Connecting the dots, genome-wide association studies in substance use
The recent genome-wide association (GWA) meta-analysis of lifetime cannabis use by the International Cannabis Consortium marks a milestone in the study of the genetics of cannabis use. Similar milestones for the genetics of substance use were the GWA meta-analyses of four smoking related traits, of coffee consumption and of alcohol consumption. Combined, 315 981 partly overlapping individuals were genotyped, phenotyped and their data analyzed in genetic association studies, reflecting a huge communal effort by the substance use/addiction genetics community. These genome-wide association study (GWAS) efforts considered different stages of substance use: lifetime use (ever versus never use) was analyzed for cannabis and smoking, quantity of use (in users) was analyzed for coffee, alcohol, and smoking and age of initiation and cessation were analyzed for smoking. There are other GWA efforts and publications in the realm of addiction, but here we limit ourselves to the largest meta-analyses per substance in order to maximize power. The GWA meta-analyses of substance-related traits identified many substance-specific genetic variants of moderate to small effect, which provided insight in the genetic etiology of substance use and its comorbidities.