53BP1, BRCA1, and the choice between recombination and end joining at DNA double-strand breaks

Abstract

When DNA double-strand breaks occur, the cell cycle stage has a major influence on the choice of the repair pathway employed. Specifically, nonhomologous end joining is the predominant mechanism used in the G1 phase of the cell cycle, while homologous recombination becomes fully activated in S phase. Studies over the past 2 decades have revealed that the aberrant joining of replication-associated breaks leads to catastrophic genome rearrangements, revealing an important role of DNA break repair pathway choice in the preservation of genome integrity. 53BP1, first identified as a DNA damage checkpoint protein, and BRCA1, a well-known breast cancer tumor suppressor, are at the center of this choice. Research on how these proteins function at the DNA break site has advanced rapidly in the recent past. Here, we review what is known regarding how the repair pathway choice is made, including the mechanisms that govern the recruitment of each critical factor, and how the cell transitions from end joining in G1 to homologous recombination in S/G2.

FIG 1

The effects of nucleolytic resection on NHEJ of DNA ends that bear 5′ or 3′ overhangs. With ends that harbor 3′ overhangs (top left), NHEJ becomes inefficient once the gap size stemming from resection becomes 2 nucleotides or greater. Accurate NHEJ of ends with 5′ overhangs (top right) is eliminated by resection. After resection, repair normally proceeds by HR (bottom right). Occasionally, and especially when a homologous template is unavailable, deletion-prone MMEJ occurs (bottom left).

FIG 2

Histone modifications involved in 53BP1 or BRCA1 recruitment to DSBs. Proteins that add or remove each modification are indicated by arrows. Dotted lines indicate protein-protein interactions. A question mark indicates a relationship that has been documented in the literature but whose mechanism is unknown.

FIG 3

Model for DSB occupancy in G₁ and S/G₂. (A) In G₁ cells, 53BP1 is phosphorylated by ATM and becomes localized at the break. RIF1 and PTIP are recruited in a phospho-dependent manner and block resection via an unknown mechanism. BRCA1 is bound to Ku but in a quantity so low that they do not appear as visual foci. (B) In S/G₂, CtIP is phosphorylated by CDK, inducing the formation of a complex with BRCA1 and MRN. This complex displaces 53BP1 and initiates resection. Ku may be removed by coordinated endo- and exonuclease activities that are initiated from a nick in a manner analogous to the postulated mechanism of Spo11 removal.