OXYGEN EFFECT FOR DNA DOUBLE-STRAND BREAK INDUCTION DETERMINED BY PULSED-FIELD GEL-ELECTROPHORESIS

The induction of DNA double-strand breaks (dsb) following irradiation under oxygenated and hypoxic conditions with and without misonidazole was measured by pulsed-field gel electrophoresis (PFGE) in a human bladder carcinoma cell line. The dose-response curve for DNA dsb detection by PFGE was biphasic with an apparent reduction in rate of dsb induced with dose. Oxygen enhancement ratios (OER) for cell survival (at a surviving fraction of 0.1) and for DNA damage assessed by PFGE (at 80% retained) were 2.0 and 3.0 respectively. Dose-modifying factors for misonidazole (15mM), of 1.9 (survival) and 2.4 (DNA damage) were found. Although the magnitude of the inter-experiment variations limit the precision with which cell survival and DNA electrophoresis can be compared, the data do support a simple correlation between these two measures of response. When DNA dsb induction frequency was assessed from the number average molecular weight, values of 2.7 (+/- 0.3), 0.7 (+/- 0.1) and 2.6 (+/- 0.5) x 10(-9) dsb/bp/Gy were found for irradiation under oxic, hypoxic alone and hypoxic + misonidazole conditions respectively. This gives an OER of 3.9 and a DMF of 3.7.