Selenium is recognized as a chemopreventive agent against various forms of cancer (e.g., colon, prostate, lung and liver), against heart disease, and as an antiviral agent. This element also likely plays a role in delaying the aging process, in delaying the progression of AIDS in HIV positive patients, in immune function, in mammalian development and in male reproduction. The beneficial effects of this element are due, at least in part, to its presence in selenoproteins as the amino acid, selenocysteine (Sec). Our program focuses on the means by which Sec is incorporated into protein and the role of specific selenoproteins in human health. As Sec tRNA is the only known tRNA to govern the expression of an entire class of proteins, the selenoproteins, we produced transgenic mice encoding mutant Sec tRNA transgenes, produced mice lacking the Sec tRNA gene (i.e., the standard Sec tRNA gene knockout mouse) and produced mice carrying a conditional knockout of the Sec tRNA gene as model systems to modulate selenoprotein synthesis, and therefore, assess the roles of these selenium-containing proteins in health. We generated transgenic mouse lines carrying mutant Sec tRNA transgenes that have either a T -> A base change in the wobble position of the anticodon or an A -> G base change at position 37. Both mutant tRNAs alter selenoprotein biosynthesis in a protein and tissue specific manner. We are now examining these mouse lines (in collaboration with M. Beck, A. Diamond and V. Gladyshev) to respond to viral infection, to carcinogens and to diets with varying levels of selenium in a genetic background encoding a cancer driver gene. We have rescued standard Sec tRNA gene knockout mice that are embryonic lethal with transgenic mice carrying wild type Sec tRNA transgenes and are presently attempting to rescue standard knockout mice with transgenic mice carrying mutant transgenes. Using loxP-Cre technology, we have mated the conditional knockout mice with different Cre mice to selectively remove the Sec tRNA gene (and therefore alter selenoprotein expression) in various tissues including liver, prostate, brain and breast. In this way, we can assess the role of selenoproteins in liver, prostate, brain and breast malignancies. This past year (in collaboration with L. Hennighausen), we have targeted mammary tissue for deletion of the floxed Sec tRNA gene (i.e., the gene encodes flanking loxP sites) with transgenic mice carrying the mouse mammary tumor virus long terminal repeat Cre driven promoter or the whey acidic protein promoter. Both these promoters target mammary epithelium. Sec tRNA levels are reduced as much as 80% and selenoproteins are selectively expressed wherein thioredoxin reductase levels are largely unaffected, while glutathinone peroxidases are reduced. In studies examining the role of specific selenoproteins in human health, we previously observed a correlation between several characteristics of the 15 kDa selenoprotein (Sep15) and cancer etiology. This past year (in collaboration with V. Gladyshev), we have generated the knockout of the Sep15 gene and are presently examining the consequences of removing this gene from the mouse genome. We have initiated a study (in collaboration with B.J. Lee) using RNA interference to specifically remove the three selenoprotein genes encoded in the Drosophila genome that we previously identified. Each of the above studies is directed at understanding the roles of selenium-containing proteins in health.
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