CTCF couples long-range loop extrusion and diffusion to mediate a diverse Igk repertoire [ChIP-Seq]

Dynamic genome folding is important for V(D)J recombination at the immunoglobulin kappa (Igk) locus, which recombines Jk and Vk gene segments across a 3.2 Mb region in both deletional and inversional orientations. Chromatin loop extrusion and diffusion are considered two key mechanisms underlying Igk folding, but how they coordinate remains unclear. Here we show that CTCF is a key regulator coupling loop extrusion and diffusion during Igk V-J rearrangement, promoting recombination in both orientations across long genomic distances. Mechanistically, the CTCF N-terminus promotes long-range loop extrusion that facilitates distal Vk usage by stabilizing cohesin against WAPL release, and also forms loop barriers enabling chromatin diffusion for inversional Vk joining. In CTCF N-terminal-deficient B cells, defects in inversional Vk joining are not restored by WAPL depletion but are instead largely rescued by a dCas9-blockade targeted to the Vk-Jk intergenic region, mimicking the CTCF barrier. Our findings thus highlight how CTCF coordinates distinct genome-folding mechanisms through its dual roles in cohesin stabilization and extrusion barrier formation to ensure the generation of a diverse Igk repertoire.