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Abstract Background Current evidence suggests that cis-regulatory elements controlling gene expression may be the predominant target of natural selection in humans and other species. Detecting selection acting on these elements is critical to understanding evolution but remains challenging because we do not know which mutations will affect gene regulation. Results To address this, we devise an approach to search for lineage-specific selection on three critical steps in transcriptional regulation: chromatin activity, transcription factor binding, and chromosomal looping. Applying this approach to lymphoblastoid cells from 831 individuals of either European or African descent, we find strong signals of differential chromatin activity linked to gene expression differences between ancestries in numerous contexts, but no evidence of functional differences in chromosomal looping. Moreover, we show that enhancers rather than promoters display the strongest signs of selection associated with sites of differential transcription factor binding. Conclusions Overall, our study indicates that some cis-regulatory adaptation may be more easily detected at the level of chromatin than DNA sequence. This work provides a vast resource of genomic interaction data from diverse human populations and establishes a novel selection test that will benefit future study of regulatory evolution in humans and other species.

Abstract The introduction of more effective and selective mRNA delivery systems is required for the advancement of many emerging biomedical technologies including the development of prophylactic and therapeutic vaccines, immunotherapies for cancer and strategies for genome editing. While polymers and oligomers have served as promising mRNA delivery systems, their efficacy in hard-to-transfect cells such as primary T lymphocytes is often limited as is their cell and organ tropism. To address these problems, considerable attention has been placed on structural screening of various lipid and cation components of mRNA delivery systems. Here, we disclose a class of charge-altering releasable transporters (CARTs) that differ from previous CARTs based on their beta-amido carbonate backbone (bAC) and side chain spacing. These bAC-CARTs exhibit enhanced mRNA transfection in primary T lymphocytes in vitro and enhanced protein expression in vivo with highly selective spleen tropism, supporting their broader therapeutic use as effective polyanionic delivery systems.

Abstract Assay for Transposase Accessible Chromatin by sequencing (ATAC-seq) accurately depicts the chromatin regulatory state and altered mechanisms guiding gene expression in disease. However, bulk sequencing entangles information from different cell types and obscures cellular heterogeneity. To address this, we developed Cellformer, a deep learning method that deconvolutes bulk ATAC-seq into cell type-specific expression across the whole genome. Cellformer enables cost-effective cell type-specific open chromatin profiling in large cohorts. Applied to 191 bulk samples from 3 brain regions, Cellformer identifies cell type-specific gene regulatory mechanisms involved in resilience to Alzheimer’s disease, an uncommon group of cognitively healthy individuals that harbor a high pathological load of Alzheimer’s disease. Cell type-resolved chromatin profiling unveils cell type-specific pathways and nominates potential epigenetic mediators underlying resilience that may illuminate therapeutic opportunities to limit the cognitive impact of the disease. Cellformer is freely available to facilitate future investigations using high-throughput bulk ATAC-seq data.

Abstract Recombinant adeno-associated viral vectors (rAAVs) are among the most commonly used vehicles for in vivo based gene therapies. However, it is hard to predict which AAV capsid will provide the most robust expression in human subjects due to the observed discordance in vector-mediated transduction between species. In our study, we use a primate specific capsid, AAV-LK03, to demonstrate that the limitation of this capsid towards transduction of mouse cells is unrelated to cell entry and nuclear transport but rather due to depleted histone H3 chemical modifications related to active transcription, namely H3K4me3 and H3K27ac, on the vector DNA itself. A single-amino acid insertion into the AAV-LK03 capsid enables efficient transduction and the accumulation of active-related epigenetic marks on the vector chromatin in mouse without compromising transduction efficiency in human cells. Our study suggests that the capsid protein itself is involved in driving the epigenetic status of the vector genome, most likely during the process of uncoating. Programming viral chromatin states by capsid design may enable facile DNA transduction between vector and host species and ultimately lead to rational selection of AAV capsids for use in humans.

Summary: The blood-brain barrier (BBB) is primarily manifested by a variety of physiological properties of brain endothelial cells (ECs), but the molecular foundation for these properties remains incompletely clear. Here, we generate a comprehensive molecular atlas of adult brain ECs using acutely purified mouse ECs and integrated multi-omics. Using RNA sequencing (RNA-seq) and proteomics, we identify the transcripts and proteins selectively enriched in brain ECs and demonstrate that they are partially correlated. Using single-cell RNA-seq, we dissect the molecular basis of functional heterogeneity of brain ECs. Using integrative epigenomics and transcriptomics, we determine that TCF/LEF, SOX, and ETS families are top-ranked transcription factors regulating the BBB. We then validate the identified brain-EC-enriched proteins and transcription factors in normal mouse and human brain tissue and assess their expression changes in mice with Alzheimer’s disease. Overall, we present a valuable resource with broad implications for regulation of the BBB and treatment of neurological disorders.

Abstract The chromodomain helicase DNA-binding protein CHD8 is the most frequently mutated gene in autism spectrum disorder. Despite its prominent disease involvement, little is known about its molecular function in the human brain. CHD8 is a chromatin regulator which binds to the promoters of actively transcribed genes through genomic targeting mechanisms which have yet to be fully defined. By generating a conditional loss-of-function and an endogenously tagged allele in human pluripotent stem cells, we investigated the molecular function and the interaction of CHD8 with chromatin in human neurons. Chromatin accessibility analysis and transcriptional profiling revealed that CHD8 functions as a transcriptional activator at its target genes in human neurons. Furthermore, we found that CHD8 chromatin targeting is cell context-dependent. In human neurons, CHD8 preferentially binds at ETS motif-enriched promoters. This enrichment is particularly prominent on the promoters of genes whose expression significantly changes upon the loss of CHD8. Indeed, among the ETS transcription factors, we identified ELK1 as being most highly correlated with CHD8 expression in primary human fetal and adult cortical neurons and most highly expressed in our stem cell-derived neurons. Remarkably, ELK1 was necessary to recruit CHD8 specifically to ETS motif-containing sites. These findings imply that ELK1 and CHD8 functionally cooperate to regulate gene expression and chromatin states at MAPK/ERK target genes in human neurons. Our results suggest that the MAPK/ERK/ELK1 axis potentially contributes to the pathogenesis caused by CHD8 mutations in human neurodevelopmental disorders.

Here the authors studied pulmonary arterial endothelial cells (PAEC) under laminar shear stress and show that this physiologic condition markedly changes chromatin accessibility at regulatory regions, when compared to cells grown in a static state. They find that KLF4 organizes chromatin by interacting with the SWI/SNF nucleosome remodeling complex to regulate vasculo-protective gene expression.

Some autosomal genes are expressed in a random monoallelic manner, but its extent and mechanisms have remained unclear. Here the authors show robust monoallelic expression in cell lines and mice, where the silent allele can be reexpressed using epidrugs. Further, they find these genes display various modalities of allelic expression with different degrees of allelic imbalance.

Studies show the cancer transcriptome correlates poorly with the cancer proteome, questioning the role of chromatin regulation. Here the authors demonstrate proximal-gene-body chromatin elements and transcription predict abundances of differentially expressed proteins in thyroid and breast cancers.

The long noncoding RNA XIST plays a central role in sex-specific gene expression in humans by silencing one of two X chromosomes in female cells. Here the authors show that higher order secondary structure creates the modular domain structure of XIST ribonucleoprotein complex and spatial separation of functions.

Systemic sclerosis (SSc) is a disease with manifestation in the skin and immune etiology, but the pathogenic immune cell types remain unidentified. Here the authors use ATAC-seq to profile chromatin landscapes of skin samples from patients with SSc to implicate skin dendritic cells for having the strongest disease-associated epigenetic changes.

Summary: Selective 2′-hydroxyl acylation analyzed by primer extension (SHAPE) structure probing techniques characterize the secondary structure of RNA molecules, which influence their functions and interactions. A variation of SHAPE, footprinting SHAPE (fSHAPE), probes RNA in the presence and absence of protein to identify RNA bases that hydrogen-bond with protein. SHAPE or fSHAPE coupled with enhanced crosslinking and immunoprecipitation (SHAPE-eCLIP or fSHAPE-eCLIP) pulls down RNAs bound by any protein of interest and returns their structure or protein interaction information, respectively. Here, we describe detailed protocols for SHAPE-eCLIP and fSHAPE-eCLIP and an analysis protocol for fSHAPE.For complete details on the use and execution of these protocols, please refer to Corley et al. (2020).

Abstract We develop PIRCh-seq, a method which enables a comprehensive survey of chromatin-associated RNAs in a histone modification-specific manner. We identify hundreds of chromatin-associated RNAs in several cell types with substantially less contamination by nascent transcripts. Non-coding RNAs are found enriched on chromatin and are classified into functional groups based on the patterns of their association with specific histone modifications. We find single-stranded RNA bases are more chromatin-associated, and we discover hundreds of allele-specific RNA-chromatin interactions. These results provide a unique resource to globally study the functions of chromatin-associated lncRNAs and elucidate the basic mechanisms of chromatin-RNA interactions.

The authors report a co-occurrence of the U2AF1 S34F splicing factor mutation and ROS1 translocations in lung adenocarcinomas and profile effects of S34F on transcriptome-wide RNA binding. They further show that U2AF1 S34F enhances invasive potential and alters splicing of ROS1 fusion transcripts

Systemic sclerosis (SSc) is a heterogeneous chronic autoimmune disease that affects the connective tissue. Here, López-Isac et al. identify 13 new risk loci for SSc as well as loci specific for limited cutaneous and diffuse SSc and, defining credible sets and performing functional annotation, highlight key pathways and cell types for SSc.

Satb1 is a master regulator of multiple cellular processes. Here the authors find that Satb1 preferentially targets nucleosome dense regions and combinatorially uses multiple selection criteria including DNA torsion, flanking DNA shape, motif density and periodicity to streamline binding choices.

Summary: The majority of the mammalian genome is transcribed into non-coding RNAs, many of which co-evolve with RNA-binding proteins (RBPs) to function as biochemically defined and tractable ribonucleoproteins (RNPs). Here, we applied icSHAPE- a robust and versatile RNA structural probing pipeline- to endogenous RNPs purified from nuclei, providing base-resolution structural rationale for RNP activity and subcellular localization. Combining with genetic and biochemical reconstitutions, structural and functional alternations can be directly attributed to a given RBP without ambiguity.For complete details on the use and execution of this protocol, please refer to Chen et al. (2018).

Summary: Human dendritic cells (DCs) comprise subsets with distinct phenotypic and functional characteristics, but the transcriptional programs that dictate their identity remain elusive. Here, we analyze global chromatin accessibility profiles across resting and stimulated human DC subsets by means of the assay for transposase-accessible chromatin using sequencing (ATAC-seq). We uncover specific regions of chromatin accessibility for each subset and transcriptional regulators of DC function. By comparing plasmacytoid DC responses to IFN-I-producing and non-IFN-I-producing conditions, we identify genetic programs related to their function. Finally, by intersecting chromatin accessibility with genome-wide association studies, we recognize DC subset-specific enrichment of heritability in autoimmune diseases. Our results unravel the basis of human DC subset heterogeneity and provide a framework for their analysis in disease pathogenesis.

Cellular heterogeneity in cancer is complex and difficult to study. Here, the authors introduce Protein-indexed Assay of Transposase Accessible Chromatin (Pi-ATAC), which combines single cell chromatin and proteomic profiling to provide deep insight into the tumor microenvironment, and reveal the role of hypoxia in shaping the regulome of a subset of breast cancer cells in vivo.

How transcription factors (TFs) reprogram one cell lineage to another remains unclear. Here, we define chromatin accessibility changes induced by the proneural TF Ascl1 throughout conversion of fibroblasts into induced neuronal (iN) cells. Thousands of genomic loci are affected as early as 12 hr after Ascl1 induction. Surprisingly, over 80% of the accessibility changes occur between days 2 and 5 of the 3-week reprogramming process. This chromatin switch coincides with robust activation of endogenous neuronal TFs and nucleosome phasing of neuronal promoters and enhancers. Subsequent morphological and functional maturation of iN cells is accomplished with relatively little chromatin reconfiguration. By integrating chromatin accessibility and transcriptome changes, we built a network model of dynamic TF regulation during iN cell reprogramming and identified Zfp238, Sox8, and Dlx3 as key TFs downstream of Ascl1. These results reveal a singular, coordinated epigenomic switch during direct reprogramming, in contrast to stepwise cell fate transitions in development.

A Correction to this paper has been published: https://doi.org/10.1038/s41467-020-20411-w

Acute myeloid leukemia (AML) is an aggressive malignancy of the bone marrow characterized by an uncontrolled proliferation of undifferentiated myeloid lineage cells. Decades of research have demonstrated that AML evolves from the sequential acquisition of genetic alterations within a single lineage of hematopoietic cells. More recently, the advent of high-throughput sequencing has enabled the identification of a premalignant phase of AML termed preleukemia. Multiple studies have demonstrated that AML can arise from the accumulation of mutations within hematopoietic stem cells (HSCs). These HSCs have been termed “preleukemic HSCs” as they represent the evolutionary ancestors of the leukemia. Through examination of the biological and clinical characteristics of these preleukemic HSCs, this review aims to shed light on some of the unexplored questions in the field. We note that some of the material discussed is speculative in nature and is presented in order to motivate future work.

Long noncoding RNAs (lncRNAs) are thought to be prevalent regulators of gene expression, but the consequences of lncRNA inactivation in vivo are mostly unknown. Here, we show that targeted deletion of mouse Hotair lncRNA leads to derepression of hundreds of genes, resulting in homeotic transformation of the spine and malformation of metacarpal-carpal bones. RNA sequencing and conditional inactivation reveal an ongoing requirement of Hotair to repress HoxD genes and several imprinted loci such as Dlk1-Meg3 and Igf2-H19 without affecting imprinting choice. Hotair binds to both Polycomb repressive complex 2, which methylates histone H3 at lysine 27 (H3K27), and Lsd1 complex, which demethylates histone H3 at lysine 4 (H3K4) in vivo. Hotair inactivation causes H3K4me3 gain and, to a lesser extent, H3K27me3 loss at target genes. These results reveal the function and mechanisms of Hotair lncRNA in enforcing a silent chromatin state at Hox and additional genes.