Down-regulation of PARP-1, but not of Ku80 or DNA-PKcs, results in higher gene targeting efficiency

The viability of non-homologous end-joining (NHEJ)-defective mice suggests that homologous recombination (HR) might take over its role in DNA repair. To test this hypothesis, we examined gene targeting frequencies (TF) in DNA-PKcs, Ku80 and poly(ADP-ribose) polymerase (PARP-1) nullizygous cells. We observed a 3-fold TF increase in PARP-1 knockout embryonic stem (ES) cells, which is consistent with the predicted role of PARP-I as a switch between HR and NHEJ. To a lesser extent, such effect could be reproduced upon chemical inhibition of PARP-1. However, TF was not enhanced in Ku80- or DNA-PKcs-defective cells. Our study also suggests an unexpected involvement of DNA-PKcs in HR. (c) 2006 International Federation for Cell Biology. Published by Elsevier Ltd. All rights reserved.