Human antibody heavy chain variable region exon interacts with 3' regulatory region to form a stable somatic hypermutation center

The V(D)J exon location is ‘‘privileged’’ for SHM targeting of diverse sequences within it. In analogy to the related HC class switch recombination center, we proposed that cohesin-mediated loop extrusion juxtaposes widely separated enhancers, including 3' Igh regulatory region ("3'RR"), with the V(D)J exon in GC B cells to establish a SHM-center "privileged" for AID access.

To test this hypothesis, we employed a human RAMOS GC B lymphoma cell SHM model with inducible AID expression in which we targeted an unmutated mouse HC V(D)J exon in place of its human counterpart. Activation of AID expression in this cycling RAMOS line for 10 days led to robust SHM accumulation in the mouse V(D)J exon with a pattern similar to that in mouse GC B cells. In this model, the mouse V(D)J exon was indeed juxtaposed to the two human 3'RRs and other elements consistent with SHM-C formation.