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

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Abstract
The WRN protein is mutated in the chromosomally unstable Werner syndrome (WS) and the Nbs1 protein is mutated in Nijmegen breakage syndrome (NBS). The Nbs1 protein is an integral component of the M/R/N complex. Although WRN is known to interact with this complex in response to γ-irradiation, the mechanism of action is unclear. Here, we show that WRN co-localizes and associates with γH2AX, a marker protein of DNA double strand breaks (DSBs), after cellular exposure to γ-irradiation. While the DNA damage-inducible Nbs1 foci formation is normal in WS cells, WRN focus formation is defective in NBS cells. Consistent with this, γH2AX colocalizes with Nbs1 in WS cells but not with WRN in NBS cells. The defective WRN-γH2AX association in NBS cells can be complemented with wild-type Nbs1, but not with an Nbs1 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 Nbs1 may recruit WRN to the site of DNA DSBs in an ATM-dependent manner