Tying the loose ends together in DNA double strand break repair with 53BP1

Abstract

To maintain genomic stability and ensure the fidelity of chromosomal transmission, cells respond to various forms of genotoxic stress, including DNA double-stranded breaks (DSBs), through the activation of DNA damage response signaling networks. In response to DSBs as induced by ionizing radiation (IR), during DNA replication, or through immunoglobulin heavy chain (IgH) rearrangements in B cells of lymphoid origin, the phosphatidyl inositol-like kinase (PIK) kinases ATM (mutated in ataxia telangiectasia), ATR (ATM and Rad3-related kinase), and the DNA-dependent protein kinase (DNA-PK) activate signaling pathways that lead to DSB repair. DSBs are repaired by either of two major, non-mutually exclusive pathways: homologous recombination (HR) that utilizes an undamaged sister chromatid template (or homologous chromosome) and non- homologous end joining (NHEJ), an error prone mechanism that processes and joins broken DNA ends through the coordinated effort of a small set of ubiquitous factors (DNA-PKcs, Ku70, Ku80, artemis, Xrcc4/DNA lig IV, and XLF/Cernunnos). The PIK kinases phosphorylate a variety of effector substrates that propagate the DNA damage signal, ultimately resulting in various biological outputs that influence cell cycle arrest, transcription, DNA repair, and apoptosis. A variety of data has revealed a critical role for p53-binding protein 1 (53BP1) in the cellular response to DSBs including various aspects of p53 function. Importantly, 53BP1 plays a major role in suppressing translocations, particularly in B and T cells. This report will review past experiments and current knowledge regarding the role of 53BP1 in the DNA damage response.

Figure 1

Human 53BP1 is composed of 1,972 amino acids and contains several noteworthy structural features as discussed throughout the text. p53 binds to the N-terminal BRCT motif and linker sequence of 53BP1. 53BP1 possesses numerous PIK phosphorylation sites (S/TQ) and is phosphorylated on serine residues 25 and 29 in vivo. Like BRCA1 and Mdc1 and the yeast Rad9 and Crb2 proteins, 53BP1 possesses two repeating C-terminal BRCT motifs. In addition, 53BP1 contains a tandem tudor domain, a stretch rich in glycine and arginine residues (1396–1403) that is methylated by the PRMT1 arginine methyltransferase in vivo and in vitro, LC8 binding sites and two potential KEN boxes (aa 54–60 and 85–91), sequences known to interact with the anaphase promoting complex (APC).

Figure 2

Class-switch recombination is a NHEJ event involving the exchange of heavy chain constant regions with the previously assembled V(D)J exons. AID induces lesions at the switch regions (circles) upstream of a given IgH gene which are converted to DSBs. These DSBs recruit various factors, including Mre11/Nbs1/Rad50, that are important for the resolution of the break. The end result of CSR is the replacement of the mu exon with another constant region gene (IgG₁ shown). 53BP1 performs an anchoring role at the IgH locus during this process that might involve interactions with nucleosomes near the break through various histone modifications (see text for details). This is thought to modulate chromatin in this area to facilitate DNA repair.

Figure 3

During class-switch recombination, AID-induced breaks are repaired by the DSB repair proteins such as Mre11/Nbs1/Rad50, H2AX, Mdc1, 53BP1, and components of the NHEJ apparatus (Ku70, Ku 80, DNA-PK). Failure to perfom DSB repair activates ATM, leading to p53 stabilization. Alternatively, breaks are resolved by translocations often involving c-myc. These translocations are suppressed by ATM or by activation of the p19-dependent pathway leading to the activation of p53, resulting in checkpoint arrest and cell death. Adapted from reference 88 with permission.