Tumor Induction as a Measure of Genetic Damage and Repair in Somatic Cells of Mice

Groups of mice were subjected to X-rays and to neutrons, followed either 1 month or 9 months later with a large dose of CCl₄. The animals were sacrificed at 20 months of age, and liver tumors were scored. The CCl₄ treatment unmasks the carcinogenic mutations produced by the radiation. It is found that the tumor incidence is about the same for X-rays and neutrons, and that tumor incidence is almost as high when the CCl₄ is given 9 months postirradiation as when it is given at 1 month. This indicates that, once a carcinogenic mutation is formed, repair is impossible. This is especially interesting, since chromosomal aberrations scored in regenerating liver cells indicate a marked long-term recovery following X-irradiation but not following neutron irradiation. It is concluded that certain structural repair of chromosomes is possible over protracted periods but repair of functional genetic damage probably is not.