Radiation-induced chromosome damage in G1 phase cells as breaks in premature chromosome condensation (PCC) and its biological meaning.

We studied radiation-induced chromosome fragmentation in the G1-phase of Syrian golden hamster embryo (SHE) cells by premature chromosome condensation (PCC). SHE cells were irradiated with 14N ions (95 MeV) and 4He ions (22 MeV) generated by the cyclotron and 137Cs gamma rays. The number of chromosome breaks in PCC (PCC break) induced by 14N ions was higher than in those by the others. High linear energy transfer (LET) radiation (heavy ions) is more effective in the induction of PCC breaks than low LET radiation (gamma-rays). 530 kev/microns 14N induced PCC 2.4 times more than 137Cs gamma rays. There is an inverse relationship between the number of chromatid deletions and the incidence of mutation or transformation. There is, however, a good correlation between the incidence of PCC breaks and frequencies of mutation and/or morphological transformation. Although over 95% of the PCC breaks induced by gamma-rays were rejoined within 8 hours of post-irradiation incubation, only 35 to 45% of PCC breaks induced by heavy ions were rejoined. These results suggest that there is a qualitative difference in the initial chromosome damage caused by heavy ions and gamma rays.