Moonlighting at replication forks – a new life for homologous recombination proteins BRCA1, BRCA2 and RAD51

Abstract

Edited by Wilhelm JustCoordination between DNA replication and DNA repair ensures maintenance of genome integrity, which is lost in cancer cells. Emerging evidence has linked homologous recombination (HR) proteins RAD51, BRCA1 and BRCA2 to the stability of nascent DNA. This function appears to be distinct from double-strand break (DSB) repair and is in part due to the prevention of MRE11-mediated degradation of nascent DNA at stalled forks. The role of RAD51 in fork protection resembles the activity described for its prokaryotic orthologue RecA, which prevents nuclease-mediated degradation of DNA and promotes replication fork restart in cells challenged by DNA-damaging agents. Here, we examine the mechanistic aspects of HR-mediated fork protection, addressing the crosstalk between HR and replication proteins.Keywords: DNA recombination; DNA replication; genome stability BRCA1, BRCA2, RAD51 and the RAD51 paralogs family, which consists of five proteins (RAD51B, RAD51C, RAD51D, XRCC2 and XRCC3) in mammalian cells, are required to repair DNA damage by homologous recombination (HR). Mutations in most of these genes predispose to cancer, indicating an important role of DNA damage repair in preventing cell transformation. Intriguingly, complete loss of function of most of HR proteins is incompatible with life in vertebrate organisms [1,2,3]. These features together with their ability to form foci in unperturbed and challenged S-phase nuclei indicate a role for HR proteins in chromosomal DNA replication even in unchallenged conditions [4,5].The mechanisms underlying the function of DNA repair proteins in unchallenged chromosomal DNA replication are poorly understood. This is in part due to the fact that many of the genes involved in DNA metabolism are essential for cell viability, which complicate their study, especially in higher eukaryotes [1,2,3]. The reasons why HR genes are essential for cell viability in higher eukaryotes are unclear. One explanation might be that they have a specific and direct role in the replication of complex eukaryotic genomes. Alternatively, complex genomes might be more vulnerable to spontaneous DNA damage, which might irreversibly halt replication progression inducing chromosomes breakage. HR factors might be therefore required to repair the damage and to complete whole genome duplication. Here we review the links between HR proteins and the DNA replication machinery, some of which appear to be conserved between prokaryotes and eukaryotes.

Homology-Directed Repair and the Role of BRCA1, BRCA2, and Related Proteins in Genome Integrity and Cancer

“…More recently, BRCA1 and BRCA2, as well as FA proteins, were discovered to be required for replication fork protection, a pathway that can be genetically separable from HDR (Kolinjivadi et al 2017; Schlacher et al 2011, 2012) (Figure 3). Under replication stress conditions leading to fork stalling, BRCA/FA proteins prevent the degradation of newly synthesized DNA strands by the MRE11 nuclease (Schlacher et al 2011, 2012; Ying et al 2012).…”

BRCA1 and BRCA2 mutations and treatment strategies for breast cancer

“…Recent work demonstrates that women with abnormal PALB2 levels have a 14% risk of developing cancer until 50 years old, and 35% risk until 70 years old [61]. Mutations in RAD51 have also been identifed [62]. A recent study utilizing a focused panel of 25 genes sequenced in more than 35,000 women with breast cancer demonstrated pathogenic variants were present in 9.3% of the tested population.…”

RADX Promotes Genome Stability and Modulates Chemosensitivity by Regulating RAD51 at Replication Forks

“…It may also promote strand invasion to restart a replication fork after cleavage by structure-specific nucleases like MUS81 (Sarbajna and West, 2014). The fork protection and HDR functions of RAD51 depend on BRCA2, which assists in replacing RPA with RAD51 at resected DSBs and stabilizes RAD51 at stalled forks (Kolinjivadi et al, 2017). Thus, BRCA2-deficiency causes genomic instability and cancer predisposition due to a failure of HDR and fork degradation (Kass et al, 2016).…”