Genetic analysis of resistance to total bromodeoxyuridine substitution in mammalian cell hybrids

Somatic cell hybrids derived from the fusion of Chinese hamster ovary cells (CHO) and mutant Syrian hamster melanoma cells (2E) were tested for their ability to grow with all of the thymidine (dThd) in their DNA replaced with 5-bromo-2'-deoxyuridine (BrdU), a phenotypic capability of the 2E cells but not of the CHO cells. Under these conditions, the 2E cells survived and grew, all of the hybrid clones survived and grew to varying degrees, and the CHO cells did not survive at all. When 2E cells were tested, they were also found to be resistant to the toxic effects of BrdU substitution and white light irradiation, relative to CHO cells. Thus, when the DNAs of 2E and CHO cells were equally (50%) substituted with BrdU, and the two cell lines irradiated with identical doses of white light, the survival of CHO cells was reduced to less than 1% of that of unirradiated cells, while 40% of the 2E cells survived. The 2E x CHO hybrid clones were found to survive at values from 10% to 40% under these identical conditions. Thus, the phenotypic characteristics of 2E cells involving total substitution and resistance to the toxic effects of BrdU substitution and white light irradiation appear to be expressed in a codominant fashion in somatic cell hybrids.