The BRCA Tumor Suppressor Network in Chromosome Damage Repair by Homologous Recombination

Abstract

Mutations in the BRCA1 and BRCA2 genes predispose afflicted individuals to breast, ovarian, and other cancers. The BRCA-encoded products form complexes with other tumor suppressor proteins and with the recombinase enzyme RAD51 to mediate chromosome damage repair by homologous recombination and also to protect stressed DNA replication forks against spurious nucleolytic attrition. Understanding how the BRCA tumor suppressor network executes its biological functions would provide the foundation for developing targeted cancer therapeutics, but progress in this area has been greatly hampered by the challenge of obtaining purified BRCA complexes for mechanistic studies. In this article, we review how recent effort begins to overcome this technical challenge, leading to functional and structural insights into the biochemical attributes of these complexes and the multifaceted roles that they fulfill in genome maintenance. We also highlight the major mechanistic questions that remain.

Keywords: BARD1; BRCA1; BRCA2; DNA damage repair; genome maintenance; homologous recombination; replication fork protection; replication fork repair; tumor suppression.

Figure 1.

DNA double-strand break repair by homologous recombination. Emphasis is placed on the roles of BRCA1-BARD1 in DNA end resection and strand invasion and on the BRCA mediator complex (DSS1-BRCA2-PALB2-BRCA1-BARD1) in presynaptic filament formation. Note that many other factors that function in different stages of the recombination reaction are not shown (see Table 1 for the list of uncited factors). The three pathways of D-loop resolution are briefly discussed in Section 4.4 and in detail in recent reviews (8, 93). The four subunits of Shieldin are REV7 (MAD2L2), SHLD1 (C20orf196), SHLD2 (FAM35A), and SHLD3 (CTC534A2.2). The three subunits of the CST complex are CTC1, SNT1, and TEN1.

Figure 2.

Structure of the RAD51-ssDNA presynaptic filament and RAD51-dsDNA post-synaptic complex. (A, D) Cartoon illustrations of the presynaptic filament (A) and post-synaptic complex (D); RAD51 is shown in yellow and DNA is depicted as dashed lines. (B, E) Atomic structure of the presynaptic filament (B) and post-synaptic complex (E), the invading ssDNA is shown in red and complementary strand in blue. (C, F) RAD51-DNA interactions in the presynaptic filament (C) and post-synaptic complex (F). Base triplets interacting with V273 in RAD51 DNA binding Loop 2 in the presynaptic filament (C), and base pair triplets with R235 in DNA binding Loop 1 in the post-synaptic complex (F) are highlighted. See reference for details.

Figure 3.

BRCA1-BARD1 structure and function. (A) Cartoon illustrating the role of BRCA1-BARD1 in the promotion of paranemic and plectonemic joint formation (89). BRCA1-BARD1 aids in the capture of the recombining duplex molecule to facilitate paranemic joint formation and the assembly of the triple-stranded synaptic complex. Subsequent plectonemic joint formation yields the D-loop intermediate. (B) Functional motifs and domains in BRCA1 (B) and BARD1 (C) including the RING (Really Interesting New Gene) domain (PDB: 1JM7), CC (Coiled Coil) domain, ANK (Ankyrin) motifs (PDB: 3C5R), and BRCT (BRCA1 Carboxy-Terminal) repeats (PDB: 1T2U for BRCTs of BRCA1; PDB: 2NTE for BRCTs of BARD1). The known atomic structures are linked by the dash lines depicting protein regions of which no structural information is available.

Figure 4.

BRCA2 structure and function. (A) Functional motifs and domains in BRCA2. The BRC (Breast Cancer) repeats, HD (helical domain), NTD (N-terminal DNA Binding Domain), DBD (DNA Binding Domain) comprising three OB (oligonucleotide/oligosaccharide binding) folds, and CTRB (C-terminal RAD51-binding) domain are shown. The HD and a portion of OB1 constitute the interface for association with DSS1 that physically interacts with RPA via a solvent-exposed acidic loop (77). The known atomic structures (PDB: 3EU7,1N0W and 1MIU) are linked by the dash lines depicting protein regions of which no structural information is available. (B) Schematic of the BRCA2-derived polypeptide composed of BRC3, BRC4 and the DBD. (C) Structure of the BRCA2-DSS1 complex ((123); PDB: 1IYJ)) with the BRCA2 parts in green. Residues in the solvent-exposed loop of DSS1 targeted for mutagenesis in the construction of a mutant defective in RPA interaction are in red (77). (D) Cartoon illustrating the HR mediator function of BRCA2-DSS1 in which BRCA2 provides the DNA binding and RAD51 interaction attributes, while DSS1 facilitates presynaptic filament assembly through its ability to interact with RPA and to act as a DNA mimetic to attenuate the affinity of RPA for ssDNA (77).