Presumptive Evidence for ADAR1 A-to-I Deamination at WA-sites as the Mutagenic Genomic Driver in Hepatocellular and Related ADAR1-Hi Cancers

In hepatocellular cancer (HCC) there is an over expression of the RNA editing enzyme ADAR1. Further, the prominent genomic somatic mutation signature in HCC is almost exclusively focused on mutations at A:T base pairs where A-to-G mutations far exceed T-to-C mutations (when read on the non-transcribed strand). A clear mechanism for this extreme transcriptional strand biased mutation signature, putatively associated with over expression of ADAR1 deaminase, is yet to be explicitly demonstrated. The standard description of this strand bias has been nominally called “Transcription Coupled Damage” (TCD) to distinguish it from more conventional “Transcription Coupled Repair” (TCR). We show that the TCD description does not satisfy all features of the molecular evidence. The conventional view is that ADAR1 is thought to target adenosines at WA-sites for editing to inosine (I) in double stranded RNA stem-loop structures in transcripts. Here we show that the totality of the molecular and cellular data on these mutation signatures provides strong presumptive evidence for a clear role for ADAR1-mediated A-to-I deamination at WA-sites as the mutagenic driver in hepatocellular and possibly other related ADAR1-Hi cancers displaying biased mutation features at A:T base pairs.