Cannabis use and Cancer: Dissecting genetic causality for site-specific risks through two-sample Mendelian Randomization

Background: The prevalence of cannabis use is increasing at an alarming rate owing to its legalization and decriminalization in some countries. Epidemiological evidence on the association between cannabis use and cancer is inconsistent and conflicting. Herein, we performed two-sample Mendelian randomization (MR) to investigate whether cannabis use is causally associated with site-specific cancers in individuals of European ancestry. Methods: We identified 22 independent genetic variants strongly associated with cannabis use (p-value < 5 x 10-8) in a large meta-analysis of genome-wide association studies of individuals of European ancestry. Genome-wide association summary-level data on site-specific cancers were obtained from individuals of European ancestry in FinnGen, Finland. MR analyses were performed using the inverse-variance weighted (IVW) and multivariable method. Sensitivity analyses were performed using the simple median, weighted median, MR-Egger, and MR pleiotropy residual sum and outlier methods. Results: Our multivariable IVW analyses adjusted for cigarette smoking found that genetic liability to cannabis use was causally associated with esophageal cancer (odds ratio [OR] =1.74, 95% confidence interval [CI] =1.29-2.15, p-value =0.013) and lung cancer (OR=1.35, 95% CI = 1.13-1.58, p-value =0.009). However, genetic liability to cannabis use exerted a protective effect against pancreatic cancer (OR=0.77, 95% CI =0.57-0.91, pvalue=0.032) in individuals of European ancestry in the FinnGen. Our sensitivity analyses found no evidence of horizontal pleiotropy between cannabis use and site-specific cancers. Conclusion: We found that genetic liability to cannabis use was associated with esophageal, lung, and pancreatic cancers in individuals of European ancestry.