Abstract P3-03-03: A mouse model of sporadic breast tumor with a conditional P53 mutation
Authors: Zhang Y , Xiong S , Pant V , El-Naggar A , Lozano G 
Published: May 2016

Abstract
Missense mutations in the tumor suppressor gene p53 are present in more than 50% of human tumors. In particular, the arginine-to-histidine mutation at codon 175 (p53R175H) has been found in more than 4% of human breast cancers. p53 mutations fall into two general categories: germline mutations that are associated with hereditary tumors, and somatic mutations that cause sporadic tumors. Mouse models for cancers induced by p53 germline mutations have been established and characterized. However, hitherto there are no accurate animal models for sporadic tumors induced by p53 somatic missense mutations, although this mechanism of inactivating p53 occurs in a large fraction of human cancers.
We have generated a mouse allele, p53WM, that carries coding sequences for both wild type (WT) and R172H mutant p53 (corresponding to R175H in humans) at the p53 endogenous locus. The coding sequence for WT p53 is flanked by loxP sites and therefore can be deleted by Cre recombinase. In the absence of Cre, following radiation, the p53WM/+ mice have the similar ability as the p53 WT mice to activate p53, as demonstrated by transcriptional activation of p53 targets and induction of apoptosis. In addition, like WT mouse embryonic fibroblasts (MEFs), the p53WM/+ MEFs also underwent cell cycle arrest following radiation. After crossing p53WM/+ mice with Zp3-Cre mice which express the Cre recombinase in oocytes, the coding sequence for WT p53 was removed, and the mutant p53R172H allele was regenerated as determined by sequencing, western blot analysis and real time reverse transcription PCR.
To investigate whether a somatic p53R172H mutation in mammary epithelium can induce breast tumors, adenoviruses expressing Cre (Ad-Cre) were injected into the mammary duct of p53WM/+ mice. Limiting the adenovirus dose allowed us to induce the p53R172H mutation in 1 of 20, 100 or 1000 cells, generating a mutant p53 surrounded by normal cells which more accurately mimics the clinical situation of sporadic tumors. Meanwhile, p53WM/+; K14-Cre and p53WM/+; WAP-Cre mice have been generated, in which p53R172H mutation are induced through Cre transgene specifically in the mammary epithelial cells. Lastly, to examine how somatic p53 mutation cooperates with an oncogene in mammary tumor induction, MMTV-neu; p53WM/+ mice were also subjected to the mammary intraductal Ad-Cre injection. We are currently monitoring mice and will further investigate the molecular mechanisms of tumorigenesis by studying the loss of heterozygosity, and the co-evolution of tumor epithelial cells and their microenvironment. This mouse model is more likely to share the underlying molecular pathology with human sporadic tumors. Therefore, it will be more predictive of human responses to drugs, and thus a more valuable tool in preclinical testing.
Citation Format: Zhang Y, Xiong S, Pant V, El-Naggar A, Lozano G. A mouse model of sporadic breast tumor with a conditional P53 mutation. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P3-03-03.