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
    Cell Cycle Checkpoint Functions of the BRCA1/BARD1 Tumor Suppressor Complex

    Scientific Abstract:
    A pressing goal of breast cancer research is to determine how loss of BRCA1 function promotes familial breast cancer. While BRCA1 is involved in multiple biological processes, it plays an especially important role in the cellular response to DNA damage. In particular, BRCA1 is essential for both the intra-S and G2/M cell cycle checkpoints induced by ionizing radiation (IR). Moreover, activation of these checkpoints requires IR-induced phosphorylation of BRCA1 by the ATM kinase. In vivo, BRCA1 exists as a stoichiometric heterodimer with the BARD1 protein and by most measures its biological functions are mediated through the BRCA1/BARD1 complex. Our preliminary data indicate that BARD1 is phosphorylated by ATM and required for the IR-induced intra-S checkpoint. Given the functional interdependence of BRCA1 and BARD1, we propose that BARD1 is also an essential component of the IR-induced cell cycle checkpoints and that ATM-dependent phosphorylation of BARD1 is required for enforcement of these checkpoints. To test this hypothesis, we will (1) identify the damage-induced phosphorylation sites of BARD1, (2) determine whether these sites are required for the IR-induced intra-S checkpoint, and (3) determine whether these sites are also essential for the IR-induced G2/M checkpoint. The phosphorylation sites of BARD1 will be identified by site-directed mutagenesis, mass spectrometric analysis, and reactivity to phospho-specific antibodies. Each site will then be evaluated for its involvement in the intra-S checkpoint by testing radioresistant DNA synthesis in isogenic cells lines that do or do not express BARD1 polypeptides bearing specific amino acid mutations. The BARD1 phosphorylation sites will also be examined for their involvement in the G2/M checkpoint by measuring mitotic entry after IR treatment in the same panels of isogenic lines. By defining the role of BARD1 in these processes, the proposed studies should elucidate the checkpoint functions of the BRCA1/BARD1 tumor suppressor complex as a whole and set the stage for mechanistic studies of checkpoint activation by the BRCA1/BARD1 complex.

    Lay Abstract:
    Germline mutations of the BRCA1 tumor suppressor gene constitute the most common cause of familial breast cancer. Thus, a pressing goal of breast cancer research is to elucidate the biological functions of the BRCA1 protein and to determine how loss of these functions promotes breast cancer. In living cells, BRCA1 exists as a in association with the BARD1 protein, and by most measures its biological functions appear to be mediated through the BRCA1/BARD1 complex. The cell cycle checkpoints induced by DNA damage are particularly common targets for oncogenic lesions in human cancer. Indeed, many tumor susceptibility syndromes are determined by inherited mutations in genes required for these checkpoints, including ataxia telangiectasia (ATM gene), Seckel syndrome (ATR), AT-like disorder (Mre11), Nijmegen breakage syndrome (NBS), Li-Fraumeni syndrome (p53 and Chk2) and Fanconi anemia (FancD2). Recent studies have established that BRCA1 is also an essential mediator of several critical cell cycle checkpoints, including the intra-S and G2/M checkpoints induced by ionizing radiation (IR), and that activation of these checkpoints requires IR-induced phosphorylation of BRCA1 by the ATM kinase. However, the role of the BARD1 protein in checkpoint regulation has not been investigated. Given the functional interdependence of BRCA1 and BARD1, we propose that BARD1 is also an essential component of the IR-induced checkpoints and that ATM-dependent phosphorylation of BARD1 is required for enforcement of these checkpoints. To test this hypothesis, we will (1) identify the damage-induced phosphorylation sites of BARD1, (2) determine whether these sites are required for the IR-induced intra-S checkpoint, and (3) determine whether these sites are also essential for the IR-induced G2/M checkpoint. By defining the role of BARD1 in these processes, the proposed studies should provide a comprehensive understanding of cell cycle checkpoint control by the BRCA1/BARD1 complex and yield new insights into the mechanism of BRCA1-mediated tumor suppression.