Pathway Map Details

DNA damage_NHEJ mechanisms of DSBs repair

view in full size
| open in MetaCore

Object list (links open in MetaCore):

WRN, Sirtuin, Ku70/80, DNA ligase IV, PNKP, XRCC4, Rad50, Artemis, Brca1, MRE11, Casein kinase II, alpha chains, Ku80, FEN1, Casein kinase II, beta chain (Phosvitin), Ku70, DNA polymerase mu, DNA-PK, Nibrin, MRN complex


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 ( Sirtuin s) may participate in DSB repair. Presence of Sirtuin s 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].


  1. Pastwa E, Blasiak J
    Non-homologous DNA end joining. Acta biochimica Polonica 2003;50(4):891-908
  2. Koch CA, Agyei R, Galicia S, Metalnikov P, O'Donnell P, Starostine A, Weinfeld M, Durocher D
    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
  3. Zhong Q, Boyer TG, Chen PL, Lee WH
    Deficient nonhomologous end-joining activity in cell-free extracts from Brca1-null fibroblasts. Cancer research 2002 Jul 15;62(14):3966-70
  4. Mahajan KN, Nick McElhinny SA, Mitchell BS, Ramsden DA
    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
  5. Li B, Comai L
    Displacement of DNA-PKcs from DNA ends by the Werner syndrome protein. Nucleic acids research 2002 Sep 1;30(17):3653-61
  6. Brosh RM Jr, Driscoll HC, Dianov GL, Sommers JA
    Biochemical characterization of the WRN-FEN-1 functional interaction. Biochemistry 2002 Oct 8;41(40):12204-16
  7. Ma Y, Pannicke U, Schwarz K, Lieber MR
    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
  8. Ma Y, Lu H, Tippin B, Goodman MF, Shimazaki N, Koiwai O, Hsieh CL, Schwarz K, Lieber MR
    A biochemically defined system for mammalian nonhomologous DNA end joining. Molecular cell 2004 Dec 3;16(5):701-13
  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