Activation-induced cytidine deaminase

Activation-induced cytidine deaminase (AID) is an enzyme that plays a crucial role in the adaptive immune response. AID is primarily involved in somatic hypermutation (SHM) and class switch recombination (CSR), processes that are essential for the diversification of the antibody repertoire. This enzyme functions by deaminating cytidine to uridine in DNA, which ultimately leads to mutations or recombination in immunoglobulin genes. AID's activity is tightly regulated within B cells to prevent unintended mutations that could lead to oncogenesis.

Function

AID is exclusively expressed in activated B cells and is a pivotal player in the immune system's ability to adapt to new threats. Its main functions include:

• Somatic Hypermutation (SHM): AID introduces point mutations in the variable regions of immunoglobulin genes, allowing for the production of antibodies with increased affinity for their antigens.
• Class Switch Recombination (CSR): AID initiates recombination events in the constant region of the immunoglobulin heavy chain locus. This recombination changes the antibody isotype without altering its specificity, enabling different effector functions.

Mechanism

The mechanism of AID involves the deamination of cytidine to uridine in DNA. This modification can be processed in several ways:

• It can be replicated over, leading to a transition mutation.
• It can be recognized and excised by the base excision repair (BER) pathway, leading to mutations or double-strand breaks (DSBs).
• DSBs are essential for CSR and are resolved through mechanisms involving non-homologous end joining (NHEJ) or alternative end joining (A-EJ).

Regulation

The expression and activity of AID are tightly regulated at multiple levels, including transcriptional, post-transcriptional, and post-translational modifications, to minimize off-target effects and potential genomic instability.

Clinical Significance

Dysregulation of AID has been implicated in various autoimmune diseases and cancers, particularly those involving B cells like lymphomas. Understanding AID's role in these pathologies is crucial for developing targeted therapies.

Research

Ongoing research aims to elucidate the detailed mechanisms of AID action, its regulation, and its role in disease. This includes studying AID's structure-function relationships, identifying cofactors that modulate its activity, and developing inhibitors.