Radiation Biology and DNA Repair, Darmstadt University of Technology, 64287 Darmstadt, Germany.
Nucleic Acids Res. 2011 Mar;39(6):2144-52. doi: 10.1093/nar/gkq1175. Epub 2010 Nov 17.
Topoisomerases class II (topoII) cleave and re-ligate the DNA double helix to allow the passage of an intact DNA strand through it. Chemotherapeutic drugs such as etoposide target topoII, interfere with the normal enzymatic cleavage/re-ligation reaction and create a DNA double-strand break (DSB) with the enzyme covalently bound to the 5'-end of the DNA. Such DSBs are repaired by one of the two major DSB repair pathways, non-homologous end-joining (NHEJ) or homologous recombination. However, prior to repair, the covalently bound topoII needs to be removed from the DNA end, a process requiring the MRX complex and ctp1 in fission yeast. CtIP, the mammalian ortholog of ctp1, is known to promote homologous recombination by resecting DSB ends. Here, we show that human cells arrested in G0/G1 repair etoposide-induced DSBs by NHEJ and, surprisingly, require the MRN complex (the ortholog of MRX) and CtIP. CtIP's function for repairing etoposide-induced DSBs by NHEJ in G0/G1 requires the Thr-847 but not the Ser-327 phosphorylation site, both of which are needed for resection during HR. This finding establishes that CtIP promotes NHEJ of etoposide-induced DSBs during G0/G1 phase with an end-processing function that is distinct to its resection function.
拓扑异构酶 II 类（topoII）切割并重新连接 DNA 双链，以允许完整的 DNA 链通过。依托泊苷等化疗药物靶向 topoII，干扰正常的酶切/重连反应，并在酶与 DNA 5'端共价结合的情况下产生 DNA 双链断裂（DSB）。这种 DSB 可以通过两种主要的 DSB 修复途径之一修复，即非同源末端连接（NHEJ）或同源重组。然而，在修复之前，需要将共价结合的 topoII 从 DNA 末端去除，这一过程需要 MRX 复合物和裂殖酵母中的 ctp1。ctp1 的哺乳动物同源物 CtIP 已知通过切除 DSB 末端来促进同源重组。在这里，我们表明，人细胞在 G0/G1 期通过 NHEJ 修复依托泊苷诱导的 DSB，令人惊讶的是，需要 MRN 复合物（MRX 的同源物）和 CtIP。CtIP 在 G0/G1 期通过 NHEJ 修复依托泊苷诱导的 DSB 的功能需要 Thr-847 但不需要 Ser-327 磷酸化位点，这两个位点都是 HR 过程中需要进行切除的。这一发现确立了 CtIP 在 G0/G1 期通过末端处理功能促进依托泊苷诱导的 DSB 的 NHEJ，该功能与它的切除功能不同。
