Iliakis G, Wang Y, Pantelias G E, Metzger L
Thomas Jefferson University Hospital, Department of Radiation Oncology and Nuclear Medicine, Philadelphia, Pennsylvania 19107.
Radiat Res. 1992 Feb;129(2):202-11.
There is evidence suggesting that radiosensitization induced in mammalian cells by substitution in the DNA of thymidine with BrdU has a component that relies on inhibition of repair and/or fixation of radiation damage. Here, experiments designed to study the mechanism of this phenomenon are described. The effect of BrdU incorporation into DNA was studied on cellular repair capability, rejoining of interphase chromosome breaks, as well as induction and rejoining of DNA double- and single-stranded breaks (DSBs and SSBs) in plateau-phase CHO cells exposed to X rays. Repair of potentially lethal damage (PLD), as measured by delayed plating of plateau-phase cells, was used to assay cellular repair capacity. Rejoining of interphase chromosome breaks was assayed by means of premature chromosome condensation (PCC); induction and rejoining of DNA DSBs were assayed by pulsed-field gel electrophoresis and induction and rejoining of DNA SSBs by DNA unwinding. A decrease was observed in the rate of repair of PLD in cells grown in the presence of BrdU, the magnitude of which depended upon the degree of thymidine replacement. The relative increase in survival caused by PLD repair was larger in cells substituted with BrdU and led to a partial loss of the radiosensitizing effect compared to cells tested immediately after irradiation. A decrease was also observed in the rate of rejoining of interphase chromosome breaks as well as in the rate of rejoining of the slow component of DNA DSBs in cells substituted with BrdU. The time constants measured for the rejoining of the slow component of DNA DSBs and of interphase chromosome breaks were similar both in the presence and in the absence of BrdU, suggesting a correlation between this subset of DNA lesions and interphase chromosome breaks. It is proposed that a larger proportion of radiation-induced potentially lethal lesions becomes lethal in cells grown in the presence of BrdU. Potentially lethal lesions are fixed via interaction with processes associated with cell cycle progression in cells plated immediately after irradiation, but can be partly repaired in cells kept in the plateau-phase. It is hypothesized that fixation of PLD is caused by alterations in chromatin conformation that occur during normal progression of cells throughout the cell cycle.(ABSTRACT TRUNCATED AT 400 WORDS)
有证据表明，用5-溴脱氧尿嘧啶核苷（BrdU）取代胸腺嘧啶核苷掺入哺乳动物细胞DNA中所诱导的放射增敏作用，有一个成分依赖于对辐射损伤修复和/或固定的抑制。在此，描述了旨在研究这一现象机制的实验。研究了BrdU掺入DNA对细胞修复能力、间期染色体断裂的重新连接，以及处于平台期的中国仓鼠卵巢（CHO）细胞在接受X射线照射后DNA双链和单链断裂（DSB和SSB）的诱导和重新连接的影响。通过对平台期细胞进行延迟铺板来检测潜在致死损伤（PLD）的修复情况，以此来测定细胞修复能力。通过早熟染色体凝集（PCC）检测间期染色体断裂的重新连接；通过脉冲场凝胶电泳检测DNA DSB的诱导和重新连接，通过DNA解旋检测DNA SSB的诱导和重新连接。观察到在含有BrdU的情况下生长的细胞中，PLD修复速率降低，其幅度取决于胸腺嘧啶核苷的取代程度。与照射后立即检测的细胞相比，由PLD修复引起的存活率相对增加在被BrdU取代的细胞中更大，并且导致放射增敏作用部分丧失。在用BrdU取代的细胞中，还观察到间期染色体断裂的重新连接速率以及DNA DSB慢成分的重新连接速率降低。在有和没有BrdU的情况下，测得的DNA DSB慢成分和间期染色体断裂重新连接的时间常数相似，这表明该DNA损伤子集与间期染色体断裂之间存在相关性。有人提出，在含有BrdU的情况下生长的细胞中，更大比例的辐射诱导潜在致死性损伤变得具有致死性。潜在致死性损伤通过与照射后立即铺板的细胞中与细胞周期进程相关的过程相互作用而固定，但在处于平台期的细胞中可以部分修复。据推测，PLD的固定是由细胞在整个细胞周期正常进程中发生的染色质构象改变引起的。（摘要截短于400字）
