The Werner Syndrome Exonuclease Facilitates DNA Degradation and High Fidelity DNA Polymerization by Human DNA Polymerase δ

DNA Polymerase delta (Pol delta) and the Werner syndrome protein, WRN, are involved in maintaining cellular genomic stability. Pol delta synthesizes the lagging strand during replication of genomic DNA and also functions in the synthesis steps of DNA repair and recombination. WRN is a member of the RecQ helicase family, loss of which results in the premature aging and cancer-prone disorder, Werner syndrome. Both Pol delta and WRN encode 3' -> 5' DNA exonuclease activities. Pol delta exonuclease removes 3'-terminal mismatched nucleotides incorporated during replication to ensure high fidelity DNA synthesis. WRN exonuclease degrades DNA containing alternate secondary structures to prevent formation and enable resolution of stalled replication forks. We now observe that similarly to WRN, Pol delta degrades alternate DNA structures including bubbles, four-way junctions, and D-loops. Moreover, WRN and Pol delta form a complex with enhanced ability to hydrolyze these structures. We also present evidence that WRN can proofread for Pol delta; WRN excises 3'-terminal mismatches to enable primer extension by Pol delta. Consistent with our in vitro observations, we show that WRN contributes to the maintenance of DNA synthesis fidelity in vivo. Cells expressing limiting amounts (similar to 10% of normal) of WRN have elevated mutation frequencies compared with wildtype cells. Together, our data highlight the importance of WRN exonuclease activity and its cooperativity with Pol delta -in preserving genome stability, which is compromised by the loss of WRN in Werner syndrome.