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Mice homozygous for an allele encoding the selenocysteine (Sec) tRNA [Ser]Sec gene (Trsp) flanked by loxP sites were generated. Cre recombinase-dependent removal of Trsp in these mice was lethal to embryos. To investigate the role of Trsp in mouse mammary epithelium, we deleted this gene by using transgenic mice carrying the Cre recombinase gene under control of the mouse mammary tumor virus (MMTV) long terminal repeat or the whey acidic protein promoter. While both promoters target Cre gene expression to mammary epithelium, MMTV-Cre is also expressed in spleen and skin. Sec tRNA[Ser]Sec amounts were reduced by more than 70% in mammary tissue with either transgene, while in skin and spleen, levels were reduced only with MMTV-Cre. The selenoprotein population was selectively affected with MMTV-Cre in breast and skin but not in the control tissue, kidney. Moreover, within affected tissues, expression of specific selenoproteins was regulated differently and often in a contrasting manner, with levels of Sep15 and the glutathione peroxidases GPx1 and GPx4 being substantially reduced. Expression of the tumor suppressor genes BRCA1 and p53 was also altered in a contrasting manner in MMTV-Cre mice, suggesting greater susceptibility to cancer and/or increased cell apoptosis. Thus, the conditional Trsp knockout mouse allows tissue-specific manipulation of Sec tRNA and selenoprotein expression, suggesting that this approach will provide a useful tool for studying the role of selenoproteins in health.Selenium is an essential micronutrient in the diet of mammals and numerous other life forms (see reference 26 for a review). Many health benefits have been attributed to this element, including a role in the prevention of cancer (10) and heart disease and other cardiovascular and muscle disorders (11), in delaying the aging process (33) and the onset of AIDS in human immunodeficiency virus-positive patients (1), in male reproduction (17), in mammalian development (5), in immune function (33), and as an antiviral agent (2). Selenium is incorporated into protein in the form of selenocysteine (Sec), and Sec has its own tRNA (designated Sec tRNA[Ser]Sec ) and its own code word, UGA (26). Sec is indeed the 21st naturally occurring amino acid in the genetic code. Most certainly, the health benefits of selenium are due in large part to its presence in protein (26).Sec tRNA [Ser]Sec is the only known tRNA that governs the expression of an entire class of proteins, the selenoproteins (26). This provides a unique opportunity to study the expression of selenoproteins by manipulating the levels and characteristics of Sec tRNA [Ser]Sec . For example, the levels of numerous selenoproteins were reduced in a protein-and tissue-specific manner in transgenic mice carrying mutant Sec tRNA [Ser]Sec transgenes lacking the highly modified base isopentenyladenosine in its anticodon (37). Glutathione peroxidase 1 (GPx1) and thioredoxin reductases 1 (TR1) and 3 (TR3) were the most and least affected selenoproteins, respectively, and selenoprotein expre...

Novel mouse models were developed in which the hepatic selenoprotein population was targeted for removal by disrupting the selenocysteine (Sec) tRNA [Ser]Sec gene (trsp), and selenoprotein expression was then restored by introducing wild type or mutant trsp transgenes. The selenoprotein population was partially replaced in liver with mutant transgenes encoding mutations at either position 34 (34T3 A) or 37 (37A3 G) in tRNA[Ser]Sec . The A34 transgene product lacked the highly modified 5-methoxycarbonylmethyl-2-O-methyluridine, and its mutant base A was converted to I34. The G37 transgene product lacked the highly modified N 6 -isopentenyladenosine. Both mutant tRNAs lacked the 2-methylribose at position 34 (Um34), and both supported expression of housekeeping selenoproteins (e.g. thioredoxin reductase 1) in liver but not stress-related proteins (e.g. glutathione peroxidase 1). Thus, Um34 is responsible for synthesis of a select group of selenoproteins rather than the entire selenoprotein population. The ICA anticodon in the A34 mutant tRNA decoded Cys codons, UGU and UGC, as well as the Sec codon, UGA. However, metabolic labeling of A34 transgenic mice with 75 Se revealed that selenoproteins incorporated the label from the A34 mutant tRNA, whereas other proteins did not. These results suggest that the A34 mutant tRNA did not randomly insert Sec in place of Cys, but specifically targeted selected selenoproteins. High copy numbers of A34 transgene, but not G37 transgene, were not tolerated in the absence of wild type trsp, further suggesting insertion of Sec in place of Cys in selenoproteins.There are 24 known selenoproteins in rodents and 25 in humans (1). The targeted removal of specific selenoproteins has shown that some are essential in development, whereas others appear to be nonessential. For example, the loss of selenoproteins glutathione peroxidase 4 (GPx4) (2) or thioredoxin reductase 1 (TR1 or Txnrd1) (3) or 2 (TR3 or Txnrd2) (4) is embryonic lethal, whereas the loss of glutathione peroxidase 1 (GPx1) (5) or 2 (GPx2) (6) appears to be of little or no consequence. Other studies, however, suggest that those selenoproteins whose loss results in little or no phenotypic change may function in protective mechanisms against certain environmental stresses (see Ref. 6 and references therein). There are selenoproteins whose removal or mutation results in dramatic effects on health. For example, knock-out of selenoprotein P (SelP) 6 causes neurological problems (7, 8), and knock-out of type 2 iodothyronine deiodinase results in a variety of defects, including an impaired adaptive thermogenesis and hypothermia in cold-exposed mice (see Ref. 9 and references therein), retarded cochlear development and hearing loss (10), and a pituitary resistance to thyroxine (11). Mutations affecting selenoprotein N (SelN) result in several muscle disorders (12, 13).LoxP-Cre technology, which allows the removal of embryonic lethal genes in specific tissues and organs (3,4,14), has been used to examine the roles of essential sele...