THE DNA METHYLTRANSFERASE DNMT3A CONTRIBUTES TO OPIOID TOLERANCE THROUGH SEX-SPECIFIC EPIGENOMIC PLASTICITY

Epigenetic modifications such as DNA methylation regulate gene expression and neuronal plasticity. Previous work has indicated that chronic exposure to psychoactive substances, such as opioids, may trigger DNA methylation changes, but the enzymatic pathways and cell types involved remain poorly defined.
Using in situ hybridization, we found that Dnmt3a, the most abundant enzyme responsible for DNA methylation in the brain, is predominantly expressed in the mouse nucleus accumbens (NAc) by neuronal cells. We then used a tamoxifen-inducible strategy to knockout Dnmt3a in NAc neurons, specifically in adulthood, without interfering with postnatal accumulation of DNA methylation (cKO). Behavioral experiments showed that, in both male and female mice, Dnmt3a cKO did not affect morphine-induced hyperlocomotion, but attenuated tolerance to this response upon drug re-exposure, revealing an implication of Dnmt3a in this phenomenon.
Next, we analyzed gene expression (RNA-sequencing) and DNA methylation (Enzymatic Methylation sequencing) changes specifically recruited by morphine in NAc neurons (isolated by flow cytometry). At transcriptomic level, we observed changes that notably affected transcription factors involved in circadian rhythms (eg Bmal1). For DNA methylation, preliminary results indicate that NAc neurons show widespread morphine-induced differentially methylated regions throughout the genome. At both transcriptomic and DNA methylation levels, these adaptations were largely different across male and female mice, suggesting that opioid tolerance relies on a common enzyme, Dnmt3a, through sex-specific molecular pathways.
We are now investigating how these neuron-specific adaptations contribute to tolerance. Findings will provide new insight into the epigenomic impact of opioids and the pathophysiology of opioid use disorder.