Induction and rejoining of DNA double-strand breaks in V79-4 mammalian cells following gamma- and alpha-irradiation.

The induction and rejoining of DNA double-strand breaks (dsbs) in V79-4 mammalian cells following irradiation by 60Co gamma-rays and 238Pu alpha-particles (average LET 120 keV microns-1) under aerobic conditions have been determined using both the sucrose sedimentation and filter elution techniques under non-denaturing conditions. Cellular inactivation was also determined. The dependence of the initial yield of dsbs at 277 K on dose under aerobic conditions is linear with a relative biological effectiveness (RBE) for alpha-particles of 0.85 +/- 0.14 (sedimentation) and 0.68 +/- 0.12 (elution) compared with 60Co gamma-rays. The ability of the cells to rejoin dsbs at 310K is significantly reduced for alpha-irradiations with only 30-50% rejoined for a 3-h incubation period. With low LET radiation, > 90% of the dsbs are rejoined within 3 h at a dose of 20 Gy. The RBE for cellular inactivation was determined to be 4.0 at the 1% survival level. From the cellular dimensions and the D0-value for cellular inactivation by alpha-particles, it is determined that, on average, 4.7 tracks traverse the cell nucleus per lethal lesion. Under hypoxic conditions, the RBE values for induction of dsbs and cellular inactivation (10% level) by alpha-particles are approximately 3.0 and approximately 11.8 respectively. From these findings, it is suggested that the residual DNA damage and not the initial damage is reflected in the cellular inactivation. It is inferred that the difference in repair of the various lesions is a reflection of the differences in the complexity of the clustered damage produced by these radiations.