Project description:In addition to mediating sister chromatid cohesion, cohesin plays a central role in DNA looping and segmentation of the genome into contact domains (TADs). Two variant cohesin complexes that contain either STAG/SA1 or SA2 are present in all cell types. Here we addressed their specific contribution to genome architecture in non-transformed human cells. We found that cohesin-SA1 drives stacking of cohesin rings at CTCF-bound sites and thereby contributes to the stabilization and preservation of TAD boundaries. In contrast, a more dynamic cohesin-SA2 promotes cell type-specific contacts between enhancers and promoters within TADs independently of CTCF. SA2 loss, a condition frequently observed in cancer cells, results in increased intra-TAD interactions, likely altering the expression of key cell identity genes.
Project description:Cohesin complex, a main organizer of mammalian genomes, exists in two versions that differ in the identity of the STAG/SA subunit, which can be SA1 or SA2. Mouse embryonic stem cell (mESC) provide a useful system to address the specific contributions of each variant to genome architecture and gene expression, since 3D organization of super- enhancers and Polycomb domains is essential to achieve transcription of pluripotency factors and repression of lineage specification genes, respectively. Hi-C analyses reveal That cohesin-SA1 preserves the integrity of topological associating domains (TAD) boundaries together with CTCF and prevents excessive segregation of same-class Compartment regions. Cohesin-SA2 is enriched within super-enhancers and Polycomb domains. Moreover, it contributes to establishment and compaction of Polycomb domains through PRC1 recruitment thereby promoting interchromosomal interactions among Hox gene promoters and favoring gene repression. In contrast, these interactions are counteracted by cohesin-SA1. The opposite effects of the two complexes on genome topology explain the distinct consequences of their ablation for the mESCs Transcriptome.