Highs and Lows in the Calicivirus Reverse Genetics

In Virology, the term reverse genetics refers to a set of methodologies in which changes are introduced into the viral genome and their effects on the generation of infectious viral progeny and their phenotypic features are assessed. Reverse genetics emerged thanks to advances in recombinant DNA technology which made the isolation, cloning, and modification of genes through mutagenesis possible. Most virus reverse genetics studies depend on our capacity to rescue an infectious wild-type virus progeny from cell cultures transfected with an “infectious clone”. This infectious clone generally consists of a circular DNA plasmid containing a functional copy of the full-length viral genome, under the control of an appropriate RNA polymerase promoter. For most DNA viruses, reverse genetics systems are very straightforward, since DNA virus genomes are relatively easy to handle and modify and are also (with few notable exceptions) infectious per se. This is not true for RNA viruses, whose genomes need to be reverse transcribed into cDNA before any modification -such as restriction enzyme digestion, gene knock-out or knock-in, site directed mutagenesis, ligation, etc.- can be done. Establishing reverse genetics systems for members of the Caliciviridae has proven exceptionally challenging, due to the low number of members of this family that propagate in cell cultures. Despite the successful rescue of Feline calicivirus (FCV) from cDNA containing plasmid more than 20 years ago, reverse genetics methods are not routine procedures than can be easily extrapolated to other caliciviruses. Reports of calicivirus reverse genetics systems have been few and far between, in this review, we discuss the main pitfalls, failures, and delays behind several successful calicivirus infectious clones.