Timing of the steps in transformation of C3H 10T½ cells by X-irradiation

Abstract

Transformation of cells in culture by chemical carcinogens or X rays seems to require at least two steps. The initial step is a frequent event; for example, after transient exposure to either methylcholanthrene or X rays, almost every cell of established lines of mouse embryo fibroblasts proved capable of yielding transformed, tumorigenic descendants^1–3. Although results were interpreted as indicating that 100% of the progeny of methylcholanthrene-treated cells were potentially transformed¹, later experiments showed that only a very small minority of the progeny of cells initiated by X rays² or methylcholanthrene³ actually produced transformed colonies. We thus concluded² that there must be a second step in transformation that is a very rare event. We assumed that this event occurred after the cultures became confluent, a time when transformed cells have a selective growth advantage. Since then, however, others have shown that transformation can occur soon after initiation⁴ and that clones of transformed cells may already be present by the time initiated cultures become confluent⁵. It has been hypothesized that the second step behaves like a spontaneous mutation in having a constant but small probability of occurring each time an initiated cell divides^3,6. We show here that the clone size distribution of transformed cells in growing cultures initiated by X rays is, indeed, exactly what would be expected on that hypothesis.