Genetic Evidence for Asymmetric Blocking of Higher-Order Chromatin Structure by CTCF/Cohesin

The architectural protein CTCF binds to mammalian insulators genome wide with directionality and recruits cohesin to mediate oriented long-distance chromatin loops. In many cases, CTCF-binding sites (CBSs) are associated with distal enhancers and target promoters. It is generally thought that CTCF binding results in spatial contacts between distal enhancers and target promoters leading to activation of specific promoters. However, how CTCF orchestrates higher-order chromatin organization to regulate gene expression is not fully understood. Here we used CRISPR DNA-fragment editing to investigate the roles of CTCF in chromatin organization of the clustered protocadherin (Pcdh) genes, which is an ideal model system with repertoires of CBSs in variable promoters and super-enhancers. We found that the CBSs at chromatin domain boundary and even single CBSs could determine the orientation of long-distance chromatin looping. In addition, deletion or inversion of large regions containing tandem CBS arrays in mice demonstrated asymmetric influences on chromatin looping between super-enhancers and target promoters. These genetic data have important implications on the assembly mechanisms of higher-order chromatin structures. circularized chromosome conformation capture (4C) experiments