Antiviral inhibition targeting the HCMV kinase pUL97 requires pUL27-dependent degradation of Tip60 acetyltransferase and cell-cycle arrest

Progress in the development of new therapies for herpesvirus infections

Resurgent interest in antiviral drugs for the treatment of herpesvirus has led to the development of new compounds that are progressing through clinical trials. This is important because there are few therapeutic options for resistant infections and some viruses such as human cytomegalovirus remain underserved. New compounds include conventional DNA polymerase inhibitors such as valomaciclovir and cyclopropavir, as well as CMX001 that has a broad spectrum of antiviral activity that includes all the herpesviruses. It also includes compounds with new molecular targets such as maribavir (MBV), FV-100, AIC361, and AIC246. Recent advances with each of these compounds will be reviewed including their virus specificity, mechanism of action, and stage of development. The potential of these new compounds to improve clinical outcome will also be discussed.

Maribavir and human cytomegalovirus-what happened in the clinical trials and why might the drug have failed?

We summarize the history of the clinical drug development of maribavir for its use as prophylaxis in stem-cell transplant recipients. We highlight key aspects in the design and interpretation of the results of the dose escalation phase II maribavir study that may have contributed to the negative findings on the phase III trials. We discuss key aspects of study design that should be considered in the study of new interventions needed to advance the prevention and treatment of CMV in transplant recipients.

Cyclopropavir inhibits the normal function of the human cytomegalovirus UL97 kinase

Cyclopropavir (CPV) is active against human cytomegalovirus (CMV), as well as both variants of human herpesvirus 6 and human herpesvirus 8. The mechanism of action of CPV against CMV is similar to that of ganciclovir (GCV) in that it is phosphorylated initially by the CMV UL97 kinase, resulting in inhibition of viral DNA synthesis. Resistance to CPV maps to the UL97 kinase but is associated primarily with H520Q mutations and thus retains good antiviral activity against most GCV-resistant isolates. An examination of CMV-infected cultures treated with CPV revealed unusual cell morphology typically associated with the absence of UL97 kinase activity. A surrogate assay for UL97 kinase activity confirmed that CPV inhibited the activity of this enzyme and that its action was similar to the inhibition seen with maribavir (MBV) in this assay. Combination studies using real-time PCR indicated that, like MBV, CPV also antagonized the efficacy of GCV and were consistent with the observed inhibition of the UL97 kinase. Deep sequencing of CPV-resistant laboratory isolates identified a frameshift mutation in UL27, presumably to compensate for a loss of UL97 enzymatic activity. We conclude that the mechanism of action of CPV against CMV is complex and involves both the inhibition of DNA synthesis and the inhibition of the normal activity of the UL97 kinase.

Resistance to maribavir is associated with the exclusion of pUL27 from nucleoli during human cytomegalovirus infection

Select mutations in the human cytomegalovirus (HCMV) gene UL27 confer low-grade resistance to the HCMV UL97 kinase inhibitor maribavir (MBV). It has been reported that the 608-amino acid UL27 gene product (pUL27) normally localizes to cell nuclei and nucleoli, whereas its truncation at codon 415, as found in a MBV-resistant mutant, results in cytoplasmic localization. We now show that in the context of full-length pUL27, diverse single amino acid substitutions associated with MBV resistance result in loss of its nucleolar localization when visualized after transient transfection, whereas substitutions representing normal interstrain polymorphism had no such effect. The same differences in localization were observed during a complete infection cycle with recombinant HCMV strains over-expressing full-length fluorescent pUL27 variants. Nested UL27 C-terminal truncation expression plasmids showed that amino acids 596-599 were required for the nucleolar localization of pUL27. These results indicate that the loss of a nucleolar function of pUL27 may contribute to MBV resistance, and that the nucleolar localization of pUL27 during HCMV infection depends not only on a carboxy-terminal domain but also on a property of pUL27 that is affected by MBV-resistant mutations, such as an interaction with component(s) of the nucleolus.

HATs on for drug resistance

Mutations in the human cytomegalovirus UL27 gene confer resistance to an inhibitor of the viral protein kinase UL97. Reitsma and colleagues (2011) demonstrate that UL27 destabilizes the Tip60 histone acetyltransferase, setting forth intriguing new mechanisms for antiviral drug resistance and for viral regulation of the cell cycle.