Table_5_Persistent DNA Double-Strand Breaks After Repeated Diagnostic CT Scans in Breast Epithelial Cells and Lymphocytes.doc

DNA double-strand break (DSB) induction and repair have been widely studied in radiation therapy (RT); however little is known about the impact of very low exposures from repeated computed tomography (CT) scans for the efficiency of repair. In our current study, DSB repair and kinetics were investigated in side-by-side comparison of RT treatment (2 Gy) with repeated diagnostic CT scans (≤20 mGy) in human breast epithelial cell lines and lymphoblastoid cells harboring different mutations in known DNA damage repair proteins. Immunocytochemical analysis of well known DSB markers γH2AX and 53BP1, within 48 h after each treatment, revealed highly correlated numbers of foci and similar appearance/disappearance profiles. The levels of γH2AX and 53BP1 foci after CT scans were up to 30% of those occurring 0.5 h after 2 Gy irradiation. The DNA damage repair after diagnostic CT scans was monitored and quantitatively assessed by both γH2AX and 53BP1 foci in different cell types. Subsequent diagnostic CT scans in 6 and/or 12 weeks intervals resulted in elevated background levels of repair foci, more pronounced in cells that were prone to genomic instability due to mutations in known regulators of DNA damage response (DDR). The levels of persistent foci remained enhanced for up to 6 months. This “memory effect” may reflect a radiation-induced long-term response of cells after low-dose x-ray exposure.

DNA双链断裂（DNA double-strand break, DSB）的诱导与修复在放射治疗（radiation therapy, RT）中已得到广泛研究，但对于反复计算机断层扫描（computed tomography, CT）产生的极低剂量辐射对修复效率的影响，目前仍知之甚少。在本研究中，我们对放射治疗剂量（2 Gy）与反复诊断性CT扫描（≤20 mGy）引发的DSB修复及其动力学进行了平行对照研究，实验对象为携带已知DNA损伤修复蛋白突变的人乳腺上皮细胞系与淋巴母细胞系。在每次处理后48小时内，对经典DSB标志物γH2AX与53BP1进行免疫细胞化学分析，结果显示灶点数量高度相关，且出现与消失的动态特征相似。诊断性CT扫描后，γH2AX与53BP1灶点的水平最高可达2 Gy辐照后0.5小时时的30%。本研究通过γH2AX与53BP1灶点，对不同细胞类型中诊断性CT扫描后的DNA损伤修复情况进行了监测与定量评估。后续每隔6周和/或12周进行的诊断性CT扫描，会导致修复灶点的本底水平升高，在因DNA损伤应答（DNA damage response, DDR）调控因子突变而易发生基因组不稳定性的细胞中，这一现象更为显著。持续性灶点的水平在长达6个月的时间里仍维持升高状态。这种“记忆效应”可能反映了细胞在低剂量X射线暴露后，辐射诱导产生的长期应答反应。