ABNORMAL PATTERN OF POST-GAMMA-RAY DNA-REPLICATION IN RADIORESISTANT FIBROBLAST STRAINS FROM AFFECTED MEMBERS OF A CANCER-PRONE FAMILY WITHLI-FRAUMENI SYNDROME

Non-malignant dermal fibroblast strains, cultured from affected member s of a Li-Fraumeni syndrome (LFS) family with diverse neoplasms associ ated with radiation exposure, display a unique increased resistance to the lethal effects of gamma-radiation. In the studies reported here, this radioresistance (RR) trait has been found to correlate strongly w ith an abnormal pattern of post-gamma-ray DNA replicative synthesis, a s monitored by radiolabelled thymidine incorporation and S-phase cell autoradiography. In particular, the time interval between the gamma-ra y-induced shutdown of DNA synthesis and its subsequent recovery was gr eater in all four RR strains examined and the post-recovery replicatio n rate was much higher and was maintained longer than in normal and sp ousal controls. Alkaline sucrose sedimentation profiles of pulse-label led cellular DNA indicated that the unusual pattern of DNA replication in irradiated RR strains may be ascribed to anomalies in both replico n initiation and DNA chain elongation processes. Moreover, the RR stra in which had previously displayed the highest post-gamma-ray clonogeni c survival was found to harbour a somatic (codon 234) mutation (presum ably acquired during culture in vitro) in the same conserved region of the p53 tumour-suppressor gene as the germline (codon 245) mutation i n the remaining three RR strains from other family members, thus coupl ing the RR phenotype and abnormal post-gamma-ray DNA synthesis pattern with faulty p53 expression. Significantly, these two aberrant radiore sponse end points, along with documented anomalies in c-myc and c-raf- 1 proto-oncogenes, are unprecedented among other LFS families carrying p53 germline mutations. We thus speculate that this peculiar cancer-p rone family may possess in its germ line a second, as yet unidentified , genetic defect in addition to the p53 mutation.