DNA double-strand breaks: signaling, repair and the cancer connection

Gallic acid: molecular rival of cancer

Gallic acid, a predominant polyphenol, has been shown to inhibit carcinogenesis in animal models and in vitro cancerous cell lines. The inhibitory effect of gallic acid on cancer cell growth is mediated via the modulation of genes which encodes for cell cycle, metastasis, angiogenesis and apoptosis. Gallic acid inhibits activation of NF-κB and Akt signaling pathways along with the activity of COX, ribonucleotide reductase and GSH. Moreover, gallic acid activates ATM kinase signaling pathways to prevent the processes of carcinogenesis. The data so far available, both from in vivo and in vitro studies, indicate that this dietary polyphenol could be promising agent in the field of cancer chemoprevention.

The C-terminal region of Rad52 is essential for Rad52 nuclear and nucleolar localization, and accumulation at DNA damage sites immediately after irradiation

Rad52 plays essential roles in homologous recombination (HR) and repair of DNA double-strand breaks (DSBs) in Saccharomyces cerevisiae. However, in vertebrates, knockouts of the Rad52 gene show no hypersensitivity to agents that induce DSBs. Rad52 localizes in the nucleus and forms foci at a late stage following irradiation. Ku70 and Ku80, which play an essential role in nonhomologous DNA-end-joining (NHEJ), are essential for the accumulation of other core NHEJ factors, e.g., XRCC4, and a HR-related factor, e.g., BRCA1. Here, we show that the subcellular localization of EYFP-Rad52(1-418) changes dynamically during the cell cycle. In addition, EYFP-Rad52(1-418) accumulates rapidly at microirradiated sites and colocalizes with the DSB sensor protein Ku80. Moreover, the accumulation of EYFP-Rad52(1-418) at DSB sites is independent of the core NHEJ factors, i.e., Ku80 and XRCC4. Furthermore, we observed that EYFP-Rad52(1-418) localizes in nucleoli in CHO-K1 cells and XRCC4-deficient cells, but not in Ku80-deficient cells. We also found that Rad52 nuclear localization, nucleolar localization, and accumulation at DSB sites are dependent on eight amino acids (411-418) at the end of the C-terminal region of Rad52 (Rad52 CTR). Furthermore, basic amino acids on Rad52 CTR are highly conserved among mammalian, avian, and fish homologues, suggesting that Rad52 CTR is important for the regulation and function of Rad52 in vertebrates. These findings also suggest that the mechanism underlying the regulation of subcellular localization of Rad52 is important for the physiological function of Rad52 not only at a late stage following irradiation, but also at an early stage.

Polymorphisms in double-strand breaks repair genes are associated with impaired fertility in Chinese population

The DNA double-strand breaks (DSBs) repair pathway plays a critical role in repairing double-strand breaks, and genetic variants in DSBs repair pathway genes are potential risk factors for various diseases. To test the hypothesis that polymorphisms in DSBs genes are associated with susceptibility to male infertility, we examined 11 single nucleotide polymorphisms in eight key DSBs genes (XRCC3, XRCC2, BRCA2, RAG1, XRCC5, LIG4, XRCC4 and ATM) in 580 infertility cases and 580 controls from a Chinese population-based case-control study (NJMU Infertility Study). Genotypes were determined using the OpenArray platform, and sperm DNA fragmentation was detected using the TUNEL assay. The adjusted odds ratio (OR) and 95% CI were estimated using logistic regression. The results indicate that LIG4 rs1805388 (Ex2+54C>T, Thr9Ile) T allele could increase the susceptibility to male infertility (adjusted OR=2.78; 95% CI, 1.77-4.36 for TT genotype; and adjusted OR=1.58; 95% CI, 1.77-4.36 for TC genotype respectively). In addition, the homozygous variant genotype GG of RAG1 rs2227973 (A>G, K820R) was associated with a significantly increased risk of male infertility (adjusted OR, 1.44; 95% CI, 1.01-2.04). Moreover, linear regression analysis revealed that carriers of LIG4 rs1805388 or RAG1 rs2227973 variants had a significantly higher level of sperm DNA fragmentation and that T allele carriers of LIG4 rs1805388 also had a lower level of sperm concentration when compared with common homozygous genotype carriers. This study demonstrates, for the first time, to our knowledge, that functional variants of RAG1 rs2227973 and LIG4 rs1805388 are associated with susceptibility to male infertility.

Clinicopathological significance of KU70/KU80, a key DNA damage repair protein in breast cancer

Although the role of BRCA1 and the homologous recombination (HR) pathway in breast cancer (BC) has been extensively studied, the alternative repair pathway for DNA double-strand breaks (DSBs), non-homologous end-joining (NHEJ) remains to be defined. Ku proteins bind to DNA DSB ends and play a key role in NHEJ. In this study we aimed to assess the expression and biological significance of the KU70/KU80 heterodimer in the different molecular classes of BC. The expression of KU70/KU80 was assessed immunohistochemically in a well-characterised and annotated series of 1302 unselected invasive BC cases with a long-term follow-up together with 25 cases with known BRCA1 mutations. The results were correlated with clinicopathological parameters, other DNA repair proteins and patient outcome. The expression of KU70/KU80 protein was further evaluated in various BC cell lines using western blotting and reverse-phase protein microarray (RPPA). Nuclear KU70/KU80 expression was correlated with features of poor prognosis including higher histological grade, lymphovascular invasion, negative oestrogen receptor expression, basal-like phenotype, P53 and CHK1 positivity. KU70/KU80 was expressed in all BRCA1-associated tumours and showed an inverse correlation with nuclear BRCA1 protein and aberrant cytoplasmic RAD51 expression. RPPA confirmed these results and showed higher expression of KU70/KU80 in BRCA1-deficient cell line compared to BRCA1-proficient cell line. KU70/KU80 expression showed an association with disease-free interval; however, it was not an independent predictor of outcome. As a conclusion, KU70/KU80 may play a role in DNA DSBs repair in HR-deficient tumours. Further study of other NHEJ markers in sporadic BC is warranted.

Working hard for recovery: mitotic kinases in the DNA damage checkpoint

Cell division in mitosis is tightly regulated via a group of protein kinases. Activation of these mitotic kinases is inhibited by the DNA damage checkpoint that arrests the cell cycle in interphase and prevents mitotic entry. Interestingly, it has been shown that the DNA damage checkpoint is feedback regulated by several mitotic kinases. These kinases are reactivated from checkpoint arrest to deactivate the checkpoint and restart cell cycle progression, thereby allowing the cell to recover from the DNA damage checkpoint. The emerging role of mitotic kinases in the DNA damage pathway provides important insights into cancer progression and treatment.

PCNA is efficiently loaded on the DNA recombination intermediate to modulate polymerase δ, η, and ζ activities

Proliferating cell nuclear antigen (PCNA) is required for DNA homologous recombination (HR), but its exact role is unclear. Here, we investigated the loading of PCNA onto a synthetic D-loop (DL) intermediate of HR and the functional interactions of PCNA with Rad51 recombinase and DNA polymerase (Pol) δ, Pol η, and Pol ζ. PCNA was loaded onto the synthetic DL as efficiently as it was loaded onto a primed DNA substrate. Efficient PCNA loading requires Replication Protein A, which is associated with the displaced ssDNA loop and provides a binding site for the clamp-loader Replication Factor C. Loaded PCNA greatly stimulates DNA synthesis by Pol δ within the DL but does not affect primer recognition by Pol δ. This suggests that the essential role of PCNA in HR is not recruitment of Pol δ to the DL but stimulation of Pol δ to displace a DNA strand during DL extension. Both Pol η and Pol ζ extended the DL more efficiently than Pol δ in the absence of PCNA, but little or no stimulation was observed in the presence of PCNA. Finally, Rad51 inhibited both the loading of PCNA onto the DL and the extension of the DL by Pol δ and Pol η. However, preloaded PCNA on the DL counteracts the Rad51-mediated inhibition of the DL extension. This suggests that the inhibition of postinvasion DNA synthesis by Rad51 occurs mostly at the step of PCNA loading.

Relative biological effectiveness of mammography X-rays at the level of DNA and chromosomes in lymphocytes

PURPOSE

In many countries, breast cancer screening programs based on periodic mammography exist, giving a large group of women regularly a small dose of ionizing radiation. In order to assess the benefit/risk ratio of those programs the relative biological effectiveness (RBE) of mammography X-rays needs to be determined.

MATERIALS AND METHODS

Blood of five healthy donors was irradiated in vitro with 30 kV X-rays and (60)Co γ-rays with doses between 5 and 2000 mGy. The phosphorylated histone subtype H2A isoform X-foci (γH2AX-foci) technique was used to quantify the number of DNA double-strand breaks (DSB) after irradiation. Chromosomal damage resulting from non- or misrepaired DNA DSB was quantified with the micronucleus (MN)-assay and the sensitivity was improved by counting only centromere negative micronuclei (MNCM-).

RESULTS

The threshold detection dose obtained with the γH2AX-foci test was 10 mGy for mammography X-rays compared to 50 mGy for γ-rays. With the MN-assay respectively MN-centromere-assay threshold detection doses of 100, respectively, 50 mGy were obtained for mammography X-rays compared to 200 respectively 100 mGy for γ-rays. An RBE of 1.4 was obtained with the γH2AX-foci assay. With the MN-assays low-dose RBE values between 3 and 4 were determined.

CONCLUSION

Our results indicate that exposure to mammography X-rays resulted in a modest increase in the induction of DSB compared to γ-rays. However, due to the higher linear energy transfer (LET) of mammography X-rays more clustered DNA damage is produced that is more difficult to repair and results in a more pronounced increase in micronucleus formation.

Molecular epidemiology of DNA repair gene polymorphisms and head and neck cancer

Although tobacco and alcohol consumption are two common risk factors of head and neck cancer (HNC), other specific etiologic causes, such as viral infection and genetic susceptibility factors, remain to be understood. Human DNA is often damaged by numerous endogenous and exogenous mutagens or carcinogens, and genetic variants in interaction with environmental exposure to these agents may explain interindividual differences in HNC risk. Single nucleotide polymorphisms (SNPs) in genes involved in the DNA damage-repair response are reported to be risk factors for various cancer types, including HNC. Here, we reviewed epidemiological studies that have assessed the associations between HNC risk and SNPs in DNA repair genes involved in base-excision repair, nucleotide-excision repair, mismatch repair, double-strand break repair and direct reversion repair pathways. We found, however, that only a few SNPs in DNA repair genes were found to be associated with significantly increased or decreased risk of HNC, and, in most cases, the effects were moderate, depending upon locus-locus interactions among the risk SNPs in the pathways. We believe that, in the presence of exposure, additional pathway-based analyses of DNA repair genes derived from genome-wide association studies (GWASs) in HNC are needed.

Intense THz pulses cause H2AX phosphorylation and activate DNA damage response in human skin tissue

Recent emergence and growing use of terahertz (THz) radiation for medical imaging and public security screening raise questions on reasonable levels of exposure and health consequences of this form of electromagnetic radiation. In particular, picosecond-duration THz pulses have shown promise for novel diagnostic imaging techniques. However, the effects of THz pulses on human cells and tissues thus far remain largely unknown. We report on the investigation of the biological effects of pulsed THz radiation on artificial human skin tissues. We observe that exposure to intense THz pulses for ten minutes leads to a significant induction of H2AX phosphorylation, indicating that THz pulse irradiation may cause DNA damage in exposed skin tissue. At the same time, we find a THz-pulse-induced increase in the levels of several proteins responsible for cell-cycle regulation and tumor suppression, suggesting that DNA damage repair mechanisms are quickly activated. Furthermore, we find that the cellular response to pulsed THz radiation is significantly different from that induced by exposure to UVA (400 nm).

DNA repair and cytotoxic drugs: the potential role of RAD51 in clinical outcome of non-small-cell lung cancer patients

Many of the cytotoxic drugs used in the treatment of non-small-cell lung carcinoma patients can interfere with DNA activity and the definition of an individual DNA repair profile could be a key strategy to achieve better response to chemotherapeutic treatment. Although DNA repair mechanisms are important factors in the prevention of carcinogenesis, these molecular pathways are also involved in therapy response. RAD51 is a crucial element in DNA repair by homologous recombination and has been shown to interfere with the prognosis of patients treated with chemoradiotherapy. There is increasing evidence that genetic polymorphisms in repair enzymes can influence DNA repair capacity and, consequently, affect chemotherapy efficacy. We conducted this review to show the possible influence of the RAD51 genetic variants in damage repair capacity and treatment response in non-small-cell lung carcinoma patients.

Human single-stranded DNA binding proteins are essential for maintaining genomic stability

The double-stranded conformation of cellular DNA is a central aspect of DNA stabilisation and protection. The helix preserves the genetic code against chemical and enzymatic degradation, metabolic activation, and formation of secondary structures. However, there are various instances where single-stranded DNA is exposed, such as during replication or transcription, in the synthesis of chromosome ends, and following DNA damage. In these instances, single-stranded DNA binding proteins are essential for the sequestration and processing of single-stranded DNA. In order to bind single-stranded DNA, these proteins utilise a characteristic and evolutionary conserved single-stranded DNA-binding domain, the oligonucleotide/oligosaccharide-binding (OB)-fold. In the current review we discuss a subset of these proteins involved in the direct maintenance of genomic stability, an important cellular process in the conservation of cellular viability and prevention of malignant transformation. We discuss the central roles of single-stranded DNA binding proteins from the OB-fold domain family in DNA replication, the restart of stalled replication forks, DNA damage repair, cell cycle-checkpoint activation, and telomere maintenance.

Outcome of BRCA1- compared with BRCA2-associated ovarian cancer: a nationwide study in the Netherlands

BACKGROUND

Recent studies suggested an improved overall survival (OS) for BRCA2- versus BRCA1-associated epithelial ovarian cancer (EOC), whereas the impact of chemotherapy is not yet clear. In a nationwide cohort, we examined the results of primary treatment, progression-free survival (PFS), treatment-free interval (TFI), and OS of BRCA1 versus BRCA2 EOC patients.

METHODS

Two hundred and forty-five BRCA1- and 99 BRCA2-associated EOC patients were identified through all Dutch university hospitals. Analyses were carried out with the Pearson's Chi-square test, Kaplan-Meier, and Cox regression methods.

RESULTS

BRCA1 patients were younger at EOC diagnosis than BRCA2 patients (51 versus 55 years; P < 0.001), without differences regarding histology, tumor grade, and International Federation of Gynecology and Obstetrics (FIGO) stage. Complete response rates after primary treatment, including chemotherapy, did not differ between BRCA1 (86%) and BRCA2 patients (90%). BRCA1 versus BRCA2 patients had a shorter PFS (median 2.2 versus 3.9 years, respectively; P = 0.006), TFI (median 1.7 versus 2.8 years; P = 0.009), and OS (median 6.0 versus 9.7 years; P = 0.04). Differences could not be explained by age at diagnosis, FIGO stage or type of treatment.

CONCLUSIONS

PFS and OS were substantially longer in BRCA2- than in BRCA1-associated EOC patients. While response rates after primary treatment were similarly high in both groups, TFI, as surrogate for chemosensitivity, was significantly longer in BRCA2 patients.

HDAC inhibitors: roles of DNA damage and repair

Histone deacetylase inhibitors (HDACis) increase gene expression through induction of histone acetylation. However, it remains unclear whether specific gene expression changes determine the apoptotic response following HDACis administration. Herein, we discuss evidence that HDACis trigger in cancer and leukemia cells not only widespread histone acetylation but also actual increases in reactive oxygen species (ROS) and DNA damage that are further increased following treatment with DNA-damaging chemotherapies. While the origins of ROS production are not completely understood, mechanisms, including inflammation and altered antioxidant signaling, have been reported. While the generation of ROS is an explanation, at least in part, for the source of DNA damage observed with HDACi treatment, DNA damage can also be independently induced by changes in the DNA repair activity and chromatin remodeling factors. Recent development of sirtuin inhibitors (SIRTis) has shown that, similar to HDACis, these drugs induce increases in ROS and DNA damage used singly, or in combination with HDACis and other drugs. Thus, induction of apoptosis by HDACis/SIRTis may result through oxidative stress and DNA damage mechanisms in addition to direct activation of apoptosis-inducing genes. Nevertheless, while DNA damage and stress responses could be of interest as markers for clinical responses, they have yet to be validated as markers for responses to HDACi treatment in clinical trials, alone, and in combination.

DNA damage activates a complex transcriptional response in murine lymphocytes that includes both physiological and cancer-predisposition programs

BACKGROUND

Double strand (ds) DNA breaks are a form of DNA damage that can be generated from both genotoxic exposures and physiologic processes, can disrupt cellular functions and can be lethal if not repaired properly. Physiologic dsDNA breaks are generated in a variety of normal cellular functions, including the RAG endonuclease-mediated rearrangement of antigen receptor genes during the normal development of lymphocytes. We previously showed that physiologic breaks initiate lymphocyte development-specific transcriptional programs. Here we compare transcriptional responses to physiological DNA breaks with responses to genotoxic DNA damage induced by ionizing radiation.

RESULTS

We identified a central lymphocyte-specific transcriptional response common to both physiologic and genotoxic breaks, which includes many lymphocyte developmental processes. Genotoxic damage causes robust alterations to pathways associated with B cell activation and increased proliferation, suggesting that genotoxic damage initiates not only the normal B cell maturation processes but also mimics activated B cell response to antigenic agents. Notably, changes including elevated levels of expression of Kras and mmu-miR-155 and the repression of Socs1 were observed following genotoxic damage, reflecting induction of a cancer-prone phenotype.

CONCLUSIONS

Comparing these transcriptional responses provides a greater understanding of the mechanisms cells use in the differentiation between types of DNA damage and the potential consequences of different sources of damage. These results suggest genotoxic damage may induce a unique cancer-prone phenotype and processes mimicking activated B cell response to antigenic agents, as well as the normal B cell maturation processes.

Imatinib radiosensitizes bladder cancer by targeting homologous recombination

Radiotherapy is a major treatment modality used to treat muscle-invasive bladder cancer, with patient outcomes similar to surgery. However, radioresistance is a significant factor in treatment failure. Cell-free extracts of muscle-invasive bladder tumors are defective in nonhomologous end-joining (NHEJ), and this phenotype may be used clinically by combining radiotherapy with a radiosensitizing drug that targets homologous recombination, thereby sparing normal tissues with intact NHEJ. The response of the homologous recombination protein RAD51 to radiation is inhibited by the small-molecule tyrosine kinase inhibitor imatinib. Stable RT112 bladder cancer Ku knockdown (Ku80KD) cells were generated using short hairpin RNA technology to mimic the invasive tumor phenotype and also RAD51 knockdown (RAD51KD) cells to show imatinib's pathway selectivity. Ku80KD, RAD51KD, nonsilencing vector control, and parental RT112 cells were treated with radiation in combination with either imatinib or lapatinib, which inhibits NHEJ and cell survival assessed by clonogenic assay. Drug doses were chosen at approximately IC40 and IC10 (nontoxic) levels. Imatinib radiosensitized Ku80KD cells to a greater extent than RAD51KD or RT112 cells. In contrast, lapatinib radiosensitized RAD51KD and RT112 cells but not Ku80KD cells. Taken together, our findings suggest a new application for imatinib in concurrent use with radiotherapy to treat muscle-invasive bladder cancer. Cancer Res; 73(5); 1611-20. ©2012 AACR.

The interaction of CtIP and Nbs1 connects CDK and ATM to regulate HR-mediated double-strand break repair

CtIP plays an important role in homologous recombination (HR)-mediated DNA double-stranded break (DSB) repair and interacts with Nbs1 and BRCA1, which are linked to Nijmegen breakage syndrome (NBS) and familial breast cancer, respectively. We identified new CDK phosphorylation sites on CtIP and found that phosphorylation of these newly identified CDK sites induces association of CtIP with the N-terminus FHA and BRCT domains of Nbs1. We further showed that these CDK-dependent phosphorylation events are a prerequisite for ATM to phosphorylate CtIP upon DNA damage, which is important for end resection to activate HR by promoting recruitment of BLM and Exo1 to DSBs. Most notably, this CDK-dependent CtIP and Nbs1 interaction facilitates ATM to phosphorylate CtIP in a substrate-specific manner. These studies reveal one important mechanism to regulate cell-cycle-dependent activation of HR upon DNA damage by coupling CDK- and ATM-mediated phosphorylation of CtIP through modulating the interaction of CtIP with Nbs1, which significantly helps to understand how DSB repair is regulated in mammalian cells to maintain genome stability.

Comprehensive SNP scan of DNA repair and DNA damage response genes reveal multiple susceptibility loci conferring risk to tobacco associated leukoplakia and oral cancer

Polymorphic variants of DNA repair and damage response genes play major role in carcinogenesis. These variants are suspected as predisposition factors to Oral Squamous Cell Carcinoma (OSCC). For identification of susceptible variants affecting OSCC development in Indian population, the "maximally informative" method of SNP selection from HapMap data to non-HapMap populations was applied. Three hundred twenty-five SNPs from 11 key genes involved in double strand break repair, mismatch repair and DNA damage response pathways were genotyped on a total of 373 OSCC, 253 leukoplakia and 535 unrelated control individuals. The significantly associated SNPs were validated in an additional cohort of 144 OSCC patients and 160 controls. The rs12515548 of MSH3 showed significant association with OSCC both in the discovery and validation phases (discovery P-value: 1.43E-05, replication P-value: 4.84E-03). Two SNPs (rs12360870 of MRE11A, P-value: 2.37E-07 and rs7003908 of PRKDC, P-value: 7.99E-05) were found to be significantly associated only with leukoplakia. Stratification of subjects based on amount of tobacco consumption identified SNPs that were associated with either high or low tobacco exposed group. The study reveals a synergism between associated SNPs and lifestyle factors in predisposition to OSCC and leukoplakia.

Overexpression of CDCA2 in human squamous cell carcinoma: correlation with prevention of G1 phase arrest and apoptosis

Cell division cycle associated 2 (CDCA2) recruits protein phosphatase 1 to chromatin to antagonize activation of ataxia telangiectasia mutated (ATM)-dependent signal transduction. ATM kinase plays a critical role in the DNA damage response and its phosphorylation cascade to inhibit the p53-MDM2 interaction, which releases p53 to induce p21 and G1 cell-cycle arrest. However, the relevance of CDCA2 to human malignancy including oral squamous cell carcinoma (OSCC) is unknown. In the current study, we found that CDCA2 expression was up-regulated in OSCC cell lines. Functional studies with shRNA system showed that knockdown of CDCA2 significantly (P<0.05) inhibited cellular proliferation compared with the control cells by arresting cell-cycle progression at the G1 phase and up-regulating the cyclin-dependent kinase inhibitors (p21^Cip1, p27^Kip1, p15^INK4B, and p16^INK4A). CDCA2 knockdown also promoted apoptosis after treatment with the DNA damage reagent, cisplatin. In clinical samples, the CDCA2 protein expression level in primary OSCCs was significantly (P<0.05) greater than in matched normal oral tissues (67/85, 79%). Furthermore, CDCA2-positive cases were correlated significantly (P<0.05) with high cancer progression. Our results showed for the first time that CDCA2 frequently is overexpressed in OSCCs and might be associated closely with OSCC progression by preventing cell-cycle arrest and apoptosis.

γH2AX as a marker of DNA double strand breaks and genomic instability in human population studies

DNA double strand breaks (DSB) are the gravest form of DNA damage in eukaryotic cells. Failure to detect DSB and activate appropriate DNA damage responses can cause genomic instability, leading to tumorigenesis and possibly accelerated aging. Phosphorylated histone H2AX (γH2AX) is used as a biomarker of cellular response to DSB and its potential for monitoring DNA damage and repair in human populations has been explored in this review. A systematic search was conducted in PubMed for articles, in English, on human studies reporting γH2AX as a biomarker of either DNA repair or DNA damage. A total of 68 publications were identified. Thirty-four studies (50.0%) evaluated the effect of medical procedures or treatments on γH2AX levels; 20 (29.4%) monitored γH2AX in specific pathological conditions with a case/control or case/case design; 5 studies (7.4%) evaluated the effect of environmental genotoxic exposures, and 9 (13.2%) were descriptive studies on cancer and aging. Peripheral blood lymphocytes (44.6%) or biopsies/tissue specimens (24.3%) were the most commonly used samples. γH2AX was scored by optical microscopy as immunostained foci (78%), or by flow cytometry (16%). Critical features affecting the reliability of the assay, including protocols heterogeneity, specimen, cell cycle, kinetics, study design, and statistical analysis, are hereby discussed. Because of its sensitivity, efficiency and mechanistic relevance, the γH2AX assay has great potential as a DNA damage biomarker; however, the technical and epidemiological heterogeneity highlighted in this review infer a necessity for experimental standardization of the assay.

Inhibition of DNA-dependent protein kinase catalytic subunit by small molecule inhibitor NU7026 sensitizes human leukemic K562 cells to benzene metabolite-induced apoptosis

Benzene is an established leukotoxin and leukemogen in humans. We have previously reported that exposure of workers to benzene and to benzene metabolite hydroquinone in cultured cells induced DNA-dependent protein kinase catalytic subunit (DNA-PKcs) to mediate the cellular response to DNA double strand break (DSB) caused by DNA-damaging metabolites. In this study, we used a new, small molecule, a selective inhibitor of DNA-PKcs, 2-(morpholin-4-yl)-benzo[h]chomen-4-one (NU7026), as a probe to analyze the molecular events and pathways in hydroquinone-induced DNA DSB repair and apoptosis. Inhibition of DNA-PKcs by NU7026 markedly potentiated the apoptotic and growth inhibitory effects of hydroquinone in proerythroid leukemic K562 cells in a dose-dependent manner. Treatment with NU7026 did not alter the production of reactive oxygen species and oxidative stress by hydroquinone but repressed the protein level of DNA-PKcs and blocked the induction of the kinase mRNA and protein expression by hydroquinone. Moreover, hydroquinone increased the phosphorylation of Akt to activate Akt, whereas co-treatment with NU7026 prevented the activation of Akt by hydroquinone. Lastly, hydroquinone and NU7026 exhibited synergistic effects on promoting apoptosis by increasing the protein levels of pro-apoptotic proteins Bax and caspase-3 but decreasing the protein expression of anti-apoptotic protein Bcl-2. Taken together, the findings reveal a central role of DNA-PKcs in hydroquinone-induced hematotoxicity in which it coordinates DNA DSB repair, cell cycle progression, and apoptosis to regulate the response to hydroquinone-induced DNA damage.

Lack of association between the 135G/C RAD51 gene polymorphism and the risk of colorectal cancer among Polish population

One of the major causes of carcinogenesis is loss of genome stability. RAD51 in process of homologous recombination (HR) played crucial role in maintenance integrity of genome through initiate of DNA double strand breaks repair. Presence of single nucleotide polymorphism (SNP) in RAD51 gene could change the capacity of DNA repair and altered the response to damaging agents. Research on potential impact of genetic variability on development and progression CRC may contribute to setting new genetic markers or/and determined individual susceptibility to CRC.The aim of the study. This study was designed to evaluate the effect of 135 G/C (rs1801320) RAD51 polymorphism located in the 5' untraslated region on the risk and progression of CRC.Material and methods. The subjects consisted of histologically confirmed colorectal cancer (n = 200) and controls (n = 200) with lack of previous history of cancer. The distribution of genotypes was determined by restriction fragment length polymorphism PCR (RFLP - PCR). Statistical analysis was based on multivariate regression model.Results and conclusion. Our study reveal no significance association of 135 G/C RAD51 polymorphism with occurrence and progression of colorectal cancer.

MicroRNA-182-5p targets a network of genes involved in DNA repair

MicroRNAs are noncoding regulators of gene expression, which act by repressing protein translation and/or degrading mRNA. Many have been shown to drive tumorigenesis in cancer, but functional studies to understand their mode of action are typically limited to single-target genes. In this study, we use synthetic biotinylated miRNA to pull down endogenous targets of miR-182-5p. We identified more than 1000 genes as potential targets of miR-182-5p, most of which have a known function in pathways underlying tumor biology. Specifically, functional enrichment analysis identified components of both the DNA damage response pathway and cell cycle to be highly represented in this target cohort. Experimental validation confirmed that miR-182-5p-mediated disruption of the homologous recombination (HR) pathway is a consequence of its ability to target multiple components in that pathway. Although there is a strong enrichment for the cell cycle ontology, we do not see primary proliferative defects as a consequence of miR-182-5p overexpression. We highlight targets that could be responsible for miR-182-5p-mediated disruption of other biological processes attributed in the literature so far. Finally, we show that miR-182-5p is highly expressed in a panel of human breast cancer samples, highlighting its role as a potential oncomir in breast cancer.

Maintenance of genomic stability in mouse embryonic stem cells: relevance in aging and disease

Recent studies have shown that mouse embryonic stem cells (mESCs) rely on a distinctive genome caretaking network. In this review, we will discuss how mESCs functionally respond to DNA damage and describe several modifications in mESC DNA damage response, which accommodate dynamic cycling and preservation of genetic information. Subsequently, we will discuss how the transition from mESCs to adult stem/progenitor cells can be involved in the decline of tissue integrity and function in the elderly.

BRCA1-Ku80 protein interaction enhances end-joining fidelity of chromosomal double-strand breaks in the G1 phase of the cell cycle

Quality control of DNA double-strand break (DSB) repair is vital in preventing mutagenesis. Non-homologous end-joining (NHEJ), a repair process predominant in the G1 phase of the cell cycle, rejoins DSBs either accurately or with errors, but the mechanisms controlling its fidelity are poorly understood. Here we show that BRCA1, a tumor suppressor, enhances the fidelity of NHEJ-mediated DSB repair and prevents mutagenic deletional end-joining through interaction with canonical NHEJ machinery during G1. BRCA1 binds and stabilizes Ku80 at DSBs through its N-terminal region, promotes precise DSB rejoining, and increases cellular resistance to radiation-induced DNA damage in a G1 phase-specific manner. These results suggest that BRCA1, as a central player in genome integrity maintenance, ensures high fidelity repair of DSBs by not only promoting homologous recombination repair in G2/M phase but also facilitating fidelity of Ku80-dependent NHEJ repair, thus preventing deletional end-joining of chromosomal DSBs during G1.

The N-terminal region of the DNA-dependent protein kinase catalytic subunit is required for its DNA double-stranded break-mediated activation

DNA-dependent protein kinase (DNA-PK) plays an essential role in the repair of DNA double-stranded breaks (DSBs) mediated by the nonhomologous end-joining pathway. DNA-PK is a holoenzyme consisting of a DNA-binding (Ku70/Ku80) and catalytic (DNA-PKcs) subunit. DNA-PKcs is a serine/threonine protein kinase that is recruited to DSBs via Ku70/80 and is activated once the kinase is bound to the DSB ends. In this study, two large, distinct fragments of DNA-PKcs, consisting of the N terminus (amino acids 1-2713), termed N-PKcs, and the C terminus (amino acids 2714-4128), termed C-PKcs, were produced to determine the role of each terminal region in regulating the activity of DNA-PKcs. N-PKcs but not C-PKcs interacts with the Ku-DNA complex and is required for the ability of DNA-PKcs to localize to DSBs. C-PKcs has increased basal kinase activity compared with DNA-PKcs, suggesting that the N-terminal region of DNA-PKcs keeps basal activity low. The kinase activity of C-PKcs is not stimulated by Ku70/80 and DNA, further supporting that the N-terminal region is required for binding to the Ku-DNA complex and full activation of kinase activity. Collectively, the results show the N-terminal region mediates the interaction between DNA-PKcs and the Ku-DNA complex and is required for its DSB-induced enzymatic activity.

NBS1 rs1805794G>C polymorphism is associated with decreased risk of acute myeloid leukemia in a Chinese population

As a key encoding protein gene of MRN (MRE11-RAD50-NBS1) complex, NBS1 plays a crucial role in maintaining genomic stability and preventing cell apoptosis, inflammation and tumorgenesis. Single nucleotide polymorphisms (rs2735383 and rs1805794) in NBS1 have been frequently studied in some cancers with discordant results in previous case-control studies. However, the relationship between these two functional polymorphisms and the susceptibility to acute myeloid leukemia (AML) in Chinese population has not been investigated. We performed a case-control study with 428 patients and 600 controls to detect the association between the two polymorphisms of NBS1 and the risk of AML in a Chinese population. The polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) method was carried out to determine the genotypes of potential functional SNPs in NBS1 gene. The results showed that compared with the homozygous carriers rs1805794CC, rs1805794GC genotype was significantly associated with decreased risk of AML in total subjects (adjusted odds ratio (OR) = 0.50; 95% CI = 0.37-0.67), the risk decreased even further in those carrying rs1805794GG genotype (OR = 0.23; 95% CI = 0.16-0.34). No significant association was found between rs2735383C>G polymorphism and the risk of AML (OR = 0.93; 95% CI = 0.71-1.22 for GC; OR = 0.78; 95% CI = 0.53-1.13 for CC, P = 0.152). These findings indicated that rs1805794G/C polymorphism in NBS1 may play a protective role in mediating the risk of AML.

FOXM1 (Forkhead box M1) in tumorigenesis: overexpression in human cancer, implication in tumorigenesis, oncogenic functions, tumor-suppressive properties, and target of anticancer therapy

FOXM1 (Forkhead box M1) is a typical proliferation-associated transcription factor and is also intimately involved in tumorigenesis. FOXM1 stimulates cell proliferation and cell cycle progression by promoting the entry into S-phase and M-phase. Additionally, FOXM1 is required for proper execution of mitosis. In accordance with its role in stimulation of cell proliferation, FOXM1 exhibits a proliferation-specific expression pattern and its expression is regulated by proliferation and anti-proliferation signals as well as by proto-oncoproteins and tumor suppressors. Since these factors are often mutated, overexpressed, or lost in human cancer, the normal control of the foxm1 expression by them provides the basis for deregulated FOXM1 expression in tumors. Accordingly, FOXM1 is overexpressed in many types of human cancer. FOXM1 is intimately involved in tumorigenesis, because it contributes to oncogenic transformation and participates in tumor initiation, growth, and progression, including positive effects on angiogenesis, migration, invasion, epithelial-mesenchymal transition, metastasis, recruitment of tumor-associated macrophages, tumor-associated lung inflammation, self-renewal capacity of cancer cells, prevention of premature cellular senescence, and chemotherapeutic drug resistance. However, in the context of urethane-induced lung tumorigenesis, FOXM1 has an unexpected tumor suppressor role in endothelial cells because it limits pulmonary inflammation and canonical Wnt signaling in epithelial lung cells, thereby restricting carcinogenesis. Accordingly, FOXM1 plays a role in homologous recombination repair of DNA double-strand breaks and maintenance of genomic stability, that is, prevention of polyploidy and aneuploidy. The implication of FOXM1 in tumorigenesis makes it an attractive target for anticancer therapy, and several antitumor drugs have been reported to decrease FOXM1 expression.

Targeting Jab1/CSN5 in nasopharyngeal carcinoma

Nasopharyngeal carcinoma (NPC) is an Epstein-Barr virus-associated head and neck cancer that is most common in eastern Asia. Epstein-Barr virus infection, environmental factors, and genetic susceptibility play important roles in NPC pathogenesis. Jab1/CSN5 is a multifunctional protein that participates in affecting integrin signaling, controlling cell proliferation and apoptosis, and regulating genomic instability and DNA repair. Correlation of Jab1/CSN5 overexpression with poor prognosis for NPC provides evidence that it is involved in the tumorigenic process. In this review, we highlight recent advances in studies of the oncogenic role of Jab1/CSN5 in NPC and its potential as a therapeutic target for this cancer.

RAD51 and breast cancer susceptibility: no evidence for rare variant association in the Breast Cancer Family Registry study

Background

Although inherited breast cancer has been associated with germline mutations in genes that are functionally involved in the DNA homologous recombination repair (HRR) pathway, including BRCA1, BRCA2, TP53, ATM, BRIP1, CHEK2 and PALB2, about 70% of breast cancer heritability remains unexplained. Because of their critical functions in maintaining genome integrity and already well-established associations with breast cancer susceptibility, it is likely that additional genes involved in the HRR pathway harbor sequence variants associated with increased risk of breast cancer. RAD51 plays a central biological function in DNA repair and despite the fact that rare, likely dysfunctional variants in three of its five paralogs, RAD51C, RAD51D, and XRCC2, have been associated with breast and/or ovarian cancer risk, no population-based case-control mutation screening data are available for the RAD51 gene. We thus postulated that RAD51 could harbor rare germline mutations that confer increased risk of breast cancer.

Methodology/Principal Findings

We screened the coding exons and proximal splice junction regions of the gene for germline sequence variation in 1,330 early-onset breast cancer cases and 1,123 controls from the Breast Cancer Family Registry, using the same population-based sampling and analytical strategy that we developed for assessment of rare sequence variants in ATM and CHEK2. In total, 12 distinct very rare or private variants were characterized in RAD51, with 10 cases (0.75%) and 9 controls (0.80%) carrying such a variant. Variants were either likely neutral missense substitutions (3), silent substitutions (4) or non-coding substitutions (5) that were predicted to have little effect on efficiency of the splicing machinery.

Conclusion

Altogether, our data suggest that RAD51 tolerates so little dysfunctional sequence variation that rare variants in the gene contribute little, if anything, to breast cancer susceptibility.

Genetic adaptation associated with genome-doubling in autotetraploid Arabidopsis arenosa

Genome duplication, which results in polyploidy, is disruptive to fundamental biological processes. Genome duplications occur spontaneously in a range of taxa and problems such as sterility, aneuploidy, and gene expression aberrations are common in newly formed polyploids. In mammals, genome duplication is associated with cancer and spontaneous abortion of embryos. Nevertheless, stable polyploid species occur in both plants and animals. Understanding how natural selection enabled these species to overcome early challenges can provide important insights into the mechanisms by which core cellular functions can adapt to perturbations of the genomic environment. Arabidopsis arenosa includes stable tetraploid populations and is related to well-characterized diploids A. lyrata and A. thaliana. It thus provides a rare opportunity to leverage genomic tools to investigate the genetic basis of polyploid stabilization. We sequenced the genomes of twelve A. arenosa individuals and found signatures suggestive of recent and ongoing selective sweeps throughout the genome. Many of these are at genes implicated in genome maintenance functions, including chromosome cohesion and segregation, DNA repair, homologous recombination, transcriptional regulation, and chromatin structure. Numerous encoded proteins are predicted to interact with one another. For a critical meiosis gene, ASYNAPSIS1, we identified a non-synonymous mutation that is highly differentiated by cytotype, but present as a rare variant in diploid A. arenosa, indicating selection may have acted on standing variation already present in the diploid. Several genes we identified that are implicated in sister chromatid cohesion and segregation are homologous to genes identified in a yeast mutant screen as necessary for survival of polyploid cells, and also implicated in genome instability in human diseases including cancer. This points to commonalities across kingdoms and supports the hypothesis that selection has acted on genes controlling genome integrity in A. arenosa as an adaptive response to genome doubling.

Polymorphism of the DNA repair genes RAD51 and XRCC2 in smoking- and drinking-related laryngeal cancer in a Polish population

INTRODUCTION

Cigarette smoke and alcohol can generate reactive oxygen species, which may induce DNA double-strand breaks (DSBs), the most serious DNA lesion. In humans, DSBs are repaired mainly by non-homologous end joining and homologous recombination repair (HRR). Several polymorphisms in the DNA repair gene have been extensively studied in the association with various human cancers. In the present work we investigated the association between polymorphisms of two HRR genes, XRCC2 and RAD51, and tobacco- and alcohol-related larynx cancer in a Polish population.

MATERIAL AND METHODS

Two polymorphisms of the XRCC2 gene, -41657C > T (rs718282) and 31479G > A (rs3218536), as well as one polymorphism of the RAD51 gene, -135G > C (rs1801320), were investigated by PCR-RFLP in 253 patients with larynx cancer and 253 age- and sex-matched non-cancer controls.

RESULTS

Analysis of the gene-smoking and -drinking interactions revealed a weak association between larynx cancer and the -41657C > T polymorphisms of the XRCC2 gene among the moderate alcohol drinkers. The C allele of the -135G > C polymorphism of RAD51 increased cancer risk in the smoker group. Increased risk was also found for heavy drinkers. Additionally, there were no significant differences between distributions of genotypes in subgroups assigned to different TNM stages and grades.

CONCLUSIONS

The results indicated that the -135G > C polymorphism of the RAD51 gene may be associated with smoking- and drinking-related larynx cancer in Poland.

Age-related susceptibility to apoptosis in human retinal pigment epithelial cells is triggered by disruption of p53-Mdm2 association

PURPOSE

Relatively little is known about the contribution of p53/Mdm2 pathway in apoptosis of retinal pigment epithelial (RPE) cells or its possible link to dysfunction of aging RPE or to related blinding disorders such as age-related macular degeneration (AMD).

METHODS

Age-associated changes in p53 activation were evaluated in primary RPE cultures from human donor eyes of various ages. Apoptosis was evaluated by activation of caspases and DNA fragmentation. Gene-specific small interfering RNA was used to knock down expression of p53.

RESULTS

We observed that the basal rate of p53-dependent apoptosis increased in an age-dependent manner in human RPE. The age-dependent increase in apoptosis was linked to alterations in several aspects of the p53 pathway. p53 phosphorylation Ser15 was increased through the stimulation of ATM-Ser1981. p53 acetylation Lys379 was increased through the inhibition of SIRT1/2. These two posttranslational modifications of p53 blocked the sequestration of p53 by Mdm2, thus resulting in an increase in free p53 and of p53 stimulation of apoptosis through increased expression of PUMA (p53 upregulated modulator of apoptosis) and activation of caspase-3. Aged RPE also had reduced expression of antiapoptotic Bcl-2, which contributed to the increase in apoptosis. Of particular interest in these studies was that pharmacologic treatments to block p53 phosphorylation, acetylation, or expression were able to protect RPE cells from apoptosis.

CONCLUSIONS

Our studies suggest that aging in the RPE leads to alterations of specific checkpoints in the apoptotic pathway, which may represent important molecular targets for the treatment of RPE-related aging disorders such as AMD.

RAD52 variants predict platinum resistance and prognosis of cervical cancer

RAD52 is an important but not well characterized homologous recombination repair gene that can bind to single-stranded DNA ends and mediate the DNA-DNA interaction necessary for the annealing of complementary DNA strands. To evaluate the role of RAD52 variants in the response of tumor cells to platinum agents, we investigated their associations with platinum resistance and prognosis in cervical cancer patients. We enrolled 154 patients with cervical squamous cell carcinoma, who had radical surgery between 2008 and 2009, and genotyped three potentially functional RAD52 variants by the SNaPshot assay. We tested in vitro platinum resistance and RAD52 expression by using the MTT and immunohistochemistry methods, respectively. In 144 cases who had genotyping data, we found that both the rs1051669 variant and RAD52 protein expression were significantly associated with carboplatin resistance (P = 0.024 and 0.028, respectively) and rs10774474 with nedaplatin resistance (P = 0.018). The rs1051669 variant was significantly associated with RAD52 protein expression (adjusted OR = 4.7, 95% CI = 1.4-16.1, P = 0.013). When these three RAD52 variants were combined, progression-free survival was lower in patients who carried at least one (≥1) variant allele compared to those without any of the variant alleles (P = 0.047). Therefore, both RAD52 variants and protein expression can predict platinum resistance, and RAD52 variants appeared to predict prognosis in cervical cancer patients. Large studies are warranted to validate these findings.

Hyperactivation of PARP triggers nonhomologous end-joining in repair-deficient mouse fibroblasts

Regulation of poly(ADP-ribose) (PAR) synthesis and turnover is critical to determining cell fate after genotoxic stress. Hyperactivation of PAR synthesis by poly(ADP-ribose) polymerase-1 (PARP-1) occurs when cells deficient in DNA repair are exposed to genotoxic agents; however, the function of this hyperactivation has not been adequately explained. Here, we examine PAR synthesis in mouse fibroblasts deficient in the base excision repair enzyme DNA polymerase β (pol β). The extent and duration of PARP-1 activation was measured after exposure to either the DNA alkylating agent, methyl methanesulfonate (MMS), or to low energy laser-induced DNA damage. There was strong DNA damage-induced hyperactivation of PARP-1 in pol β nullcells, but not in wild-type cells. In the case of MMS treatment, PAR synthesis did not lead to cell death in the pol β null cells, but instead resulted in increased PARylation of the nonhomologous end-joining (NHEJ) protein Ku70 and increased association of Ku70 with PARP-1. Inhibition of the NHEJ factor DNA-PK, under conditions of MMS-induced PARP-1 hyperactivation, enhanced necrotic cell death. These data suggest that PARP-1 hyperactivation is a protective mechanism triggering the classical-NHEJ DNA repair pathway when the primary alkylated base damage repair pathway is compromised.

DNA repair polymorphisms influence the risk of second neoplasm after treatment of childhood acute lymphoblastic leukemia

PURPOSE

Patients treated for childhood acute lymphoblastic leukemia (ALL) are considered to be at increased risk of developing second neoplasm. The aim of our study was to identify DNA repair polymorphisms contributing to the risk of second neoplasm in clinically well-characterized Slovenian patients treated for childhood ALL.

METHODS

Pediatric patients diagnosed with ALL between 1971 and 2001 were included in the study. According to the identified clinical risk factors for second neoplasm, a matched set of cases with second neoplasm and controls was selected and genotyped for 11 DNA repair polymorphisms.

RESULTS

Among 359 pediatric patients with ALL, 20 second neoplasms were observed. The dose of radiotherapy (P = 0.011), administration of epipodophyllotoxins (P = 0.006), and the dose of anthracyclines (P < 0.001) showed a significant association with the risk of second neoplasm. Among genetic factors, we observed a significant association of NBN 1197G allele with increased risk of second neoplasms (RR = 4.36; 95 % CI: 1.19-15.98; P = 0.026), while the risk was decreased in carriers of XRCC3-316G allele compared with patients with wild-type genotype (RR = 0.20; 95 % CI: 0.04-0.99; P = 0.049).

CONCLUSIONS

Our results suggest an important role of NBN 1197A>G and XRCC3-316A>G polymorphisms in the development of second neoplasm in patients treated for childhood ALL.

Alteration/deficiency in activation-3 (Ada3) plays a critical role in maintaining genomic stability

Cell cycle regulation and DNA repair following damage are essential for maintaining genome integrity. DNA damage activates checkpoints in order to repair damaged DNA prior to exit to the next phase of cell cycle. Recently, we have shown the role of Ada3, a component of various histone acetyltransferase complexes, in cell cycle regulation, and loss of Ada3 results in mouse embryonic lethality. Here, we used adenovirus-Cre-mediated Ada3 deletion in Ada3(fl/fl) mouse embryonic fibroblasts (MEFs) to assess the role of Ada3 in DNA damage response following exposure to ionizing radiation (IR). We report that Ada3 depletion was associated with increased levels of phospho-ATM (pATM), γH2AX, phospho-53BP1 (p53BP1) and phospho-RAD51 (pRAD51) in untreated cells; however, radiation response was intact in Ada3(-/-) cells. Notably, Ada3(-/-) cells exhibited a significant delay in disappearance of DNA damage foci for several critical proteins involved in the DNA repair process. Significantly, loss of Ada3 led to enhanced chromosomal aberrations, such as chromosome breaks, fragments, deletions and translocations, which further increased upon DNA damage. Notably, the total numbers of aberrations were more clearly observed in S-phase, as compared with G₁ or G₂ phases of cell cycle with IR. Lastly, comparison of DNA damage in Ada3(fl/fl) and Ada3(-/-) cells confirmed higher residual DNA damage in Ada3(-/-) cells, underscoring a critical role of Ada3 in the DNA repair process. Taken together, these findings provide evidence for a novel role for Ada3 in maintenance of the DNA repair process and genomic stability.

The BRCA1 Breast Cancer Suppressor: Regulation of Transport, Dynamics, and Function at Multiple Subcellular Locations

Inherited mutations in the BRCA1 gene predispose to a higher risk of breast/ovarian cancer. The BRCA1 tumor suppressor is a 1863 amino acid protein with multiple protein interaction domains that facilitate its roles in regulating DNA repair and maintenance, cell cycle progression, transcription, and cell survival/apoptosis. BRCA1 was first identified as a nuclear phosphoprotein, but has since been shown to contain different transport sequences including nuclear export and nuclear localization signals that enable it to shuttle between specific sites within the nucleus and cytoplasm, including DNA repair foci, centrosomes, and mitochondria. BRCA1 nuclear transport and ubiquitin E3 ligase enzymatic activity are tightly regulated by the BRCA1 dimeric binding partner BARD1 and further modulated by cancer mutations and diverse signaling pathways. This paper will focus on the transport, dynamics, and multiple intracellular destinations of BRCA1 with emphasis on how regulation of these events has impact on, and determines, a broad range of important cellular functions.

Targeting protein for xenopus kinesin-like protein 2 (TPX2) regulates γ-histone 2AX (γ-H2AX) levels upon ionizing radiation

The microtubule-associated protein targeting protein for Xenopus kinesin-like protein 2 (TPX2) plays a key role in spindle assembly and is required for mitosis in human cells. In interphase, TPX2 is actively imported into the nucleus to prevent its premature activity in microtubule organization. To date, no function has been assigned to nuclear TPX2. We now report that TPX2 plays a role in the cellular response to DNA double strand breaks induced by ionizing radiation. Loss of TPX2 leads to inordinately strong and transient accumulation of ionizing radiation-dependent Ser-139-phosphorylated Histone 2AX (γ-H2AX) at G(0) and G(1) phases of the cell cycle. This is accompanied by the formation of increased numbers of high intensity γ-H2AX ionizing radiation-induced foci. Conversely, cells overexpressing TPX2 have reduced levels of γ-H2AX after ionizing radiation. Consistent with a role for TPX2 in the DNA damage response, we found that the protein accumulates at DNA double strand breaks and associates with the mediator of DNA damage checkpoint 1 (MDC1) and the ataxia telangiectasia mutated (ATM) kinase, both key regulators of γ-H2AX amplification. Pharmacologic inhibition or depletion of ATM or MDC1, but not of DNA-dependent protein kinase (DNA-PK), antagonizes the γ-H2AX phenotype caused by TPX2 depletion. Importantly, the regulation of γ-H2AX signals by TPX2 is not associated with apoptosis or the mitotic functions of TPX2. In sum, our study identifies a novel and the first nuclear function for TPX2 in the cellular responses to DNA damage.

Complex chromosomal rearrangements mediated by break-induced replication involve structure-selective endonucleases

DNA double-strand break (DSB) repair occurring in repeated DNA sequences often leads to the generation of chromosomal rearrangements. Homologous recombination normally ensures a faithful repair of DSBs through a mechanism that transfers the genetic information of an intact donor template to the broken molecule. When only one DSB end shares homology to the donor template, conventional gene conversion fails to occur and repair can be channeled to a recombination-dependent replication pathway termed break-induced replication (BIR), which is prone to produce chromosome non-reciprocal translocations (NRTs), a classical feature of numerous human cancers. Using a newly designed substrate for the analysis of DSB-induced chromosomal translocations, we show that Mus81 and Yen1 structure-selective endonucleases (SSEs) promote BIR, thus causing NRTs. We propose that Mus81 and Yen1 are recruited at the strand invasion intermediate to allow the establishment of a replication fork, which is required to complete BIR. Replication template switching during BIR, a feature of this pathway, engenders complex chromosomal rearrangements when using repeated DNA sequences dispersed over the genome. We demonstrate here that Mus81 and Yen1, together with Slx4, also promote template switching during BIR. Altogether, our study provides evidence for a role of SSEs at multiple steps during BIR, thus participating in the destabilization of the genome by generating complex chromosomal rearrangements.

Reversion of the ErbB malignant phenotype and the DNA damage response

The ErbB or HER family is a group of membrane bound tyrosine kinase receptors that initiate signal transduction cascades, which are critical to a wide range of biological processes. When over-expressed or mutated, members of this kinase family form homomeric or heteromeric kinase assemblies that are involved in certain human malignancies. Targeted therapy evolved from studies showing that monoclonal antibodies to the ectodomain of ErbB2/neu would reverse the malignant phenotype. Unfortunately, tumors develop resistance to targeted therapies even when coupled with genotoxic insults such as radiation. Radiation treatment predominantly induces double strand DNA breaks, which, if not repaired, are potentially lethal to the cell. Some tumors are resistant to radiation treatment because they effectively repair double strand breaks. We and others have shown that even in the presence of ionizing radiation, active ErbB kinase signaling apparently enhances the repair process, such that transformed cells resist genotoxic signal induced cell death. We review here the current understanding of ErbB signaling and DNA double strand break repair. Some studies have identified a mechanism by which DNA damage is coordinated to assemblies of proteins that associate with SUN domain containing proteins. These assemblies represent a new target for therapy of resistant tumor cells.

DNA double-strand break response in stem cells: mechanisms to maintain genomic integrity

BACKGROUND

Embryonic stem cells (ESCs) represent the point of origin of all cells in a given organism and must protect their genomes from both endogenous and exogenous genotoxic stress. DNA double-strand breaks (DSBs) are one of the most lethal forms of damage, and failure to adequately repair DSBs would not only compromise the ability of SCs to self-renew and differentiate, but will also lead to genomic instability and disease.

SCOPE OF REVIEW

Herein, we describe the mechanisms by which ESCs respond to DSB-inducing agents such as reactive oxygen species (ROS) and ionizing radiation, compared to somatic cells. We will also discuss whether the DSB response is fully reprogrammed in induced pluripotent stem cells (iPSCs) and the role of the DNA damage response (DDR) in the reprogramming of these cells.

MAJOR CONCLUSIONS

ESCs have distinct mechanisms to protect themselves against DSBs and oxidative stress compared to somatic cells. The response to damage and stress is crucial for the maintenance of self-renewal and differentiation capacity in SCs. iPSCs appear to reprogram some of the responses to genotoxic stress. However, it remains to be determined if iPSCs also retain some DDR characteristics of the somatic cells of origin.

GENERAL SIGNIFICANCE

The mechanisms regulating the genomic integrity in ESCs and iPSCs are critical for its safe use in regenerative medicine and may shed light on the pathways and factors that maintain genomic stability, preventing diseases such as cancer. This article is part of a Special Issue entitled Biochemistry of Stem Cells.

Dynamics of p53 and NF-κB regulation in response to DNA damage and identification of target proteins suitable for therapeutic intervention

Background

The genome is continuously attacked by a variety of agents that cause DNA damage. Recognition of DNA lesions activates the cellular DNA damage response (DDR), which comprises a network of signal transduction pathways to maintain genome integrity. In response to severe DNA damage, cells undergo apoptosis to avoid transformation into tumour cells, or alternatively, the cells enter permanent cell cycle arrest, called senescence. Most tumour cells have defects in pathways leading to DNA repair or apoptosis. In addition, apoptosis could be counteracted by nuclear factor kappa B (NF-κB), the main anti-apoptotic transcription factor in the DDR. Despite the high clinical relevance, the interplay of the DDR pathways is poorly understood. For therapeutic purposes DNA damage signalling processes are induced to induce apoptosis in tumour cells. However, the efficiency of radio- and chemotherapy is strongly hampered by cell survival pathways in tumour cells. In this study logical modelling was performed to facilitate understanding of the complexity of the signal transduction networks in the DDR and to provide cancer treatment options.

Results

Our comprehensive discrete logical model provided new insights into the dynamics of the DDR in human epithelial tumours. We identified new mechanisms by which the cell regulates the dynamics of the activation of the tumour suppressor p53 and NF-κB. Simulating therapeutic intervention by agents causing DNA single-strand breaks (SSBs) or DNA double-strand breaks (DSBs) we identified candidate target proteins for sensitization of carcinomas to therapeutic intervention. Further, we enlightened the DDR in different genetic diseases, and by failure mode analysis we defined molecular defects putatively contributing to carcinogenesis.

Conclusion

By logic modelling we identified candidate target proteins that could be suitable for radio- and chemotherapy, and contributes to the design of more effective therapies.

Scc1 sumoylation by Mms21 promotes sister chromatid recombination through counteracting Wapl

DNA double-strand breaks (DSBs) fuel cancer-driving chromosome translocations. Two related structural maintenance of chromosomes (Smc) complexes, cohesin and Smc5/6, promote DSB repair through sister chromatid homologous recombination (SCR). Here we show that the Smc5/6 subunit Mms21 sumoylates multiple lysines of the cohesin subunit Scc1. Mms21 promotes cohesin-dependent small ubiquitin-like modifier (SUMO) accumulation at laser-induced DNA damage sites in S/G2 human cells. Cells expressing the nonsumoylatable Scc1 mutant (15KR) maintain sister chromatid cohesion during mitosis but are defective in SCR and sensitive to ionizing radiation (IR). Scc1 15KR is recruited to DNA damage sites. Depletion of Wapl, a negative cohesin regulator, rescues SCR defects of Mms21-deficient or Scc1 15KR-expressing cells. Expression of the acetylation-mimicking Smc3 mutant does not bypass the requirement for Mms21 in SCR. We propose that Scc1 sumoylation by Mms21 promotes SCR by antagonizing Wapl at a step after cohesin loading at DSBs and in a way not solely dependent on Smc3 acetylation.