Throughout its history, this grant has involved studies of the role of inheritance in individual variation in the metabolism of drugs and other xenobiotics-i.e., pharmacogenetics. The initial decades of the 21st century have seen the transformation of pharmacogenetics into pharmacogenomics, moving from candidate genes to genome-wide approaches and-with the integration of additional omics disciplines-it is now becoming pharmaco-omics. We have taken advantage of these advances and have used them to study drug metabolism, to explore underlying mechanisms responsible for variation in drug response phenotypes and to apply that understanding to the selective serotonin reuptake inhibitor (SSRI) therapy of Major Depressive Disorder (MDD). Specifically, during the most recent funding cycle, we set out to test the hypothesis that using metabolomics to inform genomics might increase our ability to discover genes, pathways and mechanisms involved in variation in SSRI clinical response. The application of this Pharmacometabolomics-informed Pharmacogenomic research strategy to studies of SSRI clinical response resulted in the identification of a series of novel candidate genes associated with tryptophan metabolism, a pathway known to be related to mood as a result its control of the balance between the formation of either serotonin or kynurenine from tryptophan. Those novel genes were identified by the use of GWAS to identify genes related to variation in the concentrations of metabolites associated with SSRI response. Those genes included AHR, TSPAN5, and SOD2--results directly related to drug metabolism because one of the key genes identified, AHR, encodes the aryl hydrocarbon receptor, a transcription factor for drug metabolizing enzymes such as CYP1A1 and CYP1A2. We also observed that single nucleotide polymorphisms (SNPs) located hundreds of base pairs from the xenobiotic response element (XRE) DNA sequences to which AHR binds can profoundly alter both AHR-DNA binding and subsequent transcription-identifying a novel pharmacogenomic mechanism. These SSRI-response related genes were then functionally validated in a series of cell lines ranging from hepatic cells to iPS-derived neurons to address concerns with regard to the relevance of peripheral metabolites for the biochemistry of the central nervous system (CNS)-although kynurenine formed in the liver is a major source of kynurenine in the CNS. The identification of genetic variation in AHR-a transcription factor known primarily for its role in xenobiotic and drug metabolism--fits within a growing body of evidence supporting an important role in MDD pathophysiology and response to drug therapy for biochemical cross-talk between a peripheral organ, the liver, and the brain. This series of novel observations provides a foundation for the experiments proposed in this application, studies that have been designed to increase our understanding of genomic mechanisms associated with individual variation in SSRI clinical response.