Common genetic variation and risk of gallbladder cancer in India: a case-control genome-wide association study

Background Gallbladder cancer is highly lethal, with notable differences in incidence by geography and ethnic background. The aim of this study was to identify common genetic susceptibility alleles for gallbladder cancer. Methods In this case-control genome-wide association study (GWAS), we did a genome-wide scan of gallbladder cancer cases and hospital visitor controls, both of Indian descent, followed by imputation across the genome. Cases were patients aged 20-80 years with microscopically confirmed primary gallbladder cancer diagnosed or treated at Tata Memorial Hospital, Mumbai, India, and enrolled in the study between Sept 12, 2010, and June 8, 2015. We only included patients who had been diagnosed less than 1 year before the date of enrolment and excluded patients with any other malignancies. We recruited visitor controls aged 20-80 years with no history of cancer visiting all departments or units of Tata Memorial Hospital during the same time period and frequency matched them to cases on the basis of age, sex, and current region of residence. We estimated association using logistic regression, adjusting for age, sex, and five eigenvectors. We recruited samples for a replication cohort from patients visiting Tata Memorial Hospital between Aug 4, 2015, and May 17, 2016, and patients visiting the Sanjay Gandhi Postgraduate Institute of Medical Sciences, Lucknow, India, between July, 2010, and May, 2015. We used the same inclusion and exclusion criteria for the replication set. We examined three of the most significant single-nucleotide polymorphisms (SNPs) in the replication cohort and did a meta-analysis of the GWAS discovery and replication sets to get combined estimates of association. Findings The discovery cohort comprised 1042 gallbladder cancer cases and 1709 controls and the replication cohort contained 428 gallbladder cancer cases and 420 controls. We observed genome-wide significant associations for several markers in the chromosomal region 7q21.12 harbouring both the ABCB1 and ABCB4 genes, with the most notable SNPs after replication and meta-analysis being rs1558375 (GWAS p= 3.8 x 10(-9); replication p= 0.01; combined p= 2.3 x 10(-10)); rs17209837 (GWAS p= 2.0 x 10(-8); replication p= 0.02; combined p= 2.3 x 10(-9)), and rs4148808 (GWAS p= 2.4 x 10(-8); replication p= 0.008; combined p= 2.7 x 10(-9)). Combined estimates of per-allele trend odds ratios were 1.47 (95% CI 1.30-1.66; p= 2.31 x 10(-9)) for rs1558375, 1.61 (1.38-1.89; p= 2.26 x 10(-9)) for rs17209837, and 1.57 (1.35-1.82; p= 2.71 x 10(-9)) for rs4148808. GWAS heritability analysis suggested that common variants are associated with substantial variation in risk of gallbladder cancer (sibling relative risk 3.15 [95% CI 1.80-5.49]). Interpretation To our knowledge, this study is the first report of common genetic variation conferring gallbladder cancer risk at genome-wide significance. This finding, along with in-silico and biological evidence indicating the potential functional significance of ABCB1 and ABCB4, underlines the likely importance of these hepatobiliary phospholipid transporter genes in the pathology of gallbladder cancer.