Adverse drug reactions (ADRs) are more common in older persons, exact a heavy economic toll on health systems and can be fatal. The reasons behind increased ADRs in the aged population are not well understood, but changes to drug metabolism are a factor. Prior research in humans and mice has shown age-associated changes to gene expression for several important genes encoding drug metabolizing enzymes. Furthermore, several human studies have shown age-associated epigenetic changes in blood at many of the same genes. Therefore, we hypothesize that epigenetic changes with age may contribute to age-related changes in drug metabolism by affecting regulation of drug metabolism pathways. To test this hypothesis, we propose a preclinical (mouse) study to map the extent and impact of age-related epigenetic changes at key genes in the liver, the primary site of xenobiotic metabolism. Specifically, we will first conduct a genome-wide DNA methylation analysis in mouse liver samples from aged mice from the National Institute of Aging rodent colonies. To assay DNA methylation, we will use reduced representation bisulfite sequencing (RRBS), a robust method that preferentially assays CpG-dense regions such as gene promoters. Use of this next-generation sequencing (NGS)-based method is important because of the high level of genome-wide coverage achievable, relative to current epigenomic microarrays, which are under-developed for model organisms. We will test for association between age and DNA methylation levels at each assayed locus, to identify age-associated differentially methylated regions (a-DMRs). Our top a-DMRs, plus important candidate genes involved in drug metabolism such as Cyp2e1 and Cyp1b1 (whose human orthologs have shown a-DMRs), will be tested for correlation with 1) local histone acetylation levels via chromatin immunoprecipitation - quantitative real-time PCR (qPCR), and 2) expression levels of neighboring genes using reverse transcription qPCR. Through these analyses, we will establish epigenetic regulatory states and functional impact of a-DMRs in the mouse liver. As a final step, we will also conduct a pilot pharmacokinetic study of Cyp2e1 gene activity in aged mice. Cyp2e1 has been shown by us and others to exhibit differential expression with age. We will phenotype CYP2E1-mediated metabolism via administration of chlorzoxazone, a CYP2E1-selective probe drug. Our expectation is that the reduced expression of the Cyp2e1 gene with age should be associated with a reduced rate of chlorzoxazone metabolism. A successful demonstration of the hypothesized effects could pave the way for future clinical studies to develop epigenetic biomarkers of pharmacokinetic pathways, which could guide dosing decisions in older patients. Finally, as an R15, this proposal includes provision for student training opportunities in functional pharmacogenomics.
Age-related changes to drug metabolism may partly explain the increased incidence of adverse drug events in older persons, but these changes are hard to predict and poorly understood. We hypothesize that epigenetic changes with age may contribute to age-related changes in drug metabolism. Here, we propose a preclinical study to identify age-related epigenetic changes that affect regulation of genes involved in drug metabolism.
