An ectopic IgH 3' superenhancer endows AID-initiated lesions to participate into distant class switching junctions

Lesions inflicted to DNA by activation-induced deaminase (AID) instrumentally initiate the processes that reshape immunoglobulin (Ig) genes in mature B-cells, from local somatic hypermutation (SHM) to junction of distant chromosomal breaks during class switch recombination (CSR). It remains incompletely understood how these divergent outcomes of similar lesions are differentially and timely focused, with CSR strictly occurring in the Ig heavy chain (IgH) locus while SHM concentrates on rearranged V(D)J regions from the IgH and Ig light (IgL) chain loci. Disruption of either the IgH locus 3' regulatory region (3'RR) super-enhancer or of switch (S) regions targeted by CSR were shown to affect CSR strongly but always incompletely. To analyze the prerequisites of CSR with an inverse strategy, we implemented a 3'RR downstream of a switchable Ig? locus also carrying a pair of transcribed and spliced S regions, together with a reporter system for “?CSR”. Our study shows that the ectopic super-enhancer both stimulates local SHM and CSR but somehow lowers the threshold of SHM necessary for the occurrence of CSR. This suggests that the 3'RR helps recruit AID for initiating DNA lesions, but then also promotes the resolution of such lesions through double strand breaks and long-distance recombination.