Activation-induced cytidine deaminase (AID) is a molecule central to initiating class switch recombination, somatic hypermutation, and gene conversion of Ig genes. However, its mechanism to initiate these genetic alterations is still unclear. AID can convert cytosine to uracil on either mRNA or DNA and is involved in DNA cleavage. Although these events are expected to take place in the nucleus, overexpressed AID was found predominantly in the cytoplasm. Here, we demonstrated that AID is a nucleocytoplasmic shuttling protein with a bipartite nuclear localization signal and a nuclear export signal in its N and C termini, respectively. In addition to previously identified genetic, structural, and biochemical similarities of AID with apolipoprotein B mRNA editing catalytic polypeptide 1, an RNA editing enzyme of ApoB100 mRNA, the present finding provides another aspect to their resemblance, suggesting that both may have homologous reaction mechanisms. T he immune system has evolved specific mechanisms to defend against numerous pathogens using a limited arsenal of Ig genes. After the formation of the primary Ig repertoire by V(D)J recombination of Ig genes during the developmental process, further diversification is achieved in antigenexperienced mature IgM ϩ B cells by three types of genetic alterations, i.e., somatic hypermutation (SHM), gene conversion (GC), and class switch recombination (CSR). SHM and GC introduce a large number of non-templated and templated point mutations, respectively, in the Ig V region genes to raise high-affinity antibodies after selection with a limited amount of antigen. CSR takes place between two S regions that locate 5Ј adjacent to each Ig heavy chain constant (CH) region gene, resulting in replacement of the most upstream C gene with another downstream CH (C␥, C, or C␣) gene. B cells can thus generate isotypes other than IgM, such as IgG, IgE, and IgA, without changing antigen specificity (1).Activation-induced cytidine deaminase (AID) is expressed almost exclusively in activated B cells (2). Disruption of the AID gene in mouse and human causes the hyper-IgM phenotype by abolishing both SHM and CSR without any other signs of lymphocyte dysfunction (3, 4). Furthermore, knockout of AID in chicken B cell line DT40 also abolishes GC, which is spontaneously taking place in this cell line (5, 6). Inversely, ectopic expression of AID in non-B cells induces CSR and SHM (7-10). These results indicate that AID is a molecule central to initiating CSR, SHM, and GC, the three types of Ig gene alterations that occur in mature B lymphocytes.Although AID is required for DNA cleavage (11), the detailed mechanism by which AID induces these genetic events is still under extensive debate. AID has the highest sequence homology with the apolipoprotein B (apoB) mRNA editing catalytic polypeptide 1 (APOBEC1), which edits a specific cytidine on mRNA of apoB100, a cholesterol carrier, converting it to mRNA of apoB 48, a triglyceride carrier. AID, like APOBEC1, conserves the catalytic motif of cytosine deami...

Activation-induced cytidine deaminase (AID) is essential for the generation of antibody memory but also targets oncogenes, among other genes. We investigated the transcriptional regulation of Aicda (which encodes AID) in class switch-inducible CH12F3-2 cells and found that Aicda regulation involved derepression by several layers of positive regulatory elements in addition to the 5' promoter region. The 5' upstream region contained functional motifs for the response to signaling by cytokines, the ligand for the costimulatory molecule CD40 or stimuli that activated the transcription factor NF-kappaB. The first intron contained functional binding elements for the ubiquitous silencers c-Myb and E2f and for the B cell-specific activator Pax5 and E-box-binding proteins. Our results show that Aicda is regulated by the balance between B cell-specific and stimulation-responsive elements and ubiquitous silencers.

Background: Histone chaperone Spt6 is required for CSR, but the mechanism remains unknown. Results: Spt6 preserves H3K4me3 mark on the target chromatin, which is required for AID induced DNA cleavage of CSR and SHM. Conclusion: Target-specific DNA breaks and AID expression are controlled by two distinct modes of histone epigenetic regulation by Spt6. Significance: Chromatin plays a key role in controlling AID-induced genomic instability.

To initiate class switch recombination (CSR) activation-induced cytidine deaminase (AID) induces staggered nick cleavage in the S region, which lies 5 to each Ig constant region gene and is rich in palindromic sequences. Topoisomerase 1 (Top1) controls the supercoiling of DNA by nicking, rotating, and religating one strand of DNA. Curiously, Top1 reduction or AID overexpression causes the genomic instability. Here, we report that the inactivation of Top1 by its specific inhibitor camptothecin drastically blocked both the S region cleavage and CSR, indicating that Top1 is responsible for the S region cleavage in CSR. Surprisingly, AID expression suppressed Top1 mRNA translation and reduced its protein level. In addition, the decrease in the Top1 protein by RNA-mediated knockdown augmented the AID-dependent S region cleavage, as well as CSR. Furthermore, Top1 reduction altered DNA structure of the S region. Taken together, AID-induced Top1 reduction alters S region DNA structure probably to non-B form, on which Top1 can introduce nicks but cannot religate, resulting in S region cleavage.camptothecin ͉ non-B DNA ͉ translation suppression

Activation-induced cytidine deaminase (AID), produced by the Aicda gene, is essential for the immunoglobulin gene (Ig) alterations that form immune memory. Using a Cre-mediated genetic system, we unexpectedly found CD4+ T cells that had expressed Aicda (exAID cells) as well as B cells. ExAID cells increased with age, reaching up to 25% of the CD4+ and B220+ cell populations. ExAID B cells remained IgM+, suggesting that class-switched memory B cells do not accumulate in the spleen. In T cells, AID was expressed in a subset that produced IFN-γ and IL-10 but little IL-4 or IL-17, and showed no evidence of genetic mutation. Interestingly, the endogenous Aicda expression in T cells was enhanced in the absence of B cells, indicating that the process is independent from the germinal center reaction. These results suggest that in addition to its roles in B cells, AID may have previously unappreciated roles in T-cell function or tumorigenesis.