RESEARCH ARTICLE Genetic Association between ERCC5 rs17655 Polymorphism and Colorectal Cancer Risk: Evidence Based on a Meta-analysis

Background: Previous studies evaluating the association between the excision repair cross complementing group 5 (ERCC5) gene rs17655 polymorphism and colorectal cancer susceptibility generated controversial results. To generate large-scale evidence on whether the ERCC5 rs17655 polymorphism might indeed be associated with colorectal cancer susceptibility, the present meta-analysis was performed. Materials and Methods: Data were collected from PubMed, Embase and Web of Science, with the last report up to Apr 03, 2015. Odds ratios (ORs) with 95% confidence intervals (CIs) were used to assess the strength of any association. Results: A total of nine studies including 5,102 cases and 6,326 controls based on the search criteria were included and significant associations were found between ERCC5 rs17655 polymorphism CG vs GG overall (OR = 1.29, 95% CI =1.18~1.40) and in the dominant model (OR=1.23, 95% CI =1.13~1.33). On subgroup analysis by ethnicity and source of controls, the ERCC5 rs17655 polymorphism was found to correlate with the pathogenesis of colorectal cancer among Asians and Caucasians and with hospital-based populations. Conclusions: This meta-analysis suggests that the ERCC5 rs17655 polymorphism might contribute to genetic susceptibility to colorectal cancer.


Introduction
Cancer has developed to be one of the most common and severe diseases that leads to high mortality worldwide in recent decades. Colorectal cancer (CRC) is the third most common malignancy and the fourth most common cause of cancer-related death in the world (Siegel et al., 2014). The incidence of CRC is increasing each year (Center et al., 2009;Ferlay et al., 2010). Generally, CRC is considered as a multifactorial disease caused by the interaction of both environmental and genetic factors (Haggar and Boushey, 2009;Rawson et al., 2011). Evidence from epidemiological studies has suggested that a wide range of environmental factors are involved in the etiology of CRC, such as diet, increasing age, and related aspects of lifestyle (Dahm et al., 2010;Ogino et al., 2010). Genetic susceptibility to this disease may result from inherited mutations in genes involved in carcinogen metabolism and DNA repair (Shields and Harris, 2000;Goode et al., 2002).
Single nucleotide polymorphisms (SNPs) are the most common sources of human genetic variation, and they maycontribute to an individual's susceptibility to cancer (Zhang et al. 2013). A recent study has revealed that approximately 35% of CRC cases can be attributed to inherited genetic (Markowitz and Bertagnolli, 2009).

Genetic Association between ERCC5 rs17655 Polymorphism and Colorectal Cancer Risk: Evidence Based on a Meta-analysis
Yong Zeng 1& , Li Wei 2& , Ya-Jie Wang 3 *, Chuan Liu 3 * Then, investigating the effects of genetic variants on CRC risk may help better understand the association between genetic variants and cancer risk.
Excision repair cross-complementing group 5 (ERCC5 or xeroderma pigmentosum group G, XPG), one of the functional genes in the base excision repair (NER) pathway, is located on chromosome 13q22-q33, consisting of 15 exons and 14 introns (Emmert et al., 2001). ERCC5 is a member of the flap structure-specific endonuclease 1(FEN1) family and encodes a protein of 1186 amino acids. It has reported that a defective ERCC5 plays a pivotal role in the initiation of carcinogenesis and leads to DNA repair defects, genomic instability, and failure of gene transcription modulation (Cheng et al., 2002;Koeppel et al., 2004;Bartolucci et al., 2009). SNPs in ERCC5 gene have been discovered in human populations, a polymorphism at codon 1104, resulting in the amino acid change of His1104Asp (rs17655, G>C), is common and regarded as a tagger, which was most frequently investigated for its association with cancer risk.
To date, rs17655 polymorphism in the ERCC5 gene has been found to be involved in the pathogenesis of CRC. However, the results from different laboratories are conflicting. Therefore, we performed a quantitative synthesis of the evidence on the association of ERCC5 rs17655 polymorphism with the developing risk of CRC.

Literature sources and search strategies
This meta-analysis was performed by searching PubMed, EMBASE, and Web of Science databases, with a time limit of Apr 03, 2015. The following key terms were used: "excision repair cross-complementing group 5", "ERCC5", "XPG ", "xeroderma pigmentosum group G", "NER", "polymorphism" , combined with "colon cancer", "rectal cancer", or "colorectal cancer". There was no language limitation in the search of databases. All of the searched studies were retrieved, and the bibliographies were checked for other relevant publications. Review articles and bibliographies of other relevant studies identified were searched by hand in order to find additional eligible studies. If more than one article was published using the same study population, only the study with largest sample size was selected.

Inclusion criteria
The following inclusion criteria were used to identify eligible articles. (1) Sufficient genotype data were presented to allow calculation of the odds ratios (ORs).
(2) Studies designed as case-controls; (3) Study clearly described the diagnosis of CRC and the sources of the cases and controls.

Data extraction and methodological assessment.
Information was extracted carefully from all eligible publications independently by two investigators according to the inclusion criteria listed above. For conflicting evaluation, an agreement was reached following discussion. The following information was extracted from each study: first author, publication year, ethnicity (country), source of controls, number of cases and controls, and the genotype frequencies of the cases and controls. Ethnicities were categorized as Asian and Caucasian. Source of control were categorized as hospital based, population based and family based. We did not define any minimum number of patients to include in our meta-analysis. Methodological quality was evaluated separately by two authors using the Newcastle-Ottawa Scale (NOS) criteria (Stang, 2010). The NOS criteria is based on three aspects: (1) subject selection: 0~4; (2) comparability of subject: 0~2; (3) clinical outcome: 0~3. NOS scores range from 0 to 9 with scores ≥7 indicating good quality.

Statistical analysis
The pooled odds ratio (OR) and 95 %CI were used to assess the association between ERCC5 rs17655 polymorphism and CRC risk for each case-control study. Heterogeneity among studies was estimated with the Cochran's Q statistic and I 2 tests (Zintzaras and Ioannidis, 2005a). If the Q test showed a P<0.05 or I 2 test exhibited >50 %, indicating significant heterogeneity, the random effects model was used. Otherwise, the fixed effects model was used (Zintzaras and Ioannidis, 2005b). We explored potential sources of heterogeneity using subgroup analyses, and the subgroup analyses were done by ethnicity and source of control. The pooled ORs were performed for co-dominant model (CC vs GG and CG vs GG), the dominant model (CC+CG vs GG), and the recessive model (CC vs CG+GG), respectively. The significance of the pooled ORs was determined by Z test. The Hardy-Weinberg equilibrium (HWE) was assessed by Fisher's exact test. Publication bias was assessed by visual inspection of funnel plots (Munafo et al., 2004), in which the standard error of log (OR) of each study was plotted against its log (OR). An asymmetric plot indicates a possible publication bias. Publication bias may be absent if the plot resembles a symmetrical inverted funnel in which smaller, less precise, and more numerous studies have increasingly large variation in the estimates of their effect size (Sutton et al., 2000). Thus, publication bias was further investigated using Begg's funnel plot and Egger's regression test (Begg and Mazumdar, 1994;Egger et al., 1997) (p<0.05 was considered to be statistically significant). All statistical analyses were performed using STATA statistical software (version 10.0). Two-sided p values less than 0.05 were considered statistically significant.

Characteristics of eligible study
We retrieved and screened the publications relevant to the keywords originally. The detailed characteristics of the included studies are listed in Table 1. A total of nine articles ( Figure 1) were collected according to the search criteria (Pardini et al., 2008;Joshi et al., 2009;Canbay et al., 2011;Gil et al., 2012;Liu et al., 2012;Du et al., 2014;Steck et al., 2014;Paszkowska-Szczur et al., 2015;Sun et al., 2015). One publication (Steck et al., 2014) contained two case-control data was considered to two separate studies, therefore, 10 articles including 5,102 cases and 6,326 controls were used for this meta-analysis. The controls were primarily healthy population. All of the cases were pathologically confirmed. In terms of ethnicity, there were three groups of Asians (Liu et al., 2012;Du et al., 2014;Sun et al., 2015), six groups of Caucasians (Pardini et al., 2008;Joshi et al., 2009;Canbay et al., 2011;Gil et al., 2012;Steck et al., 2014;Paszkowska-Szczur et al., 2015). In terms of source of control, there were six groups of hospital based (Pardini et al., 2008;Canbay et al., 2011;Liu et al., 2012;Du et al., 2014;Paszkowska-Szczur et al., 2015;Sun et al., 2015), three groups of population based (Gil et al., 2012;Steck et al., 2014), one group of family based (Joshi et al., 2009). Studies with control not in Hardy-Weinberg equilibrium (HWE) were also considered for meta-analysis, but they were excluded in the sensitivity analysis (Minelli et al., 2008).

Quantitative data synthesis
The main results of our meta-analysis under four distinct genetic models were listed in Table 2 and Table 3. Overall, significant associations were found between the ERCC5 rs17655 polymorphism and CRC susceptibility for CG vs GG (OR = 1.29, 95% CI =1.18~1.40, p =0.10 for heterogeneity, Figure 2A) and the dominant model (OR=1.22, 95% CI =1.13~1.33, p =0.24 for heterogeneity, Figure 2B). But no significant association was found Genetic Association between the ERCC5 rs17655 Polymorphism and Colorectal Cancer Risk: Evidence from a Meta-analysis.
To comprehensively evaluate the effect of ERCC5 rs17655 polymorphism on the pathogenesis of CRC, we also carried out subgroup analysis based on ethnicity and source of control. Ethnicity-stratified analysis indicated that ERCC5 rs17655 polymorphism was correlated with the pathogenesis of CRC among both Asians and Caucasians in the majority of subgroups. Furthermore, the results of stratified analyses based on source of control revealed that ERCC5 rs17655 polymorphism was closely linked to the pathogenesis of CRC in the majority in hospital based subgroup, but neither in population based nor family subgroups.

Heterogeneity and sensitivity analysis
We observed no substantial heterogeneity in the metaanalysis (P>0.05; Table 2). To check the stability of the     DOI:http://dx.doi.org/10.7314/APJCP.2015.16.13.5565 Genetic Association between the ERCC5 rs17655 Polymorphism and Colorectal Cancer Risk: Evidence from a Meta-analysis. combined effects, sensitivity analysis was performed by sequentially deleting each of the included studies and recalculating the ORs. Such a leave-one-out sensitivity analysis indicated that no single study influenced the pooled ORs qualitatively in the dominant model ( Figure  3A), the recessive model ( Figure 3B) and other models (not showed). Hence, results of the sensitivity analysis suggest that the data in this meta-analysis are relatively stable.

Publication bias
Begg's funnel plot and Egger's linear regression test were performed to assess the publication bias of included studies. The funnel plots for publication bias showed symmetry for the dominant model and the recessive model ( Figure 4A, Figure 4B). Meanwhile, results of Begg's test and Eggers's linear regression method indicated that there was no obvious publication bias (P>0.05, Table 2).

Discussion
Development of malignant tumors results from multistep accumulation of genetic and epigenetic alterations in cells, and the presence of genomic instability has a substantial effect in accelerating the carcinogenic processes. Dysfunction of DNA repair systems may make a significant contribution (Masutani et al., 2003). CRC is a common and complex multifactorial disease in which environmental and host-related factors interact (Bordignon et al., 2012;Petraki et al., 2012;Wang et al., 2013). Data has indicated an association of the descent productivity of DNA repair and the increased susceptibility of colorectal cancer (Jiricny and Marra, 2003). It is widely recognized that mismatch repair pathway is an etiological factor of individual risk to colorectal cancer (Peltomaki, 2003). NER is a crucial DNA repair mechanism, which counteracts the consequences of mutagenic exposure of cell. XPF and ERCC5 are both central players in the NER pathway, and they are involved in incision 5' and 3', respectively, to the DNA lesion.
Since the identification of ERCC5 rs17655 polymorphism, an increasing number of studies suggested that ERCC5 rs17655 polymorphism may play important roles in CRC development. Epidemiological studies of ERCC5 rs17655 polymorphism, if large and unbiased, can provide insight into the in vivo relationship between the overall, gene and CRC risk. However, these studies have resulted in apparently contradicting findings.
Meta-analysis has been recognized as an effective method to solve a wide variety of clinical questions by summarizing and reviewing the previously published quantitative research. By using meta-analysis, CTLA-4 rs231775 (Wang et al., 2015), IRS-1 rs1801278 (Li et al., 2014), hOGG1 Ser326Cys  polymorphisms have been proved associated with bladder cancer susceptibility.
In the present meta-analysis, we attempted to identify whether ERCC5 rs17655 polymorphism was a causative factor for the pathogenesis of CRC. A total of 11,428 subjects were included, the results showed that the ERCC5 rs17655 polymorphism was a risk factor for CRC in overall population, furthermore, in the subgroups by ethnicity and by source of control, ERCC5 rs17655 polymorphism was correlated with the pathogenesis of CRC among Asians, Caucasians and hospital based population.
Our meta-analysis has a number of limitations that should be addressed. First, the small sample size does not possess sufficient statistical significance to evaluate the relationship between ERCC5 rs17655 and the risk of CRC with great certainty, especially in the family based subgroup (only one article). Since the small number of studies may constrain the general applicability of our findings, the cognitive function of our meta-analysis should be regarded as preliminary. Second, the controls included in our analysis were selected variously either from populations or hospitals. Therefore, misclassification bias was possible because these studies may have included control groups who have different risks of developing GI cancers. Third, OR value was obtained with a failure to take account of important confounding factors. More accurate OR should be corrected by age, lifestyle, gender, and other environmental risk factors, after adjusting for covariates that might help explain the association between ERCC5 rs17655 polymorphism and susceptibility to CRC.
In summary, this meta-analysis provideed reliable evidence that the ERCC5 rs17655 polymorphism was a risk factor for CRC. However, due to the limitations acknowledged above, research based on a larger sample size and more detailed data is needed in order to achieve a more significant and representative statistical analysis.