DNA methylation and histone acetylation changes to cytochrome P450 2E1 regulation in normal aging and impact on rates of drug metabolism in the liver

Aging is associated with reduced liver function that may increase the risk for adverse drug reactions in older adults. We hypothesized that age-related changes to epigenetic regulation of genes involved in drug metabolism may contribute to this effect. We reviewed published epigenome-wide studies of human blood and identified the cytochrome P450 2E1 (CYP2E1) gene as a top locus exhibiting epigenetic changes with age. To investigate potential functional changes with age in the liver, the primary organ of drug metabolism, we obtained liver tissue from mice aged 4-32 months from the National Institute on Aging. We assayed global DNA methylation (5-methylcytosine, 5mC), hydroxymethylation (5-hydroxymethylcytosine, 5hmC), and locus-specific 5mC and histone acetylation changes around mouse Cyp2e1. The mouse livers exhibit significant global decreases in 5mC and 5hmC with age. Furthermore, 5mC significantly increased with age at two regulatory regions of Cyp2e1 in tandem with decreases in its gene and protein expressions. H3K9ac levels also changed with age at both regulatory regions of Cyp2e1 investigated, while H3K27ac did not. To test if these epigenetic changes are associated with varying rates of drug metabolism, we assayed clearance of the CYP2E1-specific probe drug chlorzoxazone in microsome extracts from the same livers. CYP2E1 intrinsic clearance is associated with DNA methylation and H3K9ac levels at the Cyp2e1 locus but not with chronological age. This suggests that age-related epigenetic changes may influence rates of hepatic drug metabolism. In the future, epigenetic biomarkers could prove useful to guide dosing regimens in older adults.