The Enhancer Landscape during Early Neocortical Development Reveals Patterns of Dense Regulation and Co-option

Abstract

Genetic studies have identified a core set of transcription factors and target genes that control the development of the neocortex, the region of the human brain responsible for higher cognition. The specific regulatory interactions between these factors, many key upstream and downstream genes, and the enhancers that mediate all these interactions remain mostly uncharacterized. We perform p300 ChIP-seq to identify over 6,600 candidate enhancers active in the dorsal cerebral wall of embryonic day 14.5 (E14.5) mice. Over 95% of the peaks we measure are conserved to human. Eight of ten (80%) candidates tested using mouse transgenesis drive activity in restricted laminar patterns within the neocortex. GREAT based computational analysis reveals highly significant correlation with genes expressed at E14.5 in key areas for neocortex development, and allows the grouping of enhancers by known biological functions and pathways for further studies. We find that multiple genes are flanked by dozens of candidate enhancers each, including well-known key neocortical genes as well as suspected and novel genes. Nearly a quarter of our candidate enhancers are conserved well beyond mammals. Human and zebrafish regions orthologous to our candidate enhancers are shown to most often function in other aspects of central nervous system development. Finally, we find strong evidence that specific interspersed repeat families have contributed potentially key developmental enhancers via co-option. Our analysis expands the methodologies available for extracting the richness of information found in genome-wide functional maps.

Author Summary

Sequencing based technologies provide global snapshots of transcriptional regulation. These data promise insights into gene regulation, disease susceptibility and organismal evolution. They also provide a methodological challenge in distilling specific hypotheses from large masses of data. Most work to date has focused on deriving broad biochemical insights. Here we obtain the active enhancer landscape of the dorsal cerebral wall during early neocortical development. We show that our set likely contains enhancers from both the developing neocortex, the ventricular, subventricular and intermediate zones, and develop methods to separate this mass into subsets of interest in particular contexts. We discover novel enhancers next to key neocortex development genes. We show that some known key and novel genes are regulated by dozens of enhancers each, and find known and novel enriched binding sites for key transcription factors in our set. Nearly all newly discovered enhancers are conserved in human. A quarter of loci are shared with non-mammalian vertebrates. We show that the human and zebrafish orthologs of our enhancers mostly drive expression in related nervous system contexts. We also show that particular interspersed repeats were preferentially co-opted into potentially key neocortex development enhancers.