A NOVEL TYPE OF X-RAY-SENSITIVE CHINESE-HAMSTER CELL MUTANT WITH RADIORESISTANT DNA-SYNTHESIS AND HAMPERED DNA DOUBLE-STRAND BREAK REPAIR

It has been shown that the Chinese hamster cell mutant V-C8 is sensiti ve to different DNA damaging agents, such as mitomycin C (MMC), alkyla ting agents, UV light, and X-rays. We found that V-C8 is also sensitiv e to the following radiomimetic agents: bleomycin(similar to 2-fold, b ased on D-10 values), H2O2 (similar to 2-fold), streptonigrin (similar to 11-fold), and etoposide(similar to 8-fold). Two independent sponta neous MMC-resistant revertants isolated from V-C8 cells show a level o f cell killing by X-rays, EMS, and UV light which is similar to that o f wild-type cells, suggesting that the observed pattern of cross-sensi tivity of V-C8 cells to a wide spectrum of DNA damaging agents results from a single mutation. V-C8 cells also display radioresistant DNA sy nthesis following gamma-irradiation which, however, remained almost un changed in the V-C8 revertants. The measurement of the level and rate of repair of DNA single- and double-strand breaks (SSBs and DSBs, resp ectively) by the DNA elution technique showed that the V-C8 mutant has a slower repair of DSBs induced by gamma-rays. The described unique p henotype of V-C8 cells suggested that V-C8 represents a novel type of mutant amongst X-ray-sensitive hamster cell mutants. To confirm this, complementation analysis with other X-ray-sensitive mutants was perfor med. V-C8 cells were fused with EM9, XR-1, xrs5, sxi-1, V-3, V-E5, irs 3, and BLM2 mutant cells, representing different complementation group s. All the obtained hybrids regained X-ray resistance (or bleomycin re sistance in the case of V-C8/BLM2 hybrids) similar to that of wild-typ e cells, indicating that V-C8 represents a new complementation group. The results presented indicate that V-C8 is defective in a gene involv ed in a pathway operating in the responses to different DNA damaging a gents in mammalian cells.