**4. BRCA1 and transcriptional regulation**

208 DNA Repair

CtIP, containing the consensus sequence pSer-X-X-Phe) that is involved mainly in the control of the G2/M phase checkpoint and DNA damage repair (Varma et al., 2005; Williams et al., 2004). Several cancer-predisposing mutations in the BRCA1-BRCT domain resulted in destabilization of the structural integrity at the BRCT active sites, and abolished their affinities to synthetic BACH1 and CtIP phosphopeptides (Rowling et al., 2010). These findings provide a better understanding of the pathogenic BRCA1 mutations on functional

A substantial amount of evidence that has implicated BRCA1 in the DNA damage repair pathways has been documented. BRCA1 co-localizes with RAD51 and BARD1 to nuclear foci (sites associated with repair of DNA caused by the damaging agents or γ-irradiation) (Hashizume et al., 2001; Scully et al., 1997). The nuclear foci is marked by the histone variant H2AX that was phosphorylated on Ser139 (known as γH2AX) (Rogakou et al., 1998). γH2AX is one of the initial recruiting factors for various checkpoints and DNA repair proteins, including Abraxas, RAP80, and BRCA1, at sites of DNA breaks (Foulkes, 2010). The H2AX signaling cascade begins to emerge with the finding that MDC1 (mediator of DNA damage checkpoint 1) is the main downstream factor in the pathway, and is required for the damage-induced focal accumulation of a number of DNA damage repair factors at the DNA

BRCA1 plays a role in maintaining genome integrity through its role in DNA damage repair. Several observations have implicated BRCA1 in homologous recombination (HR), non-homologous end-joining (NHEJ and nucleotide excision repair (NER). A role for BRCA1 in HR-mediated repair is involved through its stable complex formation with BRCA2, which has a well-defined role in HR through its direct interaction with RAD51 (Bhattacharyya et al., 2000). RAD51 (the mammalian homolog of the Escherichia coli RecA protein) is a DNA recombinase that catalyzes strand exchange in an early step of HR (Baumann et al., 1996). PALB2 (the partner and localizer of BRCA2) has recently been identified as the bridging factor required for the BRCA1-BRCA2 association (Rahman et al., 2007). The BRCA1-PALB2 interaction was mediated by their respective coiled-coil domains, and was found to promote HR-mediated repair (Rahman et al., 2007). Importantly, missense mutations identified in the PALB2-binding region on BRCA1 disrupted the specific interaction of BRCA1 with PALB2, and compromised DNA repair in a gene conversion assay (Sy et al., 2009). Although these studies have revealed a molecular link between BRCA1 function and HR-mediated repair, the mechanism by which BRCA1 promotes HR

As an alternative to HR, there is a growing body of evidences, to indicate that a component of NHEJ is regulated by BRCA1. The exact role of BRCA1 in NHEJ, however, has not been well defined (Zhang & Powell, 2005). In the NHEJ pathway, the DNA-dependent protein kinase catalytic subunit (DNA-PKcs) and a Ku heterodimer of Ku80 and Ku70 are recruited to the sites of DNA DSBs for preparing the DNA ends before ligation by the XRCC4 ligase IV. The most possible explanation for BRCA1 being involved in NHEJ is its association with a NHEJ factor Ku80 (Chiba & Parvin, 2001; Wei et al., 2008). Many studies have provided strong evidences that the NHEJ pathway was impaired, both in vivo and in vitro, in BRCA1-deficient mouse embryonic fibroblasts and in the human breast cancer cell line HCC1937 which carries

a homozygous mutation in the BRCA1 gene (Bau et al., 2004; Zhong et al., 2002).

mechanisms and tumorigenesis.

breaks (Stucki et al., 2005).

**3. BRCA1 and DNA damage repair** 

through the PALB2-BRCA2-RAD51 axis remains unclear.

As described earlier, BRCA1 contains a C-terminal transactivation domain as was first defined using the yeast two-hybrid system (Chapman & Verma, 1996; Monteiro et al., 1996). The transactivation domain was mapped to the region of the protein encoded by exons 21- 24 using deletion constructs of BRCA1 fused to the GAL4 DNA binding domain. The BRCA1-BRCT domain has been implicated in the regulation of transcription of several genes responsible for DNA damage. The ability of BRCA1 to act as either a co-activator or a corepressor of transcription may involve its ability to recruit the basal transcriptional machinery and other proteins that have been implicated in chromatin remodeling (Mullan et al., 2006). BRCA1 was capable of activating the p21 promoter (Somasundaram et al., 1997). One report claimed that BRCA1 participated in the stabilization of p53 in response to DNA damage, and served as a co-activator for p53 (Zhang et al., 1998). The interaction of BRCA1 and p53 potentially resulted in the redirection of p53-mediated transactivation from a proapoptotic target to genes involved in DNA repair and cell cycle arrest (Zhang et al., 1998). In addition, BRCA1 has been shown to interact with the RNA polymerase II holoenzyme (Scully et al., 1997). However, BRCA1 could repress the transcription of an estrogen receptor α (ERα) and its downstream estrogen responsive genes (Fan et al., 1999). The transcriptional repression activity of BRCA1 for ERα occurs by the association of the N-terminus of BRCA1 (residues 1-300) with the C-terminal activation function (AF-2) of ERα. Breast cancerassociated mutations of BRCA1 were found to abolish its ability to inhibit ERα activity (Fan et al., 2001). The repression activity exerted by BRCA1 involved the ability of BRCA1 to down-regulate levels of the transcriptional coactivator p300, which has also been shown to interact with the AF-2 domain of ERα (Fan et al., 2002). Further investigations revealed that overexpression of BRCA1 could inhibit the recruitment of the co-activators [steroid receptor co-activator 1 (SRC1), and amplified breast cancer 1 (AIB1)], and enhanced the recruitment of a co-repressor [histone deacetylase 1 (HDAC1)] to the progesterone response elements (PRE) of c-Myc.
