Summary The molecular mechanisms underlying folding of mammalian chromosomes remain poorly understood. The transcription factor CTCF is a candidate regulator of chromosomal structure. Using the auxin-inducible degron system in mouse embryonic stem cells, we show that CTCF is absolutely and dose-dependently required for looping between CTCF target sites and insulation of topologically associating domains (TADs). Restoring CTCF reinstates proper architecture on altered chromosomes, indicating a powerful instructive function for CTCF in chromatin folding. CTCF remains essential for TAD organization in non-dividing cells. Surprisingly, active and inactive genome compartments remain properly segregated upon CTCF depletion, revealing that compartmentalization of mammalian chromosomes emerges independently of proper insulation of TADs. Further, our data support that CTCF mediates transcriptional insulator function through enhancer-blocking but not as a direct barrier to heterochromatin spreading. Beyond defining the functions of CTCF in chromosome folding these results provide new fundamental insights into the rules governing mammalian genome organization.

Summary Topologically Associating Domains (TADs) are fundamental structural and functional building blocks of human interphase chromosomes, yet mechanisms of TAD formation remain unclear. Here we propose that loop extrusion underlies TAD formation. In this process, cis-acting loop-extruding factors, likely cohesins, form progressively larger loops, but stall at TAD boundaries due to interactions with boundary proteins, including CTCF. Using polymer simulations, we show that this model produces TADs and other fine features of Hi-C data. Contrary to typical illustrations, each TAD consists of multiple loops dynamically formed through extrusion, rather than a single static loop. Loop extrusion both explains diverse experimental observations, including the preferential orientation of CTCF motifs, enrichments of architectural proteins at TAD boundaries, and boundary deletion experiments, and makes specific predictions for depletion of CTCF versus cohesin. Finally, loop extrusion has additional, potentially far-ranging, consequences for processes including enhancer-promoter interactions, orientation-specific chromosomal looping, and compaction of mitotic chromosomes.

Extracting biologically meaningful information from chromosomal interactions obtained with genome-wide chromosome conformation capture (3C) analyses requires elimination of systematic biases. We present a pipeline that integrates a strategy for mapping of sequencing reads and a data-driven method for iterative correction of biases, yielding genome-wide maps of relative contact probabilities. We validate ICE (Iterative Correction and Eigenvector decomposition) on published Hi-C data, and demonstrate that eigenvector decomposition of the obtained maps provides insights into local chromatin states, global patterns of chromosomal interactions, and the conserved organization of human and mouse chromosomes.

Imaging and chromosome conformation capture studies have revealed several layers of chromosome organization, including segregation into megabase-large active and inactive compartments, and partitioning into sub-megabase domains (TADs). Yet, it remains unclear how these layers of organization form, interact with one another and impact genome functions. Here, we show that deletion of the cohesin-loading factor Nipbl, in mouse liver, leads to a dramatic reorganization of chromosomal folding. TADs and associated peaks vanish globally, even in the absence of transcriptional changes. In contrast, compartmental segregation is preserved and even reinforced. Strikingly, the disappearance of TADs unmasks a finer compartment structure that accurately reflects the underlying epigenetic landscape. These observations demonstrate that the 3D organization of the genome results from the interplay of two independent mechanisms: 1) cohesin-independent segregation of the genome into fine-scale compartments, defined by chromatin state; 2) cohesin-dependent formation of TADs, possibly by loop extrusion, which contributes to guide distant enhancers to their target genes.

Mitotic chromosomes fold as compact arrays of chromatin loops. To identify the pathway of mitotic chromosome formation, we combined imaging and Hi-C of synchronous DT40 cell cultures with polymer simulations. We show that in prophase, the interphase organization is rapidly lost in a condensin-dependent manner and arrays of consecutive 60 kb loops are formed. During prometaphase ~80 kb inner loops are nested within ~400 kb outer loops. The loop array acquires a helical arrangement with consecutive loops emanating from a central spiral-staircase condensin scaffold. The size of helical turns progressively increases during prometaphase to ~12 Mb. Acute depletion of condensin I or II shows that nested loops form by differential action of the two condensins while condensin II is required for helical winding.