Kinetics of formation of exchanges and rejoining of breaks in human G0 and G2 lymphocytes after low-LET radiation.

PURPOSE

To study the kinetics of chromosomal break rejoining and exchange formation in human G0 and G2 lymphocytes after low-LET irradiation using the premature chromosome condensation (PCC) technique.

METHODS

For the G0 experiments, isolated human lymphocytes were irradiated (1, 3 and 8 Gy) and immediate and delayed fusions (1, 3, 5, 8 and 24 h) were performed with CHO-cells, and 48-h cultures were set up simultaneously. Interphase and metaphase human chromosomes were hybridized with dual colour combination probes for chromosomes 2 and 4. For the G2 experiments, isolated lymphocytes from the same donor were cultured for 69 h, irradiated with 3 Gy 60Co gamma rays and fused with CHO cells immediately and after 1, 2 and 3 h. Chromatid breaks and exchanges were scored after Giemsa staining.

RESULTS

In both G0 and G2 cells, exchanges were already observed at the first fusion and, at lower doses, their frequencies remained relatively stable over time. The exchange-type aberrations seen in G0 cells were mostly incomplete, especially in the first fusions, while the exchanges seen in metaphase were mostly complete. The frequency of complete exchanges increased by time during incubation, while the frequency of incomplete exchanges showed more variation and a general decline by time in the G0-PCC experiment. The number of breaks declined rapidly by time both in G0 and G2. The break rejoining rate was higher at 8 Gy than in the two lower doses in the G0-PCC. The overall aberration frequencies in G0 and G2 cells studied by PCC were higher than in the corresponding metaphase. The initial number of breaks in G2 was higher than in G0 and the break rejoining rate in the G2 was two-fold.

CONCLUSIONS

The formation of complete exchanges in G0 is a delayed process. The incomplete exchanges appear unstable in G0. The decreasing proportion of incomplete exchanges and simultaneous increase in complete exchanges in G0-PCC indicates that formation of exchanges is not coordinated in time.