Persistence of DNA in cell cultures may jeopardize the analysis of human herpesvirus 6 dynamics by means of real-time PCR

Antiviral resistance in herpes simplex virus and varicella-zoster virus infections: diagnosis and management

PURPOSE OF REVIEW

Aciclovir (ACV) is the first-line drug for the management of herpes simplex virus (HSV) and varicella-zoster virus (VZV) infections. Long-term administration of ACV for the treatment of severe infections in immunocompromised patients can lead to the development of drug resistance. Furthermore, the emergence of isolates resistant to ACV is increasingly recognized in immunocompetent individuals with herpetic keratitis. This review describes the mechanisms involved in drug resistance for HSV and VZV, the laboratory diagnosis and management of patients with infections refractory to ACV therapy.

RECENT FINDINGS

Genotypic testing is more frequently performed for the diagnosis of infections caused by drug-resistant HSV or VZV isolates. Molecular biology-based systems for the generation of recombinant viruses have been developed to link unknown mutations with their drug phenotypes. Fast and sensitive methods based on next-generation sequencing will improve the detection of heterogeneous viral populations of drug-resistant viruses and their temporal changes during antiviral therapy, which could allow better patient management. Novel promising compounds acting on targets that differ from the viral DNA polymerase are under clinical development.

SUMMARY

Antiviral drug resistance monitoring for HSV and VZV is required for a rational use of antiviral therapy in high-risk populations.

Resistance of herpes simplex viruses to nucleoside analogues: mechanisms, prevalence, and management

Herpes simplex viruses (HSV) type 1 and type 2 are responsible for recurrent orolabial and genital infections. The standard therapy for the management of HSV infections includes acyclovir (ACV) and penciclovir (PCV) with their respective prodrugs valacyclovir and famciclovir. These compounds are phosphorylated by the viral thymidine kinase (TK) and then by cellular kinases. The triphosphate forms selectively inhibit the viral DNA polymerase (DNA pol) activity. Drug-resistant HSV isolates are frequently recovered from immunocompromised patients but rarely found in immunocompetent subjects. The gold standard phenotypic method for evaluating the susceptibility of HSV isolates to antiviral drugs is the plaque reduction assay. Plaque autoradiography allows the associated phenotype to be distinguished (TK-wild-type, TK-negative, TK-low-producer, or TK-altered viruses or mixtures of wild-type and mutant viruses). Genotypic characterization of drug-resistant isolates can reveal mutations located in the viral TK and/or in the DNA pol genes. Recombinant HSV mutants can be generated to analyze the contribution of each specific mutation with regard to the drug resistance phenotype. Most ACV-resistant mutants exhibit some reduction in their capacity to establish latency and to reactivate, as well as in their degree of neurovirulence in animal models of HSV infection. For instance, TK-negative HSV mutants establish latency with a lower efficiency than wild-type strains and reactivate poorly. DNA pol HSV mutants exhibit different degrees of attenuation of neurovirulence. The management of ACV- or PCV-resistant HSV infections includes the use of the pyrophosphate analogue foscarnet and the nucleotide analogue cidofovir. There is a need to develop new antiherpetic compounds with different mechanisms of action.

Testing the susceptibility of human herpesviruses to antivirals

Herpesviruses cause chronic lifelong infections in humans and may cause life-threatening diseases in immunosuppressed patients. Antiviral drugs targeted to viral DNA polymerase, such as acyclovir, penciclovir, ganciclovir, foscarnet and cidofovir, are currently available and have been proven to be efficient against clinical symptoms of herpesvirus infections. The resistance of herpesviruses to these drugs is associated with specific mutations of viral genes encoding either DNA polymerase or enzymes phosphorylating nucleoside analogs. Resistance is detected and characterized by means of specific susceptibility assays, which can be classified as phenotypic, genetic and functional. These tests are used both to investigate novel antiviral compounds and look for the emergence of resistant viruses in treated patients in case of clinical failure. Although susceptibility assays are often time consuming and present some limitations regarding the interpretation of their results, their use in the monitoring of antiherpetic treatments should be promoted and improved, in parallel to the development of novel efficient drugs.

Different Mutations in the HHV-6 DNA Polymerase Gene Accounting for Resistance to Foscarnet

Background

Human herpesvirus 6 (HHV-6), which is closely related to human cytomegalovirus, is sensitive to foscarnet (PFA). Up to now, the resistance of HHV-6 to PFA has not been investigated.

Objectives

The goal of the study was to isolate and characterize PFA-resistant HHV-6 mutants in order to determine the mechanisms of resistance to the drug.

Methods

PFA-resistant viruses, isolated in MT4 cell culture under increasing concentrations of PFA, were characterized phenotypically and genotypically. The mutations identified in the HHV-6 DNA polymerase gene were evaluated in a functional assay using recombinant mutated forms of the enzyme, and their effect on protein structure was analysed in a three-dimensional model derived from available structures of DNA polymerases.

Results

Two mutants were selected and were 8- and 15-fold more resistant to PFA than the wild-type strain. Four amino acid changes were detected in the HHV-6 DNA polymerase in association with PFA resistance: T435R, H507Y, C525S, located in the δC conserved domain, and F292S. Either alone or in combination, these substitutions significantly decreased the inhibitory effect of PFA at the level of the polymerase, as measured by the incorporation of radiolabeled nucleotides in a DNA elongation assay. In the three-dimensional model of HHV-6 DNA polymerase structure the four changes were not located within the putative catalytic site, but they might induce either a disturbance of local conformation or a restricted access of PFA to its target site.

Conclusion

This first characterization of HHV-6 resistance to PFA highlights the role of distinct DNA polymerase gene mutations.

Role of cell culture for virus detection in the age of technology

Viral disease diagnosis has traditionally relied on the isolation of viral pathogens in cell cultures. Although this approach is often slow and requires considerable technical expertise, it has been regarded for decades as the "gold standard" for the laboratory diagnosis of viral disease. With the development of nonculture methods for the rapid detection of viral antigens and/or nucleic acids, the usefulness of viral culture has been questioned. This review describes advances in cell culture-based viral diagnostic products and techniques, including the use of newer cell culture formats, cryopreserved cell cultures, centrifugation-enhanced inoculation, precytopathogenic effect detection, cocultivated cell cultures, and transgenic cell lines. All of these contribute to more efficient and less technically demanding viral detection in cell culture. Although most laboratories combine various culture and nonculture approaches to optimize viral disease diagnosis, virus isolation in cell culture remains a useful approach, especially when a viable isolate is needed, if viable and nonviable virus must be differentiated, when infection is not characteristic of any single virus (i.e., when testing for only one virus is not sufficient), and when available culture-based methods can provide a result in a more timely fashion than molecular methods.

Inflammation, ECG changes and pericardial effusion: whom to biopsy in suspected myocarditis?

The role of endomyocardial biopsies in patients with clinically suspected acute myocarditis, myocarditis in the past, and dilated cardiomyopathy is discussed controversially. In fact, it is still under discussion whether information obtained from endomyocardial biopsies is relevant for further clinical decisions. Therefore this Critical Perspective will deal with the question, which patient should undergo endomyocardial biopsy investigations for an etiopathogenic differentiation of the disease and for the possible choice of immunomodulatory treatment strategies.

Ether lipid ester derivatives of cidofovir inhibit polyomavirus BK replication in vitro

Polyomavirus BK is a significant pathogen in transplant recipients, but no effective antiviral therapy is available. We show that cidofovir can inhibit BK virus replication in vitro. Esterification of cidofovir with hexadecyloxypropyl, octadecyloxyethyl, and oleyloxyethyl groups results in up to a 3-log lowering of the 50% effective concentration and an increased selectivity index.