UV micro-irradiation of the Chinese hamster cell nucleus and caffeine post-treatment. Immunocytochemical localization of DNA photolesions in cells with partial and generalized chromosome shattering.

UV micro-irradiation of a small part of the Chinese hamster nucleus and caffeine post-incubation often results in shattered chromosomes at the first post-irradiation mitosis. In some of these mitotic cells, chromosome shattering is restricted to a few chromosomes spatially related in a small area of the metaphase spread; in others, shattering includes the whole chromosome complement. These 2 types of damage have been called partial and generalized chromosome shattering (PCS and GCS). Using antisera that specifically react with UV-irradiated DNA, we identified micro-irradiated chromatin in interphase nuclei and in mitotic cells with PCS or GCS by indirect immunofluorescence microscopy. In PCS, immunofluorescence staining was found in the damaged area, while the surrounding intact chromosomes were not stained. In GCS, staining was also restricted to a small region of the shattered chromosome complement. In other experiments, cells synchronized in G1 were micro-irradiated in the nucleus, pulse-labelled with [3H]thymidine and post-incubated with caffeine. Autoradiographs of cells with GCS showed unscheduled DNA synthesis restricted to a small chromatin region. Our data present direct evidence that the distribution of DNA photolesions does not coincide with the sites of chromosomal damage in GCS. As a working, hypothesis, we propose that an indirect mechanism is involved in the induction of GCS by which DNA photolesions in a small nuclear segment induce shattering of both micro-irradiated and non-irradiated chromosomes.