Activation-induced cytidine deaminase (AID) likely initiates immunoglobulin gene-conversion (GC) by deaminating cytidines within the V-region of chicken B-cells. However, the intervening DNA lesion required to initiate GC remains elusive. GC could be initiated by a single strand break or a double strand break (DSB). To distinguish between these possibilities, we examined GC in the chicken DT40 B cell line deficient in non-homologous end joining (NHEJ). It is known that the NHEJ and homologous recombination DNA repair pathways compete for DSBs. In light of this, if a DSB is the major intermediate, deficiency in NHEJ should result in increased levels of GC. Here we show that DNA–PKcs−/−/− and Ku70−/− DT40 cells had 5- to 10-fold higher levels of GC relative to wildtype DT40 as measured by surface IgM reversion and sequencing of the V-region. These data suggest that a DSB is the major DNA lesion that initiates GC.

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Antibody diversification processes play a major role in protecting humans from pathogens. Somatic hypermutation and gene conversion increase the affinity of pathogen-specific antibodies by changing the sequence within antibody variable genes, while the class switch recombination (CSR) process changes the antibody's effector function by replacing the constant region of the antibody gene with a different constant region. Activation-induced cytidine deaminase (AID) initiates each of these three processes by deaminating cytidines within antibody genes, while a host of other DNA transacting factors are involved in either creating new mutations or repairing DNA lesions introduced during these processes. This review will discuss the main features of antibody diversification and their role in lymphomagenesis, highlight outstanding issues and questions that remain in the field, and discuss our contributions to this field.