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 and in delaying the progression of AIDS in HIV positive patients. 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, the role of specific selenoproteins in human health and the role of selenium in HIV infection. As Sec tRNA is the key molecule in selenoprotein biosynthesis, we produced transgenic mice encoding wild type and mutant Sec tRNA transgenes and produced Sec tRNA standard knockout and conditional knockout mice to examine the effect of each genetically altered mouse on selenium metabolism. We observed previously that over-expression of wild type Sec tRNA does not affect selenoprotein synthesis, but that of an i6A mutant Sec tRNA, however, selectively inhibits Sec tRNA maturation and translation of specific selenoproteins. This study marked the first example of a transgenic mouse being genetically engineered to contain a functional tRNA transgene, will provide an important tool for studying the role of specific selenoproteins in health and was featured on the cover of the prestigious journal, Molecular and Cellular Biology. We have now generated a transgenic mouse line carrying mutant Sec tRNA transgenes that have a T -> A base change in the wobble position of the anticodon. This tRNA produces I in the wobble position of the expressed tRNA gene product and translates UGU and UGC in addition to the Sec codon, UGA. We are presently analyzing the effect of this tRNA on selenoprotein biosynthesis. We have recently begun to mate our standard knockout mice with our transgenic mice to rescue selenoprotein biosynthesis that is dependent on the wild type or mutant Sec tRNA transgene. In addition, we have recently begun to mate our conditional knockout mice with different Cre mice to selectively remove the Sec tRNA gene (and therefore shut off 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 malignancy. In our studies on 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. To specifically define the cellular role of this selenoprotein, we are preparing a targeting vector to knockout the Sep 15 gene for generating mice lacking this gene. In our studies involving the effects of selenium on human health, we have shown that HIV infected cells in culture have a deficient amount of selenoproteins compared to unifected cells and that Rhesus monkeys infected with simian immunodeficiency virus (SIV) develop selenium deficiency in the progression of developing AIDS as is characteristcally found in HIV positive patients. We examined SIV infected cells in culture and have found that they have very similar characteristics as HIV infected cells including selenoprotein deficiency and reduced levels and distributions of the Sec tRNA isoforms that are normally associated with selenium deficiency. As a result of these observations, we now have a model for understanding the molecular mechanism of selenium deficiency associated with AIDS. We hope to obtain funds to further pursue this model in Rhesus monkeys infected with SIV.
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