Heterogeneous interactions and polymer entropy decide organization and dynamics of chromatin domains

TL;DR

This study combines experimental contact maps with polymer physics simulations to reveal how heterogeneous interactions and entropy govern chromatin domain organization, dynamics, and their cell-type specific variability.

Contribution

It introduces a method to derive interaction parameters from contact maps and explores how these influence chromatin structure and dynamics across different epigenetic states.

Findings

Heterogeneous weak interactions are key to chromatin domain organization.

Perturbations in interactions alter the solid-like or liquid-like behavior of domains.

Polymer entropy and interaction energy competition affects chromatin loop dynamics.

Abstract

Chromatin is known to be organized into multiple domains of varying sizes and compaction. While these domains are often imagined as static structures, they are highly dynamic and show cell-to-cell variability. Since processes such as gene regulation and DNA replication occur in the context of these domains, it is important to understand their organization, fluctuation and dynamics. To simulate chromatin domains, one requires knowledge of interaction strengths among chromatin segments. Here, we derive interaction strength parameters from experimentally known contact maps and use them to predict chromatin organization and dynamics. Taking two domains on the human chromosome as examples, we investigate its 3D organization, size/shape fluctuations, and dynamics of different segments within a domain, accounting for hydrodynamic effects. Considering different cell types, we quantify changes in interaction strengths and chromatin shape fluctuations in different epigenetic states. Perturbing the interaction strengths systematically, we further investigate how epigenetic-like changes can alter the spatio-temporal nature of the domains. Our results show that heterogeneous weak interactions are crucial in determining the organization of the domains. Computing effective stiffness and relaxation times, we investigate how perturbations in interactions affect the solid-like and liquid-like nature of chromatin domains. Quantifying dynamics of chromatin segments within a domain, we show how the competition between polymer entropy and interaction energy influence the timescales of loop formation and maintenance of stable loops.