NHEJ mechanisms of DSBs
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 .
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
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
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 .
Other proteins that are involved in the end-processing are DNA
polymerase mu , exonuclease flap structure-specific
endonuclease 1 (FEN1) and Werner syndrome helicase
(WRN) . Ku70/80
interacts with WRN and stimulates WRN
exonuclease activity . 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 .
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
. 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 .
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 , .
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