title: Coordinated processing of 3' slipped (CAG)n/(CTG)n hairpins by DNA polymerases β and δ preferentially induces repeat expansions.
abstract: Expansion of CAG/CTG trinucleotide repeats causes certain familial neurological disorders. Hairpin formation in the nascent strand during DNA synthesis is considered a major path for CAG/CTG repeat expansion. However, the underlying mechanism is unclear. We show here that removal or retention of a nascent strand hairpin during DNA synthesis depends on hairpin structures and types of DNA polymerases. Polymerase (pol) δ alone removes the 3'-slipped hairpin using its 3'-5' proofreading activity when the hairpin contains no immediate 3' complementary sequences. However, in the presence of pol β, pol δ preferentially facilitates hairpin retention regardless of hairpin structures. In this reaction, pol β incorporates several nucleotides to the hairpin 3'-end, which serves as an effective primer for the continuous DNA synthesis by pol δ, thereby leading to hairpin retention and repeat expansion. These findings strongly suggest that coordinated processing of 3'-slipped (CAG)n/(CTG)n hairpins by polymerases δ and β on during DNA synthesis induces CAG/CTG repeat expansions.
authors.affiliation: Graduate Center for Toxicology and Markey Cancer Center, University of Kentucky College of Medicine, Lexington, Kentucky 40536, USA.
authors.forename: Nelson L S
authors.initials: NL
authors.lastname: Chan
authors.forename: Jinzhen
authors.initials: J
authors.lastname: Guo
authors.forename: Tianyi
authors.initials: T
authors.lastname: Zhang
authors.forename: Guogen
authors.initials: G
authors.lastname: Mao
authors.forename: Caixia
authors.initials: C
authors.lastname: Hou
authors.forename: Fenghua
authors.initials: F
authors.lastname: Yuan
authors.forename: Jian
authors.initials: J
authors.lastname: Huang
authors.forename: Yanbin
authors.initials: Y
authors.lastname: Zhang
authors.forename: Jianxin
authors.initials: J
authors.lastname: Wu
authors.forename: Liya
authors.initials: L
authors.lastname: Gu
authors.forename: Guo-Min
authors.initials: GM
authors.lastname: Li
mesh_terms: D004247:DNA / Q000096:biosynthesis* / Q000737:chemistry / Q000235:genetics; D004258:DNA Polymerase III / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D019951:DNA Polymerase beta / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D004261:DNA Replication / Q000502:physiology*; D006367:HeLa Cells; D006801:Humans; D055029:Inverted Repeat Sequences
chemical_list: D004247:DNA; D004258:DNA Polymerase III; D019951:DNA Polymerase beta
keywords: DNA Enzymes; DNA Polymerase; DNA Repair; Genomic Instability; Nucleotide Repeat Disease