Faulty DNA polymerase delta/epsilon-mediated excision repair in response to gamma radiation or ultraviolet light in p53-deficient fibroblast strains from affected members of a cancer-prone family with Li-Fraumeni syndrome
Dermal fibroblast strains cultured from affected members of a cancer-prone family with Li-Fraumeni syndrome (LFS) harbor a point mutation in one allele of the p53 tumor suppressor gene, resulting in loss of normal p53 function. In this study we have examined the ability of these p53-deficient strains to carry out the long-patch mode of excision repair, mediated by DNA polymerases delta and epsilon, after exposure to 60Co gamma radiation or far ultraviolet (UV) (chiefly 254 nm) light. Repair was monitored by incubation of the irradiated cultures in the presence of aphidicolin (apc) or 1-beta-D-arabinofuranosylcytosine (araC), each a specific inhibitor of long-patch repair, followed by measurement of drug-induced DNA strand breaks (reflecting non-ligated strand incision events) by alkaline sucrose velocity sedimentation. The LFS strains displayed deficient repair capacity in response to both gamma rays and UV light. The repair anomaly in UV-irradiated LFS cultures was manifested not only in the overall genome, but also in the transcriptionally active, preferentially repaired c-myc gene. Using autoradiography we also assessed unscheduled DNA synthesis (UDS) after UV irradiation and found this conventional measure of repair replication to be deficient in LFS strains. Moreover, both apc and araC decreased the level of UV-induced UDS by approximately 75% in normal cells, but each had only a marginal effect on LFS cells. We further demonstrated that the LFS strains are impaired in the recovery of both RNA and replicative DNA syntheses after UV treatment, two molecular anomalies of the DNA repair deficiency disorders xeroderma pigmentosum and Cockayne's syndrome. Together these results imply a critical role for wild-type p53 protein in DNA polymerase delta/epsilon-mediated excision repair, both the mechanism operating on the entire genome and that acting on expressed genes.