Double-strand breaks (DSBs), one of the most deleterious types of DNA damage along with interstrand crosslinks, are caused by ionizing radiation or certain chemicals such as bleomycin. DSBs also occur physiologically, during the processes of DNA replication, meiotic exchange, and V(D)J recombination.
DSBs are sensed (detected) by the MRN complex. Binding of the MRN complex to the DSBs usually triggers ATM kinase activation, thus initiating the DNA double strand break response. ATM phosphorylates a number of proteins involved in DNA damage checkpoint signaling, as well as proteins directly involved in the repair of DNA DSBs. DSBs are repaired via homology directed repair (HDR) or via nonhomologous end-joining (NHEJ).
HDR requires resection of DNA DSB ends. Resection creates 3'-ssDNA overhangs which then anneal with a homologous DNA sequence. This homologous sequence can then be used as a template for DNA repair synthesis that bridges the DSB. HDR preferably occurs through the error-free homologous recombination repair (HRR), but can also occur through the error-prone single strand annealing (SSA), or the least accurate microhomology-mediated end joining (MMEJ). MMEJ takes place when DSB response cannot be initiated.
While HRR is limited to actively dividing cells with replicated DNA, error-prone NHEJ pathway functions at all stages of the cell cycle, playing the predominant role in both the G1 phase and in S-phase regions of DNA that have not yet replicated. During NHEJ, the Ku70:Ku80 heterodimer (also known as the Ku complex or XRCC5:XRCC6) binds DNA DSB ends, competing away the MRN complex and preventing MRN-mediated resection of DNA DSB ends. The catalytic subunit of the DNA-dependent protein kinase (DNA-PKcs, PRKDC) is then recruited to DNA-bound Ku to form the DNA-PK holoenzyme. Two DNA-PK complexes, one at each side of the break, bring DNA DSB ends together, joining them in a synaptic complex. DNA-PK complex recruits DCLRE1C (ARTEMIS) to DNA DSB ends, leading to trimming of 3'- and 5'-overhangs at the break site, followed by ligation.
For review of this topic, please refer to Ciccia and Elledge 2010.