Clonal dominance detected in metastases but not primary tumors of retrovirally marked human breast carcinoma injected into nude mice.

Human breast cancer cell lines which grow in athymic (nude) mice provide a model of tumor cell growth and metastasis. Marking transplanted tumor cell populations with retroviral vectors provides a means of studying the dynamics of tumor cell growth in vivo. We evaluated three human breast cancer cell lines, MDA-MB-435, MDA-MB-231 and MCF-7, and found the cells were highly susceptible to retroviral gene transfer after a single 2-h exposure (90.9%, 62.7% and 72.3%, respectively). MDA-MB-435 cells (5 x 10(5)) marked with a retroviral vector containing the beta-galactosidase gene (approximately 10(4) uniquely marked clones) were injected into the mammary fat pad of athymic mice to study clonal dominance. Primary tumors resected 10 weeks after injection expressed beta-galactosidase, demonstrating persistent vector expression in vivo. Southern blot analysis did not reveal clonal dominance in the primary tumors of the five mice studied. In contrast, pulmonary metastases in each animal were monoclonal or biclonal. These results demonstrate clonal dominance in pulmonary metastases but not primary tumors of retrovirally marked MDA-MB-435 cells. Our findings suggest that this model may also be used to introduce retroviral vectors expressing oncogenes, and anti-sense oncogenes, to determine their effect on tumor cell proliferation and metastasis in vivo.