DNA damage NHEJ mechanisms of DSBs repair
NHEJ mechanisms of DSBs repair DNA double-strand breaks (DSBs) result from disruption of the phosphodiester backboneon both strands of the DNA double helix. Non-homologous end joining (NHEJ) seems to bethe primary mechanism of DSBs repair in mammalian cells. This pathway does not requirehomology and can rejoin broken DNA ends directly end-to-end. It was suggested that DSBsrepair via NHEJ is carried out in three steps: end-binding and bridging, terminalprocessing, and ligation . In the first step, Ku70/80 heterodimer binds the DNA ends (the end-bindingactivity of Ku70/80 heterodimer suggests that it may be the primary damagedetector in NHEJ), aligns them and thus prepares the ends for ligation and protects fromdegradation. Ku70/80 consists of two ATP-dependent DNA helicases II subunits, 70kDa and 80 kDa ( Ku70 and Ku80 ). This complex recruits DNA-activatedprotein kinase ( DNA-PK ) to the DSBs, activating its kinase function . Finally, DNA-PK binds to DNA ligase IV/ X-ray repair crosscomplementing protein 4 ( XRCC4 ) complex and phosphorylates it. Hereinafter,Casein kinase II - phosphorylated XRCC4 interacts with polynucleotidekinase ( PNKP ), which acts as a 5'-kinase/3'-phosphatase to create5'-phosphate/3'-hydroxyl termini, which are a necessary prerequisite for ligation duringrepair . The nuclease MRN complex also can participate in terminal processing of NHEJ,as well as in damage signaling and protection of the ends from degradation. MRNcomplex consists of double-strand break repair protein ( Mre11), Rad50homolog (S. cerevisiae) ( Rad50 ) and Nijmegen breakage syndrome 1 protein (Nibrin ). MRN complex may be activated via Brca1/ Rad50pathway . Other proteins that are involved in the end-processing are DNA polymerase mu, exonuclease flap structure-specific endonuclease 1 ( FEN1 ) andWerner syndrome helicase ( WRN ) . Ku70/80 interacts withWRN and stimulates WRN exonuclease activity . The abilityof WRN to facilitate FEN1 cleavage of DNA replication/repair intermediatesmay be important for the role of WRN in the maintenance of genomic stability. A significant fraction of DNA cross-link repair 1C protein, Artemis, exists inthe cell in complex with DNA-PK, which becomes an endonuclease after it isphosphorylated by DNA-PK . Upontrimming off an excess or damaged DNA, Artemis/ DNA-PK complex maydisassemble which permits binding of the ligase complex, XRCC4/ DNA ligaseIV, which completes the joining . In addition, silent mating type information regulation horologes ( Sirtuin s)may participate in DSB repair. Presence of Sirtuin s at DNA damage sites and itsinteraction with Ku70/80 indicate that they might influence the accessibility ofthe broken ends to DNA processing enzymes and/or to the Ku70/80 in NHEJ , .