The human cytomegalovirus terminase complex as an antiviral target: a close-up view.

Human cytomegalovirus (HCMV) is responsible for life-threatening infections in immunocompromised individuals and can cause serious congenital malformations. Available antivirals target the viral polymerase but are subject to cross-resistance and toxicity. New antivirals targeting other replication steps and inducing fewer adverse effects are therefore needed. During HCMV replication, DNA maturation and packaging are performed by the terminase complex, which cleaves DNA to package the genome into the capsid. Identified in herpesviruses and bacteriophages, and with no counterpart in mammalian cells, these terminase proteins are ideal targets for highly specific antivirals. A new terminase inhibitor, letermovir, recently proved effective against HCMV in phase III clinical trials, but the mechanism of action is unclear. Letermovir has no significant activity against other herpesvirus or non-human CMV. This review focuses on the highly conserved mechanism of HCMV DNA-packaging and the potential of the terminase complex to serve as an antiviral target. We describe the intrinsic mechanism of DNA-packaging, highlighting the structure-function relationship of HCMV terminase complex components.

Identification of a short sequence in the HCMV terminase pUL56 essential for interaction with pUL89 subunit.

The human cytomegalovirus (HCMV) terminase complex consists of several components acting together to cleave viral DNA into unit length genomes and translocate them into capsids, a critical process in the production of infectious virions subsequent to DNA replication. Previous studies suggest that the carboxyl-terminal portion of the pUL56 subunit interacts with the pUL89 subunit. However, the specific interacting residues of pUL56 remain unknown. We identified a conserved sequence in the C-terminal moiety of pUL56 (671WMVVKYMGFF680). Overrepresentation of conserved aromatic amino acids through 20 herpesviruses homologues of pUL56 suggests an involvement of this short peptide into the interaction between the larger pUL56 terminase subunit and the smaller pUL89 subunit. Use of Alpha technology highlighted an interaction between pUL56 and pUL89 driven through the peptide 671WMVVKYMGFF680. A deletion of these residues blocks viral replication. We hypothesize that it is the consequence of the disruption of the pUL56-pUL89 interaction. These results show that this motif is essential for HCMV replication and could be a target for development of new small antiviral drugs or peptidomimetics.

Contrasting effect of new HCMV pUL54 mutations on antiviral drug susceptibility: Benefits and limits of 3D analysis.

Human cytomegalovirus (HCMV) resistance to antiviral drugs is a major drawback of repeated or long-duration treatment in immunocompromised patients. Resistance testing is usually performed by genotypic assays. For accurate interpretation of these assays, the role of new mutations in HCMV resistance has to be assessed. Two previously unknown UL54 single point mutations (D515Y and V787A) were characterized for phenotypic drug-resistance by marker transfer analysis using bacterial artificial chromosome (BAC) mutagenesis. Increases in 50% inhibitory concentrations of ganciclovir and foscarnet were found for both mutated recombinant strains showing that mutations D515Y and V787A induce resistance to both antivirals. Importantly, none of those impacted the viral growth kinetics. For a better understanding of their molecular resistance mechanisms, a 3D homology model was used to localize the mutated amino-acids in functional domains of UL54 and predict their impact on UL54 function and resistance. However, 3D homology model analysis has limits and phenotypic characterization using BAC-HCMV is still essential to measure the role of unknown mutations.