Post-mitotic transcriptional activation and 3D regulatory interactions show locus- and differentiation-specific sensitivity to cohesin depletion.

Prior studies showed that structural loops collapse upon acute cohesin depletion, while regulatory enhancer-promoter (E-P) loops largely persist, consistent with minimal transcriptional changes. However, these studies, conducted in asynchronous cells, could not resolve whether cohesin is required for the establishment of regulatory interactions and transcriptional activation during cell division or cell state transitions. To address this gap, we degraded RAD21, a core cohesin subunit, in naïve mouse embryonic stem cells (ESCs) transitioning from mitosis to G1 either in self-renewal condition or during differentiation toward formative pluripotency. Although most structural loops failed to be re-established without cohesin, about 35% of regulatory loops reformed at normal or higher frequencies. Cohesin-independent loops showed characteristics of strong active enhancers and promoters and a significant association with H3K27ac mitotic bookmarks. However, inhibition of CBP/p300 during mitotic exit did not impact these cohesin-independent interactions, suggesting the presence of complex compensatory mechanisms. At the transcriptional level, cohesin depletion induced only minor changes, supporting that post-mitotic transcriptional reactivation is largely independent of cohesin. The few genes with impaired reactivation were directly bound by RAD21 at their promoters, engaged in many structural loops, and located within strongly insulated TADs with low gene density. Importantly, degrading cohesin during the M-to-G1 transition in the presence of EpiLC differentiation signals revealed a larger group of susceptible genes, including key signature genes and transcription factors. Impaired activation of these genes was partly due to the failure to establish