A systematic review of substance use disorder methylome-wide association studies in postmortem human brain

Substance use disorders (SUDs) are complex psychiatric conditions influenced by genetic and environmental factors. DNA methylation (DNAm), a dynamic and context-dependent epigenetic mechanism, can offer insight into regulatory variation associated with SUDs, particularly when examined in brain regions central to addiction-related neurobiology. In this systematic review, we compiled findings from 20 methylome-wide association studies (MWASs) of SUDs in postmortem human brain tissue, encompassing alcohol (n study =9), opioids (n study =4), cocaine (n study =3), nicotine (n study =2), methamphetamine (n study =1), and any illicit drug (n study =1). Studies varied in brain regions analyzed, methylation platforms used, and sample characteristics. Most focused on methylation at cytosines followed by guanine (i.e., mCGs). Despite methodological heterogeneity and limited overlap of findings within individual substances, consistent biological themes emerged across substances, including differential methylation in genes related to neurodevelopment, inflammatory response, signaling, and transcriptional regulation. Cross-substance comparisons revealed 525 genes implicated across independent results from at least two substances, suggesting shared methylomic signals that may reflect common neurobiological pathways, downstream adaptations to exposure, or unmeasured polysubstance use. Exploratory drug repurposing analyses based on genes overlapping three or more substances identified a small number of existing compounds with potential relevance for treating polysubstance use. This review synthesizes current results to highlight both the promise and challenges of SUD MWASs in human brain, including limited sample size, generalizability, cell-type specificity, and lack of functional validation. Future research should prioritize multi-omic integration, larger and more diverse cohorts, and advanced technologies to enhance mechanistic understanding and translational potential.