SIGNIFICANCE OF GENETIC VARIATION IN ESTROGEN METABOLISM

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Although the causes of most cancers in the human population are unknown, it is recognized that most cancers have both a genetic and an environmental component. Certain genes play a role in the body by "detoxifying" both exogenous and endogenous chemicals, and individual differences in the ability to detoxify such chemicals could be an important contributor of cancer risk. In this study, we propose to examine whether genetic differences in some of these "biotransformation" enzymes might contribute to risk of endometrial cancer. Exactly what causes endometrial cancer is uncertain, although it is widely recognized that the endogenous hormone, estrogen, plays an important role. Estrogen might increase endometrial cancer risk in two ways: 1) because of its hormonal activity, it affects the rate of cell division in endometrial tissue, which may increase the chances of damaged DNA being passed on to daughter cells, and 2) recent evidence indicates that estrogen may be metabolized in the body to chemical forms (catechol estrogens) that may directly or indirectly damage DNA in endometrial tissue. There are numerous enzymes in the body that both make and eliminate catechol estrogens, and several of these enzymes exist in slightly different forms in the human population (they exhibit "genetic polymorphism"). In this study, we propose to determine: a) if certain human polymorphic variants of cytochrome P4501A1 and cytochrome P450 1B1 produce catechol estrogens more (or less) efficiently than the normal forms of the enzymes, b) if polymorphic variants in catechol-O-methyltransferase and specific forms of glutathione S-transferases are more (or less) efficient at conjugating catechol estrogens, c) whether and to what extent these enzymes are expressed in human endometrial tissue, and d) whether people with variant forms of these enzymes exhibit increased (or decreased) DNA damage in endometrial tissue. To do these studies, individual forms of each of these enzymes will be expressed in yeast, using the cDNAs for the normal and variant forms. The expressed proteins will then be evaluated to determine their relative abilities to metabolize catechol estrogens. Human endometrial tissues will be obtained from 100 patients undergoing hysterectomies for various medical reasons, and the tissues will be analyzed for the presence of these proteins, their activities, and their mRNA levels. The tissues will also be evaluated for the extent of DNA damage that has occurred over time by two techniques: one measures the actual number of oxidatively damaged DNA bases, and the other measures the overall structure of the DNA by measuring its infrared spectral diversity. The genetic forms (genotypes) of each of the enzymes under study will also be determined in each sample. Statistical methods will be used to determine if uterine tissues from certain "susceptibility" genotypes have higher levels of DNA damage, relative to less susceptible genotypes. We hypothesize that genotypes that favor an increased rate of formation and/or decreased rate of elimination of catechol estrogens will have relatively greater levels of oxidatively damaged DNA in endometrial tissues. The in vitro work to measure the activities and relative levels of expression of these enzymes in endometrial tissues will provide important mechanistic information to justify future population-based studies to ultimately determine if these genetic polymorphisms individually or in combination are important risk factors for endometrial cancer.