Compartment-dependent chromatin interaction dynamics revealed by liquid chromatin Hi-C

Chromosomes are folded so that active and inactive chromatin domains are spatially segregated to form a variety of sub-nuclear neighborhoods. Compartmentalization is thought to occur through polymer phase/microphase separation mediated by interactions between loci of similar type. The nature and dynamics of these interactions are not known. We developed liquid chromatin Hi-C to map the stability of associations between loci genome-wide. Before fixation and Hi-C, chromosomes are fragmented removing the strong polymeric constraint to enable detection of intrinsic locus-locus interaction stabilities. We find that chromosome compartmentalization is dependent on the length of chromatin fragments. Compartmentalization is stable when fragments are over 10-25 kb. Fragmenting chromatin into pieces smaller than 6 kb leads to gradual loss of spatial genome organization. In addition to fragmentation level dissolution kinetics of chromatin interactions vary for different chromatin domains. Lamin associated domains are most stable, and speckle-associated loci are more dynamic. The polycomb-enriched B1 sub-compartment also displays highly unstable interactions. Cohesin-mediated loops dissolve after fragmentation, possibly because cohesin rings slide off nearby DNA ends. Liquid chromatin Hi-C provides a genome-wide view of chromosome interaction dynamics, revealing a range of conformational stabilities at different sub-nuclear structures. Two replicate timecourses of liquid chromatin Hi-C, Hi-C 2.0 and 5C in cells and mock nuclei, DpnII-seq timecourse