DNA-deaminase AID plays a pivotal role in adaptive immunity, antibody diversification and epigenetic regulation. AID catalyzes cytidine deamination in immunoglobulin genes, facilitating somatic hypermutation (SHM), class-switch recombination (CSR) and gene conversion (GC). However, the dysregulation of AID activity can lead to oncogenic mutations and immune disorders such as hyper-IgM syndrome type 2 (HIGM2). At present the number of studies investigating the role of AID polymorphic variants in the promotion of pathology is low. The current review examines the structural and functional aspects of AID, focusing on the impact of amino acid substitutions-both natural polymorphisms and artificial mutations-on its catalytic activity, substrate binding and interactions with regulatory proteins. Additionally, a bioinformatic analysis of single-nucleotide polymorphisms of AID deposited in the dbSNP database was performed. SNPs leading to amino acid substitutions in the primary protein structure were analyzed. The bioinformatic analysis of SNPs in the AID gene predicts that among 208 SNPs causing amino acid substitutions in the primary protein structure, 62 substitutions may have significant negative impact on the functioning of AID. The integration of computational predictions with experimental data underscores the importance of AID regulation in maintaining immune homeostasis and highlights potential markers for immune-related pathologies. This comprehensive analysis provides insights into the molecular mechanisms of AID dysfunction and its implications for disease.

B cells and their secreted antibodies are fundamental for host-defense against pathogens. The generation of high-affinity class switched antibodies results from both somatic hypermutation (SHM) of the immunoglobulin (Ig) variable region genes of the B-cell receptor and class switch recombination (CSR) which alters the Ig heavy chain constant region. Both of these processes are initiated by the enzyme activation-induced cytidine deaminase (AID), encoded by AICDA. Deleterious variants in AICDA are causal of hyper-IgM syndrome type 2 (HIGM2), a B-cell intrinsic primary immunodeficiency characterised by recurrent infections and low serum IgG and IgA levels. Biallelic variants affecting exons 2, 3 or 4 of AICDA have been identified that impair both CSR and SHM in patients with autosomal recessive HIGM2. Interestingly, B cells from patients with autosomal dominant HIGM2, caused by heterozygous variants (V186X, R190X) located in AICDA exon 5 encoding the nuclear export signal (NES) domain, show abolished CSR but variable SHM. We herein report the immunological and functional phenotype of two related patients presenting with common variable immunodeficiency who were found to have a novel heterozygous variant in AICDA (L189X). This variant led to a truncated AID protein lacking the last 10 amino acids of the NES at the C-terminal domain. Interestingly, patients' B cells carrying the L189X variant exhibited not only greatly impaired CSR but also SHM in vivo, as well as CSR and production of IgG and IgA in vitro. Our findings demonstrate that the NES domain of AID can be essential for SHM, as well as for CSR, thereby refining the correlation between AICDA genotype and SHM phenotype as well as broadening our understanding of the pathophysiology of HIGM disorders.

We report the case of a woman in her early 20s with a history of recurrent infection, atopic dermatitis, filariasis and bilateral purulent ear discharge since childhood with tonsillar enlargement on examination. She was started on supportive care and evaluated for primary immunodeficiency disease. Blood investigations revealed increased IgM levels with reduced IgG, IgA and IgE levels. Radiological imaging of the chest revealed bilateral bronchiectasis. Otoscopic examination showed features suggestive of chronic suppurative otitis media. Next-generation sequencing identified homozygous single base pair deletion in exon 2 of the activation-induced cytidine deaminase gene. Thus, a diagnosis of hyper-IgM syndrome type 2 was confirmed. The patient was started on monthly intravenous immunoglobulin replacement therapy and is currently symptomatically better, and she remains under regular follow-up.

Hyper-IgM syndrome type 2 (HIGM2) is a B cell intrinsic primary immunodeficiency caused by mutations in AICDA encoding activation-induced cytidine deaminase (AID) which impair immunoglobulin class switch recombination (CSR) and somatic hypermutation (SHM). Whereas autosomal-recessive AID-deficiency (AR-AID) affects both CSR and SHM, the autosomal-dominant form (AD-AID) due to C-terminal heterozygous variants completely abolishes CSR but only partially affects SHM. AR-AID patients display enhanced germinal center (GC) reactions and autoimmune manifestations, which are not present in AD-AID, suggesting that SHM but not CSR regulates GC reactions and peripheral B cell tolerance. Herein, we describe two siblings with HIGM2 due to a novel homozygous AICDA mutation (c.428-1G > T) which disrupts the splice acceptor site of exon 4 and results in the sole expression of a truncated AID variant that lacks 10 highly conserved amino acids encoded by exon 4 (AID-ΔE4a). AID-ΔE4a patients suffered from defective CSR and enhanced GC reactions and were therefore indistinguishable from other AR-AID patients. However, the AID-ΔE4a variant only partially affected SHM as observed in AD-AID patients. In addition, AID-ΔE4a but not AD-AID patients revealed impaired targeting of mutational hotspot motives and distorted mutational patterns. Hence, qualitative defects in AID function and altered SHM rather than global decreased SHM activity may account for the disease phenotype in these patients.

Activation-induced deaminase (AID) initiates antibody diversification in germinal center B cells by deaminating cytosines, leading to somatic hypermutation and class-switch recombination. Loss-of-function mutations in AID lead to hyper-IgM syndrome type 2 (HIGM2), a rare human primary antibody deficiency. AID-mediated deamination has been proposed as leading to active demethylation of 5-methycytosines in the DNA, although evidence both supports and casts doubt on such a role. In this study, using whole-genome bisulfite sequencing of HIGM2 B cells, we investigated direct AID involvement in active DNA demethylation. HIGM2 naïve and memory B cells both display widespread DNA methylation alterations, of which ∼25% are attributable to active DNA demethylation. For genes that undergo active demethylation that is impaired in HIGM2 individuals, our analysis indicates that AID is not directly involved. We demonstrate that the widespread alterations in the DNA methylation and expression profiles of HIGM2 naïve B cells result from premature overstimulation of the B-cell receptor prior to the germinal center reaction. Our data support a role for AID in B cell central tolerance in preventing the expansion of autoreactive cell clones, affecting the correct establishment of DNA methylation patterns.