Divide et Impera: Identification of Small-Molecule Inhibitors of HCMV Replication Interfering with Dimerization of DNA Polymerase Processivity Factor UL44

Human cytomegalovirus (HCMV) is a leading cause of severe diseases in immunocompromised individuals, including AIDS and transplanted patients, and in congenitally infected newborns. Despite the availability of several antiviral drugs, their utility is limited by poor bioavailability, toxicity, and resistant strains emergence. Therefore, it is crucial to identify new targets of therapeutic intervention. The dimerization of HCMV DNA polymerase processivity factor UL44 plays an essential role in the viral life cycle being required for ori Lyt-dependent DNA replication. We validated the existence of UL44 homodimers both in vitro and in living cells by a variety of approaches, including GST pulldown, thermal shift, FRET and BRET assays. Dimerization occurred with an affinity comparable to that of the UL54/UL44 interaction, and was impaired by amino acid substitutions at the dimerization interface. Subsequently, we performed an in-silico screening to select 18 small molecules (SMs) potentially interfering with UL44 homodimerization. Antiviral assays using recombinant HCMV TB4-UL83-YFP in the presence of the 18 selected SMs led to the identification of four active SMs. The most active one also inhibited AD169 in plaque reduction assays, and impaired replication of an AD169-GFP reporter virus and its ganciclovir-resistant counterpart to a similar extent. As assessed by Western blotting experiments, treatment of infected cells specifically reduced viral gene expression starting from 48 h post infection, consistent with activity on viral DNA synthesis. Therefore, SMs inhibitors of UL44 dimerization could represent a new class of HCMV inhibitors, alternative to those targeting the DNA polymerase catalytic subunit or the viral terminase complex. IMPORTANCE HCMV is a ubiquitous infectious agent causing life-lasting infections in humans. HCMV primary infections and reactivation in non-immunocompetent individuals often result in life-threatening conditions. Antiviral therapy mainly targets the DNA polymerase catalytic subunit UL54 and is often limited by toxicity and selection of drug-resistant viral strains, making the identification of new targets of therapeutic intervention crucial for a successful management of HCMV infections. The significance of our work is in identifying the dimerization of the DNA polymerase processivity factor UL44 as an alternative antiviral target. We could show that full length UL44 dimerizes in a cellular context with high affinity and that such interaction could be targeted by small molecules, thus inhibiting the replication of several HCMV strains, including a drug-resistant mutant. Thus, our work could pave the way to the development of a new class of anti-HCMV compounds that act by targeting UL44 dimerization.