Werner syndrome protein associates with γH2AX in a manner that depends upon Nbs1

The WRN protein is mutated in the chromosomally unstable Werner syndrome (WS) and the Nbsl protein is mutated in Nijmegen breakage syndrome (NBS). The NbsI protein is an integral component of the M/R/N complex. Although WRN is known to interact with this complex in response to gamma-irradiation, the mechanism of action is unclear. Here, we show that WRN co-localizes and associates with gammaH2AX, a marker protein of DNA double strand breaks (DSBs), after cellular exposure to gamma-irradiation. While the DNA damage-inducible Nbsl foci formation is normal in WS cells, WRN focus formation is defective in NBS cells. Consistent with this, gammaH2AX colocalizes with NbsI in WS cells but not with WRN in NBS cells. The defective WRN-yH2AX association in NBS cells can be complemented with wild-type NbsI, but not with an Nbsl S343A point mutant that lacks an ATM phosphorylation site. WRN associates with H2AX in a manner dependent upon the M/R/N complex. Our results suggest a novel pathway in which Nbsl may recruit WRN to the site of DNA DSBs in an ATM-dependent manner. (C) 2005 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved.