Analysis of the brain transcriptome for substance-associated genes: An update on large-scale genome-wide association studies

Substance use disorder (SUD) arises from the initiation to subsequent regular, irregular and harmful use of substances such as alcohol, tobacco/nicotine and cannabis. While thousands of genetic variants have been identified from recent large-scale genome-wide association studies (GWAS), understanding their functions in substance involvement and investigating the mechanisms by which they act in the addiction circuits remains challenging. In this study, we re-analysed the brain regional transcriptome data from the most comprehensive postmortem transcriptomic study, with a focus on up- or down-regulation of substance-associated protein-coding genes in the addiction circuit-related brain regions (AddictRegions), including six cortical and 11 subcortical regions. We found that substance-associated genes were overrepresented in AddictRegions. Interestingly, we observed a greater degree of genetic overlap between initiation and use and between use and SUD than between initiation and SUD. Moreover, substance initiation, use and SUD-associated genes tend to shift their enriched AddictRegions from the cortical for initiation and, to a lesser extent, substance use to subcortical regions for SUD (e.g., thalamus, substantia nigra and ventral tegmental area). We also uncovered a pattern of coordinated cortical up-regulation and subcortical down-regulation for the genes associated with tobacco initiation and alcohol use. Moreover, the Gene Ontology terms of glutamate receptor activity and neurotransmitter binding were most significantly overrepresented in AddictRegion-upregulated, substance-associated genes, with the strongest enrichment for those involved in common substance use behaviours. Overall, our analysis provides a resource of AddictRegion-related transcriptomes for studying substance-associated genes and generates intriguing insights into the genetic control of substance initiation, use and SUD.