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A modular dCas9-SunTag DNMT3A epigenome editing system overcomes pervasive off-target activity of direct fusion dCas9-DNMT3A constructs

ABSTRACT: DNA methylation is a covalent modification of the genome that plays important roles in genome regulation and vertebrate development. Although detection of this modification in the genome has been possible for several decades, the ability to deliberately and specifically manipulate local DNA methylation states in the genome has been extremely limited. Consequently, this has impeded the direct determination of the consequence of DNA methylation on transcriptional regulation and transcription factor binding in the native chromatin context. Thus, highly specific targeted epigenome editing tools are needed to address this outstanding question. Recent adaptations of genome editing technologies, such as the fusion of the DNMT3A methyltransferase catalytic domain to catalytically inactive Cas9 (dC9-D3A), have aimed to provide new tools for altering DNA methylation at desired loci. Here, we performed a deeper analysis of the performance of the tools at a genome wide level which revealed consistent off-target binding events and DNA methylation deposition throughout the genome, limiting the capacity of these tools to generate unambiguous determination of the functional consequences of DNA methylation. To address this issue, we developed a modular dCas9-SunTag (dC9Sun-D3A) system that can recruit multiple DNMT3A catalytic domains to a target site for editing DNA-methylation. dC9Sun-D3A is tunable, specific and exhibits much higher induction of DNA methylation at target sites than the dC9-D3A direct fusion protein. Importantly, genome-wide characterization of dC9Sun-D3A binding sites and DNA methylation revealed minimal off-target protein binding and induction of DNA methylation with dC9Sun-D3A, compared to pervasive off-target binding and methylation by the dC9-D3A direct fusion construct. Furthermore, we used dC9Sun-D3A to test the impact of DNA methylation upon the DNA binding of CTCF and NRF1 upon targeted methylation of their core binding sites, demonstrating the binding sensitivity of these proteins to DNA methylation in situ. Overall, this modular dC9Sun-D3A system enables precise DNA methylation deposition with the least amount of off-target DNA methylation reported to date, allowing accurate functional determination of the role of DNA methylation at single loci.

ORGANISM(S): Homo sapiens

PROVIDER: GSE107607 | GEO | 2018/06/05

REPOSITORIES: GEO

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Project description:Fusion of active protein domains to the nuclease-deficient clustered regularly interspaced short palindromic repeat (CRISPR) associated protein 9 (dCas9) has been widely used for epigenome editing, but the specificities of these engineered proteins have still not been fully investigated. Targeted methylation of specific gene loci offers a direct approach to perturb DNA methylation-associated biological processes. In this study, we generated and validated the global off-target characteristics of CRISPR-guided DNA methyltransferases (CRISPRme) by fusing the catalytic domain of DNMT3A or DNMT3B to the C terminus of the dCas9 protein from S. pyogenes. Using targeted quantitative bisulfite pyrosequencing and whole genome bisulfite sequencing (WGBS), we prove that CRISPRme can efficiently methylate the CpG dinucleotides flanking its target sites in genomic loci (uPA and TGFBR3) in human cells (HEK293T) with CpG-methylation levels exceeding 70% for some target sites. Using qPCR, fluorescent reporter cells, and RNA sequencing, we found that CRISPRme can mediate transient inhibition of gene expression which appears to result from Cas9-mediated interference with transcription rather than de novo DNA methylation. Analyses of whole genome methylation did not identify global methylation changes, however a substantial number of CRISPRme off-target differentially methylated regions (DMR, over 6000) were still identified. The majority of these DMRs were hypermethylated both in cells expressing CRISPRme alone and cells expressing CRISPRme together with gRNAs. These off-target hypermethylated sites were enriched in gene bodies, introns, 5’UTR, CGI shores, Alu sequences, open chromatin and PAM rich regions, but not correlated with off-target binding sites predicted by ChIP-seq. Our results prove that CRISPRme allows for efficient RNA-guided methylation of endogenous CpGs, however with high frequencies of off-target methylation indicating that further improvements of the specificity of CRISPR-dCas9 based DNA methylation modifiers are still required.

Project description:The Detect-seq provides an unbiased method for genome-wide identification of CBE induced off-targets inside of cells. Purpose: Cytosine base editors (CBEs) have the potential to correct human pathogenic point mutations. However, its genome-wide specificity remains poorly understood, precluding its therapeutic applications. Unbiased tools are urgently needed to comprehensively evaluate CBE off-targets at the genome-wide level. Methods: The genomic DNA from CBE transfected cells are processed with fragmentation, endogenous d5fC protection, damage repair, biotin-dUTP and 5fdCTP labeling, and pull-down steps. After the sequencing library preparation, the FASTQ files are produced by the Illumina HiSeq XTen platform. To guarantee the reproducibility, each case of the experiment is present with two biological replicates. Results: Through Detect-seq, we comprehensively profiled the genome-wide off-targets of CBE for several representative sgRNAs, and observed both weak, “random-like” off-targets as well as strong Cas9-dependent off-targets in different human cell lines. Cas9-dependent off-targets can be prevalent for several sgRNAs, and demonstrate a divergent degrees of sequence similarity to the sgRNA. These CBE-induced off-target sites differ greatly from those caused by Cas9 nuclease alone, suggesting inherently different properties among these genome editing tools. Targeted amplicon sequencing further corroborated the novel off-target sites by Detect-seq; quite a few sites showed editing ratio that is on the same order of magnitude or even comparable to the on-target sites. Detect-seq also revealed two unexpected types of Cas9-dependent off-targets, which are out-of-protospacer editing and target-strand editing. These novel off-targets were likely caused by an unstable structure at the PAM-distal side of the Cas9-sgRNA-DNA complex. Based upon such understanding to the off-target editing, we engineered several CBE variants bearing mutations in APOBEC1 and obtained improved CBEs with significantly reduced off-target editing activities.

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A modular dCas9-SunTag DNMT3A epigenome editing system overcomes pervasive off-target activity of direct fusion dCas9-DNMT3A constructs

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