Rydberg B
Life Sciences Division, Lawrence Berkeley National Laboratory, University of California, Berkeley 94720, USA.
Radiat Res. 1996 Feb;145(2):200-9.
The basic 30-nm chromatin fiber in the mammalian cell consists of an unknown (possibly helical) arrangement of nucleosomes, with about 1.2 kb of DNA per 10-nm length of fiber. Track-structure considerations suggest that interactions of single delta rays or high-LET particles with the chromatin fiber might result in the formation of multiple lesions spread over a few kilobases of DNA (see the accompanying paper: W.R. Holley and A. Chatterjee, Radiat. Res. 145, 188-199, 1996). In particular, multiple DNA double-strand breaks and single-strand breaks may form. To test this experimentally, primary human fibroblasts were labeled with [3H]thymidine and exposed at 0 degrees C to X rays or accelerated nitrogen or iron ions in the LET range of 97-440 keV/microns. DNA was isolated inside agarose plugs and subjected to agarose gel electrophoresis under conditions that allowed good separation of 0.1-2 kb size DNA. The bulk of DNA remained in the well or migrated only a small distance into the gel. It was found that DNA fragments in the expected size range were formed linearly with dose with an efficiency that increased with LET. A comparison of the yield of such fragments with the yield of total DNA double-strand breaks suggests that for the high-LET ions a substantial proportion (20-90%) of DNA double-strand breaks are accompanied within 0.1-2 kb by at least one additional DNA double-strand break. It is shown that these results are in good agreement with theoretical calculations based on treating the 30-nm chromatin fiber as the target for ionizing particles. Theoretical considerations also predict that the clusters will contain numerous single-strand breaks and base damages. It is proposed that such clusters be designated "regionally multiply damaged sites." Postirradiation incubation at 37 degrees C resulted in a decline in the number of short DNA fragments, suggesting a repair activity. The biological significance of regionally multiply damaged sites is presently unknown.
哺乳动物细胞中的基本30纳米染色质纤维由核小体的未知（可能是螺旋状）排列组成，每10纳米纤维长度约有1.2千碱基对的DNA。轨道结构方面的考虑表明，单个δ射线或高传能线密度粒子与染色质纤维的相互作用可能导致在几千碱基对的DNA上形成多个损伤（见随附论文：W.R. 霍利和A. 查特吉，《辐射研究》145，188 - 199，1996）。特别是，可能会形成多个DNA双链断裂和单链断裂。为了通过实验验证这一点，将原代人成纤维细胞用[³H]胸腺嘧啶核苷标记，并在0℃下暴露于X射线或传能线密度在97 - 440 keV/μm范围内的加速氮离子或铁离子。在琼脂糖凝胶块内分离DNA，并在能良好分离0.1 - 2千碱基大小DNA的条件下进行琼脂糖凝胶电泳。大部分DNA留在孔中或仅向凝胶内迁移一小段距离。发现预期大小范围内的DNA片段随剂量呈线性形成，且形成效率随传能线密度增加。将此类片段的产率与总DNA双链断裂的产率进行比较表明，对于高传能线密度离子，相当大比例（20 - 90%）的DNA双链断裂在0.1 - 2千碱基范围内至少伴随有一个额外的DNA双链断裂。结果表明，这些结果与将30纳米染色质纤维视为电离粒子靶标的理论计算结果高度吻合。理论考虑还预测，这些簇将包含大量单链断裂和碱基损伤。建议将此类簇命名为“区域多重损伤位点”。37℃下辐照后孵育导致短DNA片段数量减少，表明存在修复活性。区域多重损伤位点的生物学意义目前尚不清楚。
