Shanqi Guo,Xingkang Jiang,Xiaojiang Li,Yuanyuan Wang,Yingjie Jia,*

1Department of Oncology,First Teaching Hospital of Tianjin University of Traditional Chinese Medicine,Tianjin,China.

2Department of Urology,the Second Hospital of Tianjin Medical University,Tianjin Institute of Urology,Tianjin,China.

#These authorscontributed equally to this work.

Abstract DNA repair gene are important in maintaining genome stability and integrity.Xeroderma pigmentosum group D(XPD)polymorphisms have been reported with prostate cancer(PCa)risk,but the conclusion remained controversial.The aim of this study was to derive a more precise estimation of the associations of two common XPD polymorphisms with the susceptibility of PCa.The pooled odds ratios(ORs)with their 95%confidence intervals(CIs)were estimated to assess the association between XPD polymorphisms and PCa risk.A total of ten trials including 3,991 cases and 4,814 controls who were evaluated of XPD rs13181 genotype and PCa risk,but no statistical relationship was found both overall four genetic model and subgroup analyses.In addition,seven studies were identified with a total 7,642 participants to assess association between rs1799793 genetic polymorphism and PCa susceptibility.There were a slight association between PCa risk and XPD rs1799793 genetic polymorphism(CC versus TT,OR=0.52,95%CI 0.25-1.06,P=0.07;Dominant model,OR=0.56,95%CI 0.30-1.06,P=0.08 and recessive model,OR=0.74,95%CI 0.53-1.03,P=0.08).Moreover,subgroup analysis stratified by ethnicity found the protective effect of rs1799793 polymorphism against PCa risk upon Asian population(CC versus TT,OR=0.48,95%CI 0.32-0.72,P=0.00;Dominant model,OR=0.52,95%CI 0.28-0.95,P=0.03 and recessive model,OR=0.67,95%CI 0.50-0.89,P=0.01).Taken together,the existing evidence indicates XPD rs1799793 polymorphism should be viewed as a protective effect against PCa risk under CC versus TT,Dominant and Recessive model.For rs13181 polymorphism,no associations were found between this variant and the susceptibility of PCa.

Keywords:DNA repair;meta-analysis;prostatic neoplasms;polymorphism;XPD

Introduction

Prostate cancer(PCa)is one of the most common malignancy worldwide,with an estimated incidence of 233,000 cases and 29,480 deaths in the United State for 2014[1].Although its etiology remains largely unknown,genetic factors are believed to play an important role in the development of PCa[2].Many environmental component is implicated in～80%of all cancers;however,the causes for certain cancers are still unknown[3,4].Modulations in metabolism and DNA adduct formation are considered central mechanisms in environmental carcinogenesis.DNA damage is critical to carcinogenesis and unrepaired DNA damages will lead to mutations and ultimately cancer[3,5].

There are four major DNA repair pathways in mammals,including base excision repair(BER),nucleotide excision repair(NER),mismatch repair(MMR)and double strand break repair(DSBR)[6].Reduced DNA repair capacity constitutes a statistically significant risk factor for cancer[7,8].The xeroderma pigmentosum group D(XPD)gene,also known as excision repair cross-complementing rodent repair deficiency group 2 (ERCC2), is located at chromosome 19q13.3.XPD is a subunit of TFIIH complex and allows the formation of a complex between DNA polymerase beta,DNA ligase III and poly(ADP-rib)polymerase[9].There are two common single nucleotide polymorphisms (SNPs),rs13181(K751Q) and rs1799793(D312N) [10].Non-synonymous genetic polymorphisms in those DNA repair genes may influence their capacity to repair DNA damage,which could thus lead to carcinogenesis.

Over the past decade,multiple studies have evaluated the association of PCa risk with polymorphisms in the DNA repair genes XPD.However,the results from those studies are to some extent divergent,which may be partly attributed to the limited power of individual study.Therefore,it is necessary to present a meta-analysis based on the current high-level clinical evidence to assess the association between two common polymorphisms of XPD and PCarisk.

Materialsand methods

Search strategy

Two independent authors carried out the comprehensive literature searches,including Pubmed,Web of Science,Wanfang database and China National Knowledge Infrastructure database,to identify all relevant studies until 1 June 2015.Studies were considered irrespective of language status.The medical subject headings and key words used for search were:(“xeroderma pigmentosum group D”or“excision repair cross-complementing rodent repair deficiency group 2”or XPD or ERCC2)and(rs13181 or K751Q or rs1799793 or D312N)and(“prostate cancer” or “prostatic cancer” or“prostate tumor” or “prostatic tumor”) and(polymorphism or variant or mutant).All the searched studies were retrieved, and their references were also checked as well for other relevant articles.

Identification of articlesand data extractions

Studies performed with evaluation of two common XPD polymorphism (rs13181 and rs1799793)and PCa susceptibility were eligible for inclusion.When duplication trials were published,only the latest or complete study was included in this meta-analysis.Abstract,case report,editorials,and reviews were neglected.A standardized form was created and used to extract the available data from all eligible publications.The extracted data elements including the following:(1)the first author's surname,publication year and country,(2)study design, ethnicity,sample source and the characteristics of controls,(3)sample size and available genotype.Two investigators extracted data independently and an agreement was reached by discussion.

Statistical analysis

This meta-analysis was strictly performed according to the preferred reporting items of the systematic reviews and meta-analysis(PRISMA)statement.Dichotomous data were presented as odds ratio(OR)with 95%confidence intervals(CI) to assess the associations of XPD polymorphisms with PCa risk.The Mantel-Haenszel estimates were calculated according to the heterogeneity among the pooled studies.If significant heterogeneity was observed(P＜0.10,I2＞50%),a random-effects model was applied;otherwise,the fixed effects model was utilized.Moreover,we minimized the influence of heterogeneity by classifying the enrolled studies into subgroups based on similar characteristics.Sensitivity analysis were also performed to avoid biases in the result due to certain low-quality studies.The publication bias was estimated using Egger's linear regression test with a funnel plot.In addition,we also assessed the departure from the Hardy-Weinberg equilibrium(HWE)for the control group in each study using an online HWE calculator.All the statistical tests were conducted with STATA 10.0(Stata,College Station,TX,USA)and Microsoft Excel(V.2007,Microsoft Corporation,Redmond,Washington,USA).A P-value＜0.05 was considered statistically significant.

Results

Studies selection and characteristics of enrolled studies

We identified 21 relevant articles according to the search strategy(Figure 1).After screening titles and abstracts,we included eighteen studies in full text for formal review.Moreover,we also examined recent reviews,no additional relevant trial was included.In total,eleven identified studies containing 9,604 participants that met predefined inclusion criteria[10-20].The characteristics of eligible studies summarized in Table 1.In detail,there are seven studies with a total 7,390 subjects on Caucasian population,four trials were Asian descendants with 1,877 men and two were African-American with 337 participants.All of trails recruited patients from population,except three studies from hospital and one from sibling.PCR restriction fragment length polymorphism (PCR-RELP)was used to validate genotype in eight studies and Mass genotyping assay was used for other five trials.All the trials used blood samples for genotyping.The characteristics of each study such as country,ethnicity and sample size were collected and source of control were also gathered.

Overall and subgroup analysis of XPD polymorphisms and PCa riskrs13181

There were ten trials with a total of 3,991 patients and 4,814 controls who were evaluated of XPD rs13181 polymorphism and PCa susceptibility(Table 1).Overall,no statistical relationship was found with any genetic model associated with susceptibility to PCa(AA versus CC,OR=0.99,95%CI 0.86-1.15,P=0.93;AC versus CC,OR=0.97,95%CI 0.84-1.12,P=0.69;Dominant,OR=0.98,95%CI 0.86-1.12,P=0.78;Recessive, OR=1.01, 95% CI 0.92-1.11,P=0.85.).Furthermore,there was no significant statistical heterogeneity among these studies.In addition,a sensitivity analysis was also carried out to evaluate the stability of combined results.In brief,we deleted each study in the analysis and the corresponding pooled ORs were not altered,which suggesting meta-analysis relatively stable and reliable(data not shown).In subgroup analyses stratified by ethnicity and study design,there were also no significant associations between this polymorphism and PCa risk in all genetic models.Egger's test showed little publication bias of overall and any subgroup analyses(Table 2).The genotype distributions in the controls of all studies were in line with HWE.

Figure 1.Flowchart of literature search

?

rs1799793

Table 3 listed the main results of the meta-analysis of XPD rs1799793 genetic variant and PCa risk.Seven studies were identified with a total 3127 cases and 3809 controls and pooled into this analysis.There were a slight association between PCa risk and XPD rs1799793 genetic polymorphism(CC versus TT,OR=0.52,95%CI 0.25-1.06,P=0.07;Dominant model,OR=0.56,95%CI 0.30-1.06,P=0.08 and recessive model,OR=0.74,95% CI 0.53-1.03,P=0.08).However,great heterogeneity was found on Cochran Q statistics(P＜0.1)or I2＞50%in four genetic model among included studies(Table 3).Furthermore,sensitivity analysis also showed great differences when remove some of studies.Subsequently,we carried out subgroup analysis based on ethnicity to minimize the statistical and clinical heterogeneity.The results showed reduced PCa risk associated with the polymorphism upon Asian population(CC versus TT,OR=0.48,95%CI 0.32-0.72,P=0.00;Dominant model,OR=0.52,95%CI 0.28-0.95,P=0.03 and recessive model,OR=0.67,95%CI 0.50-0.89,P=0.00).When stratified studies by study design,we failed to detect any significant association in any genetic model.No publication bias was found by Egger's test in above analysis.

Discussion

DNA repair pathways are crucial to prevent accumulation of DNA damage and maintain genomic stability and integrity.Numerous polymorphisms of the NER genes have been identified and may modify the NER capacity and then contributed to the development of PCa[21].XPD plays a curial role in the opening of the DNA helix to allow the excision of the DNA fragment containing the damaged base.Mutation in the XPD gene may diminish the activity of transcription factor IIH complexes,which might influence the capacity to repair damaged DNA and increase the susceptibility to cancers[22,23].Currently,two variant model of XPD have been investigated to be associated with risks of many cancers,but the conclusion remained inclusive,especially for PCa risk[24-26].Hence,we performed a well-designed meta-analysis to provide a quantitative approach for combining the results of various studies for estimating and explaining their diversity.

In present study,we identified eleven studies with a total 9,604 participants to assess the association of two common XPD polymorphisms with PCa risk.We evaluated the quality of these trials methodology and found all the enrolled studies clearly described ethnicity,sample size,study type and design,characteristics of controls,sample source and the methods of genotype[2].Moreover,the genotype frequencies in control group of each study were all consistent with HWE.Thus,the reasonable design and high level of clinical evidence upgrade the quality of this meta-analysis.

The XPD rs13181 variant is at position 751 in exon 23 and characterized by an A to C substitution causing a lysine to be exchanged for glutamine.Previous studies reported that XPD rs13181 variant could diminish the activity of transcription factor IIH complexes,giving rise to defect in repair capability in NER capacity.In present study,we did not find any rs13181 genetic model associated with susceptibility to PCa in the overall analysis.Moreover,there were no significant heterogeneity on statistical testing in these trials.Furthermore,we stratified studies into subgroup based on similar characteristics(e.g.ethnicity,study design),there was still no significant association between the genetic variant and PCa risk in all genetic models.In addition,little publication bias was found among overall and subgroup analysis.

The XPD rs1799793 polymorphism at position 312 in exon 10 is characterized by a G to A substitution resulting in aspartic amino acid to asparagine amino acid exchange.The amino acid substitution could alter the RecQ family of DNA helicases function and result in a protein malfunction in either repair capacity or fidelity.In this analysis,there was a slight association with the susceptibility to PCa in CC versus TT,Dominant and Recessive model.However,great heterogeneity was found on both statistical testing and clinical characteristics of included studies,which could weaken the authenticity and reliability of the pooled results.In order to minimize the above heterogeneity methodologically,we further stratified studies into subgroup based on similar characteristics(e.g.ethnicity,study design).Subgroup analysis of ethnicity showed a decreased risk for Asian populations under CC versus TT,Dominant and Recessive model,while no association was detected in Caucasians and African-American population.This difference may be caused by ethnic differences in genetic background and the

environment existed among different ethnicities.In addition,the source of controls may also be partly responsible for the biases because controls from hospital-based studies might just represent samples with non-prostate cancer diseases and may have different genetic variant from healthy population,although we did not detect any evidence from the subgroup analyses stratified by study design.However,the conclusion should be interpreted with caution because the heterogeneities of subgroup analysis were still could not be removed.In addition,sensitivity analysis also demonstrated the enrolled studies have great heterogeneity and the results were unstable.

?

?

In addition,several studies have reported the different Gleason grade and bone metastasis with XPD polymorphisms.Henriquez-Hernandez et al and Mittal et al found that SNP rs13181 was not associated with poor prognosis for PCa[18,27].Mandal et al also indicated that two XPD polymorphisms were not associated with susceptibility to higher grade or bone metastasis[16].This phenomena suggested that these genes or their variants are not associated with disease progression but only the risk of disease initiation.

Moreover,cigarette smoking,drinking status,age and gene-gene interaction may have great influence on XPD polymorphisms and PCa risk.Mandal et al and Mittal et al evaluated the gene-smoking interaction to study the modulation of PCa risk with respect to these gene polymorphisms and did not observe any association with PCa risk[16,18].Sobti et al found no statistically significant association between smoking,alcoholism and the rs13181 polymorphism to overall risk of PCa,respectively[19].In addition Mirecka et al also indicated that the patients under 65 years of age who were heterozygous carriers of rs1799793 polymorphism appeared to have a slightly greater decreased PCa risk compared to the control population than those over 65 years.Multivariate logistic regression was applied to identify possible dual interaction between the 15 SNPs,but no significant interaction could be identified[20].Due to less publications according the above variant,we could not calculated the pooled OR from enrolled studies through meta-analysis quantitatively,but demonstrated the conclusion qualitatively.

However,some potential limitations should also be taken into consideration.Firstly,the number of published studies were not sufficiently large for comprehensive analysis,particularly for subgroup with Asian and African-American populations.Secondly,the meta-analysis based on multivariate model could be performed if only the individual patient data(IPD)were available,which actually beyond our capacity.Moreover,IPD could also provide more detailed clinical parameters(e.g.tumor stage,response to chemotherapy,survival data),which might more useful for clinical practice.Lastly, PSA levels,hormone-dependent type and other co-variables that may contribute to influence the association between XPD polymorphism and PCa risk should also becalculated when preformed meta-analysis.

In conclusion,this meta-analysis suggested that XPD 1799793 polymorphism present a protective effect under CC versus TT,Dominant and Recessive model.For rs13181 polymorphism,no associations were found between this variant and the susceptibility of PCa.Due to the aforementioned limitations,additional large-scale and well-designed studies are needed to further validate the risk of the present meta-analysis.