Sustained mammary gland-directed, ponasterone A-inducible expression in transgenic mice

The ability to regulate temporal- and spatial-specific expression of target genes in transgenic mice will facilitate analysis of gene function and ena ble the generation of murine models of human diseases. The genetic analysis of mammary gland tumorigenesis requires the development of mammary gland-s pecific transgenics, which are tightly regulated throughout the adult mamma ry epithelium. Analysis of genes implicated in mammary gland tumorigenesis has been hampered by mosaic transgene expression and the findings that homo zygous deletion of several candidate genes (cyclin D1, Stat5A, prolactin re ceptor) abrogates normal mammary gland development. We describe the develop ment of transgenic mouse lines in which sustained transgene expression was inducibly regulated, both specifically and homogeneously, in the adult mamm ary gland epithelium. Transgenes encoding RXR alpha and a chimeric ecdysone receptor under control of a modified MMTV-LTR, which targets mammary gland expression, were used. These transgenic 'receptor' lines were crossed with transgenic 'enhancer' lines in which the ecdysone/RXR binding site induced ligand-dependent expression of transgenic beta-galactoxidase. Pharmacokine tic analysis of a highly bioactive ligand (ponasterone A), identified throu gh screening ecdysteroids from local plants, demonstrated sustained release and transgene expression in vivo. This transgenic model with both tightly regulated and homogeneous transgene expression, which was sustained in vivo using ligands readily extracted from local flora, has broad practical appl icability for genetic analysis of mammary gland disease.