DNA damage - NHEJ mechanisms of DSBs repair

NHEJ mechanisms of DSBs repair

DNA double-strand breaks (DSBs) result from disruption of the phosphodiester backbone on both strands of the DNA double helix. Non-homologous end joining (NHEJ) seems to be the primary mechanism of DSBs repair in mammalian cells. This pathway does not require homology and can rejoin broken DNA ends directly end-to-end. It was suggested that DSBs repair via NHEJ is carried out in three steps: end-binding and bridging, terminal processing, and ligation [1].

In the first step, Ku70/80 heterodimer binds the DNA ends (the end-binding activity of Ku70/80 heterodimer suggests that it may be the primary damage detector in NHEJ), aligns them and thus prepares the ends for ligation and protects from degradation. Ku70/80 consists of two ATP-dependent DNA helicases II subunits, 70 kDa and 80 kDa (Ku70 and Ku80). This complex recruits DNA-activated protein kinase (DNA-PK) to the DSBs, activating its kinase function [1].

Finally, DNA-PK binds to DNA ligase IV / X-ray repair cross complementing protein 4 (XRCC4) complex and phosphorylates it. Hereinafter, Casein kinase II - phosphorylated XRCC4 interacts with polynucleotide kinase (PNKP), which acts as a 5'-kinase/3'-phosphatase to create 5'-phosphate/3'-hydroxyl termini, which are a necessary prerequisite for ligation during repair [2].

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. MRN complex consists of double-strand break repair protein (Mre11), Rad50 homolog (S. cerevisiae) (Rad50) and Nijmegen breakage syndrome 1 protein (Nibrin). MRN complex may be activated via Brca1/ Rad50 pathway [3].

Other proteins that are involved in the end-processing are DNA polymerase mu [4], exonuclease flap structure-specific endonuclease 1 (FEN1) and Werner syndrome helicase (WRN) [1]. Ku70/80 interacts with WRN and stimulates WRN exonuclease activity [5]. The ability of WRN to facilitate FEN1 cleavage of DNA replication/repair intermediates may be important for the role of WRN in the maintenance of genomic stability [6].

A significant fraction of DNA cross-link repair 1C protein, Artemis, exists in the cell in complex with DNA-PK, which becomes an endonuclease after it is phosphorylated by DNA-PK [7]. Upon trimming off an excess or damaged DNA, Artemis/DNA-PK complex may disassemble which permits binding of the ligase complex, XRCC4/DNA ligase IV, which completes the joining [8].

In addition, silent mating type information regulation horologes (Sirtuins) may participate in DSB repair. Presence of Sirtuins at DNA damage sites and its interaction with Ku70/80 indicate that they might influence the accessibility of the broken ends to DNA processing enzymes and/or to the Ku70/80 in NHEJ [9], [10].

References:

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   Non-homologous DNA end joining. Acta biochimica Polonica 2003;50(4):891-908
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   Xrcc4 physically links DNA end processing by polynucleotide kinase to DNA ligation by DNA ligase IV. The EMBO journal 2004 Oct 1;23(19):3874-85
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   Deficient nonhomologous end-joining activity in cell-free extracts from Brca1-null fibroblasts. Cancer research 2002 Jul 15;62(14):3966-70
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   Association of DNA polymerase mu (pol mu) with Ku and ligase IV: role for pol mu in end-joining double-strand break repair. Molecular and cellular biology 2002 Jul;22(14):5194-202
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   Hairpin opening and overhang processing by an Artemis/DNA-dependent protein kinase complex in nonhomologous end joining and V(D)J recombination. Cell 2002 Mar 22;108(6):781-94
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9. Mills KD, Sinclair DA, Guarente L
   MEC1-dependent redistribution of the Sir3 silencing protein from telomeres to DNA double-strand breaks. Cell 1999 May 28;97(5):609-20
10. Bailey SM, Meyne J, Chen DJ, Kurimasa A, Li GC, Lehnert BE, Goodwin EH
    DNA double-strand break repair proteins are required to cap the ends of mammalian chromosomes. Proceedings of the National Academy of Sciences of the United States of America 1999 Dec 21;96(26):14899-904