Project description:The genome is partitioned into Topologically Associating Domains (TADs). About half of the boundaries of these TADs exhibit transcriptional activity and are correlated with better TAD insulation. However, the association between these transcripts and TAD insulation, enhancer:promoter interactions and transcription of genes remains unknown. Here we investigate the functional roles of these bRNAs (boundary-RNA) in boundary insulation and consequent effects on enhancer-promoter interactions and TAD transcription genome-wide and more specifically on the disease relevant INK4a/ARF TAD. Using a series of CTCF site deletions and bRNA knockdown experiments at this TAD boundary, we show a direct association of CTCF with the bidirectional bRNAs where the loss of bRNA triggers the concomitant loss of: CTCF at the TAD boundary, its insulation, enhancer:promoter interactions and gene transcription within the targeted TAD. We used another series of enhancer deletions and CRISPRi on promoters within INK4a/ARF TAD to better understand the regulation and origins of bRNA itself. We observe that the enhancers interact with boundaries and positively regulate the bRNA transcription at TAD boundaries. In return, the bRNAs recruit/stabilize the CTCF even on weak motifs within these boundaries and supports CTCF binding in clusters, therefore enhancing TAD insulation. This favors the intra-TAD enhancer:promoter interactions and leads to robust gene transcription. Functionally, bRNAs are more stable than eRNAs and their knockdown exactly mimics the CTCF site deletion. Furthermore, transcribing boundaries exhibit high TAD transcription in TCGA tumour datasets. Together, these results show that active enhancers directly mediate better insulation of TADs by activating the transcription at TAD boundaries triggering CTCF clustering at the boundary which results in better insulation and favours robust intra-TAD enhancer:promoter interactions to activate the gene transcription.

Project description:Chromatin domain boundaries delimited by CTCF motifs have been shown to restrict the range of enhancers. However, disruption of domain structure often results in mild gene expression changes, and predicting the impact of boundary rearrangements on animal development remains challenging. By targeting clusters of CTCF motifs in a domain with three FGF genes—Fgf3, Fgf4, and Fgf15—we tested whether gene regulation and mouse development may be particularly sensitive to disruption of chromatin domains with multiple developmental regulators. Deletion of a cluster of four CTCF motifs that defines the centromeric boundary of this domain, resulted in ectopic interactions of the FGF genes with brain enhancers located across the deleted boundary, and a strong induction of FGF expression that led to perinatal lethality with encephalocele and orofacial cleft phenotypes. Heterozygous boundary loss was enough to cause these fully penetrant phenotypes, and strikingly, loss of the single CTCF motif within the cluster that is oriented towards the brain enhancers, was sufficient to induce ectopic FGF expression and cause encephalocele. However, such phenotypic sensitivity to perturbation of domain structure did not extend to all CTCF-mediated boundaries of this domain nor to all developmental processes controlled by these three FGF genes. Although precise control of FGF4 levels is essential to regulate blastocyst development, none of our structural perturbations affected implantation. In fact, deletion of a CTCF boundary between Fgf3 and Fgf4, showed that the ability of these neighboring genes to have fully divergent expression patterns in blastocysts is achieved through CTCF-independent mechanisms and relies on remarkable enhancer-promoter specificity in vivo. Our work highlights how small sequence variants at certain domain boundaries can have a surprisingly outsized phenotypic effect and provides important insight into how different developmental contexts affect phenotypic sensitivity to perturbation of chromatin structure.

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High-resolution TADs reveal DNA sequences underlying genome organization in flies

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