May A, Bohr V A
Laboratory of Molecular Genetics, National Institute on Aging, National Institutes of Health, 5600 Nathan Shock Drive, Baltimore, Maryland 21224, USA.
Biochem Biophys Res Commun. 2000 Mar 16;269(2):433-7. doi: 10.1006/bbrc.2000.2264.
We have measured gene-specific DNA damage and repair of alkaline-sensitive sites and DNA strand breaks after gamma-irradiation. Although fairly high doses are used in order to introduce sufficient DNA damage, we find that there is efficient and almost complete repair within 2 h. Human colon cancer cells were exposed to gamma-irradiation, and the repair was measured in various nuclear regions and in the mitochondrial genome. In the essential housekeeping gene, dihydrofolate reductase (DHFR), there was about 80% repair of the strand breaks after 2 h. There was no difference in the repair activities between the two individual DNA strands of the DHFR gene, and thus no evidence of strand bias, or transcription coupling of the repair process. There was no preferential repair of the DHFR gene compared to repair in an inactive, X-linked, noncoding gene. We can thus not detect any nuclear heterogeneity of the formation and repair of these lesions. In contrast, the formation and processing of gamma-irradiation introduced lesions differ in the mitochondrial DNA. Here, we detect about twofold more alkaline-sensitive sites and strand breaks after gamma-irradiation than observed in the DHFR gene. The repair of these lesions is deficient in the mitochondria, where only about 25% are removed within 2 h.
我们已经测量了γ射线照射后基因特异性DNA损伤以及碱性敏感位点和DNA链断裂的修复情况。尽管为了引入足够的DNA损伤使用了相当高的剂量，但我们发现2小时内存在高效且几乎完全的修复。将人结肠癌细胞暴露于γ射线照射下，并在不同的核区域和线粒体基因组中测量修复情况。在必需的管家基因二氢叶酸还原酶（DHFR）中，2小时后约80%的链断裂得到修复。DHFR基因的两条单链之间的修复活性没有差异，因此没有链偏向或修复过程转录偶联的证据。与在一个无活性的、X连锁的非编码基因中的修复相比，DHFR基因没有优先修复。因此，我们无法检测到这些损伤形成和修复的任何核异质性。相比之下，γ射线照射引入的损伤在线粒体DNA中的形成和处理有所不同。在这里，我们检测到γ射线照射后碱性敏感位点和链断裂比在DHFR基因中观察到的多约两倍。这些损伤在线粒体中的修复存在缺陷，2小时内仅约25%被去除。
