Abstract
The purpose of this systemic review and meta-analysis was to examine the relationship between VDR gene polymorphisms and breast cancer. Literature was searched through PubMed database, Google scholar, and the web of knowledge from December 2015 to January 2017 and consists of 34 studies (26,372 cases and 32,883 controls). All statistical measures were done using STATA version 11.2. The heterogeneity among studies was tested using
Introduction
Breast cancer (BC) is a multifarious group of disease, accounts 23% of all cancers among females worldwide. 1 The incidence rate of BC has been increasing annually by more than 1 million across the globe (http://www.cancerscreening.nhs.uk/) but the etiology of BC is still not clear. Genetic and environmental factors are the most influential contributing factors that lead to the development of BC.2–4 Sometimes, minor genes might become important to increase the chance of BC including Vitamin D receptor (VDR) gene. The VDR gene is situated at chromosome 12q13.11 which spans ~100 kb and have five promoters, eight coding exons, and six untranslated exons. 5 VDR produces its pleiotropic effects via binding with its ligand vitamin D and operates a cascade of signaling pathways inducing various genes activation 6 and involved in BC prevention. 7 The gene has many polymorphic variants which influence the expression/functions of VDR which is associated with complex traits or diseases such as tuberculosis, 8 osteoporosis, 9 diabetes, 10 and cancers11,12 including BC.13,14 To date, only a few VDR gene variants have been considered in relation to BC risk with contradictory results including Fok1, Bsm1, Taq1, Apa1, Cdx2, and Poly (A).13–47 This inconsistency in results may because of small sample size, low statistical power, and clinical heterogeneity in these studies. 48 In order to resolve the inconsistencies among studies and reduce the likelihood of having false-positive or false-negative relations due to random errors, we executed a meta-analysis of 34 existing studies. So, the purpose of this study is to compare VDR gene polymorphisms in BC cases and controls which were systematically recognized, observed, and assembled in a meta-analysis.
Materials and methods
Publication search strategy
The relevant research articles were searched through PubMed database, Google scholar, Science.gov and web of knowledge from December 2015 till January 2017 to determine the link between VDR gene polymorphisms and BC risk. The following key words and their combinations were used during searching: VDR, SNP, single nucleotide polymorphisms, variants, BC, breast cancer, Cdx2, Fok1, Bsm1, Apa1, Taq1, Bgl1, Poly (A), and combinations of these key words. The bibliography was also searched in each article manually to identify more appropriate eligible studies.
Inclusion criteria
The selection of studies based on following criteria includes (1) all studies were retrospective case-control studies, (2) all data were original, (3) all studies having genotype frequencies for estimating ORs and 95% confidence interval (CI), and (4) all studies which are available in English language were included in this systematic review and meta-analysis.
Exclusion criteria
Following research papers were excluded: (1) all review articles, abstracts, and repeated studies; (2) if the frequencies of genotypes were not reported in the original papers; (3) if the genetic polymorphisms performed on the animal or human cell lines; (4) if the cross-sectional studies were done and conducted only on cases, and (5) if the studies selected male patients instead of female BC patients.
Data extraction
All potentially relevant studies were gone through and data were extracted and tabulated by one investigator independently and checked by another senior investigator. The extracted information for all studies includes first author, year of publication, ethnicity of subjects included in studies, origin of subjects, study design, sources of controls, sample size, and genotyping method. The frequencies of VDR Cdx2, Fok1, Bsm1, Apa1, Taq1, Bgl1, and Poly (A) genotypes in all participants were retrieved and ORs along with 95% CI were calculated. Because the reference genotype of each VDR gene polymorphisms was different in all studies, we took homozygous genotype (AA for Cdx2, FF for Fok1, BB for Bsm1, AA for Apa1, tt for Taq1, TT for Bgl1 and SS for Poly (A)) as the reference genotype in this meta-analysis. Allelic frequencies were also calculated from the corresponding genotyping distribution.
Quality assessment criteria
The quality of selected studies was assessed by Newcastle Ottawa Scale (NOS). 49 In addition, the meta-analysis was performed in accordance with PRISMA (Reporting Items for Systematic reviews and Meta-Analyses) guidelines (Table S1).
Statistical analysis
All statistical measures were done using STATA version 11.2 software. Summary odds ratio (SOR) along with 95% CIs were calculated to determine the strength of association between the VDR gene polymorphisms and BC. The SOR significance was assessed using
Results
Baseline properties of studies
In all, 150 potentially relevant research publications were searched through electronic databases. Out of these, 35 were related to vitamin D levels and BC, 25 were related to vitamin D pathway gene polymorphisms and BC, 5 were duplicated records, and 19 were not related to BC which led to the selection of 67 and exclusion of 83 publications. Out of 67 publications, 8 were reviews, 14 were not retrieved completely, 1 was abstract, 3 had insufficient genotype data, 3 had no data for controls, 3 were on BC cell lines, and 1 was on male BC patients. This left 34 research papers, which read completely (Figure 1) and satisfied the inclusion criteria and were included in this meta-analysis. This review involved a total of 26,372 cases and 32,883 controls with mixed ethnicity. The baseline characteristics of all reviewed articles along with the HWE of each SNP are described in Table 1;

Flowchart illustrating selection of research publication.
Baseline properties of the reviewed studies on BC and VDR gene polymorphisms.
CSS: case-control studies; NCCS: nested case-control study; PB: population based; HB: hospital based.
Test of heterogeneity
The heterogeneity of all VDR gene polymorphisms allelic models, co-dominant models, dominant models, and recessive models was made to analyze in all 34 studies (Figure S1 A–G). High-estimate heterogeneity was observed in Cdx2, Fok1, Bsm1 (except bb+Bb vs BB model), Apa1, and Poly (A) indicating between-study heterogeneity. When the co-dominant comparison was applied to genotype data Bsm1 bb versus BB (SOR = 1.18, 95% CI = 1.054–1.322,
Meta-analysis and publication bias between VDR gene polymorphisms and BC.
Number of studies included in the meta-analysis.

Summary estimates for the association between breast cancer and Bsm1 bb versus BB.
SA
SA was determined by sequentially eliminating each study one by one (data not shown). It was observed that SA did not very much affect the results in all cases. SA was done using both REM and FEM.
Publication bias
Egger’s tests (Table 2) were used to determine the publication bias. Significant results of Egger’s test were obtained due to varying sample size among studies which showed publication bias was present.
Discussion
In the present meta-analysis, we examined the seven polymorphisms of VDR (Cdx2, Fok1, Bsm1, Apa1, Taq1, Bgl1, and Poly (A)) gene for its association with BC risk. Our meta-analysis showed a literature survey on the seven widely studied VDR gene polymorphisms and BC risk, whereas previous meta-analyses were restricted to few polymorphisms. By extensive analysis of heterogeneity on BC, we highlighted summary estimates via REM and FEM, association between VDR gene polymorphisms and BC, SA, and publication bias. We obtained heterogeneity in the majority of models except for Taq1 and Bgl1. In the co-dominant model, Bsm1 bb, Apa1 aa, and Poly (A) LL genotypes were associated with increased BC risk. In allelic contrast models, the Poly (A) L allele has also shown association with the BC risk. The recessive contrast models of Fok1, Apa1, and Poly (A) were at risk, whereas, in dominant contrast models, none were associated with the risk.
This meta-analysis suggested a lack of association between VDR Cdx2 gene polymorphisms and BC risk in all models. These results were not consistent with previous reports which documented a positive association between Cdx2 polymorphism and BC risk. 29 While another report was found no risk among Japanese population. 33 Our results showed that Fok1-recessive models are at BC risk which differs in scope with a recent report which suggested that Fok1 polymorphism may be associated with postmenopausal BC but not with premenopausal BC.
The other functional polymorphisms are located near 3′ end of the gene including Bsm1, Apa1, Bgl1, Taq1, and Poly (A) polymorphisms. Our meta-analysis showed that Bsm1 bb, Apa1 aa, and Poly (A) LL genotypes are associated with BC, but lack of association is found between Taq1, Bgl1 and BC which could be explained in further haplotype studies. Whereas recent meta-analyses revealed no association between Fok1, Bsm1 Apa1, Taq1, and Poly (A) with BC.57–59 It was suggested that the wild-type Poly (A) L allele corresponds to the functional marker of Bsm1 b allele. The variation in LD between these functional and marker alleles may explain the inconsistencies in the strength of associations perceived across studies. 60
The sample size has an impact on the summary estimates and power of study; if all related published data were recruited completely, the power of meta-analysis increases. HWE is another important aspect of genotyping; in present meta-analysis, majority of SNPs were in HWE. However, the deviation from HWE showed a signal that the segregation of alleles is not independent. There may be various reasons including non-randomized matching of controls with cases, biased participants selection from population, errors in genotyping, and stratification of population.
The present meta-analysis has some limitations. First, the presence of heterogeneity among patients and various confounding factors might misrepresent the meta-analysis. Second, haplotyping of genotype might provide more informative results, so it would be more influential than single SNPs. Third, due to the lack of original data of studies, we cannot be able to consider other factors, such as plasma/serum vitamin D concentration, diet, life style, body mass index (BMI), sun exposure, use of NSAIDs/Quinone, BC stages, intake of calcium, and vitamin D that could affect the subjects and modify the overall risk of BC.
Conclusion
The present meta-analysis determines that the VDR Bsm1, Apa1, Fok1, and Poly (A) polymorphisms may be susceptible for BC development. However, Cdx2, Taq1, and Bgl1 polymorphisms fail to find an association with BC incidence. Vitamin D deficiency is common throughout developed and developing countries worldwide, so vitamin D supplementation is practiced to normalize various biological processes, but the results are not promising. In fact, a mechanism may be impaired in the pathway of vitamin D or there may be an involvement of genetic polymorphisms in the VDR which may be a cause of developing BC. Thus, the findings of this study will provide a better understanding of the BC treatment since VDR SNPs along with endogenous vitamin D should be considered during therapy.
Footnotes
Acknowledgements
Mehir un Nisa Iqbal and Taseer Ahmed Khan did conception and design of study; Mehir un Nisa Iqbal did extraction, analysis, and interpretation of data; Mehir un Nisa Iqbal did drafting of the article or critical revision for important intellectual content; and Taseer Ahmed Khan did final approval of the version to be published.
Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Ethical approval
The ethical approval was not required for this study.
Funding
The author(s) received no financial support for the research, authorship, and/or publication of this article.
References
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