Trends in Cell Biology
ReviewSpecial Issue: Quality ControlRepair Pathway Choices and Consequences at the Double-Strand Break
Section snippets
Mechanisms of DNA DSB Repair
Detection and faithful repair of damaged DNA is essential for genome integrity. Many types of DNA lesions impede replication fork progression, resulting in replication fork collapse and DSB formation with loss of physical continuity of the genome [1]. The repair of DSBs involves four possible mechanisms (Figure 1). The first mechanism is C-NHEJ. In this mechanism, the DSB is repaired by blunt end ligation independently of sequence homology, but requiring many factors such as Ku70/80, DNA-PKcs,
Role of End Resection in DSB Repair Choice
Given that three of the pathways diverge at the early step of end resection and have different outcomes (Figure 1), it is likely that end resection dictates pathway choice and repair outcome [12]. The initial phase of end resection, called ‘end clipping’, is carried out by the structure-specific nuclease MRE11 and CtIP. In this phase, a relatively small number of base pairs (i.e., 20 bp in mammalian cells or 100–300 bp in yeast) are processed, making the DNA ends available for alt-EJ 13, 14. In
Homologous Recombination Control: The RAD51 Hub in Homology-Based Repair Pathway Choice
Once end resection has occurred, the C-NHEJ repair pathway is prevented and any of the three homology-based pathways, that is, SSA, alt-EJ, and HR, can be invoked. Usage of these pathways can be affected by regulation of and competition with the RAD51 presynaptic and postsynaptic steps. We focus on positive and negative regulators of RAD51 function. Upon DNA damage induction, the RPA complex initially competes with RAD51 for ssDNA binding in yeast and mammals 17, 40. However, RPA has also a
Connection between HR and alt-EJ
alt-EJ functions in S phase independently of C-NHEJ factors and, until recently, was considered a backup repair pathway for C-NHEJ for joining chromosomal DSBs, particularly during V(D)J recombination 2, 71, 72. However, recent advances have shown that the alt-EJ and HR pathways compete for the repair of DSBs 49, 73. While both alt-EJ and HR share a common initial resection mechanism [8], processing of resected ends in HR is catalyzed by the eukaryotic RecA homolog, RAD51, while PARP1 is
Annealing-Dependent DSB Repair Outcomes and Their Mutational Signatures
As the repair of DSBs, via annealing-dependent pathways, introduces insertions and/or deletions (Figure 5), one can predict that the use of such DNA repair mechanisms will leave behind a particular genomic signature, or ‘genomic scar’, distinct from that of HR-mediated repair, which usually preserves genomic integrity. As modern next-generation sequencing technologies can rapidly identify cancer risk alleles of many genes, genome-wide sequencing and loss of heterozygosity (LOH) profiling have
Concluding Remarks
Here, we have reviewed the current knowledge regarding pathways used for the repair of DSBs through the lens of clinical applications. We have discussed regulation of the four known DSB repair pathways and how error-prone DNA repair pathways can compensate for the loss of HR. Understanding the basic regulation of DNA repair mechanisms has led to advances in synthetic lethality-based drug development. For several decades, significant effort has focused on identifying unique oncoproteins or
Acknowledgments
We would like to thank Jessica C. Liu and Prabha Sarangi for editing the manuscript and helpful discussions. R.C. is a recipient of the Ovarian Cancer Research Fellowship (OCRF). B.R. is a recipient of the Italian Association for Cancer Research (AIRC) Fellowships for Abroad. This work was supported by National Institutes of Health (NIH) grants P50CA168504 and R01HL52725 and by grants from the OCRF and Breast Cancer Research Foundation (BCRF) to A.D.D.
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