Abstract
Studies of spatial genome organization have revealed the existence of chromatin loops and Topologically Associating Domains (TADs) of several hundred kilobases in size, which are created by the cohesin ring complex through a process of DNA loop extrusion. However, these structures have been characterized almost exclusively by genomic techniques and fixed cell imaging, thus their temporal dynamics are still poorly understood. For example, it is not clear whether loop extrusion creates stable or transient contacts at loop anchors and key parameters of this process, including loop lifetime and extrusion speed, remain unknown. To address this gap, my thesis aims to quantify the dynamics of cohesin-dependent loop extrusion by visualizing and tracking in time pairs of anchors at several loops and TADs in living human cells. Extrusion is expected to manifest itself as a progressive decrease in anchor-anchor distances. However, this signature is obscured by stochastic motions of the chromatin, whereby anchors can occasionally come into contact even without extrusion. Furthermore, measuring the anchor-anchor distance from fluorescent images is complicated by several sources of uncertainties, such as unavoidable random errors in the computational localization of fluorescent spots. To evaluate the experimental conditions under which one can expect to detect and quantify loop extrusion despite such complications, I first performed an analysis in silico using polymer simulations that account for loop extrusion. Using these simulations, I also tested and validated novel analysis methods to quantify chromatin loop dynamics from static imaging (e. g. from DNA-FISH images of loop anchors), and to estimate the lifetime and frequency of anchor contacts, as well as the effective loop extrusion speed from dynamic imaging in vivo. Using the simulation results as guidelines, we fluorescently labelled multiple loop and TAD anchors in human cells using the CRISPR/Cas9 system and tracked the loop anchors by live-cell imaging. Based on our analysis of the imaging data, we found that contacts between the two loop anchors are infrequent and short-lived as compared to the cell cycle duration. However, loops were found to be almost constantly extruded by cohesin. By comparing simulations and experimental data, we could estimate key biophysical parameters of loop extrusion dynamics including loop lifetimes and extrusion speed. Our results suggest that cohesin-dependent loop extrusion is a highly dynamic process, which creates transient long-range interactions rather than stable contact s. Our findings will help to quantitatively understand biological processes that involve short-lived but long-range contacts created by loop extrusion, including mechanisms of DNA repair and gene regulation.