Severe, short-duration (0-3 min) heat shocks (50-52°C) inhibit the repair of DNA damage

Purpose: The goal of this study was to determine whether short-duration (15 s-3 min) high-temperature (50 degrees C) heat shocks inhibit the repair of DNA damage. Materials and methods: Cultured HeLa cells were used. DNA damage was measured after UV exposure or X-irradiation. Three methods were used to measure DNA damage: alkaline comet assay with the endonuclease, UVDE, for single strand breaks and UV photoproducts, antibodies specific for cyclo-pyrimidine dimers (CPD) or for 6-4 photoproducts (64PP), and the appearance-resolution of gamma-H2AX foci for DNA double strand breaks. Results: Heat shocks of 15-30 s at 50 degrees C inhibited repair of DNA damage after UV exposure or X-irradiation detected by the alkaline comet assay (after UV) or by persistence of gamma-H2AX foci (after X-rays). The phosphorylation of histone, H2AX, induced by I or 4 Gy of X-rays was inhibited in a time-dependent manner after 15-45 s at 52 degrees C. When the excision of UV-induced PP was measured, heat shocks of more than 60 s at 50 degrees C were required to show measurable inhibition. Conclusion: Severe (50 degrees C) short-duration (15 s or greater) heat shocks inhibit repair of UV-induced DNA damage. The ability to detect the inhibitory effects of very short, 15-60 s, heat shocks was assay dependent. The comet assay could detect repair inhibition after a 15-s heat shock. Detection of DNA damage by specific antibodies could only detect repair inhibition after 1-3-min heat shocks. Using the gamma-H2AX foci method 30 s at 50 degrees C induced a significant delay in the repair of DNA damage after I Gy of X-rays.