Non-homologous end joining mechanismn 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) appears to be the major mechanism for repairing DSBs in mammalian cells. This pathway does not require homology and can "rejoin broken" DNA ends directly, end-to-end. It has been suggested that repair of DSBs via NHEJ proceeds in three steps:

• Terminal binding and bridging,
• terminal processing and
• Ligation (Pastwa E et al. 2003).

In the first step, the Ku70/80 heterodimer binds the DNA ends (the end-binding activity of the Ku70/80 heterodimer suggests that it may be the primary damage detector in NHEJ), aligning them and thus preparing the ends for ligation and protecting them 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 DSBs and activates their kinase function (Pastwa E et al. 2003).

Finally, DNA-PK binds to and phosphorylates the complex of DNA ligase IV and XRCC4 (X-ray repair cross complementing protein 4). Subsequently, casein kinase II-phosphorylated XRCC4 interacts with polynucleotide kinase (PNKP), which functions as a 5'-kinase/3'-phosphatase, to form 5'-phosphate/3'-hydroxyl termini that are a necessary prerequisite for ligation during repair (Koch CA et al. 2004).

The nuclease complex MRN may also participate in terminal processing of NHEJ as well as damage signaling and protection of ends from degradation. The MRN complex consists of the double-strand break repair protein (Mre11), the Rad50 homolog (S. cerevisiae) (Rad50), and the Nijmegen break syndrome 1 protein (Nibrin). The MRN complex can be activated via the Brca1/Rad50 pathway (Zhong Q et al. 2002).

Other proteins involved in end processing include DNA polymerase mu (Mahajan KN et al. (2002), exonuclease flap structure-specific endonuclease 1 (FEN1), and Werner syndrome helicase (WRN). Ku70/80 interacts with WRN and stimulates the exonuclease activity of WRN (Li B et al. 2002). The ability of WRN to facilitate cleavage of DNA replication/repair intermediates by FEN1 may be relevant to the role of WRN in maintaining genome stability.

A significant portion of the DNA cross-link repair protein 1C, Artemis, exists in the cell in complex with DNA-PK, which becomes an endonuclease after its phosphorylation by DNA-PK (Ma Y et al. 2002). After truncation of excess or damaged DNA, the Artemis/DNA-PK complex can disintegrate, allowing binding of the ligase complex XRCC4/DNA ligase IV, which completes the linkage . In addition, so-called "silent horologs" (sirtuins) regulating mating information may be involved in DSB repair. The presence of sirtuins at DNA damage sites and their interaction with Ku70/80 suggest that they may affect the accessibility of broken ends to DNA-processing enzymes and/or to Ku70/80 in NHEJ (Bailey SM et al. (1999).

1. Bailey SM et al (1999) 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 96:14899-14904.
2. Koch CA et al (2004) Xrcc4 physically links DNA end processing by polynucleotide kinase to DNA ligation by DNA ligase IV. The EMBO journal 23:3874-3885Pastwa E et al. (2003) Non-homologous DNA end joining. Acta biochimica Polonica 50:891-908.
3. Mahajan KN et al. (2002) 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 22:5194-202.
4. Pastwa E et al (2003) Non-homologous DNA end joining. Acta biochimica Polonica 50:891-908.
5. Zhong Q et al (2002) Deficient nonhomologous end-joining activity in cell-free extracts from Brca1-null fibroblasts. Cancer research 62:3966-3970