Effect of multiple irradiation with low doses of gamma-rays on morphological transformation and growth ability of human embryo cells in vitro.

We have measured expression of transformed phenotypes in human embryo (HE) cells repeatedly irradiated with a dose of 7.5 cGy per week throughout the life span of these cells in vitro. Irradiation was repeated until the cells had accumulated 195 cGy at which time the cells had reached the equivalent of their 26th passage and samples of cells at several passages were assayed for cell survival by colony formation, for mutation at hypoxanthine guanine phosphoribosyl transferase (HGPRT) locus and for transformation by focus formation. The lifespan (mean population doublings) of multiple irradiated cultures with a total dose of 97.5 cGy was slightly, but significantly, prolonged over that of controls. For example, if cells had accumulated 195 cGy, the maximum number of cell division of HE cells in vitro extended to 130-160% of non-irradiated control. Although transformed foci were not observed with cells until cells had accumulated 97.5 cGy, it increased with increasing accumulated dose. No cells, however, showed unlimited life span in vitro and also expressed tumorigenicity.