Formation and repair of clustered damaged DNA sites in high LET irradiated cells

Abstract

Purpose: Radiation with high linear energy transfer (LET) produces clustering of DNA double-strand breaks (DSB) as well as non-DSB lesions. Heat-labile sites (HLS) are non-DSB lesions in irradiated cells that may convert into DSB at elevated temperature during preparation of naked DNA for electrophoretic assays and here we studied the initial formation and repair of these clustered damaged sites after irradiation with high LET ions.

Materials and methods: Induction and repair of DSB were studied in normal human skin fibroblast (GM5758) after irradiation with accelerated carbon and nitrogen ions at an LET of 125 eV/nm. DNA fragmentation was analyzed by pulsed-field gel electrophoresis (PFGE) and by varying the lysis condition we could differentiate between prompt DSB and heat-released DSB.

Results: Before repair (t = 0 h), the 125 eV/nm ions produced a significant fraction of heat-released DSB, which appeared clustered on DNA fragments with sizes of 1 Mbp or less. These heat-released DSB increased the total number of DSB by 30–40%. This increase is similar to what has been found in low-LET irradiated cells, suggesting that the relative biological effectiveness (RBE) for DSB induction will not be largely affected by the lysis temperature. After 1–2 hours repair, a large fraction of DSB was still unrejoined but there was essentially no heat-released DSB present.

Conclusions: These results suggest that high LET radiation, as low LET gamma radiation, induces a significant fraction of heat-labile sites which can be converted into DSB, and these heat-released DSB may affect both induction yields and estimates of repair.

Keywords::

• Clustered damage
• DNA damage
• DNA DSB repair
• DNA repair
• double-strand breaks
• High LET

Acknowledgements

This research was supported by grants from the Swedish Cancer Society and Swedish Radiation Safety Authority. The authors are grateful to the Svedberg Laboratory cyclotron staff for beam delivery.

Declaration of interest

The authors report no conflict of interest. The authors alone are responsible for the content and writing of the paper.

Supplementary material available online

Supplementary data and Supplementary Figure 1 available online at http://informahealthcare.com/abs/doi/10.3109/09553002.2015.1068463.