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
    Identification of Collaborating Mutations in Brca1 and Brca2 Mouse Mammary Tumors by Genome-Wide Screening for Chromosomal Imbalances

    Scientific Abstract:
    Background: Approximately five percent of the women who develop breast cancer carry a germ-line mutation in the BRCA1 or BRCA2 breast cancer susceptibility genes. To understand how the loss of BRCA1 or BRCA2 function leads to tumor formation, mouse genetic models for BRCA1 or BRCA2 mutations have been generated. Unfortunately, Brca1 and Brca2 homozygous deletions cause early embryonic lethality, and heterozygous females are not tumor prone. To understand how the loss of BRCA1 or BRCA2 function leads to tumor formation, we have generated Brca1, Brca2, and p53 conditional knockouts, and used combinations of these mice together with an epithelial Cre transgenic line (Kcre) to generate compound mutant mice that develop mammary tumors with high incidence. Using these mice, we were able to show that BRCA1/BRCA2 and p53 loss-of-function effectively collaborate in mammary tumorigenesis, indicating a pivotal role of the p53 pathway in BRCA1- or BRCA2-associated breast cancer. We have now collected a series of approximately 150 mouse mammary tumors derived from Kcre;Brca1;p53 , Kcre;Brca2;p53 , and Kcre;p53 homozygous conditional knockout females. In addition, we have developed a high-resolution array-based comparative genomic hybridization (CGH) method for genome-wide analysis of DNA copy number changes in murine tumors. Objective: Although the vast majority of hereditary breast and ovarian tumors shows loss of BRCA1 or BRCA2, both factors are essential for normal cell proliferation, and their inactivation leads to cell death or a severe growth impairment, even in the absence of functional p53. By necessity, this lethality is somehow suppressed during BRCA-associated tumorigenesis, resulting in extreme genomic instability caused by BRCA loss of function and the concomitant arisal of tumor-promoting mutations. Hence, the acquisition of suppressor mutations that allow cells to survive in the absence of functional BRCA1 or BRCA2 may be a key step in BRCA-associated oncogenesis. The objective of this project is to identify these suppressor mutations by searching for consistent chromosomal imbalances in Brca1 and Brca2 mammary tumors. Specific Aims: (1) To perform array-CGH analysis on well-matched groups of mouse mammary tumors. This analysis will reveal signatures that are specific for BRCA1 and/or BRCA2 loss-of-function. (2) To compare the CGH profiles to CGH profiles from human BRCA1 or BRCA2 tumors. (3) To identify candidate cancer genes that are specifically amplified or deleted in the Brca1 or Brca2 tumors, and, therefore, represent potential suppressor mutations that bypass cell cycle arrest induced by loss of BRCA function. (4) To validate these potential suppressor mutations in a mouse BRCA1/2 tumor model. Study design: We will perform high-resolution array-CGH analysis on a large series of well-matched Brca1, Brca2, and control mouse mammary tumors. We will use the CGH profiles to identify recurrent regional amplifications and deletions. The minimal regions of amplification or deletion will be determined CGH analysis with tile-path arrays. By combining the mouse CGH data with the available gene expression profiles of the corresponding mouse tumors, we will further refine our search and reduce the number of candidate genes to the level where they can be tested individually in an established mouse mammary gland fat-pad transplantation system with controlled inactivation of BRCA1/2 and p53. Potential Outcomes and Benefits of the Research: The proposed studies may lead to the discovery of cancer genes and pathways that are specifically mutated in BRCA-associated tumorigenesis, and might ultimately result in the identification of new markers and therapeutic targets for diagnosis, prevention and/or treatment of human BRCA1 or BRCA2 tumors.

    Lay Abstract:
    Approximately five percent of the women who develop breast cancer carry an inherited mutation in one of the breast cancer susceptibility genes BRCA1 or BRCA2. The proteins produced by these genes play an important role in the repair of broken DNA. Even though most hereditary breast tumors arise because of the loss of BRCA1 or BRCA2 function, both factors are essential for proper function of normal cells. These cells stop growing or even die in the absence of functional BRCA protein. This growth-inhibitory effect cannot be suppressed by inactivation of the p53 tumor suppressor, which is lost in most tumors from BRCA1 or BRCA2 mutation carriers. Obviously, the growth-inhibitory effect of BRCA loss is somehow suppressed during BRCA-associated tumor development, resulting in a rapid accumulation of DNA mutations, including those that promote tumor growth. Hence, a key step in BRCA-associated cancer development may be the arisal of genetic mutations that allow cells to survive in the absence of functional BRCA1 or BRCA2 protein. The objective of this project is to identify these suppressor mutations by searching for DNA amplifications or deletions in Brca1 and Brca2 mouse mammary tumors. We have produced these tumors by generating mouse strains with mammary gland-specific deletion of Brca1 and p53 (or Brca2 and p53). Indeed, the females from these strains of mice develop mammary tumors with high incidence, demonstrating that combined loss BRCA and p53 is needed for mammary tumorigenesis. We have now collected a series of approximately 150 mouse mammary tumors derived from Brca1;p53 and Brca2;p53 females, as well as from p53 control females. In addition, we have developed a method for high-resolution, genome-wide analysis of DNA amplifications and deletions in mouse tumors. The specific aims of this proposal are: (1) To look for DNA amplifications and deletions in well-matched groups of mouse mammary tumors. This analysis will reveal signatures that are specific for BRCA1 and/or BRCA2 loss-of-function. (2) To compare the results from the mouse tumors to data from human BRCA1 or BRCA2 tumors. (3) To identify candidate cancer genes that are specifically amplified or deleted in the Brca1 or Brca2 tumors, and, therefore, could represent mutations that suppress the growth inhibition induced by loss of BRCA function. (4) To validate these potential suppressor mutations in a mouse BRCA1/2 tumor model. The proposed studies may lead to the discovery of cancer genes that are specifically mutated in BRCA-associated tumorigenesis, and might ultimately result in the identification of new markers and therapeutic targets for diagnosis, prevention and/or treatment of human BRCA1 or BRCA2 tumors. Targeted therapies for BRCA-associated breast cancer are eagerly awaited, in view of the severe physical and psychological impact of prophylactic bilateral mastectomy or oophorectomy, which are currently used as preventive treatment.