EVIDENCE FOR DNA-PK-DEPENDENT AND DNA-PK-INDEPENDENT DNA DOUBLE-STRAND BREAK REPAIR PATHWAYS IN MAMMALIAN-CELLS AS A FUNCTION OF THE CELL-CYCLE

Mice homozygous for the scid (severe combined immune deficiency) mutat ion are defective in the repair of DNA double-strand breaks (DSBs) and are consequently very X-ray sensitive and defective in the lymphoid V (D)J recombination process, Recently, a strong candidate for the scid gene has been identified as the catalytic subunit of the DNA-dependent protein kinase (DNA-PK) complex, Here, we show that the activity of t he DNA-PK complex is regulated in a cell cycle-dependent manner, with peaks of activity found at the G(1)/early S phase and again at the G(2 ) phase in wild-type cells. Interestingly, only the deficit of the G(1 )/early S phase DNA-PK activity correlated with an increased hypersens itivity to X-irradiation and a DNA DSB repair deficit in synchronized scid pre-B cells. Finally, we demonstrate that the DNA-PK activity fou nd at the G(2) phase may be required for exit from a DNA damage-induce d G(2) checkpoint arrest. These observations suggest the presence of t wo pathways (DNA-PK-dependent and -independent) of illegitimate mammal ian DNA DSB repair and two distinct roles (DNA DSB repair and G(2) che ckpoint traversal) for DNA-PK in the cellular response to ionizing rad iation.