Genome-wide maps of R-loops in Drosophila embryogenesis and cultured cells
Ontology highlight
ABSTRACT:
ORGANISM(S): Drosophila melanogaster
PROVIDER: GSE185403 | GEO | 2022/06/21
REPOSITORIES: GEO
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Project description:Strand asymmetry in the distribution of guanines and cytosines, measured by GC skew, predisposes DNA sequences towards R-loop formation upon transcription. Previous work revealed that GC skew and R-loop formation associate with a core set of unmethylated CpG island (CGI) promoters in the human genome. Here, we show that GC skew can distinguish four classes of promoters, including three types of CGI promoters, each associated with unique epigenetic and gene ontology signatures. In particular, we identify a strong and a weak class of CGI promoters and show that these loci are enriched in distinct chromosomal territories reflecting the intrinsic strength of their protection against DNA methylation. Interestingly, we show that strong CGI promoters are depleted from the X chromosome while weak CGIs are enriched, a property consistent with the acquisition of DNA methylation during dosage compensation. Furthermore, we identify a third class of CGI promoters based on its unique GC skew profile and show that this gene set is enriched for Polycomb group targets. Lastly, we show that nearly 2,000 genes harbor GC skew at their 3’ ends and that these genes are preferentially located in gene-dense regions and tend to be closely arranged. Genomic profiling of R-loops accordingly showed that a large proportion of genes with terminal GC skew form R-loops at their 3’-ends, consistent with a role for these structures in permitting efficient transcription termination. Altogether, we show that GC skew and R-loop formation offer significant insights into the epigenetic regulation, genomic organization, and function of human genes. DRIP-seq was performed on genomic DNA extracted from human pluripotent Ntera2 cells. The DNA was either fragmented using HindIII, EcoRI, BsrGI, XbaI and SspI (DRIP-seq 1) or BamHI, NcoI, ApaLI, NheI and PvuII (DRIP-seq 2, two technical replicates). Input DNA was also fragmented with each restriction enzyme cocktail and sequenced alongside.
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Project description:Strand asymmetry in the distribution of guanines and cytosines, measured by GC skew, predisposes DNA sequences towards R-loop formation upon transcription. Previous work revealed that GC skew and R-loop formation associate with a core set of unmethylated CpG island (CGI) promoters in the human genome. Here, we show that GC skew can distinguish four classes of promoters, including three types of CGI promoters, each associated with unique epigenetic and gene ontology signatures. In particular, we identify a strong and a weak class of CGI promoters and show that these loci are enriched in distinct chromosomal territories reflecting the intrinsic strength of their protection against DNA methylation. Interestingly, we show that strong CGI promoters are depleted from the X chromosome while weak CGIs are enriched, a property consistent with the acquisition of DNA methylation during dosage compensation. Furthermore, we identify a third class of CGI promoters based on its unique GC skew profile and show that this gene set is enriched for Polycomb group targets. Lastly, we show that nearly 2,000 genes harbor GC skew at their 3’ ends and that these genes are preferentially located in gene-dense regions and tend to be closely arranged. Genomic profiling of R-loops accordingly showed that a large proportion of genes with terminal GC skew form R-loops at their 3’-ends, consistent with a role for these structures in permitting efficient transcription termination. Altogether, we show that GC skew and R-loop formation offer significant insights into the epigenetic regulation, genomic organization, and function of human genes.
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Project description:R-loops serve as key modulators in various biological processes in humans and plants.Current high-throughput methodologies for R-loop profiling have their own intrinsic pros and cons. Therefore, any robust, sensitive, and amenable methods, complementary and/or advantageous to existing methodologies, are much needed for advancing R-loop biology. We here present an RNase H based cleavage combined with DNA polymerase-mediated in situ biotin-dATP/dCTP labeling of newly synthesized DNA for sequencing, termed ISRH-seq, an antibody-independent approach for strand-specific mapping of R-loops. After conducting a parallel comparison with DRIP-seq, we found that ISRH-seq can identify more R-loop peaks with the same sequencing depth. Moreover, ISRH- and DRIP-specific R-loops had distinct genomic and epigenetic features, exhibited distinct relationships with expression of overlapping genes, and functional divergence. Importantly, we found that ISRH-seq can be scalable down to 250-100ng DNA input, indicative of a robust and sensitive high-throughput methodology. In addition, it can be applied for R-loop profiling under stress conditions. Thus, our study provides a valuable orthogonal methodology for comprehensive profiling of R-loops in plants. It could be easily adapted to other non-plant systems, especially for these biologically important tissues or cell types with limited cell numbers, thereby advancing understanding of R-loop biology.
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Project description:R-loop, a three-stranded RNA/DNA structure, has been linked to induced genome instability and regulated gene expression. To enable precision analysis of R-loops in vivo, we develop an RNase H-based approach, revealing predominant R-loop formation near gene promoters with strong G/C skew and propensity to form G-quadruplex in non-template DNA, corroborating with all biochemically established properties of R-loops. Transcription perturbation experiments further indicate that R-loop induction correlates to transcriptional pausing. Interestingly, we note that most mapped R-loops are each linked to a nearby free RNA end, and by using a ribozyme to co-transcriptionally cleave nascent RNA, we demonstrate that such free RNA end coupled with a G/C-skewed sequence is necessary and sufficient to induce R-loop. These findings provide a topological solution for RNA invasion into duplex DNA and suggest an order for R-loop initiation and elongation in an opposite direction to that previously proposed.
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Project description:3D chromatin structure, enhancers, and gene transcription function together to regulate cellular differentiation, establish cellular identity, and respond to external stimuli; however, the functional interplay between these regulatory elements remains incompletely understood. To infer potential causal relationships, we mapped 3D chromatin architecture, histone H3 K27 acetylation, chromatin accessibility, and gene expression across eight narrowly-spaced time points of macrophage activation. Enhancers and genes that were connected by a loop exhibited stronger correlation between histone H3K27 acetylation and expression than can be explained by distance or proximity alone, suggesting that the presence of a chromatin loop may facilitate functional regulatory connections via methods beyond simply increasing their frequency of physical proximity. Changes in enhancer acetylation preceded changes in gene expression by 30-60 minutes further supporting a causal relationship. Changes in gene expression exhibit a directional bias at differential loop anchors. The anchors of gained enhancer-promoter loops are associated with increased gene expression of genes oriented away from the center of the loop. Surprisingly, lost enhancer-promoter loops were also associated with increased gene expression, particularly within the bounds of the loop. Analysis of absolute levels of transcription revealed that lost loops were characterized by particularly high levels of internal transcription. Taken together, these results are consistent with a reciprocal relationship between looping and transcription. In this model, loops can enhance transcription by facilitating enhancer-promoter contacts, but also high levels of transcription can negatively impact loop strength by antagonizing loop extrusion mechanisms.
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