Comparative characterization of human accelerated regions in neurons [PIPseq]

Human accelerated regions (HARs) are conserved genomic loci that have experienced rapid nucleotide substitutions following the divergence from chimpanzees. HARs are enriched in candidate regulatory regions near neurodevelopmental genes, suggesting their roles in gene regulation. However, their target genes and functional contributions to human brain development remain largely uncharacterized. Here, we elucidate the cis-regulatory functions of HARs in human and chimpanzee induced pluripotent stem cell (iPSC)-induced excitatory neurons. Using genomic and chromatin looping information, we prioritized 20 HARs and their chimpanzee orthologs for functional characterization via single-cell CRISPRi, and demonstrated their species-specific gene regulatory functions. Our findings reveal diverse functional outcomes of HAR-mediated cis-regulation in human neurons, including attenuated NPAS3 expression by altering the binding affinities of multiple transcription factors in HAR202 and maintaining iPSC pluripotency and neuronal differentiation capacities through the upregulation of PUM2 by 2xHAR.319. Finally, we use prime editing to demonstrate differential enhancer activity caused by several HAR26;2xHAR.178 variants. In particular, we link one variant in HAR26;2xHAR.178 to elevated SOCS2 expression and increased neurite outgrowth in human neurons. Thus, our study sheds new light on the endogenous gene regulatory functions of HARs and their potential contribution to human brain evolution.