COMMENTARY

Who’s who in human recombination: BRCA2 and RAD52

Jie Liua and Wolf-Dietrich Heyera,b,1 Departments of aMicrobiology and bMolecular and Cellular Biology, University of California, Davis, CA 95616

omologous recombination actions of HR: search and DNA (HR) is a key pathway to re- strand invasion (1). Surprisingly, in or- H pair complex DNA damage, ganisms containing a BRCA2 homolog, such as DNA gaps, double- such as U. maydis, chicken, and mice, strand breaks, and interstrand cross-links RAD52 inactivation causes minimal or no (1) (Fig. 1). Moreover, HR supports the HR and DNA repair defects (2, 4, 12). recovery of stalled or broken replication Caenorhabditis elegans and Drosophila forks and ensures faithful melanogaster seem to lack a RAD52 ho- segregation during meiosis. The RAD52 molog entirely (Fig. 1). in the budding yeast Saccharomyces So is RAD52 irrelevant for HR in cerevisiae encodes the lynchpin of HR in BRCA2-containing organisms? The expec- this organism. Hence, the of the tation horizon from budding yeast HR pathway are called the RAD52 epis- was so high that the absence tasis group. However, much to everybody’s of strong phenotypes in the mouse RAD52 surprise, the RAD52 gene knockout in mutants left this gene without much in- mice exhibits minimal to no phenotypes terest. However, the findings by Feng et al. in recombination, repair, and meiosis (2). (3) in BRCA2-deficient cells force another Instead, the breast and ovarian tumor look at RAD52 and its role in HR in ver- suppressor BRCA2 maintains tebrates. Already the initial reports in a central HR function, a protein that is RAD52-deficient mouse or chicken DT40 missing in budding yeast. Analyzing HR cells demonstrate a small but significant in BRCA2-deficient human cells, a study reduction in gene targeting (2, 12). Now reported in PNAS (3) redefines human Feng et al. (3) show that RAD52 depletion RAD52 by showing that it plays a critical in BRCA2-complemented EUFA423 cells role in HR in BRCA2-deficient cells. causes an increase in damage-induced This important result mandates a re- chromosomal abnormalities, such as telo- examination of vertebrate RAD52 and mere end associations and radials. The its functions, which opens perplexing reduction in the development of T-cell mechanistic conundrums. lymphomas in ATM-deficient mice by the In the study (3), Feng et al. manipulate RAD52 single knockout also indicates the BRCA2 and RAD52 status by muta- a significant in vivo function of mouse tion, complementation, or RNA-mediated RAD52 (13). In conclusion, RAD52 mu- knockdown in three different human cell tants in vertebrates do have HR-related lines expressing either normal or truncated Fig. 1. Roles of BRCA2 and RAD52 in homologous phenotypes, although they are much BRCA2 . They show that lower recombination. Upper: Schematic representation more subtle than expected from the yeast RAD52 protein levels cause decreased of the mediator and annealing functions during paradigm. levels of RAD51 foci and HR in BRCA2- recombination involving (A) invasion of both ends, How does mammalian RAD52 act: as deficient cells. These data greatly expand (B) invasion of the first end and second end capture a mediator or annealer (Fig. 1)? Several on a recent observation of a negative ge- by annealing, or (C) invasion of the first end, D- genetic results favor the model proposed netic interaction between mutants in the loop disruption, and annealing of the extended fi fi by Feng et al. (3) that RAD52 de nes an Ustilago maydis BRCA2 and RAD52 ho- rst end to the second end (SDSA, synthesis-de- pendent strand annealing). Lower: BRCA2 and alternative mediator pathway to BRCA2. mologs (4). As noted by Feng et al. (3), RAD52 functions and phenotypes in selected or- They show that BRCA2 status does not these results have interesting implications ganisms. Checkmark denotes presence of the bio- affect RAD52–RAD51 focus formation for tumor therapy and suggest a more chemical activity, − denotes absence of phenotype, and that restoring RAD52 expression en- important role of RAD52 protein in hu- ++ denotes a strong and (+) a weak phenotype. hances RAD51 focus formation in man HR than previously thought. BRCA2-deficient cells. Although it is for- Analyzing the proliferation rates in their mally possible that RAD52 acts upstream cell lines, Feng et al. (3) show that normal Why are the results by Feng et al. (3) so of BRCA2, the alternative mediator levels of RAD52 expression are required interesting from a mechanistic point of model is more consistent with additional for proliferation in cells with no or low view? Biochemical analysis shows that the synthetic phenotypes involving RAD52.In BRCA2 function. This identifies a vulner- S. cerevisiae Rad52 protein performs two chicken DT40 cells, RAD52 disruption is ability of BRCA2-deficient cells besides critical reactions in HR: (i) it mediates lethal with a defect in XRCC3, a RAD51 inhibition of poly(ADP-ribose) poly- the assembly of Rad51 filament on repli- paralog that forms a complex with merase (5), because depletion of RAD52 cation protein A (RPA)-coated ssDNA RAD51C required for RAD51 filament in such cells causes a strong proliferation (8–10), and (ii) it performs the annealing defect (synthetic lethality) associated with step in second end capture and synthesis- severe chromosomal fragility. This iden- dependent strand annealing (11) (Fig. 1). Author contributions: J.L. and W.-D.H. wrote the paper. tifies RAD52 as a potential therapeutic This dual function readily explains why The authors declare no conflict of interest. target not only in BRCA2-deficient cells rad52 mutants display even more extreme See companion article 10.1073/pnas.1010959107. (3) but possibly also in BRCA2 revertants phenotypes than defects in the Rad51 1To whom correspondence should be addressed. E-mail: that become treatment resistant (6, 7). protein, which performs the signature re- [email protected].

www.pnas.org/cgi/doi/10.1073/pnas.1016614108 PNAS Early Edition | 1of2 Downloaded by guest on September 28, 2021 formation/stability (14). In addition in U. displays strong mediator activity (15, 20). ssDNA. Finally, an entirely novel protein maydis, rad52 mutants show a synthetic Several possible solutions can be enter- might be involved. Interestingly, all five enhancement of UV and IR sensitivity tained. The most radical would be that HR human RecQ-like proteins have been re- with a in the only RAD51 paralog in vertebrates does not involve an an- ported to anneal ssDNA, but this activity is gene, REC2 (4). However, human RAD52 nealing step [i.e., no synthesis-dependent inhibited by RPA, and the biological sig- protein lacks mediator activity in recon- strand annealing pathway; (c) in Fig. 1] nificance of these activities remains un- stituted biochemical reactions (15). Possi- but involves second end strand invasion – bly, RAD52 performs its mediator func- clear (23 27). tion in conjunction with other proteins, In sum, human RAD52 functions to perhaps the RAD51 paralog complex of Lower RAD52 protein anneal RPA-coated ssDNA, and the im- RAD51B/C/D-XRCC2, whose inacti- levels cause decreased portant in vivo role of human RAD52 in vation also shows a synthetic defect with BRCA2-deficient cells uncovered by Feng an XRCC3 in chicken DT40 cells levels of RAD51 foci et al. (3) may entail RAD51 stimulation (16) just like a RAD52 defect (14). not involving classical mediation but pos- Last, the analysis of HR in humans and HR in BRCA2- sibly the RPA-independent effects seen in poses another interesting conundrum: earlier biochemical studies (28, 29). The which protein is responsible for the an- deficient cells. study by Feng et al. (3) forces renewed nealing steps in mammalian HR [(b) and attention to RAD52 in mammalian HR, (c) in Fig. 1]? RAD52 is capable of per- forming this reaction in vitro (15) and in [(a) in Fig. 1]. The resulting double Hol- with interesting implications not only for vivo (17), but the single mutant or RAD52 liday junctions (dHJs) will put a consid- the fundamental HR mechanism but also depletion lacks the expected phenotypes erable burden on the dHJ dissolution for anticancer therapy. (IR sensitivity, strong HR phenotype) pathway involving BLM-TOPO3α-RMI1- ACKNOWLEDGMENTS. Work on homologous re- (2, 3, 17). BRCA2 mutants or depletion RMI2 (21). The high incidence of ana- ’ phase bridges in mammalian cells may be combination in W.-D.H. s laboratory is supported exhibits strong HR and DNA repair phe- by National Institutes of Health Grants GM58015 notypes (3, 17). However, unlike its U. an indication of failure of dissolving the and CA92276 and Department of Defense Grant maydis and C. elegans homologs (18, 19), frequent dHJs (22). Alternatively, there BC083684/W81XWH-09-1-0116. J.L. was supported the human BRCA2 protein is incapable of might be a cofactor that endows BRCA2 by a postdoctoral fellowship from the Tobacco- annealing RPA-coated ssDNA, although it with the ability to anneal RPA-coated Related Disease Research Program (17FT-0046).

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