Rescue of Fragile X Syndrome Neurons by DNA Methylation Editing of the FMR1 Gene

Highlights

• Targeted demethylation of CGG repeats by dCas9-Tet1 reactivates FMR1 in FXS cells

• Demethylation of CGG repeats induces an active chromatin status for FMR1 promoter

• Methylation-edited FXS neurons behave similarly as wild-type neurons

• FMR1 reactivation by dCas9-Tet1 is sustainable in a human/mouse chimeric model

Summary

Fragile X syndrome (FXS), the most common genetic form of intellectual disability in males, is caused by silencing of the FMR1 gene associated with hypermethylation of the CGG expansion mutation in the 5′ UTR of FMR1 in FXS patients. Here, we applied recently developed DNA methylation editing tools to reverse this hypermethylation event. Targeted demethylation of the CGG expansion by dCas9-Tet1/single guide RNA (sgRNA) switched the heterochromatin status of the upstream FMR1 promoter to an active chromatin state, restoring a persistent expression of FMR1 in FXS iPSCs. Neurons derived from methylation-edited FXS iPSCs rescued the electrophysiological abnormalities and restored a wild-type phenotype upon the mutant neurons. FMR1 expression in edited neurons was maintained in vivo after engrafting into the mouse brain. Finally, demethylation of the CGG repeats in post-mitotic FXS neurons also reactivated FMR1. Our data establish that demethylation of the CGG expansion is sufficient for FMR1 reactivation, suggesting potential therapeutic strategies for FXS.