Induction of A:T mutations is dependent on cellular environment but independent of target gene location

The somatic hypermutation of the immunoglobulin (Ig) genes is initiated by the activation‐induced cytidine deaminase (AID), which catalyzes the deamination of cytosine to uracil and generates a U:G DNA lesion. Consistent with its substrate specificity, ectopic expression of AID in fibroblasts induces predominantly C:G mutations in a GFP reporter gene. In contrast to fibroblasts, AID induces a high proportion of A:T mutations in Ig genes in the germinal center (GC) B cells. To investigate whether the ability to generate A:T mutations is dependent on cellular environment or target gene, we introduced a GFP gene into a GC B‐like cell line Ramos and compared the AID‐induced mutations in the endogenous Ig V H and the transgenic GFP genes. Remarkably, a high proportion of A:T mutations was induced in both genes. Moreover, using a lacZ ‐transgenic system to detect endogenous genome mutations, we found that GC B cells exhibited a much higher proportion of A:T mutations as compared with naïve B, non‐GC B and cells of other tissues. These results demonstrate that the ability to generate A:T mutations is dependent on the GC B cell environment but independent of the target gene location.