DNA-PK-independent rejoining of DNA double-strand breaks in human cell extracts in vitro

To investigate the role of DNA-dependent protein kinase (DNA-PK) in the rejoining of ionizing radiation-induced DNA double-strand breaks (dsb).

This study employed previously described in vitro assays that utilize nuclei or 'naked' DNA prepared from agarose-embedded cells as a substrate and S-HeLa cell extracts as a source of enzymes. Rejoining of dsb in these assays is absolutely dependent on cell extract and it proceeds, under optimal reaction conditions, to an extent similar to that observed in intact cells. Results were confirmed in a plasmid-based assay for in vitro rejoining of dsb.

It is shown that concentrations of wortmannin completely inhibiting DNA-PK activity profoundly affect the rejoining of dsb in vivo, but have no effect on dsb rejoining in vitro. Furthermore, fractionation of cell extracts using ammonium sulphate precipitation, generates protein fractions that are able to support dsb rejoining, despite the fact that they do not contain detectable amounts of either DNA-PKcs or Ku80. Efficient rejoining of dsb in vitro is also observed with extracts of MO59J cells that lack DNA-PK activity. Finally, rejoining of dsb remains unaffected by wortmannin in a plasmid-based assay, and is also detectable with extracts of MO59J cells.

These findings are in contrast with genetic studies demonstrating a requirement for DNA-PK activity for efficient rejoining of dsb in vivo. The difference between in vitro and in vivo results may not be attributed to chromatin structure since wortmannin was without an effect when using nuclei as a substrate. It is speculated that the differences between in vivo and in vitro results can be explained either by assuming the operation of multiple pathways in dsb rejoining, some of which do not require DNA-PK, or by postulating a purely regulatory/damage-sensing role for DNA-PK in intact cells but no direct involvement in dsb rejoining.