The spatial folding of eukaryotic genome plays a key role in genome function. We report here that our recently developed method, Hi-TrAC, which specializes in detecting chromatin loops among accessible genomic regions, can detect active sub-TADs with a median size of 100 kb, most of which harbor one or two cell specifically expressed genes and regulatory elements such as super-enhancers organized into nested interaction domains. These active sub-TADs are characterized by highly enriched histone mark H3K4me1 and chromatin-binding proteins, including Cohesin complex. Deletion of selected sub-TAD boundaries have different impacts, such as decreased chromatin interaction and gene expression within the sub-TADs or compromised insulation between the sub-TADs, depending on the specific chromatin environment. We show that knocking down core subunit of the Cohesin complex using shRNAs in human cells or decreasing the H3K4me1 modification by deleting the H3K4 methyltransferase Mll4 gene in mouse Th17 cells disrupted the sub-TADs structure. Our data also suggest that super-enhancers exist as an equilibrium globule structure, while inaccessible chromatin regions exist as a fractal globule structure. In summary, Hi-TrAC serves as a highly sensitive and inexpensive approach to study dynamic changes of active sub-TADs, providing more explicit insights into delicate genome structures and functions.
Interferon-g (IFN-g) is a key cytokine produced by type 1 T helper (Th1) cells in response to viral or intracellular bacterial infections in mammals. While a number of positive regulatory elements have been well studied, no negative regulatory elements at the Ifng locus for its expression have been identified. Here we report a negative regulatory mechanism of Ifng expression by MLL4, a histone methyltransferase responsible for H3K4me1. Deletion of Mll4 resulted in loss of H3K4me1 signals at the previously unrecognized CNS-28 site and led to enhanced IFN-g expression during Th1 differentiation. GATA3 bound to the CNS-28 site and deletion of Gata3 resulted in a similar upregulation of IFN-g with Mll4 deletion. Furthermore, deletion of Mll4 compromised GATA3 binding to CNS-28 and led to a substantial reorganization of the 3D chromatin structure at the Ifng genomic locus in both naïve CD4+ T cells and Th1 cells. Thus, we conclude that MLL4 negatively regulates Ifng expression by facilitating GATA3 binding to the CNS-28 site and maintaining a special local chromatin organization.
Eukaryotic genome spatial folding plays a key role in genome function. Decoding the principles and dynamics of 3D genome organization depends on improving technologies to achieve higher resolution. Chromatin domains have been suggested as regulatory micro-environments, whose identification is crucial to understand the genome architecture. We report here that our recently developed method, Hi-TrAC, which specializes in detecting chromatin loops among genomic accessible regulatory regions, allows us to examine active domains with limited sequencing depths at a high resolution. Hi-TrAC can detect active sub-TADs with a median size of 100kb, most of which harbor one or two cell specifically expressed genes and regulatory elements such as super-enhancers organized into nested interaction domains. These active sub-TADs are characterized by highly enriched signals of histone mark H3K4me1 and chromatin-binding proteins, including Cohesin complex. We show that knocking down core subunit of the Cohesin complex using shRNAs in human cells or decreasing the H3K4me1 modification by deleting the H3K4 methyltransferase Mll4 gene in mouse Th17 cells disrupted the sub-TADs structure. In summary, Hi-TrAC serves as a compatible and highly responsive approach to studying dynamic changes of active sub-TADs, allowing us more explicit insights into delicate genome structures and functions.
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