DNA Double-Strand Breaks : Prior to but not Sufficient in Targeting Hypermutation

The activation-induced cytidine deaminase (AID) is required for somatic hypermutation (SHM) and class-switch recombination (CSR) of immunoglobulin (Ig) genes, both of which are associated with DNA double-strand breaks (DSBs). As AID is capable of deaminating deoxy-cytidine (dC) to deoxy-uracil (dU), it might induce nicks (single strand DNA breaks) and also DNA DSBs via a U-DNA glycosylase-mediated base excision repair pathway (‘DNA-substrate model’). Alternatively, AID functions like its closest homologue Apobec1 as a catalytic subunit of a RNA editing holoenzyme (‘RNA-substrate model’). Although rearranged Vλ genes are preferred targets of SHM we found that germinal center (GC) B cells of AID-proficient and -deficient Vλ1-expressing GC B cells display a similar frequency, distribution, and sequence preference of DSBs in rearranged and also in germline Vλ1 genes. The possible roles of DSBs in relation to AID function and SHM are discussed.