Somatic hypermutation mechanisms during lymphomagenesis and transformation.

B cells undergoing physiologically programmed or aberrant genomic alterations provide an opportune system to study the causes and consequences of genome mutagenesis. Activated B cells in germinal centers express activation-induced cytidine deaminase (AID) to accomplish physiological somatic hypermutation (SHM) of their antibody-encoding genes. In attempting to diversify their immunoglobulin (Ig) heavy- and light-chain genes, several B-cell clones successfully optimize their antigen-binding affinities. However, SHM can sometimes occur at non-Ig loci, causing genetic alternations that lay the foundation for lymphomagenesis, particularly diffuse large B-cell lymphoma. Thus, SHM acts as a double-edged sword, bestowing superb humoral immunity at the potential risk of initiating disease. We refer to off-target, non-Ig AID mutations - that are often but not always associated with disease - as aberrant SHM (aSHM). A key challenge in understanding SHM and aSHM is determining how AID targets and mutates specific DNA sequences in the Ig loci to generate antibody diversity and non-Ig genes to initiate lymphomagenesis. Herein, we discuss some current advances regarding the regulation of AID's DNA mutagenesis activity in B cells.