Analysis by Pulsed-field Gel Electrophoresis of DNA Double-strand Breaks Induced by Heat and/or X-irradiation in Bulk and Replicating DNA of CHO Cells

Abstract

For a given amount of cell killing, heat alone (10–80 min, 45·5°C) induced very few double-strand breaks (dsbs) compared with X-rays. Furthermore, 10 min at 45·5°C immediately prior to X-rays caused only a 1·3-fold increase in the slope of the X-ray-induced dsb dose—response curve, i.e. 0·67 ± 0·006 (95% confidence) dsbs/100Mbp/Gy for heated cells compared with 0·53 ± 0·005 for unheated control cells. However, this same heat treatment caused > 5-fold inhibition in the rate of repair of dsbs induced by 60-Gy X-rays, with the degree of inhibition being much less in thermotolerant (TT) cells than in non-tolerant (NT) cells. This reduced inhibition of repair in TT cells correlated with the more rapid removal of excess nuclear protein from nuclei isolated from TT cells than from NT cells. These results plus a TT ratio of 2–3 for both heat-induced radiosensitization and heat-inhibition of repairing dsbs are consistent with the hypothesis that heat radiosensitization results primarily from heat aggregation of nuclear protein interfering with access of repair enzymes to DNA dsbs. The selective heat-radiosensitization of S-phase cells, however, may result from an increase in radiation-induced dsbs in or near replicating regions. For example, a preferential increase in dsbs in replicating DNA compared with bulk DNA was found following either hyperthermia alone (10–30 min, 45·5°C) or a combined treatment (10 min, 45·5°C before 60 Gy). A 30-min treatment at 45·5°C induced dsbs equivalent to ∼ 10 Gy in replicating DNA compared with 3–5 Gy in bulk DNA. When cells were heated immediately before irradiation, the increase in dsbs induced in the replicating DNA by 60 Gy was equivalent to 200 Gy. We hypothesize that the observed 2-fold increase in single-stranded regions in replicating DNA after heat resulted in radiation selectively inducing dsbs at or near the replication fork where the heat-induced increase in single-stranded DNA should occur. Thus, this preferential increase in dsbs in the replicating DNA by heat alone and especially when heat was combined with radiation may explain at least in part, the high sensitivity of S-phase cells to heat killing and heat radiosensitization.