Rescue of Marker Phenotypes: Effects of BUdR Sensitization, Hypoxia and High LET

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Abstract
The survival curve of colony-forming ability of Chinese hamster wg3h cells was 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 137Cs irradiation, when a highly resistant fraction of hybrid colonies was seen at about 1% survival. The Do [mean lethal dose] 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% of the DNA content of the cell (or about 1/5-1/10 of a chromosome). The data support the idea that marker inactivation results primarily from damage occurring outside the marker gene.BUdR [bromodeoxyuridine] labeling of wg3h cells before irradiation caused slight toxicity (30% reduction in plating efficiency) and a 2-fold increase in cell sensitivity. The sensitivity of the TK+ phenotype increases by only 30%. 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.