A de novo transcription-dependent TAD boundary underpins critical multiway interactions during antibody class switch recombination [Tri-C]

Antibody class switch recombination (CSR) requires the ligation of DNA breaks in transcribed and juxtaposed donor and acceptor switch (S) sequences (S-S synapsis) spaced 50-200 kb apart within the immunoglobulin heavy chain locus (Igh). CSR is believed to occur via the association of active S regions with the 3’ regulatory region (3’RR) super-enhancer thereby coupling transcription with S-S synapsis. Here, we map the multi-way interactome underlying CSR using Tri-C. Surprisingly, S-S synapsis is strongly associated with the 3` CTCF binding element (3`CBE), but not the 3’RR, a finding also confirmed by Micro-C measurements. Instead, the 3’RR forms a self-interacting domain independently engaging with Igh genes. Additionally, transcription at acceptor S regions creates a topological barrier that insulates S-S synapsis from loop extrusion complexes. Thus, Igh gene activation and S-S synapsis occur via distinct topological states created by the interaction between transcription and the loop extrusion machinery. Examination of the multi-way interactome at the Igh locus through Tri-C.