Relationship between PCC fragments and cell killing studied in X-irradiated CHO, CHO-K1 cells and two radiosensitive mutants xrs1 and xrs5

Abstract

Purpose: To investigate the correlation between PCC fragments and cell killing. Materials and methods: Induction and repair of DNA fragments were measured in CHO, CHO-K1, xrs 1 and xrs 5 cells using the premature chromosome condensation (PCC) technique and cell survival was determined by a colony assay. Results: The number of PCC fragments measured in cells immediately fused after X-irradiation was the same for CHO (3.4 0.16/cell/Gy) and CHO-K1 (3.6 0.12/cell/Gy) cells but significantly higher for xrs 1 (4.9 0.07/cell/Gy) and xrs 5 cells (7.0 0.4/cell/Gy). The repair curve of PCC fragments studied for CHO, CHO-K1 and xrs 5 cells was best described by a monophasic exponential decline with a final plateau; the halftime of this decline was always about 30 min. The number of unrejoined PCC fragments, which was measured 14 h after irradiation, increased linearly with dose. The steepest increase was found for xrs 5 cells (5.5 0.3 fragments per cell and per Gy), the lowest for CHO/CHO-K1 (0.9 0.1; 1.0 0.1) and for xrs 1 in between (3.3 0.1). For all four cell lines the relationship between cell killing and unrejoined fragments could be described by a single curve with a D0 of 2.5 0.4 unrejoined PCC fragments per lethal event. Conclusions: The data showed that the number of unrejoined PCC fragments can be used as an indicator of cellular radiosensitivity.