New Facets of DNA Double Strand Break Repair: Radiation Dose as Key Determinant of HR versus c-NHEJ Engagement

Radiation therapy is an essential component of present-day cancer management, utilizing ionizing radiation (IR) of different modalities to mitigate cancer progression. IR functions by generating ionizations in cells that induce a plethora of DNA lesions. The most detrimental among them are the DNA double strand breaks (DSBs). In the course of evolution, cells of higher eukaryotes have evolved four major DSB repair pathways: classical non-homologous end joining (c-NHEJ), homologous recombination (HR), alternative end-joining (alt-EJ), and single strand annealing (SSA). These mechanistically distinct repair pathways have different cell cycle- and homology-dependencies but, surprisingly, they operate with widely different fidelity and kinetics and therefore contribute unequally to cell survival and genome maintenance. It is therefore reasonable to anticipate tight regulation and coordination in the engagement of these DSB repair pathway to achieve the maximum possible genomic stability. Here, we provide a state-of-the-art review of the accumulated knowledge on the molecular mechanisms underpinning these repair pathways, with emphasis on c-NHEJ and HR. We discuss factors and processes that have recently come to the fore. We outline mechanisms steering DSB repair pathway choice throughout the cell cycle, and highlight the critical role of DNA end resection in this process. Most importantly, however, we point out the strong preference for HR at low DSB loads, and thus low IR doses, for cells irradiated in the G₂-phase of the cell cycle. We further explore the molecular underpinnings of transitions from high fidelity to low fidelity error-prone repair pathways and analyze the coordination and consequences of this transition on cell viability and genomic stability. Finally, we elaborate on how these advances may help in the development of improved cancer treatment protocols in radiation therapy.