ALTERED METAPHASE CHROMOSOME STRUCTURE IN XRS-5 CELLS IS NOT RELATED TO ITS RADIATION SENSITIVITY OR DEFECTIVE-DNA BREAK REJOINING

The Chinese hamster ovary (CHO) cell line xrs-5 is a radiation-sensiti ve derivative of CHO-K1 cells. The xrs-5 cells have a defect in DNA do uble-strand break rejoining and show alterations in chromosome structu re and nuclear morphology. The relationship between radiation sensitiv ity and metaphase chromosome morphology was examined in 12 'revertant' xrs-5 clones isolated following treatment with 5-azacytidine. Nine of the clones were radioresistant while the other three retained xrs-5-l ike radiation sensitivity. Chromosome morphology reverted to CHO-K1-li ke characteristics in three of the radioresistant clones and one of th e radiosensitive clones suggesting that the over-condensed metaphase c hromosome morphology of xrs-5 cells does not underlie its radiation se nsitivity. Radiation sensitivity did correlate with DNA double-strand break rejoining ability. The radioresistant clones showing the over-co ndensed xrs-5-like chromosome morphology were also slightly more sensi tive to the topoisomerase II inhibitor etoposide (VP-16) than CHO-K1, suggesting that the over-condensed morphology might be due to alterati ons in the phosphorylation of chromatin proteins.