The rapid growth of genetic insights into causes of breast cancer continues to identify new targets for gene-specific therapies. A screen predicting their potential efficacy is needed. Transgenic mice have been under-utilized for this purpose, considering their unique ability to mimic gene-specific effects in vivo. Cyclin D1 is amplified in 15 percent of breast cancers and over-expressed in nearly 40 percent. Its expression identifies estrogen-positive breast cancers with a poor prognosis. It uniquely drives abnormal proliferation all the way to frank malignancy when expressed in transgenic mammary glands, unlike any other tissue. The mammary gland is a particular target of cyclin D1 loss in knockout studies. Reagents developed in our laboratories include MMTV-cyclin D1 mice, cyclin D1 loss in knockout studies. Reagents developed in our laboratories include MMTV-cyclin D1 mice, cyclin D1 knockouts, and a new MMTV-p16 strain. Using our mouse strains, additional MMTV- oncogene transgenic strains, p16 peptide fusions with TAT (or Antennapedia) protein transduction domains, and drugs that inhibit cyclin dependent kinase function, we seek to validate the use of transgenic models as a pre-clinical test system. We propose to identify genetic pathways where intact cyclin D1 activity is needed for tumorigenesis. 1. The phenotype of crosses between cyclin D1 knockout mice and transgenic mice targeting expression of c-myc, ras, transforming growth factor-alpha (TGFalpha), Wnt-1, Py T antigen (src pathway) and Her2/Neu to mammary epithelium will be examined. The role of cyclin dependent kinases in any protective effects will be studied using additional crosses between MMTV-p16 mice and transgenic mice expressing the same set of genes. 2. To evaluate the effects of blocking cyclin D1 in neoplastic lesions versus fully transformed lesions we will develop regulated excision of the cyclin D1 locus and evaluate loss of cyclin D1 function in spontaneously arising tumors. To extend this test to cdk function, we will also develop tetracycline-regulated, tissue-specific targeting of p16/INK4A. Using p53-/- x MMTV-cyclin D1 mice, we will expand the pre-neoplastic cyclin D1-expressing population by transplanting mammary glands into wild type recipients. 3. Finally, we will evaluate the relevance of these genetic manipulations by comparing their effects to the results of treatment using small molecule antagonists of cyclin D1/cdk. Specifically we will test protein transduction with Antennapedia or TAT-p16 peptide fusions as gene therapy in vivo. We will follow these studies by evaluating the therapeutic efficacy of chemical inhibitors of Cdks in the treatment of tumors caused by transgenic over-expression of oncogenes in mammary tissues.