Heat effects on DNA repair after ionising radiation: hyperthermia commonlyincreases the number of non-repaired double-strand breaks and structural rearrangements

After ionising radiation double-strand breaks (dsb) are lethal if not repai red or misrepaired. Cell killing is greatly enhanced by hyperthermia and it is questioned here whether heat not only affects dsb repair capacity but a lso fidelity in a chromosomal context. dsb repair experiments were designed so as to mainly score non-homologous end joining, while homologous recombi nation was largely precluded. Human male G(0) fibroblasts were either prehe ated (45 degreesC, 20 min) or not before X-irradiation, dsb induction and r epair were measured by conventional gel electrophoresis and an assay combin ing restriction digestion using a rare cutting enzyme (Notl) and Southern h ybridisation, which detects large chromosomal rearrangements (>100 kb), dsb induction rate in an X-chromosomal Notl fragment was 4.8 x 10(-3) dsb/Gy/M b. Similar values were found for the genome overall and also when cells wer e preheated. After 50 Gy, fibroblasts were competent to largely restore the original restriction fragment size. Five per cent of dsb remained non-rejo ined and 14% were misrejoined. Correct restitution of restriction fragments occurred preferably during the first hour but continued at a slow rate for 12-16 h. In addition, dsb appeared to misrejoin throughout the entire repa ir period. After hyperthermia the fractions of non-rejoined and misrejoined dsb were similarly increased to 13 and 51%, respectively. It is suggested that heat increases the probability of dsb being incorrectly rejoined but i t is not likely to interfere with one dsb repair pathway in particular.