Mre11 Is Essential for the Removal of Lethal Topoisomerase 2 Covalent Cleavage Complexes

Abstract

The Mre11/Rad50/Nbs1 complex initiates double-strand break repair by homologous recombination (HR). Loss of Mre11 or its nuclease activity in mouse cells is known to cause genome aberrations and cellular senescence, although the molecular basis forthis phenotype is not clear. To identify the originof these defects, we characterized Mre11-deficient(MRE11-/-) and nuclease-deficient Mre11 (MRE11-/H129N) chicken DT40 and human lymphoblast cell lines. These cells exhibit increased spontaneous chromosomal DSBs and extreme sensitivitytotopoisomerase 2 poisons. The defects in Mre11compromise the repair of etoposide-induced Top2-DNA covalent complexes, and MRE11-/- and MRE11-/H129N cells accumulate high levels of Top2covalent conjugates even in the absence of exogenous damage. We demonstrate that both the genomeinstability and mortality of MRE11-/- and MRE11-/H129N cells are significantly reversed by overexpression of Tdp2, an enzyme that eliminates covalent Top2 conjugates; thus, the essential role ofMre11 nuclease activity is likely to remove these lesions.