Cancer is a global health problem that results in significant morbidity and mortality (1). Recent estimates indicate that approximately 4,292,000 new cancer cases and 2,814,000 cancer deaths are reported in China each year (2). To date, the exact mechanisms of carcinogenesis are poorly understood. An increasing number of studies have reported that cancer is a complex disease influenced by various environmental and genetic factors and their interactions (3,4). In addition, several genes have been associated with cancer susceptibility (5)
Selenoproteins are a class of proteins characterized by incorporation of selenium (Se) in the form of the amino acid (6). Studies investigating the association of Se with cancer susceptibility have described the role of Secis carcinogenesis. The 15 kDa selenoprotein (SEP15) and other members of the thioredoxin family (7) are a type of catalyzing agent that can regulate the cellular redox reaction and reduce cumulative oxidative stress, which has correlates with cell death and oncogenesis (8). A recent study has shown that SEP15 is unregulated in the prostate gland (9) and binds to uridine diphosphate (UDP)-glucose:glycoprotein glucosyltransferase (10-12), a regulatory protein of N-linked glycoprotein folding in the endoplasmic reticulum, which suggests that SEP15 plays an essential function in this particular pathway.
Recent studies have described the association between SEP15 polymorphisms and the risk of various cancers, including colorectal cancer (CRC) (13,14), lung cancer (LC) (15), breast cancer (BC) (16-18), and prostate cancer (PCa) (19-21). However, the results of these studies are conﬂicting and inconclusive, possibly due to clinical heterogeneity, different ethnic populations, and small sample sizes. To circumvent these limitations, we conducted a meta-analysis of the results of relevant studies to evaluate the association between SEP15 polymorphisms and cancer susceptibility.
Publication search eligibility of relevant studies
All case-control studies included in this study were queried from PubMed, EMBASE, Cochrane Library, and Web of Science using the following keywords: “SEP15 OR 15kDa selenoprotein” AND “variant OR mutation OR SNP OR polymorphism” AND “cancer OR tumor OR carcinoma OR malignancy OR neoplasms”. Only relevant studies in humans were included, and the language was restricted to English. In addition, references of eligible publications were searched manually. When certain data were not mentioned in the report, the corresponding author of the publication was contacted by e-mail. The most recent or complete articles with the largest number of subjects were selected from overlapping data of reports by the same authors. The last search was performed on December 17, 2016.
Eligible studies included in the meta-analysis met the following inclusion criteria: assessed the association between SEP15 polymorphisms (rs5859 and rs5845) and susceptibility to cancer; were case-control studies designed for human subjects; and provided useful data on genotype frequencies. Meanwhile, the exclusion criteria were as follows: duplicate data; clinical cases, comments, series, and reviews; and insufficient data. Studies published in languages other than English were also excluded. Articles with two or more case-control cohorts were regarded as two or more different studies.
Two investigators independently reviewed the reports that fulfilled the selection criteria and extracted the following data: name of first author; year of publication; country of origin; ethnicity; and source of controls (population-based or hospital-based controls). The Newcastle-Ottawa scale (NOS) was applied to assess the quality of the studies included in our analysis. Different ethnic groups, including Caucasians, Asians, and Africans, were analyzed to assess the effects of SEP15 polymorphisms on cancer susceptibility.
The strength of the association between SEP15 polymorphisms and tumor susceptibility was assessed by the odds ratios (ORs) with 95% confidence intervals (CIs). For SEP15 polymorphisms, the susceptibility of dominant (MM + MW vs. WW), recessive (MM vs. MW + WW), co-dominant (MW vs. WW; MM vs. WW), and allele models (M vs. W) was evaluated, respectively (M: mutant allele; W: wild-type allele). Subgroup analyses were also conducted by ethnicity, source of control, and cancer type. The Hardy Weinberg equilibrium (HWE) was used to assess the genotype frequencies of SEP15 polymorphisms among the controls using the χ2 test. Meta-analysis was performed by the Mantel-Haenszel method in accordance with the Cochrane organization guidelines. The heterogeneity between datasets was evaluated by the heterogeneity index (I2) and the Cochran’s Q statistic (22). P-het <0.10 was considered as significant heterogeneity. The Fixed-effects model was applied when I2 was <50%, while random-effects model was used when I2 was >50% (23). Funnel plots were applied to test for publication bias (24). The forest plot was generated using the Review Manager software (RevMan, version 5.3; Copenhagen: The Nordic Cochrane Centre, The Cochrane Collaboration, 2014). All analyses were performed with Stata software ver. 12.0.
Power analysis for association of SEP15 polymorphisms and tumor susceptibility was performed using the Genetic Power Calculator (13).
Characteristics of studies
A total of 97 publications were identified after our initial search. After screening the titles and abstracts, 79 publications were excluded from the study, and 18 publications were selected for further full-text review. Nine studies were excluded because these were not case-control studies, did not describe SEP15 polymorphisms (rs5859 and rs5845) and cancer susceptibility, or did not provide detailed genotype data. We finally identified nine eligible publications, including 10 case-control studies (a total of 6,970 cases and 8,432 controls) that were subjected to our meta-analysis (Table 1) (14-21,25). The study selection processes are presented in Figure 1.
The publications included in this study were published from 2008 to 2016. For the SEP15 rs5859 polymorphism, seven studies comprising a total of 5,802 cases and 7,035 controls met the inclusion criteria. Six of these involved Caucasians and one included Asians. Six studies were population-based and one study was hospital-based. Additionally, two of the studies were performed on subjects with PCa, BC, and CRC, respectively, and one study was performed subjects with LC. There were three studies on the rs5859 polymorphism that did not conform to HWE (P<0.05) (14,17,21). For the SEP15 rs5845 polymorphism, three studies comprising a total of 1,168 cases and 1,397 controls were analyzed, two of which involved Caucasians, and one that included Asians; the controls in all three studies were hospital-based. In terms of cancer type, three studies were performed on PCa, BC, and CRC. In addition, we applied the NOS to evaluate the quality of these enrolled studies, which are presented in Table S1.
Results of the meta-analysis
Overall meta-analysis of the studies on two SEP15 polymorphisms (rs5859 and rs5845) did not detect any significant association with cancer susceptibility (P>0.05; Table 2). No significant associations were observed in the stratification analyses by ethnicity, cancer type, HWE status, or source of control subjects (P>0.05; Table 2).
Test of heterogeneity, sensitivity analyses, and publication bias
Overall comparison and subgroup analyses did not detect any significant heterogeneity among the studies included in the meta-analysis. We repeated our meta-analysis and omitted every study one by one to assess the effect of each eligible study on the results of our investigation. The pooled ORs for the effects of the rs5859 and rs5845 polymorphisms on cancer susceptibility indicated that our data were stable and reliable (Table S2). Begg’s funnel plot analysis indicated that our meta-analysis had significant symmetry and no publication bias (Figure 2), which was further validated by the Egger’s test (P>|t|=0.20 for the rs5859 polymorphism; P>|t|=0.12 for the rs5845 polymorphism).
Se is a dietary micro-nutrient that is essential for human health (26,27), and cancer mortality has been inversely correlated with Se intake (27,28). The biological functions of Se are strongly associated with the aminoacid Sec, which is present in ~25 selenoproteins (29,30). Sec is incorporated into selenoproteins at the stem-loop structure of the 3’ untranslated region (3’ UTR), which requires an in-frame stop UGA codon and is recognized as a Sec insertion sequence (SECIS) by various trans-acting factors (29). These selenoproteins include the family of glutathione peroxidases (31): (I) selenoprotein P (SePP) that function as a transporter (32); (II) selenoprotein S, an endoplasmic reticulum protein involved in removing unfolded proteins; and (III) SEP15, another endoplasmic reticulum protein that also involved in the unfolded protein response (33,34). Not all identified selenoproteins have been characterized. However, several selenoproteins are known to possess redox functions and behave as antioxidants that reduce oxidative stress (29,31) Thus, it is possible that genetic variations in the form of polymorphisms in selenoproteins are associated with the risk for different types of cancer and/or oxidative stress. For example, regulatory elements within the 3’ UTR are essential for Se incorporation into selenoproteins and therefore, single nucleotide polymorphisms (SNPs) in gene regions corresponding to the 3’ UTR of selenoprotein mRNA shave the potential to influence selenoprotein expression. Indeed, minor allelic variants of rs5845 and rs5859 in SEP1 have functional consequences (35). Furthermore, polymorphisms have been associated with an increase in BC risk (35) and LC in smokers (15). Penney et al. (20) screened 1,286 cases and 1,267 controls and found that SEP15 polymorphisms were not significantly associated with PCa. Jablonska et al. (17) reported that patients with the SEP15 1125 AA had higher Se intake, whereas those harboring the GG or GA genotype, and a higher Se status were more susceptible to LC. Because the results of these studies are conﬂicting and inconclusive, we conducted the present meta-analysis. Overall, nine publications comprising 10 case-controls were enrolled, and the overall meta-analyses showed no significant association between SEP15 polymorphisms and cancer susceptibility (Table 2). When subgroup analyses were performed based on the source of the control subjects or cancer type, null results were also found (Table 2). Thus, our findings may serve as a foundation for the development of future investigations.
This study had a number of advantages. First, we have conducted a comprehensive literature search to identify eligible studies, thereby rendering our analysis as more persuasive and substantial. Second, the quality of the enrolled studies was assessed by NOS, and low-quality studies were generally excluded to raise the overall quality. Third, subgroup analysis was conducted according to cancer type, HWE status, and other specific study features for the purpose of further deepening our research for sources of data heterogeneity. Fourth, our results were adjusted according to the recognized formula, ensuring the accuracy of our results. In addition, the stability of these studies was further verified by sensitivity analysis, and publication bias was assessed by the Egger’s test and Begg’s funnel plot. This study also had several limitations that should be described. First, single case-control studies can only support a small test power and often provide false-positive, false-negative, or inconsistent conclusions, and the number of cases in the eligible studies were relatively small. Furthermore, some detailed information such as gender and histological type could not be obtained from these reports, and thus a more in-depth subgroup analysis could not be performed. Second, most of the eligible studies involved Caucasian patients, only a few studies included Asians, and none included Africans. Additional studies involving various populations are needed to obtain more convincing results. Third, most of the published studies were hospital-based and genotype distributions among the controls in some studies deviated from HWE.
In conclusion, our research shows that the SEP15 polymorphisms are not significantly associated with cancer susceptibility. Considering the limited studies in both overall and subgroup meta-analyses, larger sample sizes and higher quality studies are needed to validate these findings.
We thank Dr. Aihua Lin from the Public Health School of Sun Yat-sen University for technical guidance on statistical analysis.
Conflicts of Interest: The authors have no conflicts of interest to declare.
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