Background

This study addresses a central question in chromosome biology: how the 3D genome is rebuilt when cells exit mitosis and re-enter G1 phase. During mitosis, transcription is broadly suppressed and higher-order chromatin organization is extensively disrupted. Although previous work had established that compartments, boundaries, and loops reappear after mitosis, it remained unclear which factors actively drive this rebuilding process, and whether the same mechanisms used to maintain genome structure in interphase are also required for its de novo re-establishment. The authors focus on two major candidates, CTCF and transcription, and use the mitosis-to-G1 transition as a clean system to dissect their specific contributions to post-mitotic chromatin reconfiguration.

Notes: 

CCCTC-binding factor (CTCF) is a highly conserved, ubiquitously expressed zinc-finger DNA-binding protein that plays a central role in the organization of 3D genome architecture and regulation of gene expression. It is widely regarded as a master architectural protein that integrates chromatin structure with transcriptional control. At the molecular level, CTCF contains 11 zinc-finger domains, enabling it to bind diverse DNA sequences with flexible specificity. This versatility allows CTCF to occupy tens of thousands of sites across the genome, where it functions as a key regulator of chromatin topology. One of its most prominent roles is to act as an anchor for chromatin loops, often in cooperation with the cohesin complex. Through a process known as loop extrusion, cohesin extrudes chromatin until it encounters convergently oriented CTCF binding sites, thereby stabilizing loops that define the structural framework of the genome. CTCF is also critically involved in the formation and maintenance of topologically associating domains (TADs). It is highly enriched at TAD boundaries, where it acts as an insulator, preventing inappropriate interactions between regulatory elements such as enhancers and promoters located in neighboring domains. In this way, CTCF helps ensure the specificity of gene regulation by restricting enhancer activity to appropriate targets.

Beyond its structural roles, CTCF has diverse regulatory functions. It can act as a transcriptional activator or repressor, depending on context, and is involved in processes such as genomic imprinting, X-chromosome inactivation, and enhancer blocking. Additionally, CTCF contributes to the coordination of epigenetic states by interacting with histone modifications and chromatin remodeling factors.

Importantly, CTCF function is highly dynamic across the cell cycle. During mitosis, CTCF binding is largely reduced or redistributed, and chromatin architecture is disrupted. Upon mitotic exit, CTCF rapidly rebinds to chromatin and plays a key role in re-establishing genome organization, including chromatin loops and domain boundaries, thereby helping restore proper gene regulatory programs in daughter cells.

Picture from 6.2: The Cell Cycle - Biology LibreTexts

Question

whether CTCF and active transcription are required to rebuild chromatin architecture after mitosis, and if so, which architectural layers depend on each. More specifically, the study asks how CTCF influences compartmentalization, boundary re-formation, chromatin loop establishment, and cis-regulatory element contacts during mitotic exit, and whether transcription acts as a global architectural driver or instead contributes only to more local chromatin organization. By distinguishing the establishment of structure from its steady-state maintenance, the work aims to clarify the hierarchy of mechanisms that reconstruct the genome in newborn nuclei. 

Main Finding

The main conclusion is that CTCF plays a major causal role in rebuilding post-mitotic genome architecture, but its effects are selective rather than universal. CTCF is essential for the proper re-formation of many chromatin loops, especially structural loops, and for a subset of domain boundaries. In its absence, illegitimate contacts between cis-regulatory elements persist, including transient enhancer-promoter-like interactions that are normally resolved after telophase. At the same time, not all boundaries require CTCF: a substantial fraction can re-form normally without it, particularly those associated with transitions between active and inactive chromatin states. This supports a model in which post-mitotic boundary formation occurs through at least two parallel mechanisms, one dependent on CTCF/cohesin-mediated loop extrusion and another driven by chromatin state segregation.

A second major conclusion is that active transcription is not a primary driver of higher-order genome reorganization after mitosis. Transcription inhibition had little effect on global compartment architecture or on transcription start site-associated boundaries, indicating that large-scale chromatin refolding proceeds largely independently of ongoing transcription. However, transcription does contribute substantially to the formation of gene domains, meaning regions of enriched contacts along gene bodies. Importantly, these gene domains begin to emerge already in anaphase/telophase, before the first full round of transcription is completed, suggesting that pre-existing epigenetic features along gene bodies help pre-pattern chromatin architecture and that transcription then reinforces or elaborates this local organization.

Key methods used

The study combines precise cell-cycle-resolved perturbation with genome-wide structural and chromatin profiling. The authors used an auxin-inducible degron system in a mouse erythroblast model to acutely deplete CTCF specifically during the mitosis-to-G1 transition, allowing them to test CTCF function during architectural re-establishment rather than long-term maintenance. Cells were synchronized with nocodazole, sorted at defined post-mitotic stages, and analyzed by in situ Hi-C to measure compartments, boundaries, and loops. These structural data were integrated with ChIP-seq for CTCF, cohesin, and histone marks, along with transcription inhibition experiments and computational analyses including loop calling, boundary clustering, and principal component-based chromatin state transition analysis. Together, these approaches enabled the authors to causally dissect how CTCF and transcription differentially contribute to genome folding after mitosis. 

Research article link: CTCF and transcription influence chromatin structure re-configuration after mitosis | Nature Communications