ATM- and cell cycle-dependent regulation of ATR in response to DNA double-strand breaks

The control of DNA repair by the cell cycle

The correct duplication and transmission of genetic material to daughter cells is the primary objective of the cell division cycle. DNA replication and chromosome segregation present both challenges and opportunities for DNA repair pathways that safeguard genetic information. As a consequence, there is a profound, two-way connection between DNA repair and cell cycle control. Here, we review how DNA repair processes, and DNA double-strand break repair in particular, are regulated during the cell cycle to optimize genomic integrity.

Targeting Ongoing DNA Damage in Multiple Myeloma: Effects of DNA Damage Response Inhibitors on Plasma Cell Survival

Human myeloma cell lines (HMCLs) and a subset of myeloma patients with poor prognosis exhibit high levels of replication stress (RS), leading to DNA damage. In this study, we confirmed the presence of DNA double-strand breaks (DSBs) in several HMCLs by measuring γH2AX and RAD51 foci and analyzed the effect of various inhibitors of the DNA damage response on MM cell survival. Inhibition of ataxia telangiectasia and Rad3-related protein (ATR), the main kinase mediating the response to RS, using the specific inhibitor VE-821 induced more cell death in HMCLs than in control lymphoblastoid cells and U266, an HMCL with a low level of DNA damage. The absence of ATR was partially compensated by ataxia telangiectasia-mutated protein (ATM), since chemical inhibition of both kinases using VE-821 and KU-55933 significantly increased the death of MM cells with DNA damage. We found that ATM and ATR are involved in DSB repair by homologous recombination (HR) in MM. Inhibition of both kinases resulted in a stronger inhibition that may underlie cell death induction, since abolition of HR using two different inhibitors severely reduced survival of HMCLs that exhibit DNA damage. On the other hand, inhibition of the other route involved in DSB repair, non-homologous end joining (NHEJ), using the DNA-PK inhibitor NU7441, did not affect MM cell viability. Interestingly, we found that NHEJ inhibition did not increase cell death when HR was simultaneously inhibited with the RAD51 inhibitor B02, but it clearly increased the level of cell death when HR was inhibited with the MRE11 inhibitor mirin, which interferes with recombination before DNA resection takes place. Taken together, our results demonstrate for the first time that MM cells with ongoing DNA damage rely on an intact HR pathway, which thereby suggests therapeutic opportunities. We also show that inhibition of HR after the initial step of end resection might be more appropriate for inducing MM cell death, since it prevents the occurrence of a compensatory NHEJ repair mechanism. These preclinical observations provide the rationale for its clinical evaluation.

Non-homologous DNA end joining and alternative pathways to double-strand break repair

DNA double-strand breaks (DSBs) are the most dangerous type of DNA damage because they can result in the loss of large chromosomal regions. In all mammalian cells, DSBs that occur throughout the cell cycle are repaired predominantly by the non-homologous DNA end joining (NHEJ) pathway. Defects in NHEJ result in sensitivity to ionizing radiation and the ablation of lymphocytes. The NHEJ pathway utilizes proteins that recognize, resect, polymerize and ligate the DNA ends in a flexible manner. This flexibility permits NHEJ to function on a wide range of DNA-end configurations, with the resulting repaired DNA junctions often containing mutations. In this Review, we discuss the most recent findings regarding the relative involvement of the different NHEJ proteins in the repair of various DNA-end configurations. We also discuss the shunting of DNA-end repair to the auxiliary pathways of alternative end joining (a-EJ) or single-strand annealing (SSA) and the relevance of these different pathways to human disease.

A Liposome Encapsulated Ruthenium Polypyridine Complex as a Theranostic Platform for Triple-Negative Breast Cancer

Ruthenium coordination complexes have the potential to serve as novel theranostic agents for cancer. However, a major limitation in their clinical implementation is effective tumor accumulation. In this study, we have developed a liposome-based theranostic nanodelivery system for [Ru(phen)2dppz](ClO4)2 (Lipo-Ru). This ruthenium polypyridine complex emits a strong fluorescent signal when incorporated in the hydrophobic lipid bilayer of the delivery vehicle or in the DNA helix, enabling visualization of the therapeutic agent in tumor tissues. Incubation of MDA-MB-231 breast cancer cells with Lipo-Ru induced double-strand DNA breaks and triggers apoptosis. In a mouse model of triple-negative breast cancer, treatment with Lipo-Ru dramatically reduced tumor growth. Biodistribution studies of Lipo-Ru revealed that more than 20% of the injected dose accumulated in the tumor. These results suggest that Lipo-Ru could serve as a promising theranostic platform for cancer.

The G2 checkpoint-a node-based molecular switch

Tight regulation of the eukaryotic cell cycle is paramount to ensure genomic integrity throughout life. Cell cycle checkpoints are present in each phase of the cell cycle and prevent cell cycle progression when genomic integrity is compromised. The G2 checkpoint is an intricate signaling network that regulates the progression of G2 to mitosis (M). We propose here a node-based model of G2 checkpoint regulation, in which the action of the central CDK1-cyclin B1 node is determined by the concerted but opposing activities of the Wee1 and cell division control protein 25C (CDC25C) nodes. Phosphorylation of both Wee1 and CDC25C at specific sites determines their subcellular localization, driving them either toward activity within the nucleus or to the cytoplasm and subsequent ubiquitin-mediated proteasomal degradation. In turn, this subcellular balance of the Wee1 and CDC25C nodes is directed by the action of the PLK1 and CHK1 nodes via what we have termed the 'nuclear and cytoplasmic decision states' of Wee1 and CDC25C. The proposed node-based model provides an intelligible structure of the complex interactions that govern the decision to delay or continue G2/M progression. The model may also aid in predicting the effects of agents that target these G2 checkpoint nodes.

The Checkpoint Kinase 1 Inhibitor Prexasertib Induces Regression of Preclinical Models of Human Neuroblastoma

Purpose: Checkpoint kinase 1 (CHK1) is a key regulator of the DNA damage response and a mediator of replication stress through modulation of replication fork licensing and activation of S and G₂-M cell-cycle checkpoints. We evaluated prexasertib (LY2606368), a small-molecule CHK1 inhibitor currently in clinical testing, in multiple preclinical models of pediatric cancer. Following an initial assessment of prexasertib activity, this study focused on the preclinical models of neuroblastoma.Experimental Design: We evaluated the antiproliferative activity of prexasertib in a panel of cancer cell lines; neuroblastoma cell lines were among the most sensitive. Subsequent Western blot and immunofluorescence analyses measured DNA damage and DNA repair protein activation. Prexasertib was investigated in several cell line-derived xenograft mouse models of neuroblastoma.Results: Within 24 hours, single-agent prexasertib promoted γH2AX-positive double-strand DNA breaks and phosphorylation of DNA damage sensors ATM and DNA-PKcs, leading to neuroblastoma cell death. Knockdown of CHK1 and/or CHK2 by siRNA verified that the double-strand DNA breaks and cell death elicited by prexasertib were due to specific CHK1 inhibition. Neuroblastoma xenografts rapidly regressed following prexasertib administration, independent of starting tumor volume. Decreased Ki67 and increased immunostaining of endothelial and pericyte markers were observed in xenografts after only 6 days of exposure to prexasertib, potentially indicating a swift reduction in tumor volume and/or a direct effect on tumor vasculature.Conclusions: Overall, these data demonstrate that prexasertib is a specific inhibitor of CHK1 in neuroblastoma and leads to DNA damage and cell death in preclinical models of this devastating pediatric malignancy. Clin Cancer Res; 23(15); 4354-63. ©2017 AACR.

The Intra-S Checkpoint Responses to DNA Damage

Faithful duplication of the genome is a challenge because DNA is susceptible to damage by a number of intrinsic and extrinsic genotoxins, such as free radicals and UV light. Cells activate the intra-S checkpoint in response to damage during S phase to protect genomic integrity and ensure replication fidelity. The checkpoint prevents genomic instability mainly by regulating origin firing, fork progression, and transcription of G1/S genes in response to DNA damage. Several studies hint that regulation of forks is perhaps the most critical function of the intra-S checkpoint. However, the exact role of the checkpoint at replication forks has remained elusive and controversial. Is the checkpoint required for fork stability, or fork restart, or to prevent fork reversal or fork collapse, or activate repair at replication forks? What are the factors that the checkpoint targets at stalled replication forks? In this review, we will discuss the various pathways activated by the intra-S checkpoint in response to damage to prevent genomic instability.

p53 and RAD9, the DNA Damage Response, and Regulation of Transcription Networks

The way cells respond to DNA damage is important since inefficient repair or misrepair of lesions can have deleterious consequences, including mutation, genomic instability, neurodegenerative disorders, premature aging, cancer or death. Whether damage occurs spontaneously as a byproduct of normal metabolic processes, or after exposure to exogenous agents, cells muster a coordinated, complex DNA damage response (DDR) to mitigate potential harmful effects. A variety of activities are involved to promote cell survival, and include DNA repair, DNA damage tolerance, as well as transient cell cycle arrest to provide time for repair before entry into critical cell cycle phases, an event that could be lethal if traversal occurs while damage is present. When such damage is prolonged or not repairable, senescence, apoptosis or autophagy is induced. One major level of DDR regulation occurs via the orchestrated transcriptional control of select sets of genes encoding proteins that mediate the response. p53 is a transcription factor that transactivates specific DDR downstream genes through binding DNA consensus sequences usually in or near target gene promoter regions. The profile of p53-regulated genes activated at any given time varies, and is dependent upon type of DNA damage or stress experienced, exact composition of the consensus DNA binding sequence, presence of other DNA binding proteins, as well as cell context. RAD9 is another protein critical for the response of cells to DNA damage, and can also selectively regulate gene transcription. The limited studies addressing the role of RAD9 in transcription regulation indicate that the protein transactivates at least one of its target genes, p21/waf1/cip1, by binding to DNA sequences demonstrated to be a p53 response element. NEIL1 is also regulated by RAD9 through a similar DNA sequence, though not yet directly verified as a bonafide p53 response element. These findings suggest a novel pathway whereby p53 and RAD9 control the DDR through a shared mechanism involving an overlapping network of downstream target genes. Details and unresolved questions about how these proteins coordinate or compete to execute the DDR through transcriptional reprogramming, as well as biological implications, are discussed.

Repair, Reuse, Recycle: The Expanding Role of Autophagy in Genome Maintenance

(Macro)Autophagy is a catabolic pathway that delivers excess, aggregated, or damaged proteins and organelles to lysosomes for degradation. Autophagy is activated in response to numerous cellular stressors such as increased levels of reactive oxygen species (ROS) and low levels of cellular nutrients as well as DNA damage. Although autophagy occurs in the cytoplasm, its inhibition leads to accumulation of DNA damage and genomic instability. In the past few years, our understanding of the interplay between autophagy and genomic stability has greatly increased. In this review we summarize these recent advances in understanding the molecular mechanisms linking autophagy to DNA repair.

Oridonin promotes G2/M arrest in A549 cells by facilitating ATM activation

Previous studies have demonstrated that oridonin, a tetracyclic diterpenoid compound extracted from Rabdosia rubescens, inhibits proliferation and induces apoptosis in several tumor cell lines. However, the mechanism by which oridonin inhibits the cell cycle remains poorly understood. In the present study, possible mechanisms by which oridonin affects cell cycle progression were explored in A549 lung cancer cells. Flow cytometry analysis indicated that oridonin inhibited the proliferation of A549 cells by inducing G2/M cell cycle arrest in a dose‑dependent manner. Western blot analysis revealed that in oridonin treated cells, phosphorylated (p‑)ATM serine/threonine kinase (S1981), p‑checkpoint kinase 2 (CHK2) (T68), p‑p53, and phosphorylated H2A histone family member X protein levels were visibly increased, indicating that oridonin promoted G2/M arrest in A549 cells through the ATM‑p53‑CHK2 pathway. This data suggests that oridonin promotes G2/M arrest in A549 cells by facilitating ATM activation, which is likely a common mechanism in other tumor cell types when using this drug for cancer treatment.

Regulation of non-homologous end joining via post-translational modifications of components of the ligation step

DNA double-strand breaks are the most serious type of DNA damage and non-homologous end joining (NHEJ) is an important pathway for their repair. In Saccharomyces cerevisiae, three complexes mediate the canonical NHEJ pathway, Ku (Ku70/Ku80), MRX (Mre11/Rad50/Xrs2) and DNA ligase IV (Dnl4/Lif1). Mammalian NHEJ is more complex, primarily as a consequence of the fact that more factors are involved in the process, and also because higher chromatin organization and more complex regulatory networks exist in mammals. In addition, a stronger interconnection between the NHEJ and DNA damage response (DDR) pathways seems to occur in mammals compared to yeast. DDR employs multiple post-translational modifications (PTMs) of the target proteins and mutual crosstalk among them to ensure highly efficient down-stream effects. Checkpoint-mediated phosphorylation is the best understood PTM that regulates DDR, although recently SUMOylation has also been shown to be involved. Both phosphorylation and SUMOylation affect components of NHEJ. In this review, we discuss a role of these two PTMs in regulation of NHEJ via targeting the components of the ligation step.

PCNA-Ub polyubiquitination inhibits cell proliferation and induces cell-cycle checkpoints

In response to replication-blocking lesions, proliferating cell nuclear antigen (PCNA) can be sequentially ubiquitinated at the K164 residue leading to 2 modes of DNA-damage tolerance, namely translesion DNA synthesis (TLS) and error-free lesion bypass. Ectopic expression of PCNA fused with ubiquitin (Ub) lacking the 2 C-terminal Gly residues resembles PCNA monoubiquitination-mediated TLS. However, if the fused Ub contains C-terminal Gly residues, it is further polyubiquitinated and inhibits cell proliferation. Unexpectedly, the polyubiquitination chain does not require any surface Lys residues and is likely to be head-to-tail linked. Such PCNA polyubiquitination interferes with replication, arrests cells at the S-phase and activates the p53 checkpoint pathway. The above cell-cycle arrest is reversible in an ATR-dependent manner, as simultaneous inhibition of ATR, but not ATM, induces apoptosis. Since ectopic expression of PCNA-Ub also induces double-strand breaks that colocalize with single-stranded DNA, we infer that this non-canonical PCNA poly-Ub chain serves as a signal to activate ATR checkpoint and recruit double-strand-break repair apparatus.

Coupling end resection with the checkpoint response at DNA double-strand breaks

DNA double-strand breaks (DSBs) are a nasty form of damage that needs to be repaired to ensure genome stability. The DSB ends can undergo a strand-biased nucleolytic processing (resection) to generate 3'-ended single-stranded DNA (ssDNA) that channels DSB repair into homologous recombination. Generation of ssDNA also triggers the activation of the DNA damage checkpoint, which couples cell cycle progression with DSB repair. The checkpoint response is intimately linked to DSB resection, as some checkpoint proteins regulate the resection process. The present review will highlight recent works on the mechanism and regulation of DSB resection and its interplays with checkpoint activation/inactivation in budding yeast.

Common genetic variations in cell cycle and DNA repair pathways associated with pediatric brain tumor susceptibility

Knowledge on the role of genetic polymorphisms in the etiology of pediatric brain tumors (PBTs) is limited. Therefore, we investigated the association between single nucleotide polymorphisms (SNPs), identified by candidate gene-association studies on adult brain tumors, and PBT risk.The study is based on the largest series of PBT cases to date. Saliva DNA from 245 cases and 489 controls, aged 7-19 years at diagnosis/reference date, was genotyped for 68 SNPs. Data were analyzed using unconditional logistic regression.The results showed EGFRrs730437 and EGFRrs11506105 may decrease susceptibility to PBTs, whereas ERCC1rs3212986 may increase risk of these tumors. Moreover, stratified analyses indicated CHAF1Ars243341, CHAF1Ars2992, and XRCC1rs25487 were associated with a decreased risk of astrocytoma subtype. Furthermore, an increased risk of non-astrocytoma subtype associated with EGFRrs9642393, EME1rs12450550, ATMrs170548, and GLTSCRrs1035938 as well as a decreased risk of this subtype associated with XRCC4rs7721416 and XRCC4rs2662242 were detected.This study indicates SNPs in EGFR, ERCC1, CHAF1A, XRCC1, EME1, ATM, GLTSCR1, and XRCC4 may be associated with the risk of PBTs. Therefore, cell cycle and DNA repair pathways variations associated with susceptibility to adult brain tumors also seem to be associated with PBT risk, suggesting pediatric and adult brain tumors might share similar etiological pathways.

Influence of Human p53 on Plant Development

Mammalian p53 is a super tumor suppressor and plays a key role in guarding genome from DNA damage. However, p53 has not been found in plants which do not bear cancer although they constantly expose to ionizing radiation of ultraviolet light. Here we introduced p53 into the model plant Arabidopsis and examined p53-conferred phenotype in plant. Most strikingly, p53 caused early senescence and fasciation. In plants, fasciation has been shown as a result of the elevated homologous DNA recombination. Consistently, a reporter with overlapping segments of the GUS gene (1445) showed that the frequency of homologous recombination was highly induced in p53-transgenic plants. In contrast to p53, SUPPRESSOR OF NPR1-1 INDUCIBLE 1 (SNI1), as a negative regulator of homologous recombination in plants, is not present in mammals. Comet assay and clonogenic survival assay demonstrated that SNI1 inhibited DNA damage repair caused by either ionizing radiation or hydroxyurea in human osteosarcoma U2OS cancer cells. RAD51D is a recombinase in homologous recombination and functions downstream of SNI1 in plants. Interestingly, p53 rendered the sni1 mutants madly branching of inflorescence, a phenotype of fasciation, whereas rad51d mutant fully suppressed the p53-induced phenotype, indicating that human p53 action in plant is mediated by the SNI1-RAD51D signaling pathway. The reciprocal species-swap tests of p53 and SNI1 in human and Arabidopsis manifest that these species-specific proteins play a common role in homologous recombination across kingdoms of animals and plants.

Cancer cells that survive checkpoint adaptation contain micronuclei that harbor damaged DNA

We have examined the relationship between checkpoint adaptation (mitosis with damaged DNA) and micronuclei. Micronuclei in cancer cells are linked to genomic change, and may induce chromothripsis (chromosome shattering). We measured the cytotoxicity of the cancer drug cisplatin in M059K (glioma fibroblasts, IC50 15 μM). Nearly 100% of M059K cells were positive for histone γH2AX staining after 48 h treatment with a cytotoxic concentration of cisplatin. The proportion of micronucleated cells, as confirmed by microscopy using DAPI and lamin A/C staining, increased from 24% to 48%, and the total micronuclei in surviving cells accumulated over time. Promoting entry into mitosis with a checkpoint inhibitor increased the number of micronuclei in cells whereas blocking checkpoint adaptation with a Cdk inhibitor reduced the number of micronuclei. Interestingly, some micronuclei underwent asynchronous DNA replication, relative to the main nuclei, as measured by deoxy-bromo-uracil (BrdU) staining. These micronuclei stained positive for histone γH2AX, which was linked to DNA replication, suggesting that micronuclei arise from checkpoint adaptation and that micronuclei may continue to damage DNA. By contrast the normal cell line WI-38 did not undergo checkpoint adaptation when treated with cisplatin and did not show changes in micronuclei number. These data reveal that the production of micronuclei by checkpoint adaptation is part of a process that contributes to genomic change.

Functions, Regulation, and Therapeutic Implications of the ATR Checkpoint Pathway

The ATR (ATM and rad3-related) pathway is crucial for proliferation, responding to DNA replication stress and DNA damage. This critical signaling pathway is carefully orchestrated through a multistep process requiring initial priming of ATR prior to damage, recruitment of ATR to DNA damage lesions, activation of ATR signaling, and, finally, modulation of ATR activity through a variety of post-translational modifications. Following activation, ATR functions in several vital cellular processes, including suppression of replication origin firing, promotion of deoxynucleotide synthesis and replication fork restart, prevention of double-stranded DNA break formation, and avoidance of replication catastrophe and mitotic catastrophe. In many cancers, tumor cells have increased dependence on ATR signaling for survival, making ATR a promising target for cancer therapy. Tumor cells compromised in DNA repair pathways or DNA damage checkpoints, cells reliant on homologous recombination, and cells with increased replication stress are particularly sensitive to ATR inhibition. Understanding ATR signaling and modulation is essential to unraveling which tumors have increased dependence on ATR signaling as well as how the ATR pathway can best be exploited for targeted cancer therapy.

Mdc1 modulates the interaction between TopBP1 and the MRN complex during DNA damage checkpoint responses

TopBP1 has been identified as a direct activator of ATR and interacts with the Nbs1 subunit of the Mre11-Rad50-Nbs1 (MRN) complex in egg extracts in a checkpoint-regulated manner. In this study, we show that Mdc1 associates with both TopBP1 and Nbs1 in egg extracts and human cells. We cloned a cDNA encoding the full-length version of Xenopus Mdc1. The association between Mdc1 and TopBP1 involves the first pair of BRCT repeats in TopBP1. The N-terminal region (161-230) of Mdc1 is required for this binding. The interaction between Mdc1 and Nbs1 involves the two tandem BRCT repeats of Nbs1. Functional studies with mutated forms of Mdc1, TopBP1, and Nbs1 indicate that the BRCT-dependent association of these proteins is critical for a normal checkpoint response to DSBs. TopBP1 cannot interact with Nbs1 in Mdc1-depleted egg extracts, suggesting that Mdc1 connects TopBP1 and Nbs1 together. These findings suggest that Mdc1 is a crucial mediator of the DNA damage checkpoint response.

DDB2 increases radioresistance of NSCLC cells by enhancing DNA damage responses

Radiotherapy resistance is one of the major factors limiting the efficacy of radiotherapy in lung cancer patients. The extensive investigations indicate the diversity in the mechanisms underlying radioresistance. Here, we revealed that DNA damage binding protein 2 (DDB2) is a potential regulator in the radiosensitivity of non-small cell lung cancer (NSCLC) cells. DDB2, originally identified as a DNA damage recognition factor in the nucleotide excision repair, promotes the survival and inhibits the apoptosis of NSCLC cell lines upon ionizing radiation (IR). Mechanistic investigations demonstrated that DDB2 is able to facilitate IR-induced phosphorylation of Chk1, which plays a critical role in the cell cycle arrest and DNA repair in response to IR-induced DNA double-strand breaks (DSBs). Indeed, knockdown of DDB2 compromised the G2 arrest in the p53-proficient A549 cell line and reduced the efficiency of homologous recombination (HR) repair. Taken together, our data indicate that the expression of DDB2 in NSCLC could be used as a biomarker to predict radiosensitivity of the patients. Targeting Chk1 can be used to increase the efficacy of radiotherapy in patients of NSCLC possessing high levels of DDB2.

The Transcriptional Response to DNA-Double-Strand Breaks in Physcomitrella patens

The model bryophyte Physcomitrella patens is unique among plants in supporting the generation of mutant alleles by facile homologous recombination-mediated gene targeting (GT). Reasoning that targeted transgene integration occurs through the capture of transforming DNA by the homology-dependent pathway for DNA double-strand break (DNA-DSB) repair, we analysed the genome-wide transcriptomic response to bleomycin-induced DNA damage and generated mutants in candidate DNA repair genes. Massively parallel (Illumina) cDNA sequencing identified potential participants in gene targeting. Transcripts encoding DNA repair proteins active in multiple repair pathways were significantly up-regulated. These included Rad51, CtIP, DNA ligase 1, Replication protein A and ATR in homology-dependent repair, Xrcc4, DNA ligase 4, Ku70 and Ku80 in non-homologous end-joining and Rad1, Tebichi/polymerase theta, PARP in microhomology-mediated end-joining. Differentially regulated cell-cycle components included up-regulated Rad9 and Hus1 DNA-damage-related checkpoint proteins and down-regulated D-type cyclins and B-type CDKs, commensurate with the imposition of a checkpoint at G2 of the cell cycle characteristic of homology-dependent DNA-DSB repair. Candidate genes, including ATP-dependent chromatin remodelling helicases associated with repair and recombination, were knocked out and analysed for growth defects, hypersensitivity to DNA damage and reduced GT efficiency. Targeted knockout of PpCtIP, a cell-cycle activated mediator of homology-dependent DSB resection, resulted in bleomycin-hypersensitivity and greatly reduced GT efficiency.

The DNA damage-induced cell death response: a roadmap to kill cancer cells

Upon massive DNA damage cells fail to undergo productive DNA repair and trigger the cell death response. Resistance to cell death is linked to cellular transformation and carcinogenesis as well as radio- and chemoresistance, making the underlying signaling pathways a promising target for therapeutic intervention. Diverse DNA damage-induced cell death pathways are operative in mammalian cells and finally culminate in the induction of programmed cell death via activation of apoptosis or necroptosis. These signaling routes affect nuclear, mitochondria- and plasma membrane-associated key molecules to activate the apoptotic or necroptotic response. In this review, we highlight the main signaling pathways, molecular players and mechanisms guiding the DNA damage-induced cell death response.

BTK Modulates p53 Activity to Enhance Apoptotic and Senescent Responses

p53 is a tumor suppressor that prevents the emergence of transformed cells by inducing apoptosis or senescence, among other responses. Its functions are regulated tightly by posttranslational modifications. Here we show that Bruton's tyrosine kinase (BTK) is a novel modulator of p53. We found that BTK is induced in response to DNA damage and p53 activation. BTK induction leads to p53 phosphorylation, which constitutes a positive feedback loop that increases p53 protein levels and enhances the transactivation of its target genes in response to stress. Inhibiting BTK reduced both p53-dependent senescence and apoptosis. Further, BTK expression also upregulated DNA damage signals and apoptosis. We conclude that despite being involved in oncogenic signals in blood malignancies, BTK has antineoplastic properties in other contexts, such as the enhancement of p53's tumor suppressor responses. Along with evidence that BTK expression correlates with good prognosis in some epithelial tumors, our findings may encourage a reevaluation of the clinical uses of BTK inhibitors in cancer therapy. Cancer Res; 76(18); 5405-14. ©2016 AACR.

Targeting WEE1 Kinase in Cancer

WEE1 kinase plays a crucial role in the G2-M cell-cycle checkpoint arrest for DNA repair before mitotic entry. Normal cells repair damaged DNA during G1 arrest; however, cancer cells often have a deficient G1-S checkpoint and depend on a functional G2-M checkpoint for DNA repair. WEE1 is expressed at high levels in various cancer types including breast cancers, leukemia, melanoma, and adult and pediatric brain tumors. Many of these cancers are treated with DNA-damaging agents; therefore, targeting WEE1 for inhibition and compromising the G2-M checkpoint presents an opportunity to potentiate therapy. In this review we summarize the current WEE1 inhibitors, the potential for further inhibitor development, and the challenges in the clinic for the WEE1 inhibitor strategy.

Sharpening the ends for repair: mechanisms and regulation of DNA resection

DNA end resection is a key process in the cellular response to DNA double-strand break damage that is essential for genome maintenance and cell survival. Resection involves selective processing of 5' ends of broken DNA to generate ssDNA overhangs, which in turn control both DNA repair and checkpoint signaling. DNA resection is the first step in homologous recombination-mediated repair and a prerequisite for the activation of the ataxia telangiectasia mutated and Rad3-related (ATR)-dependent checkpoint that coordinates repair with cell cycle progression and other cellular processes. Resection occurs in a cell cycle-dependent manner and is regulated by multiple factors to ensure an optimal amount of ssDNA required for proper repair and genome stability. Here, we review the latest findings on the molecular mechanisms and regulation of the DNA end resection process and their implications for cancer formation and treatment.

Distinct genetic control of homologous recombination repair of Cas9-induced double-strand breaks, nicks and paired nicks

DNA double-strand breaks (DSBs) are known to be powerful inducers of homologous recombination (HR), but single-strand breaks (nicks) have also been shown to trigger HR. Both DSB- and nick-induced HR ((nick)HR) are exploited in advanced genome-engineering approaches based on the bacterial RNA-guided nuclease Cas9. However, the mechanisms of (nick)HR are largely unexplored. Here, we applied Cas9 nickases to study (nick)HR in mammalian cells. We find that (nick)HR is unaffected by inhibition of major damage signaling kinases and that it is not suppressed by nonhomologous end-joining (NHEJ) components, arguing that nick processing does not require a DSB intermediate to trigger HR. Relative to a single nick, nicking both strands enhances HR, consistent with a DSB intermediate, even when nicks are induced up to ∼1kb apart. Accordingly, HR and NHEJ compete for repair of these paired nicks, but, surprisingly, only when 5' overhangs or blunt ends can be generated. Our study advances the understanding of molecular mechanisms driving nick and paired-nick repair in mammalian cells and clarify phenomena associated with Cas9-mediated genome editing.

Hsp90 induces increased genomic instability toward DNA-damaging agents by tuning down RAD53 transcription

The molecular mechanism behind hyperthermia coupled to radiation-induced DNA damage sensitivity is not known. The model organism Saccharomyces cerevisiae is used to establish that a transient heat shock and particularly the concomitant induction of Hsp90 lead to increased genomic instability via transcriptional regulation of the major checkpoint kinase Rad53.

It is well documented that elevated body temperature causes tumors to regress upon radiotherapy. However, how hyperthermia induces DNA damage sensitivity is not clear. We show that a transient heat shock and particularly the concomitant induction of Hsp90 lead to increased genomic instability under DNA-damaging conditions. Using Saccharomyces cerevisiae as a model eukaryote, we demonstrate that elevated levels of Hsp90 attenuate efficient DNA damage signaling and dictate preferential use of the potentially mutagenic double-strand break repair pathway. We show that under normal physiological conditions, Hsp90 negatively regulates RAD53 transcription to suppress DNA damage checkpoint activation. However, under DNA damaging conditions, RAD53 is derepressed, and the increased level of Rad53p triggers an efficient DNA damage response. A higher abundance of Hsp90 causes increased transcriptional repression on RAD53 in a dose-dependent manner, which could not be fully derepressed even in the presence of DNA damage. Accordingly, cells behave like a rad53 loss-of-function mutant and show reduced NHEJ efficiency, with a drastic failure to up-regulate RAD51 expression and manifestly faster accumulation of CLN1 and CLN2 in DNA-damaged G1, cells leading to premature release from checkpoint arrest. We further demonstrate that Rad53 overexpression is able to rescue all of the aforementioned deleterious effects caused by Hsp90 overproduction.

DNA Damage Response in Hematopoietic Stem Cell Ageing

Maintenance of tissue-specific stem cells is vital for organ homeostasis and organismal longevity. Hematopoietic stem cells (HSCs) are the most primitive cell type in the hematopoietic system. They divide asymmetrically and give rise to daughter cells with HSC identity (self-renewal) and progenitor progenies (differentiation), which further proliferate and differentiate into full hematopoietic lineages. Mammalian ageing process is accompanied with abnormalities in the HSC self-renewal and differentiation. Transcriptional changes and epigenetic modulations have been implicated as the key regulators in HSC ageing process. The DNA damage response (DDR) in the cells involves an orchestrated signaling pathway, consisting of cell cycle regulation, cell death and senescence, transcriptional regulation, as well as chromatin remodeling. Recent studies employing DNA repair-deficient mouse models indicate that DDR could intrinsically and extrinsically regulate HSC maintenance and play important roles in tissue homeostasis of the hematopoietic system. In this review, we summarize the current understanding of how the DDR determines the HSC fates and finally contributes to organismal ageing.

The shelterin complex and hematopoiesis

Mammalian chromosomes terminate in stretches of repetitive telomeric DNA that act as buffers to avoid loss of essential genetic information during end-replication. A multiprotein complex known as shelterin prevents recognition of telomeric sequences as sites of DNA damage. Telomere erosion contributes to human diseases ranging from BM failure to premature aging syndromes and cancer. The role of shelterin telomere protection is less understood. Mutations in genes encoding the shelterin proteins TRF1-interacting nuclear factor 2 (TIN2) and adrenocortical dysplasia homolog (ACD) were identified in dyskeratosis congenita, a syndrome characterized by somatic stem cell dysfunction in multiple organs leading to BM failure and other pleiotropic manifestations. Here, we introduce the biochemical features and in vivo effects of individual shelterin proteins, discuss shelterin functions in hematopoiesis, and review emerging knowledge implicating the shelterin complex in hematological disorders.

Torin2 Suppresses Ionizing Radiation-Induced DNA Damage Repair

Several classes of inhibitors of the mammalian target of rapamycin (mTOR) have been developed based on its central role in sensing growth factor and nutrient levels to regulate cellular metabolism. However, its ATP-binding site closely resembles other phosphatidylinositol 3-kinase-related kinase (PIKK) family members, resulting in reactivity with these targets that may also be therapeutically useful. The ATP-competitive mTOR inhibitor, Torin2, shows biochemical activity against the DNA repair-associated proteins ATM, ATR and DNA-PK, which raises the possibility that Torin2 and related compounds might radiosensitize cancerous tumors. In this study Torin2 was also found to enhance ionizing radiation-induced cell killing in conditions where ATM was dispensable, confirming the requirement for multiple PIKK targets. Moreover, Torin2 did not influence the initial appearance of γ-H2AX foci after irradiation but significantly delayed the disappearance of radiation-induced γ-H2AX foci, indicating a DNA repair defect. Torin2 increased the number of radiation-induced S-phase specific chromosome aberrations and reduced the frequency of radiation-induced CtIP and Rad51 foci formation, suggesting that Torin2 works by blocking homologous recombination (HR)-mediated DNA repair resulting in an S-phase specific DNA repair defect. Accordingly, Torin2 reduced HR-mediated repair of I-Sce1-induced DNA damage and contributed to replication fork stalling. We conclude that radiosensitization of tumor cells by Torin2 is associated with disrupting ATR- and ATM-dependent DNA damage responses. Our findings support the concept of developing combination cancer therapies that incorporate ionizing radiation therapy and Torin2 or compounds with similar properties.

Balancing self-renewal against genome preservation in stem cells: How do they manage to have the cake and eat it too?

Stem cells are endowed with the awesome power of self-renewal and multi-lineage differentiation that allows them to be major contributors to tissue homeostasis. Owing to their longevity and self-renewal capacity, they are also faced with a higher risk of genomic damage compared to differentiated cells. Damage on the genome, if not prevented or repaired properly, will threaten the survival of stem cells and culminate in organ failure, premature aging, or cancer formation. It is therefore of paramount importance that stem cells remain genomically stable throughout life. Given their unique biological and functional requirement, stem cells are thought to manage genotoxic stress somewhat differently from non-stem cells. The focus of this article is to review the current knowledge on how stem cells escape the barrage of oxidative and replicative DNA damage to stay in self-renewal. A clear statement on this subject should help us better understand tissue regeneration, aging, and cancer.

The structure of ends determines the pathway choice and Mre11 nuclease dependency of DNA double-strand break repair

The key event in the choice of repair pathways for DNA double-strand breaks (DSBs) is the initial processing of ends. Non-homologous end joining (NHEJ) involves limited processing, but homology-dependent repair (HDR) requires extensive resection of the 5′ strand. How cells decide if an end is channeled to resection or NHEJ is not well understood. We hypothesize that the structure of ends is a major determinant and tested this hypothesis with model DNA substrates in Xenopus egg extracts. While ends with normal nucleotides are efficiently channeled to NHEJ, ends with damaged nucleotides or bulky adducts are channeled to resection. Resection is dependent on Mre11, but its nuclease activity is critical only for ends with 5′ bulky adducts. CtIP is absolutely required for activating the nuclease-dependent mechanism of Mre11 but not the nuclease-independent mechanism. Together, these findings suggest that the structure of ends is a major determinant for the pathway choice of DSB repair and the Mre11 nuclease dependency of resection.

FHIT loss-induced DNA damage creates optimal APOBEC substrates: Insights into APOBEC-mediated mutagenesis

APOBEC cytidine deaminase activity is a major source of hypermutation in cancer. But previous studies have shown that the TC context signature of these enzymes is not observed in sizable fractions of cancers with overexpression of APOBEC, suggesting that cooperating factors that contribute to this mutagenesis should be identified. The fragile histidine triad protein (Fhit) is a tumor suppressor and DNA caretaker that is deleted or silenced in >50% of cancers. Loss of Fhit protein activity causes replication stress through reduced Thymidine Kinase 1 expression, increased DNA breaks, and global genome instability in normal and cancer cells. Using data from The Cancer Genome Atlas (TCGA), we show that FHIT-low/APOBEC3B-high expressing lung adenocarcinomas display significantly increased numbers of APOBEC signature mutations. Tumor samples in this cohort with normal FHIT expression do not exhibit APOBEC hypermutation, despite having high APOBEC3B expression. In vitro, silencing Fhit expression elevates APOBEC3B-directed C > T mutations in the TP53 gene. Furthermore, inhibition of Fhit loss-induced DNA damage via thymidine supplementation decreases the TP53 mutation burden in FHIT-low/APOBEC3B-high cells. We conclude that APOBEC3B overexpression and Fhit-loss induced DNA damage are independent events that, when occurring together, result in a significantly increased frequency of APOBEC-induced mutations that drive cancer progression.

Effects of hyperthermia as a mitigation strategy in DNA damage-based cancer therapies

Utilization of thermal therapy (hyperthermia) is defined as the application of exogenous heat induction and represents a concept that is far from new as it goes back to ancient times when heat was used for treating various diseases, including malignancies. Such therapeutic strategy has gained even more popularity (over the last few decades) since various studies have shed light into understanding hyperthermia's underlying molecular mechanism(s) of action. In general, hyperthermia is applied as complementary (adjuvant) means in therapeutic protocols combining chemotherapy and/or irradiation both of which can induce irreversible cellular DNA damage. Furthermore, according to a number of in vitro, in vivo and clinical studies, hyperthermia has been shown to enhance the beneficial effects of DNA targeting therapeutic strategies by interfering with DNA repair response cascades. Therefore, the continuously growing evidence supporting hyperthermia's beneficial role in cancer treatment can also encourage its application as a DNA repair mitigation strategy. In this review article, we aim to provide detailed information on how hyperthermia acts on DNA damage and repair pathways and thus potentially contributing to various adjuvant therapeutic protocols relevant to more efficient cancer treatment strategies.

The Adenovirus E4orf4 Protein Provides a Novel Mechanism for Inhibition of the DNA Damage Response

The DNA damage response (DDR) is a conglomerate of pathways designed to detect DNA damage and signal its presence to cell cycle checkpoints and to the repair machinery, allowing the cell to pause and mend the damage, or if the damage is too severe, to trigger apoptosis or senescence. Various DDR branches are regulated by kinases of the phosphatidylinositol 3-kinase-like protein kinase family, including ataxia-telangiectasia mutated (ATM) and ATM- and Rad3-related (ATR). Replication intermediates and linear double-stranded genomes of DNA viruses are perceived by the cell as DNA damage and activate the DDR. If allowed to operate, the DDR will stimulate ligation of viral genomes and will inhibit virus replication. To prevent this outcome, many DNA viruses evolved ways to limit the DDR. As part of its attack on the DDR, adenovirus utilizes various viral proteins to cause degradation of DDR proteins and to sequester the MRN damage sensor outside virus replication centers. Here we show that adenovirus evolved yet another novel mechanism to inhibit the DDR. The E4orf4 protein, together with its cellular partner PP2A, reduces phosphorylation of ATM and ATR substrates in virus-infected cells and in cells treated with DNA damaging drugs, and causes accumulation of damaged DNA in the drug-treated cells. ATM and ATR are not mutually required for inhibition of their signaling pathways by E4orf4. ATM and ATR deficiency as well as E4orf4 expression enhance infection efficiency. Furthermore, E4orf4, previously reported to induce cancer-specific cell death when expressed alone, sensitizes cells to killing by sub-lethal concentrations of DNA damaging drugs, likely because it inhibits DNA damage repair. These findings provide one explanation for the cancer-specificity of E4orf4-induced cell death as many cancers have DDR deficiencies leading to increased reliance on the remaining intact DDR pathways and to enhanced susceptibility to DDR inhibitors such as E4orf4. Thus DDR inhibition by E4orf4 contributes both to the efficiency of adenovirus replication and to the ability of E4orf4 to kill cancer cells.

ATRIP Deacetylation by SIRT2 Drives ATR Checkpoint Activation by Promoting Binding to RPA-ssDNA

The ataxia telangiectasia-mutated and Rad3-related (ATR) kinase checkpoint pathway maintains genome integrity; however, the role of the sirtuin 2 (SIRT2) acetylome in regulating this pathway is not clear. We found that deacetylation of ATR-interacting protein (ATRIP), a regulatory partner of ATR, by SIRT2 potentiates the ATR checkpoint. SIRT2 interacts with and deacetylates ATRIP at lysine 32 (K32) in response to replication stress. SIRT2 deacetylation of ATRIP at K32 drives ATR autophosphorylation and signaling and facilitates DNA replication fork progression and recovery of stalled replication forks. K32 deacetylation by SIRT2 further promotes ATRIP accumulation to DNA damage sites and binding to replication protein A-coated single-stranded DNA (RPA-ssDNA). Collectively, these results support a model in which ATRIP deacetylation by SIRT2 promotes ATR-ATRIP binding to RPA-ssDNA to drive ATR activation and thus facilitate recovery from replication stress, outlining a mechanism by which the ATR checkpoint is regulated by SIRT2 through deacetylation.

Sequential Combination Therapy of CDK Inhibition and Doxorubicin Is Synthetically Lethal in p53-Mutant Triple-Negative Breast Cancer

Triple-negative breast cancer (TNBC) is an aggressive malignancy in which the tumors lack expression of estrogen receptor, progesterone receptor, and HER2. Hence, TNBC patients cannot benefit from clinically available targeted therapies and rely on chemotherapy and surgery for treatment. While initially responding to chemotherapy, TNBC patients are at increased risk of developing distant metastasis and have decreased overall survival compared with non-TNBC patients. A majority of TNBC tumors carry p53 mutations, enabling them to bypass the G1 checkpoint and complete the cell cycle even in the presence of DNA damage. Therefore, we hypothesized that TNBC cells are sensitive to cell-cycle-targeted combination therapy, which leaves nontransformed cells unharmed. Our findings demonstrate that sequential administration of the pan-CDK inhibitor roscovitine before doxorubicin treatment is synthetically lethal explicitly in TNBC cells. Roscovitine treatment arrests TNBC cells in the G2-M cell-cycle phase, priming them for DNA damage. Combination treatment increased frequency of DNA double-strand breaks, while simultaneously reducing recruitment of homologous recombination proteins compared with doxorubicin treatment alone. Furthermore, this combination therapy significantly reduced tumor volume and increased overall survival compared with single drug or concomitant treatment in xenograft studies. Examination of isogenic immortalized human mammary epithelial cells and isogenic tumor cell lines found that abolishment of the p53 pathway is required for combination-induced cytotoxicity, making p53 a putative predictor of response to therapy. By exploiting the specific biologic and molecular characteristics of TNBC tumors, this innovative therapy can greatly impact the treatment and care of TNBC patients. Mol Cancer Ther; 15(4); 593-607. ©2016 AACR.

Bclaf1 is an important NF-κB signaling transducer and C/EBPβ regulator in DNA damage-induced senescence

Inducing senescence in cancer cells is an effective approach to suppress cancer growth, and it contributes significantly to the efficacy of therapeutic drugs. Previous studies indicated that transcription factors NF-κB (nuclear factor κ-light-chain-enhancer of activated B cells) and C/EBPβ (CCAAT/enhancer-binding protein-β) play a critical role in the establishment of senescence by upregulating proinflammatory cytokines, notably interleukin-6 (IL-6) and interleukin-8 (IL-8). However, it is not clear how these two factors are activated in response to senescence-inducing stimuli and subsequently regulate gene transcription. Here, we reveal Bcl-2-associated transcription factor 1 (Bclaf1) as a novel player in the therapeutic drug doxorubicin-induced senescence (TIS) in multiple cancer cells. Bclaf1 is upregulated through the ATM/Nemo/NF-κB pathway during TIS and is a direct target of p65 and c-Rel. The induction of Bclaf1 by NF-κB is essential for C/EBPβ upregulation and IL-6/IL-8 transcription during TIS. Bclaf1 can interact with the leucine zipper region of C/EBPβ and cooperate with C/EBPβ to upregulate IL-8. Furthermore, we show that Bclaf1 is required for the effectiveness of doxorubicin (Dox) treatment-induced tumor suppression in a xenograft tumor model. These finding suggest that Bclaf1 plays a crucial role in transducing the senescence-inducing signal from NF-κB to C/EBPβ during TIS, thus amplifying the signals for the establishment of senescence. Given the recent revelation that Bclaf1 is involved in tumorigenesis, our data indicate that the responsiveness of Bclaf1 to NF-κB may determine the effectiveness of therapeutic drugs.

Enrichment of Cdk1-cyclins at DNA double-strand breaks stimulates Fun30 phosphorylation and DNA end resection

DNA double-strand breaks (DSBs) are one of the most cytotoxic types of DNA lesion challenging genome integrity. The activity of cyclin-dependent kinase Cdk1 is essential for DSB repair by homologous recombination and for DNA damage signaling. Here we identify the Fun30 chromatin remodeler as a new target of Cdk1. Fun30 is phosphorylated by Cdk1 on Serine 28 to stimulate its functions in DNA damage response including resection of DSB ends. Importantly, Cdk1-dependent phosphorylation of Fun30-S28 increases upon DNA damage and requires the recruitment of Fun30 to DSBs, suggesting that phosphorylation increases in situ at the DNA damage. Consistently, we find that Cdk1 and multiple cyclins become highly enriched at DSBs and that the recruitment of Cdk1 and cyclins Clb2 and Clb5 ensures optimal Fun30 phosphorylation and checkpoint activation. We propose that the enrichment of Cdk1-cyclin complexes at DSBs serves as a mechanism for enhanced targeting and modulating of the activity of DNA damage response proteins.

CHMP4C Disruption Sensitizes the Human Lung Cancer Cells to Irradiation

Human lung cancer is highly invasive and the most malignant among human tumors. Adenocarcinoma as a specific type of non-small cell lung cancer occurs with high frequency and is also highly resistant to radiation therapy. Thus, how to avoid radiation resistance and improve radiotherapy effectiveness is a crucial question. In the present study, human lung cancer A549 and H1299 cells were irradiated using γ-rays from a Co60 irradiator. Protein expression was detected by Western blotting. Cell cycle and apoptosis were measured by flow cytometry. Surviving fraction was determined by colony formation assay. γH2AX and 53BP1 foci formation were examined by fluorescence microscopy. In the results, we show that CHMP4C, a subunit of Endosomal sorting complex-III (ESCRT-III), is involved in radiation-induced cellular response. Radiation-induced Aurora B expression enhances CHMP4C phosphorylation in non-small cell lung cancer (NSCLC) cells, maintaining cell cycle check-point and cellular viability as well as resisting apoptosis. CHMP4C depletion enhances cellular sensitivity to radiation, delays S-phase of cell cycle and reduces ionizing radiation (IR)-induced γH2AX foci formation. We found that Aurora B targets CHMP4C and inhibition of Aurora B exhibits similar effects with silencing of CHMP4C in radioresistance. We also confirm that CHMP4C phosphorylation is elevated after IR both in p53-positive and-negative cells, indicating that the close correlation between CHMP4C and Aurora B signaling pathway in mediating radiation resistance is not p53 dependent. Together, our work establishes a new function of CHMP4C in radiation resistance, which will offer a potential strategy for non-small cell lung cancer by disrupting CHMP4C.

DNA-repair defects in pancreatic neuroendocrine tumors and potential clinical applications

BACKGROUND

The role of DNA repair in pathogenesis and response to treatment is not well understood in pancreatic neuroendocrine tumors (pNETs). However, the existing literature reveals important preliminary trends and targets in the genetic landscape of pNETs. Notably, pNETs have been shown to harbor defects in the direct reversal MGMT gene and the DNA mismatch repair genes, suggesting that these genes may be strong candidates for further prospective studies.

METHODS

PubMed searches were conducted for original studies assessing the DNA repair genes MGMT and MMR in pNETs, as well as for PTEN and MEN1, which are not directly DNA repair genes but are involved in DNA repair pathways. Searches were specific to pNETs, yielding five original studies on MGMT and four on MMR. Six original papers studied PTEN in pNETs. Five studied MEN1 in pNETs, and two others implicated MEN1 in DNA repair processes.

RESULTS

The five studies on MGMT in pNET tumor samples found MGMT loss of between 24% and 51% of tumor samples by IHC staining and between 0% and 40% by promoter hypermethylation, revealing discrepancies in methods assessing MGMT expression as well as potential weaknesses in the correlation between MGMT IHC expression and promoter hypermethylation rates. Four studies on MMR in pNET tumor samples indicated similar ambiguities, as promoter hypermethylation of the MLH1 MMR gene ranged from 0% to 31% of pNETs, while IHC staining revealed loss of MMR genes in between 0% and 36% of pNETs sampled. Studies also indicated that PTEN and MEN1 are commonly mutated or underexpressed genes in pNETs, although frequency of mutation or loss of expression was again variable among different studies.

CONCLUSION

Further studies are essential in determining a more thorough repertoire of DNA repair defects in pNETs and the clinical significance of these defects. This literature review synthesises the existing knowledge of relevant DNA repair pathways and studies of the specific genes that carry out these repair mechanisms in pNETs.

Damage-induced BRCA1 phosphorylation by Chk2 contributes to the timing of end resection

The BRCA1 tumor suppressor plays an important role in homologous recombination (HR)-mediated DNA double-strand-break (DSB) repair. BRCA1 is phosphorylated by Chk2 kinase upon γ-irradiation, but the role of Chk2 phosphorylation is not understood. Here, we report that abrogation of Chk2 phosphorylation on BRCA1 delays end resection and the dispersion of BRCA1 from DSBs but does not affect the assembly of Mre11/Rad50/NBS1 (MRN) and CtIP at DSBs. Moreover, we show that BRCA1 is ubiquitinated by SCF(Skp2) and that abrogation of Chk2 phosphorylation impairs its ubiquitination. Our study suggests that BRCA1 is more than a scaffold protein to assemble HR repair proteins at DSBs, but that Chk2 phosphorylation of BRCA1 also serves as a built-in clock for HR repair of DSBs. BRCA1 is known to inhibit Mre11 nuclease activity. SCF(Skp2) activity appears at late G1 and peaks at S/G2, and is known to ubiquitinate phosphodegron motifs. The removal of BRCA1 from DSBs by SCF(Skp2)-mediated degradation terminates BRCA1-mediated inhibition of Mre11 nuclease activity, allowing for end resection and restricting the initiation of HR to the S/G2 phases of the cell cycle.

Role of the nucleus in apoptosis: signaling and execution

Since their establishment in the early 1970s, the nuclear changes upon apoptosis induction, such as the condensation of chromatin, disassembly of nuclear scaffold proteins and degradation of DNA, were, and still are, considered as the essential steps and hallmarks of apoptosis. These are the characteristics of the execution phase of apoptotic cell death. In addition, accumulating data clearly show that some nuclear events can lead to the induction of apoptosis. In particular, if DNA lesions resulting from deregulation during the cell cycle or DNA damage induced by chemotherapeutic drugs or viral infection cannot be efficiently eliminated, apoptotic mechanisms, which enable cellular transformation to be avoided, are activated in the nucleus. The functional heterogeneity of the nuclear organization allows the tight regulation of these signaling events that involve the movement of various nuclear proteins to other intracellular compartments (and vice versa) to initiate and govern apoptosis. Here, we discuss how these events are coordinated to execute apoptotic cell death.

ATM and ATR signaling at a glance

ATM and ATR signaling pathways are well conserved throughout evolution and are central to the maintenance of genome integrity. Although the role of both ATM and ATR in DNA repair, cell cycle regulation and apoptosis have been well studied, both still remain in the focus of current research activities owing to their role in cancer. Recent advances in the field suggest that these proteins have an additional function in maintaining cellular homeostasis under both stressed and non-stressed conditions. In this Cell Science at a Glance article and the accompanying poster, we present an overview of recent advances in ATR and ATM research with emphasis on that into the modes of ATM and ATR activation, the different signaling pathways they participate in - including those that do not involve DNA damage - and highlight their relevance in cancer.

Mitotic catastrophe and cancer drug resistance: A link that must to be broken

An increased tendency of genomic alterations during the life cycle of cells leads to genomic instability, which is a major driving force for tumorigenesis. A considerable fraction of tumor cells are tetraploid or aneuploid, which renders them intrinsically susceptible to mitotic aberrations, and hence, are particularly sensitive to the induction of mitotic catastrophe. Resistance to cell death is also closely linked to genomic instability, as it enables malignant cells to expand even in a stressful environment. Currently it is known that cells can die via multiple mechanisms. Mitotic catastrophe represents a step preceding apoptosis or necrosis, depending on the expression and/or proper function of several proteins. Mitotic catastrophe was proposed to be an onco-suppressive mechanism and the evasion of mitotic catastrophe constitutes one of the gateways to cancer development. Thus, stimulation of mitotic catastrophe appears to be a promising strategy in cancer treatment. Indeed, several chemotherapeutic drugs are currently used at concentrations that induce apoptosis irrespective of the cell cycle phase, yet are very efficient at triggering mitotic catastrophe at lower doses, significantly limiting side effects. In the present review we summarize current data concerning the role of mitotic catastrophe in cancer drug resistance and discuss novel strategies to break this link.

Systematic E2 screening reveals a UBE2D-RNF138-CtIP axis promoting DNA repair

Ubiquitylation is crucial for proper cellular responses to DNA double-strand breaks (DSBs). If unrepaired, these highly cytotoxic lesions cause genome instability, tumorigenesis, neurodegeneration or premature ageing. Here, we conduct a comprehensive, multilayered screen to systematically profile all human ubiquitin E2 enzymes for impacts on cellular DSB responses. With a widely applicable approach, we use an exemplary E2 family, UBE2Ds, to identify ubiquitylation-cascade components downstream of E2s. Thus, we uncover the nuclear E3 ligase RNF138 as a key homologous recombination (HR)-promoting factor that functions with UBE2Ds in cells. Mechanistically, UBE2Ds and RNF138 accumulate at DNA-damage sites and act at early resection stages by promoting CtIP ubiquitylation and accrual. This work supplies insights into regulation of DSB repair by HR. Moreover, it provides a rich information resource on E2s that can be exploited by follow-on studies.

The RNF138 E3 ligase displaces Ku to promote DNA end resection and regulate DNA repair pathway choice

DNA double-strand breaks (DSBs) are repaired mainly by non-homologous end joining or homologous recombination (HR). Cell cycle stage and DNA end resection are believed to regulate the commitment to HR repair. Here we identify RNF138 as a ubiquitin E3 ligase that regulates the HR pathway. RNF138 is recruited to DNA damage sites through zinc fingers that have a strong preference for DNA with 5'- or 3'-single-stranded overhangs. RNF138 stimulates DNA end resection and promotes ATR-dependent signalling and DSB repair by HR, thereby contributing to cell survival on exposure to DSB-inducing agents. Finally, we establish that RNF138-dependent Ku removal from DNA breaks is one mechanism whereby RNF138 can promote HR. These results establish RNF138 as an important regulator of DSB repair pathway choice.

The N-terminus of RPA large subunit and its spatial position are important for the 5'->3' resection of DNA double-strand breaks

The first step of homology-dependent repair of DNA double-strand breaks (DSBs) is the resection of the 5' strand to generate 3' ss-DNA. Of the two major nucleases responsible for resection, EXO1 has intrinsic 5'->3' directionality, but DNA2 does not. DNA2 acts with RecQ helicases such as the Werner syndrome protein (WRN) and the heterotrimeric eukaryotic ss-DNA binding protein RPA. We have found that the N-terminus of the RPA large subunit (RPA1N) interacts with both WRN and DNA2 and is essential for stimulating WRN's 3'->5' helicase activity and DNA2's 5'->3' ss-DNA exonuclease activity. A mutant RPA complex that lacks RPA1N is unable to support resection in Xenopus egg extracts and human cells. Furthermore, relocating RPA1N to the middle subunit but not to the small subunit causes severe defects in stimulating DNA2 and WRN and in supporting resection. Together, these findings suggest that RPA1N and its spatial position are critical for restricting the directionality of the WRN-DNA2 resection pathway.