Susan G Komen  
I've Been Diagnosed With Breast Cancer Someone I Know Was Diagnosed Share Your Story Join Us And Stay Informed Donate To End Breast Cancer
    Home > Research & Grants > Grants Program > Research Grants > Research Grants Awarded > Abstract
    Awarded Grants
    Using Mouse Models to Define the Pathway of P53-Mediated Tumor Suppression in Breast Cancer

    Scientific Abstract:
    Project Title: Using mouse models to define the pathway of p53-mediated tumor suppression in breast cancer. Background: The p53 tumor suppressor gene is inactivated in approximately 30% of breast cancers, suggesting the importance of p53 in inhibiting breast tumorigenesis. Although p53 can suppress cell division, apoptosis is thought to be the predominant mode of p53 tumor suppression. Thus, elucidating the details of the p53-dependent apoptotic pathway is important for understanding the process of cancer development. p53 is a transcription factor that can stimulate the expression of various pro-apoptotic target genes, but very few studies have investigated the status of these genes in breast cancer specifically. As p53 apoptosis mediators appear to be cell-type specific, the identification of p53 target genes involved specifically in apoptosis in mammary epithelium is important for understanding how p53-mediated apoptosis suppresses breast cancer development. Mouse models are of great value for understanding the pathogenesis and molecular basis of human diseases. The breast tumors that develop in telomerase-deficient mice have characteristics that are similar to spontaneous breast cancers in humans and therefore serve as a good model system to study breast tumorigenesis. In the absence of telomerase (either TERC, the RNA component, or TERT, the catalytic subunit), chromosome ends shorten, leading to activation of the p53 apoptosis response. As a result, cells from p53+/+, telomerase-deficient mice undergo apoptosis and are resistant to becoming tumorigenic. In contrast, p53-/-, telomerase-deficient mice have a defective apoptotic response, allowing rapid tumorigenesis to ensue in the mammary gland and in other tissues. Thus, telomerase deficient mice with differing p53 status provide a good system to investigate the p53-mediated apoptotic pathway in a breast cancer model. To specifically identify p53 mediators involved in apoptosis as opposed to cell cycle arrest, our laboratory previously utilized a primary mouse embryo fibroblast (MEF) system. This system was successfully used to identify a novel p53-dependent apoptotic-specific gene PERP, which has been shown to play a cell type-specific role as a mediator of p53-induced apoptosis. However, as PERP is only one of the components of the apoptotic pathway, we subsequently utilized a global genomic microarray approach and identified a limited number of additional apoptosis-specific p53 target genes. Objective: The main goal of this proposal is to define critical mediators of p53-dependent apoptosis in mammary epithelial cells using mouse models. Specific Aims (1) To examine the apoptosis-specific p53 target genes identified in the MEF microarray analysis and identify those specifically induced in apoptotic epithelial cells derived from the mammary gland of mTERT-/-, p53+/+ mice as compared to mTERT-/-, p53 -/- mice. (2) To select a few genes for further study to determine their role in apoptosis in mammary cells. (3) To generate a knockout mouse model for one candidate apoptotic mediator to characterize its role as a breast cancer tumor suppressor. Study Design: The expression profile of apoptosis-specific target genes identified previously by the microarray analysis of MEFs will be studied using northern analysis of mammary epithelial cells derived from both mTERT-/-, p53+/+ and mTERT-/-, p53-/- mice to identify p53-dependent genes that are candidate apoptotic mediators. We will examine some of these genes for the presence of p53-responsive elements, sequence similarity to known apoptotic genes and the ability to induce apoptosis when overexpressed in mammary epithelial cells from mTERT-/-, p53-/- mice to identify a few genes to pursue further. This will be complemented by an RNAi approach to knock down the expression of these genes to confirm their role in p53-mediated apoptosis. Knockout mice will be generated for one of the most interesting of the identified genes to define a role in suppressing breast cancer. This study will help us understand how apoptosis mediators of p53 suppress breast cancer and how the evasion of apoptotic leads to breast cancer.

    Lay Abstract:
    Project Title: Using mouse models to define the pathway of p53-mediated tumor suppression in breast cancer. Background: Breast cancer is the leading cause of cancer deaths among middle-aged and elderly women in America. Mutations in several genes contribute to breast cancer, such as the p53 tumor suppressor gene which is inactivated in 30% of breast cancers. This finding suggests that p53 normally impedes breast cancer. p53 suppresses tumor development either by inhibiting cellular proliferation or by activating a form of cellular suicide known as apoptosis. As this apoptotic response of p53 is thought to be the predominant means by which p53 suppresses tumor growth, elucidation of the mechanisms by which p53 activates the apoptosis pathway is of great importance. However, very little is known about the mechanism by which p53 activates apoptosis in mammary cells. Hence, identification of p53 mediators involved in apoptosis in mammary cells is important for determining how p53 suppresses breast cancer. Mouse models are of great value for understanding the pathogenesis and molecular basis of human diseases. The breast tumors that develop in mice lacking the telomerase enzyme have characteristics that are similar to spontaneous breast cancers in humans and therefore serve as a good model system to study breast tumorigenesis. The telomerase enzyme is responsible for maintaining the ends of chromosomes and, in its absence, chromosome ends shorten, leading to activation of the p53 apoptosis response. As a result, cells from telomerase-deficient mice with intact p53 function undergo apoptosis and are tumor-resistant. In contrast, telomerase-deficient mice that also lack p53 have no apoptotic response, allowing rapid tumorigenesis to ensue in the mammary gland and in other tissues. Thus, telomerase deficient mice with differing p53 status provide a good system to investigate the p53-mediated apoptotic pathway in a breast cancer model. In our laboratory, we previously identified genes activated by p53 specifically during apoptosis and not during growth arrest. We have shown that one of these apoptotic-specific genes, PERP, is important for p53 cell death activity. We also identified an additional limited set of apoptotic specific genes based on a global genomic microarray analysis using mouse embryo fibroblasts (MEFs). Objective: The main goal of this proposal is to define critical mediators of p53-dependent apoptosis and tumor suppressor function in mammary epithelial cells using mouse models. Specific Aims (1) To identify p53-dependent apoptotic genes induced specifically in telomerase-deficient mammary epithelial cells by comparing telomerase-deficient cells with and without an intact p53 gene. (2) To select a few genes for further study to determine their ability to induce apoptosis in mammary cells. (3) To generate a knockout mouse model for one candidate apoptosis mediator to characterize its role as a breast cancer tumor suppressor in telomerase deficient mice. Study Design: To identify p53-dependent apoptotic genes specifically induced in telomerase-deficient mammary epithelial cells, the expression profile of genes identified previously by microarray analysis in fibroblasts will be studied in mammary epithelial cells derived from both telomerase-deficient, p53-expressing and telomerase-deficient, p53-deficient mice. Potential mediators of p53 apoptosis function in the mammary gland will be selected for further analysis based on their sequence similarity to known components of the apoptotic pathway and based on their ability to mimic p53 activity in apoptosis in mammary epithelial cells from telomerase-deficient mice lacking p53. Generation of mice lacking one such gene of interest will define the role of that gene in the p53-dependent apoptotic pathway in vivo. The results obtained from this study will help us understand the pathway by which p53 induces apoptosis in mammary cells and thus, will define the mechanism by which p53 suppresses breast cancer. This study has the potential to provide improved models for the therapeutic treatment of breast cancer.