Re-configuration of Chromatin Structure During the Mitosis-G1 Phase Transition

Abstract

Higher-order chromatin organization such as A/B compartments, TADs and chromatin loops are temporarily disrupted during mitosis. These structures are thought to organize aspects of gene regulation, and thus it is important to understand how they are re-established after mitosis. We examined the dynamics of chromosome reorganization by Hi-C at defined time points following exit from mitosis in highly purified, synchronous cell populations. We observed that A/B compartments are rapidly established and progressively gain in strength following mitotic exit. Contact domain formation occurs from the \"bottom-up\" with smaller sub-TADs forming initially, followed by convergence into multi-domain TAD structures. CTCF is strongly retained at a significant fraction of sites on mitotic chromosomes and immediately resumes full binding at ana/telophase, the earliest tested time point. In contrast, cohesin is completely evicted from mitotic chromosomes and resumes focal binding with delayed kinetics. The formation of CTCF/cohesin co-anchored structural loops follows the kinetics of cohesin positioning. Stripe-shaped contacts anchored by CTCF grow in length, consistent with a loop extrusion process after mitosis. Interactions between cis-regulatory elements can form rapidly, preceding CTCF/cohesin anchored structural loops. Strikingly, we identified a group of rapidly emerging transient contacts between cis-regulatory elements in ana/telophase, that are dissolved upon G1 entry, co-incident with the establishment of inner boundaries or nearby interfering loops. Our findings indicate that distinct but mutually influential forces drive post-mitotic chromatin re-configuration to shape compartments, contact domains, cis-element contacts, and CTCF/cohesin dependent loops.