Rescue of marker phenotypes: effects of BUdR sensitization, hypoxia and high LET

The survival curve of colony-forming ability of Chinese hamster wg3h cells has been compared with the dose-response curve for the expression of an active thymidine kinase (TK) gene from these cells. The TK+ phenotype was measured by hybrid colony formation after fusion of wg3h (TK+) cells with Chinese hamster A23 (TK-) cells. The TK+ survival data fitted a multi-target curve up to 3 krad of 137 Cs irradiation, when a highly resistant fraction of hybrid colonies was seen at about 1 per cent survival. The Do of TK+ survival for the multi-target region was 3.1-4.0 times greater, than that of wg3h survival, even when the Do for cell survival varied between 136 and 545 rad by 14 MeV neutrons and hypoxia respectively. This parallel modification of cell and TK+ sensitivities suggests that the lesions causing cell inactivation are of the same type as those that cause marker inactivation. Using 14 MeV neutron data the approximate target size for TK inactivation was calculated to be 0.54-0.91 per cent of the DNA content of the cell (or about one-fifth to one-tenth of a chromosome). The data support the idea that marker inactivation results primarily from damage occurring outside the marker gene. BUdR labelling of wg3h cells before irradiation caused slight toxicity (30 per cent reduction in plating efficiency) and a twofold increase in cell sensitivity. However, the sensitivity of the TK+ phenotype increases by only 30 per cent. The increased cell sensitivity thus appeared to result from synergism between increased sensitivity of DNA to strand breakage and metabolic toxicity, the latter being largely overcome by fusion with normal cells.