Mutation bias implicates RNA editing in a wide range of mammalian RNA viruses

The rapid evolution of RNA viruses has been long considered to result from a combination of high copying error frequencies during RNA replication, short generation times and the consequent extensive fixation of neutral or adaptive changes over short periods. While both the identities and sites of mutations are typically modelled as being random, recent investigations of sequence diversity of SARS coronavirus 2 (SARS-CoV-2) have identified a preponderance of C->U transitions, potentially driven by an APOBEC-like RNA editing process. The current study investigated whether this phenomenon could be observed in the more genetically diverse datasets of other RNA viruses. Using a 5% divergence filter to infer directionality, 18 from 32 datasets of aligned coding region sequences from a diverse range of mammalian RNA viruses (including Picornaviridae, Flaviviridae, Matonaviridae, Caliciviridae and Coronaviridae) showed a >2-fold base composition normalised excess of C->U transitions compared to U->C (range 2.1x-7.5x). C->U transitions showed a weak favoured 5 U upstream context consistent with previous analyses of APOBEC-mediated RNA targeting. Amongst several genomic compositional and structural parameters, the presence of genome scale RNA secondary structure (GORS) was associated with C->U/U->C transition asymmetries (p < 0.001), potentially reflecting the documented structure dependence of APOBEC-mediated RNA editing. Using the association index metric, C->U changes were specifically over-represented at phylogenetically uninformative sites, consistent with extensive homoplasy documented in SARS-CoV-2. Excess C->U substitutions accounted for 15-20% of standing sequence variability of HCV and other RNA viruses; RNA editing may therefore represent a potent driver of RNA virus sequence diversification and longer term evolution. Author SummaryThe rapid evolution of RNA viruses is thought to arise from high mutation frequencies during replication and the rapid accumulation of genetic changes over time in response to its changing environments. This study describes an additional potent factor that contributes to the evolution of RNA infecting mammals, the deliberate mutation of the viral genome by host antiviral pathways active within the cell when it becomes infected. This so called "genome editing" by one or more APOBEC enzymes leads to characteristic C->U mutations that damage the viruss ability to replicate. While this pathway is well characterised as an antiviral defence against HIV and other retroviruses, this study provides evidence for its activity against a wide range of human and veterinary viruses, including HCV and foot and mouth disease virus. APOBEC-driven mutations accounted for 15-20% of standing sequence variability of RNA virus groups, representing a potent driver of RNA virus sequence diversification.