Activation Induced Cytidine Deaminase

Induced Cytidine Deaminase Activation (AID ) is an enzyme that is expressed in B lymphocytes of vertebrates. It catalyzes the hydrolysis of cytidine residues are highly specific to uridine residues which are bound to single-stranded DNA. AID has the task to mutate the antibody sequences of the B- cells in the course of hypermutation and thereby alter the binding characteristics of the antibody produced therefrom. This is the AID an important component in the maturation of high-affinity antibodies and a key enzyme of the innate immune response.

Mutations of AICDA gene in humans are responsible for AID deficiency and this for the rare genetic disorder Hyper- IgM syndrome type 2 ( HIGM2 ), an immune deficiency, responsible.

Role of AID in the acquired immune system

The main function of the AID is the change of the immunoglobulin genes ( Ig genes). There are three options: hypermutation, class -switch recombination and gene conversion. All three processes are centrally regulated and controlled by the AID. All three processes have different effects on the antibody sequences:

  • Hypermutation: The hypermutation is the central process of antibody maturation. The antibody sequences are mutated by AID. The hypermutation is found only in B lymphocytes but not in T lymphocytes.
  • Class switch recombination: The class-switch recombination is the change of the effector in the constant regions of the antibodies. Thereby recombine certain sequence regions between constant regions of the antibody sequences and result in the change in effector function.
  • Gene conversion: The gene conversion occurs, for example in chickens to produce immunoglobulin diversity. This is induced by the AID homologous recombination with pseudo - V- genes.

Structure

The AID is a 28 kDa protein and closely related to APOBEC1 (English apolipoprotein B messenger RNA editing enzyme catalytic polypeptides ). The structure of the AID is highly conserved. It contains a cytidine deaminase motif, a C-terminal APOBEC -like motif, a C-terminal NES (English Nuclear Export Sequence) and a non-functional N- termiale NLS ( engl. Nuclear Import Sequence). The three-dimensional structure of the AID is not yet clarified.

Specificity of the AID

About the specificity of AID, there is still only conflicting views. It is certain that the AID only recognizes single-stranded DNA. For this reason, even highly transcribed DNA sequences are highly mutated, since these are increasingly single-stranded. Due to the non-functional NLS and NES of AID is propagated from the nucleus, the site of transcription, transported. The reason why they are still present in the nucleus, is the small size of 28 kDa. Thus, the AID diffuse through the nuclear pore complexes. It has been shown that the AID propagating cytidine mutated in the so-called hotspots. These hotspots are short sequence motifs with the sequence: WRCY, wherein W represents A or T, and Y and R for pyrimidines purines. Furthermore, the chromatin structure and interaction with other factors for the specificity of AID responsible. A final determination is still pending.

Application of the AID

AID is already being used for the induction of antibodies maturations in CHO cells. In this case, the cells are transfected with the AID and the cells that bind better about their antibodies to the antigen selected. The antigen is labeled with a fluorescent dye to the cells and sorted using FACS (English fluorescence activated cell sorting).

The AID is a central component of the iGEM 2012 project of the University of Potsdam

Also the iGEM Team 2012 University of Potsdam has been devoted to this topic. The team's goal is to develop a system, make with the CHO cells, triggered by viral impact, high-affinity antibodies. The system should be selected so that cells having the highest affinity antibody, which is an optimal adaptation to the selection pressure. To the AID transiently transfected into the CHO cells, and thus caused the hypermutation. The cells that produce an antibody with a higher affinity of binding to the virus, and thus better maintain a survival signal. Characterized it is (theoretically) possible to form high-affinity antibodies. The advantage of the system is that immunization of a mammal such as a mouse, so that the production of antibodies becomes unnecessary. Whether the cells produce really higher affinity antibodies by the selection pressure of the viruses has yet to be proven.

Human Genome Organisation B cell Somatic hypermutation Immunoglobulin class switching Signal peptide Nuclear localization sequence Chinese hamster ovary cell International Genetically Engineered Machine Digital Object Identifier
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