MutSβ promotes trinucleotide repeat expansion by recruiting DNA polymerase β to nascent (CAG)n or (CTG)n hairpins for error-prone DNA synthesis

Abstract

Expansion of (CAG)•(CTG) repeats causes a number of familial neurodegenerative disorders. Although the underlying mechanism remains largely unknown, components involved in DNA mismatch repair, particularly mismatch recognition protein MutSβ (a MSH2-MSH3 heterodimer), are implicated in (CAG)•(CTG) repeat expansion. In addition to recognizing small insertion-deletion loop-outs, MutSβ also specifically binds DNA hairpin imperfect heteroduplexes formed within (CAG)n•(CTG)n sequences. However, whether or not and how MutSβ binding triggers expansion of (CAG)•(CTG) repeats remain unknown. We show here that purified recombinant MutSβ physically interacts with DNA polymerase β (Polβ) and stimulates Polβ-catalyzed (CAG)n or (CTG)n hairpin retention. Consistent with these in vitro observations, MutSβ and Polβ interact with each other in vivo, and colocalize at (CAG)•(CTG) repeats during DNA replication. Our data support a model for error-prone processing of (CAG)n or (CTG)n hairpins by MutSβ and Polβ during DNA replication and/or repair: MutSβ recognizes (CAG)n or (CTG)n hairpins formed in the nascent DNA strand, and recruits Polβ to the complex, which then utilizes the hairpin as a primer for extension, leading to (CAG)•(CTG) repeat expansion. This study provides a novel mechanism for trinucleotide repeat expansion in both dividing and non-dividing cells.

Regulation of DNA Alkylation Damage Repair: Lessons and Therapeutic Opportunities

“…It is proposed that continued rounds of repair may lead to the formation of double-strand breaks and eventually cell death [27–30]. Recently, it has also been suggested that DNA Polymerase ß (Polß) may play a role in the processing of these MMR substrates, providing a level of crosstalk between MMR and BER [31, 32]. In the direct DNA damage signaling model, MutSα binds to the O 6 meG:T mispair and without processing, recruits MutLα and the DNA damage response proteins ATR and ATRIP [NN1]to initiate DNA damage checkpoints [33].…”

Close encounters: Moving along bumps, breaks, and bubbles on expanded trinucleotide tracts

“…Recent, emerging evidence reveals that BER and MMR cooperate in a hybrid pathway to correct oxidative lesion in TNR tracts and expand them [98,99] (Fig. 3B).…”

“…3B). Specifically, MSH2-MSH3 physically interacts with DNA and stimulates pol β-catalyzed synthesis on CAG and GAA repeats to complete long patch repair and strand displacement gap filling synthesis [98,99] (Fig. 3A).…”

Replication stalling and DNA microsatellite instability

“…Alternatively, displacement synthesis by Pol beta or Pol delta in conjunction with other DNA repair proteins may stabilize expanded hairpin-containing 5’ flaps that resist FEN1 cleavage [64,150–153]. Recent work has shown that recombinant MutS beta (MSH2/MSH3 heterodimer) binds to CAG imperfect hairpins in vitro [154], physically interacts with DNA Pol beta, and stimulates CAG or CTG hairpin retention catalyzed by Pol beta [155]. MutS beta and Pol beta also interact with each other in vivo, and co-localize to ectopic (CTG/CAG) 45 repeats during DNA replication [155].…”

“…Recent work has shown that recombinant MutS beta (MSH2/MSH3 heterodimer) binds to CAG imperfect hairpins in vitro [154], physically interacts with DNA Pol beta, and stimulates CAG or CTG hairpin retention catalyzed by Pol beta [155]. MutS beta and Pol beta also interact with each other in vivo, and co-localize to ectopic (CTG/CAG) 45 repeats during DNA replication [155]. …”