Submitted to: Virology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/16/2004
Publication Date: 3/1/2005
Citation: Cheung, A.K. 2005. Detection of rampant nucleotide reversion at the origin of DNA replication of porcine circovirus type 1. Virology. 333(1):22-30. Interpretive Summary: Porcine circovirus type 2 (PCV2) is a newly emerged viral pathogen of swine that is associated with a wasting syndrome in weaned pigs. While clinical signs of disease and postmortem lesions associated with PCV2 are known, there is little information on the temporal pathogenesis and epidemiology of the virus. Standardized diagnostic tests have not developed and vaccines are not available. In previous work, we examined the genetic elements synthesized by PCV type 1 (PCV1) and PCV2 in tissue culture cells. We have identified several new PCV2 genetic elements that are different from the non-pathogenic PCV1 and have determined the essential and non-essential genetic elements required for PCV1 and PCV2 replication. We also proposed a novel 'melting-pot' model to account for its replication. In this work, we examined the mechanisms involved in PCV1 DNA replication and identified a signature DNA sequence that is essential for viral protein, DNA and progeny virus biosynthesis. Thus, this work provides insight into the life-cycle of PCV and a general frame work to generate attenuated viruses that might prove useful as vaccines for PCV or vaccine vectors for other viral diseases.
Technical Abstract: Mutational analysis was conducted to investigate the involvement of the "loop-sequence" (which is flanked by a pair of 11-nucleotide inverted repeats) at the origin of DNA replication of porcine circovirus type 1 with respect to viral protein synthesis, DNA self-replication and progeny virus production. The results demonstrated that an octanucleotide (A1G2T3A4T5T6A7C8) embedded in the loop is essential for viral DNA replication. Similar to previous work with porcine circovirus type 2, this octanucleotide can be further condensed to an essential core element represented by AxTAxTAC. Mutations introduced into the positions indicated by x (positions 2 and 5) were retained in the progeny viruses, while mutations engineered into the positions specified by the indicated nucleotides either did not yield any progeny virus (positions 6, 7 and 8) or they reverted back to wild-type nucleotide to generate infectious progeny viruses (positions 1, 3 and 4). In comparison to porcine circovirus type 2, porcine circovirus type 1 mutant genomes with perturbed octanucleotide exhibited higher propensity to revert to wild-type under similar experimental conditions.