DNA double-strand break repair is decided by molecular sensors and effectors that favor either homologous recombination (HR) or non-homologous end joining (NHEJ). Choice hinges on how DNA ends are processed, the cell cycle stage, and chromatin context. Key proteins act as gatekeepers, promoting one pathway while blocking the other.
Core regulators
BRCA1 and 53BP1 form a central antagonistic pair that steers pathway choice. BRCA1 promotes DNA end resection, a process that generates single-stranded DNA required for RAD51-mediated strand invasion and HR. Research by Maria Jasin at Memorial Sloan Kettering demonstrated the central role of RAD51 in strand exchange during HR, establishing how BRCA-related functions support accurate repair. In contrast, 53BP1 protects DNA ends from excessive resection and thereby favors classical NHEJ. Work by Daniel Durocher at Lunenfeld-Tanenbaum Research Institute characterized ubiquitin-dependent recruitment pathways that help position 53BP1 at breaks, illustrating how chromatin signaling enforces pathway choice.
Downstream of 53BP1, proteins such as RIF1 and the shieldin complex consolidate end protection, while BRCA1 operates with cofactors like PALB2 and BRCA2 to load RAD51. These antagonistic modules respond to post-translational modifications and local chromatin marks so that the same break can be routed differently depending on context.
End-processing factors and cell cycle control
Initial recognition of broken ends by the MRN complex composed of MRE11, RAD50, and NBS1 recruits kinases ATM and ATR that coordinate signaling and control. The MRN complex together with nucleases and the endonuclease cofactor CtIP initiates resection, a step that commits the lesion toward HR. Cell cycle kinases such as cyclin-dependent kinases increase CtIP activity in S and G2 phases, making HR the favored route when a sister chromatid is available. Conversely, the DNA end-binding heterodimer Ku70/Ku80 and the catalytic subunit DNA-PKcs rapidly bind blunt ends to promote c-NHEJ, often with minimal processing.
Alternative end-joining pathways involve PARP1 and the translesion polymerase POLQ. These mechanisms act when classical machinery is compromised and typically use microhomologies, producing mutational scars. Clinical and experimental studies by Chris J. Lord at The Institute of Cancer Research highlighted the therapeutic relevance of these relationships, showing that BRCA-deficient tumors, unable to perform HR, are sensitive to PARP inhibition because they rely on alternative repair.
Clinical and biological relevance
Pathway choice has consequences for genome stability, cancer risk, and therapy. Germline defects in BRCA1 or BRCA2 shift the balance away from HR and raise breast and ovarian cancer susceptibility while rendering tumors vulnerable to synthetic lethality strategies targeting PARP. Tissue-specific chromatin landscapes and differences in replication timing across human populations can modulate the balance between resection and end protection, influencing mutation patterns in different organs and geographic regions. Environmental factors that increase replication stress amplify reliance on particular repair routes, with ecological and territorial disparities in exposure shaping cancer incidence and outcomes.
Understanding the network of BRCA1, 53BP1, RAD51, Ku70/80, MRN, CtIP, PARP1, POLQ, ATM, and ATR clarifies how cells choose between precise and rapid but error-prone repair. This knowledge underpins modern targeted therapies and informs risk assessment in hereditary cancer syndromes.