53BP1: pro choice in DNA repair

Abstract

The DNA damage response factor 53BP1 functions at the intersection of two major double strand break (DSB) repair pathways--promoting nonhomologous end-joining (NHEJ) and inhibiting homology-directed repair (HDR)--and integrates cellular inputs to ensure their timely execution in the proper cellular contexts. Recent work has revealed that 53BP1 controls 5' end resection at DNA ends, mediates synapsis of DNA ends, promotes the mobility of damaged chromatin, improves DSB repair in heterochromatic regions, and contributes to lethal mis-repair of DSBs in BRCA1-deficient cells. Here we review these aspects of 53BP1 and discuss new data revealing how 53BP1 is loaded onto chromatin and uses its interacting factors Rif1 and PTIP to promote NHEJ and inhibit HDR.

Keywords: 53BP1; BRCA1; CSR; HDR; NHEJ; PARPi; PTIP; Rif1; V(D)J; resection; telomere.

Figure 1

The domain structures of 53BP1 and Rif1 and the mechanism by which 53BP1 is recruited to DSBs. The binding of 53BP1 requires two histone modifications. The constitutive H4K20^Me2 mark (black) is bound by the Tudor domain and the DDR-induced H2A(X)K15^Ub marks is bound by the UDR domain. The binding of 53BP1 to damaged chromatin is also promoted by its oligomerization domain. The N-terminal ST/Q phosphorylation sites are important for the interaction of 53BP1 with PTIP and Rif1. See text for details on the mechanism by which the 53BP1 binds to damaged chromatin and Text Box 1 for details on the domain structure of 53BP1. See Text Box 2 for how the spreading of 53BP1 from sites of DNA damage is limited.

Figure 2

53BP1 contributes to c-NHEJ in four contexts. 53BP1 is involved in the c-NHEJ in G1 in three specialized contexts: (A) CSR, (B) long range V(D)J recombination, and (C) the fusion of dysfunctional telomeres deprived of TRF2 protection. In (A) the Switch regions where AID induces DSBs are shown by ovals and their associated Constant regions by rectangles with the same color. Joining Sμ to Sε results in productive CSR, yielding IgE and a deleted segment (circle). Iμ and Iε are transcription start sites that promote switching. In (B), and example of long-range V(D)J recombination is given where RAG-dependent recombination joins a Variable (V) region to a D/J region 172 kb away (adapted from [47]). In (C), the shelterin complex at telomeres is shown before and after the deletion of TRF2, which results in ATM signaling and c-NHEJ of the dysfunctional telomeres in G1. These fusions are visualized as the chromosome-type fusions (drawn) in the following metaphase. In (D), the role of 53BP1 in BRCA1-deficient cells is shown. In S/G2, 53BP1 promotes the formation of mis-rejoined chromosome aberrations and inhibits HDR, leading to radial chromosomes and chromatid breaks as shown. This role of 53BP1 is only observed in BRCA1-deficient cells and is enhanced by treatment with PARP1 inhibitors, which lead to DSB formation in S/G2 (as depicted). Not shown is the fifth setting, DSB repair in heterochromatin, where 53BP1 acts through an unknown mechanism that involves its BRCT domains, which are not required for the processes shown in A–D.

Figure 3

Cell cycle-dependent mutual exclusion of BRCA1 and Rif1 from DSBs. In G1, Rif1 prevents the accumulation of BRCA1 at DSBs and acts downstream of 53BP1 to block resection, thereby promoting c-NHEJ. In S/G2, phosphorylated CtIP blocks Rif1 from binding to 53BP1, thereby allowing resection and promoting HDR. CtIP also plays a role in promoting resection but this attribute appears to be independent of its binding to BRCA1.

Figure 4. 53BP1 stimulates chromatin mobility and mediates synapsis

Top: 53BP1 is required for the increased mobility of dysfunctional telomeres. Rif1 inhibits resection at the dysfunctional telomeres while a Rif1-independent 53BP1 function promotes the mobility of the damaged telomeric chromatin. Both processes are thought to enhance the c-NHEJ of dysfunctional telomeres in G1. Bottom: 53BP1 is proposed to mediate synapsis of distal DNA ends during CSR (shown) and long-range V(D)J (not shown). Oligomerization of 53BP1 is required for this function. 53BP1 may act by synapsis of the intervening DNA fragment thereby removing the competing DNA ends and promoting productive CSR. Alternatively, 53BP1 may be acting by bringing the DNA ends together that will lead to productive CSR.