ATM activation in response to DNA DSBs. ATM kinase activity increases immediately after DSBs occur in DNA following exposure to IR. ATM mediates the early stages of the rapid induction of several signaling pathways, which include activation of the DNA-DSB pathway, regulation of the cell-cycle checkpoint controls, activation of stress responses and maintenance of telomeres. ‘P’ with solid arrows indicates reported phosphorylation events; dashed arrows represent possible signaling steps and do not imply direct interaction between proteins; ‘C’ indicates sequestering in cytoplasm; ‘R’ indicates repair complexes; and ‘T’ indicates a role for the protein in telomere metabolism. BRCA1, breast cancer susceptibility gene product 1; c-Abl, Abelson protein tyrosine kinase; CDK, cyclin-dependent kinase; CHK, checkpoint kinase; FANCD2, Fanconi anaemia protein; JNK, Jun N-terminal protein kinase; MRE11, meiotic recombination 11 gene product; MDM2, ‘mouse double minute 2’ (p53-binding protein); NBS1, Nijmegen breakage syndrome 1 protein (p95); SMC1, ‘structural maintenance of chromosome’ 1; RAD50, a radiation-damage-repair-associated protein; TRF1, telomere-repeat-finding factor 1; hTERT, human catalytic unit of telomerase; hMOF, the human ortholog of the Drosophila MOF gene (males absent on the first); hSSB1, the human ssDNA-binding protein 1; KAP1, KRAB-associated protein.

Major regulatory steps in DSB repair. DNA damage repair is accompanied by recognition of damage, modification of chromatin at the site of DNA damage, recruitment of repair factors and cell-cycle checkpoints. Multiple proteins with different activity for posttranslational modifications of histones present at the DNA DSB allow to open the chromatin in order to make the DNA accessible to DNA repair machinery. Histone modifications are necessary to remodel the nucleosomes during the repair process. Several proteins have been reported to have multiple functions that are involved in the regulation of the DNA DSB repair, whether the damage requires to be repaired by NHEJ or HR. A two-ended DNA DSB induced by IR are substrates for binding of the Ku70/Ku80 heterodimer. Ku70/Ku80 bound to DNA ends recruits DNA-PKcs to the ends and promotes their juxtaposition. If no further processing of the ends is required, the additional core components of nonhomologous DNA end-joining, XRCC4, DNA ligase IV and XLF (XRCC4-like factor also known as Cernunnos) can promote the rejoining reaction. If the two-end DNA DSB require end processing, then such processing may require the activities of the nuclease Artemis and/or the DNA polymerase TdT, pol lambda and pol mu. The Ku heterodimer likely plays a central role in orchestrating the activities of the proteins involved in NHEJ. The exact nature of the active complex is currently undefined, but the transient reversible interaction of the processing factors with the core components provides great flexibility in the combination of broken ends that can be rejoined because the process does not require strict order in which the processing factors engage or in which the four strands will be processed. In general, the final stage of NHEJ is the ligation of DNA ends catalyzed by XRCC4-ligase IV, and this process is promoted by Cernunnos-XLF in an unknown way. DNA DSBs repaired by HR involve various steps. For the promotion of invasion, several proteins like Rad51, Dmc1 are loaded on the replicated DNA of the intact homolog by a single-strand tail of the resected DSB. The close pair of parallel lines represent the two strands of duplex DNA. The left-hand side of the top most strand has 3′ polarity. One of the two sister chromatids has damage-induced DSB. Processing results in single-stranded tails at the break with 3′-hydroxyl ends. The tails are substrates for nucleoprotein filament formation, which are directed for homology recognition and DNA strand exchange lead to joint molecule formation between the broken DNA and the intact sister chromatid.