Rad52 mediates class-switch DNA recombination to IgD

Abstract

Abstract 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 mainly expressed together with IgM through alternative splicing of primary VHDJH-Cμ-s-m-Cδ-s-m RNAs, but also independently through IgD class switch DNA recombination (CSR) via double-strand DNA breaks (DSBs) and synapse of Sμ with σδ. How such DSBs, however, are resolved is still unknown. 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 downregulates Zfp318 and promotes Rad52 phosphorylation and recruitment to Sμ and σδ, leading to alternative end-joining (A-EJ)-mediated Sμ-σδ recombination of resected DSB ends with extensive microhomologies, VHDJH-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. Rad52 knockdown in human B cells also abrogated IgD CSR. Finally, Rad52 phosphorylation was associated with high levels of IgD CSR and anti-nuclear IgD autoantibodies in lupus-prone mice and patients with lupus. Our findings thus show that Rad52 mediates IgD CSR through microhomology-mediated A-EJ in concert with Zfp318 downregulation. This is a previously unrecognized and critical role of Rad52 in mammalian DNA repair that provides a mechanistic underpinning to CSR A-EJ. Supported by NIH grants AI 079705, AI 105813, AAI 167416 and LRA grant 641363 to PC.