Checkpoint Functions of the BRCA1/BARD1 Tumor Suppressor

PUBLIC ABSTRACT

Almost half of the cases of hereditary breast cancer can be attributed to germline mutations of the BRCA1 tumor suppressor gene. Thus, a pressing goal of cancer research is to elucidate the cellular functions of the BRCA1 tumor suppressor gene and determine how loss of these functions promotes breast cancer. The major product of BRCA1 is a polypeptide of 1,863 amino acids that contains two recognizable amino acid motifs: An N-terminal RING domain and two C-terminal BRCT motifs. In living cells, BRCA1 exists as a heterodimer with the BARD1 protein, and by most measures its biological functions appear to be mediated through the BRCA1/BARD1 complex.

BRCA1 plays an especially important role in the cellular response to DNA damage. In particular, it is required for several of the cell cycle checkpoints induced by DNA damage. As a key regulator of the DNA damage response, BRCA1 presumably suppresses tumor formation by preserving genomic stability. However, the molecular mechanisms by which it carries out these functions are not understood and, as a consequence, it is still unclear why inherited mutations of the BRCA1 gene predispose women to breast cancer.

The cell cycle checkpoints induced by DNA damage are particularly common targets for oncogenic lesions in human cancer. Indeed, many of the tumor susceptibility syndromes are determined by inherited mutations in genes that encode components of these checkpoint pathways, including ataxia telangiectasia (ATM gene), Seckel syndrome (ATR gene), AT-like disorder (Mre11 gene), Nijmegen breakage syndrome (NBS gene), Li-Fraumeni syndrome (p53 and Chk2 genes) and Fanconi anemia (FancD2 gene). Therefore, the checkpoint functions of the BRCA1/BARD1 heterodimer are likely to represent an especially important aspect of its tumor suppression activity. Recent studies have established that BRCA1 is essential for several of these checkpoints and identified specific post-translational modifications of BRCA1 that are required for these checkpoints. However, the role of the BARD1 protein in these checkpoints has not been investigated. Here we provide preliminary data implicating BARD1 in at least one of these checkpoints. Moreover, we have developed novel isogenic systems that will allow the checkpoint functions of BARD1 to be dissected at the molecular level. By defining the specific contributions of BARD1 to these pathways, the proposed studies should provide a comprehensive view of the BRCA1/BARD1 tumor suppressor complex and its role in the damage-induced cell cycle checkpoints. In addition, the specific BARD1 phosphorylation required for the intra-S and G2/M checkpoints are likely to represent binding sites for proteins that mediate BRCA1/BARD1 checkpoint functions. The identification of these sites should provide important insights into the molecular mechanisms by which BRCA1 suppresses breast cancer.