Postreplication Repair of Ultraviolet Damage to DNA in Xeroderma Pigmentosum, Other Human and Mouse Cells in Culture

Abstract

Two XP cell lines used, XP 1 and XP 3, are defective in unscheduled DNA synthesis and have Do values of 12 and 19 ergs/mm², respectively. These values are much smaller than the Do of about 100 ergs/mm² found for HeLa S 3 and human embryonic diploid F 1000 cells. This indicates that excision repair predominates in normal human cells. Mouse L 5 cells possess the reduced ability of unscheduled DNA synthesis which is only one third of that in HeLa and F 1000 cells, but are as UV-resistant as human cells of HeLa and F 1000. This suggests that L 5 cells may possess an enhanced ability of postreplication repair. Molecular evidence for this is that the change with UV dose of the average size of post-UV synthesized DNA pieces in L 5 cells is more similar to that in HeLa cells than that in XP 3 cells. The characteristics of postreplication repair as revealed by alkaline sucrose sedimentation of DNA pieces taken from UV-irradiated human cells (XP, HeLa and F 1000) are as follows : Gaps are formed in newly synthesized DNA and subsequently filled in rather quickly. The gaps fully patched with BrdUrd chase are reformed by photolysis of the BrdUrd patches with near UV irradiation. The gap-filling is prevented by 2 × 10^-3M caffeine in human cells as is known for L 5 cells. All the cell lines tested, including two XP lines, show the postreplication repair ability. These results may support a model that the gaps which have been formed in newly synthesized DNA presumably opposite pyrimidine dimers in the parental DNA are filled in by de novo elongation of DNA chains. A possible aspect of UV carcinogenesis in XP is discussed in relation to DNA repair.