SV40 T T-ANTIGEN INDUCES PREMATURE MAMMARY-GLAND INVOLUTION BY APOPTOSIS AND SELECTS FOR P53 MISSENSE MUTATION IN MAMMARY-TUMORS/

We recently established transgenic animals (WAP-SV-T/t) carrying the e arly coding region of Simian Virus 40 (SV40) under the transcriptional control of the whey acidic milk protein promoter (WAP), which restric ts the expression of the transgene to mammary gland epithelial cells ( ME-cells). SV40 T/t-antigen synthesis causes premature mammary gland i nvolution during late pregnancy by inducing apoptosis and leads to dev elopment of mammary tumors after the first lactation period in both p5 3 positive (WAP-SV-T/t) and p53 negative double transgenic animals (WA P-SV-T/t.p53 -/-), The high apoptotic rate persists in all of the T/t- antigen positive breast tumor cells, as well as in established ME-tiss ue culture cell lines. ME-cells which spontaneously switch off the exp ression of the WAP-SV-T/t transgene do not undergo apoptosis, However, these cells again exhibit an extensive DNA fragmentation when SV40 T/ t-antigen synthesis is reintroduced, which indicates that it is the ex pression of T/t antigen which is the critical factor for induction of apoptosis, In addition, we isolated several ME-cell lines from differe nt breast tumors which hale spontaneously lost the T/t-antigen yet rem ain maximally transformed. Strikingly, these cells contain a missense mutation of the p53 gene at codon 242 (p53(242)), which substitutes al anine for glycine, This mutation increases p53 stability and it reduce s the transactivating function of p53, albeit without affecting the ab ility of the protein to interact with the DNA. This indicates that p53 missense mutations are selected for in breast tumors initially expres sing T/t-antigen. Therefore, the p53(242) mutation is sufficient to ma intain the transformed state after the ME-cells have switched off the WAP-SV-T/t transgene, Interestingly, the p53 minus state per se is not sufficient to induce ME-cell transformation since homozygous null mic e for the p53 gene (p53-/-) fail to develop breast cancer.