Infrequent p53 mutations in 7,12‐dimethylbenz[a]anthracene–induced mammary tumors in BALB/c and p53 hemizygous mice

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Abstract
We conducted experiments to determine if p53 alterations, which are frequent in human breast cancers, were also common in murine mammary tumors. In 13 mammary tumors from 7,12‐dimethylbenz[a]anthracene (DMBA)‐treated BALB/c mice were immunohistochemically analyzed for overexpression of p53; p53 protein was not detectable. Three of the tumors were established as cell lines in vitro. p53 protein was rarely detected at passage 4 in these lines but was overexpressed by passage 8 in two of them. The p53 nucleotide sequence was shown to be wild type in one primary mammary tumor and in the two p53‐overexpressing cell lines. One cell line that overexpressed p53 in vitro was implanted into BALB/c mice. The resulting tumors retained the wild‐type p53 nucleotide sequence but no longer expressed detectable levels of p53 protein, suggesting that the overexpression of wild‐type p53 was related to in vitro culture conditions. The effect of DMBA on mammary‐tumor development was also tested in mice rendered hemizygous for p53. These mice and wild‐type littermate controls had no differences in susceptibility to induction of mammary tumors by oral administration of DMBA. Furthermore, Southern blot hybridizations detected no gross alterations in the wild‐type p53 allele in mammary tumors from the p53‐deficient mice. Point mutation of the wild‐type p53 allele was also infrequent in the DMBA‐induced mammary tumors from hemizygous p53 mice; it occured in only one of seven tumors. Thus, the p53 gene is apparently not a primary target for genetic alterations in DMBA‐induced mammary tumors. Next, we examined mammary tumors derived from D1 and D2 transplantable hyperplastic alveolar nodule (HAN) outgrowths, which rapidly form tumors containing Ha‐ras mutations after DMBA treatment. As ras and p53 mutants can cooperate in transformation, we examined whether D1 and D2 HAN outgrowths have p53 mutations. Unlike in the DMBA‐induced primary mammary tumors, nuclear p53 accumulation was observed frequently (10 of 14) in tumors that arose from D1 and D2 HAN outgrowths. Direct sequencing of the entire coding region of the p53 cDNA from six D1 and D2 tumors confirmed that the sequence was wild type. Although wild‐type p53 was retained in both DMBA‐induced mammary tumors and mammary tumors derived from D1 and D2 preneoplastic outgrowths, wild‐type p53 overexpression was detected only in D1 and D2 tumors. Therefore, D1 and D2 tumors appear to arise by a pathway in which p53 expression is altered, whereas DMBA induction affects a different pathway that does not require such alteration.