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BRCA2: a Universal Recombinase Regulator

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BRCA2: a Universal Recombinase Regulator Oncogene (2007) 26, 7720–7730 & 2007 Nature Publishing Group All rights reserved 0950-9232/07 $30.00 www.nature.com/onc REVIEW BRCA2: a universal recombinase regulator T Thorslund and SC West London Research Institute, Cancer Research UK, Clare Hall Laboratories, South Mimms, Herts, UK Homologous recombination has a dual role in eukaryotic In yeast, it has been shown that a single un-repaired organisms. Firstly, it is responsible for the creation of double-strand break (DSB) in DNA can lead to cell death, genetic variability during meiosis by directing the forma- indicating that DSBs pose one of the most dangerous tion of reciprocal crossovers that result in random forms of DNA lesions (Bennett et al., 1993). As a combinations of alleles and traits. Secondly, in mitotic consequence, eukaryotic cells contain several DNA repair cells, it maintains the integrity of the genome by promoting pathways that can specifically recognize DSBs and initiate the faithful repair of DNA double-strand breaks (DSBs). the efficient repair of the DNA backbone (Figure 1). There In vertebrates, it therefore plays a key role in tumour are two primary mechanisms of DSB repair in vertebrates, avoidance. Mutations in the tumour suppressor protein known as non-homologous end joining and homologous BRCA2 are associated with predisposition to breast and recombination (HR). Non-homologous end joining occurs ovarian cancers, and loss of BRCA2 function leads to by a relatively simple end-splicing mechanism, but is genetic instability. BRCA2 protein interacts directly with potentially error-prone and can lead to the loss of genetic the RAD51 recombinase and regulates recombination- information (Hoeijmakers, 2001). It is primarily utilized mediated DSB repair, accounting for the high levels of during the G1 phase of the cell cycle. In contrast, DSB spontaneous chromosomal aberrations seen in BRCA2- repair by HR is generally error free and functional during defective cells. Recent observations indicate that BRCA2 the S and G2 phases. The difference in cell cycle timing also plays a critical role in meiotic recombination, this time reflects the role that HR plays in the repair of collapsed through direct interactions with the meiosis-specific recom- replication forks, whereas both HR and non-homologous binase DMC1. The interactions of BRCA2 with RAD51 end joining can promote the repair of DSBs caused by and DMC1 lead us to suggest that the BRCA2 tumour ionizing radiation (IR) or genotoxic stress. HR therefore suppressor is a universal regulator of recombinase actions. plays a critical role in the maintenance of genome integrity Oncogene (2007) 26, 7720–7730; doi:10.1038/sj.onc.1210870 in replicating cells. Two simplified mechanisms for HR are shown in Keywords: tumour suppressor; RAD51; DMC1; DNA Figure 1. The process is initiated by the resection of the repair; genome instability; meiotic recombination DSB termini to expose protruding 30-single-stranded DNA (ssDNA) tails, to which the essential recombinase RAD51 will bind and formactive nucleoprotein filaments (Figure 1c). RAD51 loading is facilitated by the ssDNA-binding protein RPA and by RAD52. The The dual functions of recombination RAD51 nucleoprotein filament mediates a search for homology within the sister chromatid, which in turn The structural integrity of DNA is continually being serves as a template for DNA synthesis to restore any challenged due to the presence of physical and chemical lost sequences at the break site and ultimately regenerate carcinogens in our environment. In addition to DNA intact DNA (West, 2003; Sung and Klein, 2006). The lesions caused by exogenous agents, DNA suffers tumour suppressor and breast cancer-susceptibility gene spontaneous decay, replication errors, and oxidative BRCA2 is required for normal levels of HR-mediated and other damages that result from normal metabolic DSB repair (Yu et al., 2000; Moynahan et al., 2001), processes (Lindahl, 1993). The repair of damaged DNA providing an important link between the ability of a cell is therefore crucial for the maintenance of genome to maintain genome stability and tumorigenesis. integrity, and as a result, all organisms have evolved a The creation of a DSB in DNA is not always an wide variety of DNA repair pathways that can restore accidental and problematic event. A large number of DNA structure and the information encoded within it DSBs are deliberately created in the DNA of germ-line (Hoeijmakers, 2001). cells undergoing meiosis, where they serve as sites for the initiation of genetic recombination between parental chromosomes. Meiosis is a key event in the life cycle of Correspondence: Dr SC West, London Research Institute, Cancer Research UK, Clare Hall Laboratories, South Mimms, Herts EN6 all sexually reproducing organisms. It promotes the 3LD, UK. transition fromdiploid to haploid phase, and during this E-mail: [email protected] process it is essential that the maternal and paternal Regulation of recombination by BRCA2 T Thorslund and SC West 7721 NHEJ HR Predominant in G1 Predominant in S/G2 Error-prone Resection RPA MRN complex 5’ 3’ Filament RAD51 RAD52/BRCA2 Formation Invasion into RAD51 RAD52/RAD54 sister chromatid SDSA DNA synthesis & RAD51 RAD52/RAD54 Second-end capture Polymerase DNA synthesis & Polymerase Ligation Ligase Holliday Junction Resolution Figure 1 Schematic illustration of DNA double-strand break (DSB) repair. (a) In the S/G2 phases of the cell cycle, DSB repair is predominantly carried out by homologous recombination (HR). (b) DNA resection produces 30 single-stranded DNA (ssDNA) tails that serve as the initiation site for RAD51 filament formation (c). (d) Homologous pairing and strand invasion reactions take place with the sister chromatid to form a D-loop structure. Two possible mechanisms of HR can then take place involving the formation and resolution of Holliday junctions (e–g) or synthesis-dependent strand annealing (SDSA) (i–k). The products of mitotic recombination are generally non-crossovers. Non-homologous end joining can take place at all times throughout the cell cycle but is particularly useful in G1 when HR is inactive (h). MRN complex, MRE11/RAD50/NBS1 complex; NHEJ, non-homologous end joining chromosomes recombine before the production of Cellular recombinases progeny (Neale and Keeney, 2006). The basic mechan- ism of recombination during meiosis is remarkably In all organisms, proteins belonging to the conserved similar to that which can occur in somatic cells, but a family of RecA/RAD51 recombinases mediate the few key features mark important differences (Figure 2). DNA–DNA interactions required for mitotic and meiotic Firstly, the introduction of DSBs during meiotic recombination. They have the unique ability to search for recombination is a deliberate and tightly controlled homologous sequences and to catalyse the exchange of process that is promoted by the SPO11 DNA endo- DNA strands fromone molecule to another. Most nuclease. Secondly, meiotic recombination involves the eukaryotes possess two recombinases: the ubiquitously actions of two recombinases, with DMC1 supplement- expressed RAD51 (Rad51 in Saccharomyces cerevisiae, ing the actions of RAD51. Thirdly, the search for Rhp51 in S. pombe, and RAD51 in vertebrates), and its homologous sequences by nucleoprotein filaments is meiosis-specific homologue DMC1. RAD51 is essential directed towards homologous chromosomes rather than in mammals, as indicated by the embryonic lethality the sister chromatids. Finally, while mitotic recombina- associated with the Rad51À/À k/o in the mouse (Lim and tion usually leads to the formation of non-crossover Hasty, 1996; Tsuzuki et al., 1996). In contrast, Dmc1À/À products, the processing of meiotic recombination mice develop normally but are infertile due to defects in intermediates results in at least one crossover per chromosome synapsis during meiotic recombination chromosome pair. (Pittman et al., 1998; Yoshida et al., 1998). Oncogene Regulation of recombination by BRCA2 T Thorslund and SC West 7722 E. coli RecA SPO11 Human RAD51 339 aa Human DMC1 340 aa Identity RAD51/DMC1 35% 40% 60% N-terminal Helix-hairpin-Helix RecA/RAD51 core domain E. coli C-terminus Walker A+B motifs DMC1/RAD51 Homologous Chromosome Figure 3 The RecA/RAD51 family of recombinases. (a) Sche- matic representation of E. coli RecA, human RAD51 and human DMC1, showing their characteristic motifs. (b) These proteins are active as nucleoprotein filaments that form on DNA, as indicated schematically and in this electron microscope image. represent sites on chromatin where DNA repair reac- tions take place. Similarly, RAD51 forms foci on meiotic chromosomes, where it colocalizes with DMC1 (Barlow et al., 1997; Tarsounas et al., 1999). These foci appear similar to the RAD51 foci observed in somatic cells after DNA damage and presumably identify sites of Crossover formation meiotic recombination (Haaf et al., 1995; Raderschall et al., 1999). It is not known why meiotic recombination requires two recombinases, while RAD51 is sufficient to mediate mitotic recombination. Indeed, the proteins appear to be Figure 2 Schematic illustration of meiotic recombination. Meiotic so similar, certainly in terms of their molecular structure recombination is similar to mitotic recombination, as described in and reactions catalysed, that it has been difficult to Figure 1a–g; however, there are a few important differences: during meiosis, double-strand breaks (DSBs) are introduced by SPO11 elucidate the specific role played by DMC1 during
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