Immunoglobulin (Ig) genes specifically recruit activation-induced deaminase (AID) for 'on-target' DNA deamination, initiating either variable (V) region somatic hypermutation, or double-strand break intermediates of class switch recombination (CSR). Such breaks overwhelmingly undergo legitimate intra-Ig repair rather than rare illegitimate and potentially oncogenic junctions outside of Ig loci. We show that in human B cells, legitimate synapsis and repair efficiently join Ig genes whether physically linked on one chromosome or located apart on both alleles. This indicates mechanisms faithfully recognizing and/or pairing loci with homology in structure and accessibility, thus licensing interchromosomal trans-CSR junctions while usually preventing illegitimate interchromosomal recombination with AID off-target genes. Physical linkage of IgH genes in cis on the same allele just increases the likelihood of legitimate repair by another fourfold. The strongest force driving CSR might thus be recognition of legitimate target genes. Formation of IgH intra-allelic loops along this process would then constitute a consequence rather than a pre-requisite of this gene-pairing process.
DNA lesions inflicted by activation-induced deaminase (AID) instrumentally initiate the processes reshaping immunoglobulin genes in mature B-cells, from local somatic hypermutation (SHM) to junctions of distant breaks during class switch recombination (CSR). It remains incompletely understood how these divergent outcomes of AID attacks are differentially and temporally focused, with CSR strictly occurring in the Ig heavy chain (IgH) locus while SHM concentrates on rearranged V(D)J regions in the IgH and Ig light chain loci. In the IgH locus, disruption of either the 3’Regulatory Region (3’RR) super-enhancer or of switch (S) regions preceding constant genes, profoundly affects CSR. Reciprocally, we now examined if these elements are sufficient to induce CSR in a synthetic locus based on the Igκ locus backbone. Addition of a surrogate “core 3’RR” (c3’RR) and of a pair of transcribed and spliced Switch regions, together with a reporter system for “κ-CSR” yielded a switchable Igκ locus. While the c3’RR stimulated SHM at S regions, it also lowered the local SHM threshold necessary for switch recombination to occur. The 3’RR thus both helps recruit AID to initiate DNA lesions, but then also promotes their resolution through long-distance synapses and recombination following double-strand breaks.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.