WRN Exonuclease activity is blocked by specific oxidatively induced base lesions positioned in either DNA strand

Werner syndrome (WS) is a premature aging disorder caused by mutations in the WS gene (WRN). Although WRN has been suggested to play an important role in DNA metabolic pathways, such as recombination, replication and repair, its precise role still remains to be determined. WRN possesses ATPase, helicase and exonuclease activities. Previous studies have shown that the WRN exonuclease is inhibited in vitro by certain lesions induced by oxidative stress and positioned in the digested strand of the substrate. The presence of the 70/86 Ku heterodimer (Ku), participating in the repair of double-strand breaks (DSBs), alleviates WRN exonuclease blockage imposed by the oxidatively induced DNA lesions. The current study demonstrates that WRN exonuclease is inhibited by several additional oxidized bases, and that Ku stimulates the WRN exonuclease to bypass these lesions. Specific lesions present in the non-digested strand were shown also to inhibit the progression of the WRN exonuclease; however, Ku was not able to stimulate WRN exonuclease to bypass these lesions. Thus, this study considerably broadens the spectrum of lesions which block WRN exonuclease progression, shows a blocking effect of lesions in the non-digested strand, and supports a function for WRN and Ku in a DNA damage processing pathway.