Genetic and epigenetic determinants of reactivation of Mecp2 and the inactive X chromosome in neural stem cells

Rett Syndrome is a neurodevelopmental disorder in girls that is caused by heterozygous inactivation of the chromatin remodeler gene MECP2. Rett Syndrome may therefore be treated by reactivation of the wild type copy of MECP2 from the inactive X chromosome. Most studies that model Mecp2 reactivation have used mouse fibroblasts rather than neural cells, which would be critical for phenotypic reversal, and rely on fluorescent reporters that lack adequate sensitivity. Here, we present a mouse model system for monitoring Mecp2 reactivation that is more sensitive and versatile than any bioluminescent and fluorescent system currently available. The model consists of neural stem cells derived from female mice with a dual reporter system where MECP2 is fused to NanoLuciferase and TdTomato on the inactive X chromosome. We show by bioluminescence and fluorescence that Mecp2 is synergistically reactivated by 5-Aza treatment and Xist knockdown. As expected, other genes on the inactive X chromosome are also reactivated, the majority of which overlaps with genes reactivated early during reprogramming of mouse embryonic fibroblasts to iPSCs. Genetic and epigenetic features such as CpG density, SINE elements, distance to escapees and CTCF binding are consistent indicators of reactivation, whereas different higher order chromatin areas are either particularly prone or resistant to reactivation. Our MeCP2 reactivation monitoring system thereby suggests that genetic and epigenetic features on the inactive X chromosome affect reactivation of its genes, irrespective of cell type or procedure of reactivation.