XRCC 177 T > C Polymorphism and Cancer Risk : A Meta-analysis

Certain genetic variants associated with repair of DNA substantially increase the risk of cancer in carriers because of defined biochemical alterations caused by the polymorphisms. Base excision repair (BER) is the predominant DNA damage repair pathway for the processing of small base lesions, derived from oxidation and alkylation damage (Almeidaet al., 2007). One of the most important proteins is X-ray repair crosscomplementation group 1 (XRCC1), a scaffold protein closely associated with BER pathway coordination by interacting with most components of the BER short-patch pathway (Vidalet al., 2001; Campalanset al., 2005; Daset al., 2006). The XRCC1 gene is 33 kb in length, and is located on chromosome 19q13.2-13.3. More than 60 validated single nucleotide polymorphisms in XRCC1 gene are listed in Ensemble database, and most extensively studied are genetic changes Arg194Trp, Arg280His, Arg399Gln and -77T>C (Ginsberget al., 2011). Recently, a variant in the 5’untranslated region (UTR) of XRCC1 (-77 T>C, rs3213245) has been identified, which appeared to lower XRCC1 levels by decreasing gene expression and was shown to be significantly associated with risks of esophageal squamous cell carcinoma and lung cancer in a Chinese population . Mutations of XRCC1 may increase the risk of cancers by impairing the


Introduction
Certain genetic variants associated with repair of DNA substantially increase the risk of cancer in carriers because of defined biochemical alterations caused by the polymorphisms.Base excision repair (BER) is the predominant DNA damage repair pathway for the processing of small base lesions, derived from oxidation and alkylation damage (Almeidaet al., 2007).One of the most important proteins is X-ray repair crosscomplementation group 1 (XRCC1), a scaffold protein closely associated with BER pathway coordination by interacting with most components of the BER short-patch pathway (Vidalet al., 2001;Campalanset al., 2005;Daset al., 2006).The XRCC1 gene is 33 kb in length, and is located on chromosome 19q13.2-13.3.More than 60 validated single nucleotide polymorphisms in XRCC1 gene are listed in Ensemble database, and most extensively studied are genetic changes Arg194Trp, Arg280His, Arg399Gln and -77T>C (Ginsberget al., 2011).
Recently, a variant in the 5'untranslated region (UTR) of XRCC1 (-77 T>C, rs3213245) has been identified, which appeared to lower XRCC1 levels by decreasing gene expression and was shown to be significantly associated with risks of esophageal squamous cell carcinoma and lung cancer in a Chinese population .Mutations of XRCC1 may increase the risk of cancers by impairing the Department of Chemotherapy, Cancer Center, Qilu Hospital, Shandong University, Jinan, China *For correspondence: hzhengtysd@ yahoo.cn

XRCC1-77T>C Polymorphism and Cancer Risk: A Metaanalysis
Yong-Gang Wang, Tian-Ying Zheng* interaction of XRCC1 with the other enzymatic proteins and consequently altering DNA repair activity (Tudek, 2007).Previous studies investigating the association between XRCC1 -77T>C polymorphism and risk of different cancers have provided inconsistent results (Haoet al., 2004;Haoet al., 2006;Bassoet al., 2007).Most of those studies involved no more than a few hundred cancer cases, which is too few to assess reliably any genetic effects.Furthermore, the interpretation of available studies may be complicated by different ethnicities, different population sampling strategies or different genotyping procedures.Therefore, there was a role for meta-analysis in pooling these studies, particularly to clarify the effect of XRCC1 -77T>C genotype on cancer risk.Hence, we carried out a meta-analysis of available data from all relevant studies.

Search strategy and selection criteria
Computer searches of PubMed, Embase and Chinese Bio-medicine Database (CBM) used the following search criterion: ("cancer" or "carcinoma" or "tumor") and ("polymorphism" or "polymorphisms") and ("XRCC1" or "X-ray repair cross-complementation group 1") without language restriction.All eligible articles were retrieved and their references were checked for other relevant articles.When the same patient population was included in different articles, only the most recent or complete study was used in this meta-analysis.The inclusion criteria were: (1) case-control studies which evaluated the association of XRCC1 -77T>C polymorphism with cancer risk; (2) based on unrelated cancer individuals; (3) sufficient published genotype data for estimating an odds ratio (OR) with 95% confidence interval (CI); (4) genotype distribution of the control population reported was in Hardy-Weinberg equilibrium (HWE).

Data extraction
The following information was extracted from each study, according to a fixed protocol: study design, ethnicity of participants, numbers of cases and controls, DNA extraction and genotyping methods and frequency of genotypes.Confirmation of the extracted information was sought by correspondence with investigators.

Statistical Analysis
A chi-square test (http://ihg2.helmholtz-muenchen.de/ cgi-bin/hw/hwa1.pl)was used to determine if genotype distribution of the control population reported conformed to HWE (P < 0.05 was considered significant).Statistical heterogeneity across the various trials was tested with the use of Cochran's Q statistic (Cochran, 1954).A P value of more than the nominal level of 0.10 for the Q statistic indicated a lack of heterogeneity across trials, allowing for the use of a fixed-effects model (the Mantel-Haenszel method) (Mantelet al., 1959); otherwise, the randomeffects model (the DerSimonian and Laird method) was used (DerSimonianet al., 1986).The pooled ORs were performed on the Allele gene model (C vs. T), homozygote model (CC vs. TT), dominant model (CC+TC vs. TT), and recessive model (CC vs. TC+TT) respectively.The significance of pooled OR was tested by Z test (P < 0.05 was considered significant).Subgroup analyses were performed by ethnicity, cancer type and sample size.Ethnic group was defined as Caucasian, Asian, African or others.An estimate of potential publication bias was carried out using funnel plot and Egger's linear regression test.The significance of the intercept was determined by the t test suggested by Egger, and P < 0.05 was considered representative of a statistically significant publication bias (Eggeret al., 1997).Data were analyzed with the use of STATA (version 10.0; Stata Corporation, College Station, TX) and Review Manager (version 5.0; Oxford, England).All the P values were two-sided.
In subgroup analyses based on ethnicity, the association was still significant in the Asian population (All p values < 0.001) but not in Caucasians (All p values > 0.05).Subgroup analyses based on cancer type showed XRCC1 -77C variant was significantly associated with increased risk of breast cancer and lung cancer (Table 3).Subgroup analyses of other kinds of cancers were not performed owing to the lack of relevant studies.
The between-study heterogeneity was not obvious in most comparisons except the Allele gene model (C vs. T) comparison analysis based on total studies (I 2 =50.4%).Interestingly, the heterogeneity remarkably decreased in subgroup analyses based on ethnicity (I 2 =0.0% in Caucasians and 27.7% in Asians), indicating the difference between various ethnicities led to the between-study heterogeneity (Table 3).

Publication Bias
Funnel plot and Egger's test were both performed to assess the publication bias of this meta-analysis.The shape of the funnel plots for most genetic contrast models seemed symmetrical, and most P values of Egger's tests were more than 0.05 (Table 3, Figure 3), providing statistical evidence of funnel plot symmetry.The results above suggested that publication bias was not evident in our meta-analyses.

Discussion
The present meta-analysis of 13 case-control studies, involving a total of 5,898 cancer cases and 6,080 controls, provides the most comprehensive assessment so far of the association between XRCC1 -77T>C polymorphism and cancer risk.The findings of this meta-analysis indicate that variant homozygote CC of XRCC1 -77T>C is significantly associated with increased cancer risk (OR=1.28,95%CI 1.07-1.52;P = 0.007).Particularly, individuals with the CC genotype has a 128% higher odd of cancer risk compared with individuals with TT carriers in Asians (OR (95%CI) =2.28(1.52-3.42);P < 0.001).In the subgroup analysis by ethnicity, the association is still significant in the Asian population (All p values < 0.001) but not in the Caucasian population (All p values > 0.05), suggesting a possible role of ethnic differences in genetic backgrounds and the environment (Table 3).Therefore, those gene-variant associations vary in different ethnicities and the reason may be different genetic backgrounds among various ethnicities.
Heterogeneity is a potential problem when interpreting the results of all meta-analyses, and finding of the source of heterogeneity is one of the most important goals of meta-analysis.The present meta-analysis showed that there was strong heterogeneity between studies in the Allele gene model (C vs. T) comparison analysis based on total studies (I 2 =50.4%).Therefore, we first stratified studies according to ethnicity.Heterogeneity between studies remarkably decreased or removed in subgroup analyses by ethnicity (I 2 =0.0% in Caucasians and 27.7% in Asians; Table 3), which indicated between-study heterogeneity mainly come from ethnic differences in genetic backgrounds and also further indicated differences of genetic backgrounds among different ethnicities in mechanisms of carcinogenesis.
Our results show that the association between XRCC1 -77T>C polymorphism and cancer risk are obvious in lung cancer and breast cancer but not in the others, indicating that XRCC -77T>C polymorphism may exert different effects in different kinds of cancer.However, it also likely that the observed different effects may be due to chances because studies with small sample size may have insufficient statistical power to detect a slight effect or may have generated a fluctuated risk estimate.For example, the sample size of gastric cancer in this metaanalysis was only 456 individuals which were too small to generate an acceptable risk estimate.Thus, large and carefully designed case-control studies among other kinds of cancers are needed to provide the best evidence for such a possible association in other ethnicity or cancers.
Efficient DNA repair represents an important defense mechanism in neutralizing mutagenic damage, and XRCC1 supports DNA repair by binding to the site where damaged bases have been removed or where single strand breaks (SSBs) have occurred for other reasons, which facilitates the activity of polymerase and ligase enzymes and so is central to the proper functioning of BER (Ginsberg and Angle, 2011).XRCC1 gene variants, which appeared to influence XRCC1 levels by decreasing gene expression, might be significantly associated with cancer risk.A variety of cancer outcomes have been evaluated according to XRCC1 polymorphism, and several largescale meta-analyses combining data from multiple studies have been published to investigate the association between XRCC1 polymorphism and various cancers, such as cervical cancer, gastric cancer, lung cancer and breast cancer.Huang Y et al suggested obvious association was found between breast cancer and XRCC1 Arg399Gln polymorphism (Huanget al., 2009).Dai L et al found that Gln/Gln genotype might be associated with esophageal squamous cell carcinoma risk in Asians, while Lao T et al found that Gln variant of XRCC1 Arg399Gln might decrease the risk of bladder cancer among ever-smokers (Laoet al., 2008;Daiet al., 2009).But there were no large-scale meta-analyses combining data from multiple studies published to investigate the association between XRCC1 -77T/C polymorphism and cancer risk.Thus, this present meta-analysis is the fist meta-analysis assessing the association between XRCC1 -77T/C polymorphism and cancer risk, and suggests that C variant of XRCC1 -77T/C is an important genetic hallmark contributing to cancer susceptibility.
However, there are still some limitations in this meta-analysis.First, misclassification bias was possible.For example, most studies could not exclude latent cancer cases in the control group.The controls in some studies were selected from non-cancer patients, while DOI:http://dx.doi.org/10.7314/APJCP.2012.13.1.111 XRCC1 -77T>C Polymorphism and Cancer Risk: A Meta-analysis the controls in other several studies were just selected from asymptomatic individuals.Second, the eligibility criteria for inclusion of subjects and sources of controls were different from each other.For example, some studies were population-based, and some were hospitalbased.The allele distribution in the hospital control groups might not have been representative of the general population.Therefore, using a proper and representative population-based control subjects is very important to reduce biases in such genetic association studies.Third, gene-gene and gene-environmental interactions were not addressed in this meta-analysis.As we know, aside genetic factor, smoking is a major risk factor for cancer; however we didn't perform subgroup analyses in smokers or nonsmokers owing to the limited reported information on such associations in the included studies.Considering these limitations, our results should be interpreted with caution.
Despite of those limitations, this meta-analysis suggests the C variant of XRCC1 -77T>C is an important genetic hallmark contributing to cancer risk, but these gene-variant associations vary in different ethnicities.Besides, large and carefully designed case-control studies among other kinds of cancers need performing to provide the best evidence for such a possible association in other ethnicity or cancers.

Figure 3 .
Figure 3. Funnel Plot for Publication Bias Test in the Meta-analysis Investigating for Associations Between XRCC1 -77T>C Polymorphism and Cancer Risk.(C vs. T, Each point represents a separate study for the indicated association.LogOR, natural logarithm of odds ratio; Horizontal line, mean effect size)

Forest Plots of Pooled OR with 95% CI for Associations Between XRCC1 -77T>C Polymorphism and
Cancer Risk.(The size of the data markers is inversely proportional to the variance of the log ORs; horizontal lines represent the 95% CIs.The pooled ORs and the subtotals for each region and their 95% CIs are indicated by the squares; A. C vs. T; B. Homozygote comparison model; C. Recessive genetic comparison model)

Table 2 . Distribution of XRCC1 -77T>C Genotypes and Alleles Among Cancer Cases and Controls in the Meta- analysis
† P HWE, P values for Hardy-Weinberg equilibrium

Table 3 . Summary of Pooled Odds Ratios (OR) with Confidence Interval (CI) in the Meta-analysis
* M, model of meta-analysis; R, random-effects model; F, Fixed-effects model; † PH, the P value of heterogeneity test