X reactivation during neurodevelopment
While random X-chromosome inactivation in female cells of placental mammals silences one allele of the majority of X-chromosomal genes, a considerable fraction is only incompletely and variably inactivated. Human model systems to study the dynamics of incomplete X-inactivation are limited mostly to postmortem tissue, thereby disregarding developmental trajectories. Here we used clonal human female induced pluripotent stem cells to track allele-specific expression of X-chromosomal genes along neural differentiation. We discovered dynamic reactivation and late-silencing of gene expression from the inactive X-chromosome leading to differentiation- induced locus- and lineage-specific usage of the two X-chromosomal alleles. In brain organoids modeling Opitz BBB/G syndrome, an X-linked neurodevelopmental disorder, reactivation of alleles from the inactive X-chromosome rescued cellular phenotypes and led to intermediate manifestations in female tissue. Taken together, our data demonstrate that alleles on the inactive X-chromosome can serve as a critical reservoir dynamically used during differentiation, thereby enhancing resilience of female neural tissue.