Roles for uniquely human enhancers in brain development and WNT signaling

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ABSTRACT Humans diverge from other primates in numerous ways including their neuroanatomy and cognitive capacities. Human-specific features are particularly prominent in the cerebral cortex, which has undergone an expansion in size and acquired unique cellular composition and circuitry. Many of these features arose through modifications to cortical development, explained by human-specific gene expression. However, how human-specific gene expression explains divergent brain development is poorly understood. This proposal aims to fill that gap by investigating how non-coding regulatory loci impact human-specific brain development. Specifically, we focus on human accelerated regions (HARs), which are ultra-conserved sequences which have rapidly acquired mutations in the human lineage. HARs frequently physically associate with neurodevelopmental genes, and at least 50% of HARs have enhancer activity in human neural cells. Further, HARs are broadly associated with neurological disorders. Yet, biological functions for HARs in brain development remain largely unknown. Our group discovered HARE5, which shows divergent human-chimpanzee (Hs-Pt) enhancer activity in the developing mouse brain, due to just 4 mutations over 600 conserved nucleotides. HARE5 activates expression of Fzd8, a receptor in the WNT signaling pathway which is implicated in brain size and neurological disease. We have generated humanized HARE5 mouse models which exhibit expanded progenitor and neuron number and enlarged brains. We have additionally discovered new HARs, which like HARE5, are predicted to impact WNT signaling. This proposal will test the central hypothesis that evolutionary modifications of HAR enhancer activity modulate WNT signaling to control neural progenitor dynamics in the developing brain. Our proposal leverages our expertise and unique genetic tools, including mouse models, and human and non-human primate iPSCs and organoids. We will investigate mechanisms of HARE5 function in mouse models (Aim 1) and in cortical organoids generated from human and non-human primate iPSCs (Aim 2). We will then test roles for 12 WNT-associated HARs in neurogenesis (aim 3). Upon completion of this study, we will gain valuable insights into the developmental underpinnings of human cognitive capacities which can inform the basis for neuropsychiatric diseases.