Mechanism of somatic hypermutation: critical analysis of strand biased mutation signatures at A:T and G:C base pairs.

The DNA sequence data of the somatic hypermutation (SHM) field published since 1984 has been critically reviewed. The analysis has revealed three strand biased mutation signatures. The first concerns the mutations generated at G:C base pairs in mice genetically deficient in uracil-DNA glycosylase and MSH2-MSH6-mediated mismatch repair. Such mice display the AID deaminase footprint and here C mutations exceed G mutations at least 1.5-fold. This supports earlier and more recent studies claiming that dC-to-dU deaminations occur preferentially in the single stranded DNA regions of the displaced nontranscribed strand (NTS) during transcription. The second concerns the signature generated in immunised mice where G mutations exceed C mutations by at least 1.7-fold. This is a newly identified strand bias which has previously gone undetected. It is consistent with the polynucleotide polymerisation signature of RNA polymerase II copying the template DNA strand carrying AID-mediated lesions generated at C bases, viz. uracils and abasic sites. A reverse transcription step would then need to intervene to fix the mutation pattern in DNA. The third concerns the long recognised strand biased signature generated in normal aged or actively immunised mice whereby A mutations exceed T mutations by two- to three-fold. It is argued that this pattern is best understood as a combination of adenosine-to-inosine (A-to-I) RNA editing followed by a reverse transcription step fixing the A-to-G, as well as A-to-T and A-to-C, as strand biased mutation signatures in DNA. The reasons why the AID-linked RNA polymerase II mutation signature had previously gone undetected are discussed with regard to limitations of standard PCR-based SHM assay techniques. It is concluded that the most economical SHM mechanism involves both DNA and RNA deaminations coupled to a reverse transcription process, most likely involving DNA polymerase eta acting in its reverse transcriptase mode. Experimental approaches to differentiate this RNA-based model from the standard DNA deamination model are discussed.