Britten R A, Murray D
Department of Oncology, University of Alberta, Edmonton, Canada.
Radiat Res. 1997 Oct;148(4):308-16.
An important approach to understanding the role of the various DNA repair pathways in the cellular response to DNA-damaging agents is through the study of repair-deficient mutant cell lines. In the present study we used this strategy to assess the relative importance of four of these pathways for the repair of DNA damage induced by low-linear energy transfer (LET) gamma rays and intermediate-LET 42 MeV (p-->Be+) fast neutrons. The panel of hamster cell mutants that we characterized for their relative sensitivity to fast neutrons and gamma rays includes cell lines with defects in the nucleotide excision repair pathway; these can be further subdivided into mutants which are defective in nucleotide excision repair alone [UV5 (ERCC2-), UV24 (ERCC3-), UV135 (ERCC5-) and UV61 (ERCC6-)] compared to those which have an associated defect in the distinct but overlapping pathway for the repair of DNA crosslinks [UV20 (ERCC1-) and UV41 (ERCC4-)]. We also examined mutants with defects in the base excision repair pathway [EM9 (XRCC1-)] and the DNA-dependent protein kinase (DNA-PK)-mediated DNA double-strand break (DSB) repair pathway [xrs5 (XRCC5-)]. None of the mutants defective in nucleotide excision repair was differentially sensitized to fast neutrons or gamma rays; in fact, the slight radiosensitivity of these mutants under aerated conditions may be secondary to their defect in nucleotide excision repair. In contrast, deficiency in the base excision repair pathway resulted in a significant primary sensitization to both types of radiation (1.95-fold to gamma rays and 1.79-fold to neutrons). Deficiency in the DSB repair pathway mediated by DNA-PK resulted in a marked, but again similar, primary sensitization to gamma rays (4.2-fold) and neutrons (5.1-fold). Thus none of the repair pathways examined here exhibited a preferential role for the repair of damage induced by low-LET compared to intermediate-LET radiations; this resulted in an essentially constant relative biological effectiveness (RBE) of approximately 2 among the cell lines studied, independent of their DNA repair proficiency. However, consideration of these data along with data published previously for high-LET alpha particles suggests that, whereas the DNA-PK pathway is important for the repair of DSBs induced by low- and intermediate-LET radiations, it becomes less important as the LET increases beyond 100 keV/microm; thus this pathway may not be involved in repairing the more complex lesions induced by densely ionizing high-LET particles.
了解各种DNA修复途径在细胞对DNA损伤剂反应中作用的一个重要方法是研究修复缺陷的突变细胞系。在本研究中，我们采用这一策略来评估其中四条途径对低线性能量传递（LET）γ射线和中等LET 42 MeV（p→Be+）快中子诱导的DNA损伤修复的相对重要性。我们鉴定了一组仓鼠细胞突变体对快中子和γ射线的相对敏感性，其中包括核苷酸切除修复途径有缺陷的细胞系；与那些在DNA交联修复的不同但重叠途径中有相关缺陷的突变体[UV20（ERCC1-）和UV41（ERCC4-）]相比，这些突变体可进一步细分为仅在核苷酸切除修复方面有缺陷的突变体[UV5（ERCC2-）、UV24（ERCC3-）、UV135（ERCC5-）和UV61（ERCC6-）]。我们还研究了碱基切除修复途径有缺陷的突变体[EM9（XRCC1-）]和DNA依赖性蛋白激酶（DNA-PK）介导的DNA双链断裂（DSB）修复途径有缺陷的突变体[xrs5（XRCC5-）]。核苷酸切除修复有缺陷的突变体对快中子或γ射线均未表现出差异敏感性；事实上，这些突变体在通气条件下的轻微放射敏感性可能继发于其核苷酸切除修复缺陷。相比之下，碱基切除修复途径的缺陷导致对两种辐射均有显著的原发敏感性增加（对γ射线增加1.95倍，对中子增加1.79倍）。DNA-PK介导的DSB修复途径的缺陷导致对γ射线（4.2倍）和中子（5.1倍）有显著但同样相似的原发敏感性增加。因此，与中等LET辐射相比，这里研究的任何修复途径在修复低LET诱导的损伤方面均未表现出优先作用；这导致在所研究的细胞系中相对生物学效应（RBE）基本恒定，约为2，与它们的DNA修复能力无关。然而，将这些数据与先前发表的关于高LETα粒子的数据相结合表明，虽然DNA-PK途径对低LET和中等LET辐射诱导的DSB修复很重要，但随着LET增加超过100 keV/μm，其重要性降低；因此该途径可能不参与修复由密集电离的高LET粒子诱导的更复杂损伤。
