Restoration of ligatable "clean" double-strand break ends is the rate-limiting step in the rejoining of ionizing-radiation-induced DNA breakage.

Radiotherapy kills malignant cells by generating double-strand breaks (DSBs). Ionizing- radiation (IR) generates "dirty" DSBs, which associates with blocking chemical adducts at DSB ends. Homologous-directed repair (HDR) efficiently removes IR-induced blocking adducts from both 3' and 5' ends of DSBs. Nonhomologous end-joining (NHEJ) rejoins virtually all DSBs in G ₁ phase and ∼80 % of DSBs in G ₂ phase. However, DNA Ligase IV, an essential NHEJ factor, rejoins only "clean" ligatable DSBs carrying 3'-OH and 5'-phosphate DSB ends but not dirty DSBs. Recent studies have identified a number of nucleases, especially the MRE11 nuclease, as key factors performing the removal of blocking chemical adducts to restore clean ligatable DSBs for subsequent NHEJ. This restoration, but not subsequent NHEJ, is the rate-limiting step in the rejoining of IR- induced DSBs. This review describes repair factors that contribute to the restoration of clean DSBs before NHEJ.
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