The Function of AID in Somatic Mutation and Class Switch Recombination

The immune system has evolved specific mechanisms to combat a potentially limitless number of foreign pathogens using a limited arsenal of Ig genes. To diversify the coding potential of the Ig genes, B cells undergo several processes of regulated genetic alterations. Early in their development, B cells in the bone marrow undergo V(D)J recombination to juxtapose variable region V, D, and J segments in different combinations, creating a large repertoire of antibodies (1). Later in B cell development, usually after antigen-dependent activation of B cells, the genetic alteration processes of somatic mutation (SM), class switch recombina-tion (CSR), and gene conversion further diversify the antigen-recognition repertoire as well as the effector function of encoded antibodies. In SM, which is the dominant means of secondary alteration of variable region gene sequences in humans and mice, mutations are introduced in the Ig variable region genes at a tremendous rate, which allows for evolution of high affinity antibodies (2). In some vertebrates, such as chickens and pigs, diversification of assembled Ig variable regions occurs by a gene conversion mechanism rather than SM (3). In CSR, to diversify the effector function of specific antibodies, recombination occurs within the downstream portion of the IgH locus to join variable region genes with different constant (C H) region genes (4). SM introduces mutations, small deletions, and insertions at a high rate in a ‫ف‬ 2 kb region downstream of the Ig promoter , altering the specificities of the encoded antibodies (2). SM usually occurs within the specific microenviron-ment of germinal centers, which is thought to be critical for this process. Within germinal centers, antibodies with high affinity for antigen are then selected, while low-affinity antibodies are weeded out in a process termed affinity maturation. The SM mutations commonly occur at conserved sequence motifs (hotspots). The mechanism of SM has been proposed to involve generation of DNA breaks followed by a repair process that involves an error-prone polymerase (5). In gene conversion, the assembled variable region sequences are altered via homologous recombina-tion using other unrearranged variable region genes or pseudogenes as templates. DNA breaks that occur during SM were first detected by overexpressing the enzyme terminal deoxynucleotidyl transferase (TdT), which catalyzes nontemplated addition of nucleotides to free DNA ends, in a constitutively hyper-mutating B cell line (6). This study revealed that nucleotides were specifically inserted at SM hotspots, suggesting that these hotspots were sites of DNA breaks. Subsequently , …