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

作者：Slean Meghan M; Panigrahi Gagan B; Lopez Castel Arturo; Pearson August B; Tomkinson Alan E; Pearson Christopher E^*

来源：DNA Repair, 2016, 42: 107-118.

DOI：10.1016/j.dnarep.2016.04.002

• Huntington's disease
• Trinucleotide repeats
• Slippage
• Repeat instability
• Mismatch repair
• MSH2
• MSH3
• Ligase I
• PCNA
• MutSbeta
• Repeat expansions
• Repeat contractions

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 MutS beta mismatch repair complex is required for CAG/CTG expansions in mice and patients. Oddly, by unknown mechanisms MutS beta-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 MutS beta- and MutS alpha-deficient human cells or cell extracts produced slipped-DNA heteroduplexes in the contraction- but not expansion-biased replication direction. Replication in the presence of MutS beta disfavoured the retention of replication products harbouring slipped-DNA heteroduplexes. Post-replication repair of slipped-DNAs by MutS beta-proficient extracts eliminated slipped-DNAs. Thus, a MutS beta-deficiency likely enhances repeat contractions because MutS beta protects against contractions by repairing template strand slip-outs. Replication deficient in Ligasel or PCNA-interaction mutant Ligasel revealed slipped-DNA formation at lagging strands. Our results reveal that distinct mechanisms lead to expansions or contractions and support inhibition of MutS beta as a therapeutic strategy to enhance the contraction of expanded repeats.

• 出版日期2016-6