BRCA1 and BRCA2 are tumor suppressor genes whose protein products play a significant role in the maintenance of chromosomal stability during cell division (6). Therefore, BRCA1 and BRCA2 are classified as caretaker tumor suppressor genes. This section will look more in depth into the role of BRCA1 and BRCA2 as tumor suppressor genes, while Chapter 15 explicitly lays out the role these two genes play in breast cancer susceptibility.
Breast Cancer type 1 Susceptibility Protein (BRCA1)
BRCA1 with 24 exons, is localized to 17q and encodes a 1863 amino acid protein with multiple functional domains including a conserved N-terminal RING finger, two nuclear localization signals (NLS), and a C-terminal BRCT domain (3). In addition to its role in chromosomal stability, BRCA1 has been implicated in DNA replication, cell cycle checkpoint control, transcription regulation, apoptosis, and chromatin unfolding (1). This wide array of functionality has been attributed to the vast network of interaction between BRCA1 and tumor suppressors, oncogenes, DNA repair proteins, regulators, and transcription factors (3).
Part of the BRCA response to DNA damage involves activation of checkpoints: the G1/S checkpoint prevents cell entry into the division cycle, the S phase checkpoint prevents replication of damaged DNA and the G2/M checkpoint prevents initiation of mitotic cells in cells with damaged DNA (3). The phosphorylation of BRCA1 whilst bound to BARD1 (BRCA1-associated RING domain protein 1) results in phosphorylation of p53 and a signalling pathway terminating in G1/S checkpoint activation through p21 (1). Alternatively, BRCA1 may follow a p53 independent pathway terminating in p21-linked checkpoint activation (3). In line with these findings, expression of BRCA1 is highest during the G1 phase (7). The role of BRCA1 in G2/M checkpoint activation appears to be dependent upon it’s interaction with Chk1, a known regulator of the G2/M phase, as well as various Cdc2/CyclinB kinase inhibitors (3). Despite these findings, the role of BRCA1 in S phase is, as of yet, uncertain (1).
Breast Cancer type 2 Susceptibility Protein (BRCA2)
BRCA2 with 27 exons, is localized to chromosome 13q and encodes a 3418 amino acid protein with an N-terminal transactivation domain, a BRC repeat motif, a C-terminal DNA binding domain and 2 NLSs. The DNA binding domain binds single and double stranded DNA while the BRC repeat motif binds RAD51, which is important at sites that require DNA repair (1). Cells with mutations in the BRC domain fail to draw RAD51 to sites of DSBs or mount repair efforts (1). Contrary to the overwhelming functionality of BRCA1, only BRCA2 has been implicated in facilitating homologous recombination (HR) as part of the DNA damage response (DDR) (6).
HR is a process involving the use of undamaged sister chromatids as a template in the repair of replication-associated DNA double strand breaks (DSBs) (1). Animation of this process can be found here. Genome stability is primarily regulated by BRCA genes through this process. DSBs pose a significant danger to the cell because both strands are simultaneously destroyed. BRCA1 and BRCA2 act as mediators in the HR pathway, interacting with effector RAD51, which recognizes and binds to DNA at the break site, forming a protein sheath (1). BRCA2 specifically transports RAD51 to the site and BRCA1 is thought to activate this effector molecule (2). However the two proteins are indirectly connected through PALB2, whose initial binding to BRCA1 is essential for the recruitment of BRCA2 and RAD51 as well as the loading of RAD51 on DNA strands (2). The overlapping function of the BRCA proteins, centering on RAD51 activity, has been demonstrated in studies of BRCA-deficient mouse embryos. In such cases, the embryos display the same phenotype as that of embryos with mutations in RAD51 (8). A very comprehensive review of HR can be found starting at 13:46 of this lecture by Jim Haber of the Department of Biology at Brandeis University.