title: The Werner syndrome exonuclease facilitates DNA degradation and high fidelity DNA polymerization by human DNA polymerase δ.
abstract: DNA Polymerase δ (Pol δ) and the Werner syndrome protein, WRN, are involved in maintaining cellular genomic stability. Pol δ 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 δ and WRN encode 3' → 5' DNA exonuclease activities. Pol δ 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 δ degrades alternate DNA structures including bubbles, four-way junctions, and D-loops. Moreover, WRN and Pol δ form a complex with enhanced ability to hydrolyze these structures. We also present evidence that WRN can proofread for Pol δ; WRN excises 3'-terminal mismatches to enable primer extension by Pol δ. Consistent with our in vitro observations, we show that WRN contributes to the maintenance of DNA synthesis fidelity in vivo. Cells expressing limiting amounts (∼10% of normal) of WRN have elevated mutation frequencies compared with wild-type cells. Together, our data highlight the importance of WRN exonuclease activity and its cooperativity with Pol δ in preserving genome stability, which is compromised by the loss of WRN in Werner syndrome.
authors.affiliation: Department of Pathology, University of Washington, Seattle, Washington 98195, USA. laloeb@uw.edu
authors.forename: Ashwini S
authors.initials: AS
authors.lastname: Kamath-Loeb
authors.forename: Jiang-Cheng
authors.initials: JC
authors.lastname: Shen
authors.forename: Michael W
authors.initials: MW
authors.lastname: Schmitt
authors.forename: Lawrence A
authors.initials: LA
authors.lastname: Loeb
mesh_terms: D020137:Base Pair Mismatch; D020029:Base Pairing; D002478:Cells, Cultured; D004247:DNA / Q000737:chemistry; D053837:DNA Cleavage; D004258:DNA Polymerase III / Q000737:chemistry*; D004261:DNA Replication; D024202:Electrophoretic Mobility Shift Assay; D005090:Exodeoxyribonucleases / Q000737:chemistry* / Q000172:deficiency; D006801:Humans; D006868:Hydrolysis; D046912:Multiprotein Complexes / Q000737:chemistry; D016296:Mutagenesis; D009690:Nucleic Acid Conformation; D011485:Protein Binding; D053484:RecQ Helicases / Q000737:chemistry* / Q000172:deficiency; D000071657:Werner Syndrome Helicase
chemical_list: D046912:Multiprotein Complexes; D004247:DNA; D004258:DNA Polymerase III; D005090:Exodeoxyribonucleases; D053484:RecQ Helicases; C098785:WRN protein, human; D000071657:Werner Syndrome Helicase