The N-end rule pathway of protein degradation targets proteins with destabilizing N-terminal residues. Ubr2 is one of the E3 ubiquitin ligases of the mouse N-end rule pathway. We have previously shown that Ubr2-/- male mice are infertile, owing to the arrest of spermatocytes between the leptotene/zygotene and pachytene of meiosis I, the failure of chromosome pairing, and subsequent apoptosis. Here, we report that mouse fibroblast cells derived from Ubr2-/- embryos display genome instability. The frequency of chromosomal bridges and micronuclei were much higher in Ubr2-/- fibroblasts than in +/+ controls. Metaphase chromosome spreads from Ubr2-/- cells revealed a high incidence of spontaneous chromosomal gaps, indicating chromosomal fragility. These fragile sites were generally replicated late in S phase. Ubr2-/- cells were hypersensitive to mitomycin C, a DNA cross-linking agent, but displayed normal sensitivity to gamma-irradiation. A reporter assay showed that Ubr2-/- cells are significantly impaired in the homologous recombination repair of a double strand break. In contrast, Ubr2-/- cells appeared normal in an assay for non-homologous end joining. Our results therefore unveil the role of the ubiquitin ligase Ubr2 in maintaining genome integrity and in homologous recombination repair.
|Original language||English (US)|
|Number of pages||12|
|Journal||Mutation Research - Fundamental and Molecular Mechanisms of Mutagenesis|
|Issue number||1-2 SPEC. ISS.|
|State||Published - Apr 11 2006|
Bibliographical noteFunding Information:
We would like to thank Robert Schiestl (UCLA Department of Radiation Oncology) and Alexander Varshavsky (California Institute of Technology Division of Biology) for critically reading the manuscript. We also thank Rita Cantor (UCLA Department of Human Genetics) for help with the statistical analyses and Maria Jasin (Memorial Sloan-Kettering Cancer Center) for p59xDR-GFP6 and pCBASce. This work was supported by the National Institutes of Health R01 HD41451:01 (Y.M.), R01 GM61007-01A1 (Y.M.), R01 GM69482 (Y.T.K.), R01GM073981 (M.A.T), R01CA74929 (M.A.T.), R01CA107300 (M.A.T.), and CMISE, a NASA URETI Institute (NCC 2-1364: M.A.T.). M.A.T. is a Scholar of the Leukemia and Lymphoma Society.
- Fragile sites
- Genome instability
- Homologous recombination repair
- Mitomycin C sensitivity
- N-end rule proteolytic pathway
- Ubr2 ubiquitin ligase