Evaluation of the Causal Relationship Between Smoking and Schizophrenia in Asia

Cigarette smoking has been suggested to be associated with the risk of schizophrenia (SCZ) in observational studies. A significant causal effect of smoking on SCZ has been reported in the European population using the Mendelian randomization (MR) approach; however, no evidence of causality was found in participants from East Asia (EAS). Using the Taiwan Biobank (TWBB, sample size up to 79,989), we conducted genome-wide association studies (GWAS) to identify susceptibility loci for smoking behavior, which included the initiation of smoking and the onset age. To maximize the power of genetic discovery in the EAS population, we meta-analyzed GWAS from the TWBB and Biobank Japan (BBJ, sample size up to 165,436) for smoking traits. The GWAS for SCZ was taken from the Asia Psychiatric Genomics Consortium, which included 22,778 cases and 35,362 controls. We performed a two-sample MR to estimate the causality of smoking behavior on SCZ in the EAS population. In TWBB, we identified one novel locus that met genome-wide significance for onset age. In a meta-analysis of TWBB and BBJ, we identified two novel loci for smoking initiation. In MR, a marginal significance was found for the causality of smoking initiation on SCZ (odds ratio (OR) = 4.00, 95% confidence interval (CI) = 0.89-18.01, P = 0.071). Later onset age for smoking was causally associated with a lower risk of SCZ (OR for a per-year increase in onset = 0.96, 95% CI = 0.91-1.01) with a marginal significance (P = 0.098).