Absence of MutSβ leads to the formation of slipped-DNA for CTG/CAG contractions at primate replication forks

Meghan M Slean; Gagan B Panigrahi; Arturo López Castel; August B Pearson; Alan E Tomkinson; Christopher E Pearson

doi:10.1016/j.dnarep.2016.04.002

Absence of MutSβ leads to the formation of slipped-DNA for CTG/CAG contractions at primate replication forks

DNA Repair (Amst). 2016 Jun:42:107-18. doi: 10.1016/j.dnarep.2016.04.002. Epub 2016 Apr 16.

Authors

Meghan M Slean¹, Gagan B Panigrahi², Arturo López Castel², August B Pearson³, Alan E Tomkinson⁴, Christopher E Pearson⁵

Affiliations

¹ 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.
² Program of Genetics and Genome Biology, The Hospital for Sick Children, Toronto, Ontario M5G 1L7, Canada.
³ 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.
⁴ 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.
⁵ 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.

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.

Keywords: Huntington’s disease; Ligase I; MSH2; MSH3; Mismatch repair; MutSbeta; PCNA; Repeat contractions; Repeat expansions; Repeat instability; Slippage; Trinucleotide repeats.

Publication types

Research Support, N.I.H., Extramural
Research Support, Non-U.S. Gov't

MeSH terms

Animals
DNA / genetics*
DNA Mismatch Repair
DNA Replication / genetics*
Haplorhini
HeLa Cells
Humans
MutS DNA Mismatch-Binding Protein / deficiency*
Trinucleotide Repeat Expansion / genetics*

Substances

DNA
MutS DNA Mismatch-Binding Protein

Abstract

Publication types

MeSH terms

Substances

Grants and funding