title: Absence of MutSβ leads to the formation of slipped-DNA for CTG/CAG contractions at primate replication forks.
abstract: Typically disease-causing CAG/CTG repeats expand, but rare affected families can display high levels of contraction of the expanded repeat amongst offspring. Understanding instability is important since arresting expansions or enhancing contractions could be clinically beneficial. The MutSβ mismatch repair complex is required for CAG/CTG expansions in mice and patients. Oddly, by unknown mechanisms MutSβ-deficient mice incur contractions instead of expansions. Replication using CTG or CAG as the lagging strand template is known to cause contractions or expansions respectively; however, the interplay between replication and repair leading to this instability remains unclear. Towards understanding how repeat contractions may arise, we performed in vitro SV40-mediated replication of repeat-containing plasmids in the presence or absence of mismatch repair. Specifically, we separated repair from replication: Replication mediated by MutSβ- and MutSα-deficient human cells or cell extracts produced slipped-DNA heteroduplexes in the contraction- but not expansion-biased replication direction. Replication in the presence of MutSβ disfavoured the retention of replication products harbouring slipped-DNA heteroduplexes. Post-replication repair of slipped-DNAs by MutSβ-proficient extracts eliminated slipped-DNAs. Thus, a MutSβ-deficiency likely enhances repeat contractions because MutSβ protects against contractions by repairing template strand slip-outs. Replication deficient in LigaseI or PCNA-interaction mutant LigaseI revealed slipped-DNA formation at lagging strands. Our results reveal that distinct mechanisms lead to expansions or contractions and support inhibition of MutSβ as a therapeutic strategy to enhance the contraction of expanded repeats.
authors.affiliation: Program of Genetics and Genome Biology, The Hospital for Sick Children, Toronto, Ontario M5G 1L7, Canada; Department of Molecular Genetics, University of Toronto, Toronto, Ontario M5G 1L7, Canada.
authors.forename: Meghan M
authors.initials: MM
authors.lastname: Slean
authors.affiliation: Program of Genetics and Genome Biology, The Hospital for Sick Children, Toronto, Ontario M5G 1L7, Canada.
authors.forename: Gagan B
authors.initials: GB
authors.lastname: Panigrahi
authors.affiliation: Program of Genetics and Genome Biology, The Hospital for Sick Children, Toronto, Ontario M5G 1L7, Canada.
authors.forename: Arturo López
authors.initials: AL
authors.lastname: Castel
authors.affiliation: Program of Genetics and Genome Biology, The Hospital for Sick Children, Toronto, Ontario M5G 1L7, Canada; Department of Microbiology & Immunology, Dalhousie University, Nova Scotia, Canada.
authors.forename: August B
authors.initials: AB
authors.lastname: Pearson
authors.affiliation: Department of Internal Medicine, University of New Mexico School of Medicine, Albuquerque, NM 87131, USA; Department of Molecular Genetics and Microbiology, University of New Mexico School of Medicine, Albuquerque, NM 87131, USA; University of New Mexico Cancer Center,University of New Mexico School of Medicine, Albuquerque, NM 87131, USA.
authors.forename: Alan E
authors.initials: AE
authors.lastname: Tomkinson
authors.affiliation: Program of Genetics and Genome Biology, The Hospital for Sick Children, Toronto, Ontario M5G 1L7, Canada; Department of Molecular Genetics, University of Toronto, Toronto, Ontario M5G 1L7, Canada. Electronic address: cepearson.sickkids@gmail.com.
authors.forename: Christopher E
authors.initials: CE
authors.lastname: Pearson
mesh_terms: D000818:Animals; D004247:DNA / Q000235:genetics*; D053843:DNA Mismatch Repair; D004261:DNA Replication / Q000235:genetics*; D000882:Haplorhini; D006367:HeLa Cells; D006801:Humans; D051719:MutS DNA Mismatch-Binding Protein / Q000172:deficiency*; D019680:Trinucleotide Repeat Expansion / Q000235:genetics*
chemical_list: D004247:DNA; D051719:MutS DNA Mismatch-Binding Protein
keywords: Huntington’s disease; Ligase I; MSH2; MSH3; Mismatch repair; MutSbeta; PCNA; Repeat contractions; Repeat expansions; Repeat instability; Slippage; Trinucleotide repeats