Distribution of UV-induced unscheduled DNA synthesis in metaphase chromosomes of Chinese hamster cells

Fully functional global genome repair of (6-4) photoproducts and compromised transcription-coupled repair of cyclobutane pyrimidine dimers in condensed mitotic chromatin

During mitosis, chromatin is highly condensed, and activities such as transcription and semiconservative replication do not occur. Consequently, the condensed condition of mitotic chromatin is assumed to inhibit DNA metabolism by impeding the access of DNA-transacting proteins. However, about 40 years ago, several researchers observed unscheduled DNA synthesis in UV-irradiated mitotic chromosomes, suggesting the presence of excision repair. We re-examined this subject by directly measuring the removal of UV-induced DNA lesions by an ELISA and by a Southern-based technique in HeLa cells arrested at mitosis. We observed that the removal of (6-4) photoproducts from the overall genome in mitotic cells was as efficient as in interphase cells. This suggests that global genome repair of (6-4) photoproducts is fully functional during mitosis, and that the DNA in mitotic chromatin is accessible to proteins involved in this mode of DNA repair. Nevertheless, not all modes of DNA repair seem fully functional during mitosis. We also observed that the removal of cyclobutane pyrimidine dimers from the dihydrofolate reductase and c-MYC genes in mitotic cells was very slow. This suggests that transcription-coupled repair of cyclobutane pyrimidine dimers is compromised or non-functional during mitosis, which is probably the consequence of mitotic transcriptional repression.

Phototoxicity testing in vitro: evaluation of mammalian cell culture techniques

We have studied the effects of ultraviolet light on cells, and the phototoxic actions of a range of UV- or visible light-absorbing compounds in three separate in vitro tests (photolysis of red blood cells and phototoxicity to macrophages and to Chinese hamster CHV79 cells). Exposure to UV light for short periods was non-toxic to primary macrophages and red blood cells, but produced extensive killing of CHV79 cells. CHV79 cells can repair a considerable amount of UV-induced damage by unscheduled DNA synthesis but scheduled DNA synthesis is seriously disrupted. In vitro phototoxic compounds selectively damage discrete cellular components, such as the nucleus, plasma membrane or cytoplasmic organelles. Cellular damage can be shown to occur via two distinct photochemical pathways, either oxygen dependent or oxygen independent.

The influence of inhibitors on dimer removal and repair of single-strand breaks in normal and bromodeoxyuridine substituted DNA of HeLa cells

The elimination of cyclobutane pyrimidine dimers from the nuclear DNA of ultraviolet irradiated HeLa cells has been examined by means of chromatography and immunoautoradiography. The extent and duration of the process was similar when dimers were assayed by both methods, proving that the anti-sera recognized pyrimidine dimers. The rate of dimer excision did not differ through the cell cycle with the exception of mitosis during which no dimers were removed. Dimer excision is a relatively fast process which is terminated within a few hours, but it leaves many dimers in the DNA. Excision is depressed by inhibitors of semiconservative DNA synthesis that affect the DNA precursor pool or DNA polymerases. Cells whose DNA is partly substituted with bromodeoxyuridine instead of thymidine, repair single-strand breaks and remove dimers at the same rate but to different extents. On the other hand, inhibitors limit repair of breaks and removal of dimers to the same degree suggesting that the repair of the two types of lesion is coordinated.