motif GIV binds and activates Gi and enhances the "free" Gβγ→PI-3-kinase→Akt pathway, which promotes survival and is known to suppress HR, favor NHEJ. Absence of GIV, or loss of either of its C-terminal motifs enhanced cell death upon genotoxic stress. Because GIV selectively binds other BRCT-containing proteins suggests that G-proteins may fine-tune sensing, repair, and survival after diverse types of DNA damage.

Disruption of Chromatin Dynamics by Hypotonic Stress Suppresses HR and Shifts DSB Processing to Error-Prone SSA

The processing of DNA double-strand breaks (DSBs) depends on the dynamic characteristics of chromatin. To investigate how abrupt changes in chromatin compaction alter these dynamics and affect DSB processing and repair, we exposed irradiated cells to hypotonic stress (HypoS). Densitometric and chromosome-length analyses show that HypoS transiently decompacts chromatin without inducing histone modifications known from regulated local chromatin decondensation, or changes in Micrococcal Nuclease (MNase) sensitivity. HypoS leaves undisturbed initial stages of DNA-damage-response (DDR), such as radiation-induced ATM activation and H2AX-phosphorylation. However, detection of ATM-pS1981, γ-H2AX and 53BP1 foci is reduced in a protein, cell cycle phase and cell line dependent manner; likely secondary to chromatin decompaction that disrupts the focal organization of DDR proteins. While HypoS only exerts small effects on classical nonhomologous end-joining (c-NHEJ) and alternative end-joining (alt-EJ), it markedly suppresses homologous recombination (HR) without affecting DNA end-resection at DSBs, and clearly enhances single-strand annealing (SSA). These shifts in pathway engagement are accompanied by decreases in HR-dependent chromatid-break repair in the G₂-phase, and by increases in alt-EJ and SSA-dependent chromosomal translocations. Consequently, HypoS sensitizes cells to ionizing radiation (IR)-induced killing. We conclude that HypoS-induced global chromatin decompaction compromises regulated chromatin dynamics and genomic stability by suppressing DSB-processing by HR, and allowing error-prone processing by alt-EJ and SSA.

Regulation of DNA damage response by trimeric G-protein Signaling.. [DOI: 10.1101/2021.07.21.452842] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Upon sensing DNA double-strand breaks (DSBs), eukaryotic cells either die or repair DSBs via one of two competing pathways, i.e., non-homologous end-joining (NHEJ) or homologous recombination (HR). We show that cell fate after DNA damage hinges on the guanine nucleotide-exchange modulator of heterotrimeric G-protein, Giα•βγ, GIV/Girdin. GIV suppresses HR by binding and sequestering BRCA1, a key coordinator of multiple steps within the HR pathway, away from DSBs; it does so using a C-terminal motif that binds BRCA1’s BRCT-modules via both phospho-dependent and -independent mechanisms. GIV promotes NHEJ, and binds and activates Gi and enhances the ‘free’ Gβγ→PI-3-kinase→Akt pathway, thus revealing the enigmatic origin of prosurvival Akt signals during dsDNA repair. Absence of GIV, or the loss of either of its two functions impaired DNA repair, and induced cell death when challenged with numerous cytotoxic agents. That GIV selectively binds few other BRCT-containing proteins suggests convergent signaling such that heterotrimeric G-proteins may finetune sensing, repair, and outcome after DNA damage.

GRAPHIC ABSTRACT

HIGHLIGHTS

Non-receptor G protein modulator, GIV/Girdin binds BRCA1

Binding occurs in both canonical and non-canonical modes

GIV sequesters BRCA1 away from dsDNA breaks, suppresses HR

Activation of Gi by GIV enhances Akt signals, favors NHEJ

IN BRIEF

In this work, the authors show that heterotrimeric G protein signaling that is triggered by non-receptor GEF, GIV/Girdin, in response to double-stranded DNA breaks is critical for decisive signaling events which favor non-homologous end-joining (NHEJ) and inhibit homologous recombination (HR).

HIM-17 regulates the position of recombination events and GSP-1/2 localization to establish short arm identity on bivalents in meiosis

The position of recombination events established along chromosomes in early prophase I and the chromosome remodeling that takes place in late prophase I are intrinsically linked steps of meiosis that need to be tightly regulated to ensure accurate chromosome segregation and haploid gamete formation. Here, we show that RAD-51 foci, which form at the sites of programmed meiotic DNA double-strand breaks (DSBs), exhibit a biased distribution toward off-centered positions along the chromosomes in wild-type Caenorhabditis elegans, and we identify two meiotic roles for chromatin-associated protein HIM-17 that ensure normal chromosome remodeling in late prophase I. During early prophase I, HIM-17 regulates the distribution of DSB-dependent RAD-51 foci and crossovers on chromosomes, which is critical for the formation of distinct chromosome subdomains (short and long arms of the bivalents) later during chromosome remodeling. During late prophase I, HIM-17 promotes the normal expression and localization of protein phosphatases GSP-1/2 to the surface of the bivalent chromosomes and may promote GSP-1 phosphorylation, thereby antagonizing Aurora B kinase AIR-2 loading on the long arms and preventing premature loss of sister chromatid cohesion. We propose that HIM-17 plays distinct roles at different stages during meiotic progression that converge to promote normal chromosome remodeling and accurate chromosome segregation.

Structural insights into DNA double-strand break signaling

Genomic integrity is most threatened by double-strand breaks, which, if left unrepaired, lead to carcinogenesis or cell death. The cell generates a network of protein-protein signaling interactions that emanate from the DNA damage which are now recognized as a rich basis for anti-cancer therapy development. Deciphering the structures of signaling proteins has been an uphill task owing to their large size and complex domain organization. Recent advances in mammalian protein expression/purification and cryo-EM-based structure determination have led to significant progress in our understanding of these large multidomain proteins. This review is an overview of the structural principles that underlie some of the key signaling proteins that function at the double-strand break site. We also discuss some plausible ideas that could be considered for future structural approaches to visualize and build a more complete understanding of protein dynamics at the break site.

A Paradigm Revolution or Just Better Resolution-Will Newly Emerging Superresolution Techniques Identify Chromatin Architecture as a Key Factor in Radiation-Induced DNA Damage and Repair Regulation?

DNA double-strand breaks (DSBs) have been recognized as the most serious lesions in irradiated cells. While several biochemical pathways capable of repairing these lesions have been identified, the mechanisms by which cells select a specific pathway for activation at a given DSB site remain poorly understood. Our knowledge of DSB induction and repair has increased dramatically since the discovery of ionizing radiation-induced foci (IRIFs), initiating the possibility of spatiotemporally monitoring the assembly and disassembly of repair complexes in single cells. IRIF exploration revealed that all post-irradiation processes-DSB formation, repair and misrepair-are strongly dependent on the characteristics of DSB damage and the microarchitecture of the whole affected chromatin domain in addition to the cell status. The microscale features of IRIFs, such as their morphology, mobility, spatiotemporal distribution, and persistence kinetics, have been linked to repair mechanisms. However, the influence of various biochemical and structural factors and their specific combinations on IRIF architecture remains unknown, as does the hierarchy of these factors in the decision-making process for a particular repair mechanism at each individual DSB site. New insights into the relationship between the physical properties of the incident radiation, chromatin architecture, IRIF architecture, and DSB repair mechanisms and repair efficiency are expected from recent developments in optical superresolution microscopy (nanoscopy) techniques that have shifted our ability to analyze chromatin and IRIF architectures towards the nanoscale. In the present review, we discuss this relationship, attempt to correlate still rather isolated nanoscale studies with already better-understood aspects of DSB repair at the microscale, and consider whether newly emerging "correlated multiscale structuromics" can revolutionarily enhance our knowledge in this field.

A New Twist in Protein Kinase B/Akt Signaling: Role of Altered Cancer Cell Metabolism in Akt-Mediated Therapy Resistance

Cancer resistance to chemotherapy, radiotherapy and molecular-targeted agents is a major obstacle to successful cancer therapy. Herein, aberrant activation of the phosphatidyl-inositol-3-kinase (PI3K)/protein kinase B (Akt) pathway is one of the most frequently deregulated pathways in cancer cells and has been associated with multiple aspects of therapy resistance. These include, for example, survival under stress conditions, apoptosis resistance, activation of the cellular response to DNA damage and repair of radiation-induced or chemotherapy-induced DNA damage, particularly DNA double strand breaks (DSB). One further important, yet not much investigated aspect of Akt-dependent signaling is the regulation of cell metabolism. In fact, many Akt target proteins are part of or involved in the regulation of metabolic pathways. Furthermore, recent studies revealed the importance of certain metabolites for protection against therapy-induced cell stress and the repair of therapy-induced DNA damage. Thus far, the likely interaction between deregulated activation of Akt, altered cancer metabolism and therapy resistance is not yet well understood. The present review describes the documented interactions between Akt, its target proteins and cancer cell metabolism, focusing on antioxidant defense and DSB repair. Furthermore, the review highlights potential connections between deregulated Akt, cancer cell metabolism and therapy resistance of cancer cells through altered DSB repair and discusses potential resulting therapeutic implications.

Studying DNA Double-Strand Break Repair: An Ever-Growing Toolbox

To ward off against the catastrophic consequences of persistent DNA double-strand breaks (DSBs), eukaryotic cells have developed a set of complex signaling networks that detect these DNA lesions, orchestrate cell cycle checkpoints and ultimately lead to their repair. Collectively, these signaling networks comprise the DNA damage response (DDR). The current knowledge of the molecular determinants and mechanistic details of the DDR owes greatly to the continuous development of ground-breaking experimental tools that couple the controlled induction of DSBs at distinct genomic positions with assays and reporters to investigate DNA repair pathways, their impact on other DNA-templated processes and the specific contribution of the chromatin environment. In this review, we present these tools, discuss their pros and cons and illustrate their contribution to our current understanding of the DDR.

Pan-mammalian analysis of molecular constraints underlying extended lifespan

Although lifespan in mammals varies over 100-fold, the precise evolutionary mechanisms underlying variation in longevity remain unknown. Species-specific genetic changes have been observed in long-lived species including the naked mole-rat, bats, and the bowhead whale, but these adaptations do not generalize to other mammals. We present a novel method to identify associations between rates of protein evolution and continuous phenotypes across the entire mammalian phylogeny. Unlike previous analyses that focused on individual species, we treat absolute and relative longevity as quantitative traits and demonstrate that these lifespan traits affect the evolutionary constraint on hundreds of genes. Specifically, we find that genes related to cell cycle, DNA repair, cell death, the IGF1 pathway, and immunity are under increased evolutionary constraint in large and long-lived mammals. For mammals exceptionally long-lived for their body size, we find increased constraint in inflammation, DNA repair, and NFKB-related pathways. Strikingly, these pathways have considerable overlap with those that have been previously reported to have potentially adaptive changes in single-species studies, and thus would be expected to show decreased constraint in our analysis. This unexpected finding of increased constraint in many longevity-associated pathways underscores the power of our quantitative approach to detect patterns that generalize across the mammalian phylogeny.

The Transmembrane Protein Semi1 Positions Gamete Nuclei for Reciprocal Fertilization in Tetrahymena

During sexual reproduction in the ciliate, Tetrahymena thermophila, cells of complementary mating type pair ("conjugate") undergo simultaneous meiosis and fertilize each other. In both mating partners only one of the four meiotic products is "selected" to escape autophagy, and this nucleus divides mitotically to produce two pronuclei. The migrating pronucleus of one cell translocates to the mating partner and fuses with its stationary pronucleus and vice versa. Selection of the designated gametic nucleus was thought to depend on its position within the cell because it always attaches to the junction with the partner cell. Here we show that a transmembrane protein, Semi1, is crucial for attachment. Loss of Semi1 causes failure to attach and consequent infertility. However, a nucleus is selected and gives rise to pronuclei regardless of Semi1 expression, indicating that attachment of a nucleus to the junction is not a precondition for selection but follows the selection process.

Advances in Sphingolipidoses: CRISPR-Cas9 Editing as an Option for Modelling and Therapy

Sphingolipidoses are inherited genetic diseases characterized by the accumulation of glycosphingolipids. Sphingolipidoses (SP), which usually involve the loss of sphingolipid hydrolase function, are of lysosomal origin, and represent an important group of rare diseases among lysosomal storage disorders. Initial treatments consisted of enzyme replacement therapy, but, in recent decades, various therapeutic approaches have been developed. However, these commonly used treatments for SP fail to be fully effective and do not penetrate the blood-brain barrier. New approaches, such as genome editing, have great potential for both the treatment and study of sphingolipidoses. Here, we review the most recent advances in the treatment and modelling of SP through the application of CRISPR-Cas9 genome editing. CRISPR-Cas9 is currently the most widely used method for genome editing. This technique is versatile; it can be used for altering the regulation of genes involved in sphingolipid degradation and synthesis pathways, interrogating gene function, generating knock out models, or knocking in mutations. CRISPR-Cas9 genome editing is being used as an approach to disease treatment, but more frequently it is utilized to create models of disease. New CRISPR-Cas9-based tools of gene editing with diminished off-targeting effects are evolving and seem to be more promising for the correction of individual mutations. Emerging Prime results and CRISPR-Cas9 difficulties are also discussed.

Chromatin Remodeling and Epigenetic Regulation in Plant DNA Damage Repair

DNA damage response (DDR) in eukaryotic cells is initiated in the chromatin context. DNA damage and repair depend on or have influence on the chromatin dynamics associated with genome stability. Epigenetic modifiers, such as chromatin remodelers, histone modifiers, DNA (de-)methylation enzymes, and noncoding RNAs regulate DDR signaling and DNA repair by affecting chromatin dynamics. In recent years, significant progress has been made in the understanding of plant DDR and DNA repair. SUPPRESSOR OF GAMMA RESPONSE1, RETINOBLASTOMA RELATED1 (RBR1)/E2FA, and NAC103 have been proven to be key players in the mediation of DDR signaling in plants, while plant-specific chromatin remodelers, such as DECREASED DNA METHYLATION1, contribute to chromatin dynamics for DNA repair. There is accumulating evidence that plant epigenetic modifiers are involved in DDR and DNA repair. In this review, I examine how DDR and DNA repair machineries are concertedly regulated in Arabidopsis thaliana by a variety of epigenetic modifiers directing chromatin remodeling and epigenetic modification. This review will aid in updating our knowledge on DDR and DNA repair in plants.

γH2AX prefers late replicating metaphase chromosome regions

DNA damage response (DDR) constitutes a protein pathway to handle eukaryotic DNA lesions in the context of chromatin. DDR engages the recruitment of signaling, transducer, effector, chromatin modifiers and remodeling proteins, allowing cell cycle delay, DNA repair or induction of senescence or apoptosis. An early DDR-event includes the epigenetic phosphorylation of the histone variant H2AX on serine 139 of the C-termini, so-called gammaH2AX. GammaH2AX foci detected by immunolabeling on interphase nuclei have been largely studied; nonetheless gammaH2AX signals on mitotic chromosomes are less understood. The CHO9 cell line is a subclone of CHO (Chinese hamster ovary) cells with original and rearranged Z chromosomes originated during cell line transformation. As a result, homologous chromosome regions have been relocated in different Z-chromosomes. In a first quantitative analysis of gammaH2AX signals on immunolabeled mitotic chromosomes of cytocentrifuged metaphase spreads, we reported that gammaH2AX¹³⁹ signals of both control and bleomycin-exposed cultures showed statistically equal distribution between CHO9 homologous chromosome regions, suggesting a possible dependence on the structure/function of chromatin. We have also demonstrated that bleomycin-induced gammaH2AX foci map preferentially to DNA replicating domains in CHO9 interphase nuclei. With the aim of understanding the role of gammaH2AX signals on metaphase chromosomes, the relation between 5-ethynyl-2'-deoxyuridine (EdU) labeled replicating chromosome regions and gammaH2AX signals in immunolabeled cytocentrifuged metaphase spreads from control and bleomycin-treated CHO9 cultures was analyzed in the present work. A quantitative analysis of colocalization between EdU and gammaH2AX signals based on the calculation of the Replication Related Damage Distribution Index (RDDI) on confocal metaphase images was performed. RDDI revealed a colocalization between EdU and gammaH2AX signals both in control and bleomycin-treated CHO9 metaphases, suggesting that replication may be involved in H2AX phosphorylation. The possible mechanisms implicated are discussed.

Implication of the VRK1 chromatin kinase in the signaling responses to DNA damage: a therapeutic target?

DNA damage causes a local distortion of chromatin that triggers the sequential processes that participate in specific DNA repair mechanisms. This initiation of the repair response requires the involvement of a protein whose activity can be regulated by histones. Kinases are candidates to regulate and coordinate the connection between a locally altered chromatin and the response initiating signals that lead to identification of the type of lesion and the sequential steps required in specific DNA damage responses (DDR). This initiating kinase must be located in chromatin, and be activated independently of the type of DNA damage. We review the contribution of the Ser-Thr vaccinia-related kinase 1 (VRK1) chromatin kinase as a new player in the signaling of DNA damage responses, at chromatin and cellular levels, and its potential as a new therapeutic target in oncology. VRK1 is involved in the regulation of histone modifications, such as histone phosphorylation and acetylation, and in the formation of γH2AX, NBS1 and 53BP1 foci induced in DDR. Induction of DNA damage by chemotherapy or radiation is a mainstay of cancer treatment. Therefore, novel treatments can be targeted to proteins implicated in the regulation of DDR, rather than by directly causing DNA damage.

The role of ubiquitin-dependent segregase p97 (VCP or Cdc48) in chromatin dynamics after DNA double strand breaks

DNA double strand breaks (DSBs) are the most cytotoxic DNA lesions and, if not repaired, lead to chromosomal rearrangement, genomic instability and cell death. Cells have evolved a complex network of DNA repair and signalling molecules which promptly detect and repair DSBs, commonly known as the DNA damage response (DDR). The DDR is orchestrated by various post-translational modifications such as phosphorylation, methylation, ubiquitination or SUMOylation. As DSBs are located in complex chromatin structures, the repair of DSBs is engineered at two levels: (i) at sites of broken DNA and (ii) at chromatin structures that surround DNA lesions. Thus, DNA repair and chromatin remodelling machineries must work together to efficiently repair DSBs. Here, we summarize the current knowledge of the ubiquitin-dependent molecular unfoldase/segregase p97 (VCP in vertebrates and Cdc48 in worms and lower eukaryotes) in DSB repair. We identify p97 as an essential factor that regulates DSB repair. p97-dependent extraction of ubiquitinated substrates mediates spatio-temporal protein turnover at and around the sites of DSBs, thus orchestrating chromatin remodelling and DSB repair. As p97 is a druggable target, p97 inhibition in the context of DDR has great potential for cancer therapy, as shown for other DDR components such as PARP, ATR and CHK1.This article is part of the themed issue 'Chromatin modifiers and remodellers in DNA repair and signalling'.

Poly(ADP-ribose) Polymerase 1, PARP1, modifies EZH2 and inhibits EZH2 histone methyltransferase activity after DNA damage

The enzyme Poly(ADP-ribose) polymerase 1 (PARP1) plays a very important role in the DNA damage response, but its role in numerous aspects is not fully understood. We recently showed that in the absence of DNA damage, PARP1 regulates the expression of the chromatin-modifying enzyme EZH2. Work from other groups has shown that EZH2 participates in the DNA damage response. These combined data suggest that EZH2 could be a target of PARP1 in both untreated and genotoxic agent-treated conditions. In this work we tested the hypothesis that, in response to DNA damage, PARP1 regulates EZH2 activity. Here we report that PARP1 regulates EZH2 activity after DNA damage. In particular, we find that EZH2 is a direct target of PARP1 upon induction of alkylating and UV-induced DNA damage in cells and in vitro. PARylation of EZH2 inhibits EZH2 histone methyltransferase (H3K27me) enzymatic activity. We observed in cells that the induction of PARP1 activity by DNA alkylating agents decreases the association of EZH2 with chromatin, and PARylation of histone H3 reduces EZH2 affinity for its target histone H3. Our findings establish that PARP1 and PARylation are important regulators of EZH2 function and link EZH2-mediated heterochromatin formation, DNA damage and PARylation. These findings may also have clinical implications, as they suggest that inhibitors of EZH2 can improve anti-tumor effects of PARP1 inhibitors in BRCA1/2-deficient cancers.

Chromatin stiffening underlies enhanced locus mobility after DNA damage in budding yeast

DNA double-strand breaks (DSBs) induce a cellular response that involves histone modifications and chromatin remodeling at the damaged site and increases chromosome dynamics both locally at the damaged site and globally in the nucleus. In parallel, it has become clear that the spatial organization and dynamics of chromosomes can be largely explained by the statistical properties of tethered, but randomly moving, polymer chains, characterized mainly by their rigidity and compaction. How these properties of chromatin are affected during DNA damage remains, however, unclear. Here, we use live cell microscopy to track chromatin loci and measure distances between loci on yeast chromosome IV in thousands of cells, in the presence or absence of genotoxic stress. We confirm that DSBs result in enhanced chromatin subdiffusion and show that intrachromosomal distances increase with DNA damage all along the chromosome. Our data can be explained by an increase in chromatin rigidity, but not by chromatin decondensation or centromeric untethering only. We provide evidence that chromatin stiffening is mediated in part by histone H2A phosphorylation. Our results support a genome-wide stiffening of the chromatin fiber as a consequence of DNA damage and as a novel mechanism underlying increased chromatin mobility.

Post-meiotic DNA double-strand breaks occur in Tetrahymena, and require Topoisomerase II and Spo11

Based on observations of markers for DNA lesions, such as phosphorylated histone H2AX (γH2AX) and open DNA ends, it has been suggested that post-meiotic DNA double-strand breaks (PM-DSBs) enable chromatin remodeling during animal spermiogenesis. However, the existence of PM-DSBs is unconfirmed, and the mechanism responsible for their formation is unclear. Here, we report the first direct observation of programmed PM-DSBs via the electrophoretic separation of DSB-generated DNA fragments in the ciliate Tetrahymena thermophila. These PM-DSBs are accompanied by switching from a heterochromatic to euchromatic chromatin structure in the haploid pronucleus. Both a topoisomerase II paralog with exclusive pronuclear expression and Spo11 are prerequisites for PM-DSB induction. Reduced PM-DSB induction blocks euchromatin formation, characterized by histone H3K56 acetylation, leading to a failure in gametic nuclei production. We propose that PM-DSBs are responsible for histone replacement during the reprogramming of generative to undifferentiated progeny nuclei.

DOI:http://dx.doi.org/10.7554/eLife.26176.001

Gcn5-mediated Rph1 acetylation regulates its autophagic degradation under DNA damage stress

Histone modifiers regulate proper cellular activities in response to various environmental stress by modulating gene expression. In budding yeast, Rph1 transcriptionally represses many DNA damage or autophagy-related gene expression. However, little is known how Rph1 is regulated during these stress conditions. Here, we report that Rph1 is degraded upon DNA damage stress conditions. Notably, this degradation occurs via the autophagy pathway rather than through 26S proteasome proteolysis. Deletion of ATG genes or inhibition of vacuole protease activity compromises Rph1 turnover. We also determine that Rph1 and nuclear export protein Crm1 interact, which is required for Rph1 translocation from the nucleus to the cytoplasm. More importantly, Gcn5 directly acetylates Rph1 in vitro and in vivo, and Gcn5-containing complex, SAGA, is required for autophagic degradation of Rph1. Gcn5-mediated Rph1 acetylation is essential for the association of Rph1 with the nuclear pore protein Nup1. Finally, we show that sustaining high levels of Rph1 during DNA damage stress results in cell growth defects. Thus, we propose that Gcn5-mediated acetylation finely regulates Rph1 protein level and that autophagic degradation of Rph1 is important for cell homeostasis. Our findings may provide a general connection between DNA damage, protein acetylation and autophagy.

Effects of Florida Red Tides on histone variant expression and DNA methylation in the Eastern oyster Crassostrea virginica

Massive algal proliferations known as Harmful Algal Blooms (HABs) represent one of the most important threats to coastal areas. Among them, the so-called Florida Red Tides (FRTs, caused by blooms of the dinoflagellate Karenia brevis and associated brevetoxins) are particularly detrimental in the southeastern U.S., causing high mortality rates and annual losses in excess of $40 million. The ability of marine organisms to cope with environmental stressors (including those produced during HABs) is influenced by genetic and epigenetic mechanisms, the latter resulting in phenotypic changes caused by heritable modifications in gene expression, without involving changes in the genetic (DNA) sequence. Yet, studies examining cause-effect relationships between environmental stressors, specific epigenetic mechanisms and subsequent responses are still lacking. The present work contributes to increase this knowledge by investigating the effects of Florida Red Tides on two types of mechanisms participating in the epigenetic memory of Eastern oysters: histone variants and DNA methylation. For that purpose, a HAB simulation was conducted in laboratory conditions, exposing oysters to increasing concentrations of K. brevis. The obtained results revealed, for the first time, the existence of H2A.X, H2A.Z and macroH2A genes in this organism, encoding histone variants potentially involved in the maintenance of genome integrity during responses to the genotoxic effect of brevetoxins. Additionally, an increase in H2A.X phosphorylation (γH2A.X, a marker of DNA damage) and a decrease in global DNA methylation were observed as the HAB simulation progressed. Overall, the present work provides a basis to better understand how epigenetic mechanisms participate in responses to environmental stress in marine invertebrates, opening new avenues to incorporate environmental epigenetics approaches into management and conservation programs.

[Epigenetic effects of human breastfeeding]

Breastfeeding is well-known for its benefits of preventing communicable and non-communicable diseases. Human breastmilk consists not only of nutrients, but also of bioactive substances. What's more, the epigenetic effects of human breast milk may also play an important role. Alterations in the epigenetic regulation of genes may lead to profound changes in phenotype. Clarifying the role of human breast milk on genetic expression can potentially benefit the infant's health and his later life. This review article makes a brief summary of the epigenetic mechanism of breast milk, and its epigenetic effects on neonatal necrotizing enterocolitis, infectious diseases, metabolism syndrome, cognitive function and anaphylactic diseases.

SIRT7 is a histone desuccinylase that functionally links to chromatin compaction and genome stability

Although SIRT7 is a member of sirtuin family proteins that are described as NAD⁺-dependent class III histone deacetylases, the intrinsic enzymatic activity of this sirtuin protein remains to be investigated and the cellular function of SIRT7 remains to be explored. Here we report that SIRT7 is an NAD⁺-dependent histone desuccinylase. We show that SIRT7 is recruited to DNA double-strand breaks (DSBs) in a PARP1-dependent manner and catalyses desuccinylation of H3K122 therein, thereby promoting chromatin condensation and DSB repair. We demonstrate that depletion of SIRT7 impairs chromatin compaction during DNA-damage response and sensitizes cells to genotoxic stresses. Our study indicates SIRT7 is a histone desuccinylase, providing a molecular basis for the understanding of epigenetic regulation by this sirtuin protein. Our experiments reveal that SIRT7-catalysed H3K122 desuccinylation is critically implemented in DNA-damage response and cell survival, providing a mechanistic insight into the cellular function of SIRT7.

SIRT7 is a member of sirtuin family proteins that are described as NAD⁺-dependent class III histone deacetylases. Here, the authors show that SIRT7 is histone desuccinylase catalysing H3K122 desuccinylation, thereby promoting chromatin condensation and repair of DNA double strand breaks.

Reprint of "DNA, the central molecule of aging"

Understanding the molecular mechanism of aging could have enormous medical implications. Despite a century of research, however, there is no universally accepted theory regarding the molecular basis of aging. On the other hand, there is plentiful evidence suggesting that DNA constitutes the central molecule in this process. Here, we review the roles of chromatin structure, DNA damage, and shortening of telomeres in aging and propose a hypothesis for how their interplay leads to aging phenotypes.

DNA, the central molecule of aging

Understanding the molecular mechanism of aging could have enormous medical implications. Despite a century of research, however, there is no universally accepted theory regarding the molecular basis of aging. On the other hand, there is plentiful evidence suggesting that DNA constitutes the central molecule in this process. Here, we review the roles of chromatin structure, DNA damage, and shortening of telomeres in aging and propose a hypothesis for how their interplay leads to aging phenotypes.

Controlled DNA double-strand break induction in mice reveals post-damage transcriptome stability

DNA double-strand breaks (DSBs) and their repair can cause extensive epigenetic changes. As a result, DSBs have been proposed to promote transcriptional and, ultimately, physiological dysfunction via both cell-intrinsic and cell-non-autonomous pathways. Studying the consequences of DSBs in higher organisms has, however, been hindered by a scarcity of tools for controlled DSB induction. Here, we describe a mouse model that allows for both tissue-specific and temporally controlled DSB formation at ∼140 defined genomic loci. Using this model, we show that DSBs promote a DNA damage signaling-dependent decrease in gene expression in primary cells specifically at break-bearing genes, which is reversed upon DSB repair. Importantly, we demonstrate that restoration of gene expression can occur independently of cell cycle progression, underlining its relevance for normal tissue maintenance. Consistent with this, we observe no evidence for persistent transcriptional repression in response to a multi-day course of continuous DSB formation and repair in mouse lymphocytes in vivo Together, our findings reveal an unexpected capacity of primary cells to maintain transcriptome integrity in response to DSBs, pointing to a limited role for DNA damage as a mediator of cell-autonomous epigenetic dysfunction.

The emerging role of lysine demethylases in DNA damage response: dissecting the recruitment mode of KDM4D/JMJD2D to DNA damage sites

KDM4D is a lysine demethylase that removes tri- and di- methylated residues from H3K9 and is involved in transcriptional regulation and carcinogenesis. We recently showed that KDM4D is recruited to DNA damage sites in a PARP1-dependent manner and facilitates double-strand break repair in human cells. Moreover, we demonstrated that KDM4D is an RNA binding protein and mapped its RNA-binding motifs. Interestingly, KDM4D-RNA interaction is essential for its localization on chromatin and subsequently for efficient demethylation of its histone substrate H3K9me3. Here, we provide new data that shed mechanistic insights into KDM4D accumulation at DNA damage sites. We show for the first time that KDM4D binds poly(ADP-ribose) (PAR) in vitro via its C-terminal region. In addition, we demonstrate that KDM4D-RNA interaction is required for KDM4D accumulation at DNA breakage sites. Finally, we discuss the recruitment mode and the biological functions of additional lysine demethylases including KDM4B, KDM5B, JMJD1C, and LSD1 in DNA damage response.

Dynamics of polycomb proteins-mediated histone modifications during UV irradiation-induced DNA damage

Polycomb group (PcG) complexes are known to be chromatin modifiers and transcriptional repressors. In this work, we reported that the histone-modifying PcG complexes are able to participate in the repair process of ultraviolet (UV)-induced DNA lesions in the silkworm, Bombyx mori. The silkworm cells with depletion of PcG genes showed hypersensitive to UV-C irradiation and increased inhibition of cell proliferation. Interestingly, an SQ site in the silkworm-human chimeric H2A protein synthesized here was phosphorylated rapidly upon UV-C exposure, which could be used as a marker for monitoring the response to DNA damage in silkworm cells. Under these UV-C irradiated conditions, we found that PRC1-mediated ubiquitylation of H2AX, but not of H2AZ, were decreased and this deubiquitylation was independent of its phosphorylation event. In contrast, UV-C irradiation induced the increase of trimethylation of lysine 27 on histone H3 (H3K27me3), a mark of transcriptionally silent chromatin catalyzed by another PcG subcomplex, PRC2. Collectively, we provided the first evidence on chromatin remodeling in response to UV-C lesion in silkworm and revealed another layer role for PcG complexes-mediated histone modifications in contributing to creating an open chromatin structure for the efficient repair of DNA damages.

Activation of DNA damage response signaling by condensed chromatin

The DNA damage response (DDR) occurs in the context of chromatin, and architectural features of chromatin have been implicated in DNA damage signaling and repair. Whereas a role of chromatin decondensation in the DDR is well established, we show here that chromatin condensation is integral to DDR signaling. We find that, in response to DNA damage chromatin regions transiently expand before undergoing extensive compaction. Using a protein-chromatin-tethering system to create defined chromatin domains, we show that interference with chromatin condensation results in failure to fully activate DDR. Conversely, forced induction of local chromatin condensation promotes ataxia telangiectasia mutated (ATM)- and ATR-dependent activation of upstream DDR signaling in a break-independent manner. Whereas persistent chromatin compaction enhanced upstream DDR signaling from irradiation-induced breaks, it reduced recovery and survival after damage. Our results demonstrate that chromatin condensation is sufficient for activation of DDR signaling and is an integral part of physiological DDR signaling.

Dynamics of double strand breaks and chromosomal translocations

Chromosomal translocations are a major cause of cancer. At the same time, the mechanisms that lead to specific chromosomal translocations that associate different gene regions remain largely unknown. Translocations are induced by double strand breaks (DSBs) in DNA. Here we review recent data on the mechanisms of generation, mobility and repair of DSBs and stress the importance of the nuclear organization in this process.

P53 and Sirt1: routes of metabolism and genome stability

The tumor suppressor p53 is a transcription factor that regulates key processes. But, the outcomes of the p53 response go beyond its role as a nuclear transcription factor. Sirtuin (SIRT1) regulates p53 functions as transcription factor. At the same time, SIRT1 protects the genome under stress conditions. The link between p53 and SIRT1 responses is unique. Both regulate metabolism, stress signaling, cell survival, cell cycle control and genome stability. Recent studies have proposed cancer as a metabolic disease. This is due to the switch from aerobic to anaerobic metabolism during tumor development. Yet, the complex molecular circuits (in and out of the nucleus) of tumor progression remain elusive. In this review, we will focus on the interplay between p53 and SIRT1. We will discuss their roles as nodes for possible therapeutic intervention.

Chromatin modifications and DNA repair: beyond double-strand breaks

DNA repair must take place in the context of chromatin, and chromatin modifications and DNA repair are intimately linked. The study of double-strand break repair has revealed numerous histone modifications that occur after induction of a DSB, and modification of the repair factors themselves can also occur. In some cases the function of the modification is at least partially understood, but in many cases it is not yet clear. Although DSB repair is a crucial activity for cell survival, DSBs account for only a small percentage of the DNA lesions that occur over the lifetime of a cell. Repair of single-strand gaps, nicks, stalled forks, alternative DNA structures, and base lesions must also occur in a chromatin context. There is increasing evidence that these repair pathways are also regulated by histone modifications and chromatin remodeling. In this review, we will summarize the current state of knowledge of chromatin modifications that occur during non-DSB repair, highlighting similarities and differences to DSB repair as well as remaining questions.

A macrohistone variant links dynamic chromatin compaction to BRCA1-dependent genome maintenance

Appropriate DNA double-strand break (DSB) repair factor choice is essential for ensuring accurate repair outcome and genomic integrity. The factors that regulate this process remain poorly understood. Here, we identify two repressive chromatin components, the macrohistone variant macroH2A1 and the H3K9 methyltransferase and tumor suppressor PRDM2, which together direct the choice between the antagonistic DSB repair mediators BRCA1 and 53BP1. The macroH2A1/PRDM2 module mediates an unexpected shift from accessible to condensed chromatin that requires the ataxia telangiectasia mutated (ATM)-dependent accumulation of both proteins at DSBs in order to promote DSB-flanking H3K9 dimethylation. Remarkably, loss of macroH2A1 or PRDM2, as well as experimentally induced chromatin decondensation, impairs the retention of BRCA1, but not 53BP1, at DSBs. As a result, mac-roH2A1 and/or PRDM2 depletion causes epistatic defects in DSB end resection, homology-directed repair, and the resistance to poly(ADP-ribose) polymerase (PARP) inhibition—all hallmarks of BRCA1-deficient tumors. Together, these findings identify dynamic, DSB-associated chromatin reorganization as a critical modulator of BRCA1-dependent genome maintenance.

Induction of recombination between diverged sequences in a mammalian genome by a double-strand break

To investigate whether mammalian cells can carry out recombinational double-strand break (DSB) repair between highly diverged sequences, mouse fibroblasts were transfected with DNA substrates that contained a "recipient" thymidine kinase (tk) gene disrupted by the recognition site for endonuclease I-SceI. Substrates also contained a linked "donor" tk gene sequence. Following DSB induction by I-SceI, selection for tk-expressing clones allowed recovery of repair events occurring by nonhomologous end-joining or recombination with the donor sequence. Although recombinational repair was most efficient when donor and recipient shared near-perfect homology, we recovered recombination events between recipient and donor sequences displaying 20 % nucleotide mismatch. Recombination between such imperfectly matched ("homeologous") sequences occurred at a frequency of 1.7 × 10(-7) events per cell and constituted 3 % of the DSB repair events recovered with the pair of homeologous sequences. Additional experiments were done with a substrate containing a donor sequence comprised of a region sharing high homology with the recipient and an adjacent region homeologous to the recipient. Recombinational DSB repair tracts initiating within high homology propagated into homeology in 11 of 112 repair events. These collective results contrasted with our earlier work in which spontaneous recombination (not intentionally induced by a DSB) between homeologous sequences occurred at an undetectable frequency of less than 10(-9) events per cell, and in which events initiating within high homology propagated into adjoining homeology in one of 81 events examined. Our current work suggests that homology requirements for recombination are effectively relaxed in proximity to a DSB in a mammalian genome.

Histone demethylase KDM5B is a key regulator of genome stability

Maintenance of genomic stability is essential for normal organismal development and is vital to prevent diseases such as cancer. As genetic information is packaged into chromatin, it has become increasingly clear that the chromatin environment plays an important role in DNA damage response. However, how DNA repair is controlled by epigenetic mechanisms is not fully understood. Here we report the identification and characterization of lysine-specific histone demethylase 5B (KDM5B), a member of the JmjC domain-containing histone demethylases, as an important player in multiple aspects of DNA double-strand break (DSB) response in human cells. We found that KDM5B becomes enriched in DNA-damage sites after ironizing radiation and endonuclease treatment in a poly(ADP ribose) polymerase 1- and histone variant macroH2A1.1-dependent manner. We showed that KDM5B is required for efficient DSB repair and for the recruitment of Ku70 and BRCA1, the essential component of nonhomologous end-joining and homologous recombination, respectively. Significantly, KDM5B deficiency disengages the DNA repair process, promotes spontaneous DNA damage, activates p53 signaling, and sensitizes cells to genotoxic insults. Our results suggest that KDM5B is a bona fide DNA damage response protein and indicate that KDM5B is an important genome caretaker and a critical regulator of genome stability, adding to the understanding of the roles of epigenetics in the maintenance of genetic fidelity.

Epigenetic control of mobile DNA as an interface between experience and genome change

Mobile DNA in the genome is subject to RNA-targeted epigenetic control. This control regulates the activity of transposons, retrotransposons and genomic proviruses. Many different life history experiences alter the activities of mobile DNA and the expression of genetic loci regulated by nearby insertions. The same experiences induce alterations in epigenetic formatting and lead to trans-generational modifications of genome expression and stability. These observations lead to the hypothesis that epigenetic formatting directed by non-coding RNA provides a molecular interface between life history events and genome alteration.

ATM alters the otherwise robust chromatin mobility at sites of DNA double-strand breaks (DSBs) in human cells

Ionizing radiation induces DNA double strand breaks (DSBs) which can lead to the formation of chromosome rearrangements through error prone repair. In mammalian cells the positional stability of chromatin contributes to the maintenance of genome integrity. DSBs exhibit only a small, submicron scale diffusive mobility, but a slight increase in the mobility of chromatin domains by the induction of DSBs might influence repair fidelity and the formation of translocations. The radiation-induced local DNA decondensation in the vicinity of DSBs is one factor potentially enhancing the mobility of DSB-containing chromatin domains. Therefore in this study we focus on the influence of different chromatin modifying proteins, known to be activated by the DNA damage response, on the mobility of DSBs. IRIF (ionizing radiation induced foci) in U2OS cells stably expressing 53BP1-GFP were used as a surrogate marker of DSBs. Low angle charged particle irradiation, known to trigger a pronounced DNA decondensation, was used for the defined induction of linear tracks of IRIF. Our results show that movement of IRIF is independent of the investigated chromatin modifying proteins like ACF1 or PARP1 and PARG. Also depletion of proteins that tether DNA strands like MRE11 and cohesin did not alter IRIF dynamics significantly. Inhibition of ATM, a key component of DNA damage response signaling, resulted in a pronounced confinement of DSB mobility, which might be attributed to a diminished radiation induced decondensation. This confinement following ATM inhibition was confirmed using X-rays, proving that this effect is not restricted to densely ionizing radiation. In conclusion, repair sites of DSBs exhibit a limited mobility on a small spatial scale that is mainly unaffected by depletion of single remodeling or DNA tethering proteins. However, it relies on functional ATM kinase which is considered to influence the chromatin structure after irradiation.

DNA repair in cancer: emerging targets for personalized therapy

Genomic deoxyribonucleic acid (DNA) is under constant threat from endogenous and exogenous DNA damaging agents. Mammalian cells have evolved highly conserved DNA repair machinery to process DNA damage and maintain genomic integrity. Impaired DNA repair is a major driver for carcinogenesis and could promote aggressive cancer biology. Interestingly, in established tumors, DNA repair activity is required to counteract oxidative DNA damage that is prevalent in the tumor microenvironment. Emerging clinical data provide compelling evidence that overexpression of DNA repair factors may have prognostic and predictive significance in patients. More recently, DNA repair inhibition has emerged as a promising target for anticancer therapy. Synthetic lethality exploits intergene relationships where the loss of function of either of two related genes is nonlethal, but loss of both causes cell death. Exploiting this approach by targeting DNA repair has emerged as a promising strategy for personalized cancer therapy. In the current review, we focus on recent advances with a particular focus on synthetic lethality targeting in cancer.

Metabolic modulation of chromatin: implications for DNA repair and genomic integrity

The maintenance of genomic integrity in response to DNA damage is tightly linked to controlled changes in the damage-proximal chromatin environment. Many of the chromatin modifying enzymes involved in DNA repair depend on metabolic intermediates as cofactors, suggesting that changes in cellular metabolism can have direct consequences for repair efficiency and ultimately, genome stability. Here, we discuss how metabolites may contribute to DNA double-strand break repair, and how alterations in cellular metabolism associated with both aging and tumorigenesis may affect the integrity of our genomes.

RNA splicing: a new player in the DNA damage response

It is widely accepted that tumorigenesis is a multistep process characterized by the sequential accumulation of genetic alterations. However, the molecular basis of genomic instability in cancer is still partially understood. The observation that hereditary cancers are often characterized by mutations in DNA repair and checkpoint genes suggests that accumulation of DNA damage is a major contributor to the oncogenic transformation. It is therefore of great interest to identify all the cellular pathways that contribute to the response to DNA damage. Recently, RNA processing has emerged as a novel pathway that may contribute to the maintenance of genome stability. In this review, we illustrate several different mechanisms through which pre-mRNA splicing and genomic stability can influence each other. We specifically focus on the role of splicing factors in the DNA damage response and describe how, in turn, activation of the DDR can influence the activity of splicing factors.

Abscisic acid suppresses the highly occurred somatic homologous recombination in Arabidopsis rfc1 mutant

The phytohormone abscisic acid (ABA) regulates many aspects of plant growth, including seed germination, root growth and cell division. Previous study indicates that ABA treatment increases DNA damage and somatic homologous recombination (HR) in Arabidopsis abo4/pol ɛ (aba overly-sensitive 4 /DNA polymerase ɛ) mutants. DNA replication factor C (RFC) complex is required for loading PCNA (Proliferating Cell Nuclear Antigen) during DNA replication. The defect in RFC1, the largest subunit of RFC, causes the high HR and DNA damage sensitivity in Arabidopsis. Here we found that like pol ε/abo4, rfc1 is sensitive to ABA in both ABA-inhibiting seed germination and root growth. However, ABA treatment greatly reduces HR and also reduces the expression of the DNA-damaged marker genes in rfc1. These results suggest that RFC1 plays critical roles in ABA-mediated HR in Arabidopsis.

Drosophila MOF controls Checkpoint protein2 and regulates genomic stability during early embryogenesis

Background

In Drosophila embryos, checkpoints maintain genome stability by delaying cell cycle progression that allows time for damage repair or to complete DNA synthesis. Drosophila MOF, a member of MYST histone acetyl transferase is an essential component of male X hyperactivation process. Until recently its involvement in G2/M cell cycle arrest and defects in ionizing radiation induced DNA damage pathways was not well established.

Results

Drosophila MOF is highly expressed during early embryogenesis. In the present study we show that haplo-insufficiency of maternal MOF leads to spontaneous mitotic defects like mitotic asynchrony, mitotic catastrophe and chromatid bridges in the syncytial embryos. Such abnormal nuclei are eliminated and digested in the yolk tissues by nuclear fall out mechanism. MOF negatively regulates Drosophila checkpoint kinase 2 tumor suppressor homologue. In response to DNA damage the checkpoint gene Chk2 (Drosophila mnk) is activated in the mof mutants, there by causing centrosomal inactivation suggesting its role in response to genotoxic stress. A drastic decrease in the fall out nuclei in the syncytial embryos derived from mof¹/+; mnk^p6/+ females further confirms the role of DNA damage response gene Chk2 to ensure the removal of abnormal nuclei from the embryonic precursor pool and maintain genome stability. The fact that mof mutants undergo DNA damage has been further elucidated by the increased number of single and double stranded DNA breaks.

Conclusion

mof mutants exhibited genomic instability as evidenced by the occurance of frequent mitotic bridges in anaphase, asynchronous nuclear divisions, disruption of cytoskeleton, inactivation of centrosomes finally leading to DNA damage. Our findings are consistent to what has been reported earlier in mammals that; reduced levels of MOF resulted in increased genomic instability while total loss resulted in lethality. The study can be further extended using Drosophila as model system and carry out the interaction of MOF with the known components of the DNA damage pathway.

DNA damage, chromatin, and transcription: the trinity of aging

Aging brings about numerous cellular defects. Amongst the most prominent are elevated levels of persistent DNA damage, changes to chromatin structure and epigenetic modifications, and alterations of global transcription programs. These are not independent events and recent work begins to shed light on the intricate interplay between these aging-related defects.