“Let sleeping dogs lie” is sound advice for those who want to stay out of trouble! However, an eye-opening epigenome editing study now suggests that rousing the “sleeping” methyl CpG-binding protein 2 (MECP2) gene from the inactive X chromosome may represent a therapeutic opportunity for girls with Rett Syndrome – an X-linked neurodevelopmental disorder that impacts the development of motor skills and communication.
A somewhat daring team led by Rudolf Jaenisch (MIT) and X. Shawn Liu (Columbia University) knew that loss-of-function heterozygous mutations in the MECP2 gene cause Rett syndrome in young females; however, they hypothesized that reactivation (or “rousing”) of the silent wild-type MECP2 allele from the inactive X chromosome using a combination of epigenome editing tools might represent a potentially exciting solution, given their recent success in similar research focused on fragile X syndrome.
Let’s hear from Qian, Guan, and Colleagues on rousing a “sleeping dog” as a therapeutic solution to this problem:
- MECP2 promoter DNA demethylation using dCas9-Tet1 with a target single-guide RNA moderately reactivates MECP2 expression from the inactive X chromosome in Rett syndrome-human embryonic stem cells without detectable off-target transcriptional effects
- Neurons differentiated from dCas9-Tet1-edited Rett syndrome human embryonic stem cells maintain MECP2 reactivation, which supports the reversal of the smaller soma size and electrophysiological abnormalities associated with Rett syndrome
- Multiple epigenetic events typically mediate X chromosome inactivation; here, additional DNAinsulation of the dCas9-Tet1-edited MECP2 locus using a catalytically dead Cpf1 fused with CCCTC-binding factor (dCpf1-CTCF) with target CRISPR RNA (crRNA) in Rett syndrome neurons further enhancesMECP2 reactivation without detectable off-target effects at the transcriptional level
- This multiplexed approach better rescues Rett syndrome-associated cellular and electrophysiological defects compared to DNA methylation editing alone
- The study finds more significant reactivation when using Rett syndrome human embryonic stem cell-derived neurons compared to directly editing Rett syndrome neurons
- However, low-level MECP2 re-expression (15 to 20% of wild type) restores a normal-like lifespan to Mecp2-null male mice, while 1 to 5% has a measurable effect on neurologic symptoms/lifespan
These apparently astute epigeneticists chose not to let sleeping dogs lie; fortunately, their multiplexed “rousing” of the sleeping MECP2 gene led to a potentially exciting therapeutic solution rather than more problems! Their findings now pave the way for the development of in vivo therapeutic approaches to Rett syndrome and other X-linked diseases; however, the authors do note that critical challenges must be overcome first.
For more on rousing sleeping dogs with multiplexed epigenome editing, see Science Translational Medicine, January 2023.