gamma-H2AX formation in response to interstrand crosslinks requires XPF in human cells

Multiparametric in vitro genotoxicity assessment of different variants of amorphous silica nanomaterials in rat alveolar epithelial cells

The hazard posed to human health by inhaled amorphous silica nanomaterials (aSiO2 NM) remains uncertain. Herein, we assessed the cyto- and genotoxicity of aSiO2 NM variants covering different sizes (7, 15, and 40 nm) and surface modifications (unmodified, phosphonate-, amino- and trimethylsilyl-modified) on rat alveolar epithelial (RLE-6TN) cells. Cytotoxicity was evaluated at 24 h after exposure to the aSiO2 NM variants by the lactate dehydrogenase (LDH) release and WST-1 reduction assays, while genotoxicity was assessed using different endpoints: DNA damage (single- and double-strand breaks [SSB and DSB]) by the comet assay for all aSiO2 NM variants; cell cycle progression and γ-H2AX levels (DSB) by flow cytometry for those variants that presented higher cytotoxic and DNA damaging potential. The variants with higher surface area demonstrated a higher cytotoxic potential (SiO2_7, SiO2_15_Unmod, SiO2_15_Amino, and SiO2_15_Phospho). SiO2_40 was the only variant that induced significant DNA damage on RLE-6TN cells. On the other hand, all tested variants (SiO2_7, SiO2_15_Unmod, SiO2_15_Amino, and SiO2_40) significantly increased total γ-H2AX levels. At high concentrations (28 µg/cm2), a decrease in G0/G1 subpopulation was accompanied by a significant increase in S and G2/M sub-populations after exposure to all tested materials except for SiO2_40 which did not affect cell cycle progression. Based on the obtained data, the tested variants can be ranked for its genotoxic DNA damage potential as follows: SiO2_7 = SiO2_40 = SiO2_15_Unmod > SiO2_15_Amino. Our study supports the usefulness of multiparametric approaches to improve the understanding on NM mechanisms of action and hazard prediction.

Cell Models for Birth Defects Caused by Chloroethyl Nitrosourea-Induced DNA Lesions

ABSTRACT

Birth defects have been linked to administration of alkylating agents during pregnancy. The anti-tumor efficacy of alkylating agents correlate with their ability to induce DNA lesions, especially interstrand crosslinks (ICLs). Yet the role of DNA damages in birth defects remains to be clarified, owing, in part, to a lack of cell models. Here we generate DNA lesions in NIH/3T3 cells to mimic defects in fetus triggered by 3-Bis(2-chloroethyl)-1-nitrosourea (BCNU, carmustine). CCK-8 assay suggests that BCNU-induced cell death was dose-dependent. Alkaline comet tests and γ-H2AX staining confirm DNA ICLs and other forms of DNA damages caused by BCNUs. The cell cycle analysis shows cells arrest in G2/M phase until crosslinks repair is complete. Taken together, all these experiments demonstrate we have successfully established normal cell models for birth defects caused by BCNU-mediated DNA damages. The model can not only guide the development of effective and low-toxicity anticancer drugs, but also be of great significance for the study of neonatal malformation triggered by BCNUs.

The DNA damage response network in the treatment of head and neck squamous cell carcinoma

BACKGROUND

We sought to determine whether DNA damage response (DDR)-related aberrations predict therapeutic benefit in cisplatin-treated head and neck squamous cell carcinoma (HNSCC) patients and how DDR pathways are modulated after treatment with olaparib alone or in combination with cisplatin or durvalumab.

PATIENTS AND METHODS

Oxidative stress, abasic sites and DDR-related parameters, including endogenous DNA damage, DNA repair mechanisms and apoptosis rates, were evaluated in HNSCC cell lines and peripheral blood mononuclear cells from 46 healthy controls (HC) and 70 HNSCC patients at baseline and following treatment with cisplatin-containing chemoradiation or nivolumab or enrolled in the OPHELIA phase II trial (NCT02882308; olaparib alone, olaparib plus cisplatin, olaparib plus durvalumab).

RESULTS

HNSCC patients at diagnosis exhibited deregulated DDR-related parameters and higher levels of oxidative stress and abasic sites compared with HC (all P < 0.05). Accordingly, nucleotide excision repair (NER; ERCC1, ERCC2/XPD, XPA, XPC) and base excision repair (APEX1, XRCC1) genes were downregulated in patients versus HC whereas double-strand breaks repair (MRE11A, RAD50, RAD51, XRCC2) and mismatch repair (MLH1, MSH2, MSH3) genes were overexpressed. Corresponding results were obtained in cell lines (all P < 0.001). Excellent correlations were observed between individual ex vivo and in vivo/therapeutic results, with cisplatin non-responders showing higher levels of endogenous DNA damage, augmented oxidative stress and abasic sites, increased NER capacities and reduced apoptosis than responders (all P < 0.05). Also, longer progression-free survival correlated with lower NER capacity (P = 0.037) and increased apoptosis (P = 0.029). Interestingly, treatment with olaparib-containing regimens results in the accumulation of cytotoxic DNA damage and exerts an extra antitumor effect by elevating oxidative stress (all P < 0.05). Nivolumab induced no significant changes in the DDR parameters examined.

CONCLUSIONS

Aberrations in DDR signals are implicated in the response to HNSCC chemotherapy and can be exploited as novel therapeutic targets, sensitive/effective non-invasive biomarkers as well as for the design of novel clinical trials.

Encapsulation of Nedaplatin in Novel PEGylated Liposomes Increases Its Cytotoxicity and Genotoxicity against A549 and U2OS Human Cancer Cells

Following the discovery of cisplatin over 50 years ago, platinum-based drugs have been a widely used and effective form of cancer therapy, primarily causing cell death by inducing DNA damage and triggering apoptosis. However, the dose-limiting toxicity of these drugs has led to the development of second and third generation platinum-based drugs that maintain the cytotoxicity of cisplatin but have a more acceptable side-effect profile. In addition to the creation of new analogs, tumor delivery systems such as liposome encapsulated platinum drugs have been developed and are currently in clinical trials. In this study, we have created the first PEGylated liposomal form of nedaplatin using thin film hydration. Nedaplatin, the main focus of this study, has been exclusively used in Japan for the treatment of non-small cell lung cancer, head and neck, esophageal, bladder, ovarian and cervical cancer. Here, we investigate the cytotoxic and genotoxic effects of free and liposomal nedaplatin on the human non-small cell lung cancer cell line A549 and human osteosarcoma cell line U2OS. We use a variety of assays including ICP MS and the highly sensitive histone H2AX assay to assess drug internalization and to quantify DNA damage induction. Strikingly, we show that by encapsulating nedaplatin in PEGylated liposomes, the platinum uptake cytotoxicity and genotoxicity of nedaplatin was significantly enhanced in both cancer cell lines. Moreover, the enhanced platinum uptake as well as the cytotoxic/antiproliferative effect of liposomal nedaplatin appears to be selective to cancer cells as it was not observed on two noncancer cell lines. This is the first study to develop PEGylated liposomal nedaplatin and to demonstrate the superior cell delivery potential of this product.

Histone H1 eviction by the histone chaperone SET reduces cell survival following DNA damage

Many chromatin remodeling and modifying proteins are involved in the DNA damage response, where they stimulate repair or induce DNA damage signaling. Interestingly, we identified that downregulation of the histone H1 (H1)-interacting protein SET results in increased resistance to a wide variety of DNA damaging agents. We found that this increased resistance does not result from alleviation of an inhibitory effect of SET on DNA repair but, rather, is the consequence of a suppressed apoptotic response to DNA damage. Furthermore, we provide evidence that the histone chaperone SET is responsible for the eviction of H1 from chromatin. Knockdown of H1 in SET-depleted cells resulted in re-sensitization of cells to DNA damage, suggesting that the increased DNA damage resistance in SET-depleted cells is the result of enhanced retention of H1 on chromatin. Finally, clonogenic survival assays showed that SET and p53 act epistatically in the attenuation of DNA damage-induced cell death. Taken together, our data indicate a role for SET in the DNA damage response as a regulator of cell survival following genotoxic stress.This article has an associated First Person interview with the first author of the paper.

Limited Capacity or Involvement of Excision Repair, Double-Strand Breaks, or Translesion Synthesis for Psoralen Cross-Link Repair in Escherichia coli

DNA interstrand cross-links are complex lesions that covalently bind complementary strands of DNA and whose mechanism of repair remains poorly understood. In Escherichia coli, several gene products have been proposed to be involved in cross-link repair based on the hypersensitivity of mutants to cross-linking agents. However, cross-linking agents induce several forms of DNA damage, making it challenging to attribute mutant hypersensitivity specifically to interstrand cross-links. To address this, we compared the survival of UVA-irradiated repair mutants in the presence of 8-methoxypsoralen-which forms interstrand cross-links and monoadducts-to that of angelicin-a congener forming only monoadducts. We show that incision by nucleotide excision repair is not required for resistance to interstrand cross-links. In addition, neither RecN nor DNA polymerases II, IV, or V is required for interstrand cross-link survival, arguing against models that involve critical roles for double-strand break repair or translesion synthesis in the repair process. Finally, estimates based on Southern analysis of DNA fragments in alkali agarose gels indicate that lethality occurs in wild-type cells at doses producing as few as one to two interstrand cross-links per genome. These observations suggest that E. coli may lack an efficient repair mechanism for this form of damage.

XPF knockout via CRISPR/Cas9 reveals that ERCC1 is retained in the cytoplasm without its heterodimer partner XPF

The XPF/ERCC1 heterodimeric complex is essentially involved in nucleotide excision repair (NER), interstrand crosslink (ICL), and double-strand break repair. Defects in XPF lead to severe diseases like xeroderma pigmentosum (XP). Up until now, XP-F patient cells have been utilized for functional analyses. Due to the multiple roles of the XPF/ERCC1 complex, these patient cells retain at least one full-length allele and residual repair capabilities. Despite the essential function of the XPF/ERCC1 complex for the human organism, we successfully generated a viable immortalised human XPF knockout cell line with complete loss of XPF using the CRISPR/Cas9 technique in fetal lung fibroblasts (MRC5Vi cells). These cells showed a markedly increased sensitivity to UVC, cisplatin, and psoralen activated by UVA as well as reduced repair capabilities for NER and ICL repair as assessed by reporter gene assays. Using the newly generated knockout cells, we could show that human XPF is markedly involved in homologous recombination repair (HRR) but dispensable for non-homologous end-joining (NHEJ). Notably, ERCC1 was not detectable in the nucleus of the XPF knockout cells indicating the necessity of a functional XPF/ERCC1 heterodimer to allow ERCC1 to enter the nucleus. Overexpression of wild-type XPF could reverse this effect as well as the repair deficiencies.

Cho Endonuclease Functions during DNA Interstrand Cross-Link Repair in Escherichia coli

DNA interstrand cross-links are complex lesions that covalently link both strands of the duplex DNA. Lesion removal is proposed to be initiated via the UvrABC nucleotide excision repair complex; however, less is known about the subsequent steps of this complex repair pathway. In this study, we characterized the contribution of nucleotide excision repair mutants to survival in the presence of psoralen-induced damage. Unexpectedly, we observed that the nucleotide excision repair mutants exhibit differential sensitivity to psoralen-induced damage, with uvrC mutants being less sensitive than either uvrA or uvrB We show that Cho, an alternative endonuclease, acts with UvrAB and is responsible for the reduced hypersensitivity of uvrC mutants. We find that Cho's contribution to survival correlates with the presence of DNA interstrand cross-links, rather than monoadducts, and operates at a step after, or independently from, the initial incision during the global repair of psoralen DNA adducts from the genome.

IMPORTANCE

DNA interstrand cross-links are complex lesions that covalently bind to both strands of the duplex DNA and whose mechanism of repair remains poorly understood. In this study, we show that Cho, an alternative endonuclease, acts with UvrAB and participates in the repair of DNA interstrand cross-links formed in the presence of photoactivated psoralens. Cho's contribution to survival correlates with the presence of DNA interstrand cross-links and operates at a step after, or independently from, the initial incision during the repair process.

CITED2 silencing sensitizes cancer cells to cisplatin by inhibiting p53 trans-activation and chromatin relaxation on the ERCC1 DNA repair gene

In this study, we show that silencing of CITED2 using small-hairpin RNA (shCITED2) induced DNA damage and reduction of ERCC1 gene expression in HEK293, HeLa and H1299 cells, even in the absence of cisplatin. In contrast, ectopic expression of ERCC1 significantly reduced intrinsic and induced DNA damage levels, and rescued the effects of CITED2 silencing on cell viability. The effects of CITED2 silencing on DNA repair and cell death were associated with p53 activity. Furthermore, CITED2 silencing caused severe elimination of the p300 protein and markers of relaxed chromatin (acetylated H3 and H4, i.e. H3K9Ac and H3K14Ac) in HEK293 cells. Chromatin immunoprecipitation assays further revealed that DNA damage induced binding of p53 along with H3K9Ac or H3K14Ac at the ERCC1 promoter, an effect which was almost entirely abrogated by silencing of CITED2 or p300. Moreover, lentivirus-based CITED2 silencing sensitized HeLa cell line-derived tumor xenografts to cisplatin in immune-deficient mice. These results demonstrate that CITED2/p300 can be recruited by p53 at the promoter of the repair gene ERCC1 in response to cisplatin-induced DNA damage. The CITED2/p300/p53/ERCC1 pathway is thus involved in the cell response to cisplatin and represents a potential target for cancer therapy.

Aberrant DNA damage response pathways may predict the outcome of platinum chemotherapy in ovarian cancer

Ovarian carcinoma (OC) is the most lethal gynecological malignancy. Despite the advances in the treatment of OC with combinatorial regimens, including surgery and platinum-based chemotherapy, patients generally exhibit poor prognosis due to high chemotherapy resistance. Herein, we tested the hypothesis that DNA damage response (DDR) pathways are involved in resistance of OC patients to platinum chemotherapy. Selected DDR signals were evaluated in two human ovarian carcinoma cell lines, one sensitive (A2780) and one resistant (A2780/C30) to platinum treatment as well as in peripheral blood mononuclear cells (PBMCs) from OC patients, sensitive (n = 7) or resistant (n = 4) to subsequent chemotherapy. PBMCs from healthy volunteers (n = 9) were studied in parallel. DNA damage was evaluated by immunofluorescence γH2AX staining and comet assay. Higher levels of intrinsic DNA damage were found in A2780 than in A2780/C30 cells. Moreover, the intrinsic DNA damage levels were significantly higher in OC patients relative to healthy volunteers, as well as in platinum-sensitive patients relative to platinum-resistant ones (all P<0.05). Following carboplatin treatment, A2780 cells showed lower DNA repair efficiency than A2780/C30 cells. Also, following carboplatin treatment of PBMCs ex vivo, the DNA repair efficiency was significantly higher in healthy volunteers than in platinum-resistant patients and lowest in platinum-sensitive ones (t_1/2 for loss of γH2AX foci: 2.7±0.5h, 8.8±1.9h and 15.4±3.2h, respectively; using comet assay, t_1/2 of platinum-induced damage repair: 4.8±1.4h, 12.9±1.9h and 21.4±2.6h, respectively; all P<0.03). Additionally, the carboplatin-induced apoptosis rate was higher in A2780 than in A2780/C30 cells. In PBMCs, apoptosis rates were inversely correlated with DNA repair efficiencies of these cells, being significantly higher in platinum-sensitive than in platinum-resistant patients and lowest in healthy volunteers (all P<0.05). We conclude that perturbations of DNA repair pathways as measured in PBMCs from OC patients correlate with the drug sensitivity of these cells and reflect the individualized response to platinum-based chemotherapy.

γ-H2AX foci formation as a pharmacodynamic marker of DNA damage produced by DNA cross-linking agents: results from 2 phase I clinical trials of SJG-136 (SG2000)

PURPOSE

To evaluate γ-H2AX foci as a pharmacodynamic marker for DNA damage induced by DNA interstrand cross-linking drugs.

EXPERIMENTAL DESIGN

γ-H2AX foci formation was validated preclinically in comparison with the Comet assay, and evaluated pharmacodynamically in two phase I studies of different dosing schedules of the novel cross-linking agent SJG-136 (SG2000).

RESULTS

The measurement of γ-H2AX foci in human fibroblasts and lymphocytes in vitro was more than 10-fold more sensitive than Comet assay measurement of cross-linking, with peak γ-H2AX response 24 hours after the peak of cross-linking. In lymphocytes from a phase I study (every three week schedule), γ-H2AX foci were detectable 1 hour following the end of administration, and in all patients, maximum response was observed at 24 hours. Significant levels of foci were still evident at days 8 and 15 consistent with the known persistence of the DNA damage produced by this agent. In two tumor biopsy samples, foci were detected 4 hours postinfusion with levels higher than in lymphocytes. Extensive foci formation was also observed before the third dose in cycle 1 in lymphocytes from a second phase I study (daily × 3 schedule). These foci also persisted with a significant level evident before the second cycle (day 21). An increased γ-H2AX response was observed during the second cycle consistent with a cumulative pharmacodynamic effect. No clear relationship between foci formation and administered drug dose was observed.

CONCLUSION

This is the first use of γ-H2AX as a pharmacodynamic response to a DNA cross-linking agent in a clinical trial setting.

Oxaliplatin as a radiosensitiser for upper and lower gastrointestinal tract malignancies: what have we learned from a decade of translational research?

Some of the greatest advances in the treatment of solid malignancies have resulted from the combination of chemotherapy and radiotherapy treatments. This article comprehensively reviews the current clinical evidence for oxaliplatin-based chemo-radiotherapy that may improve local control and survival. In order to understand how clinical studies should be designed, the pre-clinical evidence for the use of oxaliplatin chemotherapy as a radiosensitising agent is appraised. Particular focus is placed on oxaliplatin's biological mechanisms of action, including cell cycle effects, the formation of DNA adducts and interstrand cross-links and the role of DNA repair proteins. At a clinical level, there is currently no evidence to suggest that oxaliplatin provides an additional benefit to concurrent chemo-radiation regimes that utilise fluoropyrimidines; we evaluate the reasons for this observation, the limitations of clinical trial design and the opportunities that currently exist to design clinical trials which are underpinned by an understanding of the basic biology.

Involvement of MRE11A and XPA gene polymorphisms in the modulation of DNA double-strand break repair activity: a genotype-phenotype correlation study

DNA double-strand breaks (DSB) are the most lethal form of ionizing radiation-induced DNA damage, and failure to repair them results in cell death. In order to see if any associations exist between DNA repair gene polymorphisms and phenotypic profiles of DSB repair (DSBR) we performed a genotype-phenotype correlation study in 118 young healthy subjects (mean age 25.8±6.7years). Subjects were genotyped for 768 single nucleotide polymorphisms (SNPs) with a custom Illumina Golden Gate Assay, and an H2AX histone phosphorylation assay was done to test DSBR capacity. We found that H2AX phosphorylation at 1h was significantly lower in subjects heterozygous (no variant homozygotes were observed) for the XPA gene SNP rs3176683 (p-value=0.005), while dephosphorylation was significantly higher in subjects carrying the variant allele in three MRE11A gene SNPs: rs1014666, rs476137 and rs2508784 (p-value=0.003, 0.003 and 0.008, respectively). An additive effect of low-activity DNA repair alleles was associated with altered DSBR activity, as demonstrated by both H2AX phosphorylation at 1 h (p-trend <0.0001) and γH2AX dephosphorylation at 3h (p-trend <0.0001). Our study revealed that in addition to SNPs of genes that are well-established players in DSBR, non-DSBR genes, such as the XPA gene that is mainly involved in the nucleotide excision repair pathway, can also influence DSBR in healthy subjects. This suggests that successful DSBR may require both DSBR and non-DSBR mechanisms.

Phosphorylation of histone H2AX is a powerful tool for detecting chemical photogenotoxicity

Several light-absorbing chemicals are known to show phototoxic effects involving many kinds of DNA damage, and are suspected of initiating skin cancer. In this study, we clarified that phosphorylated histone H2AX (γ-H2AX) (phosphorylated histone H2AX), which was produced with the induction of DNA double-strand breaks, is a sensitive photogenotoxic marker. The immortal human keratinocyte line HaCaT was treated with a library of 11 chemicals (including known strong and weak phototoxic chemicals, and nonphototoxic chemicals) and/or UVA exposure. γ-H2AX was generated after treatments with all phototoxic chemicals and UVA. The limit of detection using γ-H2AX was 100-1,000 times lower than that using cell viability and DNA gel electrophoresis. γ-H2AX was not generated following treatments with nonphototoxic chemicals and UVA. These results indicated that γ-H2AX is a powerful tool for detecting chemical photogenotoxicity.

Novel role of base excision repair in mediating cisplatin cytotoxicity

Using isogenic mouse embryonic fibroblasts and human cancer cell lines, we show that cells defective in base excision repair (BER) display a cisplatin-specific resistant phenotype. This was accompanied by enhanced repair of cisplatin interstrand cross-links (ICLs) and ICL-induced DNA double strand breaks, but not intrastrand adducts. Cisplatin induces abasic sites with a reduced accumulation in uracil DNA glycosylase (UNG) null cells. We show that cytosines that flank the cisplatin ICLs undergo preferential oxidative deamination in vitro, and AP endonuclease 1 (APE1) can cleave the resulting ICL DNA substrate following removal of the flanking uracil. We also show that DNA polymerase β has low fidelity at the cisplatin ICL site after APE1 incision. Down-regulating ERCC1-XPF in BER-deficient cells restored cisplatin sensitivity. Based on our results, we propose a novel model in which BER plays a positive role in maintaining cisplatin cytotoxicity by competing with the productive cisplatin ICL DNA repair pathways.

Targeted gene correction using psoralen, chlorambucil and camptothecin conjugates of triplex forming peptide nucleic acid (PNA)

Gene correction activation effects of a small series of triplex forming peptide nucleic acid (PNA) covalently conjugated to the DNA interacting ligands psoralen, chlorambucil and camptothecin targeted proximal to a stop codon mutation in an EGFP reporter gene were studied. A 15-mer homopyrimidine PNA conjugated to the topoisomerase I inhibitor camptothecin was found to increase the frequency of repair domain mediated gene correctional events of the EGFP reporter in an in vitro HeLa cell nuclear extract assay, whereas PNA psoralen or chlorambucil conjugates both of which form covalent and also interstrand crosslinked adducts with dsDNA dramatically decreased the frequency of targeted repair/correction. The PNA conjugates were also studied in mammalian cell lines upon transfection of PNA bound EGFP reporter vector and scoring repair of the EGFP gene by FACS analysis of functional EGFP expression. Consistent with the extract experiments, treatment with adduct forming PNA conjugates (psoralen and chlorambucil) resulted in a decrease in background correction frequencies in transiently transfected cells, whereas unmodified PNA or the PNA-camptothecin conjugate had little or no effect. These results suggest that simple triplex forming PNAs have little effect on proximal gene correctional events whereas PNA conjugates capable of forming DNA adducts and interstrand crosslinks are strong inhibitors. Most interestingly the PNA conjugated to the topoisomerase inhibitor, camptothecin enhanced repair in nuclear extract. Thus the effects and use of camptothecin conjugates in gene targeted repair merit further studies.

DNA interstrand crosslink repair in mammalian cells: step by step

Interstrand DNA crosslinks (ICLs) are formed by natural products of metabolism and by chemotherapeutic reagents. Work in E. coli identified a two cycle repair scheme involving incisions on one strand on either side of the ICL (unhooking) producing a gapped intermediate with the incised oligonucleotide attached to the intact strand. The gap is filled by recombinational repair or lesion bypass synthesis. The remaining monoadduct is then removed by nucleotide excision repair (NER). Despite considerable effort, our understanding of each step in mammalian cells is still quite limited. In part this reflects the variety of crosslinking compounds, each with distinct structural features, used by different investigators. Also, multiple repair pathways are involved, variably operative during the cell cycle. G(1) phase repair requires functions from NER, although the mechanism of recognition has not been determined. Repair can be initiated by encounters with the transcriptional apparatus, or a replication fork. In the case of the latter, the reconstruction of a replication fork, stalled or broken by collision with an ICL, adds to the complexity of the repair process. The enzymology of unhooking, the identity of the lesion bypass polymerases required to fill the first repair gap, and the functions involved in the second repair cycle are all subjects of active inquiry. Here we will review current understanding of each step in ICL repair in mammalian cells.

XPF/ERCC4 and ERCC1: their products and biological roles

ERCC4 is the gene mutated in XPF cells and also in rodent cells representing the mutant complementation groups ERCC4 and ERCC 11. The protein functions principally as a complex with ERCC1 in a diversity of biological pathways that include NER, ICL repair, telomere maintenance and immunoglobulin switching. Sorting out these roles is an exciting and challenging problem and many important questions remain to be answered. The ERCC1/ERCC4 complex is conserved across most species presenting an opportunity to examine some functions in model organisms where mutants can be more readily generated and phenotypes more quickly assessed.

Rad54B targeting to DNA double-strand break repair sites requires complex formation with S100A11

S100A11 is involved in a variety of intracellular activities such as growth regulation and differentiation. To gain more insight into the physiological role of endogenously expressed S100A11, we used a proteomic approach to detect and identify interacting proteins in vivo. Hereby, we were able to detect a specific interaction between S100A11 and Rad54B, which could be confirmed under in vivo conditions. Rad54B, a DNA-dependent ATPase, is described to be involved in recombinational repair of DNA damage, including DNA double-strand breaks (DSBs). Treatment with bleomycin, which induces DSBs, revealed an increase in the degree of colocalization between S100A11 and Rad54B. Furthermore, S100A11/Rad54B foci are spatially associated with sites of DNA DSB repair. Furthermore, while the expression of p21(WAF1/CIP1) was increased in parallel with DNA damage, its protein level was drastically down-regulated in damaged cells after S100A11 knockdown. Down-regulation of S100A11 by RNA interference also abolished Rad54B targeting to DSBs. Additionally, S100A11 down-regulated HaCaT cells showed a restricted proliferation capacity and an increase of the apoptotic cell fraction. These observations suggest that S100A11 targets Rad54B to sites of DNA DSB repair sites and identify a novel function for S100A11 in p21-based regulation of cell cycle.

3-Methyladenine DNA glycosylase is important for cellular resistance to psoralen interstrand cross-links

DNA interstrand cross-links (ICLs), widely used in chemotherapy, are cytotoxic lesions because they block replication and transcription. Repair of ICLs involves proteins from different repair pathways however the precise mechanism is still not completely understood. Here, we report that the 3-methyladenine DNA glycosylase (Aag), an enzyme that initiates base excision repair at a variety of alkylated bases, is also involved in the repair of ICLs. Aag(-/-) mouse embryonic stem cells were shown to be more sensitive to the cross-linking agent 4,5',8-trimethylpsoralen than wild-type cells, but no more sensitive than wild-type to the psoralen derivative Angelicin that forms only monoadducts. We show that gamma-H2AX foci formation, a marker for double strand breaks that are formed during ICL repair, is impaired in psoralen treated Aag(-/-) cells in both quantity and kinetics. However, in our in vitro system, purified human AAG can neither bind to the ICL nor cleave it. Taken together, our results suggest that Aag is important for the resistance of mouse ES cells to psoralen-induced ICLs.

Histone H2AX phosphorylation as a molecular pharmacological marker for DNA interstrand crosslink cancer chemotherapy

The aims of this study were to investigate mechanisms of action involved in H2AX phosphorylation by DNA interstrand crosslinking (ICL) agents and determine whether gammaH2AX could be a suitable pharmacological marker for identifying potential ICL cellular chemosensitivity. In normal human fibroblasts, after treatment with nitrogen mustard (HN2) or cisplatin, the peak gammaH2AX response was detected 2-3 h after the peak of DNA ICLs measured using the comet assay, a validated method for detecting ICLs in vitro or in clinical samples. Detection of gammaH2AX foci by immunofluorescence microscopy could be routinely detected with 6-10 times lower concentrations of both drugs compared to detection of ICLs using the comet assay. A major pathway for repairing DNA ICLs is the initial unhooking of the ICL by the ERCC1-XPF endonuclease followed by homologous recombination. HN2 or cisplatin-induced gammaH2AX foci persisted significantly longer in both, ERCC1 or XRCC3 (homologous recombination) defective Chinese hamster cells that are highly sensitive to cell killing by ICL agents compared to wild type or ionising radiation sensitive XRCC5 cells. An advantage of using gammaH2AX immunofluorescence over the comet assay is that it appears to detect ICL chemosensitivity in both ERCC1 and HR defective cells. With HN2 and cisplatin, gammaH2AX foci also persisted in chemosensitive human ovarian cancer cells (A2780) compared to chemoresistant (A2780cisR) cells. These results show that gammaH2AX can act as a highly sensitive and general marker of DNA damage induced by HN2 or cisplatin and shows promise for predicting potential cellular chemosensitivity to ICL agents.

DNA polymerase eta reduces the gamma-H2AX response to psoralen interstrand crosslinks in human cells

DNA interstrand crosslinks are processed by multiple mechanisms whose relationships to each other are unclear. Xeroderma pigmentosum-variant (XP-V) cells lacking DNA polymerase eta are sensitive to psoralen photoadducts created under conditions favoring crosslink formation, suggesting a role for translesion synthesis in crosslink repair. Because crosslinks can lead to double-strand breaks, we monitored phosphorylated H2AX (gamma-H2AX), which is typically generated near double-strand breaks but also in response to single-stranded DNA, following psoralen photoadduct formation in XP-V fibroblasts to assess whether polymerase eta is involved in processing crosslinks. In contrast to conditions favoring monoadducts, conditions favoring psoralen crosslinks induced gamma-H2AX levels in both XP-V and nucleotide excision repair-deficient XP-A cells relative to control repair-proficient cells; ectopic expression of polymerase eta in XP-V cells normalized the gamma-H2AX response. In response to psoralen crosslinking, gamma-H2AX as well as 53BP1 formed coincident foci that were more numerous and intense in XP-V and XP-A cells than in controls. Psoralen photoadducts induced gamma-H2AX throughout the cell cycle in XP-V cells. These results indicate that polymerase eta is important in responding to psoralen crosslinks, and are consistent with a model in which nucleotide excision repair and polymerase eta are involved in processing crosslinks and avoiding gamma-H2AX associated with double-strand breaks and single-stranded DNA in human cells.

Replication blocking lesions present a unique substrate for homologous recombination

Homologous recombination (HR) plays a critical role in the restart of blocked replication forks, but how this is achieved remains poorly understood. We show that mutants in the single Rad51 paralog in Caenorhabditis elegans, rfs-1, permit discrimination between HR substrates generated at DNA double-strand breaks (DSBs), or following replication fork collapse from HR substrates assembled at replication fork barriers (RFBs). Unexpectedly, RFS-1 is dispensable for RAD-51 recruitment to meiotic and ionizing radiation (IR)-induced DSBs and following replication fork collapse, yet, is essential for RAD-51 recruitment to RFBs formed by DNA crosslinking agents and other replication blocking lesions. Deletion of rfs-1 also suppresses the accumulation of toxic HR intermediates in him-6; top-3 mutants and accelerates deletion formation at presumed endogenous RFBs formed by poly G/C tracts in the absence of DOG-1. These data suggest that RFS-1 is not a general mediator of HR-dependent DSB repair, but acts specifically to promote HR at RFBs. HR substrates generated at conventional DSBs or following replication fork collapse are therefore intrinsically different from those produced during normal repair of blocked replication forks.