Physiological and Pathological Roles of RAD52 at DNA Replication Forks

Abstract

Understanding basic molecular mechanisms underlying the biology of cancer cells is of outmost importance for identification of novel therapeutic targets and biomarkers for patient stratification and better therapy selection. One of these mechanisms, the response to replication stress, fuels cancer genomic instability. It is also an Achille's heel of cancer. Thus, identification of pathways used by the cancer cells to respond to replication-stress may assist in the identification of new biomarkers and discovery of new therapeutic targets. Alternative mechanisms that act at perturbed DNA replication forks and involve fork degradation by nucleases emerged as crucial for sensitivity of cancer cells to chemotherapeutics agents inducing replication stress. Despite its important role in homologous recombination and recombinational repair of DNA double strand breaks in lower eukaryotes, RAD52 protein has been considered dispensable in human cells and the full range of its cellular functions remained unclear. Very recently, however, human RAD52 emerged as an important player in multiple aspects of replication fork metabolism under physiological and pathological conditions. In this review, we describe recent advances on RAD52's key functions at stalled or collapsed DNA replication forks, in particular, the unexpected role of RAD52 as a gatekeeper, which prevents unscheduled processing of DNA. Last, we will discuss how these functions can be exploited using specific inhibitors in targeted therapy or for an informed therapy selection.Cancers 2020, 12, 402 2 of 17 for all known homology-directed DNA repair mechanisms in yeast [11,13,14], mammalian RAD52 was previously considered to be dispensable for recombination-based DNA repair and its knock out in mouse does not show any lethality or other remarkable phenotypes as those associated with the RAD51 knock out [15,16]. Based on the studies in chicken DT40 cells [16,17], it has been proposed that the vertebrate RAD52 may be involved only in a limited subset of the recombinational DNA repair events. Indeed, the most crucial recombination mediator function of yeast Rad52 is played by BRCA2 tumor suppressor protein in higher eukaryotes, including mammals, fungi, and plants [18][19][20].Remarkably, much interest in RAD52 function arose since its deficiency has been shown to be synthetic lethal with biallelic mutations in or absence of BRCA1/2 or BRCA-related genes [13,[21][22][23], which are found mutated or de-regulated in many hereditary and sporadic breast and ovarian cancers [24,25].While biochemical features of RAD52, its more novel double-strand break (DSB) repair functions and its modifications are the focus of other reviews in this issue [12,[26][27][28], here, we will discuss the unexpected roles that RAD52 plays at the DNA replication fork and their implications for cancer therapy. Role of RAD52 in DSBs Repair

The function of RAD52 N-terminal domain is essential for viability of BRCA-deficient cells

“…RAD52 CTD possesses the RPA- and RAD51-interacting regions, and our results with RAD52 1–209 may raise a query regarding the role of these interactions for the RAD52 cellular function. Apart from above mentioned functions, recent reports show that RAD52 plays a role in various pathways of DNA repair including break-induced replication, transcription-coupled HR, replication fork restart, and alternative lengthening of telomeres ( 25 , 45–49 ). One may suggest that RAD52 interaction with RPA and RAD51 may be important for these functions of RAD52, but further studies are needed for better understanding of the role of these interactions.…”

Current Understanding of RAD52 Functions: Fundamental and Therapeutic Insights

“…(D) DSB-independent roles of RAD52 were also reported in S phase. RAD52 prevents unleashed fork reversal, but once forks have reversed, it can facilitate MRE11-dependent degradation of newly synthesized DNA [12,18,19]. Template DNA: black, copied strand: red and violet strands: telomeric regions.…”

“…Xiaohua Wu also discusses reasons why to consider RAD52 as a potential target for cancer therapies, which genetics backgrounds might benefit from this treatment, and which limitations exist in the design of appropriate RAD52 inhibitors [28,37]. The review by Maria Spies, Pietro Pichierri, and colleagues entitled, "Physiological and Pathological Roles of RAD52 at DNA Replication" contains entire sections devoted to discussing functions of RAD52 that are independent from the resolution of DSBs, but may influence their formation [12]. One such function is a gatekeeper role of RAD52 at DNA replication forks, which prevents their excessive reversal by SMARCAL1 (SWI/SNF Related, Matrix Associated, Actin Dependent Regulator of Chromatin, Subfamily A-Like 1).…”

“…These observations suggest that once loaded at stall forks, RAD52 may perform different and apparently opposed functions, before and upon fork reversal. Since many other proteins also regulate fork reversal [12], the authors thoroughly compared previously described modes of action with the one of RAD52. Also, while the synthetic lethality reported for BRCA2 and RAD52 has been ascribed to the contribution of RAD52 to the repair of DSB [38,39], this review speculates on the contribution that DSB-independent functions of RAD52 may have on such synthetic lethality.…”

“…Notably, RAD52 cooperates with various nucleases, namely, MUS81 during BIR, ERCC1/XPF during SSA, and XPG during TA-HR. It also cooperates with MRE11 and MUS81/EME1 at de-protected forks that have reverted to process these structures into HR substrates, ultimately enabling the cell to continue DNA replication [10][11][12]. The multiple activities of RAD52 known to date are summarized in Figure 1.Cancers 2020, 12, x FOR PEER REVIEW 2 of 8…”