DNA double-strand break repair proteins are required to cap the ends of mammalian chromosomes

Abstract

Recent findings intriguingly place DNA double-strand break repair proteins at chromosome ends in yeast, where they help maintain normal telomere length and structure. In the present study, an essential telomere function, the ability to cap and thereby protect chromosomes from end-to-end fusions, was assessed in repairdeficient mouse cell lines. By using fluorescence in situ hybridization with a probe to telomeric DNA, spontaneously occurring chromosome aberrations were examined for telomere signal at the points of fusion, a clear indication of impaired end-capping. Telomeric fusions were not observed in any of the repair-proficient controls and occurred only rarely in a p53 null mutant. In striking contrast, chromosomal end fusions that retained telomeric sequence were observed in nontransformed DNA-PKcs-deficient cells, where they were a major source of chromosomal instability. Metacentric chromosomes created by telomeric fusion became even more abundant in these cells after spontaneous immortalization. Restoration of repair proficiency through transfection with a functional cDNA copy of the human DNA-PKcs gene reduced the number of fusions compared with a negative transfection control. Virally transformed cells derived from Ku70 and Ku80 knockout mice also displayed end-to-end fusions. These studies demonstrate that DNA double-strand break repair genes play a dual role in maintaining chromosomal stability in mammalian cells, the known role in repairing incidental DNA damage, as well as a new protective role in telomeric end-capping.

DNA-PKcs and ATM influence generation of ionizing radiation-induced bystander signals

“…DNA-PKcs, the catalytic subunit of DNA-PK, orchestrates NHEJ in response to DSBs. It is also critical in V(D)J recombination, and is essential for effective mammalian telomeric end-capping function (Bailey et al, 1999;Lieber, 1999 The reverse experiment revealed a significant increase in SCE in nonirradiated ATM À/À (recipients) when irradiated 5C HDF donor cells were added, whereas no significant change was seen in the direct irradiation of ATM À/À human fibroblasts. **Significant at the 0.01 level.…”

“…DNA-PK is critical for double-strand break (DSB) repair and for V(D)J recombination (Jackson and Jeggo, 1995). It has also been shown that DNA-PK is important for the protection of mammalian telomeres by helping to maintain effective end-capping and preventing inappropriate fusions (Bailey et al, 1999(Bailey et al, , 2001(Bailey et al, , 2004. Like DNA-PKcs, ATM is a member of the phosphoinositide 3-kinase-like kinase family.…”

Cancer and aging: the importance of telomeres in genome maintenance

“…Alternatively, or in addition, the repair proteins, in concert with the telomere-associated proteins with which they interact, may suppress the inappropriate fusion or recombination of telomeres. In support of this possibility, mammalian cells deficient in Ku, components of the RMN complex, or WRN all show an increase in telomere erosion, fusions or other signs of dysfunction [80,[97][98][99].…”

Non-homologous end-joining, a sticky affair

“…Obviously, these chromosome ends are DNA ends that should not be joined to another DNA end. However, the Ku70/80 heterodimer, DNA-PK CS and the MRN complex have been found to preferentially localize to these structures (Bailey et al, 1999;Slijepcevic, 2006). These interactions do not promote end-joining and even seem to prevent end-to-end fusion of chromosomes.…”