Fragile X Syndrome (FXS), the most common genetic form of intellectual disability in males is caused by a CGG trinucleotide repeat (>200) expansion mutation at the 5’-UTR of FMR1, accompanied by DNA hypermethylation that is thought to result in heterochromatin formation at the FMR1 promoter and subsequent silencing of FMR1 expression in FXS. Recently, DNA methylation editing tools have been developed to reverse the 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. A wild-type phenotype was restored in neurons derived from methylation-edited FXS iPSCs. FMR1 expression in edited neurons was maintained in vivo after engrafting in the mouse brain. Finally, demethylation of the CGG repeats in post-mitotic FXS neurons also reactivated FMR1. Off-target effects associated with the dCas9-Tet/CGG sgRNA system were minimal suggesting a highly specific effect by targeted methylation editing. This study shows that reversion of gene inactivation by epigenetic editing may be a valid therapeutic strategy, holding great potential for future research of novel therapies in FXS as well as in other neurodegenerative disorders where gene hypermethylation and silencing has a causative role.
Key points:
- Targeted demethylation of CGG repeats by dCas9-Tet1 reactivates FMR1 in FXS cells
- Demethylation of CGG repeats induces an active chromatin status for FMR1 promote
- DNA methylation editing holds a potential for future research in neurodegenerative disorders where gene hypermethylation and silencing has a causative role.
References
Liu XS, Wu H, Krzisch M, Wu X, Graef J, Muffat J, Hnisz D,Li CH, Yuan B, Xu C, Li Y, Vershkov D, Cacace A, Young RA,Jaenisch R. Rescue of Fragile X Syndrome Neurons by DNA Methylation Editing of the FMR1 Gene. Cell. 2018; 172:979-992.e6. doi: 10.1016/j.cell.2018.01.012. Epub 2018 Feb 15.
Avitzour M, Mor-Shaked H, Yanovsky-Dagan S, Aharoni S, AltarescuG, Renbaum P, Eldar-Geva T, Schonberger O, Levy-Lahad E,Epsztejn-Litman S, Eiges R. FMR1 epigenetic silencing commonly occurs in undifferentiated fragile X-affected embryonic stem cells. Stem Cell Reports. 2014; 3:699-706. doi: 10.1016/j.stemcr.2014.09.001.Epub 2014 Oct 3.