Research Article

Chromatin accessibility dynamics in a model of human forebrain development

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Science  24 Jan 2020:
Vol. 367, Issue 6476, eaay1645
DOI: 10.1126/science.aay1645

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Organoids recapitulate brain development

Gene expression changes and their control by accessible chromatin in the human brain during development is of great interest but limited accessibility. Trevino et al. avoided this problem by developing three-dimensional organoid models of human forebrain development and examining chromatin accessibility and gene expression at the single-cell level. From this analysis, they matched developmental profiles between the organoid and fetal samples, identified transcription factor binding profiles, and predicted how transcription factors are linked to cortical development. The researchers were able to correlate the expression of neurodevelopmental disease risk loci and genes with specific cell types during development.

Science, this issue p. eaay1645

Structured Abstract

INTRODUCTION

The cerebral cortex is responsible for higher-order functions in the nervous system and has undergone substantial expansion in size in primates. The development of the forebrain, including the assembly of the expanded human cerebral cortex, is a lengthy process that involves the diversification and expansion of neural progenitors, the generation and positioning of layer-specific glutamatergic neurons, the cellular migration of γ-aminobutyric acid (GABA)–ergic neurons, and the formation and maturation of glial cells. Disruption of these cellular events by either genetic or environmental factors can lead to neurodevelopmental disease, including autism spectrum disorders and intellectual disability.

RATIONALE

Human forebrain development is, to a large extent, inaccessible for cellular-level study, direct functional investigation, or manipulation. The lack of availability of primary brain tissue samples—in particular, at later stages—as well as the limitations of conventional in vitro cellular models have precluded a detailed mechanistic understanding of corticogenesis in healthy and disease states. Therefore, tracking epigenetic changes in specific forebrain cell lineages over long time periods, has the potential to unravel the molecular programs that underlie cell specification in the human cerebral cortex and, by temporally mapping disease risk onto these changes, to identify cell types and periods of increased disease susceptibility.

RESULTS

We used three-dimensional (3D) directed differentiation of human pluripotent stem cells into dorsal and ventral forebrain domains and applied the assay for transposase-accessible chromatin with high-throughput sequencing (ATAC-seq) in combination with RNA-sequencing (RNA-seq) to map the epigenetic and gene expression signatures of neuronal and glial cell lineages over 20 months in vitro. We show, through direct comparison with primary brain tissue from our study and several epigenetic datasets, that human stem cell–derived 3D forebrain organoids recapitulated in vivo chromatin accessibility patterns over time. We then integrated these data to discover putative enhancer-gene linkages and lineage-specific transcription factor regulators, including a diverse repertoire of factors that may control cortical specification. We validated protein expression of some of these transcription factors using immunofluorescence, confirming cellular and temporal dynamics in both primary tissue and forebrain organoids. Next, we used this resource to map genes and genetic variants associated with schizophrenia and autism spectrum disorders to distinct accessibility patterns to reveal cell types and periods of susceptibility. Last, we identified a wave of chromatin remodeling during cortical neurogenesis, during which a quarter of regulatory regions are active, and then highlighted transcription factors that may drive these developmental changes.

CONCLUSION

Using long-term 3D neural differentiation of stem cells as well as primary brain tissue samples, we found that organoids intrinsically undergo chromatin state transitions in vitro that are closely related to human forebrain development in vivo. Leveraging this platform, we identified epigenetic alterations putatively driven by specific transcription factors and discovered a dynamic period of chromatin remodeling during human cortical neurogenesis. Finally, we nominated several key transcription factors that may coordinate over time to drive these changes and mapped cell type–specific disease-associated variation over time and in specific cell types.

Developing a human cellular model of forebrain development to study chromatin dynamics.

ATAC-seq and RNA-seq studies over long-term differentiation of human pluripotent stem cells into forebrain organoids and in primary brain tissue samples reveal dynamic changes during human corticogenesis, including a wave associated with neurogenesis, and identify disease-susceptible cell types and stages.

Abstract

Forebrain development is characterized by highly synchronized cellular processes, which, if perturbed, can cause disease. To chart the regulatory activity underlying these events, we generated a map of accessible chromatin in human three-dimensional forebrain organoids. To capture corticogenesis, we sampled glial and neuronal lineages from dorsal or ventral forebrain organoids over 20 months in vitro. Active chromatin regions identified in human primary brain tissue were observed in organoids at different developmental stages. We used this resource to map genetic risk for disease and to explore evolutionary conservation. Moreover, we integrated chromatin accessibility with transcriptomics to identify putative enhancer-gene linkages and transcription factors that regulate human corticogenesis. Overall, this platform brings insights into gene-regulatory dynamics at previously inaccessible stages of human forebrain development, including signatures of neuropsychiatric disorders.

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