Association between HLA-DP Gene Polymorphisms and Cervical Cancer Risk: A Meta-Analysis

Objective. We aimed to derive a more precise estimation of the associations between human leukocyte antigens DP (HLA-DP) gene polymorphisms and cervical cancer risk by meta-analysis. Methods. PubMed, EMBASE, ScienceDirect, Web of Science, Chinese National Knowledge Infrastructure (CNKI), and Wanfang databases were systematically searched to identify studies investigating the relationship between HLA-DP gene polymorphisms and cervical cancer. The associations between them were evaluated by pooled OR and 95% CI. Results. A total of 11 studies including 5008 cases and 9322 controls with 11 HLA-DP alleles were included in the current meta-analysis. Results. The results showed that HLA-DPB1⁎03:01 was significantly associated with an increased risk of cervical cancer (OR=1.252, 95%CI: 1.116-1.403, Pz=0.001), while HLA-DPB1⁎04:02 and HLA-DP rs3117027 G allele were significantly associated with a decreased risk of cervical cancer (OR=0.744, 95%CI: 0.652-0.848, Pz=0.001; OR=0.790, 95%CI: 0.745-0.837, Pz=0.001), and HLA-DP rs9277535 G allele was significantly associated with a decreased risk of cervical cancer in Asia (OR=0.802, 95%CI: 0.753-0.855, Pz=0.001). Subgroup analyses based on race system showed that HLA-DPB1⁎13:01 was significantly associated with an increased risk of cervical cancer in Asia (OR=1.834, 95%CI: 1.107-3.039, Pz=0.019). No significant association was established for the HLA-DP following alleles: DPB1⁎02:01, DPB1⁎02:02, DPB1⁎04:01, DPB1⁎05:01, rs4282438, and rs3077. Conclusion. HLA-DP gene polymorphisms (HLA-DPB1⁎03:01, DPB1⁎04:02, DPB1⁎13:01, rs9277535, and rs3117027) were significantly associated with cervical cancer.


Introduction
Cervical cancer is the second most commonly diagnosed cancer and third leading cause of cancer-related mortality among women in less developed countries [1]. In only one year, 2012, there were an estimated 527,600 new cervical cancer cases and 265,700 deaths worldwide [2]. Human papillomavirus (HPV) has been widely accepted as a risk factor of cervical carcinogenesis [3]; however, HPV infections have developed to persistent infection in only a very few women and to cervical cancer in an even smaller proportion [4]. Many infected women have spontaneously HPV clear through immune response [5], suggesting that other factors such as host factors may contribute to the progression of the disease [6]. Understanding the relationship between host factors and cervical cancer risk is necessary to comprehend the striking heterogeneity in anti-HPV and provide evidence for rational design of host-directed therapy.
Human leukocyte antigens (HLA) Class II genes, which encoded by DR, DQ, and DP genes, are mainly expressed in antigen presentation cells. They are essential for the presentation of viral peptides to the immune system, including HPV [18,19]. HLA II genes are highly polymorphic, and genetic variability of the HLA II alleles may lead to variations of the antigen-recognition of antigen presentation cells, thus resulting in the body being susceptible or resistant to HPV infection and consequentially affecting the results of the infection [19][20][21]. So far, more than 200 articles focusing on the relationship of HLA and cervical cancer have been published in the past 10 years; meanwhile many researches have investigated the relationship of HLA gene polymorphisms and cervical cancer risk [22,23]. More importantly, HLA-DQA1, DQB1, and DRB1 gene polymorphisms have been found associated with the risk of cervical cancer in metaanalysis [24][25][26].

Search Strategy.
To identify eligible studies, we systematically searched PubMed, EMBASE, ScienceDirect, Web of Science, Chinese National Knowledge Infrastructure (CNKI), and Wanfang databases. The keywords used for search were as follows: "cervical cancer" or "cervical carcinoma" or "uterine cervical carcinoma" and "HLA-DP" or "human leukocyte antigen" or "HLA antigen". There were no limitations on language and publication year. The last search was updated on July 30, 2017. We also retrieved the references of all relevant articles to identify additional eligible studies.
Exclusion criteria of studies were as follows: (a) letters, reviews, and case reports; (b) lack of genotype frequency data; and (c) duplicate publication. In addition, if multiple studies had overlapping data, only those with complete data were included.

Data Extraction.
Two authors independently selected the relevant articles and extracted the following data: first author's name, publication year, country, genotyping methods, number of cases and controls, genotype and allele frequency, and evidence of HWE in controls. Any controversy was resolved by discussion between the authors.

Sensitivity Analysis and Publication Bias.
The sensitivity analysis showed that no single study altered the pooled ORs The HLA-DP rs9277535 and rs3077 were just studied among Asian population.   qualitatively, which provided the evidence of the stability of the meta-analysis (Figures 6-9). As shown in Tables  2, 3, and 4, there was no publication bias for any of the alleles.
In order to clarify interactions between HLA-DP gene polymorphisms and cervical cancer in Asia, we conducted a subgroup analysis. The results showed that HLA-DPB1 * 13:01 was significantly associated with cervical cancer, and HLA-DPB1 * 03:01 showed a tendency of association with an increased risk of cervical cancer in Asia, while HLA-DPB1 * 02:01, DPB1 * 04:01, and DPB1 * 05:01 were not associated with cervical cancer in Asia either, which implies that some alleles have the same effect in Asia and Europe. The results of subgroup analysis of seven alleles based on genotyping method also showed that HLA-DPB1 * 03:01 was associated with an increased risk of cervical cancer, and HLA-DPB1 * 04:02 and rs9277535 G allele were associated with a decreased risk of cervical cancer, which implicated that genotyping method may not affect the results.
Although the correlation of cervical cancer with HLA-DP genes has been demonstrated by various studies, the molecular mechanisms underlying the association are still not elucidated. Tumor development is preceded by chronic inflammation and immune responses, whether towards the infectious agent itself or against tumor antigens. Human tumor cells express diverse types of antigens, depending on the etiology and pathogenesis of the disease. HLA-DP alleles may affect the way the human body involved in the immune system and in cell cycle. Some alleles are considered protective while others increase the risk of developing a certain condition. On the basis of our results, it is reasonable to infer that HLA-DPB1 * 03:01, DPB1 * 13:01, DPB1 * 04:02, and rs9277535 alleles are able to affect the antigen presentation and cellular expression of HLA-DP molecules. These specific HLA-DP alleles are therefore likely to associate with persistent HPV infections and thus increase or decrease the risk of cervical cancer. Some limitations existed in the present meta-analysis: First, not all alleles were reported in each study. For example, in a GWAS investigating susceptibility loci for cervical cancer in Han Chinese, the associations of HLA-DPB1 * 03:01, DPB1 * 04:01, DPB1 * 05:01, and DPB1 * 13:01 with cancer risk were also investigated [13], but we did not include the four alleles in the meta-analysis, because we could not obtain the precise allele frequency. Second, our results were not adjusted. Since age, ethnicity, family history, environmental factors, and HPV infection type are important factors for development of cervical cancer, it is better to conduct the precise analysis adjusted by the above varieties. However, not all studies included have reported age, family history, and the situation of HPV infection, etc. Approximately 200 HPV types have been identified to date, and HPV types are associated with the malignant of disease; for example, HPV types 16 and 18 are responsible for approximately 60%-80% of all cervical cancer cases, while types 31 and 52 account for the majority of the remaining cases [7,31,32]. Third, not all alleles were included in the current meta-analysis, such as rs3117027 [8,10,11] and rs4282438of HLA-DPB2 [7,13]. Finally, some studies included in the meta-analysis took cervical intraepithelial neoplasia III (CINIII) females as cases into analysis. Considering that CINIII is the important stage for developing cervical cancer, we also enrolled it.

Conclusion
To the best of our knowledge, no previous meta-analysis has comprehensively assessed the associations between the eleven alleles and cervical cancer risk. Since the number of cases and controls included in the current meta-analysis is relative huge, our results would be relatively reliable, and we could   conclude that HLA-DP gene polymorphisms (DPB1 * 03:01, DPB1 * 04:02, DPB1 * 13:01, rs9277535, and rs3117027) were significantly associated with cervical cancer, which would be regarded as early warning factors. More well-designed largescale studies including individuals from various countries and regions are still needed to determine the associations between HLA-DP gene polymorphisms and the risk of cervical cancer.

Disclosure
The funding source had no involvement in study design, data collection, or data analysis and interpretation.

Conflicts of Interest
The authors declared no conflicts of interest.

Authors' Contributions
Lin Cheng and Peiyuan Xia conceived and designed the experiments, Lin Cheng and Yan Guo performed the experiments and analyzed the data, and Lin Cheng wrote the paper. All authors read and approved the final manuscript.