BRCA1 tumor suppressor network: focusing on its tail

Germline mutations of the BRCA1 tumor suppressor gene are a major cause of familial breast and ovarian cancer [1,2]. BRCA1 plays critical roles in a number of diverse cellular processes that ensure genome integrity and the increase risk of breast and ovarian cancer caused by mutation of BRCA1 has been attributed to increased genomic instability. To safeguard genome, cells have evolved a defensive mechanism, called the DNA damage response (DDR), to coordinate multiple cellular responses including DNA repair, cell cycle checkpoint regulation, transcription, senescence or apoptosis etc., to counteract genotoxic stress [3-6]. BRCA1 appears to act as a central mediator of the cellular response to DNA damage that regulates the activities of multiple repair and checkpoint pathways [3,5,7-10]. BRCA1 is a substrate of the central DNA damage response kinases ATM/ATR that control the DDR. It is required for homology directed repair, a pathway that facilitates error-free repair of double-strand breaks (DSBs) and resolution of stalled DNA replication forks through homologous recombination (HR) [9-11] as well as postreplicative repair in response to UV damage [12]. Recently it is suggested that much of BRCA1's role in maintaining genome stability is accounted for by its role in maintaining heterochromatin integrity via H2A ubiquitination [13].BRCA1 associates with multiple repair proteins and cell cycle regulators and such a capability to form multiple protein complexes contributes to its role in maintaining chromosome stability and tumor suppression (Figure 1). BRCA1 is a large protein of 1,863 amino acids. It contains two important domains at each end of the protein, a RING domain at the N-terminus and two BRCT domains at the C-terminus. Many clinically important mutations of BRCA1 gene frequently target these two domains. BRCA1 dimerizes with BARD1 through the RING domain present on each or the protein, forming an ubiquitin E3 ligase [14,15]. Earlier studies suggested that the