Comparison of F ratios generated from interphase and metaphase chromosome damage induced by high doses of low- and high-LET radiation

Although biophysical models predict a difference in the ratio of interchrom osomal to intrachromosomal interarm exchanges (F ratio) for low- and high-L ET radiations, few experimental data support this prediction. However, the F ratios in experiments to date have been generated using data on chromosom e aberrations in samples collected at the first postirradiation mitosis, wh ich may not be indicative of the aberrations formed in interphase after exp osure to high-LET radiations. In the present study, we exposed human lympho cytes in vitro to 2 and 5 Gy of gamma rays and 3 Gy of 1 GeV/nucleon iron i ons (LET = 140 keV/mum), stimulated the cells to grow with phytohemagglutin in (PHA), and collected the condensed chromosomes after 48 h of incubation using both chemically induced premature chromosome condensation (PCC) and t he conventional metaphase techniques. The PCC technique used here condenses chromosomes mostly in the G(2) phase of the cell cycle. The F ratio was ca lculated using data on asymmetrical chromosome aberrations in both the PCC and metaphase samples. It was found that the F ratios were similar for the samples irradiated with low- and high-LET radiation and collected at metaph ase. However, for irradiated samples assayed by PCC, the F ratio was found to be 8.2 +/- 2.0 for 5 Gy gamma rays and 5.2 +/- 0.9 for 3 Gy iron ions. T he distribution of the aberrations indicated that, in the PCC samples irrad iated with iron ions, most of the centric rings occurred in spreads contain ing five or more asymmetrical aberrations. These heavily damaged cells, whi ch were either less likely to reach mitosis or may reach mitosis at a later time, were responsible for the difference in the F ratios generated from i nterphase and metaphase analysis after exposure to iron ions. (C) 2001 by R adiation Research Society.