Comparison of mutation spectra induced by gamma-rays and carbon ion beams

Abstract

The ionizing radiation with high linear energy transfer (LET), such as a heavy ion beam, induces more serious biological effects than low LET ones, such as gamma- and X-rays. This indicates a difference in the DNA damage produced by low and high LET radiations and their biological effects. We have been studying the differences in DNA damage produced by gamma-rays and carbon ion beams. Therefore, we analyze mutations induced by both ionizing radiations to discuss the differences in their biological effects in this study. pUC19 plasmid DNA was irradiated by carbon ion beams in the solution containing 1M dimethyl sulfoxide to mimic a cellular condition. The irradiated DNA was cloned in competent cells of Escherichia coli. The clones harboring some mutations in the region of lacZα were selected, and the sequence alterations were analyzed. A one-deletion mutation is significant in the carbon-irradiated DNA, and the C:G↔T:A transition is minor. On the other hand, the gamma-irradiated DNA shows mainly G:C↔T:A transversion. These results suggest that carbon ion beams produce complex DNA damage, and gamma-rays are prone to single oxidative base damage, such as 8-oxoguanine. Carbon ion beams can also introduce oxidative base damage, and the damage species is 5-hydroxycytosine. This was consistent with our previous results of DNA damage caused by heavy ion beams. We confirmed the causal DNA damage by mass spectrometry for these mutations.

Keywords: base damage; gamma-rays; heavy ion beam; mutation.

Fig. 1

The typical images of agarose gel electrophorese for pUC19 DNA irradiated by gamma-rays (A) and carbon ion beams (B). Plots of the yields of Types I, II and III of pUC19 DNA irradiated by gamma-rays (C) and carbon ion beams (D) for the doses from the gel image analyzed by Image J. Respective plots were from the average of two independent experiments.

Fig. 2

Mutation map of the lacZα regions of pUC19 DNA irradiated by 25 and 50 Gy of gamma-rays (A) and carbon ion beams (B). We performed sequencing on the entire lacZα ORF, which is represented in this figure. As the numbering of Genbank indicates this sequence: M77789.2, this ORF is shown reverse and complemented. The start and stop codons are underlined at CAT and CTA at the tail and leading end of the sequence, respectively. The 25 and 50 Gy mutations are shown at the top and bottom of the sequence in respective figures. The numbers to the left of the sequence are the official numbers for the pUC19 sequence in the nucleotide sequence database.

Fig. 3

Yields of 8-OH-G and 5-HO-C in pUC19 DNA determined by LC–MS/MS for dose of gamma-rays (A) and carbon ion beams (B). The horizontal axis shows the dose (Gy), and the vertical axis shows the damaged bases (8-OH-G or 5-HO-C) to 10⁶ of G or C in DNA. Open circles and straight lines indicate 8-OH-G, and closed circles and dotted lines indicate 5-HO-C. The results represent the mean of at least three experiments with their standard deviations. If the standard deviation is small, the line indicating it may be hidden from view in the plot.