Instability in vitro of a PCV2 infectious clone containing an insertion between ORF1 and ORF2.

Porcine circovirus 2 (PCV2) is a small circular single-stranded DNA virus that causes postweaning multisystemic wasting syndrome in pigs. We obtained an infectious clone, derived from PCV2 strain 1147 (size 1,767 bp), with a 60 bp insertion in the cap-rep intergenic region at the 3' end of the rep. In vitro propagation and sequencing of virus showed mixed sequence for the cap-rep intergenic region. Cloning of this region of virus from the passages 2 and 20 showed deletions of various size of the 60 bp insertion. Viruses from the passage 20 showed deletions of the 60 bp insertion and had extra (+1 to +5) or less (-1 or -2) bp in the intergenic region as compared to 1,147. These findings suggest that insertion of a 60-bp DNA sequence at the 3' end of the rep is unstable in vitro. This finding can have implications for the genetic engineering of PCV2.