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:
- Pastwa E, Blasiak J
Non-homologous DNA end joining.
Acta biochimica Polonica 2003;50(4):891-908
- 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
- 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
- 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
- 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
- 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
- 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
- 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
- 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
- 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