The mismatch repair system protects against intergenerational GAA repeat instability in a Friedreich ataxia mouse model

Abstract

Friedreich ataxia (FRDA) is an autosomal recessive neurodegenerative disorder caused by a dynamic GAA repeat expansion mutation within intron 1 of the FXN gene. Studies of mouse models for other trinucleotide repeat (TNR) disorders have revealed an important role of mismatch repair (MMR) proteins in TNR instability. To explore the potential role of MMR proteins on intergenerational GAA repeat instability in FRDA, we have analyzed the transmission of unstable GAA repeat expansions from FXN transgenic mice which have been crossed with mice that are deficient for Msh2, Msh3, Msh6 or Pms2. We find in all cases that absence of parental MMR protein not only maintains transmission of GAA expansions and contractions, but also increases GAA repeat mutability (expansions and/or contractions) in the offspring. This indicates that Msh2, Msh3, Msh6 and Pms2 proteins are not the cause of intergenerational GAA expansions or contractions, but act in their canonical MMR capacity to protect against GAA repeat instability. We further identified differential modes of action for the four MMR proteins. Thus, Msh2 and Msh3 protect against GAA repeat contractions, while Msh6 protects against both GAA repeat expansions and contractions, and Pms2 protects against GAA repeat expansions and also promotes contractions. Furthermore, we detected enhanced occupancy of Msh2 and Msh3 proteins downstream of the FXN expanded GAA repeat, suggesting a model in which Msh2/3 dimers are recruited to this region to repair mismatches that would otherwise produce intergenerational GAA contractions. These findings reveal substantial differences in the intergenerational dynamics of expanded GAA repeat sequences compared with expanded CAG/CTG repeats, where Msh2 and Msh3 are thought to actively promote repeat expansions.

Role of Mismatch Repair Enzymes in GAA·TTC Triplet-repeat Expansion in Friedreich Ataxia Induced Pluripotent Stem Cells

“…Several studies have pointed to the involvement of MutS␤ (MSH2/MSH3) in triplet-repeat instability in transgenic mice (8,15,39). Our previous ChIP assays (16) also show that at a resolution of ϳ1 kb, there is an increased occupancy of MSH2 and MSH3 downstream of the GAA⅐TTC triplet repeats in FRDA iPSCs compared with an unaffected iPSC line, but, in contrast, not 1254 bp upstream of the FXN transcriptional start site or directly upstream of the GAA⅐TTC triplet repeats.…”

“…Mammalian cell models using a plasmid construct with GAA⅐TTC triplet repeats have shown that the repeat expansions are transcription-, replication-, and position-dependent (10 -12). In addition, mouse models with expanded GAA⅐TTC triplet repeats show somatic instability in different tissues (13)(14)(15). These systems differ substantially from humans and from each other in terms of DNA replication rate, the cell type in which GAA⅐TTC triplet-repeat expansion occurs, and chromatin structure.…”

Tissue specificity in DNA repair: lessons from trinucleotide repeat instability

“…The mismatch repair proteins MSH2, MSH3, MLH1, MLH3, and PMS2 have all been shown to modulate TNR instability in mouse models with CAG, CTG, GAA, and CGG expansions [21,[29][30][31][32][33][34][35][36][37][38][39][50][51][52][53]. In addition, MSH6 is involved in GAA repeat instability but appears to have little effect on CAG/CTG instability [35,38,[51][52][53].…”

“…In addition, MSH6 is involved in GAA repeat instability but appears to have little effect on CAG/CTG instability [35,38,[51][52][53]. These results prompted a systematic investigation of the levels of MSH2, MSH3 and MSH6 in several murine tissues [54], but no obvious correlation was found between instability phenotypes and protein levels.…”

DNA Mismatch Repair Complex MutSβ Promotes GAA·TTC Repeat Expansion in Human Cells

“…Recently however, it was determined that knockdown of MSH2 in FRDA patient-derived induced pluripotent stem cells (iPSCs) caused a slower rate of GAA⅐TTC expansion (26). Furthermore, it has also been reported that the MMR system plays a role in intergenerational GAA⅐TTC repeat dynamics in an FRDA mouse model (27). Thus it appears that regardless of the ability of a particular trinucleotide sequence to form hairpin structures, the contribution of DNA mismatch repair may be a common denominator in TNR expansion disorders.…”