Error-Prone Candidates Vie for Somatic Mutation

Antibodies play a major role in the resistance of higher organisms to disease. This is made possible because mice, humans, and other species have evolved three unusual molecular mechanisms that allow the generation of an enormously diverse repertoire of antibodies from a relatively small amount of genetic material. First, each antibody-forming B cell assembles different combinations of variable (V), diversity (D), and joining (J) minigenes to create an antibody with a unique antigen-binding site. Second, to generate the high-affinity antibodies that are required for survival, B cells target many point mutations to the V(D)J regions of the H and VJ region of the L chain immunoglo-bulin (Ig) genes. Those B cells that are making higher affinity antibodies are selected for further growth and differentiation , resulting in the affinity maturation of the antibody response. Third, B cells rearrange the H chain V region to various downstream C regions that encode the different isotypes. This makes it possible for each of the many anti-gen-binding sites to mediate the effector functions that are encoded in the different C region genes and to be distributed throughout the body (1). Even though the somatic hypermutation of antibody V regions was first described in 1970, the mechanisms responsible for its regulation, targeting, and biochemistry have been remarkably elusive. This is especially surprising because the sequences of thousands of mutated H and L chain V regions have been determined and the general characteristics of the mutational process are known. The rate of mutation of antibody V regions is estimated to be one million times higher than the rate of mutation in most other genes, with V regions accumulating 5–10 mutations during the secondary antibody response. Somatic mutation begins a few hundred bases downstream from the promoter of rearranged V regions and continues for ‫ف‬ 1.5 kb (2) but not further downstream to the intronic enhancer and the constant region. Mutations are largely single base changes, although deletions and insertions occur (3). Transitions occur more frequently than transversions, and hot spot motifs such as RGYW (A/G, G, C/T, A/T) and its complementary sequence on the other strand are preferentially targeted. Although mutations are targeted to both strands, there is some controversy about whether there is strand bias (4). Some of the cis-acting sequences responsible for the regulation and targeting of V region hypermutation have been identified through deletion analysis of Ig transgenes. In ec-topically integrated L chain …