Associations between XPD Asp312Asn Polymorphism and Risk of Head and Neck Cancer: A Meta-Analysis Based on 7,122 Subjects

Background To investigate the association between XPD Asp312Asn polymorphism and head and neck cancer risk through this meta-analysis. Methods We performed a meta-analysis of 9 published case-control studies including 2,670 patients with head and neck cancer and 4,452 controls. An odds ratio (OR) with a 95% confidence interval (CI) was applied to assess the association between XPD Asp312Asn polymorphism and head and neck cancer risk. Results Overall, no significant association between XPD Asp312Asn polymorphism and head and neck cancer risk was found in this meta-analysis (Asn/Asn vs. Asp/Asp: OR = 0.95, 95%CI = 0.80–1.13, P = 0.550, P heterogeneity = 0.126; Asp/Asn vs. Asp/Asp: OR = 1.11, 95%CI = 0.99–1.24, P = 0.065, P heterogeneity = 0.663; Asn/Asn+Asp/Asn vs. Asp/Asp: OR = 1.07, 95%CI = 0.97–1.19, P = 0.189, P heterogeneity = 0.627; Asn/Asn vs. Asp/Asp+Asp/Asn: OR = 0.87, 95%CI = 0.68–1.10, P = 0.243, P heterogeneity = 0.089). In the subgroup analysis by HWE, ethnicity, and study design, there was still no significant association detected in all genetic models. Conclusions This meta-analysis demonstrates that XPD Asp312Asn polymorphism may not be a risk factor for developing head and neck cancer.


Introduction
Head and neck cancers (HNC) constitute about 5% of all cancers recorded in the US, and the incidence is increasing in most developed and developing countries. These cancers have been estimated to be about six times more common among smokers than non-smokers and are most common in males over 50 years old [1,2], which increases to about 15 times if the smokers are also heavy drinkers [3,4]. Although many measures had been done to improve the diagnosis and treatments, the prognosis was still poor.
Many environmental factors, such as radiation, diet, smoking, and endogenous or exogenous estrogens, are associated with DNA damage. Unrepaired or misrepaired DNA results in gene mutations, chromosomal alterations, and genomic instability. Several studies have suggested that genes involved in DNA repair play a crucial role in protecting against mutations. Patients with certain cancers have reduced capacities for DNA repair. Similarly, the enzymes of the nucleotide excision repair (NER) pathway have been implicated in cancer. Associations between polymorphisms in several DNA repair genes and the risks of several types of cancer have been extensively examined. Many epidemiologic cancer studies have focused on single nucleotide polymorphisms (SNPs) in genes in the NER pathway such as XPD, ERCC1, and XPC [5]. The XPD protein is a DNA helicase and is an essential part of the TFIIH transcription factor complex. Some studies have suggested that XPD polymorphisms may be associated with reduced DNA repair because of a possible reduction in helicase activity [6,7]. One of the common XPD polymorphisms in the coding regions is Asp312Asn in exon 10. The functional significance is not yet completely clear, although the amino acid mutations in exon 10 give rise to a loss of an acidic residue and a complete change in the electronic configuration of the amino acid [8,9].
The first study on the relationship between HNC and XPD Asp312Asn polymorphism was conducted by Sturgis et al. [10].
They found a borderline significant association between XPD Asp312Asn polymorphism and HNC. Since then, a lot of studies have confirmed or refuted this finding [11][12][13][14][15][16][17][18]. In 2010, a recent meta-analysis was conducted by Flores-Obando et al. [19] demonstrated that increased HNC risk is associated with XPD Asp312Asn polymorphism. Worthy of note, that meta-analysis included five studies were conducted in Caucasian populations and one in an Asian population [10][11][12][13][14][15]. Today, nine case-control studies on XPD Asp312Asn polymorphism and HNC risk have been published. A comprehensive meta-analysis is needed to provide an updated approach on the overall relationship. Subgroup analyses were also performed on Caucasian and Asian populations to investigate ethnicity-specific effects.

Search strategy
The PubMed database was searched with terms ''head and neck cancer'', ''oral cancer'', ''oropharyngeal cancer'', ''laryngeal cancer'', ''pharyngeal cancer'', ''XPD'', ''excision repair crosscomplementing group 2'', ''polymorphism'', and the combined phrases for all genetic studies on the relationship between XPD polymorphism and HNC risk from 2000, when the first study of the association between XPD Asp312Asn polymorphism and HNC risk was reported, to October 2011. We also used the ''Related Articles'' option in PubMed to identify additional studies on the same topic. Reference lists in retrieved articles were also screened for. All selected studies complied with the following three criteria: (a) case-control study on the XPD Asp312Asn polymorphism and HNC risk; (b) sufficient published data for estimating the odds ratio (OR) with 95% confidence interval (CI); (c) For multiple publications reporting on the same data or overlapping data, the largest or most recent publication was selected [20].

Data extraction
Two investigators (Hu and Yuan) independently extracted the following data from each included publication: the first author's name, publication data, sources of controls, racial descent of the study population (categorized as either Asian or Caucasian), genotyping method, number of cases, cases and controls with different genotypes, and Hardy-Weinberg equilibrium(HWE).

Statistical analysis
Crude ORs with 95% CIs were computed to assess the strength of the correlation between the XPD Asp312Asn polymorphism and HNC risk. The pooled ORs were performed for codominant model (Asn/Asn vs. Asp/Asp,Asp/Asn vs. Asp/Asp), dominant model (Asn/Asn+Asp/Asn vs. Asp/Asp), and recessive model (Asn/Asn vs. Asp/Asp+Asp/Asn), respectively. In the subgroup analysis, statistical analysis was conducted on Asians and Caucasians. Heterogeneity assumption was assessed by the chisquare based Q-test [21]. The pooled OR estimation of each study was calculated by the fixed-effects model (the Mantel-Haenszel method) when P.0.10. Otherwise, the random-effects model (the DerSimonian and Laird method) was used [22]. The potential publication bias was estimated by the modified Egger's linear regression test, which proposed by Harbord et al. [23]. Statistical analysis was performed using STATA version 11.0 (Stata Corporation, College Station, TX, USA) and Review Manage (v.4.2; Oxford, England), using two-sided P-values, with P,0.05 considered statistically significant.

Meta-analysis
The main results of this meta-analysis and the heterogeneity test are shown in Table 2. Overall, no significant relationship was observed between XPD Asp312Asn polymorphism and HNC risk Similarly, in the succeeding analysis of HWE studies, no significant association was found between XPD Asp312Asn polymorphism and Finally, in the stratified analysis of ethnicity and study design, we also did not find any significant association between XPD Asp312Asn polymorphism and HNC.

Sensitivity analysis
A single study involved in the meta-analysis was deleted each time to reflect the influence of the individual dataset to the pooled ORs. The analysis results demonstrate a borderline increased risk after excluding the studies that in Asp/Asn vs. Asp/Asp model [14,16,18] (Figure 2). The other corresponding pooled ORs were not materially altered (data not shown), indicating that our results are statistically robust.

Publication bias
Funnel plot and modified Egger's test were performed to estimate the publication bias of literature. The shapes of the funnel plots in all genetic models did not reveal any evidence of obvious asymmetry. Figure

Discussion
Today, genetic susceptibility to cancer has attracted growing attention to the study of gene polymorphisms involved in tumorigenesis. The XPD gene has been mapped to chromosome 19q13.3 and it is composed of 23 exons. Germline mutations in the XPD gene can result in xeroderma pigmentosum and other diseases. The XPD protein is involved in transcription-coupled NER and is an integral member of the basal transcription factor BTF2/TFIIH complex.
The Asp to Asn change at position 312 of XPD changes the electronic configuration of amino acid and alters the interaction between XPD protein and its helicase activator [6]. Wolfe et al.  [26]. Correlations between the polymorphisms and some cancer risks have been studied, but the results remain controversial. The XPD Asp312Asn polymorphism has been shown to increase the risk of bladder cancer and lung cancer, but it is not associated with breast cancer [27][28][29].
The present meta-analysis of nine eligible studies, including 2670 cases and 4452 controls focused on XPD Asp312Asn polymorphism and HNC risk, was performed to derive a more precise estimate of the association, but no significant association was found in the total population when all the studies were pooled. Similarly, no significant association was detected in all genetic models during the satisfied analysis based on the HWE, ethnicity and study design. Our finding is not in accordance with that previously published by Flores-Obando et al [19]. A marginally significant association was observed between the XPD Asp312Asn heterozygous and combined variants and HNC in their study. The considerably larger sample size of our study may account for this difference relative to the previous study.
Despite the considerable efforts to test for possible association between XPD Asp312Asn polymorphism and HNC risk, some limitations should be addressed. First, these results are based on unadjusted estimates that lack the original data from the eligible studies, which limits the evaluation of the effects of the gene-gene and gene-environment interactions during HNC development. Second, the sample size is still relatively small. Thus, we could not have enough statistical data to find the true relationship between XPD Asp312Asn polymorphism and HNC risk. Finally, each gene is known to have a moderate effect on HNC development. The combinations of certain genotypes may be more discriminating as risk factors than a single locus genotype. In our meta-analysis, linkage disequilibrium (LD) and haplotype analysis were not performed. In spite of these limitations, no publication bias was observed, and a large number of subjects still significantly guarantee the statistical power of the analysis.
In conclusion, despite these limitations,our meta-analysis suggests that XPD Asp312Asn polymorphism may not be associated with HNC development. In the future, large-scale case-control and population-based association studies are necessary to validate the risks identified in the present meta-analysis and to investigate the potential gene-gene and gene-environment interactions between XPD Asp312Asn polymorphism and HNC cancer.