Reverse genetics for Borna disease virus

Two papers have described a reverse genetics system for Borna disease virus (BDV), a non-cytolytic neurotropic virus that has been connected with neuropsychiatric disorders in humans.

Investigation of the molecular basis for BDV persistence in the central nervous system has been hampered by the lack of a genetic system to study its genome and gene products. BDV is a non-segmented, enveloped virus with an 8.9-kb negative-strand RNA genome. Like all negative-strand RNA viruses, the minimal infectious unit of BDV is a ribonucleoprotein complex comprising the viral RNA genome tightly associated with viral proteins.

In the strategy described by Perez et al., a BDV minigenome was constructed that comprised a reporter gene flanked upstream by the 5′ untranslated region (UTR) of the BDV genome and a murine RNA polymerase I promoter, and downstream by the BDV 3′ UTR and the murine RNA polymerase I terminator sequence. In the strategy described by Schneider et al., the reporter gene was flanked upstream by the BDV 5′ UTR and the T7 promoter, and transcription termination downstream was controlled by the hepatitis delta virus ribozyme.

The roles of trans-acting BDV proteins in transcription of the BDV genome were investigated by co-transfecting cells with plasmid vectors encoding these proteins under control of the chicken β-actin promoter or a modified version. Both groups found that the stoichiometry of the ratio of BDV nucleocapsid (N) protein to phosphoprotein (P) was a key factor in the regulation of the BDV polymerase complex. In addition, both reports documented that the BDV X protein (also called p10) is a negative regulator of the viral polymerase.

Genomic manipulations of the negative-strand RNA viruses have lagged behind that of viruses with either DNA or positive-strand RNA genomes. However, progress has been made in the past decade and reverse genetics systems are now available for several negative-strand RNA viruses. The development of these analytical tools for BDV will allow the investigation of both the cis-acting signals and trans-acting factors that are involved in transcription and replication of the BDV genome.