Impact of genetic and epigenetic variability in response to two test drugs 5-fluorouracil and lansoprazole

Pharmacogenetics has proven to be an invaluable tool in predicting variability in drug response; however, there are numerous incidences where genetics cannot fully explain interindividual drug variability. Other factors, such as the epigenetic mechanism of DNA methylation, may offer an alternate explanation. Therefore, this dissertation focuses on both genetic and epigenetic regulation in order to gain a better understanding of the molecular basis behind drug response variability.;Initial investigations focused on the antineoplastic agent 5-Fluorouracil (5-FU), which can produce severe toxicity in patients with deficiency in dihydropyrimidine dehydrogenase (DPD). Genotypic studies have identified >32 sequence variants in the DPYD gene; however, sequence variants could not explain the molecular basis of DPD deficiency in a number of cases. In a cohort of samples phenotypically characterized by enzyme assay and breath test, aberrant methylation of the DPYD promoter was detected using denaturing high performance liquid chromatography (DHPLC) in all six DPD-deficient individuals without inactivating mutations in their DPYD gene. Out of four DPD-deficient individuals genetically characterized as heterozygous for either DPYD*2A or DPYD*13, two showed varying DPYD promoter methylation, indicating that methylation of the DPYD promoter is associated with decreased DPD activity in clinical samples and may act separately or in concert with genetic variants to down-regulate DPD activity.;Genetic and epigenetic factors in drug response were also analyzed in 50 gastric acid hypersecretors requiring a wide dose range of a commonly prescribed proton pump inhibitor (PPI), Lansoprazole, for therapeutic acid suppression. Genetic analysis revealed that patients homozygous for the CYP2C19*17 allele were unable to achieve acid suppression at high Lansoprazole doses, while heterozygotes achieved moderate to complete acid suppression at elevated Lansoprazole doses. Methylation analysis of the ATP4B gene showed associations between increased site specific methylation levels, decreased ATP4B gene expression, and elevated acid output (BAO).;Using two different examples, these studies further expand our knowledge of genetics and epigenetics in drug response variability, and show that both genetics and epigenetics can act separately or in concert to provide a better understanding of the molecular mechanisms in drug response.