Pharmacological Reactivation of X Chromosome for Rett Syndrome Therapeutics

Rett syndrome (RTT) is a rare neurodevelopmental disorder that is primarily caused by loss-of-function mutations or truncations in an X-linked gene, MECP2, encoding methyl CpG-binding protein 2. This pediatric disease is characterized by its almost exclusive occurrence in girls, as boys are more severely affected and do not survive beyond infancy. The severity of the phenotype in males is due to a single copy of the X chromosome. But in females, the mutant MECP2 allele is diluted due to random X chromosome inactivation (XCI), an epigenetic process essential for dosage compensation between sexes. Because of XCI, ~50% of the cells in RTT girls are deficient for functional MECP2. However, these mutant cells also carry a wild type MECP2 allele on the inactive X (Xi), thus providing a potential source of wild type MECP2-if it can be effectively reactivated.

We have reactivated Xi by leveraging 13 X Chromosome Inactivation Factors (XCIFs) that we identified through a genome-wide RNA interference screen. These XCIFs are selectively required for silencing of X-linked genes and not for general transcriptional repression. Indeed, knockdown of any of the 13 XCIFs abolishes XCI, resulting in the reactivation of Xi-linked genes. These findings also pointed to lead pharmacological inhibitors of XCIFs (PDPK1, ACVR1 and AURKA) that reactivate Xi-linked genes, including MECP2 in ex vivo models.

Currently, there is no cure for RTT. We hypothesize that since XCI can be reversed, reactivation of the Xi-linked genes is possible. Therefore, we propose to reactivate Xi-MECP2 by pharmacological intervention and test previously identified lead compounds in vivo. In light of the fact that the function of MECP2 and its downstream targets are poorly understood, our approach represents an attractive therapeutic opportunity. This modality is especially attractive for RTT given the evidence that disease reversal is possible after onset in animal models by the ectopic expression of MECP2. Our long-term goal is to treat RTT by reactivating the endogenous MECP2 from Xi, in turn, compensating for its deficit in RTT girls.