Sustained Epigenetic Reactivation in Fragile X Neurons with an RNA-Binding Small Molecule

Fragile X syndrome (FXS) is a disease of pathologic epigenetic silencing induced by RNA. In FXS, an expanded CGG repeat tract in the FMR1 gene induces epigenetic silencing during embryogenesis. FMR1 silencing can be reversed with 5-aza-deoxyctidine (5-aza-dC), a nonspecific epigenetic reactivator; however, continuous administration of 5-aza-dC is problematic due to its toxicity. We describe an approach to restore FMR1 expression in FXS neurons by transient treatment with 5-aza-dC, followed by treatment with 2HE-5NMe, which binds the CGG repeat expansion in the FMR1 mRNA and could block the resilencing of the FMR1 gene after withdrawal of 5-aza-dC.

This study uses immunofluorescence and fluorescent in situ hybridization (FISH) to measure whether FMR1 expression is maintained in FXS post-mitotic neurons treated with 2HE-5NMe. Genome-wide profiling of histone marks was used to monitor epigenetic changes and drug selectivity in response to 5-aza-dC followed by 2HE-5NMe treatment. Changes to dendritic morphology were visualized using confocal microscopy.

In this study, we find that 2HE-5Nme maintains FMR1 in a reactivated state after reactivation using 5-aza-dC in post-mitotic neurons. FMR1 reactivation in neurons results in the re-expression of FMRP and reversal of FXS-associated dendritic spine defects.

These results demonstrate that an RNA-binding small molecule can achieve gene-specific epigenetic control and provide an approach for the restoration of FMRP in FXS neurons.