P53 MUTATIONS IN MOUSE MAMMARY EPITHELIAL-CELLS - INSTABILITY IN CULTURE AND DISCORDANT SELECTION OF MUTATIONS IN-VITRO VERSUS IN-VIVO

The phenotypes of p53 mutations found in human and murine tumors often are analyzed using a variety of transformation assays in vitro, but d ata have not been available to correlate in vitro effects with in vivo activities. We have assessed the effects of p53 mutations using mouse mammary epithelial cell lines which can be analyzed both in vitro and in vivo. Parental mammary epithelial cell lines (FSK series) injected into cleared mammary fat pads of syngeneic mice frequently give rise to preneoplastic lesions (HAN) which can be reestablished in culture ( TM lines) to permit analysis of genetic changes important in the devel opment of preneoplasia. Characterization of the FSK3 cell line reveale d a cell population mixed with respect to p53 genotypes. One subpopula tion of mutant (Ser233-234) p53-expressing cells was selected in a pre neoplastic mammary outgrowth in vivo (TM3), whereas another minor popu lation of mutant (Pro135) p53-expressing cells was selected during cul turing of FSK3 cells in vitro. When FSK3 cells were subdivided and pas saged in parallel in vitro, p53 genotypes continued to evolve and dive rge. These findings reveal that selective pressures exerted on mammary epithelial cell populations in vivo are different from pressures pres ent in vitro. Selective forces may vary from one cell culture environm ent to another. The growth advantage conferred by a specific p53 mutat ion appears to differ in vivo versus in vitro. We propose that caution should be exercised when attempting to correlate the effects of p53 m utations assayed in cell culture systems with the in situ phenotypes o f mutant p53 in cancer.