FMR1 RNA interaction with DNMT1 blocks DNA methylation at the FMR1 locus.

In the FMR1 gene, expansion of the CGG triplet beyond 200 repeats triggers DNA methylation, resulting in the Fragile X Syndrome (FXS). There exist rare individuals who carry a CGG expansion >200 that remains unmethylated, rescuing them from expressing the FXS phenotype. We tested the hypothesis that active FMR1 transcription regulates DNA methylation of the locus through the binding of its mRNA to DNMT1 enzyme. Our results show that DNMT1 binds FMR1-mRNA in transcriptionally active cells preventing them from being methylated, whereas it binds to the FMR1 locus in FXS cells, resulting in gene silencing. DNMT1 binds to the transcript or to the locus after reactivating or blocking FMR1 transcription using specific drugs, respectively. As proof of concept, aptamers capable of binding and inhibiting DNMT1 were shown to reactivate the silenced FMR1 gene. We propose that DNMT1 represents a specific molecular target to reactivate the FMR1 gene expression.