cancers Editorial Current Understanding of RAD52 Functions: Fundamental and Therapeutic Insights Vanesa Gottifredi 1,* and Lisa Wiesmüller 2,* 1 Fundación Instituto Leloir, IIBBA-Consejo Nacional de Investigaciones Científicas y Técnicas. Av. Patricias Argentinas 435, 1405 Buenos Aires, Argentina 2 Division of Gynecological Oncology, Department of Obstetrics and Gynecology of the University of Ulm Prittwitzstrasse 43, 89075 Ulm, Germany * Correspondence: [email protected] (V.G.); [email protected] (L.W.); Tel.: +54-11-52387500 (V.G.); +49-731-500 58800 (L.W.) Received: 12 March 2020; Accepted: 13 March 2020; Published: 17 March 2020 Abstract: In this Special Issue, we would like to focus on the various functions of the RAD52 helicase-like protein and the current implications of such findings for cancer treatment. Over the last few years, various laboratories have discovered particular activities of mammalian RAD52—both in S and M phase—that are distinct from the auxiliary role of yeast RAD52 in homologous recombination. At DNA double-strand breaks, RAD52 was demonstrated to spur alternative pathways to compensate for the loss of homologous recombination functions. At collapsed replication forks, RAD52 activates break-induced replication. In the M phase, RAD52 promotes the finalization of DNA replication. Its compensatory role in the resolution of DNA double-strand breaks has put RAD52 in the focus of synthetic lethal strategies, which is particularly relevant for cancer treatment. Keywords: DNA double-strand break repair; common fragile site; stalled replication fork; telomeres; fork reversal; R loops; nucleases; genome integrity 1. Introduction This Special Issue gathers experts in the field to convey both comprehensive, insightful, and current perspectives on the different functions of human RAD52 in the maintenance of genomic integrity and the current implications of such findings for cancer treatment. Historically, RAD52 was described as an auxiliary factor of RAD51 in homologous recombination (HR) in yeast [1]. Around 2000, it became clear that in mammalian cells, BRCA2 had taken over the RAD51-chaperoning activity, stimulating filament formation on single-stranded DNA (ssDNA), while RAD52 was left with the single-strand annealing (SSA) activity [2]. Later, limited contributions of RAD52 to the repair of DNA double-strand breaks (DSBs)—such as during alternative non-homologous end-joining (A-NHEJ) and in compensating for the loss of HR functions—were also detected [3]. Full-length RAD52 forms a heptameric ring, whereby N- and C-terminal parts of each monomer form a positively charged ssDNA binding groove around the heptamer [4,5]. Oligomerization in the cytoplasm stimulates nuclear import, granted by the combined action of individually weak nuclear localization signals. Its RPA binding domains underlie RAD52´s biochemical functions in binding RPA-coated ssDNA, annealing, and homology-directed repair. Even though many details regarding the contribution of posttranslational modification to the function of RAD52 remain to be explored, we know that RAD52 acetylation is required for its accumulation at DSBs, sumoylation of yeast RAD52 for the choice of SSA over canonical recombination at repeats, and tyrosine phosphorylation of RAD52 for ssDNA, rather than dsDNA binding and the choice of SSA [6–9]. Cancers 2020, 12, 705; doi:10.3390/cancers12030705 www.mdpi.com/journal/cancers Cancers 2020, 12, x FOR PEER REVIEW 2 of 8 Cancers 12 Over2020 ,the, 705last years, new functions of RAD52 have been identified. For example, various2 of 8 laboratories have discovered the involvement of mammalian RAD52 and RNA templates and RNA- DNAOver hybrid the structures last years, like new R functions loops in ofunprecedented RAD52 have been homology-directed identified. For example, DSB repair various events laboratories [9]. First, RNAhave discoveredwas discovered the involvement to serve as ofa mammalianbridging template RAD52 in and RAD52- RNA templates and RPA-mediated and RNA-DNA homology- hybrid directedstructures DSB like Rrepair, loops inwhich unprecedented may play homology-directeda role during transcription, DSB repair eventsreplication, [9]. First, class-switch RNA was recombination,discovered to serveand at as telomeres. a bridging Second, template in in tran RAD52-scription-coupled and RPA-mediated HR (TC-HR) homology-directed in G0/G1 cells, DSB R loopsrepair, generated which may during play transcription a role during were transcription, found to be replication, bound by CSB, class-switch followed recombination, by RAD52-mediated, and at BRCA-independenttelomeres. Second, inRAD51 transcription-coupled recruitment, and HRHR. (TC-HR) Third, in in transcription-activated G0/G1 cells, R loops generated HR (TA-HR) during in S/G2transcription cells, RAD52 were recruits found tothe be XPG bound cleaving by CSB, R loops, followed which by gene RAD52-mediated,rates ssDNA overhangs BRCA-independent for BRCA- dependentRAD51 recruitment, HR [9]. and HR. Third, in transcription-activated HR (TA-HR) in S/G2 cells, RAD52 recruitsSurprisingly, the XPG cleaving RAD52 Ralso loops, acts whichduring generates events ot ssDNAher than overhangs canonical forDSB BRCA-dependent repair—namely, HR during [9]. DNASurprisingly, replication when RAD52 it counteracts also acts during excessive events fork other regression, than canonical which DSB may repair—namely, exhaust protection during factors, DNA causingreplication breakage when itof counteracts reversed forks. excessive At collapsed fork regression, replication which forks, may exhaust RAD52 protection activates break-induced factors, causing replicationbreakage of (BIR), reversed a specialized forks. At collapsedpathway replicationthat repairs forks, single-ended RAD52 activates DSBs. During break-induced M phase, replication RAD52- mediated(BIR), a specialized BIR also promotes pathway thatthe finalization repairs single-ended of DNA replication DSBs. During (MiDAS- M phase, mitotic RAD52-mediated DNA synthesis). BIR Notably,also promotes RAD52 the cooperates finalization with of DNAvarious replication nucleases, (MiDAS- namely, mitotic MUS81 DNA during synthesis). BIR, ERCC1/XPF Notably, during RAD52 SSA,cooperates and XPG with during various TA-HR. nucleases, It also namely,cooperates MUS81 with MRE11 during and BIR, MUS81/EME1 ERCC1/XPF during at de-protected SSA, and forks XPG thatduring have TA-HR. reverted It alsoto process cooperates these with structures MRE11 into and HR MUS81 substrates,/EME1 atultimately de-protected enabling forks the that cell have to continuereverted DNA to process replication these structures [10–12]. The into multiple HR substrates, activities ultimately of RAD52 enabling known theto date cell toare continue summarized DNA inreplication Figure 1. [10–12]. The multiple activities of RAD52 known to date are summarized in Figure1. FigureFigure 1. 1. MultipleMultiple roles roles of of RAD52 RAD52 during during DNA DNA replication replication and and repair. repair. ( (AA)) RAD52 RAD52 participates participates in in variousvarious DNA DNA double-strand double-strand break break (DSB) (DSB) repair repair proce processessses by by means means of of its its strand-annealing strand-annealing activities. activities. InIn some some pathways, pathways, RAD52 RAD52 acts acts as as a a backup backup factor factor (e.g., (e.g., HR), HR), while while in in others, others, it it is is absolutely absolutely required, required, e.g.e.g. single-strand single-strand annealing annealing (SSA) (SSA) at at DSBs DSBs and and brea break-inducedk-induced replication replication (BIR) (BIR) at at single-ended single-ended DSBs DSBs (which(which are are not not shown in thisthis scheme)scheme) [[8,9,13],8,9,13], ((BB)) RAD52 RAD52 participates participates in in the the alternative alternative lengthening lengthening of oftelomeres. telomeres. RAD52 RAD52 plays plays a rolea role in in BIR-mediated BIR-mediated elongation elongation of telomeresof telomeres during during pro-metaphase, pro-metaphase, but but also alsopromotes promotes spontaneous spontaneous telomere telomere elongation elongation in in G2, G2, independently independently ofof thethe SLX4 nuclease nuclease [11,14,15]. [11,14,15]. ((CC)) RAD52 RAD52 has has also also been been implicated implicated in in the the facilita facilitationtion of of DSB DSB formation formation by by MUS81 MUS81 in in Chk1-depleted Chk1-depleted cellscells and and during during MiDAS MiDAS [10,16,17]. [10,16,17]. (D (D) DSB-independent) DSB-independent roles roles of ofRAD RAD5252 were were also also reported reported in inS phase.S phase. RAD52 RAD52 prevents prevents unleashed unleashed fork reversal, fork reversal, but once but forks once have forks reversed, have reversed, it can facilitate it can facilitateMRE11- dependentMRE11-dependent degradation degradation of newly ofsynthesized newly synthesized DNA [12,18,19]. DNA [12 Template,18,19]. Template DNA: black, DNA: copied black, strand: copied redstrand: and violet red and strands: violet strands:telomeric telomeric regions. regions. Cancers 2020, 12, 705 3 of 8 Therefore, compensatory roles in the resolution of DSBs, as well as at replication forks, put RAD52 in the focus of synthetic lethal strategies, which is particularly relevant for cancer treatment. Consequently, efforts were made to identify RAD52-inhibitory compounds to kill HR-defective tumor cells in a synthetic lethal fashion [8]. The specific mode of action of these compounds is determined by the above-mentioned biochemical