Brain region and cell type-specific DNA methylation profiles in association with ADHD

Previous studies identified DNA methylation (DNAm) associations with ADHD in peripheral tissue and the brain. Given that DNAm is highly cell type-specific, it is crucial to understand which cell types are driving the DNAm differences observed in ADHD. Here, we report the first brain cell type-specific epigenome-wide association study (EWAS) for ADHD (25 individuals with ADHD, 33 individuals without ADHD) in postmortem anterior cingulate cortex (ACC) and caudate nucleus (CN) based on epigenomic deconvolution. We identified distinct cell type-specific DNAm patterns in both brain regions. On the single site level, we identified significant associations with ADHD in microglia. We identified that in the ACC most differentially methylated regions (DMRs) differences were driven by glutamatergic neurons, whereas differences in the CN were mainly driven by GABAergic neurons. Enrichment of DMRs implicated genes involved in brain development, both in bulk and on the cell type-specific level. Genome-wide DNAm differences in microglia and GABAergic neurons were enriched in genetic risk variants for ADHD. Lastly, the results of EWAS were dependent on cell type reference panels used in statistical analyses. Altogether, these results could provide new insights into the molecular mechanisms underlying ADHD and considerations for EWAS in brain tissue.