31Dynamic changes in chromatin accessibility coincide with important aspects of neuronal differentiation, such as 32 fate specification and arealization and confer cell type-specific associations to neurodevelopmental disorders. 33However, studies of the epigenomic landscape of the developing human brain have yet to be performed at single-34 cell resolution. Here, we profiled chromatin accessibility of >75,000 cells from eight distinct areas of developing 35human forebrain using single cell ATAC-seq (scATACseq). We identified thousands of loci that undergo 36 extensive cell type-specific changes in accessibility during corticogenesis. Chromatin state profiling also reveals 37 novel distinctions between neural progenitor cells from different cortical areas not seen in transcriptomic profiles 38 and suggests a role for retinoic acid signaling in cortical arealization. Comparison of the cell type-specific 39 chromatin landscape of cerebral organoids to primary developing cortex found that organoids establish broad 40 cell type-specific enhancer accessibility patterns similar to the developing cortex, but lack many putative 41 regulatory elements identified in homologous primary cell types. Together, our results reveal the important 42 contribution of chromatin state to the emerging patterns of cell type diversity and cell fate specification and 43 provide a blueprint for evaluating the fidelity and robustness of cerebral organoids as a model for cortical 44 development. 45 46Main text 47The diverse cell types of the human cerebral cortex (Fig. 1a) have been mostly classified based on a handful of 48 morphological, anatomical, and physiological features. Recent innovations in single cell genomics, such as single 49 cell mRNA sequencing (scRNA-seq), have enabled massively parallel profiling of thousands of molecular 50 features in every cell, uncovering the remarkable molecular diversity of cell types previously considered 51 homologous, such as excitatory neurons located in different areas of the cerebral cortex 1-6 . However, the 52 developmental mechanisms underlying the emergence of distinct cellular identities are largely unknown, as most 53 cortical neurons are generated at stages that are inaccessible to experimentation 5 . 54 55Over 60 years ago, Conrad Waddington introduced the concept of an epigenomic landscape to account for the 56 emergence of distinct cell fates 7 . In particular, chromatin state defines the functional architecture of the genome 57
Genetic risk for complex traits is strongly enriched in non-coding genomic regions involved in gene regulation, especially enhancers. However, we lack adequate tools to connect the characteristics of these disruptions to genetic risk. Here, we propose RWAS (Regulome Wide Association Study), a new framework to identify the characteristics of enhancers that contribute to genetic risk for disease. Applying our technique to interrogate genetic risk for schizophrenia, we found that risk-associated enhancers in this disease are predominantly active in the brain, evolutionarily conserved, and AT-rich. The association between AT percentage and risk corresponds to an overrepresentation in risk-associated enhancers for the binding sites of transcription factors that recognize AT-rich cis-regulatory motifs. Several of the TFs identified in our model as being overrepresented in risk-associated enhancers, including MEF2C, are master regulators of neuronal development. The genes that encode several of these TFs are themselves located at genetic risk loci for schizophrenia. This list also includes brain-expressed TFs that have not previously been linked to schizophrenia. In summary, we developed a generalizable approach that integrates GWAS summary statistics with enhancer characteristics to identify risk factors in tissue-specific regulatory regions.
2Transcriptional changes occur presymptomatically and throughout Huntington's Disease (HD), 3 motivating the study of transcriptional regulatory networks (TRNs) in HD. We reconstructed a 4 genome-scale model for the target genes of 718 TFs in the mouse striatum by integrating a model 5 of the genomic binding sites with transcriptome profiling of striatal tissue from HD mouse 6 models. We identified 48 differentially expressed TF-target gene modules associated with age-7and Htt allele-dependent gene expression changes in the mouse striatum, and replicated many of 8 these associations in independent transcriptomic and proteomic datasets. Strikingly, many of 9 these predicted target genes were also differentially expressed in striatal tissue from human 10 disease. We experimentally validated a key model prediction that SMAD3 regulates HD-related 11 gene expression changes using chromatin immunoprecipitation and deep sequencing (ChIP-seq) 12 of mouse striatum. We found Htt allele-dependent changes in the genomic occupancy of 13 SMAD3 and confirmed our model's prediction that many SMAD3 target genes are down-14 regulated early in HD. Importantly, our study provides a mouse and human striatal-specific TRN 15and prioritizes a hierarchy of transcription factor drivers in HD. 16 17 peer-reviewed)
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