Abolished clustering of MeCP2T158M can be partially reverted with small molecules

Abstract

Abstract Rett syndrome (OMIM 312750) is a rare neurodevelopmental disorder caused by de novo mutations in the Methyl-CpG Binding Protein 2 (MeCP2) gene located on the X-chromosome, typically affecting girls. Rett syndrome symptoms, characterized by microcephaly and lack of motor coordination, first appear between 6 to 18 months of age. The disease continues to progress until adulthood at which point it reaches a stationary phase. Currently, available therapy for Rett Syndrome is only symptomatic. More than 800 mutations causing Rett syndrome have been described, the most common being T158M (9% prevalence) located in the Methyl-Binding Domain (MBD) of MeCP2. Due to its importance for DNA binding through recognition of methylated CpG, mutations in the MBD have a significant impact on the stability and function of MeCP2. MeCP2 is a nuclear protein and accumulates in liquid–liquid phase condensates visualized as speckles in NIH3T3 cells by microscopy. This speckled pattern is lost with MeCP2 mutations in the MBD such as T158M. We developed a high content phenotypic assay, detecting fluorescent MeCP2 speckles in NIH3T3 cells. The assay allows the identification of small molecules that stabilize MeCP2-T158M and phenotypically rescue speckle formation. To validate the assay, a collection of 3572 drugs was screened, including FDA-approved drugs, compounds in clinical trials and biologically annotated tool compounds. 18 hits showed at least 25% rescue of speckles in the mutant cell line while not affecting wild-type MeCP2 speckles. Primary hits were confirmed in a dose response assay, a thermal shift assay with recombinant MeCP2 and by testing the MeCP2 expression levels. One class of identified hits represents histone deacetylase inhibitors (HDACis) showing 25% speckle rescue of mutant MeCP2 without toxicity. This screening strategy can be expanded to additional compound libraries and will support novel drug discovery.

Keywords

• MeCP2
• Rett Syndrome
• HDACi
• High content screen