In vitro and in vivo studies of murine cytomegalovirus mutated in M34 and M35 ORFs
Cytomegalovirus is an important human pathogen causing life-threatening and debilitating disorders in some immunocompromised individuals. This double-stranded DNA betaherpesvirus is one of the largest and most complex viruses which establishes latency in the host. Treatment available for symptomatic patients is limited and development of new antiviral strategies is highly desired. Understanding of the virulence and pathogenesis of HCMV requires functional analysis of at least 164 gene products. Due to the species-specificity of HCMV and its inability to replicate in animals, functional analysis of HCMV encoded gene products relies on studies of animal CMVs in their natural hosts. Murine CMV (MCMV) shares a high degree of sequence homology with HCMV and has a similar biology in causing acute and latent infection and disease in mice.
Analysis of gene function became more practical with the availability of MCMV cloned into a bacterial artificial chromosome (BAC) plasmid. Phenotypic characterisation of recombinant viruses disrupted in the M34 or M35 ORF, the homologues of HCMV UL34 and UL35 ORF respectively, is presented here. Infectious viruses reconstructed from the mutated BAC plasmids, the mM34 and mM35, had the expected genome rearrangements as indicted by restriction enzyme analysis, PCR and partial sequencing. In vitro, mM34 and mM35 viruses were attenuated in their replication when inoculated at a low and a high multiplicity of infection when compared to the parental virus. Similarly, these viruses were severely restricted in their replication in immunodeficient SCID mice and did not kill mice up to 28 days post-inoculation. Comparison of the predicted M34 and M35 gene products with related betaherpesviruses suggests that the M34 protein plays a role in transcriptional regulation of viral replication and the M35 protein is a component of the tegument.