An interaction between the mammalian DNA repair protein XRCC1 and DNA ligase III

Study of interaction of XRCC1 with DNA and proteins of base excision repair by photoaffinity labeling technique

The X-ray repair cross-complementing group 1 (XRCC1) protein plays a central role in base excision repair (BER) interacting with and modulating activity of key BER proteins. To estimate the influence of XRCC1 on interactions of BER proteins poly(ADP-ribose) polymerase 1 (PARP1), apurinic/apyrimidinic endonuclease 1 (APE1), flap endonuclease 1 (FEN1), and DNA polymerase beta (Pol beta) with DNA intermediates, photoaffinity labeling using different photoreactive DNA was carried out in the presence or absence of XRCC1. XRCC1 competes with APE1, FEN1, and PARP1 for DNA binding, while Pol beta increases the efficiency of XRCC1 modification. To study the interactions of XRCC1 with DNA and proteins at the initial stages of BER, DNA duplexes containing a photoreactive group in the template strand opposite the damage were designed. DNA duplexes with 8-oxoguanine or dihydrothymine opposite the photoreactive group were recognized and cleaved by specific DNA glycosylases (OGG1 or NTH1, correspondingly), although the rate of oxidized base excision in the photoreactive structures was lower than in normal substrates. XRCC1 does not display any specificity in recognition of DNA duplexes with damaged bases compared to regular DNA. A photoreactive group opposite a synthetic apurinic/apyrimidinic (AP) site (3-hydroxy-2-hydroxymethyltetrahydrofuran) weakly influences the incision efficiency of AP site analog by APE1. In the absence of magnesium ions, i.e. when incision of AP sites cannot occur, APE1 and XRCC1 compete for DNA binding when present together. However, in the presence of magnesium ions the level of XRCC1 modification increased upon APE1 addition, since APE1 creates nicked DNA duplex, which interacts with XRCC1 more efficiently.

Base excision repair processing of abasic site/single-strand break lesions within clustered damage sites associated with XRCC1 deficiency

Ionizing radiation induces clustered DNA damage, which presents a challenge to the cellular repair machinery. The repair efficiency of a single-strand break (SSB) is ∼4× less than that for repair of an abasic (AP) site when in a bistranded cluster containing 8-oxoG. To explore whether this difference in repair efficiency involves XRCC1 or other BER proteins, synthetic oligonucleotides containing either an AP site or HAP1-induced SSB (HAP1-SSB) 1 or 5 bp 5′ or 3′ to 8-oxoG on the opposite strand were synthesized and the repair investigated using either nuclear extracts from hamster cells proficient (AA8) or deficient (EM7) in XRCC1 or purified BER proteins. XRCC1 is important for efficient processing of an AP site in clustered damage containing 8-oxoG but does not affect the already low repair efficiency of a SSB. Ligase I partly compensates for the absence of the XRCC1/ligaseIII during short-patch BER of an AP site when in a cluster but only weakly if at all for a HAP1-SSB. The major difference between the repair of an AP site and a HAP1-SSB when in a 8-oxoG containing cluster is the greater efficiency of short-patch BER with the AP site compared with that for a HAP1-SSB.

Binary PAH mixtures cause additive or antagonistic effects on gene expression but synergistic effects on DNA adduct formation

Polycyclic aromatic hydrocarbons (PAHs) cover a wide range of structurally related compounds which differ greatly in their carcinogenic potency. PAH exposure usually occurs through mixtures rather than individual compounds. Therefore, we assessed whether the effects of binary PAH mixtures on gene expression, DNA adduct formation, apoptosis and cell cycle are additive compared with the effects of the individual compounds in human hepatoma cells (HepG2). Equimolar and equitoxic mixtures of benzo[a]pyrene (B[a]P) with either dibenzo[a,l]pyrene (DB[a,l]P), dibenzo[a,h]anthracene (DB[a,h]A), benzo[b]fluoranthene (B[b]F), fluoranthene (FA) or 1-methylphenanthrene (1-MPA) were studied. DB[a,l]P, B[a]P, DB[a,h]A and B[b]F dose-dependently increased apoptosis and blocked cells cycle in S-phase. PAH mixtures showed an additive effect on apoptosis and on cell cycle blockage. DNA adduct formation in mixtures was higher than expected based on the individual compounds, indicating a synergistic effect of PAH mixtures. Equimolar mixtures of B[a]P and DB[a,l]P (0.1, 0.3 and 1.0 microM) were assessed for their effects on gene expression. Only at 1.0 microM, the mixture showed antagonism. All five compounds were also tested as a binary mixture with B[a]P in equitoxic concentrations. The combinations of B[a]P with B[b]F, DB[a,h]A or FA showed additivity, whereas B[a]P with DB[a,l]P or 1-MPA showed antagonism. Many individual genes showed additivity in mixtures, but some genes showed mostly antagonism or synergism. Our results show that the effects of binary mixtures of PAHs on gene expression are generally additive or slightly antagonistic, suggesting no effect or decreased carcinogenic potency, whereas the effects on DNA adduct formation show synergism, which rather indicates increased carcinogenic potency.

Advances in the understanding of susceptibility to treatment-related acute myeloid leukaemia

Treatment-related acute myeloid leukaemia (t-AML) is a devastating complication following exposure to the cytotoxic and genotoxic agents used to treat a primary malignancy. Whilst the incidence of t-AML is rising, it still only occurs in a minority of patients who have received chemotherapy and/or radiotherapy treatment and hence it is important to identify factors that may confer susceptibility to the development of the condition. This paper reviews the literature and discusses the advances and limitations in our understanding of susceptibility factors to t-AML. In particular, it concentrates upon genetic polymorphisms in detoxification genes and in genes belonging to the major DNA repair pathways. This review also considers more novel susceptibility factors, such as those proposed to determine stem cell number. Increased understanding of t-AML susceptibility may enable steps to be taken to prevent its development and increase the effectiveness of treatment of the disease.

XRCC1 down-regulation in human cells leads to DNA-damaging agent hypersensitivity, elevated sister chromatid exchange, and reduced survival of BRCA2 mutant cells

Previous studies using rodent cells indicate that a deficiency in XRCC1 results in reduced single-strand break repair, increased sensitivity to DNA-damaging agents, and elevated levels of sister chromatid exchange (SCE). Epidemiological studies have suggested an association of certain human XRCC1 polymorphisms with genetic instability and cancer susceptibility. However, investigations on the molecular functions of XRCC1 in human cells are limited. To determine the contributions of this nonenzymatic scaffold protein, we suppressed XRCC1 levels in several human cell lines using small interfering RNA (siRNA) technology. We report that XRCC1 down-regulation in HeLa cells leads to a concomitant decrease in the DNA ligase 3 protein level and an impaired nick ligation capacity. In addition, depletion of XRCC1 resulted in a significantly increased sensitivity to the alkylating agent methyl methanesulfonate and the thymidine base analog 5-hydroxymethyl-2'-deoxyuridine, a slightly increased sensitivity to ethyl methanesulfonate and 1,3-bis(2-chloroethyl)-1-nitrosourea, and no change in the response to camptothecin. We also discovered that a 70-80% reduction in XRCC1 protein leads to an elevated level of SCE in both HeLa cells and normal human fibroblasts, but does not affect chromosome aberrations in the diploid fibroblasts. Last, XRCC1 siRNA transfection led to an approximately 40% decrease in the survival of BRCA2-deficient cells, supporting a model whereby the accumulation of unrepaired SSBs leads to the accumulation of cytotoxic DNA double strand breaks following replication fork collapse in cells defective in homologous recombination.

Targeting base excision repair to improve cancer therapies

Most commonly used cancer therapies, particularly ionizing radiation and certain classes of cytotoxic chemotherapies, cause cell death by damaging DNA. Base excision repair (BER) is the major system responsible for the removal of corrupt DNA bases and repair of DNA single strand breaks generated spontaneously and induced by exogenous DNA damaging factors such as certain cancer therapies. In this review, the physico-chemical properties of the proteins involved in BER are discussed with particular emphasis on molecular mechanisms coordinating repair processes. The aim of this review is to apply extensive knowledge that currently exists regarding the biochemical mechanisms involved in human BER to the molecular biology of current therapies for cancer. It is anticipated that the application of this knowledge will translate into the development of novel effective therapies for improving existing treatments such as radiation therapy and oxaliplatin chemotherapy.

Base excision repair proteins are required for integrin-mediated suppression of bleomycin-induced DNA breakage in murine lung endothelial cells

Engagement of integrin cell adhesion receptors suppresses bleomycin (BLM)-induced DNA strand breakage in endothelial cells. Previous investigation of cells from poly(ADP-ribose) polymerase (PARP)-1 knockout mice and with an inhibitor of the enzyme indicated that this facilitator of base excision repair (BER) is required for integrin-mediated suppression of DNA strand breakage. Here, small inhibitory RNA (siRNA) was used to assess the requirement for the BER proteins, DNA ligase III (Lig3) alpha, PARP-1, and X-ray repair complementing defective repair in Chinese hamster cells 1 (XRCC1), and for the long-patch BER ligase, DNA ligase I (Lig1), in integrin-mediated protection from BLM-induced DNA breakage. Murine lung endothelial cells (MLECs) were transfected with siRNA, treated with anti-beta1 integrin antibody, and then BLM. 3'-OH in DNA and accumulation of phosphorylated histone H2AX (gammaH2AX), which reflects double-strand breakage, were measured. Integrin antibody inhibited the increases in 3'-OH caused by BLM in MLECs transfected with either control or Lig1 siRNA. However, after knockdown of Lig3alpha, PARP-1, or XRCC1, suppression of DNA breakage by integrin antibody was limited. BLM increased gammaH2AX levels, and integrin treatment inhibited this by 57 to 73% in MLECs transfected with control siRNA. Integrin engagement also inhibited increases in gammaH2AX in BLM-treated cells transfected with Lig1 siRNA. In contrast, Lig3alpha, PARP-1, and XRCC1 siRNAs prevented integrin-mediated inhibition of BLM-induced gammaH2AX levels. The results suggest that the BER proteins, Lig3alpha, PARP-1, and XRCC1, are required for integrin-mediated suppression of BLM-induced DNA breakage.

A unified view of base excision repair: lesion-dependent protein complexes regulated by post-translational modification

Base excision repair (BER) proteins act upon a significantly broad spectrum of DNA lesions that result from endogenous and exogenous sources. Multiple sub-pathways of BER (short-path or long-patch) and newly designated DNA repair pathways (e.g., SSBR and NIR) that utilize BER proteins complicate any comprehensive understanding of BER and its role in genome maintenance, chemotherapeutic response, neuro-degeneration, cancer or aging. Herein, we propose a unified model of BER, comprised of three functional processes: Lesion Recognition/Strand Scission, Gap Tailoring and DNA Synthesis/Ligation, each represented by one or more multi-protein complexes and coordinated via the XRCC1/DNA Ligase III and PARP1 scaffold proteins. BER therefore may be represented by a series of repair complexes that assemble at the site of the DNA lesion and mediates repair in a coordinated fashion involving protein-protein interactions that dictate subsequent steps or sub-pathway choice. Complex formation is influenced by post-translational protein modifications that arise from the cellular state or the DNA damage response, providing an increase in specificity and efficiency to the BER pathway. In this review, we have summarized the reported BER protein-protein interactions and protein post-translational modifications and discuss the impact on DNA repair capacity and complex formation.

The DNA repair gene XRCC1 and genetic susceptibility of lung cancer in a northeastern Chinese population

To evaluate the effect of DNA repair gene XRCC1 polymorphisms on the risk of lung cancer in a northeastern Chinese population, we studied five cSNPs in the XRCC1 gene, three that lead to non-synonymous changes: Arg194Trp, Arg280 His and Arg399Gln and two that lead to synonymous changes: Pro206Pro and Gln632Gln. A hospital-based case-control study consisted of 247 lung cancer cases and 253 cancer-free controls matched on age, gender and ethnicity. PCR-RFLP was used for genotyping. Carriers of the minor G-allele of Pro206Pro were at significantly increased risk of lung cancer (adjusted OR=1.96, 95% CI=1.26-3.06, P=0.003). Stratified analyses revealed a significantly decreased risk of lung cancer associated with the AG/AA genotype of Arg280His (AG+AA versus GG, OR=0.38, 95% CI=0.19-0.75, P=0.005) among never smokers, although there was no interaction between Arg280His and smoking. In a haplotype analysis, a haplotype defined by Arg194Trp(C)-Pro206Pro(G)-Arg280His(G)-Arg399Gln(G)-Gln632Gln(G) was associated with increased risk of lung cancer (OR=28.60, 95% CI=2.49-331.31, P=4.45x10(-5)). No associations were observed for the other polymorphisms or haplotypes. Our results suggest that the XRCC1 Pro206Pro polymorphism or the haplotype encompassing the minor allele may contribute to genetic susceptibility for lung cancer in this northeastern Chinese population. To our knowledge, this is first report that XRCC1 Pro206Pro influences cancer risk.

Cancer-related mutations in BRCA1-BRCT cause long-range structural changes in protein-protein binding sites: a molecular dynamics study

Cancer-associated mutations in the BRCT domain of BRCA1 (BRCA1-BRCT) abolish its tumor suppressor function by disrupting interactions with other proteins such as BACH1. Many cancer-related mutations do not cause sufficient destabilization to lead to global unfolding under physiological conditions, and thus abrogation of function probably is due to localized structural changes. To explore the reasons for mutation-induced loss of function, the authors performed molecular dynamics simulations on three cancer-associated mutants, A1708E, M1775R, and Y1853ter, and on the wild type and benign M1652I mutant, and compared the structures and fluctuations. Only the cancer-associated mutants exhibited significant backbone structure differences from the wild-type crystal structure in BACH1-binding regions, some of which are far from the mutation sites. Backbone differences of the A1708E mutant from the liganded wild type structure in these regions are much larger than those of the unliganded wild type X-ray or molecular dynamics structures. These BACH1-binding regions of the cancer-associated mutants also exhibited increases in their fluctuation magnitudes compared with the same regions in the wild type and M1562I mutant, as quantified by quasiharmonic analysis. Several of the regions of increased fluctuation magnitude correspond to correlated motions of residues in contact that provide a continuous path of fluctuating amino acids in contact from the A1708E and Y1853ter mutation sites to the BACH1-binding sites with altered structure and dynamics. The increased fluctuations in the disease-related mutants suggest an increase in vibrational entropy in the unliganded state that could result in a larger entropy loss in the disease-related mutants upon binding BACH1 than in the wild type. To investigate this possibility, vibrational entropies of the A1708E and wild type in the free state and bound to a BACH1-derived phosphopeptide were calculated using quasiharmonic analysis, to determine the binding entropy difference DeltaDeltaS between the A1708E mutant and the wild type. DeltaDeltaS was determined to be -4.0 cal mol(-1) K(-1), with an uncertainty of 2 cal mol(-1) K(-1); that is, the entropy loss upon binding the peptide is 4.0 cal mol(-1) K(-1) greater for the A1708E mutant, corresponding to an entropic contribution to the DeltaDeltaG of binding (-TDeltaDeltaS) 1.1 kcal mol(-1) more positive for the mutant. The observed differences in structure, flexibility, and entropy of binding likely are responsible for abolition of BACH1 binding, and illustrate that many disease- related mutations could have very long-range effects. The methods described here have potential for identifying correlated motions responsible for other long-range effects of deleterious mutations.

Organ and cell specificity of base excision repair mutants in mice

Genetically modified mouse models are a powerful approach to study the relation of a single gene-deletion to processes such as mutagenesis and carcinogenesis. The generation of base excision repair (BER) deficient mouse models has resulted in a re-examination of the cellular defence mechanisms that exist to counteract DNA base damage. This review discusses novel insights into the relation between specific gene-deletions and the organ and cell specificity of visible and molecular phenotypes, including accumulation of base lesions in genomic DNA and carcinogenesis. Although promising models exist, there is still a need for new models. These models should comprise combined deficiencies of DNA glycosylases which initiate the BER pathway, to elaborate on the repair redundancy, as well as conditional models of the intermediate steps of BER.

Genome instability and DNA damage accumulation in gene-targeted mice

Six major pathways for DNA repair have been identified. These include (1) DNA repair by direct reversal, (2) base excision repair, (3) mismatch repair, (4) nucleotide excision repair, (5) homologous recombination, and (6) non-homologous end-joining. In addition, several other cellular processes influence the response to DNA damage. The generation of gene-targeted organisms is crucial for assessing the relative contribution of single DNA repair proteins and DNA repair pathways in maintaining genome stability. In particular, the accumulation of DNA damage, mutations and cancer in unexposed gene-targeted animals illuminates the spontaneous load of a particular lesion and the relative significance of a single gene in a specific pathway. Strategies for the generation of gene-targeted mice have been available for 15 years and more than 100 different genes relevant to DNA repair have been targeted. This review describes some important progress made toward understanding spontaneous DNA damage and its repair, exemplified through one, or a few, gene-targeted mice from each major DNA repair pathway.

Differing effects of T4 DNA ligase in the modulation of the damage induced in mammalian cells by either X-rays or restriction endonucleases

BACKGROUND

Of the different lesions induced by X-rays, DNA double-strand breaks (DSBs) are considered the main cause of chromosomal aberrations and cell death. Restriction endonucleases (REs) induce only DNA DSBs and have frequently been used to mimic the effects of ionizing radiation in the study of DNA damage and repair.

METHODS

The present work makes use of clonogenic and cytogenetic assays to study the effect of T4 DNA ligase on modulating the damage induced by either X-rays or an RE (MspI) that produces breaks with cohesive ends. A CHO cell line defective in ligase III activity (EM9) and its corresponding parental line (AA8) were used.

RESULTS

Our results show that T4 DNA ligase increased cell survival and decreased chromosomal aberrations in cells treated with MspI, suggesting that most RE-induced DSBs can be repaired by a simple ligation. This enzyme was, however, unable to promote repair of the DNA damage induced by X-rays. Analysis of the ratios of exchange-type aberrations to chromatid break-type aberrations indicated that T4 ligase increased misrejoining of the DNA damage induced by X-rays. The results were similar in EM9 and AA8 cells, although the effect was greater in the cells deficient in DNA strand break rejoining. In addition, depending on whether the end strand break structure is 3'-hydroxyl and 5'-phosphoryl (REs) or more complex (X-rays), T4 DNA ligase could either promote the correct repair or, conversely, increase misrejoining.

CONCLUSION

The present results confirm the idea that DNA DSBs induced by cohesive cutting RE are repaired by different mechanisms than those induced by X-rays causing cell lethality.

Molecular mechanisms of sister-chromatid exchange

Sister-chromatid exchange (SCE) is the process whereby, during DNA replication, two sister chromatids break and rejoin with one another, physically exchanging regions of the parental strands in the duplicated chromosomes. This process is considered to be conservative and error-free, since no information is generally altered during reciprocal interchange by homologous recombination. Upon the advent of non-radiolabel detection methods for SCE, such events were used as genetic indicators for potential genotoxins/mutagens in laboratory toxicology tests, since, as we now know, most forms of DNA damage induce chromatid exchange upon replication fork collapse. Much of our present understanding of the mechanisms of SCE stems from studies involving nonhuman vertebrate cell lines that are defective in processes of DNA repair and/or recombination. In this article, we present a historical perspective of studies spearheaded by Dr. Anthony V. Carrano and colleagues focusing on SCE as a genetic outcome, and the role of the single-strand break DNA repair protein XRCC1 in suppressing SCE. A more general overview of the cellular processes and key protein "effectors" that regulate the manifestation of SCE is also presented.

XRCC1 interactions with base excision repair DNA intermediates

Abasic (AP) sites in DNA arise either spontaneously, or through glycosylase-catalyzed excision of damaged bases. Their removal by the base excision repair (BER) pathway avoids their mutagenic and cytotoxic consequences. XRCC1 coordinates and facilitates single-strand break (SSB) repair and BER in mammalian cells. We report that XRCC1, through its NTD and BRCT1 domains, has affinity for several DNA intermediates in BER. As shown by its capacity to form a covalent complex via Schiff base, XRCC1 binds AP sites. APE1 suppresses binding of XRCC1 to unincised AP sites however, affinity was higher when the DNA carried an AP-lyase- or APE1-incised AP site. The AP site binding capacity of XRCC1 is enhanced by the presence of strand interruptions in the opposite strand. Binding of XRCC1 to BER DNA intermediates could play an important role to warrant the accurate repair of damaged bases, AP sites or SSBs, in particular in the context of clustered DNA damage.

ATM mediates oxidative stress-induced dephosphorylation of DNA ligase IIIalpha

Among the three mammalian genes encoding DNA ligases, only the LIG3 gene does not have a homolog in lower eukaryotes. In somatic mammalian cells, the nuclear form of DNA ligase IIIalpha forms a stable complex with the DNA repair protein XRCC1 that is also found only in higher eukaryotes. Recent studies have shown that XRCC1 participates in S phase-specific DNA repair pathways independently of DNA ligase IIIalpha and is constitutively phosphorylated by casein kinase II. In this study we demonstrate that DNA ligase IIIalpha, unlike XRCC1, is phosphorylated in a cell cycle-dependent manner. Specifically, DNA ligase IIIalpha is phosphorylated on Ser123 by the cell division cycle kinase Cdk2 beginning early in S phase and continuing into M phase. Interestingly, treatment of S phase cells with agents that cause oxygen free radicals induces the dephosphorylation of DNA ligase IIIalpha. This oxidative stress-induced dephosphorylation of DNA ligase IIIalpha is dependent upon the ATM (ataxia-telangiectasia mutated) kinase and appears to involve inhibition of Cdk2 and probably activation of a phosphatase.

Induction of ROS formation, poly(ADP-ribose) polymerase-1 activation, and cell death by PCB126 and PCB153 in human T47D and MDA-MB-231 breast cancer cells

The primary purpose of this research is to investigate whether exposure to polychlorinated biphenyls (PCBs), i.e. PCB153 and PCB126, is associated with induction of reactive oxygen species (ROS), poly(ADP-ribose) polymerase-1 (PARP-1) activation, and cell death in human T47D and MDA-MB-231 breast cancer cells. Results indicated that PCB153 and PCB126 induced concentration- and time-dependent increases in cytotoxic response and ROS formation in both T47D and MDA-MB-231 cells. At non-cytotoxic concentrations both PCB153 and PCB126 induced decreases in intracellular NAD(P)H and NAD+ in T47D and MDA-MB-231 cells where T47D cells were more resistant to PCB-induced reduction in intracellular NAD(P)H than MDA-MB-231 cells. Further investigation indicated that three specific PARP inhibitors completely blocked PCB-induced decreases in intracellular NAD(P)H in both T47D and MDA-MB-231 cells. These results imply that decreases in intracellular NAD(P)H in PCB-treated cells may be, in part, due to depletion of intracellular NAD+ pool mediated by PARP-1 activation through formation of DNA strand breaks. Overall, the extent of cytotoxic response, ROS formation, and PARP-1 activation generated in T47D and MDA-MB-231 cells was greater for PCB153 than for PCB126. In addition, the cytotoxicity induced by PCB153 and PCB126 in both T47D and MDA-MB-231 cells was completely blocked by co-treatment of catalase, dimethylsulfoxide, cupper (I)-/iron (II)-specific chelators, and CYP1A/2B inhibitors. This evidence suggests the involvement of ROS, Cu(I), Fe(II), and CYP1A/2B enzymes in mediating the induction of cell death by PCB153 and PCB126. Further, antagonism was observed between PCB126 and PCB153 for effects on cytotoxic response and ROS formation in T47D and MDA-MB-231 cells. Antagonism was also observed between PCB153 and PCB126 in the induction of NAD(P)H depletion at lower concentration (<10 microM) in T47D cells, but not in MDA-MB-231 cells. In conclusions, results from our investigation suggest that ROS formation induced by PCBs is a significant determinant factor in mediating the DNA damage and cell death in human breast cancer cells. The data also suggests that the status of estrogen receptor alpha may play a role in modulating the PCB-induced oxidative DNA damage and cell death in human breast cancer cells.

Rapid analysis of XRCC1 polymorphisms using real-time polymerase chain reaction

DNA repair plays a critical role in protecting the genome from carcinogens or ionizing radiation. Three coding polymorphisms at codons 194, 280, and 399 in X-ray cross-complementing group 1 (XRCC1) DNA repair gene have been identified that may affect DNA repair and alter cancer susceptibility. In order to study their role in molecular-epidemiology studies we developed a single-step procedure for genotyping these polymorphisms using real-time polymerase chain reaction (rt-PCR) and subsequent melting curve analysis. Genotypes of 622 unrelated Caucasians without prior history of cancer were determined by real-time PCR and compared to genotypes obtained by restriction fragment length polymorphism PCR. In the population studied, the allele frequency of the XRCC1 26304 site (C-->T) of codon 194 in exon 6 was 0.065, the allele frequency of the XRCC1 27466 site (G-->A) of codon 280 in exon 9 was 0.048 and of the XRCC1 28152 site (G-->A) of codon 399 in exon 10 was 0.35. There was no disagreement between the two methods. These findings confirm the real-time fluorescence PCR method as a rapid and reliable assay for the analysis of large numbers of samples.

Condensin I interacts with the PARP-1-XRCC1 complex and functions in DNA single-strand break repair

Condensins are essential protein complexes critical for mitotic chromosome organization. Little is known about the function of condensins during interphase, particularly in mammalian cells. Here we report the interphase-specific interaction between condensin I and the DNA nick-sensor poly(ADP-ribose) polymerase 1 (PARP-1). We show that the association between condensin I, PARP-1, and the base excision repair (BER) factor XRCC1 increases dramatically upon single-strand break damage (SSB) induction. Damage-specific association of condensin I with the BER factors flap endonuclease 1 (FEN-1) and DNA polymerase delta/epsilon was also observed, suggesting that condensin I is recruited to interact with BER factors at damage sites. Consistent with this, DNA damage rapidly stimulates the chromatin association of PARP-1, condensin I, and XRCC1. Furthermore, depletion of condensin in vivo compromises SSB but not double-strand break (DSB) repair. Our results identify a SSB-specific response of condensin I through PARP-1 and demonstrate a role for condensin in SSB repair.

Single nucleotide polymorphisms of RecQ1, RAD54L, and ATM genes are associated with reduced survival of pancreatic cancer

PURPOSE

Our goal was to determine whether single nucleotide polymorphisms (SNPs) in DNA repair genes influence the clinical outcome of pancreatic cancer.

PATIENTS AND METHODS

We evaluated 13 SNPs of eight DNA damage response and repair genes in 92 patients with potentially resectable pancreatic adenocarcinoma. All patients were treated with neoadjuvant concurrent gemcitabine and radiotherapy with or without a component of induction gemcitabine/cisplatin at The University of Texas M.D. Anderson Cancer Center (Houston, TX) from February 1999 to August 2004 and observed through August 2005. Response to the pretreatment was assessed by evaluating time to tumor progression and overall survival. Kaplan-Meier plot, log-rank test, and Cox regression were used to compare survival of patients according to genotype.

RESULTS

The RecQ1 A159C, RAD54L C157T, XRCC1 R194W, and ATM T77C genotypes had a significant effect on the overall survival with log-rank P values of .001, .004, .001, and .02, respectively. A strong combined effect of the four genotypes was observed. Patients with none of the adverse genotypes had a mean survival time of 62.1 months, and those with one, two, or three or more at-risk alleles had median survival times of 27.5, 14.4, and 9.9 months, respectively (log-rank P < .001). There is a significant interaction between the RecQ1 gene and other genotypes. All four genes except XRCC1 remained as independent predictors of survival in multivariate Cox regression models adjusted for other clinical predictors.

CONCLUSION

These observations support the hypothesis that polymorphic variants of DNA repair genes affect clinical prognosis of patients with pancreatic cancer.

Roles of DNA ligase III and XRCC1 in regulating the switch between short patch and long patch BER

Damaged DNA bases are repaired by base excision repair (BER), which can proceed via two pathways: short patch and long patch BER. During the latter, a stretch of several nucleotides is replaced by strand displacement DNA synthesis. We recently demonstrated that the ATP concentration may govern the decision between these BER sub-pathways. Employing a reconstituted BER complex containing among others DNA polymerase beta (Pol beta), DNA ligase III (Lig III) and XRCC1, here we show that Lig III and XRCC1 are essential mediators of this regulation. XRCC1 stimulates Pol beta strand displacement activity and releases inhibition of Pol beta by DNA-bound Lig III if ligation is prevented. XRCC1 is thus able to strongly promote strand displacement and long patch BER under conditions of ATP shortage. If sufficient ATP is available, ligation by Lig III prevents strand displacement, leading to short patch BER. Ligation-inactive mutants of Lig III do not prevent strand displacement by Pol beta under the same conditions. Consequently, the preferred use of short patch BER depends on the ligation competence of Lig III. Accordingly, lowering the levels of the XRCC1/Lig III complex in HeLa cells using siRNA decreases ligation capacity but enhances Pol beta-dependent DNA synthesis.

DNA repair gene polymorphisms and risk of second primary neoplasms and mortality in oral cancer patients

OBJECTIVES/HYPOTHESIS

We tested the hypothesis that polymorphisms in genes involved in DNA repair pathways are associated with the development of second primary neoplasms of the upper aerodigestive tract (UADT), as well as mortality, in patients previously diagnosed with oral squamous cell cancer (OSCC).

METHODS

DNA specimens from 279 OSCC patients who had participated in two previous population-based case-control studies were assayed for the following polymorphisms: X-ray repair cross-complementing (XRCC) 1 Arg399Gln, XRCC3 Thr241Met, xeroderma pigmentosum complementing group D (XPD) Lys751Gln, and O-methylguanine- DNA methyltransferase (MGMT) Leu84Phe and Val143Ile. Baseline demographic information was obtained from personal interviews and tumor characteristics and treatment were obtained from cancer registry files. Cox proportional hazards models were used to calculate hazards ratio (HR) estimates for each polymorphism in relation to the risk of developing second primary neoplasms at any site, UADT, and head and neck. HRs were also determined for associations with all-cause mortality and oral cancer specific mortality.

RESULTS

A significant increased risk of second neoplasms (all sites combined, as well as for UADT sites and for head and neck squamous cell cancers) was observed among XRCC3 241Met allele homozygotes (HR 2.65-3.44, P < .02). No significant association with the development of second neoplasms was observed for the XRCC1 399Gln, XPD 751Gln, or MGMT 84Phe or 143Ile alleles. Although no associations with oral cancer-specific mortality were observed, we found a significant inverse association between all-cause mortality and possessing at least one copy of the XRCC1 399Gln allele (HR 0.68, 95% confidence interval [CI] 0.47-0.97, P = .03), as well as a suggestion of a direct association between all-cause mortality and having one copy of the XRCC3 241Met allele (HR 1.39, 95% CI 0.95-2.03, P = .09).

CONCLUSIONS

Polymorphisms in the DNA repair enzyme gene XRCC3 241Met was associated with an increased risk of second neoplasms, and polymorphisms of the XRCC1 399Gln gene were associated with a decreased risk of all-cause mortality in patients with primary OSCC. These findings require confirmation in other populations before the clinical implications can be considered.

Human DNA ligases I, III, and IV-purification and new specific assays for these enzymes

The joining of DNA strand breaks by DNA ligases is required to seal Okazaki fragments during DNA replication and to complete almost all DNA repair pathways. In human cells, there are multiple species of DNA ligase encoded by the LIG1, LIG3, and LIG4 genes. Here we describe protocols to overexpress and purify recombinant DNA ligase I, DNA ligase IIIbeta, and DNA ligase IV/XRCC4 and the assays used to purify and distinguish between these enzymes. In addition, we describe a fluorescence-based ligation assay that can be used for high throughput screening of chemical libraries.

Single nucleotide polymorphisms of DNA repair genes XRCC1 and XPD and its molecular mapping in Indian oral cancer

Tobacco users with diminished ability to repair somatic mutations may be more susceptible to tobacco attributable cancers. The distribution of single nucleotide polymorphisms (SNPs) in DNA repair genes XRCC1 and XPD in 110 oral carcinoma cases, 84 leukoplakia and 110 controls belonging to the Travancore South Indian population were examined. SNPs investigated included Arg194Trp, Arg280His, and Arg399Gln of the XRCC1 gene and Lys751Gln of the XPD gene. In addition, one of the variants positions, A399G, was mapped onto the BRCT I domain model built by comparative modeling (threading). Presence of the polymorphic variant of XRCC1 codon 194 and 399 and XPD was associated with increased risk of oral cancer compared to the wild genotype. Smokers and betel quid chewers with the variant allele of XRCC1 399 codon and XPD also exhibited increased risk of oral cancer. The A399G variant position mapped onto the surface of the BRCT I domain provides a possible rationale for altered XRCC1 function. These results suggest that polymorphisms in functionally important repair genes, specifically, those that map onto the protein surface may alter protein function without significantly affecting its structure.

The polymorphism and haplotypes of XRCC1 and survival of non–small-cell lung cancer after radiotherapy

PURPOSE

The X-ray repair cross-complementing Group 1 (XRCC1) protein is involved mainly in the base excision repair of the DNA repair process. This study examined the association of 3 polymorphisms (codon 194, 280, and 399) of XRCC1 and lung cancer in terms of whether or not these polymorphisms have an effect on the survival of lung cancer patients who have received radiotherapy.

METHODS AND MATERIALS

Between January 2000 and April 2004, 229 lung cancer patients with non-small-cell lung cancer in Stages I-III were enrolled. Genotyping was performed by single base primer extension assay using the SNP-IT Kit with genomic DNA samples from all patients. The haplotype of the XRCC1 polymorphisms was estimated by PHASE version 2.1.

RESULTS

The patients consisted of 191 (83.4%) males and 38 (16.6%) females with a median age of 62 (range, 26-88 years). Sixty percent of the patients were included in Stage I-IIIa. The median progression-free and overall survival was 13 months and 16 months, respectively. The XRCC1 codon 194, histology, and stage were shown to be significant predictors of the progression-free survival. The 6 haplotypes among the XRCC1 polymorphisms (194, 280, and 399) were estimated by PHASE v.2.1. The patients with haplotype pairs other than the homozygous TGG haplotype pairs survived significantly longer (p = 0.04).

CONCLUSIONS

Polymorphisms of XRCC1 have an effect on the survival of lung cancer patients treated with radiotherapy, and this effect seems to be more significant after the haplotype pairs are considered.

XRCC1 interactions with multiple DNA glycosylases: a model for its recruitment to base excision repair

Repair of chemically modified bases in DNA is accomplished through base excision repair (BER). This pathway is initiated by a specific DNA glycosylase that recognizes and excises the altered base to yield an abasic (AP) site. After cleavage of the AP site by APE1, repair proceeds through re-synthesis and ligation steps. In mammalian cells, the XRCC1 protein, essential for the maintenance of genomic stability, is involved in both base excision and single-strand break repair. XRCC1 participates in the first step of BER by interacting with the human DNA glycosylases hOGG1 and NEIL1. To analyze the possibility of a general mechanism involving the interaction of XRCC1 with DNA glycosylases we used XRCC1 to pull-down DNA glycosylases activities from human cell extracts. XRCC1 co-purifies with DNA glycosylase activities capable of excising hypoxanthine and dihydrothymine, in addition to 8-oxoguanine, but not uracil. Biochemical analyses with the purified proteins confirmed the interactions between XRCC1 and MPG, hNTH1 or hNEIL2. Furthermore, XRCC1 stimulates the activities of these enzymes. In vivo localization studies show that after genotoxic treatments these DNA glycosylases can be found associated with XRCC1 foci. Our results support a BER model in which XRCC1 is recruited to the repair of alkylated or oxidized bases by the enzyme recognizing the lesion. XRCC1 would then coordinate the subsequent enzymatic steps and modulate the activities of all the proteins involved.

Polymorphisms of XRCC1 gene, alcohol consumption and colorectal cancer

To evaluate contribution of polymorphisms of the XRCC1 gene to the risk of colorectal cancer, we conducted a case-control study of 209 colorectal cancer cases and 209 age- and gender-matched controls in the Korean population. We tested the hypothesis by constructing allele combinations with known SNP. Allelic variants of the XRCC1 gene at codons 194, 280 and 399 were analyzed in lymphocyte DNA by PCR-RFLP. We observed an increased risk of colorectal cancer associated with the 399Gln allele. The odds ratio (OR) was 1.61 (95% confidence interval [CI] 1.09-2.39) for the 399Gln allele. When combined allele-specific OR were calculated after estimating frequencies, 3 common allele combinations were found to be associated with an increased risk of colorectal cancer. The OR for the 194Trp-280Arg-399Arg was 1.48 (95% CI = 1.06-2.07) using 194Arg-280Arg-399Arg as the reference. The OR for the 194Arg-280His-399Arg and the 194Arg-280Arg-399Gln were 1.78 (95% CI = 1.09-2.89) and 1.78 (95% CI = 1.23-2.59), respectively. Analysis after controlling for smoking, exercise and dietary habits indicated that alcohol consumption (> or =80 g/week) is a significant risk factor of colorectal cancer (OR = 2.60, 95% CI = 1.46-4.62). An increased risk for colorectal cancer was identified in alcohol drinkers with the risky allele combinations. Our results suggest that polymorphisms in the XRCC1 genes may contribute to colorectal cancer susceptibility, and some evidence was obtained of a genetic modification for the relationship between alcohol intake and colorectal cancer.

Radiosensitization by a dominant negative to DNA polymerase β is DNA polymerase β-independent and XRCC1-dependent

BACKGROUND AND PURPOSE

DNA base damages and single strand breaks after ionizing radiation are repaired by base excision repair (BER) and single strand break repair (SSBR), with both DNA polymerase beta (polbeta) and XRCC1 playing key roles. We previously showed that a dominant negative to polbeta (polbetaDN) sensitized human tumor cells to ionizing radiation. However, polbeta-deficient cells, in contrast to XRCC1-deficient cells, are not more radiosensitive. The purpose of the present study was to further elucidate the mechanism of action of the polbetaDN to better understand the roles of BER and SSBR in determining radiosensitivity.

MATERIALS AND METHODS

Mouse embryonic fibroblasts, both polbeta wildtype and knockout, and hamster XRCC1-deficient EM9 cells together with its parental line, were transfected with the polbetaDN. Clones with equal polbetaDN expression levels were selected and used in clonogenic assays to determine radiosensitivity.

RESULTS

Radiosensitization of polbeta deficient cells by the polbetaDN is shown here, demonstrating inhibition of a polbeta-independent pathway. In addition, we observed radiosensitization of wildtype hamster cells but no radiosensitization of the XRCC1-deficient EM9 cells.

CONCLUSIONS

The polbetaDN acts independently of polbeta status and inhibits a pathway, which is dependent on XRCC1, consistent with inhibition of BER and/or SSBR. The data further indicate involvement of other polymerases, which are inhibited by polbetaDN. Finally, they demonstrate that inhibition of BER and SSBR can increase radiosensitivity, with potential clinical relevance.

Specificity of protein interactions mediated by BRCT domains of the XRCC1 DNA repair protein

Protein interactions critical to DNA repair and cell cycle control systems are often coordinated by modules that belong to a superfamily of structurally conserved BRCT domains. Because the mechanisms of BRCT interactions and their significance are not well understood, we sought to define the affinity and specificity of those BRCT modules that orchestrate base excision repair and single-strand break repair. Common to these pathways is the essential XRCC1 DNA repair protein, which interacts with at least nine other proteins and DNA. Here, we characterized the interactions of four purified BRCT domains, two from XRCC1 and their two partners from DNA ligase IIIalpha and poly(ADP-ribosyl) polymerase 1. A monoclonal antibody was selected that recognizes the ligase IIIalpha BRCT domain, but not the other BRCT domains, and was used to capture the relevant ligase IIIalpha BRCT complex. To examine the assembly states of isolated BRCT domains and pairwise domain complexes, we used size-exclusion chromatography coupled with on-line light scattering. This analysis indicated that isolated BRCT domains form homo-oligomers and that the BRCT complex between the C-terminal XRCC1 domain and the ligase IIIalpha domain is a heterotetramer with 2:2 stoichiometry. Using affinity capture and surface plasmon resonance methods, we determined that specific heteromeric interactions with high nanomolar dissociation constants occur between pairs of cognate BRCT domains. A structural model for a XRCC1 x DNA ligase IIIalpha heterotetramer is proposed as a core base excision repair complex, which constitutes a scaffold for higher order complexes to which other repair proteins and DNA are brought into proximity.

Micronuclei in EM9 cells expressing polymorphic forms of human XRCC1

X-ray repair cross-complementing gene 1 (XRCC1) is involved in base excision repair (BER) through interaction with other BER enzymes, and polymorphisms in XRCC1 appear to increase the risk of various cancers. We evaluated how three XRCC1 polymorphisms, Arg194Trp, Arg280His and Arg399Gln, affect the extent of DNA damage and repair using the micronucleus assay. XRCC1 cDNAs containing the wild-type sequence and the three polymorphisms were overexpressed in EM9 cells, which lack the full sequence needed to perform XRCC1 functions. Normal human XRCC1 cDNA corrected the defect in EM9 cells. Only XRCC1 cDNA containing the Arg399Gln polymorphism did not fully correct the DNA repair defect in EM9 cells. These results indicate that the Arg399Gln polymorphism, but not the Arg194Trp or Arg280His polymorphism, influences the ability of XRCC1 to repair DNA. This study may provide a model that can be used to evaluate the functional significance of polymorphisms in DNA repair genes.

Defective DNA single-strand break repair in spinocerebellar ataxia with axonal neuropathy-1

Spinocerebellar ataxia with axonal neuropathy-1 (SCAN1) is a neurodegenerative disease that results from mutation of tyrosyl phosphodiesterase 1 (TDP1). In lower eukaryotes, Tdp1 removes topoisomerase 1 (top1) peptide from DNA termini during the repair of double-strand breaks created by collision of replication forks with top1 cleavage complexes in proliferating cells. Although TDP1 most probably fulfils a similar function in human cells, this role is unlikely to account for the clinical phenotype of SCAN1, which is associated with progressive degeneration of post-mitotic neurons. In addition, this role is redundant in lower eukaryotes, and Tdp1 mutations alone confer little phenotype. Moreover, defects in processing or preventing double-strand breaks during DNA replication are most probably associated with increased genetic instability and cancer, phenotypes not observed in SCAN1 (ref. 8). Here we show that in human cells TDP1 is required for repair of chromosomal single-strand breaks arising independently of DNA replication from abortive top1 activity or oxidative stress. We report that TDP1 is sequestered into multi-protein single-strand break repair (SSBR) complexes by direct interaction with DNA ligase IIIalpha and that these complexes are catalytically inactive in SCAN1 cells. These data identify a defect in SSBR in a neurodegenerative disease, and implicate this process in the maintenance of genetic integrity in post-mitotic neurons.

Alterations in gene expression profiles and the DNA-damage response in ionizing radiation-exposed TK6 cells

Identifying genes that are differentially expressed in response to DNA damage may help elucidate markers for genetic damage and provide insight into the cellular responses to specific genotoxic agents. We utilized cDNA microarrays to develop gene expression profiles for ionizing radiation-exposed human lymphoblastoid TK6 cells. In order to relate changes in the expression profiles to biological responses, the effects of ionizing radiation on cell viability, cloning efficiency, and micronucleus formation were measured. TK6 cells were exposed to 0.5, 1, 5, 10, and 20 Gy ionizing radiation and cultured for 4 or 24 hr. A significant (P < 0.0001) decrease in cloning efficiency was observed at all doses at 4 and 24 hr after exposure. Flow cytometry revealed significant decreases in cell viability at 24 hr in cells exposed to 5 (P < 0.001), 10 (P < 0.0001), and 20 Gy (P < 0.0001). An increase in micronucleus frequency occurred at both 4 and 24 hr at 0.5 and 1 Gy; however, insufficient binucleated cells were present for analysis at the higher doses. Gene expression profiles were developed from mRNA isolated from cells exposed to 5, 10, and 20 Gy using a 350 gene human cDNA array platform. Overall, more genes were differentially expressed at 24-hr than at the 4-hr time point. The genes upregulated (> 1.5-fold) or downregulated (< 0.67-fold) at 4 hr were those primarily involved in the cessation of the cell cycle, cellular detoxification pathways, DNA repair, and apoptosis. At 24 hr, glutathione-associated genes were induced in addition to genes involved in apoptosis. Genes involved in cell cycle progression and mitosis were downregulated at 24 hr. Real-time quantitative PCR was used to confirm the microarray results and to evaluate expression levels of selected genes at the low doses (0.5 and 1.0 Gy). The expression profiles reflect the cellular and molecular responses to ionizing radiation related to the recognition of DNA damage, a halt in progression through the cell cycle, activation of DNA-repair pathways, and the promotion of apoptosis.

XRCC1 and DNA polymerase β interaction contributes to cellular alkylating-agent resistance and single-strand break repair

X-ray cross complementing 1 (XRCC1) protein has been suggested to bind to DNA single-strand breaks (SSBs) and organize protein interactions that facilitate efficient DNA repair. Using four site-specifically modified human XRCC1 mutant expression systems and functional complementation assays in Chinese hamster ovary (CHO) XRCC1-deficient EM9 cells, we evaluated the cellular contributions of XRCC1s proposed N-terminal domain (NTD) DNA binding and DNA polymerase beta (POLbeta) interaction activities. Results within demonstrate that the interaction with POLbeta is biologically important for alkylating agent resistance and SSB repair, whereas the proposed DNA binding function is not critical to these phenotypes. Our data favor a model where the interaction of XRCC1 with POLbeta contributes to efficient DNA repair in vivo, whereas its interactions with target DNA is biologically less relevant.

X-ray repair cross-complementing group 1 polymorphisms and cancer risks in Asian populations: A mini review

X-ray repair cross-complementing group 1 (XRCC1) is an important DNA repair protein. Arg194Trp, Arg280His, and Arg399Gln are three polymorphisms of XRCC1 that commonly exist in human. In this context, we obtained the relevant articles through a PubMed search and examined the association of XRCC1 polymorphisms and the risk of cancer in Asian populations. Generally, a single XRCC1 polymorphism is weakly associated with cancer in Asian populations. However, when combined with other genetic polymorphisms or such lifestyle factors as smoking, XRCC1 polymorphisms show a stronger association with the risk of cancer. The interaction of the 399Gln/Gln genotype and smoking might be associated with a three-fold increase in the risk of cancer. In this paper we provide some important information for practical future cancer prevention programs. To further clarify the association of XRCC1 polymorphisms and cancer risks, additional studies are required from the perspectives of epidemiology and in vitro.

Involvement of Poly(ADP-ribose) Polymerase-1 and XRCC1/DNA Ligase III in an Alternative Route for DNA Double-strand Breaks Rejoining

The efficient repair of DNA double-strand breaks (DSBs) is critical for the maintenance of genomic integrity. In mammalian cells, the nonhomologous end-joining process that represents the predominant repair pathway relies on the DNA-dependent protein kinase (DNA-PK) and the XRCC4-DNA ligase IV complex. Nonetheless, several in vitro and in vivo results indicate that mammalian cells use more than a single end-joining mechanism. While searching for a DNA-PK-independent end-joining activity, we found that the pretreatment of DNA-PK-proficient and -deficient rodent cells with an inhibitor of the poly(ADP-ribose) polymerase-1 enzyme (PARP-1) led to increased cytotoxicity of the highly efficient DNA double-strand breaking compound calicheamicin gamma1. In addition, the repair kinetics of the DSBs induced by calicheamicin gamma1 was delayed both in PARP-1-proficient cells pretreated with the PARP-1 inhibitor and in PARP-1-deficient cells. In order to get new insights into the mechanism of an alternative route for DSBs repair, we have established a new synapsis and end-joining two-step assay in vitro, operating on DSBs with either nuclear protein extracts or recombinant proteins. We found an end-joining activity independent of the DNA-PK/XRCC4-ligase IV complex but that actually required a novel synapsis activity of PARP-1 and the ligation activity of the XRCC1-DNA ligase III complex, proteins otherwise involved in the base excision repair pathway. Taken together, these results strongly suggest that a PARP-1-dependent DSBs end-joining activity may exist in mammalian cells. We propose that this mechanism could act as an alternative route of DSBs repair that complements the DNA-PK/XRCC4/ligase IV-dependent nonhomologous end-joining.

Evidence for induction of DNA double strand breaks in the bystander response to targeted soft X-rays in CHO cells

This study investigated the role of DNA double strand breaks and DNA base damage in radiation-induced bystander responses in Chinese hamster ovary (CHO) cell lines. Two CHO repair-deficient clones, xrs5 (DNA double strand break repair-deficient) and EM9 (DNA base excision repair-deficient) were used in addition to the wild type (CHO). The Gray Cancer Institute ultrasoft X-ray microprobe is a powerful tool for investigating the bystander response, because it permits the irradiation of only a single nucleus of a cell, as reported previously. In order to investigate the bystander effect in each repair-deficient cell line, we irradiated a single cell within a population and scored the formation of micronuclei. When a single nucleus in the population was targeted with 1 Gy, elevated numbers of micronuclei were induced in the neighbouring unirradiated cells in the EM9 and xrs5 cell lines, whereas induction was not observed in CHO. The induction of micronuclei in xrs5 was significantly higher than that in EM9. Under these conditions, the surviving fraction in the neighbouring cells was significantly lower in xrs5 than in the other cell lines, showing a higher cell killing effect in xrs5. To confirm that bystander factors secreted from irradiated cells caused these effects, we carried out medium transfer experiments using conventional X-irradiation. Medium conditioned for 24 h with irradiated cells was transferred to unirradiated cells and elevated induction of micronuclei was observed in xrs5. These results suggest that DNA double strand breaks rather than base damage are caused by factors secreted in the medium from irradiated cells.

The ataxia–oculomotor apraxia 1 gene product has a role distinct from ATM and interacts with the DNA strand break repair proteins XRCC1 and XRCC4

Ataxia-oculomotor apraxia 1 (AOA1) is an autosomal recessive neurodegenerative disease that is reminiscent of ataxia-telangiectasia (A-T). AOA1 is caused by mutations in the gene encoding aprataxin, a protein whose physiological function is currently unknown. We report here that, in contrast to A-T, AOA1 cell lines exhibit neither radioresistant DNA synthesis nor a reduced ability to phosphorylate downstream targets of ATM following DNA damage, suggesting that AOA1 lacks the cell cycle checkpoint defects that are characteristic of A-T. In addition, AOA1 primary fibroblasts exhibit only mild sensitivity to ionising radiation, hydrogen peroxide, and methyl methanesulphonate (MMS). Strikingly, however, aprataxin physically interacts in vitro and in vivo with the DNA strand break repair proteins XRCC1 and XRCC4. Aprataxin possesses a divergent forkhead associated (FHA) domain that closely resembles the FHA domain present in polynucleotide kinase, and appears to mediate the interactions with CK2-phosphorylated XRCC1 and XRCC4 through this domain. Aprataxin is therefore physically associated with both the DNA single-strand and double-strand break repair machinery, raising the possibility that AOA1 is a novel DNA damage response-defective disease.

Association of DNA repair gene XRCC1 polymorphisms with head and neck cancer in Korean population

Squamous cell carcinoma of the head and neck (SCCHN), which is relatively prevalent in Korea, is believed to be induced by environmental carcinogens and host genetic factors. Accumulating evidence has shown that genetic differences in DNA repair capacity resulting from genetic polymorphism influence the risk of environmental carcinogenesis. We therefore examined the associations of genetic polymorphisms in the DNA repair genes XRCC1 with the risk of SCCHN in a Korean population (hospital-based, case-control study; 147 cases and 168 controls). Three known polymorphisms in the XRCC1 gene were genotyped: R194W(C>T) in exon 6, R280H(G>A) in exon 9 and R399G(G>A) in exon 10. Although no significant associations were apparent with R280H(G>A) and R399G(G>A), a highly significant association (p = 0.0005) of R194W(C>T) with the increased risk (OR = 2.61; 95% CI 1.53-4.46) of SCCHN was detected among patients and normal controls under dominant model. The frequency of minor allele-containing genotypes (TT and CT) was much higher in SCCHN patients (51.8%) compared to that in normal controls (30.3%) (p = 0. 0005). When considering a relatively small number of cases (n = 147) and controls (n = 168) in our study, larger studies are needed to validate the genetic effects of XRCC1 polymorphisms in Asian populations. In conclusion, the result from our study provides additional evidence of an association of the XRCC1 polymorphism (Arg194Trp) with SCCHN as markers of genetic susceptibility in the Korean population.

Rescue of Xrcc1 knockout mouse embryo lethality by transgene-complementation

Xrcc1 knockout embryos show increased DNA breakage and apoptosis in tissues of the embryo proper prior to death at embryonic day E6.5. An additional deficiency in Trp53 allows Xrcc1(-/-) embryos to enlarge slightly and initiate gastrulation although ultimately death is delayed by less than 24h. Death presumably results from DNA damage that reaches toxic levels in the post-implantation mouse embryo. To investigate the level of XRCC1 protein needed for successful mouse development, we derived Xrcc1 transgene-complemented Xrcc1(-/-) mice that express Xrcc1 within the normal range or at a greatly reduced level (<10% normal). The greatly reduced XRCC1 protein level destabilized the XRCC1 partner protein DNA ligase III (LIG3) but still allowed for successful mouse development and healthy, fertile adults. Fibroblasts from these animals exhibited almost normal alkylation sensitivity measured by differential cytotoxicity. Thus, a large reduction of both XRCC1 and DNA ligase III has no observable effect on mouse embryogenesis and post-natal development, and no significant effect on cellular sensitivity to DNA alkylation. The presence of XRCC1, even at reduced levels of expression, is therefore capable of supporting mouse development and DNA repair.

Lack of association between DNA base excision repair gene XRCC1 Gln399Arg polymorphism and risk of malignant lymphoma in Japan

Growing evidence suggests that the polymorphism of DNA base excision repair gene XRCC1 Arg399Gln is associated with altered DNA repair proficiency and subsequent cancer susceptibility; however, no evidence is available for malignant lymphoma. We therefore conducted a case-control study (372 cases, 500 controls) to evaluate links with malignant lymphoma risk in Japan. The risk was evaluated in terms of odds ratio (OR) and 95% confidence interval (CI) adjusted for age and sex in an unconditional logistic regression model. There was no statistical risk change with the Arg/Gln (adjusted OR 0.89; 0.65-1.23, P = 0.492) or the Gln/Gln (0.57; 0.27-1.17, P = 0.127) compared with the Arg/Arg of the XRCC1 Arg399Gln polymorphism. The results were unchanged in analyses according to histological subtype (diffuse large lymphoma, follicular lymphoma, low-grade lymphoma of mucosa-associated lymphoid tissue, and others). These data suggest that XRCC1 Gln399Arg polymorphism plays a limited role in lymphomagenesis. Further study on the interaction between the polymorphism and environmental exposure is required.

The Protein Kinase CK2 Facilitates Repair of Chromosomal DNA Single-Strand Breaks

CK2 was the first protein kinase identified and is required for the proliferation and survival of mammalian cells. Here, we have identified an unanticipated role for CK2. We show that this essential protein kinase phosphorylates the scaffold protein XRCC1 and thereby enables the assembly and activity of DNA single-strand break repair protein complexes in vitro and at sites of chromosomal breakage. Moreover, we show that inhibiting XRCC1 phosphorylation by mutation of the CK2 phosphorylation sites or preventing CK2 activity using a highly specific inhibitor ablates the rapid repair of cellular DNA single-strand breaks by XRCC1. These data identify a direct role for CK2 in the repair of chromosomal DNA strand breaks and in maintaining genetic integrity.

Role of XRCC1 in the coordination and stimulation of oxidative DNA damage repair initiated by the DNA glycosylase hOGG1

XRCC1 participates in DNA single strand break and base excision repair (BER) to preserve genetic stability in mammalian cells. XRCC1 participation in these pathways is mediated by its interactions with several of the acting enzymes. Here, we report that XRCC1 interacts physically and functionally with hOGG1, the human DNA glycosylase that initiates the repair by BER of the mutagenic oxidized base 8-oxoguanine. This interaction leads to a 2- to 3-fold stimulation of the DNA glycosylase activity of hOGG1. XRCC1 stimulates the formation of the hOGG1 Schiff-base DNA intermediate without interfering with the endonuclease activity of APE1, the second enzyme in the pathway. On the contrary, the stimulation in the appearance of the incision product seems to reflect the addition of the effects of XRCC1 on the two first enzymes of the pathway. The data presented support a model by which XRCC1 will pass on the DNA intermediate from hOGG1 to the endonuclease APE1. This results in an acceleration of the overall repair process of oxidized purines to yield an APE1-cleaved abasic site, which can be used as a substrate by DNA polymerase beta. More importantly, the results unveil a highly coordinated mechanism by which XRCC1, through its multiple protein-protein interactions, extends its orchestrating role from the base excision step to the resealing of the repaired DNA strand.

Phosphopeptide binding specificities of BRCA1 COOH-terminal (BRCT) domains

Protein phosphorylation by protein kinases may generate docking sites for other proteins. It thus allows the assembly of signaling complexes in response to kinase activation. Several protein domains that bind phosphoserine or phosphothreonine residues have been identified, including the 14-3-3, PIN1, FHA, KIX, WD-40 domain, and polo box (Yaffe, M. B., and Elia, A. E. (2001) Curr. Opin. Cell Biol. 13, 131-138; Elia, A. E., Cantley, L. C., and Yaffe, M. B. (2003) Science 299, 1228-1231). The BRCA1 COOH-terminal (BRCT) domains are protein modules found in many proteins that regulate DNA damage responses (Koonin, E. V., Altschul, S. F., and Bork, P. (1996) Nat. Genet. 13, 266-268). Whether BRCT domains can mediate phosphorylation-dependent interactions has not been systematically investigated. We report here that the BRCT domains also recognize phosphopeptides. Oriented peptide library analysis indicated that the BRCT domains from BRCA1, MDC1, BARD1, and DNA Ligase IV preferred distinct phosphoserine-containing peptides. In addition, the interaction between BRCA1 and the BRCT binding motif of BACH1 was required for BACH1 checkpoint activity. Furthermore, BRCT domains of the yeast DNA repair protein Rad9 could bind phosphopeptides, suggesting that the BRCT domains represent a class of ancient phosphopeptide-binding modules. Potential targets of BRCT domains were identified through data base search. Structural analysis of BRCA1 BRCT repeats also predicted conserved residues that may form the phosphopeptide-binding pocket. Thus, the BRCT repeats are a new family of phosphopeptide-binding domains in DNA damage responses.

XRCC1 and DNA strand break repair

DNA single-strand breaks can arise indirectly, as normal intermediates of DNA base excision repair, or directly from damage to deoxyribose. Because single-strand breaks are induced by endogenous reactive molecules such as reactive oxygen species, these lesions pose a continuous threat to genetic integrity. XRCC1 protein plays a major role in facilitating the repair of single-strand breaks in mammalian cells, via an ability to interact with multiple enzymatic components of repair reactions. Here, the protein-protein interactions facilitated by XRCC1, and the repair processes in which these interactions operate, are reviewed. Models for the repair of single-strand breaks during base excision repair and at direct breaks are presented.

From genotype to phenotype: correlating XRCC1 polymorphisms with mutagen sensitivity

This study correlated the extent of induced in vitro chromosomal damage, assessed by the mutagen sensitivity assay, with genotypes of the X-ray repair cross complementing group 1 (XRCC1) gene, which encodes for a base excision repair protein. There are two common polymorphisms that cause amino acid substitutions in XRCC1, one at codon 194 in exon 6 and another at codon 399 in exon 10. We genotyped these two polymorphisms in 524 healthy subjects and performed mutagen sensitivity assays using both bleomycin and benzo[a]pyrene-diol-epoxide (BPDE) as challenge mutagens. Our results showed that individuals with the wildtype exon 6 Arg/Arg exhibited significantly higher values of chromosomal breaks per cell (b/c) than those with one or two variant Trp alleles (P=0.005 for bleomycin and P=0.05 for BPDE). For the exon 10 polymorphism, subjects who were Gln/Gln homozygotes had higher b/c than did those with other genotypes, with evidence of a gene dosage effect. When we combined the two polymorphic sites and used the exon 6 Arg/Trp and Trp/Trp and exon 10 Arg/Arg genotypes as the reference category, these differences were enhanced for bleomycin sensitivity (P for trend = 0.032), but not for BPDE sensitivity (P for trend = 0.821). These data are biologically plausible since codon 399 is located within the BRCA1 C-terminus functional domain and codon 194 is in the linker region of the XRCC1 N-terminal functional domain. To our knowledge, this is the largest study conducted evaluating the functional relevance of these polymorphisms.

The mammalian XRCC genes: their roles in DNA repair and genetic stability

Analysis of the XRCC genes has played an important part in understanding mammalian DNA repair processes, especially those involved in double-strand break (DSB) repair. Most of these genes were identified through their ability to correct DNA damage hypersensitivity in rodent cell lines, and they represent components of several different repair pathways including base-excision repair, non-homologous end joining, and homologous recombination. We document the phenotypic effects of mutation of the XRCC genes, and the current state of our knowledge of their functions. In addition to their continuing importance in discovering mechanisms of DNA repair, analysis of the XRCC genes is making a substantial contribution to the understanding of specific human disorders, including cancer.