Biphasic survival curves for XRS radiosensitive cells: subpopulations or transient expression of repair competence?

Four of the most radiosensitive xrs variants of CHO-K1 cells, obtained after mutagenizing treatment with EMS, have been studied in detail over three to five decades of cell survival. Although these lines were initially reported to have very steep exponential survival curves, and to vary in sensitivity between themselves by a factor of two, we found in each case a similar biphasic response. The initial sensitivity was similar for all four lines, with a D0 of 0.5-0.7 Gy. A subpopulation, representing between 0.4 and 12 per cent of the cells, showed a resistant response, characterized by a D0 of 1.5-2.0 Gy. The previously reported variation in sensitivity seems to result from differences in the fraction of resistant cells rather than from differences in the D0. The consequence of such phenotypic variants within each cloned line is considerable, both for radiobiological studies of repair, and for molecular biology studies of the repair genes. Attempts were made to clone the sensitive and resistant subpopulations from each xrs cell line. Simple cloning from an untreated population was expected to yield pure sensitive cells, but these cells also gave biphasic responses in most cases. Only the cell line with the lowest resistant fraction (xrs5) gave a completely sensitive response in two of its subclones. Cells selected as survivors after high radiation doses were expected to yield resistant populations. However, for xrs4, 5 and 7 these subclones all gave biphasic responses. Three of the subclones from xrs6 gave biphasic responses but others gave a resistant response close to the wild type. We present a model in which transient gene expression may be seen in each individual cell if the silent copy of the xrs repair gene is temporarily hemimethylated. This transient gene transcription should occur during DNA synthesis, in the interval between synthesis of the gene and maintenance methylation. This interval may vary from cell line to cell line, resulting in different fractions of resistant cells.