53BP1: Keeping It under Control, Even at a Distance from DNA Damage

Abstract

Double-strand breaks (DSBs) are toxic lesions that can be generated by exposure to genotoxic agents or during physiological processes, such as during V(D)J recombination. The repair of these DSBs is crucial to prevent genomic instability and to maintain cellular homeostasis. Two main pathways participate in repairing DSBs, namely, non-homologous end joining (NHEJ) and homologous recombination (HR). The P53-binding protein 1 (53BP1) plays a pivotal role in the choice of DSB repair mechanism, promotes checkpoint activation and preserves genome stability upon DSBs. By preventing DSB end resection, 53BP1 promotes NHEJ over HR. Nonetheless, the balance between DSB repair pathways remains crucial, as unscheduled NHEJ or HR events at different phases of the cell cycle may lead to genomic instability. Therefore, the recruitment of 53BP1 to chromatin is tightly regulated and has been widely studied. However, less is known about the mechanism regulating 53BP1 recruitment at a distance from the DNA damage. The present review focuses on the mechanism of 53BP1 recruitment to damage and on recent studies describing novel mechanisms keeping 53BP1 at a distance from DSBs.

Keywords: 53BP1; BRCA1; PARP inhibitors; double-strand break repair; homologous recombination; lamins; non-homologous end joining; shieldin.

Figure 2

Implication of 53BP1 in biological processes: 53BP1 has been implicated in (A) the p53 response in different settings; (B) C-NHEJ-dependent joining of DSB ends; (C) long-range V(D)J recombination; (D) multiple steps in CSR: loop formation between switch regions, regulation in the order of DSB induction by AID in switch regions and joining of C-NHEJ-dependent long-range CSR; (E) response to replicative stress and stalled fork restart; and (F) protecting underreplicated DNA from degradation in 53BP1 nuclear bodies (see text for details).

Figure 3

53BP1 prevents DSB end resection. 53BP1 and its partners protect against resection. RIF1-PP1 and DYNLL1 prevent the early step of resection (mediated by the MRN complex and CtIP). Shieldin (SHLD) along with CTC1–STN1–TEN1 (CST)/POLα/PRIMASE and ASTE1 protects against the late step of resection (mediated by DNA2, EXO1, etc.) (see text for details).

Figure 1

The importance of 53BP1 in the choice of double-strand break (DSB) repair pathway. 53BP1 plays a pivotal role in the balance of the DSB repair choice by limiting DSB end resection, thus promoting canonical non-homologous end joining (C-NHEJ) over homologous recombination (HR) and the mutagenic alternative NHEJ (A-NHEJ), which both require a first resection step.

Figure 4

Domains and interactions of 53BP1. Upon DSBs, activation of ATM, unmasking of dimethylated lysine 20 of histone H4 (H4K20me2) and ubiquitination of histone H2A on lysine 15 of (H2AK15ub) by RNF168 and RNF8 allow the recruitment of 53BP1 to damaged chromatin. 53BP1 recruitment requires the focus-forming region (FFR), which includes the oligomerization domain (OD), the glycine-arginine rich motif (GAR), the tandem Tudor domain (TUDOR), the ubiquitin-dependent recognition motif (UDR), the dynein light chain-binding domain (LC8) and the nuclear localization sequence (NLS). DSB end resection is inhibited by RIF1, shieldin, CST, POLα and PRIMASE, which interact with the S/T-Q sites on the N-terminal part of 53BP1. RIF1 recognizes three consensus sequences, each containing two leucine and two S/T-Q sites (3x {LxL[xx](pS/pT)xpS}) [72]. Serine 25 (S25) allows the interaction of 53BP1with PTIP. The mobility domain (MOB) also encompasses S/T-Q sites and regulates DNA end mobility, but its interactors remain to be identified. The C-terminal part of 53BP1 contains two BRCA-carboxyterminal (BRCT) repeats, allowing 53BP1 interaction with p53, γH2AX, EXPAND and USP28.

Figure 5

Factors participating in the regulation of 53BP1 recruitment to the chromatin, at a distance from the DSB. Left panel: In the absence of DSBs, several factors such as TIRR and NuMA interact with 53BP1 to prevent its recruitment to chromatin by binding region essentials for 53BP1 recruitment. Upon DSBs, mechanisms regulated by ATM take place to dissociate these factors from 53BP1 and allow its recruitment to the damaged chromatin. Nuclear envelope proteins also play an important role in genome stability and 53BP1 regulation. Lamin A interacts with 53BP1 and increases its stability by preventing 53BP1 degradation by the proteasome. Furthermore, 53BP1 interacts with lamin B1 via its FFR domain, impeding its recruitment to DSBs. Upon DNA damage, an ATM-dependent dissociation occurs, thus enabling 53BP1 to be recruited to damaged chromatin. Right panel: In the absence of DNA damage, NuMA associates with 53BP1, reducing its mobility and its access to chromatin. Following DNA damage, ATM enhances the FOXK1–53BP1 interaction in the S phase, preventing 53BP1 recruitment to DSBs. On the other hand, prelamin A expression impedes 53BP1 importation to the nucleus via NUP153. 53BP1 interactors’ deregulation, such as TIRR or lamin B1 overexpression, also inhibits 53BP1 recruitment to damaged chromatin through its sequestration.