Antisense GADD45 expression results in decreased DNA repair and sensitizes cells to u.v.-irradiation or cisplatin.

Loss of p53 function in cancer cells commonly results in a condition of genomic instability. This is believed to emanate from a loss of the G1 checkpoint response to DNA damage. While the role of p53 in the induction of a G1 arrest is well-accepted, additional p53 functions are being discovered. Cell cycle checkpoints presumably function to allow additional time for DNA repair after damage is incurred, however, genetic studies in yeast suggest that components of the checkpoint pathway may also be involved in DNA lesion processing (Lydall and Weinert, 1995). Recent evidence suggests that this may also be the case for p53, as suggested by numerous reports linking p53 function to DNA repair. Thus, loss of p53 function might contribute to genomic instability independent of G1-arrest. In the present study, we explored the effect of p53 disruption and consequences of antisense GADD45 expression on the DNA repair capacity of human colon carcinoma RKO cells. DNA repair was assayed using host-cell reactivation of u.v.-damaged reporter plasmids and unscheduled DNA synthesis experiments in transiently-transfected cells. We show that a number of transfected genes that suppress p53 function reduce the ability of cells to repair u.v.-induced DNA damage. Moreover, cells in which expression of the p53-regulated gene GADD45 was blocked by antisense vectors, also showed altered levels of DNA repair. Blocking Gadd45 expression by constitutive antisense expression sensitized cells to killing by u.v.-radiation or by cis-platinum (II) diamine-dichloride (CDDP, or cisplatin), a cancer chemotherapy drug which produces DNA cross-links. These findings suggest the involvement of downstream effectors of the p53 pathway in the coordination of cell cycle arrest and DNA repair.