Aberrant AID expression and human cancer development.

Cancer develops via a multistep process that occurs through the accumulation of somatic mutations of tumor-related genes that govern cell proliferation, regeneration, and apoptosis. The question how normal cells acquire the genetic changes that lead to malignant transformation is, however, unknown at present. Activation-induced cytidine deaminase (AID) produces immune-diversity by inducing somatic hypermutations and class-switch recombinations in human immunoglobulin genes. Unfortunately, this function of AID as a genome mutator could aim at the generation of somatic mutations in various host genes of non-lymphoid tissues and contribute to tumorgenesis. Notably, aberrant AID expression can be triggered by several pathogenic factors, including Helicobacter pylori infection and proinflammatory cytokine stimulation, in human epithelial cells, whereas AID expression is absent in those cells under physiologic conditions. Thus, aberrant AID activity in epithelial tissues may provide the critical link between inflammation, somatic mutations, and cancer development.