Comparison of clonogenic cell survival and DNA damage induced by ¹⁸⁸Re and X-rays in rat thyroid cells

Summary

Aim: Ionizing radiation produces DNA lesions among which DNA double strand breaks (DSB) are the most critical events. Radiation of various energy types might differ in their biological effectiveness. Here, we compared cell survival and DNA damage induced by ¹⁸⁸Re and X-rays using YH2AX foci as a measure of DSB. The correlation between survival and residual foci was also analyzed. Methods: PCCl3 cells were irradiated with 200 kV X-rays (1.2 Gy/min) or 0.5-25 MBq/ml ¹⁸⁸Re (1 h irradiation) achieving doses up to 10 Gy. By blocking of sodium iodide sym- porter (NIS) essentially extracellular activity could be guaranteed. Survival fractions (SF) were detected by colony forming assay. Initial and residual YH2AX foci (15 min and 24 h after irradiation) were assessed by immunos- taining. The relationship between SF and residual radiation induced YH2AX foci (RIF) was evaluated by Spearman and Pearson correlation tests. Results: We did not find significant differences between the survival curves in terms of the radiation quality. The D₃₇ values were 4.6 Gy and 4.2 Gy for ¹⁸⁸Re or X-ray, respectively. The initial foci numbers were in the same range for ¹⁸⁸Re and X-ray, but higher levels of residual foci persisted after X-rays in comparison to ¹⁸⁸Re (1 Gy_X-ray 6.5 ± 0.2; 1 Gy_Re-188 4.8 ± 0.2 RIF). Accordingly, for ¹⁸⁸Re a higher extent of DSB repair was found. The Spearman test revealed a significant (p < 0.01) correlation between SF and residual RIF for both radiation modalities. Conclusion: No differences in terms of radiation were found for SF and initial foci. However, residual foci were lower for ¹⁸⁸Re than for X-rays. A prediction of SF by residual foci should consider the properties of the radiation qualities that influence foci removal and DSB repair.

Zusammenfassung

Ziel: Ionisierende Strahlung induziert DNA Schäden. DNA-Doppelstrangbrüche (DSB) sind dabei die kritischsten Ereignisse für das Zellüberleben. In Abhängigkeit von den Strahlungseigenschaften und der Dosis variiert der radiotoxische Effekt. Ziel der Studie war ein quantitativer Vergleich von Zellüberleben und YH2AX Foci als DSB-Marker nach Bestrahlung mit ¹⁸⁸Re und Röntgenstrahlung. Weiterhin wurde die Korrelation zwischen Zellüberleben und residualen YH2AX Foci untersucht. Methodik: PCCl3-Schilddrüsenzellen wurden mit Röntgenstrahlung oder ¹⁸⁸Re mit 0,5-25 MBq/ml über 1 h im Dosisbereich von 0-10 Gy bestrahlt. Durch Blockade des Natriumiodid- Symporters (NIS) wurde eine Radionuklidakkumulation in die Zellen verhindert. Das Zellüberleben wurde mit dem Koloniebildungstest bestimmt. Die initialen (15 min nach Bestrahlung) und residualen YH2AX Foci (24 h nach Bestrahlung) wurden mit dem YH2AX- Immunfluoreszenz-Assay analysiert. Die Beziehung zwischen Zellüberleben und residualen Foci wurde mit den Korrelationstests nach Spearman und Pearson untersucht. Ergebnisse: Die Dosis-Wirkungs-Kurven für ¹⁸⁸Re und Röntgenstrahlung zeigten keine signifikanten Unterschiede. Die D₃₇-Werte betrugen 4,6 Gy für ¹⁸⁸Re und 4,3 Gy für Röntgenstrahlung. Zwischen beiden Strahlenmodalitäten wurden keine Unterschiede hinsichtlich initialer YH2AX-Foci gefunden. Die Anzahl der residualen YH2AX-Foci war nach Röntgenbestrahlung höher als nach ¹⁸⁸Re-Exposition, z.B. wurden bei 1 Gy Röntgen 6,5 ± 0.2 bzw. 1 Gy ¹⁸⁸Re 4,8 ± 0.2 Foci gezählt. Mit dem Spearman-Test wurde eine sehr gute Korrelation (p < 0,01) zwischen dem Zellüberleben und den residualen Foci gefunden. Schlussfolgerung: Mit der Bestimmung von Zellüberleben und Anzahl der initialen YH2AX-Foci konnte kein Unterschied zwischen den Strahlern gefunden werden. Eine Vorhersage des Zellüberlebens - basierend auf der Anzahl residualer YH2AX-Foci - sollte die Eigenschaften der Strahlenqualitäten und deren Einfluss auf das Verschwinden der Foci und somit die Reparatur der DSB in Betracht ziehen.

Keywords

PCCl3 cells - ¹⁸⁸Re - X-ray - γH2AX foci - cell survival - correlation

Schlüsselwörter

PCCl3-Zellen - ¹⁸⁸Re - Röntgenstrahlung - γH2AX-Foci - Zellüberleben - Korrelation

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