Aspirin, UGT1A6 Genotype, and Colon Gene Expression

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DESCRIPTION (provided by applicant): Colorectal cancers are thought to arise as the result of a series of molecular changes that transform normal epithelial cells into a colorectal carcinoma, with an adenomatous polyp as an intermediate step in this process. One way to reduce mortality from this cancer involves the use of oral agents that prevent neoplasms from developing in the large bowel. Regular use of aspirin and other non-steroidal anti-inflammatory drugs (NSAIDs) reduces the incidence of colon adenomas as well as carcinomas by approximately 50 percent. Enzymes prominently involved in metabolizing aspirin are UDP-glucuronosyltransferases (UGT). UGT1A6 is a polymorphic UGT and its variant alleles metabolize aspirin less efficiently. We showed that in NSAIDs users, and aspirin users in particular, the risk for colon neoplasia is reduced only in individuals who are UGT1A6 heterozygous and homozygous variant, but not homozygous wild-type. Aspirin and its metabolite, salicylic acid, can inhibit growth through the inhibition of cyclooxygenases (catalysts of prostaglandin synthesis), the promotion of apoptosis, and other as yet unidentified pathways. The goal of this project is to determine the effect of UGT1A6 genotype on aspirin metabolism and on aspirin-induced changes in colonic gene expression and protein markers of apoptosis (Bax and Bcl-2). We propose to study: 1) urinary excretion of aspirin metabolites in 380 healthy individuals with different UGT1A6 genotypes in a cross-sectional study and 2) changes in colon gene expression induced by aspirin supplementation in a randomized, placebo-controlled trial of 40 individuals who are either homozygous wild-type or homozygous variant for UGT1A6. We hypothesize that in slow metabolizers the proportion of glucuronidated metabolites is smaller and that a smaller proportion of the aspirin dose is excreted within a specific time. Alterations in colonic gene expression will be determined using cDNA microarray analysis of biopsy RNA from the sigmoid colon and rectum and Bax and Bcl-2 expression in the colonic crypts will be measured by immunohistochemistry. We hypothesize that expression of e.g. growth-promoting genes will be reduced and apoptotic genes increased during aspirin use and that this reduction may be stronger in slow metabolizers than fast metabolizers. This project will also allow the identification of other genes whose expression is affected by aspirin supplementation. Ultimately, information obtained from this project will be useful in developing chemopreventive drugs that specifically target pathways involved in colon neoplasia. By targeting relevant pathways, side effects caused by the use of NSAIDs or specific Cox-2 inhibitors may be avoided.