Goedecke W, Eijpe M, Offenberg H H, van Aalderen M, Heyting C
Laboratory of Genetics, Wageningen Agricultural University, Dreijenlaan 2, 6703 HA Wageningen, The Netherlands.
Nat Genet. 1999 Oct;23(2):194-8. doi: 10.1038/13821.
Double-strand DNA breaks (DSBs) pose a major threat to living cells, and several mechanisms for repairing these lesions have evolved. Eukaryotes can process DSBs by homologous recombination (HR) or non-homologous end joining (NHEJ). NHEJ connects DNA ends irrespective of their sequence, and it predominates in mitotic cells, particularly during G1 (ref. 3). HR requires interaction of the broken DNA molecule with an intact homologous copy, and allows restoration of the original DNA sequence. HR is active during G2 of the mitotic cycle and predominates during meiosis, when the cell creates DSBs (ref. 4), which must be repaired by HR to ensure proper chromosome segregation. How the cell controls the choice between the two repair pathways is not understood. We demonstrate here a physical interaction between mammalian Ku70, which is essential for NHEJ (ref. 5), and Mre11, which functions both in NHEJ and meiotic HR (Refs 2,6). Moreover, we show that irradiated cells deficient for Ku70 are incapable of targeting Mre11 to subnuclear foci that may represent DNA-repair complexes. Nevertheless, Ku70 and Mre11 were differentially expressed during meiosis. In the mouse testis, Mre11 and Ku70 co-localized in nuclei of somatic cells and in the XY bivalent. In early meiotic prophase, however, when meiotic recombination is most probably initiated, Mre11 was abundant, whereas Ku70 was not detectable. We propose that Ku70 acts as a switch between the two DSB repair pathways. When present, Ku70 destines DSBs for NHEJ by binding to DNA ends and attracting other factors for NHEJ, including Mre11; when absent, it allows participation of DNA ends and Mre11 in the meiotic HR pathway.
双链DNA断裂（DSBs）对活细胞构成重大威胁，因此进化出了几种修复这些损伤的机制。真核生物可以通过同源重组（HR）或非同源末端连接（NHEJ）来处理DSBs。NHEJ连接DNA末端时不考虑其序列，并且在有丝分裂细胞中占主导地位，尤其是在G1期（参考文献3）。HR需要断裂的DNA分子与完整的同源拷贝相互作用，并允许恢复原始DNA序列。HR在有丝分裂周期的G2期活跃，在减数分裂期间占主导地位，此时细胞会产生DSBs（参考文献4），这些DSBs必须通过HR修复以确保正确的染色体分离。细胞如何控制这两种修复途径之间的选择尚不清楚。我们在此证明了哺乳动物Ku70（NHEJ所必需的）与Mre11（在NHEJ和减数分裂HR中均起作用）之间存在物理相互作用（参考文献2、6）。此外，我们表明缺乏Ku70的受辐射细胞无法将Mre11靶向可能代表DNA修复复合物的亚核焦点。然而，Ku70和Mre11在减数分裂过程中差异表达。在小鼠睾丸中，Mre11和Ku70共定位于体细胞的细胞核和XY二价体中。然而，在减数分裂前期早期，当减数分裂重组最有可能启动时，Mre11丰富，而Ku70则无法检测到。我们提出Ku70作为两种DSB修复途径之间的开关。当存在时，Ku70通过与DNA末端结合并吸引其他NHEJ因子（包括Mre11），将DSBs导向NHEJ；当不存在时，它允许DNA末端和Mre11参与减数分裂HR途径。
