The importance of codon context for understanding the Ig-like somatic hypermutation strand-biased patterns in TP53 mutations in breast cancer

Evidence already exists that the activation-induced deaminase (AID)/APOBEC family constitutes a set of differentially expressed enzymes capable of deaminating cytosines (C to U) in single-stranded DNA (ssDNA) and that they are potentially powerful mutagens. The mutagenic processes involved are believed to be activated in many nonlymphoid tissue types—for example, initiating some cancers and/or leading to further somatic mutagenesis. To investigate the extent that codon context might be important in influencing the likely location of TP53 mutations in breast cancer, the codon-bias patterns resulting from the ssDNA target specificities of cytidine deaminases of the AID/APOBEC family were analyzed. The data indicate that codon context strongly influences the likely location of mutations at motifs for AID/APOBEC1/APOBEC3G, and at WA sites. An unexpected finding is a highly significant preference for transitions of cytosine to occur at the first nucleotide position and for transitions of guanosine to occur at the second nucleotide position in the mutated codon (read 3′ to 5′). Thus, the mechanisms involved appear to be sensitive to codon reading frames and to have an intrinsic ability to differentiate between the cytosines on the nontranscribed strand and those on the transcribed strand in the context of an open “transcription bubble.”