Interstrain variation in the human cytomegalovirus DNA polymerase sequence and its effect on genotypic diagnosis of antiviral drug resistance. Adult AIDS Clinical Trials Group CMV Laboratories

Drug resistance mutations and associated phenotypes detected in clinical trials of maribavir for treatment of cytomegalovirus infection

Background

In separate phase 2 trials, 120 patients received maribavir for cytomegalovirus (CMV) infection failing conventional therapy (trial 202) and 119 received maribavir for asymptomatic infection (trial 203). Overall, 172 cleared their CMV infection (CMV DNA <200 copies/mL) within 6 weeks.

Methods

Baseline and posttreatment plasma samples were tested for mutations in viral genes UL97, UL54, and/or UL27. Selected viral mutants were phenotyped for drug susceptibility.

Results

Baseline samples revealed UL54 mutations newly phenotyped as conferring resistance to standard DNA polymerase inhibitor(s), including K493N, P497S, K513T, L565V, V823A, A987V, and E989D. Of 29 patients (including 25 from trial 202) who cleared but later experienced recurrent CMV infection while on maribavir, 23 had available UL97 genotyping data; 17 had known resistance mutations (T409M or H411Y) and 5 additional had UL97 C480F alone. The newly phenotyped mutation C480F conferred high-grade maribavir resistance and low-grade ganciclovir resistance. Among 25 who did not respond to >14 days of therapy, 9 showed T409M or H411Y and 4 others showed C480F alone.

Conclusions

After maribavir therapy (400–1200 mg twice daily), UL97 mutations T409M, H411Y, or C480F emerge to confer maribavir resistance in patients with recurrent CMV infection while on therapy or no response to therapy.

Clinical Trials Registration

NCT01611974 and EudraCT 2010-024247-32.

Biological relevance of Cytomegalovirus genetic variability in congenitally and postnatally infected children

BACKGROUND

Human cytomegalovirus (HCMV) is the leading cause of congenital infections resulting in severe morbidity and mortality among infected children. Although the virus is highly polymorphic, particularly in genes contributing to immune evasion, the mechanisms underlying its genetic variability and pathogenicity are only partially understood.

OBJECTIVES

We aimed to characterize different HCMV clinical strains isolated from 21 congenitally- or postnatally-infected children for in vitro growth properties and genetic polymorphisms.

STUDY DESIGN

The growth of various HCMV isolates was analyzed in different cell culture models. Genetic polymorphism was assessed by genetic and phylogenetic analysis of viral genes involved in virulence (UL144, US28, and UL18), latency (UL133-138), or drug resistance (UL54 and UL97).

RESULTS

Here, we report a high degree of genetic and phenotypic diversity in distinct HCMV clinical isolates, as shown by their in vitro growth properties. In particular, HCMV isolates displayed the highest degree of genetic variability in the UL144 gene, where we were able to define four distinct genotypes within the cohort based on UL144 heterogeneity. Lastly, among all isolates we were able to identify 36 mutations in UL54 and 2 in UL97.

CONCLUSIONS

Our findings indicate that surprisingly high levels of genetic HCMV variability correlate with a high degree of phenotypic polymorphism, which in turn might differentially influence the growth, fitness, and drug susceptibility of HCMV.

HCMV UL97 phosphotransferase gene mutations may be associated with antiviral resistance in immunocompromised patients in Belém, PA, Northern Brazil

INTRODUCTION

Human cytomegalovirus is one of the causes of opportunist infections in immunocompromised patients, and is triggered by factors such as state of viral latency, weakened immune responses, and development of antiviral resistance to ganciclovir, the only drug offered by the public health system in Brazil to treat the infection. The goal of this study was to identify mutations that may be associated with antiviral resistance in immunocompromised patients.

METHODS

Molecular analysis was performed in 82 blood samples and subjected to genomic DNA extraction by a silica-based method. Three sequences of the HCMV UL97 gene, which encodes a phosphotransferase protein required for activation of ganciclovir, were amplified by polymerase chain reaction. Pyrosequencing methods were applied to one external 2096-bp segment DNA and two internal sequences between nucleotides 1087 to 1828 to detect mutations in this gene.

RESULTS

Approximately 10% of sequences contained mutations between nucleotides 377 and 594, in conserved regions of the UL97 gene, leading to amino acid changes. Eleven coding mutations were identified, including changes leading to amino acid substitutions, E596K and S604F, which were observed in 100% of samples and are described for the first time in Brazil. In addition, one mutation (A594V) that is associated with ganciclovir resistance was detected in a kidney transplant patient.

CONCLUSIONS

Further studies to detect mutations associated with HCMV resistance to antiviral drugs are required to demonstrate the need to increase the variety and availability of drugs used to treat viral infections in the public health care system in Brazil.

Experience with leflunomide as treatment and as secondary prophylaxis for cytomegalovirus infection in lung transplant recipients: A case series and review of the literature

BACKGROUND

Data concerning the use of leflunomide-a drug approved for rheumatoid arthritis with in vitro anticytomegalovirus (CMV) activity-in lung transplant (LT) recipients are scarce.

AIMS

To report the use of leflunomide in LT recipients diagnosed with CMV infection/disease.

MATERIAL AND METHODS

We performed a single-center retrospective study including LT recipients who received leflunomide for CMV infection or as secondary prophylaxis after viremia clearance. We also conducted a full systematic PubMed search until June 30, 2017.

RESULTS

We identified 5 LT recipients in our center plus 7 patients reported in the literature. All patients had previously received ganciclovir (GCV) and foscarnet (FOS), with drug-induced adverse effects described in 6 recipients (50%). Antiviral resistance mutations were observed in 8 patients (66.7%). Leflunomide was prescribed for CMV infection in 9 of 12 patients (75%) and as secondary prophylaxis in 3 patients (25%). Initial decrease of CMV viremia after starting leflunomide was observed in 7 of 9 recipients (77.7%), although this response was only transient in 2 patients. Long-term suppression of CMV viremia was reported in 7 of 12 patients (58.3%). In 3 recipients (25%), leflunomide was discontinued due to adverse effects.

DISCUSSION

Our study has some limitations, such as the small number of patients included, its retrospective nature, and absence of leflunomide drug monitoring in serum. Notwithstanding, in our experience, leflunomide proved to be particularly effective as an anti-CMV secondary prophylaxis treatment and for clearing low-grade viremia. Moreover, leflunomide combined with a short course of GCV or intravitreal FOS also proved to be very effective in some patients.

CONCLUSION

Leflunomide, alone or in combination, could be an effective treatment in selected LT recipients with GCV-resistant CMV infection and as secondary prophylaxis. Further studies are necessary.

Herpesvirus DNA polymerases: Structures, functions and inhibitors

Human herpesviruses are large double-stranded DNA viruses belonging to the Herpesviridae family. These viruses have the ability to establish lifelong latency into the host and to periodically reactivate. Primary infections and reactivations of herpesviruses cause a large spectrum of diseases and may lead to severe complications in immunocompromised patients. The viral DNA polymerase is a key enzyme in the lytic phase of the infection by herpesviruses. This review focuses on the structures and functions of viral DNA polymerases of herpes simplex virus (HSV) and human cytomegalovirus (HCMV). DNA polymerases of HSV (UL30) and HCMV (UL54) belong to B family DNA polymerases with which they share seven regions of homology numbered I to VII as well as a δ-region C which is homologous to DNA polymerases δ. These DNA polymerases are multi-functional enzymes exhibiting polymerase, 3'-5' exonuclease proofreading and ribonuclease H activities. Furthermore, UL30 and UL54 DNA polymerases form a complex with UL42 and UL44 processivity factors, respectively. The mechanisms involved in their polymerisation activity have been elucidated based on structural analyses of the DNA polymerase of bacteriophage RB69 crystallized under different conformations, i.e. the enzyme alone or in complex with DNA and with both DNA and incoming nucleotide. All antiviral agents currently used for the prevention or treatment of HSV and HCMV infections target the viral DNA polymerases. However, long-term administration of these antivirals may lead to the emergence of drug-resistant isolates harboring mutations in genes encoding viral enzymes that phosphorylate drugs (i.e., nucleoside analogues) and/or DNA polymerases.

How I treat resistant cytomegalovirus infection in hematopoietic cell transplantation recipients

Cytomegalovirus (CMV) infection is a significant complication in hematopoietic cell transplantation (HCT) recipients. Four antiviral drugs are used for preventing or treating CMV: ganciclovir, valganciclovir, foscarnet, and cidofovir. With prolonged and repeated use of these drugs, CMV can become resistant to standard therapy, resulting in increased morbidity and mortality, especially in HCT recipients. Antiviral drug resistance should be suspected when CMV viremia (DNAemia or antigenemia) fails to improve or continue to increase after 2 weeks of appropriately dosed and delivered antiviral therapy. CMV resistance is diagnosed by detecting specific genetic mutations. UL97 mutations confer resistance to ganciclovir and valganciclovir, and a UL54 mutation confers multidrug resistance. Risk factors for resistance include prolonged or previous anti-CMV drug exposure or inadequate dosing, absorption, or bioavailability. Host risk factors include type of HCT and degree of immunosuppression. Depending on the genotyping results, multiple strategies can be adopted to treat resistant CMV infections, albeit no randomized clinical trials exist so far, after reducing immunosuppression (if possible): ganciclovir dose escalation, ganciclovir and foscarnet combination, and adjunct therapy such as CMV-specific cytotoxic T-lymphocyte infusions. Novel therapies such as maribavir, brincidofovir, and letermovir should be further studied for treatment of resistant CMV.

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.

A Herpesvirus Specific Motif of Epstein-Barr Virus DNA Polymerase Is Required for the Efficient Lytic Genome Synthesis

Epstein-Barr virus (EBV) is associated with several malignancies, including Burkitt lymphoma and nasopharyngeal carcinoma. To overcome such disorders, understanding the molecular mechanisms of the EBV replication is important. The EBV DNA polymerase (Pol) is one of the essential factors for viral lytic DNA replication. Although it is well known that its C-terminal half, possessing DNA polymerase and 3’-5’ exonuclease activity, is highly conserved among Family B Pols, the NH₂-terminal half has yet to be characterized in detail. In this study, we show that a stretch of hydrophobic amino acids within the pre-NH₂-terminal domain of EBV Pol plays important role. In addition, we could identify the most essential residue for replication in the motif. These findings will shed light on molecular mechanisms of viral DNA synthesis and will help to develop new herpesviruses treatments.

Recent developments in human cytomegalovirus diagnosis

Since the early 1990s, great efforts have been made in the field of human cytomegalovirus (HCMV) diagnosis. Besides HCMV diagnosis in immunosuppressed patients (solid organ transplant recipients, hematopoietic stem cell transplant patients and AIDS patients), diagnosis in connection with congenital HCMV infection is of great interest. This review focuses on the development and clinical utility of serological assays, as well as on virological tests (molecular and nonmolecular assays). Interpretation of these tests is strongly dependent on the patient group (solid organ transplant recipients and hematopoietic stem cell transplant patients) and whether the tests are used for screening, risk stratification or diagnosis. Furthermore, a better understanding of HCMV infection has led to new approaches in HCMV diagnosis and monitoring. Thus, assays for viral resistance testing and assays for monitoring the HCMV-specific cellular immune response are increasingly important for the guidance of antiviral therapy.

Phenotypic evaluation of previously uncharacterized cytomegalovirus DNA polymerase sequence variants detected in a valganciclovir treatment trial

BACKGROUND

In a large randomized trial comparing oral valganciclovir and intravenous ganciclovir for treatment of cytomegalovirus disease in solid organ transplantation, confirmed genotypic drug resistance was uncommon (<5%), but definitive interpretation was limited by the detection of 110 uncharacterized UL54 viral DNA polymerase sequence variants.

METHODS

Based on treatment history and genetic locus of the sequence changes, 39 of the sequence variants were prioritized for recombinant phenotyping by construction of cloned viral mutants and drug susceptibility testing in cell culture.

RESULTS

Four amino acid substitutions were newly confirmed to alter ganciclovir susceptibility: A505V and I726T conferred a borderline decrease in ganciclovir and cidofovir susceptibility, while Q578L and G841S conferred slightly decreased ganciclovir and foscarnet susceptibility. A nonviable phenotype was found for 8 mutations distributed among amino terminal, exonuclease and catalytic domains. Retesting of stored study specimens could not confirm the original detection of >20 sequence variants, including the nonviable mutations and several resistance mutations.

CONCLUSIONS

Newly phenotyped UL54 sequence variants did not significantly change the reported incidence of drug resistance in the clinical trial. Unrecognized sequence variants in diagnostic genotyping reports should be confirmed by additional testing in order to improve clinical decision making.

Novel method based on "en passant" mutagenesis coupled with a gaussia luciferase reporter assay for studying the combined effects of human cytomegalovirus mutations

Human cytomegalovirus (HCMV) resistance to antivirals is a major problem in immunocompromised patients. Drug resistance is characterized by phenotypic testing or genotypic analysis of the phosphotransferase (UL97) and DNA polymerase (UL54) genes. However, genotypic assays require further characterization of unknown mutations in the drug resistance phenotype. Here, we describe a novel method for generating single or multiple mutations anywhere in the HCMV genome and for studying their effects on drug susceptibility. This method is based on cloning of the reference AD169 strain in a bacterial artificial chromosome and the use of "en passant" mutagenesis in bacteria to introduce mutations in recombinant HCMV without scar sequences. We also used this methodology to introduce the Gaussia luciferase reporter gene into the genome of the recombinant virus. To validate our system, the well-characterized single mutants with UL97 A594V and UL54 E756K mutations as well as the undescribed A594V/E756K double mutant were generated and their drug susceptibility profiles were determined by measuring the luciferase activity in cell culture supernatants. Drug susceptibility phenotypes for the A594V (8.2-fold increase in ganciclovir 50% effective concentration [EC50]) and E756K (1.9-, 3.9-, and 3.0-fold increases in ganciclovir, foscarnet, and cidofovir EC50s, respectively) mutants were similar to those previously reported, while the double mutant exhibited 10.8-, 4.1-, and 2.0-fold increases in ganciclovir, foscarnet, and cidofovir EC50s, respectively. The combination of the Gaussia luciferase reporter-based assay with the markerless "en passant" mutagenesis methodology constitutes an efficient system for studying phenotypes with single or multiple HCMV mutations.

The time course of development and impact from viral resistance against ganciclovir in cytomegalovirus infection

(Val)ganciclovir is used to treat cytomegalovirus (CMV) infection following solid organ (SOT) or hematopoietic stem cell (HSCT) transplantation. Treatment failures occur, but the contribution from 39 known ganciclovir-related mutations (GRMs) in the CMV-UL97 gene remains controversial. We propose a categorization of these GRMs potentially useful when interpreting sequence analyses in clinical settings. The UL97 gene was sequenced from first/recurrent CMV infections among consecutive SOT or HSCT recipients during 2004-2009. GRMs were categorized as: Signature GRM (sGRM) if in vitro ganciclovir IC(50) ratio for mutated versus wild-type virus >2 (n = 24); polymorphic GRM (pGRM) if ratio <2 (n = 15). (Val)ganciclovir treatment failure was defined as persistent viremia for 30 days or switch to foscarnet within this period. Of 99 (49 HSCT and 50 SOT) recipients with one CMV infection episode, 15 (13 HSCT and 2 SOT) experienced a total of 19 recurrent infection episodes. The prevalence of sGRM was 0% at start of first episode, whereas at start of recurrent episodes, prevalence was 37%. Only one sGRM was present at a time in individual patients. Patients with CMV containing an sGRM (vs. wild type)-but not with a pGRM-were at excess risk of treatment failure (odds ratio = 70.6 [95% CI:8.2-609.6]; p < 0.001). sGRMs emerged only following longer termed use of antiherpetic drugs and usually in recurrent CMV infection episodes. Risk of ganciclovir treatment failure was raised if an sGRM was detected.

The pre-NH(2)-terminal domain of the herpes simplex virus 1 DNA polymerase catalytic subunit is required for efficient viral replication

The catalytic subunit of herpes simplex virus 1 DNA polymerase (HSV-1 Pol) has been extensively studied; however, its full complement of functional domains has yet to be characterized. A crystal structure has revealed a previously uncharacterized pre-NH(2)-terminal domain (residues 1 to 140) within HSV-1 Pol. Due to the conservation of the pre-NH(2)-terminal domain within the herpesvirus Pol family and its location in the crystal structure, we hypothesized that this domain provides an important function during viral replication in the infected cell distinct from 5'-3' polymerase activity. We identified three pre-NH(2)-terminal Pol mutants that exhibited 5'-3' polymerase activity indistinguishable from that of wild-type Pol in vitro: deletion mutants PolΔN43 and PolΔN52 that lack the extreme N-terminal 42 and 51 residues, respectively, and mutant PolA(6), in which a conserved motif at residues 44 to 49 was replaced with alanines. We constructed the corresponding pol mutant viruses and found that the polΔN43 mutant displayed replication kinetics similar to those of wild-type virus, while polΔN52 and polA(6) mutant virus infection resulted in an 8-fold defect in viral yield compared to that achieved with wild type and their respective rescued derivative viruses. Additionally, both polΔN52 and polA(6) viruses exhibited defects in viral DNA synthesis that correlated with the observed reduction in viral yield. These results strongly indicate that the conserved motif within the pre-NH(2)-terminal domain is important for viral DNA synthesis and production of infectious virus and indicate a functional role for this domain.

The genetic basis of human cytomegalovirus resistance and current trends in antiviral resistance analysis

Infections due to resistant human cytomegalovirus (CMV) are an emerging problem, particularly in immunocompromised hosts. When managing such patients, clinicians should be aware of the possibility of developing CMV antiviral resistance, especially while on prolonged therapy or if severe immunosuppression is present. CMV resistance to current antiviral agents is mediated by alterations in either the UL97 kinase or DNA polymerase, encoded by the UL97 and UL54 genes, respectively. UL97 mutations are capable of conferring resistance to ganciclovir, while UL54 mutations can impart resistance to ganciclovir, cidofovir, and foscarnet. If treatment failure is suspected to be due to antiviral resistance, CMV resistance analysis should be obtained. Phenotypic resistance assays performed on clinical isolates measure antiviral susceptibilities directly, but are laborious and time-consuming. Therefore, genotypic resistance analysis has become the more common means of diagnosing CMV resistance. Mutations in UL97 or UL54 may be clinically associated with resistance, but their effect on antiviral susceptibility must be confirmed by marker transfer techniques such as recombinant phenotyping.

Recombinant phenotyping of cytomegalovirus UL54 mutations that emerged during cell passages in the presence of either ganciclovir or foscarnet

Selection of human cytomegalovirus variants in the presence of ganciclovir or foscarnet led to 18 DNA polymerase mutations, 14 of which had not been previously studied. Using bacterial artificial chromosome technology, each of these mutations was individually transferred into the genome of a reference strain. Following reconstitution of infectious viral stocks, each mutant was assessed for its drug susceptibility and growth kinetics in cell culture. Computer-assisted three-dimensional (3D) modeling of the polymerase was also used to position each of the mutations in one of four proposed structural domains and to predict their influence on structural stability of the protein. Among the 10 DNA polymerase mutations selected with ganciclovir, 7 (P488R, C539R, L545S, V787L, V812L, P829S, and L862F) were associated with ganciclovir resistance, whereas 2 (F595I and V946L) conferred only foscarnet resistance. Among the eight mutations selected with foscarnet, only two (T552N and S585A) conferred foscarnet resistance, whereas four (N408D, K500N, L802V, and L957F) had an impact on ganciclovir susceptibility. Surprisingly, the combination of mutations, some of which were not associated with resistance for a specific antiviral, resulted in increasing resistance effects. 3D modeling suggested that none of the mutated residues were directly involved in the polymerase catalytic site but rather had an influence on drug susceptibility by modifying the structural flexibility of the protein. Our study significantly adds to the number of DNA polymerase mutations conferring in vitro drug resistance and emphasizes the point that evaluation of individual mutations may not accurately reflect the phenotype conferred by multiple mutations.

Interferon successfully inhibited refractory cytomegalovirus infection and resulted in CD4+ T-cells increase in a patient with AIDS

We report on a young male HIV-positive patient with cytomegalovirus (CMV) retinitis who received HAART and ganciclovir but could not gain immune restoration. One and a half years later, his CMV reoccurred. He was treated again with ganciclovir treatment combined with foscarnet treatment but failed. Finally, he was treated with recombinant human interferon alpha-2b (IFN-α2b) alone and acquired sustained inhibition to CMV. His CD4+ T-cell count increased to 186 cell/mm3 and gained partial immune restoration. Retinitis progression has stopped.

Genotypic characterization of human cytomegalovirus UL97 phosphotransferase natural polymorphism in the era of ganciclovir and maribavir

The molecular mechanisms of human cytomegalovirus (CMV) resistance to both ganciclovir and maribavir reported so far rely predominantly on the presence of mutations within UL97 phosphotransferase. The accurate interpretation of genotypic antiviral resistance assay results requires the clear distinction between resistance mutations and natural interstrain sequence variations. The objective of this work was to extend the catalog of CMV UL97 phosphotransferase natural polymorphisms. The full-length UL97 gene sequence analysis from 4 laboratory strains and 35 clinical samples from patients who had not received any previous anti-CMV treatment was performed. At the nucleotide level, the interstrain identity was >98%. At the amino acid level, ten natural polymorphisms never previously described were identified. Together with all previous results reported in the literature, a new map of UL97 phosphotransferase natural polymorphism could be settled in the era of ganciclovir and maribavir.

Cytomegalovirus retinitis and the acquired immunodeficiency syndrome--bench to bedside: LXVII Edward Jackson Memorial Lecture

PURPOSE

To update information on cytomegalovirus (CMV) retinitis in patients with acquired immunodeficiency syndrome (AIDS) and to integrate information on its pathogenesis and clinical outcomes.

DESIGN

Literature review.

METHODS

Selected articles from the medical literature, particularly large epidemiologic studies, including the Johns Hopkins Cytomegalovirus Retinitis Cohort Study, the Longitudinal Study of the Ocular Complications of AIDS, and the Cytomegalovirus Retinitis and Viral Resistance Study, were reviewed. Clinical information is discussed in light of knowledge on CMV, its pathogenesis, and its interactions with human immunodeficiency virus (HIV).

RESULTS

Cytomegalovirus uses several mechanisms to evade the immune system and establish latent infection in immunologically normal hosts. With immune deficiency, such as late-stage AIDS, CMV reactivates, is disseminated to the eye, and establishes a productive infection, resulting in retinal necrosis. HIV and CMV potentiate each other: CMV accelerates HIV disease, and CMV retinitis is associated with increased mortality. Randomized clinical trials have demonstrated the efficacy of treatments for CMV retinitis. Systemically administered treatment for CMV retinitis decreases AIDS mortality. Highly active antiretroviral therapy (HAART) effectively suppresses HIV replication, resulting in immune recovery, which, if sufficient, controls retinitis without anti-CMV therapy. Resistant CMV, detected in the blood, correlates with resistant virus in the eye and is associated with worse clinical outcomes, including mortality. Host factors, including host genetics and access to care, play a role in the development of CMV retinitis.

CONCLUSIONS

Clinical outcomes of CMV retinitis in patients with AIDS are dependent on characteristics of the virus and host and on HIV-CMV interactions.

Antiviral drug resistance of human cytomegalovirus

The study of human cytomegalovirus (HCMV) antiviral drug resistance has enhanced knowledge of the virological targets and the mechanisms of antiviral activity. The currently approved drugs, ganciclovir (GCV), foscarnet (FOS), and cidofovir (CDV), target the viral DNA polymerase. GCV anabolism also requires phosphorylation by the virus-encoded UL97 kinase. GCV resistance mutations have been identified in both genes, while FOS and CDV mutations occur only in the DNA polymerase gene. Confirmation of resistance mutations requires phenotypic analysis; however, phenotypic assays are too time-consuming for diagnostic purposes. Genotypic assays based on sequencing provide more rapid results but are dependent on prior validation by phenotypic methods. Reports from many laboratories have produced an evolving list of confirmed resistance mutations, although differences in interpretation have led to some confusion. Recombinant phenotyping methods performed in a few research laboratories have resolved some of the conflicting results. Treatment options for drug-resistant HCMV infections are complex and have not been subjected to controlled clinical trials, although consensus guidelines have been proposed. This review summarizes the virological and clinical data pertaining to HCMV antiviral drug resistance.

Diversity of antiviral-resistant human cytomegalovirus in heart and lung transplant recipients

Immunocompromised transplant recipients are at high risk for human cytomegalovirus (CMV)-related infection and disease. Antiviral prophylaxis and treatment have reduced CMV morbidity and mortality, but at times promote development of antiviral-resistant CMV strains that can significantly contribute to adverse clinical outcomes in transplant recipients. We have investigated CMV genotypes in transplant recipients (bone marrow, stem cell, kidney, heart, lung, and liver) receiving antiviral prophylaxis or preemptive therapy or treatment, to determine the viral characteristics and clinical impact of antiviral-resistant CMV in these different groups. Antiviral-resistant CMV strains were detected by polymerase chain reaction sequencing of the CMV protein kinase (UL97) and viral DNA polymerase (UL54) genes from clinical specimens. A trend toward more frequent detection of multidrug resistance and co-circulation of multiple resistant strains was seen in heart and lung transplant recipients compared with other transplantation types. A greater diversity and number of UL97 and UL54 mutations were observed in heart and lung transplant recipients; whereas antiviral-resistant CMV infections in other transplant recipients were predominantly the result of a single mutant genotype. Furthermore, 43% (6/14) of CMV-positive heart and lung transplant recipients were infected with CMV strains containing UL54 mutations conferring multidrug resistance compared with only 6% (1/18) of CMV-positive recipients of other transplanted organs or stem cells. Emergence of CMV strains containing previously unrecognized UL54 mutations (F412S and D485N) also occurred in 1 lung and 1 heart transplant recipient. The development of these mutations under antiviral selective pressure, and clinical outcome of infection suggests these mutations are likely to confer antiviral resistance. Emergence of CMV antiviral resistance remains a significant issue in immunocompromised patients treated with antiviral agents, and emphasizes the relevance of regular antiviral resistance testing when designing optimal patient-management strategies.

Differentiation between polymorphisms and resistance-associated mutations in human cytomegalovirus DNA polymerase

Specific mutations in the human cytomegalovirus (HCMV) DNA polymerase (pUL54) are known to confer resistance against all currently licensed drugs for treatment of HCMV infection and disease. Following the widespread use of antivirals, the occurrence of HCMV drug resistance is constantly increasing. Recently, diagnostic laboratories have started to replace phenotypic drug resistance testing with genotypic resistance testing. However, the reliability and success of genotypic testing highly depend on the availability of high-quality phenotypic resistance data for each individual mutation and for combinations of mutations, with the latter being increasingly found in patients' HCMV isolates. We performed clonal marker transfer experiments to investigate the impacts of 7 different UL54 point mutations and also of combinations of these mutations on drug susceptibility and viral replicative fitness. We show that several mutations-S695T, A972V, K415R, S291P, and A692V-of suspected but uncertain drug susceptibility phenotype, either alone or in combination, were not relevant to antiviral drug resistance. In contrast, the combination of two mutations individually characterized previously-E756K and D413E-conferred high-grade loss of susceptibility to all three antivirals. Our results have been added to the newly available database of all published HCMV resistance mutations (http://www.informatik.uni-ulm.de/ni/mitarbeiter/HKestler/hcmv/index.html). These data will allow better interpretation of genotypic data and further improve the basis for drug resistance testing.

Contrasting effects on ganciclovir susceptibility and replicative capacity of two mutations at codon 466 of the human cytomegalovirus UL97 gene

BACKGROUND

Human cytomegalovirus (HCMV) infections cause significant morbidity in immunocompromised hosts. Resistance to ganciclovir is predominantly associated with alterations in the HCMV UL97 kinase and, more occasionally, with mutations in the HCMV DNA polymerase gene.

OBJECTIVES

The aim of this study was to investigate the impact of two different mutations found at the same UL97 codon on drug susceptibility and viral replicative capacity. Mutation V466G was observed in a solid organ transplant recipient whereas mutation V466M was observed in a patient with AIDS.

STUDY DESIGN

Two HCMV UL97 mutations, V466M and V466G, were transferred to recombinant viruses using a bacterial artificial chromosome system. Susceptibility testing of the recombinant wild-type and mutant viruses was performed using a standard plaque reduction assay. Replication kinetics of recombinant viruses was investigated using a yield assay.

RESULTS

Mutant V466G was resistant to ganciclovir and had significant replicative defect whereas mutant V466M was drug susceptible and had unaltered replication kinetics. Furthermore, mutant V466G formed small viral plaques with intracellular inclusions.

CONCLUSIONS

To our knowledge, this is the first report of such contrasting phenotypes for drug susceptibility and replicative capacity for HCMV mutations found at the same codon of the UL97 gene.

International consensus guidelines on the management of cytomegalovirus in solid organ transplantation

Cytomegalovirus (CMV) remains one of the most common infections after solid organ transplantation, resulting in significant morbidity, graft loss, and occasional mortality. Management of CMV varies considerably among transplant centers. A panel of experts on CMV and solid organ transplant was convened by The Infectious Diseases Section of The Transplantation Society to develop evidence and expert opinion-based consensus guidelines on CMV management including diagnostics, immunology, prevention, treatment, drug resistance, and pediatric issues.

Stability of murine cytomegalovirus genome after in vitro and in vivo passage

While large DNA viruses are thought to have low mutation rates, only a small fraction of their genomes have been analyzed at the single-nucleotide level. Here, we defined the genetic stability of murine cytomegalovirus (MCMV) by whole-genome sequencing. Independently assembled sequences of three sister plaques showed only two single-base-pair substitutions after in vitro passage. In vivo-passaged MCMV likewise demonstrated low mutation rates, comparable to those after in vitro passage, indicating high genome stability of MCMV at the single-nucleotide level in the absence of obvious selection pressure.

Genetic analysis and putative role in resistance to antivirals of the human cytomegalovirus DNA polymerase UL44 processivity factor

BACKGROUND

The human cytomegalovirus (HCMV) DNA polymerase is composed of the UL54 catalytic subunit and the UL44 accessory protein. UL44 increases the processivity of polymerase along the DNA template during replication and, incidentally, is a substrate for the UL97 phosphotransferase. The molecular mechanisms of HCMV resistance to antiviral drugs interfering with viral DNA synthesis reported so far only rely on the presence of amino acid changes within the UL97 and UL54 viral enzymes. We aimed to describe the natural polymorphism of UL44 and to analyse the changes of its amino acids potentially associated with HCMV resistance to antivirals.

METHODS

The full-length UL44 gene sequence was compared to that of four reference strains (including the AD169 strain) and 43 clinical strains from patients who had not received any previous anti-HCMV treatment, and 25 blood samples from 15 HCMV-infected patients experiencing therapeutic failure and exhibiting genotypic traits of HCMV resistance to antivirals.

RESULTS

Overall, seven different amino acid changes associated with natural polymorphisms were identified among the 433 residues of the UL44 protein, occurring at a frequency of 2.1% for five of them and 10.6% for the double change G296S+L319I. The analysis of the HCMV strains exhibiting genotypic resistance to antivirals did not show any changes in UL44 that had significant association with resistance mutations of UL97 and/or UL54.

CONCLUSIONS

UL44 processivity factor exhibits a very low polymorphism that does not concern the assumed functional domains of the protein. From this preliminary study, UL44 does not seem to be involved in HCMV resistance to antivirals.

Single and multiple mutations in the human cytomegalovirus UL97 gene and their relationship to the enzymatic activity of UL97 kinase for ganciclovir phosphorylation

In this study we determined that the double mutant M460V/D605E in the UL97 gene of an HCMV isolate from an immunocompromised patient (MMT isolate) is related to resistance to ganciclovir (GCV) therapy. Our results suggest that the aspartic acid-to-glutamic acid substitution at codon 605 may be associated with a natural polymorphism of the UL97 gene, and not with positive selection pressure exerted by the antiviral drug. We also determined that GCV resistance due to the M460V mutation in the HCMV UL97 gene is not offset by a second mutation (D605E) at codon 605. Furthermore, we showed that when the two mutations related to GCV resistance were simultaneously detected in the same HCMV construct, virus-drug resistance might be enhanced in comparison to that of the single mutants studied separately. To our knowledge for the first time, seven of 12 amino acid changes (F102L, D118V, M330T, T400A, R507P and C511R and I533V) in the UL97 gene of an isolate are herein reported.

Cytomegalovirus resistance in solid organ transplant recipients treated with intravenous ganciclovir or oral valganciclovir

Background

The rate of cytomegalovirus (CMV) mutations conferring ganciclovir resistance was assessed in a trial comparing intravenous ganciclovir and oral valganciclovir for treatment of CMV disease in solid organ transplant (SOT) recipients.

Methods

Viral genes ( UL97 and UL54) conferring ganciclovir resistance were amplified and sequenced from blood samples collected at days 0 (before therapy), 21 (end of induction) and 49 (end of maintenance).

Results

The overall risk of developing a confirmed or probable ganciclovir resistance mutation during treatment was similar for patients treated with ganciclovir (2.3%) and valganciclovir (3.6%; P=0.51). A persistent viral load at day 21 was associated with a significant risk of ganciclovir resistance by day 49 (odds ratio 11.83; P=0.022). In multivariate analyses, presence of a confirmed ganciclovir resistance mutation was independently associated with virological failure (viral load ≥600 copies/ml) at days 21 and 49. One-third (3/9) of patients with confirmed CMV resistance mutations had recurrent CMV disease. The plasma half-life of confirmed ganciclovir-resistant UL97 mutants was significantly longer than that of wild-type strains, polymorphic variants and strains with mutations of unknown significance ( P=0.045). Multiple UL54 mutations of unknown significance were found in clinical strains. Viral kinetic analysis of these latter strains revealed no effect (negative or positive) on in vivo viral fitness.

Conclusions

Treatment with oral valganciclovir or intravenous ganciclovir results in similar and low rates of resistance mutations in SOT recipients. Patients with drug-resistant CMV strains often have virological failure and might have unfavourable clinical outcomes.

Fluorescence-based assay for phenotypic characterization of human cytomegalovirus polymerase mutations regarding drug susceptibility and viral replicative fitness

One essential prerequisite for genotypic drug susceptibility testing of human cytomegalovirus (HCMV) is the phenotypic characterization of mutations identified in the viral protein kinase gene UL97 and the viral DNA polymerase gene UL54 regarding their quantitative impact on drug susceptibility. We developed a new method for phenotypic characterization of UL54 mutations with regard to polymerase activity, viral replication, and drug susceptibility. To determine the most suitable viral indicator gene, enhanced green fluorescence protein was C-terminally fused to the HCMV early-late protein UL83 (pp65) or the late proteins UL32 (pp150) and UL99 (pp28), resulting in reporter viruses vTB65g, vTB150g, and vTB28g. vTB65g proved to be superior to the other constructs due to its favorable signal-to-noise ratio and was therefore used to establish the optimum conditions for our assay. The UL54 E756K and D413E mutations were introduced into vTB65g by markerless bacterial artificial chromosome mutagenesis, resulting in virus strains vE756Kg and vD413Eg. The drug susceptibility phenotypes of vE756Kg and vD413Eg were comparable to those previously reported. Furthermore, we found a reduced replicative fitness of vE756Kg by measuring fluorescence intensity as well as by conventional virus growth kinetics. Decreased fluorescence signals of vE756Kg- and vD413Eg-infected cells at late times of infection suggested a reduced polymerase activity, which was confirmed by real-time PCR quantification of the newly synthesized viral DNAs. This new fluorescence-based assay is a highly reproducible method for the phenotypic characterization of mutations potentially influencing drug susceptibility, viral replicative fitness, and polymerase activity of HCMV after marker transfer.

Long-term outcomes of CMV disease treatment with valganciclovir versus IV ganciclovir in solid organ transplant recipients

Though an important cause of morbidity and mortality in solid organ transplantation (SOT), the long-term outcomes of cytomegalovirus (CMV) disease treatment have not been well studied. In a randomized trial, 321 SOT recipients with CMV disease were followed 1 year after treatment with either twice daily intravenous ganciclovir or oral valganciclovir (for 21 days) followed by once daily valganciclovir until day 49 in all patients. Clinical and viral eradication of CMV disease was similar between groups. Clinical recurrence beyond day 49 was found in 15.1% and virological recurrence in 30.0%, no difference between groups (p > 0.77). In a multivariable logistic regression analysis, the only independent predictor for recurrence was failure to eradicate DNAemia by day 21 (clinical: OR 3.9 [1.3-11.3], p = 0.012; virological: OR 5.6 [2.5-12.6], p < 0.0001). Eight patients developed ganciclovir resistance, with no difference between groups (p = 0.62). Twenty patients (valganciclovir: 11, ganciclovir: 9, p = 0.82) died, 12 due to infections, two involving CMV disease. There were no differences in long-term outcomes between treatment arms, further supporting the use of oral valganciclovir for treatment of CMV disease. Persistent DNAemia at day 21, CMV IgG serostatus and development of resistance may be relevant factors for further individualization of treatment.

Resistance pattern of cytomegalovirus (CMV) after oral valganciclovir therapy in transplant recipients at high-risk for CMV infection

In transplant recipients, cytomegalovirus (CMV) resistance to antivirals causes an increasing problem. Here we report the clinical, therapeutic, and virological characteristics of 11 cases of CMV resistance among transplant recipients at high-risk for CMV infection and receiving valganciclovir as a prophylactic, preemptive or maintenance therapy. Active CMV infection was monitored by viral DNA quantification in whole blood, and CMV resistance was assessed by UL97 and UL54 viral gene sequencing. For 10 patients, ganciclovir resistance detected after valganciclovir therapy was associated with one mutation within UL97 phosphotransferase located at codons 460 and 592-603, which constitutes a similar pattern of resistance to what has been reported previously in AIDS patients treated with valganciclovir. For the last patient, two mutations in UL97 and UL54 genes were identified. The start of valganciclovir maintenance treatment after an intravenous curative treatment while CMV DNA is still detectable in peripheral blood might represent a risk factor for the emergence of CMV resistance. The possible emergence of CMV resistance in transplant recipients at high-risk for CMV infection who receive valganciclovir therapy should be taken into account. Among those patients, CMV infection has to be closely monitored in order to detect promptly the emergence of drug-resistance.

Detection of human cytomegalovirus in dental plaque from individual periodontal sites by real-time polymerase chain reaction

OBJECTIVE

The aim was to evaluate three primer-probe sets and real-time polymerase chain reaction (PCR) for the detection of human cytomegalovirus (HCMV) in dental plaque from individual periodontal sites.

STUDY DESIGN

Fifty subgingival plaque specimens from 13 healthy subjects (on average at least 2 healthy and 2 periodontal disease sites per subject) and 50 saliva specimens from 24 subjects, including 16 controls, were assessed using 3 primer-probe sets (polymerase [POL], glycoprotein B [gB], and US14) and real-time PCR. Kappa statistics were performed to measure agreement between the primer-probe sets.

RESULTS

There was excellent agreement between the gB and POL primers in the detection of HCMV (kappa statistic = 0.85 [95% confidence interval 0.71-0.99]), yielding a prevalence of 4% (2 out of 50) at individual periodontal disease sites and a similar rate of 8.8% (3 out of 34) in saliva.

CONCLUSION

Human cytomegalovirus was infrequently detected in dental plaque. Of 3 primer-probe sets evaluated, those targeting the POL and gB genes were more accurate in the detection of HCMV than that targeting US14.

Efficient inhibition of human cytomegalovirus DNA polymerase expression by small hairpin RNA in vitro

The aim of this study is to try to develop an RNA interference (RNAi) approach to human cytomegalovirus (HCMV) therapy by inhibition of viral DNA polymerase (product of the UL54 gene) expression in vitro. The fusion protein expression plasmid pEGFP-UL54 was constructed and cotransfected into AD293 cells with the lentiviral vector pGCL-GFP expressing small hairpin RNA (shRNA) especially targeting the UL54 gene. At 24, 48, and 72 h posttransfection, expression of the fusion protein was detected by fluorescence microscopy and Western blot. The level of UL54 mRNA was semiquantitated by reverse transcription polymerase chain reaction. It was found that, compared with the negative control, expression of the EGFP-UL54 fusion protein was efficiently inhibited and the mRNA level of the UL54 gene decreased significantly in cells introduced with a shRNA producing plasmid called pGCL-UL54-1479. However, the plasmid pGCL-UL54-2884 had no significant inhibitive effect on protein expression or mRNA level. It may be concluded that introduction of shRNA targeting UL54 gene of HCMV could specifically inhibit the expression of viral DNA polymerase in vitro. Nucleotides 1479-1499 of the UL54 gene seems to be an effective target site of RNAi.

Resistance to combined ganciclovir and foscarnet therapy in a liver transplant recipient with possible dual-strain cytomegalovirus coinfection

We present a case report of a cytomegalovirus (CMV)-seronegative, 58-year-old male who received a CMV-seropositive donor liver transplant without CMV prophylaxis. On postoperative day 30, the patient developed primary CMV disease that responded to ganciclovir. On postoperative day 114, however, he was diagnosed with recurrent CMV infection. Despite aggressive, combined antiviral treatment with ganciclovir and foscarnet and reduction of immunosuppression, viral clearance was never achieved. Serum samples were collected throughout the infectious process for viral DNA analysis. Portions of the UL97 and UL54 genes were amplified and compared to the AD169 wild-type strain. Sequencing studies revealed the presence of mutations in viral isolates obtained after clinical resistance was observed. These mutations were not present in samples obtained during the primary CMV infection. Our findings suggest the presence of coinfection with at least 2 different strains of CMV rather than induction of mutations after ganciclovir therapy.

[Different human cytomegalovirus glycoprotein B (gB) genotype distribution]

The glycoprotein B (gB) is the major glycoprotein of the envelope of the human cytomegalovirus, it is encoded by UL55 open reading frame, implicated in host cell, entry cell to cell virus transmission and fusion of infected cells and a significant is an important target for immuno-reactions humoral and cellular. A prospective analysis of cytomegalovirus glycoprotein B genotype was conducted on 31 immunodepressed (transplant recipients and AIDS patients). The DNA was extracted directly from the bronchoalveolar liquid (BAL) of these patients. The gB genotypes of CMV was determined by using the polymerase chain reaction, followed by the digestion of two enzymes of restriction HinfI and RsaI. The distribution of the gB genotype of the CMV was: gB1 38,70%; gB2 25,80%; gB3 16,12% and gB4 19,35%. The analysis of the peptide sequence of this region (codons 437-520), indicate the variation was more frequent between codons 448-480.

The interplay between host and viral factors in shaping the outcome of cytomegalovirus infection

Cytomegalovirus (CMV) remains a major human pathogen causing significant morbidity and mortality in immunosuppressed or immunoimmature individuals. Although significant advances have been made in dissecting out certain features of the host response to human CMV (HCMV) infection, the strict species specificity of CMVs means that most aspects of antiviral immunity are best assessed in animal models. The mouse model of murine CMV (MCMV) infection is an important tool for analysis of in vivo features of host-virus interactions and responses to antiviral drugs that are difficult to assess in humans. Important studies of the contribution of host resistance genes to infection outcome, interplays between innate and adaptive host immune responses, the contribution of virus immune evasion genes and genetic variation in these genes to the establishment of persistence and in vivo studies of resistance to antiviral drugs have benefited from the well-developed MCMV model. In this review, we discuss recent advances in the immunobiology of host-CMV interactions that provide intriguing insights into the complex interplay between host and virus that ultimately facilitates viral persistence. We also discuss recent studies of genetic responses to antiviral therapy, particularly changes in DNA polymerase and protein kinase genes of MCMV and HCMV.

Polymorphisms of human cytomegalovirus UL148A, UL148B, UL148C, UL148D genes in clinical strains

BACKGROUND

Clinical isolates of human cytomegalovirus (HCMV) display polymorphisms in multiple genes. Some authors have suggested that polymorphisms are implicated in HCMV-induced immunopathogenesis, as well as in strain-specific behaviors, such as tissue-tropism and the ability to establish persistent or latent infections.

OBJECTIVE

To describe the features of HCMV UL148A, UL148B, UL148C and UL148D open reading frames (ORFs) and the variable sites within the frames in clinical strains.

STUDY DESIGN

PCR was performed to amplify these ORFs in 22 clinical strains. PCR amplification products were sequenced directly and analyzed.

RESULTS

The nucleotide diversity of UL148A, UL148B, UL148C and UL148D ORFs in studied strains is 0.5-8.3%, 0.5-4.6%, 0.5-3% and 1.7-8.1%, respectively; the amino acid diversity of their putative proteins is 1.3-6.3%, 1.3-5.0%, 1.3-3.9% and 1.7-8.1%, respectively, related to the Merlin strain. The modification sites of UL148A, UL148B, UL148C and UL148D predicted proteins from strains in unpassaged urine samples were conserved, except for strain U96, compared with that of the Merlin strain. By phylogenetic and statistical analysis, the UL148A and UL148D sequences of clinical strains were classified into three groups.

CONCLUSION

Compared to the UL148A, UL148B and UL148D ORFs, the UL148C ORF was relatively conserved, as was the amino acid sequence of the UL148C putative protein. Isolates that have been passaged several times in human embryonic lung fibroblasts (HELF) showed some changes of modification sites, however. A discrete linkage was found between the groups of UL148A gene and those of UL148D gene.

Analysis of equid herpesvirus 1 strain variation reveals a point mutation of the DNA polymerase strongly associated with neuropathogenic versus nonneuropathogenic disease outbreaks

Equid herpesvirus 1 (EHV-1) can cause a wide spectrum of diseases ranging from inapparent respiratory infection to the induction of abortion and, in extreme cases, neurological disease resulting in paralysis and ultimately death. It has been suggested that distinct strains of EHV-1 that differ in pathogenic capacity circulate in the field. In order to investigate this hypothesis, it was necessary to identify genetic markers that allow subgroups of related strains to be identified. We have determined all of the genetic differences between a neuropathogenic strain (Ab4) and a nonneuropathogenic strain (V592) of EHV-1 and developed PCR/sequencing procedures enabling differentiation of EHV-1 strains circulating in the field. The results indicate the occurrence of several major genetic subgroups of EHV-1 among isolates recovered from outbreaks over the course of 30 years, consistent with the proposal that distinct strains of EHV-1 circulate in the field. Moreover, there is evidence that certain strain groups are geographically restricted, being recovered predominantly from outbreaks occurring in either North America or Europe. Significantly, variation of a single amino acid of the DNA polymerase is strongly associated with neurological versus nonneurological disease outbreaks. Strikingly, this variant amino acid occurs at a highly conserved position for herpesvirus DNA polymerases, suggesting an important functional role.

New reporter cell line to evaluate the sequential emergence of multiple human cytomegalovirus mutations during in vitro drug exposure

We developed a new reporter cell line for human cytomegalovirus (HCMV) drug susceptibility testing. This cell line was obtained by incorporating the luciferase reporter gene under the control of an HCMV-specific promoter into the genome of astrocytoma cells (U373MG). We then used our reporter cell line to evaluate phenotypic changes conferred by the sequential emergence of HCMV UL54 and UL97 mutations following long-term drug exposure. The laboratory strain AD169 was passaged in the presence of increasing concentrations of ganciclovir (one viral line) or foscarnet (two viral lines). Resistant viruses were plaque purified at five different concentrations of ganciclovir and at three different concentrations of foscarnet. In addition to the previously described M460I and L595S UL97 mutations and the L545S and V812L UL54 mutations, exposition to ganciclovir (up to 3,000 microM) resulted in the selection of two unreported UL54 mutations (P829S and D879G). Passages in the presence of foscarnet (up to 3,000 microM) resulted in the selection of seven not previously described UL54 mutations (K500N, T552N, S585A, N757K, L802V, L926V, and L957F) in addition to the N408D mutation that has been associated with ganciclovir and cidofovir resistance. Long-term exposure of HCMV to either ganciclovir or foscarnet ultimately resulted in the selection of multiple UL54 mutations that conferred high levels of resistance to all approved HCMV DNA polymerase inhibitors, i.e., ganciclovir, cidofovir, and foscarnet. Emergence of each viral mutation conferred stepwise increases in drug 50% inhibitory concentrations that could be objectively measured with the new reporter cell assay.

[A novel colorimetric test to study the susceptibility of human cytomegalovirus DNA polymerase to foscarnet]

We described a colorimetric method to determine the biochemical phenotype of wild-type and mutated cytomegalovirus (HCMV) DNA polymerases by measuring the incorporation of digoxigenin-labelled nucleotides into the growing DNA chain. Mutations V715M and E756K, which are known to confer foscarnet-resistance, were used as controls. Mutation N495K and a combination of changes K415R and S291P, both observed in foscarnet-resistant isolates, were studied. The mutations were introduced by site-directed mutagenesis into wild-type gene UL54 cloned in an expression vector and then polymerases were synthesised by using a commercially available coupled transcription-translation system. The polymerase activity was measured with and without foscarnet. The activity of polymerases containing the V715M or E756K mutations was inhibited by foscarnet at concentrations 70- and 30-fold higher than that of wild-type polymerase, respectively. Change N495K and combination of K415R and S291P, induced a five- and ten-fold decrease in susceptibility to foscarnet, respectively. The results of this non-radioactive assay were consistent with those obtained with the conventional radioactive assay. Therefore, this novel phenotypic method could be useful for the characterisation of mutations that confer HCMV resistance to foscarnet.

Murine cytomegalovirus resistant to antivirals has genetic correlates with human cytomegalovirus

Human cytomegalovirus (HCMV) resistance to antivirals is a significant clinical problem. Murine cytomegalovirus (MCMV) infection of mice is a well-described animal model for in vivo studies of CMV pathogenesis, although the mechanisms of MCMV antiviral susceptibility need elucidation. Mutants resistant to nucleoside analogues aciclovir, adefovir, cidofovir, ganciclovir, penciclovir and valaciclovir, and the pyrophosphate analogue foscarnet were generated by in vitro passage of MCMV (Smith) in increasing concentrations of antiviral. All MCMV antiviral resistant mutants contained DNA polymerase mutations identical or similar to HCMV DNA polymerase mutations known to confer antiviral resistance. Mapping of the mutations onto an MCMV DNA polymerase three-dimensional model generated using the Thermococcus gorgonarius Tgo polymerase crystal structure showed that the DNA polymerase mutations potentially confer resistance through changes in regions surrounding a catalytic aspartate triad. The ganciclovir-, penciclovir- and valaciclovir-resistant isolates also contained mutations within MCMV M97 identical or similar to recognized GCV-resistant mutations of HCMV UL97 protein kinase, and demonstrated cross-resistance to antivirals of the same class. This strongly suggests that MCMV M97 has a similar role to HCMV UL97 in the phosphorylation of nucleoside analogue antivirals. All MCMV mutants demonstrated replication-impaired phenotypes, with the lowest titre and plaque size observed for isolates containing mutations in both DNA polymerase and M97. These findings indicate DNA polymerase and protein kinase regions of potential importance for antiviral susceptibility and replication. The similarities between MCMV and HCMV mutations that arise under antiviral selective pressure increase the utility of MCMV as a model for in vivo studies of CMV antiviral resistance.

Phenotypic characterisation of cytomegalovirus DNA polymerase: a method to study cytomegalovirus isolates resistant to foscarnet

A phenotypic method was developed to test mutations in the human cytomegalovirus (HCMV) DNA polymerase gene (UL54) suspected to confer resistance to foscarnet. This method was used to determine the biochemical phenotype of wild-type and mutated HCMV DNA polymerases that had been synthesised in vitro as follows. The UL54 genes were amplified from foscarnet-resistant and -sensitive isolates by PCR and the products were cloned into an expression vector under the control of a T7 promoter. Mutations were introduced by site-directed mutagenesis into wild-type gene UL54 and then polymerases were synthesised by using a commercially available coupled transcription/translation system. Polymerase activity was measured with and without foscarnet by detecting the incorporation of digoxigenin-labelled nucleotides into the growing DNA chain. The results of this non-radioactive assay were consistent with those obtained with the conventional radioactive assay. It was found that the activity of polymerases containing the V715M or E756K mutations was inhibited by foscarnet at concentrations 70- and 30-fold higher than that of wild-type polymerase, respectively. Change N495K and a combination of changes K415R and S291P, both observed in foscarnet-resistant isolates, induced a 5- and 10-fold decrease in susceptibility to foscarnet, respectively. This non-radioactive phenotypic assay could be useful for the characterisation of mutations that confer HCMV resistance to foscarnet.