Advanced Science Research Center, Japan Atomic Energy Agency, 2-4 Shirakata-shirane, Tokai, Ibaraki 319-1195, Japan.
Radiat Res. 2012 May;177(5):614-27. doi: 10.1667/rr2701.1. Epub 2011 Dec 29.
We measured the yield and spectrum of strand breaks and nucleobase lesions produced in fully hydrated plasmid DNA films to determine the linear energy transfer (LET) dependence of DNA damage induced by ion-beam irradiation in relation to the change in the atomic number of ions. The yield of isolated damage was revealed as a decrease in prompt SSBs with increasing LET of He(2+), C(5+,6+) and Ne(8+,10+) ions. On the other hand, the yields of prompt DSBs increased with increasing ion LET. SSBs were additionally induced in ion-irradiated DNA film by treatment with two kinds of base excision repair proteins (glycosylases), Nth and Fpg, indicating that base lesions are produced in the hydrated DNA film. This result shows that nucleobase lesions are produced via both chemical reactions with diffusible water radicals, such as OH radicals, and direct energy deposition onto DNA and the hydrated water layer. Nth-sensitive sites deduced to be pyrimidine lesions, such as 5,6-dihydrothymine (DHT), showed a relatively larger yield than Fpg-sensitive sites deduced to be purine lesions, such as 7,8-dihydro-8-oxo-2'deoxyguanine (8-oxoGua), for all ion exposures tested. The yield of SSBs or DSBs observed by enzyme treatment decreased noticeably with increasing LET for all tested ions. These results indicated that higher-LET ions preferentially produce a complex type of damage that might compromise the activities of the glycosylases used in this study. These findings are biologically important since, under cell mimicking conditions, persistent DNA damage occurs in part due to direct energy deposition on the DNA or hydrated water shell that is specifically induced by dense ionization in the track.
我们测量了完全水合质粒 DNA 薄膜中产生的链断裂和碱基损伤的产额和光谱，以确定与离子原子数变化相关的离子束辐照诱导的 DNA 损伤的线性能量转移 (LET) 依赖性。随着 He(2+)、C(5+、6+)和 Ne(8+、10+)离子 LET 的增加，孤立损伤的产额表现为瞬时 SSB 的减少。另一方面，瞬时 DSB 的产额随着离子 LET 的增加而增加。用两种碱基切除修复蛋白（糖苷酶）Nth 和 Fpg 处理离子辐照的 DNA 薄膜，还诱导了 SSB 的产生，表明碱基损伤是在水合 DNA 薄膜中产生的。这一结果表明，碱基损伤是通过与可扩散水自由基（如 OH 自由基）的化学反应以及直接能量沉积到 DNA 和水合水层而产生的。Nth 敏感位点推断为嘧啶损伤，如 5,6-二氢胸腺嘧啶（DHT），比 Fpg 敏感位点推断为嘌呤损伤，如 7,8-二氢-8-氧代-2'-脱氧鸟嘌呤（8-氧代鸟嘌呤），在所有测试的离子暴露中都有较大的产额。用酶处理观察到的 SSB 或 DSB 的产额随着所有测试离子 LET 的增加而明显下降。这些结果表明，较高 LET 的离子优先产生一种复杂类型的损伤，这可能会影响本研究中使用的糖苷酶的活性。这些发现具有重要的生物学意义，因为在细胞模拟条件下，部分 DNA 损伤的持续存在是由于 DNA 或水合水壳中的直接能量沉积，这是由轨迹中的密集电离所特异地诱导的。
