MSH2 is not required for either maintenance of DNA methylation or repeat contraction at the FMR1 locus in fragile X syndrome

BackgroundRepeat-induced epigenetic changes are observed in many repeat expansion disorders (REDs). These changes result in transcriptional deficits and/or silencing of the associated gene. MSH2, a mismatch repair protein that is required for repeat expansion in the REDs, has been implicated in the maintenance of DNA methylation seen in the region surrounding expanded CTG repeats at the DMPK locus in myotonic dystrophy type 1 (DM1). Here, we investigated the role of MSH2 in aberrant DNA methylation in two additional REDs, fragile X syndrome (FXS) that is caused by a CGG repeat expansion in the 5 untranslated region (UTR) of the fragile X messenger ribonucleoprotein 1 (FMR1) gene, and Friedreichs ataxia (FRDA) that is caused by GAA repeat expansion in intron 1 of the frataxin (FXN) gene. ResultsIn contrast to what is seen at the DMPK locus in DM1, loss of MSH2 did not decrease DNA methylation at the FMR1 promoter in FXS embryonic stem cells (ESCs) or increase FMR1 transcription. This difference was not due to the differences in the CpG density of the two loci as a decrease in DNA methylation was also not observed in a less CpG dense region upstream of the expanded GAA repeats in the FXN gene in MSH2 null induced pluripotent stem cells (iPSCs) derived from FRDA patient fibroblasts. Surprisingly given previous reports, we found that FMR1 reactivation was associated with a high frequency of MSH2- independent repeat contractions that resulted a permanent loss of DNA methylation. ConclusionsOur results suggest that there are mechanistic differences in the way that DNA methylation is maintained in the vicinity of expanded repeats among different REDs even though they share a similar mechanism of repeat expansion. The high frequency of transcription- induced MSH2-independent contractions we have observed may contribute to the mosaicism that is frequently seen in carriers of FMR1 alleles with expanded CGG-repeat tracts. Given the recent interest in the therapeutic use of transcription-driven repeat contractions, our data may have interesting mechanistic, prognostic, and therapeutic implications. Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=118 SRC="FIGDIR/small/629815v1_ufig1.gif" ALT="Figure 1000"> View larger version (20K): org.highwire.dtl.DTLVardef@1a6a59aorg.highwire.dtl.DTLVardef@1c25fdborg.highwire.dtl.DTLVardef@23394eorg.highwire.dtl.DTLVardef@876445_HPS_FORMAT_FIGEXP M_FIG C_FIG