Breast cancer evolves from a complex, multistep process involving an accumulation of defects in gene structure and/or expression. Using the mouse model of chemically-induced mammary carcinogenesis, our research has three long-term objectives: l) defining the steps required for neoplastic progression of mouse mammary epithelial cells (MMEC) to malignancy, 2) identifying the factors and events responsible for advancement from one stage to the next, and 3) developing markers to detect cells in different stages of progression. We have recently shown that many preneoplastic and malignant MMEC overexpress the endogenous retrotransposons, intracisternal A particles (IAPs) and murine leukemia virus-related elements (MLVEs), compared to normal cells. Retrotransposons are mobile elements in mouse DNA capable of contributing to neoplastic transformation by transposing and causing insertional mutagenesis, which can alter gene expression and increase genetic instability of the host genome. The hypothesis of the proposed research is that amplified expression with subsequent transposition of MLVE sequences are key events in at least one pathway of mouse mammary carcinogenesis induced by 7,12-dimethylbenzanthracene (DMBA). To investigate the possible role of MLVEs in development of mammary cancer, the proposed research will use three approaches: 1) mouse mammary carcinogenesis induced by DMBA will be analyzed to determine how frequently and when altered expression of MLVEs occurs during neoplastic progression, the identity of the cells affected, and which class(es) of MLVEs is involved; 2) a probe specific for the main class of MLVEs expressed in DMBA tumors will be used to determine if any MLVE copies are located in new sites of the host genome and if so, what effect this has had on any nearby genes; and 3) a cDNA library prepared from a DMBA tumor with high levels of MLVE RNA will be screened for cDNAs corresponding to cellular transcripts which either originate in or are polyadenylated by a MLVE LTR. These studies will include the use of polymerase chain reaction, Northern and Southern analyses, in situ hybridization, and DNA sequencing. As ten percent of the human genome consists of transposable elements, several of which have been implicated in mutagenizing human DNA, these elements might be operative in human, as well as mouse, breast cancer. In fact, oncogene activation due to transposition and insertional mutation by a retroelement has already been documented in a human breast ductal carcinoma.