RAI1 safeguards fidelity and tempo of human neurodevelopmental gene expression.

Human brain development proceeds on an unusually long timeline, fostering the species' cognitive advantages. Retinoic Acid Induced 1 (RAI1) gene encodes a nucleosome-binding protein haploinsufficient in Smith–Magenis Syndrome (SMS), a neurodevelopmental disorder characterized by cognitive deficits with autistic features. However, the role of RAI1 in human neurodevelopment remains unexplored experimentally. Here, we generated isogenic heterozygous and homozygous RAI1 loss-of-function human embryonic stem cell lines and interrogated the roles of RAI1 in neurodevelopmental gene regulation. A longitudinal transcriptome study during in vitro cortical development revealed that RAI1 deficiency accelerates developmental transcriptome progression, as indicated by the induction of synaptic genes. Single-cell RNA-seq analysis revealed that RAI1-deficient neuroprogenitors acquire a transient mesoderm-like gene expression signature followed by pro-neuronal maturation gene expression in postmitotic neurons. Unexpectedly, the developmental acceleration signature was exacerbated during NGN2-induced excitatory neuron differentiation, suggesting functional interplay between RAI1 and NGN2-driven programs. Together, these results identify RAI1 as a suppressor of the mesodermal lineage program and as a novel brake that slows the tempo of human neurodevelopmental gene expression.