Drinking and DNA: How Epigenetic Modifications Reinforce Alcohol Dependency 

Alcohol dependency is not only restricted to behaviour. Epigenetic modifications such as histone acetylation implicate specific metabolic factors in neural functions that drive addictive behaviour in alcohol dependency (Berkel and Pandey 2017). Histones are proteins around which DNA is wrapped to form chromatins. At the tails of histones, they possess lysine residues which can be acetylated. Histone acetyltransferases are a class of enzymes that facilitate the addition of acetyl groups onto histones to enable gene expression, examples include CBP/P300 and GCN5 (Steunou, Rossetto, and Côté 2013). Another enzyme that plays an important role in alcohol metabolism is Acetyl-CoA Synthetase 2 (ACSS2). Contributing to a major step of epigenetic modification, ACSS2 influences histone acetylation at genes associated with reward, processing, memory, and learning as it is often found in the nucleus of hippocampal cells. Knockdown and loss of this enzyme can lead to decline in histone acetylation and repression of certain genes (Creighton et al. 2020). 

Mews et al. used mice to investigate histone acetylation rates in the brain after ethanol ingestion. The main rationale for this paper was to research how alcohol metabolism influences gene expression in the brain through histone acetylation. They hypothesized that acetate (ethanol metabolite), which is converted to acetyl-CoA by ACSS2, is a major part of histone acetylation in the brain. They employed different scientific methods for this study, which include in-vivo stable-isotope labelling of deuterated ethanol using mice, mass spectrometry, enzyme knockdown, and ChIP (Chromatin Immunoprecipitation) and RNA sequencing (Mews et al. 2019).  

They compared histone acetylation at specific markers (H3K9K14, H3K18K23, H4) in the dorsal hippocampal cells (dHPC) following ethanol ingestion between wild-type mice (normal levels of ACSS2) and knockdown mice (reduced levels of ACSS2) (Figure 1). Prior to ingestion, the levels of acetylation for both mouse types were similar but after ingestion, the rates of acetylation remained the same for knockdown mice, and increased for the wild-type mice, showing the importance of ACSS2 in the incorporation of acetyl on histones to enhance gene expression. The same experiment was performed but in the ventral hippocampal cells (vHPC) (where ACSS2 levels are normal), they found no difference in the rate of histone acetylation in both mouse types, showing that knockdown was only specific to the dorsal hippocampal cells (Mews et al. 2019). ChIP-sequencing was done at the Fstl1 locus, where H3K9ac and H3K27ac mark active regulatory regions primarily by the promoter and nearby enhancers, indicating ethanol-induced chromatin activation. They only observed peaks at wild-type mice treated with ethanol, which shows that ACSS2 facilitates ethanol driven histone acetylation at certain gene loci (Mews et al. 2019). 

Figure 1. The statistical results of the research done by Mews et al. using mass spectrometry. Panel A and B depict radar charts showing acetyl incorporation rates in histone markers (H3K9K14, H3K18K23, H4) in the dHPC and vHPC respectively, for wild-type and knockdown mice. Panel C shows the results of ChIP-sequencing tracks done at the Fstl1 locus, where peaks can be observed at wild-type mice treated with ethanol. Panels D, E, F, and G are box plots essentially showing the effects of ethanol on histone acetylation in wild-type mice, and how histone acetylation is reduced in knockdown mice compared to wild type mice (Mews et al. 2019).  

In the context of the broader epigenetic implications of alcohol consumption, alcohol intake increases histone acetylation in certain regions of the brain, which in turn enhances expression of genes linked to reward and memory, increasing the chances of alcohol dependency. Due to the hippocampal region of the brain that is associated with reward and learning, alcohol intake could reinforce drinking behaviour (Ghezzi et al. 2013). ACSS2 serves as a potential pathway to target in clinical research to reduce the risk of dependency (Egervari et al. 2024). Overall, the epigenetic effects of alcohol intake allow for positive reinforcement, contributing to compulsive drinking and the potential for alcohol dependency. 

References

Anne-Lise Steunou, Dorine Rossetto, and Jacques Côté. 2013. “Regulating Chromatin by Histone Acetylation.” Springer EBooks, November, 147–212. https://doi.org/10.1007/978-1-4614-8624-4_4.

Berkel, Tiffani D.M., and Subhash C. Pandey. 2017. “Emerging Role of Epigenetic Mechanisms in Alcohol Addiction.” Alcoholism: Clinical and Experimental Research 41 (4): 666–80. https://doi.org/10.1111/acer.13338.

Creighton, Samantha D., Gilda Stefanelli, Anas Reda, and Iva B. Zovkic. 2020. “Epigenetic Mechanisms of Learning and Memory: Implications for Aging.” International Journal of Molecular Sciences 21 (18): 6918. https://doi.org/10.3390/ijms21186918.

Egervari, Gabor, Greg Donahue, Natalia A.Quijano Cardé, Desi C. Alexander, Connor Hogan, Jessica K. Shaw, Erica M. Periandri, Vanessa Fleites, Mariella De Biasi, and Shelley L. Berger. 2024. “Decreased Voluntary Alcohol Intake and Ventral Striatal Epigenetic and Transcriptional Remodeling in Male Acss2 KO Mice.” Neuropharmacology 265 (December): 110258. https://doi.org/10.1016/j.neuropharm.2024.110258.

Ghezzi, Alfredo, Harish R. Krishnan, Linda Lew, Francisco J. Prado, Darryl S. Ong, and Nigel S. Atkinson. 2013. “Alcohol-Induced Histone Acetylation Reveals a Gene Network Involved in Alcohol Tolerance.” Edited by Andrzej Z. Pietrzykowski. PLoS Genetics 9 (12): e1003986. https://doi.org/10.1371/journal.pgen.1003986.

Mews, P., G. Egervari, R. Nativio, S. Sidoli, G. Donahue, S. I. Lombroso, D. C. Alexander, et al. 2019. “Alcohol Metabolism Contributes to Brain Histone Acetylation.” Nature 574 (7780): 717–21. https://doi.org/10.1038/s41586-019-1700-7.