BRCA1 and its toolbox for the maintenance of genome integrity

Abstract

The breast and ovarian cancer type 1 susceptibility protein (BRCA1) has pivotal roles in the maintenance of genome stability. Studies support that BRCA1 exerts its tumour suppression function primarily through its involvement in cell cycle checkpoint control and DNA damage repair. In addition, recent proteomic and genetic studies have revealed the presence of distinct BRCA1 complexes in vivo, each of which governs a specific cellular response to DNA damage. Thus, BRCA1 is emerging as the master regulator of the genome through its ability to execute and coordinate various aspects of the DNA damage response.

Figure 1. BRCA1 domain organization and interaction partners

The human breast and ovarian cancer type 1 susceptibility gene (BRCA1) encodes a protein of 1,863 amino acids. BRCA1 has two highly conserved motifs at its termini: the RING domain and tandem BRCT domains. The RING domain sequence encodes a folded protein structure and confers the E3 ubiquitin ligase activity of BRCA1, and the BRCT domains bind phosphorylated proteins that are primarily involved in the DNA damage response. BRCA1 exists as a stable heterodimer through its RING-mediated interaction with BRCA1-associated RING domain protein 1 (BARD1), which is inhibited by BRCA1-associated protein 1 (BAP1). The sites at which other BRCA1-interacting proteins bind are also shown. Important clinical mutations routinely used in research settings are indicated, such as Cys61Gly and Cys64Gly, which target the RING domains, and Met1775Arg, which disrupts the BRCA1 BRCT domain interaction with phosphorylated proteins and the accumulation of BRCA1 at damage-induced foci. The IlE26Ala mutation abrogates the BRCA1 E3 ubiquitin ligase activity and is predicted to maintain the RING structure. In addition, mutations from patients with cancer at conserved residues Leu1407Pro and Met1411Thr on the BRCA1 coiled-coil domain were found to abrogate its interaction with partner and localizer of BRCA2 (PALB2; also known as FANCN) and to compromise homologous recombination-mediated DNA repair. BACH1, BRCA1-interacting protein carboxy-terminal helicase 1; CtIP, CtBP-interacting protein; NLS, nuclear localization sequence.

Figure 2. BRCA1 participates in DNA damage signalling

A model for targeting the breast and ovarian cancer type 1 susceptibility protein (BRCA1) to damage-induced foci at DNA double-stranded breaks that involves a ubiquitin-dependent signal transduction pathway. The current hypothesis is that DNA damage (1) first triggers the ataxia telangiectasia mutated (ATM)–ataxia telangiectasia and RAD3-related protein (ATR)-dependent phosphorylation of histone variant H2AX (2). Subsequently, phosphorylated H2AX directly recruits mediator of DNA damage checkpoint 1 (MDC1), which in turn promotes the accumulation of the E3 ubiquitin ligase complex RING finger protein 8 (RNF8)–ubiquitin-conjugating enzyme 13 (UBC13; also known as UBE2N) to mediate ubiquitylation of histones and other yet-to-be-identified substrates at or near the sites of DNA breaks (3). The newly identified E3 ubiquitin ligase RNF168 recognizes ubiquitylated histones and, in concert with UBC13, amplifies the local ubiquitylation events involving Lys63-linked ubiquitin chains (4). These ubiquitin chains are recognized by the ubiquitin-interacting motif (UIM)-containing receptor-associated protein 80 (RAP80; also known as UIMC1), which recruits the BRCA1A complex (including breast cancer type 1 susceptibility protein (BRCA1)) to DNA double-stranded breaks (5). Chromatin retention of BRCA1 promotes checkpoint kinase 1 (CHK1; also known as CHEK1) phosphorylation and G2–M checkpoint activation. Ub, ubiquitin.

Figure 3. The connection between the Fanconi anaemia pathway and the BRCA1-linked homologous recombination repair pathway

Repair of inter-strand DNA cross links can involve both Fanconi anaemia (FANC) and breast cancer susceptibility (BRCA) proteins. When the replication machinery encounters cross-linked DNA, replication fork stalling and/or collapsing occurs (1). This type of replication stress might signal for the monoubiquitylation of FANCD2–FANCI and its consequent association with chromatin, which is mediated by the Fanconi anaemia core complex (FANCA–FANCC, FANCE–FANCG, FANCL and FANCM) (2). It is thought that the Fanconi anaemia core and FANCD2–FANCI may be involved in the repair or removal of DNA cross links. Subsequently, the BRCA1 core complex (BRCA1, BRCA2 (also known as FANCD1), partner and localizer of BRCA2 (PALB2; also known as FANCN) and BRCA1-interacting protein carboxy-terminal helicase 1 (BACH1; also known as FANCJ and BRIP1)) would be loaded onto sites of DNA damage (3). This would facilitate the accumulation and nucleation of RAD51 filaments (4), ultimately leading to the repair of inter-strand DNA cross links and the subsequent reinitiation of DNA replication (5).