Department of Molecular Medicine/Institute of Biotechnology, University of Texas Health Science Center, San Antonio, Texas 78245-3207, USA.
Genetics. 2011 Aug;188(4):787-97. doi: 10.1534/genetics.111.129833. Epub 2011 May 5.
Trex2 is a 3' → 5' exonuclease that removes 3'-mismatched sequences in a biochemical assay; however, its biological function remains unclear. To address biology we previously generated trex2(null) mouse embryonic stem (ES) cells and expressed in these cells wild-type human TREX2 cDNA (Trex2(hTX2)) or cDNA with a single-amino-acid change in the catalytic domain (Trex2(H188A)) or in the DNA-binding domain (Trex2(R167A)). We found the trex2(null) and Trex2(H188A) cells exhibited spontaneous broken chromosomes and trex2(null) cells exhibited spontaneous chromosomal rearrangements. We also found ectopically expressed human TREX2 was active at the 3' ends of I-SceI-induced chromosomal double-strand breaks (DSBs). Therefore, we hypothesized Trex2 participates in DNA DSB repair by modifying 3' ends. This may be especially important for ends with damaged nucleotides. Here we present data that are unexpected and prompt a new model. We found Trex2-altered cells (null, H188A, and R167A) were not hypersensitive to camptothecin, a type-1 topoisomerase inhibitor that induces DSBs at replication forks. In addition, Trex2-altered cells were not hypersensitive to γ-radiation, an agent that causes DSBs throughout the cell cycle. This observation held true even in cells compromised for one of the two major DSB repair pathways: homology-directed repair (HDR) or nonhomologous end joining (NHEJ). Trex2 deletion also enhanced repair of an I-SceI-induced DSB by both HDR and NHEJ without affecting pathway choice. Interestingly, however, trex2(null) cells exhibited reduced spontaneous sister chromatid exchanges (SCEs) but this was not due to a defect in HDR-mediated crossing over. Therefore, reduced spontaneous SCE could be a manifestation of the same defect that caused spontaneous broken chromosomes and spontaneous chromosomal rearrangements. These unexpected data suggest Trex2 does not enable DSB repair and prompt a new model that posits Trex2 suppresses the formation of broken chromosomes.
Trex2 是一种 3'→5'外切核酸酶，在生化测定中能去除 3'错配序列；然而，其生物学功能尚不清楚。为了解决生物学问题，我们之前生成了 trex2(null) 小鼠胚胎干细胞，并在这些细胞中表达野生型人 TREX2 cDNA（Trex2(hTX2)）或在催化结构域中单个氨基酸发生改变的 cDNA（Trex2(H188A)）或在 DNA 结合结构域中单个氨基酸发生改变的 cDNA（Trex2(R167A)）。我们发现 trex2(null) 和 Trex2(H188A) 细胞表现出自发的断裂染色体，而 trex2(null) 细胞表现出自发的染色体重排。我们还发现异位表达的人 TREX2 在 I-SceI 诱导的染色体双链断裂 (DSB) 的 3' 末端具有活性。因此，我们假设 Trex2 通过修饰 3' 末端参与 DNA DSB 修复。这对于带有受损核苷酸的末端可能尤为重要。在这里，我们提出了一些出乎意料的数据，并提出了一个新的模型。我们发现 Trex2 改变的细胞（null、H188A 和 R167A）对喜树碱（一种诱导复制叉处 DSB 的拓扑异构酶 I 抑制剂）不敏感。此外，Trex2 改变的细胞对 γ 辐射不敏感，γ 辐射是一种在整个细胞周期中引起 DSB 的试剂。即使在两种主要 DSB 修复途径之一（同源重组修复（HDR）或非同源末端连接（NHEJ））受损的细胞中，这一观察结果也是如此。Trex2 缺失还增强了 I-SceI 诱导的 DSB 的 HDR 和 NHEJ 修复，而不影响途径选择。然而，有趣的是，trex2(null) 细胞表现出减少的自发姐妹染色单体交换（SCE），但这不是由于 HDR 介导的交叉所致。因此，自发 SCE 的减少可能是导致自发断裂染色体和自发染色体重排的同一缺陷的表现。这些出乎意料的数据表明 Trex2 不能促进 DSB 修复，并提出了一个新的模型，即 Trex2 抑制断裂染色体的形成。
