Following-up on findings suggesting an association between inflammation-related genes and biliary stones and cancer , in this analysis we expanded the gene coverage of four previously identified genes and examined another gene not previously studied in the Shanghai Biliary Tract Cancer Study. We observed suggestive associations for SNPs in VEGFA with biliary stones, RNASEL with ampulla of Vater cancer, and IL8 with gallbladder and ampulla of Vater cancers after correcting for multiple comparisons (FDR≤0.2).
Genetic susceptibility to biliary stones was linked to two SNPs in VEGFA. These two SNPs, rs9367173 and rs6905288, are located downstream of VEGFA and are neither in linkage disequilibrium (LD) with each other, nor in LD with the other VEGFA SNPs we examined. VEGFA is a signal protein that is fundamental in vascular permeability and angiogenesis . Thus, the role of VEGFA in gallstones susceptibility could be attributed to the process of blood vascularization during acute and/or chronic inflammation; however, the functional effects of these variants are unknown. To our knowledge, this study is the first to report an association between VEGFA and biliary stones. In our previous study, VEGFA was not associated with biliary stones, but rather VEGFA rs3025039 was associated with gallbladder cancer. This variant is not in LD with the two VEGFA SNPs we identified in this analysis.
We also found an increased risk of ampulla of Vater cancer with RNASEL rs672527, which is located in an intronic region of the gene. RNASEL is a key component of the innate immune system and participates in the process of apoptosis , but the functional effect of rs672527 is unknown. Other polymorphisms in the RNASEL gene have been associated with an increased risk of such cancers as prostate, head and neck, uterine cervix and breast [20, 21]; however, this is the first report to our knowledge of an association of rs672527 with cancer. We have previously shown that another RNASEL SNP (rs486907) was associated with biliary stones. These two SNPs are not in LD.
IL8 rs10805066, which was linked with increased risks of ampulla of Vater and gallbladder cancers, is located outside the promoter region of the gene. No functional effects of the variant have been reported. IL8 chemokine has been recognized as a potent mitogenic/angiogenic and inflammatory factor , which could support its participation in biliary tract cancer development. In our previous study, other variants in IL8 (rs4073, rs2227307 and rs27306, all in LD) were associated with bile duct stones . These three SNPs are not in LD with rs10805066.
The associations between RNASEL and IL8 variants with biliary tract cancers were independent of biliary stones. In our previous study, some, but not all genetic variants interacted significantly with biliary stone status to effect biliary cancer risk (Hsing, 2008). It has been suggested that most of the inflammatory processes of biliary tract cancer are linked to biliary stones; however, not all biliary cancer cases have prior stones, and other inflammatory conditions such as cholecystitis, history of gastric or duodenal ulcers have been reported to play a role . The cascade of inflammatory events in relation to the genes detected in this or previous studies, with or without the presence of biliary stones is unclear. In general, in both humans and mouse models, inflammatory processes, such as edema of the gallbladder, increased organ wall thickness, inflammatory infiltrates (the presence of inflammatory cells), and increases in transforming growth factor (TGF)-β production ) lead to chronic inflammation [25, 26], which may eventually lead to carcinogenesis.
Although we extended the analysis of common genetic variants to approximately 80% coverage for VEGFA (chr6: 43827369–43870265), 90% for RNASEL (chr1: 180805238–180826133) and 75% for IL8 (chr4: 74831698–74808648), none of the overall effects for each of the tested genes resulted in significant associations at the 0.05 level with either biliary stones or cancer. Our findings from this analysis, together with our findings from our previous study conducted in the same population do not strongly support an association between a particular gene and a particular biliary disease. It may be possible that each of the current and previously examined SNPs have independent effects on stones or biliary tract susceptibility, which is supported by the absence of LD (D’ or r2) between the SNPs. In addition, these variants may influence expression of different genes, which it is supported by the findings of recent follow-up of genome-wide association studies and the rare candidate genes [27, 28]; however, we did not find evidence of regulatory consequences for the associated variants.
Strengths of this study include the population-based design, the high case ascertainment and response rates, and the detailed review of pathology and clinical data to confirm the diagnosis of cancer cases. The use of ultrasound among controls also minimized misclassification of gallstones. Although, this study is the largest population-based study of biliary tract cancer to date, we have limited statistical power to detect a modest association, in particular for cancer of the ampulla of Vater.