Neoplastic transformation of human cells in culture associated with deficient repair of light‐induced chromosomal DNA damage

Abstract

AbstractTwo lines of normal human skin fibroblasts and five derivative lines transformed in culture to neoplastic cells by chemical carcinogens were compared with respect to chromatid breakage produced by exposure to low‐intensity fluorescent light (cool‐white, 4.6 W/m2) during pre‐S, S, and late S‐G2 periods of the cell cycle. Five additional normal human cell lines and a line derived from a lung adenocarcinoma were also examined for chromatid damage following light exposure during pre‐S and late S‐G2. On the assumption that each chromatid (half chromosome) contains a single continuous DNA double helix, chromatid breaks seen in the first post‐treatment metaphase must represent unrepaired DNA doublestrand breaks. The frequency of chromatid breaks induced by pre‐S light exposure in the neoplastic cell is not significantly different from that in the normal cells. However, addition of caffeine, an inhibitor of DNA repair, to the culture medium following light exposure significantly increases the frequency of chromatid breaks in the neoplastic but not in the normal cells. One possible explanation for these observations is that the normal cells, known to have effective excision repair, remove the DNA damage, whereas the neoplastic cells, presumably deficient in excision repair, handle the damage during S by a caffeinesensitive mechanism. Furthermore, the neoplastic but not the normal cells, show significant increases in chromatid breaks when exposed during S or late S‐G2The chromatid damage produced by late S‐G2 exposure can be completely prevented by addition of mannitol, a scavenger of free hydroxyl radicals. Addition of caffeine to the culture medium during late S‐G2 light exposure has no effect on the frequency of chromatid breaks in the neoplastic cells but signincantly increases chromatid breakage in the normal cells. These results suggest that the normal cells repair the late S‐G2 light‐induced damage by a caffeine‐sensitive repair mechanism that is absent or deficient in the neoplastic cells. It thus appears that neoplastic cells are deficient in mechanisms for repair of DNA damage induced by light both during pre‐S and late S‐G2periods of the cell cycle.