Abstract
Activation-induced cytidine deaminase (AID) is essential for antibody diversification, namely somatic hypermutation (SHM) and class switch recombination (CSR). The deficiency of apurinic/apyrimidinic endonuclease 1 (Ape1) in CH12F3-2A B cells reduces CSR to ∼20% of wild-type cells, whereas the effect of APE1 loss on SHM has not been examined. Here we show that, although APE1's endonuclease activity is important for CSR, it is dispensable for SHM as well as IgH/c-myc translocation. Importantly, APE1 deficiency did not show any defect in AID-induced S-region break formation, but blocked both the recruitment of repair protein Ku80 to the S region and the synapse formation between Sμ and Sα. Knockdown of end-processing factors such as meiotic recombination 11 homolog (MRE11) and carboxy-terminal binding protein (CtBP)-interacting protein (CtIP) further reduced the remaining CSR in Ape1-null CH12F3-2A cells. Together, our results show that APE1 is dispensable for SHM and AID-induced DNA breaks and may function as a DNA end-processing enzyme to facilitate the joining of broken ends during CSR.
Highlights
Activation-induced cytidine deaminase (AID) is essential for antibody diversification, namely somatic hypermutation (SHM) and class switch recombination (CSR)
Three mutant proteins include (i) APE1K6R/K7R that is defective in the acetylation-mediated gene regulation function [29]; (ii) APE1C64S mutant that lacks the redox activity [36]; and (iii) APE1Y170F mutant that is devoid of the endonuclease activity [37] (Fig. 1A)
Western blotting analysis showed that WT and all apyrimidinic endonuclease 1 (APE1) mutant proteins were almost expressed in transiently transfected apyrimidinic endonuclease 1 (Ape1)-null CH12F32A cells (Fig. 1B, Upper)
Summary
Activation-induced cytidine deaminase (AID) is essential for antibody diversification, namely somatic hypermutation (SHM) and class switch recombination (CSR). Upon encountering antigens in the periphery, mature B cells undergo two types of genetic alterations, somatic hypermutation (SHM) and class switch recombination (CSR), in the Ig gene during the G1 phase [1, 2] Mechanistically different, both events are initiated by the activation-induced cytidine deaminase (AID) [3,4,5], which introduces single-strand DNA break (SSB) in the target DNA, namely the V region for SHM and the S region for CSR [6,7,8,9]. The A-EJ pathway is used for the joining of DSB with long overhangs and is involved in aberrant chromosomal translocations, which depend on uracil DNA glycosylase (UNG) [18] Both SHM and CSR take place during the G1 phase and are transcription-dependent [19].
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