Facultative CTCF sites moderate mammary super-enhancer activity and regulate juxtaposed gene in non-mammary cells

Precise spatiotemporal gene regulation is paramount for the establishment and maintenance of cell-specific programmes. Although there is evidence that chromatin neighbourhoods, formed by the zinc-finger protein CTCF, can sequester enhancers and their target genes, there is limited in vivo evidence for CTCF demarcating super-enhancers and preventing cross talk between distinct regulatory elements. Here, we address these questions in the Wap locus with its mammary-specific super-enhancer separated by CTCF sites from widely expressed genes. Mutational analysis demonstrates that the Wap super-enhancer controls Ramp3, despite three separating CTCF sites. Their deletion in mice results in elevated expression of Ramp3 in mammary tissue through augmented promoter-enhancer interactions. Deletion of the distal CTCF-binding site results in loss of Ramp3 expression in non-mammary tissues. This suggests that CTCF sites are porous borders, allowing a super-enhancer to activate a secondary target. Likewise, CTCF sites shield a widely expressed gene from suppressive influences of a silent locus.