Strange Stable Replicators Generated From Mumps Virus cDNA Clones

In reverse genetic experiments we have isolated recombinant mumps viruses (rMuV) based on a recent clinical isolate that carry large numbers of mutations clustered in small parts of their genome and which are not caused by biased hyper-mutation. In two separate experiments we obtained such rMuV: one virus had 19 mutations in the V/P region of the genome; the other, which also contained an extra transcription unit encoding green fluorescent protein (EGFP), had 32 mutations in the N gene. These specific constellations of mutations have not been observed in naturally occurring MuV isolates. The vast majority of the mutations (48/51) are synonymous. On passage in Vero cells and human B-LCL cells, a B lymphocyte-like cell Line, these mutations appear stable as no reversal occurs to the original consensus sequences, though mutations in other genes occur and change in frequency during passage. Defective Interfering RNAs accumulate in passage in Vero cells but not in B-LCL cells. Interestingly, in all passaged samples the level of variation in the EGFP gene is the same as in the viral genes, though it is unlikely that this gene is under any functionality constraint. The stability in repeated high multiplicity passage indicates that the constellation of mutations is placing the virus on a fitness peak from which it cannot escape. What mechanism gave rise to these mutant viruses and their stability remain open questions of interest to a wider field than mumps reverse genetics alone.