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AID in adaptive immunity and beyond

Final Report Summary - AID AND BEYOND (AID in adaptive immunity and beyond)

Since its discovery in 1999, activation-induced deaminase (AID) has attracted the attention of immunologists and geneticists. AID is a cytidine deaminase that edits the immunoglobulin genes of activated B cells. During somatic hypermutation, it introduces the point mutations required to generate the diversity that fuels the process of affinity maturation, eventually improving the affinity of the antibodies. During class switch recombination (CSR), it triggers recombination events leading to the replacement of the constant region of the immunoglobulin heavy chain (class switch recombination), thus producing antibodies with different effector functions. In addition to its essential role in adaptive immunity, AID has an oncogenic potential due to the generation of double-strand breaks in the genome that can result in translocations. More recently, the expression of AID in non-lymphoid cells raised a number of new hypotheses concerning its role in the global demethylation of the genome and in meiotic recombination.

We have performed complementary approaches to elucidate the role of AID in the germ-line and in CSR, combining clinical results, mice models, genetically engineered molecules and functional assays. Specifically, we have shown that the expression of AID in the germ-line has no impact on the frequency of meiotic recombination, which is a significant contribution for the community working on AID and genome stability. In addition, we have found in females that the AID protein is expressed mainly in the somatic granulosa cells surrounding the oocyte, an observation that raises a number of new hypotheses concerning the role of AID in the protection of the genome of the gametes. On a different front, we have tested AID orthologs from a shark and a sturgeon using functional assays, thus showing that the specific ability of the C-terminus of AID to drive CSR evolved at least 50 million years before the appearance of CSR in Tetrapods (amphibians, reptiles, birds and mammals) and that such function was conserved in independent evolutionary branches for 450 million years. This paradoxical finding suggested that a conserved role of the C-terminus of AID was coopted in the context of CSR and using chimeric molecules we have shown that such role is not only conserved in AID orthologs but shared with the other enzyme that introduces physiologic double-strand breaks in lymphocytes. Future experiments will detail the exact nature of this function, but the available evidence points to the active recruitment of a DNA repair pathway.
1. Trancoso I, Cortesao CS, Zhao Y, Hammarstr?m QP, Barreto VM (in preparation). The RAG1/RAG2 complex and Activation-Induced Cytidine Deaminase converged for the recruitment of the Non-Homologous End Joining repair pathway.
2. Carat?o N, Reis PH, Freitas RF, Cortes?o CS, Jacob CMA, Pastorino AC, Carneiro-Sampaio M, Barreto VM (ready for submission) A Novel Activation-Induced Cytidine Deaminase (AID) Mutation in patients with hyper-IgM type 2 syndrome.
3. Cortes?o SCS, Freitas RF, Barreto VM (submitted) Activation-Induced Cytidine Deaminase does not impact on murine meiotic recombination.
4. Barreto VM and Magor B. (2011) Activation-induced cytidine deaminase structure and functions: A species comparative view. " Developmental & Comparative Immunology. 35 (9): 991-1007.
5. Pavri R, Gazumyan A, Jankovic M, Di Virgilio M, Klein I, Ansarah-Sobrinho C, Resch W, Yamane A, Reina San-Martin B, Barreto VM, Nieland TJ, Root DE, Casellas R, Nussenzweig MC. (2010) Activation-induced cytidine deaminase targets DNA at sites of RNA polymerase II stalling by interaction with Spt5. Cell 143 (1): 122-33