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.

Keywords: RAD52; genome stability; replication fork recovery; replication fork reversal; target therapy.

Figure 1

RAD52 is involved in multiple DNA repair pathways activated by double-strand breaks (DSBs). RAD52 is involved in different DSBs repair pathways. Mainly, RAD52 plays essential functions in single-strand annealing (SSA) or break-induced replication (BIR). MMEJ: Microhomology mediated end-joining; HR: Homologous Recombination.

Figure 2

RAD52 is a gatekeeper that limits excessive replication fork reversal. Upon fork stalling, RAD52 can associate with parental ssDNA at fork. Binding of RAD52 at fork can induce a rearrangement of the DNA closing the Y structure to counteract recruitment of fork reversal enzymes such as SMARCAL1 or ZRANB3. This gatekeeper function probably put the stalled fork on hold until it is ready to restart. If RAD52 is absent or its association with ssDNA is abrogated, the recruitment of fork reversal enzymes is abnormally increased leading to exhaustion of fork protection factors, such as RAD51. Such exhaustion of fork protection factors triggers degradation by MRE11 and eventually leads to pathological fork recovery and genome instability.

Figure 3

Multiple roles of RAD52 in response to normal or pathological fork stalling. RAD52 can perform multiples and sequential roles at perturbed replication forks in wild-type or BRCA2-deficient cells. Under normal conditions, such as in BRCA2 wild-type cells, RAD52 mainly functions as gatekeeper and limits unscheduled fork reversal (left arm). However, it might be also involved in MRE11 recruitment at a subset of deprotected forks. When BRCA2 is absent or mutated, RAD52 may still play its gatekeeper role but it is expected to be much more required downstream fork reversal (right arm). At deprotected forks, RAD52 is required to recruit MRE11 and initiate pathological fork degradation. After fork degradation, RAD52 would persist at degraded forks to recruit the MUS81 complex through direct protein–protein interaction. Hence, RAD52 would stimulate formation of MUS81-dependent DSBs through, at least, two independent mechanisms: recruitment of MRE11 and recruitment/stimulation of MUS81. Finally, RAD52 would promote repair of MUS81-induced DSBs through break-induced replication (BIR).