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MCM complexes are barriers that restrict cohesin-mediated loop extrusion [snHi-C]

ABSTRACT: Eukaryotic genomes are compacted into loops and topologically associating domains (TADs), which contribute to transcription, recombination and genomic stability. Cohesin extrudes DNA into loops that are thought to lengthen until CTCF boundaries are encountered. Little is known about whether loop extrusion is impeded by DNA-bound macromolecular machines. We demonstrate that the replicative helicase MCM is a barrier that restricts loop extrusion in G1 phase. Single-nucleus Hi-C of one-cell embryos revealed that MCM loading reduces CTCF-anchored loops and decreases TAD boundary insulation, suggesting loop extrusion is impeded before reaching CTCF. Single-molecule imaging shows that MCMs are physical barriers that constrain cohesin translocation in vitro. Simulations are consistent with MCMs as abundant, random barriers. We conclude that distinct loop extrusion barriers contribute to shaping 3D genomes.

ORGANISM(S): Mus musculus

PROVIDER: GSE196300 | GEO | 2022/03/08

REPOSITORIES: GEO

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Project description:Cohesin organizes mammalian interphase chromosomes by reeling chromatin fibers into dynamic loops. "Loop extrusion" is obstructed when cohesin encounters a properly oriented CTCF protein. It has been proposed that transcription relocalizes or interferes with cohesin, and that active transcription start sites function as cohesin loading sites, but how these effects, and transcription in general, shape chromatin is unknown. To determine whether transcription can modulate loop extrusion, we studied cells in which the primary extrusion barriers could be removed by CTCF depletion and cohesin’s residence time and abundance on chromatin could be increased by Wapl knockout. We found evidence that transcription directly interacts with loop extrusion through a novel "moving barrier" mechanism, but not by loading cohesin at active promoters. Hi-C experiments showed intricate, cohesin-dependent genomic contact patterns near actively transcribed genes, and in CTCF-Wapl double knockout (DKO) cells, genomic contacts were enriched between sites of transcription-driven cohesin localization ("cohesin islands"). Similar patterns also emerged in polymer simulations in which transcribing RNA polymerases (RNAPs) acted as "moving barriers" by impeding, slowing, or pushing loop-extruding cohesins. The model predicts that cohesin does not load preferentially at promoters and instead accumulates at TSSs due to the barrier function of RNAPs. We tested this prediction by new ChIP-seq experiments, which revealed that the "cohesin loader" Nipbl co-localizes with cohesin, but, unlike in previous reports, Nipbl did not accumulate at active promoters. We propose that RNAP acts as a new type of barrier to loop extrusion that, unlike CTCF, is not stationary in its precise genomic position, but is itself dynamically translocating and relocalizes cohesin along DNA. In this way, loop extrusion could enable translocating RNAPs to maintain contacts with distal regulatory elements, allowing transcriptional activity to shape genomic functional organization.

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MCM complexes are barriers that restrict cohesin-mediated loop extrusion [snHi-C]

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