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

How Do ROS Induce NETosis? Oxidative DNA Damage, DNA Repair, and Chromatin Decondensation

Neutrophil extracellular traps (NETs) are intricate, DNA-based, web-like structures adorned with cytotoxic proteins. They play a crucial role in antimicrobial defense but are also implicated in autoimmune diseases and tissue injury. The process of NET formation, known as NETosis, is a regulated cell death mechanism that involves the release of these structures and is unique to neutrophils. NETosis is heavily dependent on the production of reactive oxygen species (ROS), which can be generated either through NADPH oxidase (NOX) or mitochondrial pathways, leading to NOX-dependent or NOX-independent NETosis, respectively. Recent research has revealed an intricate interplay between ROS production, DNA repair, and NET formation in different contexts. UV radiation can trigger a combined process of NETosis and apoptosis, known as apoNETosis, driven by mitochondrial ROS and DNA repair. Similarly, in calcium ionophore-induced NETosis, both ROS and DNA repair are key components, but only play a partial role. In the case of bacterial infections, the early stages of DNA repair are pivotal. Interestingly, in serum-free conditions, spontaneous NETosis occurs through NOX-derived ROS, with early-stage DNA repair inhibition halting the process, while late-stage inhibition increases it. The intricate balance between DNA repair processes and ROS production appears to be a critical factor in regulating NET formation, with different pathways being activated depending on the nature of the stimulus. These findings not only deepen our understanding of the mechanisms behind NETosis but also suggest potential therapeutic targets for conditions where NETs contribute to disease pathology.

Divalent and multivalent cations control liquid-like assembly of poly(ADP-ribosyl)ated PARP1 into multimolecular associates in vitro

The formation of nuclear biomolecular condensates is often associated with local accumulation of proteins at a site of DNA damage. The key role in the formation of DNA repair foci belongs to PARP1, which is a sensor of DNA damage and catalyzes the synthesis of poly(ADP-ribose) attracting repair factors. We show here that biogenic cations such as Mg2+, Ca2+, Mn2+, spermidine3+, or spermine4+ can induce liquid-like assembly of poly(ADP-ribosyl)ated [PARylated] PARP1 into multimolecular associates (hereafter: self-assembly). The self-assembly of PARylated PARP1 affects the level of its automodification and hydrolysis of poly(ADP-ribose) by poly(ADP-ribose) glycohydrolase (PARG). Furthermore, association of PARylated PARP1 with repair proteins strongly stimulates strand displacement DNA synthesis by DNA polymerase β (Pol β) but has no noticeable effect on DNA ligase III activity. Thus, liquid-like self-assembly of PARylated PARP1 may play a critical part in the regulation of i) its own activity, ii) PARG-dependent hydrolysis of poly(ADP-ribose), and iii) Pol β-mediated DNA synthesis. The latter can be considered an additional factor influencing the choice between long-patch and short-patch DNA synthesis during repair.

Regulation of base excision repair during adipogenesis and osteogenesis of bone marrow-derived mesenchymal stem cells

Bone marrow-derived human mesenchymal stem cells (hMSCs) can differentiate into various lineages, such as chondrocytes, adipocytes, osteoblasts, and neuronal lineages. It has been shown that the high-efficiency DNA-repair capacity of hMSCs is decreased during their differentiation. However, the underlying its mechanism during adipogenesis and osteogenesis is unknown. Herein, we investigated how alkyl-damage repair is modulated during adipogenic and osteogenic differentiation, especially focusing on the base excision repair (BER) pathway. Response to an alkylation agent was assessed via quantification of the double-strand break (DSB) foci and activities of BER-related enzymes during differentiation in hMSCs. Adipocytes showed high resistance against methyl methanesulfonate (MMS)-induced alkyl damage, whereas osteoblasts were more sensitive than hMSCs. During the differentiation, activities, and protein levels of uracil-DNA glycosylase were found to be regulated. In addition, ligation-related proteins, such as X-ray repair cross-complementing protein 1 (XRCC1) and DNA polymerase β, were upregulated in adipocytes, whereas their levels and recruitment declined during osteogenesis. These modulations of BER enzyme activity during differentiation influenced DNA repair efficiency and the accumulation of DSBs as repair intermediates in the nucleus. Taken together, we suggest that BER enzymatic activity is regulated in adipogenic and osteogenic differentiation and these alterations in the BER pathway led to different responses to alkyl damage from those in hMSCs.

Impact of XRCC1 genetic variants on its tissue expression and breast cancer risk: A case-control study

X-ray repair cross-complementing group 1 (XRCC1), a coordinator protein of the DNA repair complex, is thought to be involved in cancer progression. This case-control study aimed to investigate the association of two biallelic single-nucleotide polymorphisms (SNPs; Arg399Gln, Arg194Trp) of the XRCC1 gene with its tissue expression level and breast cancer (BC) risk in Egyptian women. This study included 100 BC female patients (case group 1) and 100 healthy females (control group 2). The XRCC1 tissue expression was assessed by immunohistochemistry (IHC). Genotyping of the two XRCC1 SNPs (Arg399Gln, Arg194Trp) using real-time polymerase chain reaction (PCR) was also conducted. The XRCC1 expression level was significantly lower in cancerous tissues than adjacent non-cancerous tissues (p < .001). The XRCC1 399Gln/Gln genotype, 399Gln allele, the dominant, and recessive models were significantly associated with lower XRCC1 expression in breast cancerous tissues and increased risk for BC (3.390-, 1.965-, 2.241-, and 2.429-folds, respectively). The XRCC1 399Gln/Gln genotype was associated with lower incidence of advanced tumor grade (OR: 0.06; 95%CI: 0.01-0.74; p = .028). Conversely, the XRCC1 Arg194Trp polymorphism did not show any significant association with either XRCC1 expression in breast cancer tissues or BC risk in all genetic models. The XRCC1 haplotypes, 399Gln/194Arg and 399Gln/194Trp, were associated with 1.800- and 1.675-folds risk for BC, respectively. The XRCC1 gene polymorphism (Arg399Gln) is associated with reduced XRCC1 tissue expression and enhanced BC risk with a well-differentiated nature in Egyptian women. Moreover, XRCC1 haplotypes, 399Gln/194Arg and 399Gln/194Trp, were associated with increased BC risk.

Significant Association Between XRCC1 Expression and Its rs25487 Polymorphism and Radiotherapy-Related Cancer Prognosis

Background/Aims

XRCC1 (X-ray repair cross-complementing protein 1) expression and its single nucleotide polymorphism XRCC1 rs25487 (G>A) may be related to radiotherapy-related cancer prognosis or radiation-induced side effects. However, this association is controversial. We performed a bioinformatic analysis and a meta-analysis to obtain comprehensive results.

Results

Sixty nine articles with 10232 patients and 17 TCGA data sets with 2705 patients were included in the analysis. We observed that high XRCC1 expression was associated with an increased risk of minor treatment response and poor overall survival, XRCC1 rs25487 was associated with reduced risk of minor treatment response in esophageal cancer and an increased risk of high-grade side effects in head and neck cancer.

Conclusion

The results suggest that XRCC1 expression and rs25487 polymorphism are prognostic factors for patients receiving radiotherapy-related treatment. Considering the insufficient treatment parameters provided and the various sample sizes in most of the studies, we suggest that genetic association studies related to radiation-based treatment should include more cancer types with sufficient statistical power and more detailed clinical parameters.

The Therapeutic Potential of DNA Damage Repair Pathways and Genomic Stability in Lung Cancer

Despite advances in our understanding of the molecular biology of the disease and improved therapeutics, lung cancer remains the most common cause of cancer-related deaths worldwide. Therefore, an unmet need remains for improved treatments, especially in advanced stage disease. Genomic instability is a universal hallmark of all cancers. Many of the most commonly prescribed chemotherapeutics, including platinum-based compounds such as cisplatin, target the characteristic genomic instability of tumors by directly damaging the DNA. Chemotherapies are designed to selectively target rapidly dividing cells, where they cause critical DNA damage and subsequent cell death (1, 2). Despite the initial efficacy of these drugs, the development of chemotherapy resistant tumors remains the primary concern for treatment of all lung cancer patients. The correct functioning of the DNA damage repair machinery is essential to ensure the maintenance of normal cycling cells. Dysregulation of these pathways promotes the accumulation of mutations which increase the potential of malignancy. Following the development of the initial malignancy, the continued disruption of the DNA repair machinery may result in the further progression of metastatic disease. Lung cancer is recognized as one of the most genomically unstable cancers (3). In this review, we present an overview of the DNA damage repair pathways and their contributions to lung cancer disease occurrence and progression. We conclude with an overview of current targeted lung cancer treatments and their evolution toward combination therapies, including chemotherapy with immunotherapies and antibody-drug conjugates and the mechanisms by which they target DNA damage repair pathways.

Redox Regulation in the Base Excision Repair Pathway: Old and New Players as Cancer Therapeutic Targets

BACKGROUND

Reactive Oxygen Species (ROS) are by-products of normal cellular metabolic processes, such as mitochondrial oxidative phosphorylation. While low levels of ROS are important signalling molecules, high levels of ROS can damage proteins, lipids and DNA. Indeed, oxidative DNA damage is the most frequent type of damage in the mammalian genome and is linked to human pathologies such as cancer and neurodegenerative disorders. Although oxidative DNA damage is cleared predominantly through the Base Excision Repair (BER) pathway, recent evidence suggests that additional pathways such as Nucleotide Excision Repair (NER) and Mismatch Repair (MMR) can also participate in clearance of these lesions. One of the most common forms of oxidative DNA damage is the base damage 8-oxoguanine (8-oxoG), which if left unrepaired may result in G:C to A:T transversions during replication, a common mutagenic feature that can lead to cellular transformation.

OBJECTIVE

Repair of oxidative DNA damage, including 8-oxoG base damage, involves the functional interplay between a number of proteins in a series of enzymatic reactions. This review describes the role and the redox regulation of key proteins involved in the initial stages of BER of 8-oxoG damage, namely Apurinic/Apyrimidinic Endonuclease 1 (APE1), human 8-oxoguanine DNA glycosylase-1 (hOGG1) and human single-stranded DNA binding protein 1 (hSSB1). Moreover, the therapeutic potential and modalities of targeting these key proteins in cancer are discussed.

CONCLUSION

It is becoming increasingly apparent that some DNA repair proteins function in multiple repair pathways. Inhibiting these factors would provide attractive strategies for the development of more effective cancer therapies.

Androgen Receptor and Poly(ADP-ribose) Glycohydrolase Inhibition Increases Efficiency of Androgen Ablation in Prostate Cancer Cells

There is mounting evidence of androgen receptor signaling inducing genome instability and changing DNA repair capacity in prostate cancer cells. Expression of genes associated with base excision repair (BER) is increased with prostate cancer progression and correlates with poor prognosis. Poly(ADP-ribose) polymerase (PARP) and poly(ADP-ribose) glycohydrolase (PARG) are key enzymes in BER that elongate and degrade PAR polymers on target proteins. While PARP inhibitors have been tested in clinical trials and are a promising therapy for prostate cancer patients with TMPRSS2-ERG fusions and mutations in DNA repair genes, PARG inhibitors have not been evaluated. We show that PARG is a direct androgen receptor (AR) target gene. AR is recruited to the PARG locus and induces PARG expression. Androgen ablation combined with PARG inhibition synergistically reduces BER capacity in independently derived LNCaP and LAPC4 prostate cancer cell lines. A combination of PARG inhibition with androgen ablation or with the DNA damaging drug, temozolomide, significantly reduces cellular proliferation and increases DNA damage. PARG inhibition alters AR transcriptional output without changing AR protein levels. Thus, AR and PARG are engaged in reciprocal regulation suggesting that the success of androgen ablation therapy can be enhanced by PARG inhibition in prostate cancer patients.

OGG1 rs1052133 Polymorphism and Genetic Susceptibility to Chronic Myelogenous Leukaemia

Background:

In some cancer cells, the OGG1 gene is somatically mutated and highly populated. This study was conducted to examine whether OGG1 rs1052133 polymorphism is associated with the genetic background of chronic myelogenous leukaemia (CML) in Sudan.

Methods:

A total of 332 CML patients and 70 healthy controls were included in this study. Overall, the genotypes (P=0.0000) and allele (C vs. G, P=0.0007) differed considerably in the frequencies of OGG1 rs1052133 polymorphism between CML patients and controls. Our study is the first to evaluate the association of polymorphism with CML risk with OGG1 rs1052133.

Results:

A statistically significant association was observed between the genotype distribution of OGG1 rs1052133 polymorphism and CML (P=0.0000) patients. A similar result was also observed in the allele distribution (C vs. G, P=0.0007) compared with healthy controls when compared OGG1 rs1052133 genotypes with CML stages.

Results:

Genotype and allele frequencies of OGG1 rs1052133 among CML patients. A statistically significant association was observed between the genotype distribution of the OGG1 rs1052133 polymorphism and CML patients (P=0.0000). A similar result was also observed in the allele distribution (C vs. G, P=0.0007) compared with healthy controls with stages of CML in OGG1 rs1052133 genotypes.

Conclusion:

The results suggest that single nucleotide polymorphism in the gene involved in the restoration of DNA base excision (OGG1 rs1052133) can play a key role in the risk of appearance of CML. To clarify the role of OGG1 in the genetic basis of CML, further case control with larger sample sizes and fine-mapping is required.

Polymorphism rs1052536 in Base Excision Repair Gene Is a Risk Factor in a High-Risk Area of Neural Tube Defects in China

Background

DNA Base Excision Repair Gene-DNA LigaseIII (LIG3) is an important repair gene in the repair pathway and plays an important role in maintaining the integrity of mitochondria. Rs1052536 and rs3135967 polymorphisms of the gene are associated with lung cancer, keratoconus, and Fuchs endothelial corneal dystrophy. There is no previously published report on the relationship between the polymorphisms and neural tube defects (NTDs).

Material/Methods

Mass ARRAY iPLEX was used to determine the distribution of the polymorphisms in the case group of 108 NTD pregnant women and a control group of 233 normal healthy pregnant women to examine the relevance of their polymorphisms and NTD occurrence.

Results

The homozygotes of rs1052536 TT were associated with an increased risk for NTDs than CC (P=0.014, OR=2.31, 95%CI [1.17–4.54]), and variants of rs1052536 T were associated with an increased risk of NTDs (P=0.024, OR=1.50, 95%CI [1.06–2.13]). The stratified analysis showed that TT genotype of rs1052536 increased the risk of anencephaly (P=0.016, OR=2.69, 95%CI [1.18–6.10]) and the T allele significantly increased the risk of cranial NTDs (P=0.033, OR=1.56, 95%CI [1.04–2.35]).

Conclusions

Rs1052536 in LIG3 gene might be a potential genetic risk factor in a high-risk area of NTDs in China.

The Relationship Between Single-Nucleotide Polymorphisms, the Expression of DNA Damage Response Genes, and Hepatocellular Carcinoma in a Polish Population

The molecular mechanism of hepatocellular carcinoma (HCC) is related to DNA damage caused by oxidative stress products induced by hepatitis B virus (HBV) or C (HCV) infection and exposure to environmental pollutants. Single-nucleotide polymorphisms (SNPs) of DNA damage response (DDR) genes may influence individual susceptibility to environmental risk factors and affect DNA repair efficacy, which, in turn, can influence the risk of HCC. The study evaluates a panel of 15 SNPs in 11 DDR genes (XRCC1, XRCC3, XPD, MUTYH, LIG1, LIG3, hOGG1, PARP1, NFIL1, FEN1, and APEX1) in 65 HCC patients, 50 HBV- and 50 HCV-infected non-cancerous patients, and 50 healthy controls. It also estimates the mRNA expression of nine DDR genes in cancerous and adjacent healthy liver tissues. Two of the investigated polymorphisms (rs1052133 and rs13181) were associated with HCC risk. For all investigated genes, the level of mRNA was significantly lower in HCC cancer tissue than in non-cancerous liver tissue. Seven of the investigated polymorphisms were statistically related to gene expression in cancer tissues. The disruption of DDR genes may be responsible for hepatocellular transformation in HCV-infected patients.

Repair of 8-oxoG:A mismatches by the MUTYH glycosylase: Mechanism, metals and medicine

Reactive oxygen and nitrogen species (RONS) may infringe on the passing of pristine genetic information by inducing DNA inter- and intra-strand crosslinks, protein-DNA crosslinks, and chemical alterations to the sugar or base moieties of DNA. 8-Oxo-7,8-dihydroguanine (8-oxoG) is one of the most prevalent DNA lesions formed by RONS and is repaired through the base excision repair (BER) pathway involving the DNA repair glycosylases OGG1 and MUTYH in eukaryotes. MUTYH removes adenine (A) from 8-oxoG:A mispairs, thus mitigating the potential of G:C to T:A transversion mutations from occurring in the genome. The paramount role of MUTYH in guarding the genome is well established in the etiology of a colorectal cancer predisposition syndrome involving variants of MUTYH, referred to as MUTYH-associated polyposis (MAP). In this review, we highlight recent advances in understanding how MUTYH structure and related function participate in the manifestation of human disease such as MAP. Here we focus on the importance of MUTYH's metal cofactor sites, including a recently discovered "Zinc linchpin" motif, as well as updates to the catalytic mechanism. Finally, we touch on the insight gleaned from studies with MAP-associated MUTYH variants and recent advances in understanding the multifaceted roles of MUTYH in the cell, both in the prevention of mutagenesis and tumorigenesis.

Single-Nucleotide Polymorphisms of Genes Involved in Repair of Oxidative DNA Damage and the Risk of Recurrent Depressive Disorder

Background

Depressive disorder, including recurrent type (rDD), is accompanied by increased oxidative stress and activation of inflammatory pathways, which may induce DNA damage. This thesis is supported by the presence of increased levels of DNA damage in depressed patients. Such DNA damage is repaired by the base excision repair (BER) pathway. BER efficiency may be influenced by polymorphisms in BER-related genes. Therefore, we genotyped nine single-nucleotide polymorphisms (SNPs) in six genes encoding BER proteins.

Material/Methods

Using TaqMan, we selected and genotyped the following SNPs: c.-441G>A (rs174538) of FEN1, c.2285T>C (rs1136410) of PARP1, c.580C>T (rs1799782) and c.1196A>G (rs25487) of XRCC1, c.*83A>C (rs4796030) and c.*50C>T (rs1052536) of LIG3, c.-7C>T (rs20579) of LIG1, and c.-468T>G (rs1760944) and c.444T>G (rs1130409) of APEX1 in 599 samples (288 rDD patients and 311 controls).

Results

We found a strong correlation between rDD and both SNPs of LIG3, their haplotypes, as well as a weaker association with the c.-468T>G of APEXI which diminished after Nyholt correction. Polymorphisms of LIG3 were also associated with early onset versus late onset depression, whereas the c.-468T>G polymorphism showed the opposite association.

Conclusions

The SNPs of genes involved in the repair of oxidative DNA damage may modulate rDD risk. Since this is an exploratory study, the results should to be treated with caution and further work needs to be done to elucidate the exact involvement of DNA damage and repair mechanisms in the development of this disease.

Interaction between Helicobacter pylori and host genetic variants in gastric carcinogenesis

Helicobacter pylori (H. pylori) is the definite carcinogen of gastric cancer. H. pylori infection induces chronic inflammation, causes DNA damage and aberrant methylation of genes and these pathways are involved in H. pylori-related gastric carcinogenesis. Polymorphisms of the genes involved in these pathways could alter susceptibility to gastric cancer. In this mini review, we focused on the role of polymorphisms in these genes on the susceptibility to gastric cancer, with a particular emphasis on their possible interactions with H. pylori infection. We found that many studies on this theme did not simultaneously report H. pylori infection and the interactions remained inconclusive.

The current state of eukaryotic DNA base damage and repair

DNA damage is a natural hazard of life. The most common DNA lesions are base, sugar, and single-strand break damage resulting from oxidation, alkylation, deamination, and spontaneous hydrolysis. If left unrepaired, such lesions can become fixed in the genome as permanent mutations. Thus, evolution has led to the creation of several highly conserved, partially redundant pathways to repair or mitigate the effects of DNA base damage. The biochemical mechanisms of these pathways have been well characterized and the impact of this work was recently highlighted by the selection of Tomas Lindahl, Aziz Sancar and Paul Modrich as the recipients of the 2015 Nobel Prize in Chemistry for their seminal work in defining DNA repair pathways. However, how these repair pathways are regulated and interconnected is still being elucidated. This review focuses on the classical base excision repair and strand incision pathways in eukaryotes, considering both Saccharomyces cerevisiae and humans, and extends to some important questions and challenges facing the field of DNA base damage repair.

The structural basis of XRCC1-mediated DNA repair

Scaffold proteins play a central role in DNA repair by recruiting and organizing sets of enzymes required to perform multi-step repair processes. X-ray cross complementing group 1 protein (XRCC1) forms enzyme complexes optimized for single-strand break repair, but participates in other repair pathways as well. Available structural data for XRCC1 interactions is summarized and evaluated in terms of its proposed roles in DNA repair. Mutational approaches related to the abrogation of specific XRCC1 interactions are also discussed. Although substantial progress has been made in elucidating the structural basis for XRCC1 function, the molecular mechanisms of XRCC1 recruitment related to several proposed roles of the XRCC1 DNA repair complex remain undetermined.

Base excision repair capacity in informing healthspan

Base excision repair (BER) is a frontline defense mechanism for dealing with many common forms of endogenous DNA damage, several of which can drive mutagenic or cell death outcomes. The pathway engages proteins such as glycosylases, abasic endonucleases, polymerases and ligases to remove substrate modifications from DNA and restore the genome back to its original state. Inherited mutations in genes related to BER can give rise to disorders involving cancer, immunodeficiency and neurodegeneration. Studies employing genetically defined heterozygous (haploinsufficient) mouse models indicate that partial reduction in BER capacity can increase vulnerability to both spontaneous and exposure-dependent pathologies. In humans, measurement of BER variation has been imperfect to this point, yet tools to assess BER in epidemiological surveys are steadily evolving. We provide herein an overview of the BER pathway and discuss the current efforts toward defining the relationship of BER defects with disease susceptibility.

Single nucleotide polymorphisms of DNA base-excision repair genes (APE1, OGG1 and XRCC1) associated with breast cancer risk in a Chinese population

Altered DNA repair capacity can result in increased susceptibility to cancer. The base excision repair (BER) pathway effectively removes DNA damage caused by ionizing radiation and reactive oxidative species (ROS). In the current study, we analyzed the possible relation of polymorphisms in BER genes, including 8-oxoguanine DNA glycosylase (OGG1), apurinic/apyrimidinic endonuclease 1 (APE1), and X-ray repair cross-complementing group 1 protein (XRCC1), with breast cancer risk in Chinese Han women. This case-control study examined 194 patients with breast cancer and 245 cancer-free hospitalized control subjects. Single nucleotide polymorphisms (SNPs) of OGG1 (Ser326Cys), XRCC1 (Arg399Gln), and APE1 (Asp148Glu and -141T/G) were genotyped and analyzed for their association with breast cancer risk using multivariate logistic regression models. We found that XRCC1 Arg399Gln was significantly associated with an increased risk of breast cancer. Similarly, the XRCC1 Gln allele was significantly associated with an elevated risk in postmenopausal women and women with a high BMI (≥ 24 kg/m2). The OGG1 Cys allele provided a significant protective effect against developing cancer in women with a low BMI (< 24 kg/m2). When analyzing the combined effects of these alleles on the risk of breast cancer, we found that individuals with ≥ 2 adverse genotypes (XRCC1 399Gln, APE1 148Asp, and OGG1 326Ser) were at a 2.18-fold increased risk of breast cancer (P = 0.027). In conclusion, our data indicate that Chinese women with the 399Gln allele of XRCC1 have an increased risk of breast cancer, and the combined effects of polymorphisms of BER genes may contribute to tumorigenesis.

Damage response of XRCC1 at sites of DNA single strand breaks is regulated by phosphorylation and ubiquitylation after degradation of poly(ADP-ribose)

Single-strand breaks (SSBs) are the most common type of oxidative DNA damage and they are related to aging and many genetic diseases. The scaffold protein for repair of SSBs, XRCC1, accumulates at sites of poly(ADP-ribose) (pAR) synthesized by PARP, but it is retained at sites of SSBs after pAR degradation. How XRCC1 responds to SSBs after pAR degradation and how this affects repair progression are not well understood. We found that XRCC1 dissociates from pAR and is translocated to sites of SSBs dependent on its BRCTII domain and the function of PARG. In addition, phosphorylation of XRCC1 is also required for the proper dissociation kinetics of XRCC1 because (1) phosphorylation sites mutated in XRCC1 (X1 pm) cause retention of XRCC1 at sites of SSB for a longer time compared to wild type XRCC1; and (2) phosphorylation of XRCC1 is required for efficient polyubiquitylation of XRCC1. Interestingly, a mutant of XRCC1, LL360/361DD, which abolishes pAR binding, shows significant upregulation of ubiquitylation, indicating that pARylation of XRCC1 prevents the poly-ubiquitylation. We also found that the dynamics of the repair proteins DNA polymerase beta, PNK, APTX, PCNA and ligase I are regulated by domains of XRCC1. In summary, the dynamic damage response of XRCC1 is regulated in a manner that depends on modifications of polyADP-ribosylation, phosphorylation and ubiquitylation in live cells.

Association of DNA base-excision repair XRCC1, OGG1 and APE1 gene polymorphisms with nasopharyngeal carcinoma susceptibility in a Chinese population

BACKGROUND

Numerous carcinogens and reactive oxygen species (ROS) may cause DNA damage including oxidative base lesions that lead to risk of nasopharyngeal carcinoma. Genetic susceptibility has been reported to play a key role in the development of this disease. The base excision repair (BER) pathway can effectively remove oxidative lesions, maintaining genomic stability and normal expression, with X-ray repair crosscomplementing1 (XRCC1), 8-oxoguanine glycosylase-1 (OGG1) and apurinic/apyimidinic endonuclease 1 (APE1) playing important roles.

AIMS

To analyze polymorphisms of DNA BER genes (OOG1, XRCC1 and APE1) and explore their associations, and the combined effects of these variants, with risk of nasopharyngeal carcinoma.

MATERIALS AND METHODS

We detected SNPs of XRCC1 (Arg399Gln), OGG1 (Ser326Cys), APE1 (Asp148Glu and -141T/G) using the polymerase chain reaction (PCR) with peripheral blood samples from 231 patients with NPC and 300 healthy people, furtherly analyzing their relations with the risk of NPC in multivariate logistic regression models.

RESULTS

After adjustment for sex and age, individuals with the XRCC1 399Gln/Gln (OR=1.96; 95%CI:1.02- 3.78; p=0.04) and Arg/Gln (OR=1.87; 95%CI:1.29-2.71; p=0.001) genotype variants demonstrated a significantly increased risk of nasopharyngeal carcinoma compared with those having the wild-type Arg/Arg genotype. APE1- 141G/G was associated with a significantly reduced risk of NPC (OR=0.40;95%CI:0.18-0.89) in the smoking group. The OR calculated for the combination of XRCC1 399Gln and APE1 148Gln, two homozygous variants ,was significantly additive for all cases (OR=2.09; 95% CI: 1.27-3.47; p=0.004).

CONCLUSION

This is the first study to focus on the association between DNA base-excision repair genes (XRCC1, OGG1 and APE1) polymorphism and NPC risk. The XRCC1 Arg399Gln variant genotype is associated with an increased risk of NPC. APE1- 141G/G may decrease risk of NPC in current smokers. The combined effects of polymorphisms within BER genes of XRCC1 399Gln and APE1 148Gln may contribute to a high risk of nasopharyngeal carcinoma.

The role of microRNA-binding site polymorphisms in DNA repair genes as risk factors for bladder cancer and breast cancer and their impact on radiotherapy outcomes

MicroRNAs (miRNAs) are involved in post-transcriptional regulation of gene expression through binding to messenger RNAs (mRNA) thereby promoting mRNA degradation or altered translation. A single-nucleotide polymorphism (SNP) located within a miRNA-binding site could thus alter mRNA translation and influence cancer risk and treatment response. The common SNPs located within the 3'-untranslated regions of 20 DNA repair genes were analysed for putative miRNA-binding sites using bioinformatics algorithms, calculating the difference in Gibbs free binding energy (ΔΔG) for each wild-type versus variant allele. Seven SNPs were selected to be genotyped in germ line DNAs both from a bladder cancer case-control series (752 cases and 704 controls) and 202 muscle-invasive bladder cancer radiotherapy cases. The PARP-1 SNP rs8679 was also genotyped in a breast cancer case-control series (257 cases and 512 controls). Without adjustment for multiple testing, multivariate analysis demonstrated an association with increased bladder cancer risk with PARP1 rs8679 (P(trend) = 0.05) while variant homozygotes of PARP1 rs8679 were also noted to have an increased breast cancer risk (P = 0.03). In the radiotherapy cases, carriers of the RAD51 rs7180135 minor allele had improved cancer-specific survival (hazard ratio 0.52, 95% confidence interval 0.31-0.87, P = 0.01). This is the first report of associations between DNA repair gene miRNA-binding site SNPs with bladder and breast cancer risk and radiotherapy outcomes. If validated, these findings may give further insight into the biology of bladder carcinogenesis, allow testing of the RAD51 SNP as a potential predictive biomarker and also reveal potential targets for new cancer treatments.

Base excision repair and cancer

Base excision repair is the system used from bacteria to man to remove the tens of thousands of endogenous DNA damages produced daily in each human cell. Base excision repair is required for normal mammalian development and defects have been associated with neurological disorders and cancer. In this paper we provide an overview of short patch base excision repair in humans and summarize current knowledge of defects in base excision repair in mouse models and functional studies on short patch base excision repair germ line polymorphisms and their relationship to cancer. The biallelic germ line mutations that result in MUTYH-associated colon cancer are also discussed.

Genetic polymorphism of DNA base-excision repair genes (APE1, OGG1 and XRCC1) and their correlation with risk of lung cancer in a Chinese population

BACKGROUND AND AIMS

Reactive oxygen species (ROS) and numerous carcinogens may cause DNA damage including oxidative base lesions that contribute to the risk of lung cancer. The base excision repair (BER) pathway could effectively remove oxidative lesions in which 8-oxoguanine glycosylase-1 (OGG1), x-ray repair cross-complementing 1 (XRCC1), and apurinic/apyimidinic endonuclease 1 (APE1) play key roles. The aim of this study was to analyze the polymorphisms of DNA BER genes (OOG1, XRCC1 and APE1) and explore their associations, and the combined effects of these variants, with risk of lung cancer.

METHODS

In a hospital-based, case-control study of 455 lung cancer cases and 443 cancer-free hospital controls, the SNPs of OGG1 (Ser326Cys), XRCC1 (Arg399Gln), APE1 (Asp148Glu and -141T/G) were genotyped and analyzed for their correlation with the risk of lung cancer in multivariate logistic regression models.

RESULTS

Individuals homozygous for the variants APE1 -141GG showed a protective effect for lung cancer overall (OR=0.62; 95% CI: 0.42-0.91; p=0.02) and for lung adenocarcinoma (OR=0.65; 95% CI, 0.44-0.96; p=0.03). When analyzing the combined effects of variant alleles, 84 patients and controls were identified who were homozygous for two or three of the potential protective alleles (i.e., OGG1 326Cys, XRCC1 399Gln and APE1 -141G). ORs were significantly reduced when all patients were analyzed (OR=0.62; 95% CI: 0.38-0.99; p=0.05).

CONCLUSIONS

The combined effects of polymorphisms within BER genes may contribute to the tumorigenesis of lung cancer.

Nucleosome disruption by DNA ligase III-XRCC1 promotes efficient base excision repair

Each day, approximately 20,000 oxidative lesions form in the DNA of every nucleated human cell. The base excision repair (BER) enzymes that repair these lesions must function in a chromatin milieu. We have determined that the DNA glycosylase hNTH1, apurinic endonuclease (APE), and DNA polymerase β (Pol β), which catalyze the first three steps in BER, are able to process their substrates in both 601- and 5S ribosomal DNA (rDNA)-based nucleosomes. hNTH1 formed a discrete ternary complex that was displaced by the addition of APE, suggesting an orderly handoff of substrates from one enzyme to the next. In contrast, DNA ligase IIIα-XRCC1, which completes BER, was appreciably active only at concentrations that led to nucleosome disruption. Ligase IIIα-XRCC1 was also able to bind and disrupt nucleosomes containing a single base gap and, because of this property, enhanced both its own activity and that of Pol β on nucleosome substrates. Collectively, these findings provide insights into rate-limiting steps that govern BER in chromatin and reveal a unique role for ligase IIIα-XRCC1 in enhancing the efficiency of the final two steps in the BER of lesions in nucleosomes.

Uracil-DNA glycosylase of Thermoplasma acidophilum directs long-patch base excision repair, which is promoted by deoxynucleoside triphosphates and ATP/ADP, into short-patch repair

Hydrolytic deamination of cytosine to uracil in DNA is increased in organisms adapted to high temperatures. Hitherto, the uracil base excision repair (BER) pathway has only been described in two archaeons, the crenarchaeon Pyrobaculum aerophilum and the euryarchaeon Archaeoglobus fulgidus, which are hyperthermophiles and use single-nucleotide replacement. In the former the apurinic/apyrimidinic (AP) site intermediate is removed by the sequential action of a 5'-acting AP endonuclease and a 5'-deoxyribose phosphate lyase, whereas in the latter the AP site is primarily removed by a 3'-acting AP lyase, followed by a 3'-phosphodiesterase. We describe here uracil BER by a cell extract of the thermoacidophilic euryarchaeon Thermoplasma acidophilum, which prefers a similar short-patch repair mode as A. fulgidus. Importantly, T. acidophilumcell extract also efficiently executes ATP/ADP-stimulated long-patch BER in the presence of deoxynucleoside triphosphates, with a repair track of ∼15 nucleotides. Supplementation of recombinant uracil-DNA glycosylase (rTaUDG; ORF Ta0477) increased the formation of short-patch at the expense of long-patch repair intermediates, and additional supplementation of recombinant DNA ligase (rTalig; Ta1148) greatly enhanced repair product formation. TaUDG seems to recruit AP-incising and -excising functions to prepare for rapid single-nucleotide insertion and ligation, thus excluding slower and energy-costly long-patch BER.

Polymorphism in the DNA repair enzyme XRCC1: utility of current database and implications for human health risk assessment

Genetic polymorphisms are increasingly recognized as sources of variability not only in toxicokinetic but also in toxicodynamic response to environmental agents. XRCC1 is involved in base excision repair (BER) of DNA; it has variant genotypes that are associated with modified repair function. This analysis focuses on four polymorphisms: three in the coding region that affect protein structure and one in an upstream regulatory sequence that affects gene expression. The Arg399Gln variant is the most widely studied with evidence supporting a quantitative effect of genotype on phenotype. The homozygous variant (Gln/Gln) can have 3-4-fold diminished capacity to remove DNA adducts and oxidized DNA damage. This variant is relatively common in Caucasians and Asians where approximately 10% are homozygous variant. In contrast, the Arg194Trp variant appears to protect against genotoxic effects although the degree to which DNA repair is enhanced by this polymorphism is uncertain. The homozygous variant is rare in Caucasians and African Americans but it is present at 7% in Asians. A third coding region polymorphism at codon 280 appears to decrease repair function but additional quantitative information is needed and the homozygous variant is rare across populations studied. A polymorphism in an upstream promoter binding sequence (-77T>C) appears to lower XRCC1 levels by decreasing gene expression. Based upon genotype effect on phenotype and allele frequency, the current analysis finds that the codon 399 and upstream (-77) polymorphisms have the greatest potential to affect the toxicodynamic response to DNA damaging agents. However, the implications for risk assessment are limited by the likelihood that polymorphisms in multiple BER genes interact to modulate DNA repair.

Functional capacity of XRCC1 protein variants identified in DNA repair-deficient Chinese hamster ovary cell lines and the human population

XRCC1 operates as a scaffold protein in base excision repair, a pathway that copes with base and sugar damage in DNA. Studies using recombinant XRCC1 proteins revealed that: a C389Y substitution, responsible for the repair defects of the EM-C11 CHO cell line, caused protein instability; a V86R mutation abolished the interaction with POLβ, but did not disrupt the interactions with PARP-1, LIG3α and PCNA; and an E98K substitution, identified in EM-C12, reduced protein integrity, marginally destabilized the POLβ interaction, and slightly enhanced DNA binding. Two rare (P161L and Y576S) and two frequent (R194W and R399Q) amino acid population variants had little or no effect on XRCC1 protein stability or the interactions with POLβ, PARP-1, LIG3α, PCNA or DNA. One common population variant (R280H) had no pronounced effect on the interactions with POLβ, PARP-1, LIG3α and PCNA, but did reduce DNA-binding ability. When expressed in HeLa cells, the XRCC1 variants—excluding E98K, which was largely nucleolar, and C389Y, which exhibited reduced expression—exhibited normal nuclear distribution. Most of the protein variants, including the V86R POLβ-interaction mutant, displayed normal relocalization kinetics to/from sites of laser-induced DNA damage: except for E98K and C389Y, and the polymorphic variant R280H, which exhibited a slightly shorter retention time at DNA breaks.

JWA regulates XRCC1 and functions as a novel base excision repair protein in oxidative-stress-induced DNA single-strand breaks

JWA was recently demonstrated to be involved in cellular responses to environmental stress including oxidative stress. Although it was found that JWA protected cells from reactive oxygen species-induced DNA damage, upregulated base excision repair (BER) protein XRCC1 and downregulated PARP-1, the molecular mechanism of JWA in regulating the repair of DNA single-strand breaks (SSBs) is still unclear. Our present studies demonstrated that a reduction in JWA protein levels in cells resulted in a decrease of SSB repair capacity and hypersensitivity to DNA-damaging agents such as methyl methanesulfonate and hydrogen peroxide. JWA functioned as a repair protein by multi-interaction with XRCC1. On the one hand, JWA was translocated into the nucleus by the carrier protein XRCC1 and co-localized with XRCC1 foci after oxidative DNA damage. On the other hand, JWA via MAPK signaling pathway regulated nuclear factor E2F1, which further transcriptionally regulated XRCC1. In addition, JWA protected XRCC1 protein from ubiquitination and degradation by proteasome. These findings indicate that JWA may serve as a novel regulator of XRCC1 in the BER protein complex to facilitate the repair of DNA SSBs.

Gene expression profiles in radiation workers occupationally exposed to ionizing radiation

Ionizing radiation (IR) imposes risks to human health and the environment. IR at low doses and low dose rates has the potency to initiate carcinogenesis. Genotoxic environmental agents such as IR trigger a cascade of signal transduction pathways for cellular protection. In this study, using cDNA microarray technique, we monitored the gene expression profiles in lymphocytes derived from radiation-exposed individuals (radiation workers). Physical dosimetry records on these patients indicated that the absorbed dose ranged from 0.696 to 39.088 mSv. Gene expression analysis revealed statistically significant transcriptional changes in a total of 78 genes (21 up-regulated and 57 down-regulated) involved in several biological processes such as ubiquitin cycle (UHRF2 and PIAS1), DNA repair (LIG3, XPA, ERCC5, RAD52, DCLRE1C), cell cycle regulation/proliferation (RHOA, CABLES2, TGFB2, IL16), and stress response (GSTP1, PPP2R5A, DUSP22). Some of the genes that showed altered expression profiles in this study can be used as biomarkers for monitoring the chronic low level exposure in humans. Additionally, alterations in gene expression patterns observed in chronically exposed radiation workers reinforces the need for defining the effective radiation dose that causes immediate genetic damage as well as the long-term effects on genomic instability, including cancer.

Mutagenesis is elevated in male germ cells obtained from DNA polymerase-beta heterozygous mice

Gametes carry the DNA that will direct the development of the next generation. By compromising genetic integrity, DNA damage and mutagenesis threaten the ability of gametes to fulfill their biological function. DNA repair pathways function in germ cells and serve to ameliorate much DNA damage and prevent mutagenesis. High base excision repair (BER) activity is documented for spermatogenic cells. DNA polymerase-beta (POLB) is required for the short-patch BER pathway. Because mice homozygous null for the Polb gene die soon after birth, mice heterozygous for Polb were used to examine the extent to which POLB contributes to maintaining spermatogenic genomic integrity in vivo. POLB protein levels were reduced only in mixed spermatogenic cells. In vitro short-patch BER activity assays revealed that spermatogenic cell nuclear extracts obtained from Polb heterozygous mice had one third the BER activity of age-matched control mice. Polb heterozygosity had no effect on the BER activities of somatic tissues tested. The Polb heterozygous mouse line was crossed with the lacI transgenic Big Blue mouse line to assess mutant frequency. The spontaneous mutant frequency for mixed spermatogenic cells prepared from Polb heterozygous mice was 2-fold greater than that of wild-type controls, but no significant effect was found among the somatic tissues tested. These results demonstrate that normal POLB abundance is necessary for normal BER activity, which is critical in maintaining a low germline mutant frequency. Notably, spermatogenic cells respond differently than somatic cells to Polb haploinsufficiency.

XRCC1 polymorphisms, cooking oil fume and lung cancer in Chinese women nonsmokers

X-ray repair cross-complementing group 1 (XRCC1) is one of the major DNA repair proteins involved in the base excision repair (BER) and single-strand break repair (SSBR) pathway. Single nucleotide polymorphisms (SNPs) in XRCC1 may alter protein function and repair capacity, thus lead to genetic instability and carcinogenesis. To establish our understanding of possible relationships between XRCC1 polymorphisms (5'UTR -77T>C, Arg194Trp, Arg280His and Arg399Gln) and the susceptibility to lung cancer among women nonsmokers, we performed a hospital-based case-control study of 350 patients with newly diagnosed lung cancer and 350 cancer-free controls, frequency matched by age. Our results showed that exposure to cooking oil fume was associated with increased risk of lung cancer in Chinese women nonsmokers [odds ratio (OR)=2.51, 95% confidence interval (CI) [1.80-3.51], P<0.001]. Individuals with homozygous XRCC1 399Gln/Gln genotype (OR=1.75, 95% CI [1.02-3.01]) and XRCC1 -77 combined TC and CC genotype (OR=1.66, 95% CI [1.13-2.42]) showed a slightly higher risk for lung cancer overall. In the subgroup of adenocarcinoma cases, adjusted ORs were increased for individuals with homozygous XRCC1 399Gln/Gln genotype (OR=2.62, 95% CI [1.44-4.79]) and XRCC1 -77 combined TC and CC genotype (OR=1.85, 95% CI [1.19-2.86]). Haplotype analysis showed that T-Trp-Arg-Gln haplotypes were associated with an increased risk of lung cancer among women nonsmokers (OR=2.26, 95% CI [1.38-3.68]), however, we did not observe a statistically significant joint effect of cooking oil fume and 399Gln or -77C variant allele on lung cancer among women nonsmokers. In conclusion, XRCC1 Arg399Gln and T-77C polymorphisms may alter the risk of lung cancer in women nonsmokers in China.

Modulation of DNA polymerase beta-dependent base excision repair in cultured human cells after low dose exposure to arsenite

Base excision repair (BER) is crucial for development and for the repair of endogenous DNA damage. However, unlike nucleotide excision repair, the regulation of BER is not well understood. Arsenic, a well-established human carcinogen, is known to produce oxidative DNA damage, which is repaired primarily by BER, whilst high doses of arsenic can also inhibit DNA repair. However, the mechanism of repair inhibition by arsenic and the steps inhibited are not well defined. To address this question we have investigated the regulation of DNA polymerase beta (Pol beta) and AP endonuclease (APE1), in response to low, physiologically relevant doses of arsenic. GM847 lung fibroblasts and HaCaT keratinocytes were exposed to sodium arsenite, As(III), and mRNA, protein levels and BER activity were assessed. Both Pol beta and APE1 mRNA exhibited significant dose-dependant down regulation at doses of As(III) above 1 microM. However, at lower doses Pol beta mRNA and protein levels, and consequently, BER activity were significantly increased. In contrast, APE1 protein levels were only marginally increased by low doses of As(III) and there was no correlation between APE1 and overall BER activity. Enzyme supplementation of nuclear extracts confirmed that Pol beta was rate limiting. These changes in BER correlated with overall protection against sunlight UV-induced toxicity at low doses of As(III) and produced synergistic toxicity at high doses. The results provide evidence that changes in BER due to low doses of arsenic could contribute to a non-linear, threshold dose response for arsenic carcinogenesis.

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.

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.

Untangling the relationships between DNA repair pathways by silencing more than 20 DNA repair genes in human stable clones

Much effort has long been devoted to unraveling the coordinated cellular response to genotoxic insults. In view of the difficulty of obtaining human biological samples of homogeneous origin, I have established a set of stable human clones where one DNA repair gene has been stably silenced by means of RNA interference. I used pEBVsiRNA plasmids that greatly enhance long-term gene silencing in human cells. My older clones reached >500 days in culture. Knock-down HeLa clones maintained a gene silencing phenotype for an extended period in culture, demonstrating that I was able to mimic cells from cancer-prone syndromes. I have silenced >20 genes acting as sensors/transducers (ATM, ATR, Rad50, NBS1, MRE11, PARG and KIN17), or of different DNA repair pathways. In HeLa cells, I have switched off the expression of genes involved in nucleotide excision repair (XPA, XPC, hHR23A, hHR23B, CSA and CSB), nonhomologous end-joining (DNA-PKcs, XRCC4 and Ligase IV), homologous recombination repair (Rad51 and Rad54), or base excision repair (Ogg1 and Ligase III). These cells displayed the expected DNA repair phenotype. We could envisage untangling the complex network between the different DNA repair pathways. In this study, no viral vehicles, with their attendant ethical and safety concerns, were used.

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.

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.