The role of BRCA1 in homologous recombination repair in response to replication stress: significance in tumorigenesis and cancer therapy

Abstract

Germ line mutations in breast cancer gene 1 (BRCA1) predispose women to breast and ovarian cancers. Although BRCA1 is involved in many important biological processes, the function of BRCA1 in homologous recombination (HR) mediated repair is considered one of the major mechanisms contributing to its tumor suppression activity, and the cause of hypersensitivity to poly(ADP-ribose) polymerase (PARP) inhibitors when BRCA1 is defective. Mounting evidence suggests that the mechanism of repairing DNA double strand breaks (DSBs) by HR is different than the mechanism operating when DNA replication is blocked. Although BRCA1 has been recognized as a central component in HR, the precise role of BRCA1 in HR, particularly under replication stress, has remained largely unknown. Given the fact that DNA lesions caused by replication blockages are the primary substrates for HR in mitotic cells, functional analysis of BRCA1 in HR repair in the context of replication stress should benefit our understanding of the molecular mechanisms underlying tumorigenesis associated with BRCA1 deficiencies, as well as the development of therapeutic approaches for cancer patients carrying BRCA1 mutations or reduced BRCA1 expression. This review focuses on the current advances in this setting and also discusses the significance in tumorigenesis and cancer therapy.

Figure 1

DSBs can be repaired by several HR repair pathways including DSBR (double-strand break repair) and SDSA (synthesis-dependent strand annealing). HR is initiated by resection of a DSB to provide 3’ ssDNA overhangs. Strand invasion by these 3’ ssDNA overhangs into a homologous sequence is followed by DNA synthesis at the invading end. After strand invasion and synthesis, the second DSB end can be captured to form an intermediate with two HJs. After gap-repair DNA synthesis and ligation, the structure is resolved at the HJs in a non-crossover (red arrow heads at both HJs) or crossover mode (orange arrow heads at one HJ and red arrow heads at the other HJ). Alternatively, the reaction can proceed to SDSA by strand displacement, annealing of the extended single-strand end to the ssDNA on the other break end, followed by gap-filling DNA synthesis and ligation. The repair product from SDSA is always non-crossover.

Figure 2

Pathways of HR repair at stalled/ collapsed replication forks. (A,B,C) Possible pathways resolving leading-strand blockages by HR. Stalled replication forks can be cleaved by an endonuclease to generate a one-sided DSB (A) which can be repaired by HR and re-establishment of a functional fork. Resolution of the single HJ in the orientation shown by the orange arrows results in SCE. Alternatively, a one sided DSBs can be converted into two sided DSBs by encountering a second replication fork; subsequently two end DSBs trigger HR by formation of double HJs (B). Moreover, uncoupling of lagging-strand synthesis can lead to downstream re-initiation of leading strand synthesis, resulting in a leading strand gap, which can be repaired by HR. In this situation, no DSBs are created (C). (D) Possible pathway resolving lagging strand blockage. Downstream re-initiation of lagging-strand synthesis after blockage leaves a gap on the lagging strand which can be repaired by HR.

Figure 3

DNA lesions caused by PARP inhibitors lead to increased crossovers. DNA breaks are detected by PARP1 and PARP1 is active in response to DNA breaks. In the cells with intact PARP1 activity, the ssDNA is efficiently repaired (A). However, when the PARP1 activity is inhibited, unrepaired ssDNA breaks can be converted into elongated ssDNA (B) or subsequently into DSBs due to replication collapse (C). Both DNA structures stimulate SCE via HR.