Urothelial carcinoma (UC) is a common genitourinary disease, which represents the fourth most common malignancy worldwide. It originates exclusively from the urothelium and may occur in the lower urinary tract (bladder and urethra) or the upper urinary tract (renal pelvis and ureter) (1,2). Bladder cancer (BC) is the most common type of UC, whereas upper urinary tract urothelial carcinoma (UTUC) is relatively uncommon and accounts for only 5–10% of all UCs (3). To date, the pathogenesis of UC has not been fully elucidated. Known risk factors include cigarette smoking, exposure to workplace-related chemicals and intake of drugs, such as phenacetine, chlornaphrazine and cyclophosphamide (4,5).

Sulfotransferases (SULT), a family of multifunctional enzymes, transfer the sulfo group from the cofactor 5′-phosphoadenosine-3′-phosphosulfate to the nucleophilic sites of their substrates (6). It was previously demonstrated that SULTs are key contributors to the conjugation and removal of several phenolic xenobiotics and a variety of endogenous compounds, including neurotransmitters and steroid hormones (7). At least 11 different SULT enzymes were recently identified in humans. Among these, sulfotransferase isoform 1A1 (SULT1A1) appears to be a key phenol SULT, as it has been detected in a wide range of human tissues, including liver, lung and kidney (8).

The SULT1A1 gene, located on chromosome 16p12.1-p11.2, contains several genetic polymorphisms that possibly affect individual susceptibility to cancer (9). A functional polymorphism in exon 7 of the SULT1A1 gene, with a G→A substitution, results in a change in the amino acid sequence from arginine to histidine, leading to a decrease in enzymatic activity and variable rates of activation or detoxification of procarcinogens (10,11). Carcinogenic aromatic amines, such as 4-aminobiphenyl, which is contained in tobacco smoke, are one of the causal factors of UC. Furthermore, SULT1A1 has been shown to efficiently catalyze the metabolic activation of an N-hydroxyderivative of 4-aminobiphenyl (12). Therefore, it is hypothesized that the Arg213His polymorphism in the SULT1A1 gene may be closely associated with high susceptibility to UC.

To the best of our knowledge, several molecular epidemiological studies have investigated the association between the Arg213His polymorphism and UC risk in diverse populations. However, the results of those studies were inconsistent or even contradictory and, to date, no study has conducted a quantitative assessment to confirm this association. Therefore, we performed a meta-analysis of all eligible studies to derive a more precise estimation of the association between the Arg213His polymorphism and UC risk.

SULT1A1 is a phase II detoxification enzyme, which has the ability to sulphate phenolic and steroid compounds (24). SULT1A1 activity varies several fold among individuals and exhibits a strong familial segregation, suggesting that genetic factors play a significant role in determining the enzymatic activity of SULT1A1 (25). A common single-nucleotide polymorphism in the coding region of SULT1A1 (Arg213His) has been identified and was reported to be associated with decreased platelet enzymatic activity and thermostability. This mutation may affect an individual's capacity to efficiently sulfate endogenous compounds, drugs and xenobiotics and, consequently, increase an individual's susceptibility to cancer (26). To date, a number of case-control studies have been conducted to investigate the association of this polymorphism with UC in humans. Unfortunately, the link remains unclear and ambiguous. To address this issue, we conducted the present meta-analysis based on 10 studies, including a total of 2,495 cases and 2,905 controls. A meta-analysis is a powerful tool, as it may provide more reliable results compared to a single study and reduce random error (27). Finally, our meta-analysis indicated that the Arg213His polymorphism in the SULT1A1 gene may contribute to a significant decrease in the overall risk of BC. However, compelling evidence on a consistent association in UC is currently lacking.

UC is a highly aggressive malignancy that is associated with significant morbidity and mortality. UC may be mainly classified as UTUC and BC, according to the anatomical location. We observed that individuals with the A/A genotype were possibly associated with a decreased risk of UC when all the eligible studies were pooled into the meta-analysis. As anatomical differences may affect the results from meta-analyses, we also limited our analyses to BC and UTUC. A total of 4 studies in our meta-analysis enrolled cases including BC as well as UTUC patients. We were unable to consult the authors for the detailed original data of their studies; however, through careful reading, we found that the majority of the cases in 2 Japanese studies (16,17) were BC patients. Thus, we decided to combine these 2 studies with 5 studies (19–23) that only included BC patients, to probe the association between the Arg213His polymorphism and BC. Consequently, our findings demonstrated that the Arg213His polymorphism was significantly associated with a decreased risk of BC. To eliminate the potential effect of UTUC cases and determine the association more precisely, we excluded the 2 Japanese studies in the subgroup analysis by cancer type. As expected, the results were in accordance with the abovementioned observations. Of note, a contradictory and interesting association was observed in the UTUC subgroup. Our results suggested that individuals with the A/A genotype had a higher risk of UTUC compared to subjects carrying the wild genotype, raising the question of why our results on UTUC did not confirm the protective effect of the Arg213His polymorphism with respect to BC risk. This discrepancy may be attributed to the differences between BC and UTUC. Although UTUC and BC both originate from the urothelium and may share certain risk factors or molecular disruption pathways, each has its own distinct characteristics. In comparison to UC of the bladder, primary UTUCs are less common, representing only 5% of all UCs and <10% of renal tumors (28). The annual incidence of UTUC is low compared to that of BC. Catto et al (29,30) provided evidence that the extent of the mutator and methylator phenotypes in UC differed with tumor location and suggested that carcinogens may affect the urinary tract in different ways. Furthermore, several studies demonstrated that patients with UTUC have a higher risk, compared to those with BC, for hereditary non-polyposis colorectal cancer and microsatellite instability is more common in UTUC compared to BC (31,32). Green et al (33) described UTUC and BC as disparate twins in their recent review and revealed that there were practical, anatomical, biological and molecular differences between these two types of cancer. In addition, the study of Liang et al (34) strongly suggested that the urothelia of the bladder and upper urinary tract represent two separate cell lineages and are most likely maintained by distinct stem cell populations. Therefore, we hypothesized that genetic polymorphisms of enzymes metabolizing carcinogens may yield products of different activity and lead to opposite cancer risks. In other words, histologically identical tumors may arise through different molecular mechanisms. In two Japanese studies, although the cases included BC as well as UTUC patients, the sum of the UTUC patients was inadequate to have statistical power affecting the overall results. It is noteworthy that certain other factors may also result in this discrepancy. For UTUC, there was only one study included in the analysis with limited sample sizes and the subjects were all French. In addition, we found that the DNA of the cases in this UTUC study was extracted from tissues, whereas the DNA in the majority of the other studies was extracted from the blood. It is widely accepted that DNA from white blood cells rather than tissue should be used for determining genetic polymorphisms, as loss of heterozygosity has been noted in exfoliated cells (35). Taken together, a relatively small sample size, differences in country and race and diverse methods were likely to affect our results. Therefore, a larger sample size and a higher number of articles are required to verify the association between the Arg213His polymorphism and UTUC.

Different ethnicities may have disparate genetic backgrounds, which affect the association between polymorphism and cancer susceptibility. We performed a stratified analysis by ethnicity; however, we were unable to identify any positive association among Asians and Caucasians, whether 10 or 7 articles were included in the analyses. The number of subjects was relatively limited, without sufficient statistical power to investigate the true association. Furthermore, in view of the diversity of possible comparisons and the unavoidable flexibility in selecting and defining the correlations, the associations may not be necessarily reliable. For example, selection bias, matching criteria, misclassifications on disease status and genotyping may play a role. In addition, we mentioned that only 5 studies investigated Asian populations and only 4 studies included Caucasian populations. Therefore, larger-scale studies and combined analysis are required to further investigate ethnic differences in the effect of the Arg213His polymorphism on the risk of UC. In the subgroup analysis by source of controls, a significantly decreased UC risk was observed among studies using population-based controls. Some bias may exist in hospital-based studies, as such controls may represent a sample of an indistinct reference population instead of the general population, particularly when the genotypes investigated were associated with the condition of the hospital-based controls. Thus, a proper and representative cancer-free control sample is crucial for reducing bias in such genotype association studies.

Thus far, the single most important risk factor in the development of UC is exposure to cigarette smoke. It is estimated that ~80% of all UC may be attributed to cigarette smoking (34). It was previously reported that smoking increases the risk of developing BC and UTUC by as high as 4- and 6-fold, respectively (37). Arylamines, which are found in tobacco smoke, proceed via a two-step pathway involving cytochrome P450 1A2-catalyzed N-hydroxylation followed by an O-esterification step catalyzed by N-acetyltransferases (NATs) and/or SULTs. As a consequence of these reactions, in UCs, the aryl nitrenium ions generated from N-hydroxylamines are considered to be the ultimate reactive intermediates responsible for carcinogenic activity. In this study, we performed a stratified analysis by smoking status based on 6 studies that presented detailed data of smoking status and drew the conclusion that the Arg213His polymorphism was associated with a decreased risk of UC in non-smokers, but not in smokers. A possible explanation is that the association between this polymorphism and the decreased risk of UC may be masked by the overwhelming accumulated exposure to tobacco carcinogens, so that the association is more evident in non-smokers. In addition, passive smoking should be taken into consideration. Non-smokers may be exposed to second-hand smoking. When tobacco smoke is inhaled, some of the carcinogens are absorbed through the lungs into the blood. However, these results require confirmation by further large-scale studies.

The significance of heterogeneity, which may affect the results of this meta-analysis, must be addressed. Obvious heterogeneity between studies was observed when we included all eligible studies. Consistently, through meta-regression analysis and creation of a Galbraith plot, we observed that the study of Roupret et al (18), which was the only study that included UTUC patients as case group, was the main source of heterogeneity. The degree of heterogeneity significantly decreased after this study was excluded. The most likely interpretation may be the differences between BC and UTUC, as mentioned above. In addition, the differences in the pathological stages of the patients and the genotyping methods, such as Taqman and PCR-RFLP, may also lead to heterogeneity.

Furthermore, despite the overall robust statistical evidence generated through this analysis, certain limitations were identified. First, our results were based on unadjusted estimates, while a more precise analysis should be conducted if all individual raw data were available, which would allow for adjustment by other covariates, including age, gender, alcohol consumption, cigarette consumption and other lifestyle habits. Second, only studies that were indexed by the selected databases were included for the meta-analysis and some relevant published studies or unpublished studies with null results were missed, which may have biased our results. Third, UC is a multifactorial disease that results from complex interactions between several genetic and environmental factors, suggesting that there is not a single genetic or environmental factor significantly affecting the susceptibility to UC. Thus, the combined effects of different gene polymorphisms should be further analyzed. In our meta-analysis, as SULT1A1 and NAT2 are both involved in the metabolism of arylamines and NAT2 has been reported to play a role in carcinogenesis, we aimed to discuss the combined effects of SULT1A1 and NAT2 genetic polymorphisms. However, we were unable to do so, as only 3 studies provided detailed data of these two genes. Fourth, to the best of our knowledge, this is the first meta-analysis regarding the comprehensive assessment of the association between the Arg213His polymorphism and the risk of UC. Thus, the number of published studies was not sufficient for a comprehensive analysis, particularly for UTUC. Therefore, more studies with larger sample sizes and detailed information are required. In addition, the patients exhibited different histological stages and grades or other coexisting conditions, which may have affected our results. Furthermore, other pathological types of urinary BC should be considered, which may be an interference factor.

In conclusion, the results from the present meta-analysis indicated an association between the Arg213His polymorphism and a decreased risk of BC. However, there was insufficient evidence to support a consistent association between this polymorphism and UC, partly due to the differences between BC and UTUC. To advance the understanding of this association, the following recommendations have been made: First, decrease false-positive and -negative results through stratifying large samples by age, gender, dietary habits, lifestyle and ethnicity. Second, histopathological and clinical data may be used to subclassify the type and stage of UC to obtain a more homogeneous population for the analysis. Thirdly, given the fact that UC is a type of polygenic disorder, the combined effects of different gene polymorphisms require further analysis. Finally, more case-control studies or updated meta-analyses should be conducted to elucidate the possible roles of the Arg213His polymorphism in the etiology of UC.