Extrahelical (CAG)/(CTG) triplet repeat elements support proliferating cell nuclear antigen loading and MutLα endonuclease activation

Pluciennik, Anna; Burdett, Vickers; Baitinger, Celia; Iyer, Ravi; Shi, Kevin; Modrich, Paul

doi:10.1073/pnas.1311325110

Cited by 69 publications

(121 citation statements)

References 44 publications

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“…Intriguingly, Rogacheva et al (2014) demonstrated that yeast Mlh1-Mlh3 enhances Msh2-Msh3 DNA-binding activity, while Msh2-Msh3 stimulates Mlh1-Mlh3 endonuclease activity. Therefore, enhanced Msh2-Msh3 binding to TNR structures could inappropriately recruit Mlh1-Mlh3 during replication or outside of S phase and stimulate its endonuclease activity, leading to gap formation and DNA repair synthesis that incorporates incremental expansions (Gomes-Pereira et al 2004;Pluciennik et al 2013). …”

Section: Discussionmentioning

confidence: 99%

MSH3 Promotes Dynamic Behavior of Trinucleotide Repeat Tracts In Vivo

Williams

Surtees

2015

Genetics

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Trinucleotide repeat (TNR) expansions are the underlying cause of more than 40 neurodegenerative and neuromuscular diseases, including myotonic dystrophy and Huntington's disease, yet the pathway to expansion remains poorly understood. An important step in expansion is the shift from a stable TNR sequence to an unstable, expanding tract, which is thought to occur once a TNR attains a threshold length. Modeling of human data has indicated that TNR tracts are increasingly likely to expand as they increase in size and to do so in increments that are smaller than the repeat itself, but this has not been tested experimentally. Genetic work has implicated the mismatch repair factor MSH3 in promoting expansions. Using Saccharomyces cerevisiae as a model for CAG and CTG tract dynamics, we examined individual threshold-length TNR tracts in vivo over time in MSH3 and msh3D backgrounds. We demonstrate, for the first time, that these TNR tracts are highly dynamic. Furthermore, we establish that once such a tract has expanded by even a few repeat units, it is significantly more likely to expand again. Finally, we show that threshold-length TNR sequences readily accumulate net incremental expansions over time through a series of small expansion and contraction events. Importantly, the tracts were substantially stabilized in the msh3D background, with a bias toward contractions, indicating that Msh2-Msh3 plays an important role in shifting the expansioncontraction equilibrium toward expansion in the early stages of TNR tract expansion.KEYWORDS Saccharomyces cerevisiae; Msh2-Msh3; mismatch repair; trinucleotide repeat tract T HE EXPANSION of trinucleotide repeat (TNR) sequences is the underlying cause of over 40 neurodegenerative and neuromuscular diseases (Castel et al. 2010;McMurray 2010). TNR sequences made of (CNG) n repeats are of particular interest because of their role in causing Huntington's disease (HD) and myotonic dystrophy type 1 (DM1), as well as a number of other diseases (McMurray 2010). TNR tracts within the normal range (which is tract dependent) are stably maintained within that range (Figure 1). However, through a mechanism(s) that remains unclear, a TNR tract can expand, increasing the number of repeats within the tract. Initially, this brings the tract into a threshold-length range (Gannon et al. 2012;Concannon and Lahue 2014) (Figure 1 and Figure 2), in which these somewhat longer tracts are not pathogenic but are increasingly susceptible to expansion; individuals with this phenomenon are carriers for disease. Once a tract has expanded sufficiently, it crosses a threshold; tracts above this threshold (which is disease specific) are pathogenic and cause disease ( Figure 1). As the size of the tract increases, it becomes increasingly unstable and prone to changes in length, particularly expansions.The dynamic behavior of TNR tracts that are within the threshold range (i.e., more susceptible to expansions) and the manner in which they occur in vivo remain unclear. Studies of TNR tract length changes...

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Section: Discussionmentioning

confidence: 99%

MSH3 Promotes Dynamic Behavior of Trinucleotide Repeat Tracts In Vivo

Williams

Surtees

2015

Genetics

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“…In addition, the Msh2 ATPase domain (abolishing ATPase activity but not DNA-binding ability) was shown to be directly involved in trinucleotide repeat expansions, strongly suggesting that a functional MutS complex was indeed required for such expansions [35]. Finally, it was recently demonstrated that two or three extrahelical CAG or CTG triplets were sufficient to trigger MutL activation, and depended on MutS function [39]. All these data point to a model in which MMR complexes recognize and bind secondary structures associated to CAG/CTG repeats, maybe stabilizing these structures, and that such binding leads to expansions, by a process requiring a functional MMR activity.…”

Section: Introductionmentioning

confidence: 99%

Replication stalling and heteroduplex formation within CAG/CTG trinucleotide repeats by mismatch repair

et al. 2016

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2 Highlights-CAG/CTG triplet repeats block replication in both orientations on a yeast chromosome -Replication fork stalling depends on mismatch repair integrity -Msh2p is enriched at CAG/CTG triplet repeats in both orientations -MSH2 overexpression favors or stabilizes the formation of heteroduplex molecules 3 AbstractTrinucleotide repeat expansions are responsible for at least two dozen neurological disorders.Mechanisms leading to these large expansions of repeated DNA are still poorly understood. It was proposed that transient stalling of the replication fork by the repeat tract might trigger slippage of the newly-synthesized strand over its template, leading to expansions or contractions of the triplet repeat. However, such mechanism was never formally proven. Here we show that replication fork pausing and CAG/CTG trinucleotide repeat instability are not linked, stable and unstable repeats exhibiting the same propensity to stall replication forks when integrated in a yeast natural chromosome. We found that replication fork stalling was dependent on the integrity of the mismatch-repair system, especially the Msh2p-Msh6p complex, suggesting that direct interaction of MMR proteins with secondary structures formed by trinucleotide repeats in vivo, triggers replication fork pauses. We also show by chromatin immunoprecipitation that Msh2p is enriched at trinucleotide repeat tracts, in both stable and unstable orientations, this enrichment being dependent on MSH3 and MSH6.Finally, we show that overexpressing MSH2 favors the formation of heteroduplex regions, leading to an increase in contractions and expansions of CAG/CTG repeat tracts during replication, these heteroduplexes being dependent on both MSH3 and MSH6. These heteroduplex regions were not detected when a mutant msh2-E768A gene in which the ATPase domain was mutated was overexpressed. Our results unravel two new roles for mismatch-repair proteins: stabilization of heteroduplex regions and transient blocking of replication forks passing through such repeats. Both roles may involve direct interactions between MMR proteins and secondary structures formed by trinucleotide repeat tracts, although indirect interactions may not be formally excluded.

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“…Recent in vitro studies of eukaryotic MMR indicate that in these MutH-free organisms, detection of a mismatch by MutS or MutSα [MutS(α)] licenses MutL(α) to interact with the processivity factor (β-clamp/PCNA), which in turn activates the latent endonuclease activity of MutL(α) to incise the daughter DNA strand on both the 3′ and 5′ sides of the error (8)(9)(10)(11). The interaction between MutL and the β-clamp (or between MutLα and PCNA) provides the strand discrimination signal because the β-clamp (or PCNA) is loaded asymmetrically at the replication fork or at a nick in DNA (10,12).…”

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confidence: 99%

“…Other models include trapping of MutS(α) clamps near the mismatch by MutL(α) followed by DNA looping or, alternately, MutS(α)-induced polymerization of MutL(α) along the DNA to reach the strand-discrimination signal (6,7,15,18,22). Some degree of localization to the mismatch is suggested by in vitro studies of eukaryotic MMR proteins, indicating that although MutLα can introduce nicks across long stretches of DNA, they occur preferentially in the vicinity of the mismatch (9,11,12).…”

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confidence: 99%

MutL traps MutS at a DNA mismatch

Qiu

Sakato

Sacho

et al. 2015

Proc. Natl. Acad. Sci. U.S.A.

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DNA mismatch repair (MMR) identifies and corrects errors made during replication. In all organisms except those expressing MutH, interactions between a DNA mismatch, MutS, MutL, and the replication processivity factor (β-clamp or PCNA) activate the latent MutL endonuclease to nick the error-containing daughter strand. This nick provides an entry point for downstream repair proteins. Despite the well-established significance of strand-specific nicking in MMR, the mechanism(s) by which MutS and MutL assemble on mismatch DNA to allow the subsequent activation of MutL's endonuclease activity by β-clamp/PCNA remains elusive. In both prokaryotes and eukaryotes, MutS homologs undergo conformational changes to a mobile clamp state that can move away from the mismatch. However, the function of this MutS mobile clamp is unknown. Furthermore, whether the interaction with MutL leads to a mobile MutS-MutL complex or a mismatch-localized complex is hotly debated. We used single molecule FRET to determine that Thermus aquaticus MutL traps MutS at a DNA mismatch after recognition but before its conversion to a sliding clamp. Rather than a clamp, a conformationally dynamic protein assembly typically containing more MutL than MutS is formed at the mismatch. This complex provides a local marker where interaction with β-clamp/PCNA could distinguish parent/daughter strand identity. Our finding that MutL fundamentally changes MutS actions following mismatch detection reframes current thinking on MMR signaling processes critical for genomic stability.DNA mismatch repair | MutS | MutL | FRET

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Extrahelical (CAG)/(CTG) triplet repeat elements support proliferating cell nuclear antigen loading and MutLα endonuclease activation

Cited by 69 publications

References 44 publications

MSH3 Promotes Dynamic Behavior of Trinucleotide Repeat Tracts In Vivo

MSH3 Promotes Dynamic Behavior of Trinucleotide Repeat Tracts In Vivo

Replication stalling and heteroduplex formation within CAG/CTG trinucleotide repeats by mismatch repair

MutL traps MutS at a DNA mismatch

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