Repair of Psoralen-induced Cross-links and Monoadducts in Normal and Repair-deficient Human Fibroblasts

SV40-transformed normal, xeroderma pigmentosum (XP) and Fanconi's anemia (FA) fibroblasts have distinct repair capacities for monoadducts and DNA interstrand cross-links produced by exposure to near-UV (320-400 nm) light in the presence of 8-methoxypsoralen or angelicin. Excision repair of monoadducts occurred rapidly in normal and FA cells after exposure but not in XP cells. Cross-links were repaired in normal cells with a t1/2 of about 10 h but not in XP or FA cells. When the total number of adducts induced by 8-methoxypsoralen in normal cells was kept constant, the amount of repair replication decreased as the ratio of cross-links to monoadducts increased. This suggests either that cross-link repair is significantly different from monoadduct repair, involving smaller patches and a much slower rate of patching or that cross-links can inhibit monoadduct repair. Our results show that XP group A and FAH12 cell lines are deficient in cross-link repair. The data also suggest that the mechanism of cross-link repair in human cells involves several enzymes and that different ones may be deficient in XP and FA cells.