[RESULTS] We leveraged an alignment of hundreds of mammalian genomes plus a Nextflow pipeline that we wrote for automating the detection of lineage-specific accelerated regions to identify 312 high-confidence HARs (zooHARs). Through massively parallel reporter assays and machine learning integration of hundreds of epigenomic datasets, we showed that many zooHARs function as neurodevelopmental enhancers and that their human substitutions alter transcription factor binding sites, consistent with previous studies. We further mapped zooHARs to specific cell types and tissues using single-cell open chromatin and gene expression data, and we found that they represent a more diverse set of neurodevelopmental processes than a parallel set of chimpanzee accelerated regions. To test the enhancer hijacking hypothesis, we first examined the three-dimensional neighborhoods of zooHARs using publicly available chromatin capture (Hi-C) data, finding a significant enrichment of zooHARs in domains with hsSVs. This motivated us to use deep learning to predict how hsSVs changed genome folding in the human versus the chimpanzee genomes. We found that 30% of zooHARs occur within 500 kb of an hsSV that substantially alters local chromatin interactions, and we confirmed this association in Hi-C data that we generated in human and chimpanzee neural progenitor cells. Finally, we showed that chromatin domains containing zooHARs and hsSVs are enriched for genes differentially expressed in human versus chimpanzee neurodevelopment.

[RATIONALE] Because HARs acquired many substitutions in our ancestors after millions of years of extreme constraint across diverse mammals, we reasoned that their conserved roles in regulating development of the brain and other organs must have changed during human evolution. One mechanism that could drive such a functional shift is enhancer hijacking, whereby the target gene repertoire of a noncoding sequence is changed through alterations in three-dimensional genome folding. The regulatory information encoded in a hijacked enhancer would likely need to change to avoid deleterious expression of the altered target gene while also possibly supporting modified expression patterns. Structural variants—large genomic insertions, deletions, and rearrangements—are the greatest sources of sequence differences between the human and chimpanzee genomes, and they have the potential to affect how a region of the genome folds and localizes in the nucleus. We therefore hypothesized that some HARs were generated through enhancer hijacking triggered by nearby human-specific structural variants (hsSVs).

[INTRODUCTION] Human accelerated regions (HARs) are evolutionarily conserved sequences that acquired an unexpectedly high number of nucleotide substitutions in the human genome since divergence from our common ancestor with chimpanzees. Prior work has established that many HARs are gene regulatory enhancers that function during embryonic development, particularly in neurodevelopment, and that most HARs show signatures of positive selection. However, the events that caused the sudden change in selective pressures on HARs remain a mystery.

This study received support from a Discovery Fellowship (K.C.K.), National Institute of Mental Health grants R01MH109907 and U01MH116438 (N.A., K.S.P., and K.C.K.), National Institute of Mental Health grant DP2MH122400-01 (A.P. and T.F.), National Institute of Human Genome Research grant R01HG008742 (E.K.), Gladstone Institutes (K.S.P.), the Schmidt Futures Foundation (A.P. and T.F.), the Shurl and Kay Curci Foundation (A.P. and T.F.), and a Swedish Research Council Distinguished Professor Award (K.L.-T.).

[CONCLUSION] The origin of many HARs may be explained by human-specific structural variants that altered three-dimensional genome folding, causing evolutionarily conserved enhancers to adapt to different target genes and regulatory domains.

Peer reviewed