Rad52 mediates class-switch DNA recombination to IgD

While the biology of IgD begins to be better understood, the mechanism of expression of this phylogenetically old and highly conserved Ig remains unknown. In B cells, IgD is expressed together with IgM as transmembrane receptor for antigen through alternative splicing of long primary V H DJ H -Cμ-s-m-Cδ-s-m RNAs, which also underpin secreted (s)IgD. IgD is also expressed through class switch DNA recombination (CSR), as initiated by AID-mediated double-strand DNA breaks (DSBs) in Sμ and σδ, and resolution of such DSBs by a still unknown mechanism. This synapses Sμ with σδ region DSB resected ends leading to insertion of extensive S-S junction microhomologies, unlike Ku70/Ku86-dependent NHEJ which resolves DSB blunt ends in CSR to IgG, IgA and IgE with little or no microhomologies. Our previous demonstration of a novel role of Rad52 in a Ku70/Ku86-independent “short-range” microhomology-mediated synapsis of intra-Sμ region DSBs led us to hypothesize that this homologous recombination DNA annealing factor is also involved in short-range microhomology-mediated alternative endjoining (A-EJ) recombination of Sμ with σδ. We found that induction of IgD CSR by selected stimuli downregulated Zfp318 (the suppressor of Cμ-s-m transcription termination), promoted Rad52 phosphorylation and Rad52 recruitment to Sμ and σδ, leading to Sμ-σδ recombination with extensive microhomologies, V H DJ H -Cδs transcription and sustained IgD secretion. Rad52 ablation in mouse Rad52 −/− B cells aborted IgD CSR in vitro and in vivo and dampened the specific IgD antibody response to OVA. Further, Rad52 knockdown in human B cells virtually abrogated IgD CSR. Finally, Rad52 phosphorylation was associated with high levels of IgD CSR and anti-nuclear IgD autoantibodies in lupus-prone mice and lupus patients. Thus, Rad52 effects CSR to IgD through microhomology-mediated A-EJ and in concert with Zfp318 modulation. This is a previously unrecognized, critical and dedicated role of Rad52 in mammalian DNA repair that provides a mechanistic underpinning to CSR A-EJ.