Regulation of Activation induced deaminase (AID) by lysine modifications

Activation induced deaminase (AID) mediates class switch recombination and somatic hypermutation of immunoglobulin (Ig) genes in germinal centre B cells. In order to regulate its specific activity and as a means to keep off-target mutations low, several mechanisms have evolved, including binding to specific cofactors, phosphorylation and destabilization of nuclear AID protein. Although ubiquitination at lysine residues of AID is recognized as an essential step in initiating degradation of nuclear AID, any functional relevance of lysine modifications has remained elusive. In our preliminary experiments, we addressed functional implications of lysine modifications of the human AID protein by generating a panel of lysine to arginine mutants of AID and assessment of their catalytic class switch activity. We found that only mutation of Lys22 to Arg (AID mutant K3) resulted in a significant reduction of class switching to IgG1 in transfected primary mouse B cells. Importantly, this decrease in activity was neither recapitulated in the hypermutation of Ig and non-Ig sites in a K3 transfected DT40 B cell line nor in bacterial deamination assays, pointing to involvement of post-translational modification of Lys22 for AID activity specifically for class switching. By analyzing AID mutants purified from transfected cells, we could show that Lys22 is a target of ubiquitin modification. Our preliminary data strongly suggest that lysine modification represents a novel level of AID regulation and that Lys22 is important for effective AID mediated class switch recombination. In this project, we aim at elucidating post-translational modification of Lys22 and how it influences AID activity. The results gained by this project will not only be important for a better understanding of AID regulation but will also be crucial for comprehending AID- dependent genome-damage and lymphomagenesis.

Activation induced deaminase (AID) is an important enzyme of the AID/APOBEC family which mediates antibody diversity required for adaptive immunity. AID achieves this by mutating the genomic loci which harbour the antibody genes. To avoid cancerogenic off-target mutations, AID activity is tightly controlled by various mechanisms. In this project, which aimed at investigating posttranslational modifications of AID, we gained important insight into protein stability of splice variants and on/off-target activity of various AID/APOBEC members in B cell leukaemia. Our data contribute to a better understanding of the genesis of mutations in context of cancer, which is particularly relevant for clonal cancer evolution and treatment resistance.