Biochemical mechanism of DSB end resection and its regulation

Abstract

DNA double-strand breaks (DSBs) in cells can undergo nucleolytic degradation to generate long 3' single-stranded DNA tails. This process is termed DNA end resection, and its occurrence effectively commits to break repair via homologous recombination, which entails the acquisition of genetic information from an intact, homologous donor DNA sequence. Recent advances, prompted by the identification of the nucleases that catalyze resection, have revealed intricate layers of functional redundancy, interconnectedness, and regulation. Here, we review the current state of the field with an emphasis on the major questions that remain to be answered. Topics addressed will include how resection initiates via the introduction of an endonucleolytic incision close to the break end, the molecular mechanism of the conserved MRE11 complex in conjunction with Sae2/CtIP within such a model, the role of BRCA1 and 53BP1 in regulating resection initiation in mammalian cells, the influence of chromatin in the resection process, and potential roles of novel factors.

Keywords: Double-strand breaks; Helicases; Nucleases; Recombination; Resection.

Figure 1

(A) Inappropriate NHEJ of replication-associated DSBs in S phase leads to dicentric chromosomes and complex chromosome rearrangements. (B) Extensive resection of DNA ends in G1 cells can lead to deletion-associated MMEJ events.

Figure 2

Crosstalk among the three nucleases in DNA end resection. Dotted lines indicate protein-protein interactions and red arrows indicate a stimulatory effect. BLM interacts directly with EXO1, and in yeast MRX interacts with Sgs1 (the BLM ortholog). RPA, BLM, and MRN are all capable of stimulating the nuclease activity of EXO1, and MRN(X) enhances the helicase activity of BLM/Sgs1.

Figure 3

Model for nick-initiated resection by MRN-CtIP at protein-occluded DSBs. Sae2/CtIP activates the MRX/MRN endonuclease activity to generate a nick, followed by bidirectional resection catalyzed by MRN in the 3′ to 5′ direction and EXO1 in the 5′ to 3′ direction.