Massively parallel dissection of human accelerated regions in human and chimpanzee neural progenitors

How mutations in gene regulatory elements lead to evolutionary changes remains largely unknown. Human accelerated regions (HARs) are ideal for exploring this question, because they are associated with human-specific traits and contain multiple human-specific variants at sites conserved across mammals, suggesting that they alter or compensate to preserve function. We performed massively parallel reporter assays on all human and chimpanzee HAR sequences in human and chimpanzee iPSC-derived neural progenitors at two differentiation stages. Forty-three percent (306/714) of HARs function as neuronal enhancers, with two-thirds (204/306) showing consistent changes in activity between human and chimpanzee sequences. These changes were almost all sequence dependent and not affected by cell species or differentiation stage. We tested all evolutionary intermediates between human and chimpanzee sequences of seven HARs, finding variants that interact both positively and negatively. This study shows that variants acquired during human evolution interact to buffer and amplify changes to enhancer function.