Novel Insights into RAD52's Structure, Function, and Druggability for Synthetic Lethality and Innovative Anticancer Therapies

Abstract

In recent years, the RAD52 protein has been highlighted as a mediator of many DNA repair mechanisms. While RAD52 was initially considered to be a non-essential auxiliary factor, its inhibition has more recently been demonstrated to be synthetically lethal in cancer cells bearing mutations and inactivation of specific intracellular pathways, such as homologous recombination. RAD52 is now recognized as a novel and critical pharmacological target. In this review, we comprehensively describe the available structural and functional information on RAD52. The review highlights the pathways in which RAD52 is involved and the approaches to RAD52 inhibition. We discuss the multifaceted role of this protein, which has a complex, dynamic, and functional 3D superstructural arrangement. This complexity reinforces the need to further investigate and characterize RAD52 to solve a challenging mechanistic puzzle and pave the way for a robust drug discovery campaign.

Keywords: RAD52; drug discovery; precision medicine; synthetic lethality.

Figure 1

Schematic representation of the RAD52-mediated SSA DNA repair mechanism. RAD52 facilitates homology search and strand annealing.

Figure 2

Schematic representation of stalled replication fork steps involving RAD52. (Left): RAD52 acts as a gatekeeper for the replicative fork to prevent unscheduled MRE11-mediated degradation and to facilitate enzyme loading only when required. (Right): RAD52 mediates the fork break mechanism for stall resolution and may mediate fork recovery through BIR.

Figure 3

Domain map of human RAD52: the N-terminal domain contains the DNA-binding region and a self-associating region; the C-terminal domain contains RPA and RAD51 interacting regions and a nuclear localization signal.

Figure 4

Multiple alignments of RAD52 sequences. (a) Alignment of N-terminal and (b) C-terminal domain sequences of different organisms. In contrast to the N-terminal domain, the C-terminal domain is not conserved.

Figure 4

Multiple alignments of RAD52 sequences. (a) Alignment of N-terminal and (b) C-terminal domain sequences of different organisms. In contrast to the N-terminal domain, the C-terminal domain is not conserved.

Figure 5

Side and bottom views of the mushroom-like structure of the undecameric ring of RAD52 (1–212) (PDB 1KN0). Molecular graphics and analyses were performed with UCSF Chimera, developed by the Resource for Biocomputing, Visualization, and Informatics at the University of California, San Francisco [61].

Figure 6

Schematic representation of RAD52 (1–212) monomer folding. Rods and arrows indicate helices and strands, respectively.

Figure 7

Schematic representation of RAD52 (1–212) monomer surface electrostatic potentials (PDB 1KN0). The representation was carried out using the Coulombic tool of UCSF Chimera, where −10 was set as the minimum (red) and 10 was set as the maximum (blue).

Figure 8

(a) Two views of the RAD52 (1–212) monomer in complex with an ssDNA molecule inside its inner binding cleft (PDB 5XRZ); (b) two views of the RAD52 (1–212) monomer in complex with an ssDNA molecule inside its outer binding cleft (PDB 5XS0). Structures were prepared using UCSF Chimera software.

Figure 8

(a) Two views of the RAD52 (1–212) monomer in complex with an ssDNA molecule inside its inner binding cleft (PDB 5XRZ); (b) two views of the RAD52 (1–212) monomer in complex with an ssDNA molecule inside its outer binding cleft (PDB 5XS0). Structures were prepared using UCSF Chimera software.

Figure 9

Schematic representation of the two postulated cis and trans mechanisms of RAD52-mediated homology search. The dashed line circle represents the original position of one of the two DNA strands in the deep inner binding groove (1) of one of the two nucleoproteins that is then temporarily placed in the second binding site (2) of the second nucleoprotein upon homology search (solid line circle).

Figure 10

Structures of most recent and relevant inhibitors targeting RAD52. Inhibitors names are in bold, with their binding pockets with the amino acids of interest in brackets; A or B following the amino acid numbers indicates if the amino acid belongs to RAD52 protomer A or B; functional groups that establish specific interactions with amino acids are reported in red; Phe-Phenylalanine; Gln-Glutamine; Arg-Arginine; Tyr-Tyrosine; Glu-Glutamate; Val-Valine; Asp-Aspartate; Lys-Lysine; Gly-Glycine; Asn-Asparagine; Trp-Tryptophan; His-Histidine; Ala-Alanine.