The Cys607-->Tyr change in the UL97 phosphotransferase confers ganciclovir resistance to two human cytomegalovirus strains recovered from two immunocompromised patients

Specialization for Cell-Free or Cell-to-Cell Spread of BAC-Cloned Human Cytomegalovirus Strains Is Determined by Factors beyond the UL128-131 and RL13 Loci

It is widely held that clinical isolates of human cytomegalovirus (HCMV) are highly cell associated, and mutations affecting the UL128-131 and RL13 loci that arise in culture lead to the appearance of a cell-free spread phenotype. The bacterial artificial chromosome (BAC) clone Merlin (ME) expresses abundant UL128-131, is RL13 impaired, and produces low infectivity virions in fibroblasts, whereas TB40/e (TB) and TR are low in UL128-131, are RL13 intact, and produce virions of much higher infectivity. Despite these differences, quantification of spread by flow cytometry revealed remarkably similar spread efficiencies in fibroblasts. In epithelial cells, ME spread more efficiently, consistent with robust UL128-131 expression. Strikingly, ME spread far better than did TB or TR in the presence of neutralizing antibodies on both cell types, indicating that ME is not simply deficient at cell-free spread but is particularly efficient at cell-to-cell spread, whereas TB and TR cell-to-cell spread is poor. Sonically disrupted ME-infected cells contained scant infectivity, suggesting that the efficient cell-to-cell spread mechanism of ME depends on features of the intact cells such as junctions or intracellular trafficking processes. Even when UL128-131 was transcriptionally repressed, cell-to-cell spread of ME was still more efficient than that of TB or TR. Moreover, RL13 expression comparably reduced both cell-free and cell-to-cell spread of all three strains, suggesting that it acts at a stage of assembly and/or egress common to both routes of spread. Thus, HCMV strains can be highly specialized for either for cell-free or cell-to-cell spread, and these phenotypes are determined by factors beyond the UL128-131 or RL13 loci.IMPORTANCE Both cell-free and cell-to-cell spread are likely important for the natural biology of HCMV. In culture, strains clearly differ in their capacity for cell-free spread as a result of differences in the quantity and infectivity of extracellular released progeny. However, it has been unclear whether "cell-associated" phenotypes are simply the result of poor cell-free spread or are indicative of particularly efficient cell-to-cell spread mechanisms. By measuring the kinetics of spread at early time points, we were able to show that HCMV strains can be highly specialized to either cell-free or cell-to-cell mechanisms, and this was not strictly linked the efficiency of cell-free spread. Our results provide a conceptual approach to evaluating intervention strategies for their ability to limit cell-free or cell-to-cell spread as independent processes.

The Thrombopoietin Receptor Agonist Eltrombopag Inhibits Human Cytomegalovirus Replication Via Iron Chelation

The thrombopoietin receptor agonist eltrombopag was successfully used against human cytomegalovirus (HCMV)-associated thrombocytopenia refractory to immunomodulatory and antiviral drugs. These effects were ascribed to the effects of eltrombopag on megakaryocytes. Here, we tested whether eltrombopag may also exert direct antiviral effects. Therapeutic eltrombopag concentrations inhibited HCMV replication in human fibroblasts and adult mesenchymal stem cells infected with six different virus strains and drug-resistant clinical isolates. Eltrombopag also synergistically increased the anti-HCMV activity of the mainstay drug ganciclovir. Time-of-addition experiments suggested that eltrombopag interfered with HCMV replication after virus entry. Eltrombopag was effective in thrombopoietin receptor-negative cells, and the addition of Fe³⁺ prevented the anti-HCMV effects, indicating that it inhibits HCMV replication via iron chelation. This may be of particular interest for the treatment of cytopenias after hematopoietic stem cell transplantation, as HCMV reactivation is a major reason for transplantation failure. Since therapeutic eltrombopag concentrations are effective against drug-resistant viruses, and synergistically increase the effects of ganciclovir, eltrombopag is also a drug-repurposing candidate for the treatment of therapy-refractory HCMV disease.

An overview of letermovir: a cytomegalovirus prophylactic option

Introduction: Human cytomegalovirus (HCMV) or human herpesvirus 5 (HHV-5) is a β-herpesvirus that causes widespread infection in nearly all members of the human population worldwide. Its persistence in humans after primary infection in a latent phase as well as a partial non-protective immune response is the basis for repeated re-activation/re-infection episodes occurring both in immunocompetent and immunocompromised subjects. In the latter patient populations, which include hematopoietic stem cell transplant (HSCT) recipients, HCMV reactivation episodes may be particularly severe, leading to both systemic and end-organ diseases. Since the 90s, at least four antiviral drugs targeting the DNA polymerase complex have been developed for the prevention and treatment of HCMV infections in transplant recipients, used as first-line (ganciclovir and valganciclovir) and second-line therapy (foscarnet and cidofovir). However, due to their toxicity and drug-resistance induction, new drugs with different targets were needed. Areas covered: In 2017, a new drug named letermovir (LTV), which targets the HCMV DNA terminase complex, was licensed for prophylaxis of HCMV infections in HSCT recipients. This is the focus of this review. Expert opinion: LTV safety and efficacy are promising. However, long-term adverse events and the emergence of drug-resistant HCMV strains must be investigated in extended clinical trials prior to drawing final conclusions.

Inhibition of human cytomegalovirus major capsid protein expression and replication by ribonuclease P-associated external guide sequences

External guide sequences (EGSs) signify the short RNAs that induce ribonuclease P (RNase P), an enzyme responsible for processing the 5' termini of tRNA, to specifically cleave a target mRNA by forming a precursor tRNA-like complex. Hence, the EGS technology may serve as a potential strategy for gene-targeting therapy. Our previous studies have revealed that engineered EGS variants induced RNase P to efficiently hydrolyze target mRNAs. In the present research, an EGS variant was designed to be complementary to the mRNA coding for human cytomegalovirus (HCMV) major capsid protein (MCP), which is vital to form the viral capsid. In vitro, the EGS variant was about 80-fold more efficient in inducing human RNase P-mediated cleavage of the target mRNA than a natural tRNA-derived EGS. Moreover, the expressed variant and natural tRNA-originated EGSs led to a decrease of MCP expression by 98% and 73%-74% and a decrease of viral growth by about 10,000- and 200-fold in cells infected with HCMV, respectively. These results reveal direct evidence that the engineered EGS variant has higher efficiency in blocking the expression of HCMV genes and viral growth than the natural tRNA-originated EGS. Therefore, our findings imply that the EGS variant can be a potent candidate agent for the treatment of infections caused by HCMV.

Detailed polymorphism study on cytomegalovirus DNA polymerase gene to reveal the most suitable genomic targets for quantitative Real-time PCR

The human cytomegalovirus (HCMV) is an important human pathogen primarily affecting immunocompromised patients, like transplant recipients or HIV- infected individuals. Early diagnosis of cytomegalovirus (CMV) infection in high-risk patients is essential in order to start preemptive treatments. pol (UL54) gene encoding for HCMV viral DNA polymerase is a well-defined target for HCMV detection in clinical samples and identifying most highly conserved regions for primer design remains crucial. Though real-time polymerase chain reaction (qPCR) is a rapid and sensitive method for HCMV detection, failure to detect some HCMV strains due to primer and target mismatches have led the researchers to explore more sensitive and reliable methods. Hence, to understand the broader diversity of the pol mutations in HCMV and to specify the most suitable region for primer-probe design to be used in qPCR assay, we studied both nucleotide and amino acid heterogeneities in 60 HCMV positive samples that were collected to represent national mutational prevalence of pol gene of HCMV in Turkey. The test was designed with a new set of primers- probe for HCMV detection and quantification based on the sequencing data which revealed the most conserved region on the pol gene. Statistical probit analysis was applied on qPCR studies which revealed a 95% detection limit of 100 copies/mL. In addition, linearity, reproducibility, and precision of the new test were assessed for diagnostic purposes.

Detection of Two Drug-Resistance Mutants of the Cytomegalovirus by High-Resolution Melting Analysis

BACKGROUND

Human cytomegalovirus (CMV) is an opportunistic pathogen that can be treated with ganciclovir. Mutations in the UL97 gene of CMV render the virus ganciclovir resistance. These include H520Q and C603W mutations, against which we developed a novel genotyping assay for their identification.

METHODS

PCR reactions were performed to amplify fragments of the UL97 gene containing H520Q or C603W mutations. High resolution melting analysis (HRMA) coupled with unlabeled DNA probes was employed to identify the shift in melting temperature of the probe-template complex, which reflexes the presence of point mutations.

RESULTS

Melting point analysis performed on the dimeric DNA of PCR products of UL97 gene could not identify mutations in the gene. When coupled to unlabeled probes, point mutations in UL97 can be identified by analyzing the melting curve of probe-template complex. When WT and mutant UL97 DNAs were mixed together to mimic heterogeneous viral population in clinical samples, the genotyping assay is sensitive enough to detect H520Q and C603W mutants that constitute 10% of total DNA input.

CONCLUSION

Probe-based HRMA is effective in detecting H520Q and C603W mutations in the UL97 gene of CMV.

Antiviral Drug- and Multidrug Resistance in Cytomegalovirus Infected SCT Patients

In pediatric and adult patients after stem cell transplantation (SCT) disseminated infections caused by human cytomegalovirus (HCMV) can cause life threatening diseases. For treatment, the three antivirals ganciclovir (GCV), foscarnet (PFA) and cidofovir (CDV) are approved and most frequently used. Resistance to all of these antiviral drugs may induce a severe problem in this patient cohort. Responsible for resistance phenomena are mutations in the HCMV phosphotransferase-gene (UL97) and the polymerase-gene (UL54). Most frequently mutations in the UL97-gene are associated with resistance to GCV. Resistance against all three drugs is associated to mutations in the UL54-gene. Monitoring of drug resistance by genotyping is mostly done by PCR-based Sanger sequencing. For phenotyping with cell culture the isolation of HCMV is a prerequisite. The development of multidrug resistance with mutation in both genes is rare, but it is often associated with a fatal outcome. The manifestation of multidrug resistance is mostly associated with combined UL97/UL54-mutations. Normally, mutations in the UL97 gene occur initially followed by UL54 mutation after therapy switch. The appearance of UL54-mutation alone without any detection of UL97-mutation is rare. Interestingly, in a number of patients the UL97 mutation could be detected in specific compartments exclusively and not in blood.

Improved detection of emerging drug-resistant mutant cytomegalovirus subpopulations by deep sequencing

In immunosuppressed hosts, the development of multidrug resistance complicates the treatment of cytomegalovirus (CMV) infection. Improved genotypic detection of impending drug resistance may follow from recent technical advances. A severely T-cell-depleted patient with chronic lymphocytic leukemia developed CMV pneumonia and high plasma viral loads that were poorly responsive to antiviral therapy. Serial plasma specimens were analyzed for mutant viral populations by conventional and high-throughput deep-sequencing methods. Uncharacterized mutations were phenotyped for drug resistance using recombinant viruses. Conventional genotyping detected viruses with the UL97 kinase substitution C607Y after ganciclovir treatment, a transient subpopulation of UL54 polymerase L773V mutants first detected 8 weeks after foscarnet was started, and a subpopulation of a mutant with deletion of UL54 codons 981 and 982 2 months after the addition of cidofovir. Deep sequencing of the same serial specimens revealed the same UL54 mutants sooner, along with a more complex evolution of known and newly recognized mutant subpopulations missed by conventional sequencing. The UL54 exonuclease substitutions D413N, K513R, and C539G were newly shown to confer ganciclovir-cidofovir resistance, while L773V was shown to confer foscarnet resistance and add to the ganciclovir resistance conferred by UL97 C607Y. Increased sequencing depth provided a more timely and detailed diagnosis of mutant viral subpopulations that evolved with changing anti-CMV therapy.

Rapid detection and quantitation of ganciclovir resistance in cytomegalovirus quasispecies

Human cytomegalovirus (HCMV) may cause severe or fatal disease among immunocompromised patients. The first line prophylaxis and systemic HCMV disease therapy is ganciclovir (GCV). The presence of GCV-resistant virus has been linked to fatal HCMV disease. The implementation of rapid and sensitive techniques for the early detection and monitoring of GCV-resistance may be helpful to support antiviral therapy management. A pyrosequencing assay for the detection and quantitation of the most frequent mutations conferring moderate- and high-grade GCV resistance was implemented. The pyrosequencing achieved an analytical sensitivity for adequate interpretation of ≥10(3)  copies/ml. The assay was validated with 18 whole blood samples taken over a 6-month period from an umbilical cord blood recipient infected persistently with HCMV and allowed the detection and monitoring of the M460I and A594V GCV-resistant mutations. The percentage of resistant quasispecies ranged from 7.9% to 55.2% for the M460I mutation and from 19.8% to 43% for the A594V mutation. Clearance of the M460I mutation occurred in parallel with a decrease in the HCMV viremia, while the A594V mutation persisted. The pyrosequencing method for detection of GCV is sensitive enough to be used directly on clinical samples for the early identification of resistance mutations and allows the quantitation of resistant and wild type virus quasispecies within hours. The quantitation of minor resistant variants is an important issue to understand their relationship with viral load modification, and potentially anticipate treatment adjustment.

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.

Ribonuclease P-mediated inhibition of human cytomegalovirus gene expression and replication induced by engineered external guide sequences

External guide sequences (EGSs) are RNA molecules that can bind to a target mRNA and direct ribonuclease P (RNase P), a tRNA processing enzyme, for specific cleavage of the target mRNA. Using an in vitro selection procedure, we have previously generated EGS variants that efficiently direct human RNase P to cleave a target mRNA in vitro. In this study, we constructed EGSs from a variant to target the overlapping region of the mRNAs coding for human cytomegalovirus (HCMV) capsid scaffolding protein (CSP) and assemblin, which are essential for viral capsid formation. The EGS variant was about 40-fold more active in directing human RNase P to cleave the mRNA in vitro than the EGS derived from a natural tRNA. Moreover, a reduction of about 98% and 75% in CSP/assemblin gene expression and a reduction of 7000- and 250-fold in viral growth were observed in HCMV-infected cells that expressed the variant and the tRNA-derived EGS, respectively. Our study shows that the EGS variant is more effective in blocking HCMV gene expression and growth than the tRNA-derived EGS. Moreover, these results demonstrate the utility of highly active EGS RNA variants in gene targeting applications including anti-HCMV therapy.

Genotyping cytomegalovirus UL97 mutations by high-resolution melting analysis with unlabeled probe

Human cytomegalovirus (CMV) is an opportunistic pathogen, and infections with this virus can be treated with ganciclovir (GCV). Most GCV-resistant clinical CMV isolates contain a mutation in the UL97 gene. Genotypic assays for diagnostic screening of GCV-resistant CMV have been developed. High-resolution melting analysis (HRMA) with unlabeled probe is considered a perfect tool for this purpose. In this study, we have developed an HRMA-based genotypic test for the detection of UL97 mutations. Wild type and M460V/I mutants of UL97 were constructed. HRMA with unlabeled probe was used as a genotyping method for the detection of M460V/I mutations. The melting peaks obtained directly from PCR products did not enable us to distinguish the wild type from M460 mutants. The sensitivity and accuracy of HRMA were dramatically improved by using unlabeled probe. HRMA with unlabeled probe successfully distinguished M460V from M460I and served well for the detection of M460V/I mutations in clinical samples. HRMA with unlabeled probe proves to be a sensitive and cost-effective genotyping method for the detection of M460 mutations.

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.

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.

Dynamics of the emergence of a human cytomegalovirus UL97 mutant strain conferring ganciclovir resistance in a pediatric stem-cell transplant recipient

Stem-cell transplant recipients are at risk of developing ganciclovir-resistant human cytomegalovirus (HCMV) infection caused by mutations in the viral UL97 gene. Knowledge of the relative proportions of coexisting HCMV wild-type and mutant strains may contribute to a better understanding of the dynamics of in vivo mutant strain selection under ganciclovir. Currently, genotype resistance screening for UL97 is routinely performed by restriction fragment length polymorphism detection and sequencing. We present here the longitudinal course of a pediatric recipient of an allogeneic stem-cell transplant infected with a ganciclovir-resistant HCMV strain. EDTA-treated blood samples were analyzed longitudinally. The patient acquired a primary HCMV infection shortly before transplantation and reactivated the virus following allogeneic hematopoietic stem cell transplantation, thus receiving an intensive antiviral treatment schedule. Three different methods for UL97 mutation analysis, restriction fragment length polymorphism detection, sequencing, and a new, real-time PCR approach were performed. In conclusion, for our pediatric patient, during peak viral load, the UL97 wild-type strain predominates, while during clinical deterioration with low viral load, the predominant mutant strain persists.

Antiviral treatment of cytomegalovirus infection and resistant strains

This review discusses the management of resistant cytomegalovirus and prevention strategies for fatal therapy failures. Five drugs, ganciclovir/valganciclovir, cidofovir, foscarnet and fomivirsen, have been approved so far for the treatment of human cytomegalovirus (HCMV) diseases. Except for fomivirsen, all of the approved drugs share the same target molecule, the viral DNA polymerase. The emergence of drug-resistant HCMV has also been reported for all of them. For optimal care of patients, the clinical virologist has to provide the most meaningful assays for monitoring of therapy and early detection of emerging drug-resistant HCMV. Additionally, a quantitative drug monitoring would be helpful. New antiviral agents are urgently needed with less adverse effects, good oral bioavailability and possibly novel targets or mechanisms of action to avoid cross-resistance and to improve the ability to suppress the selection of resistant virus strains by combination therapy. Compounds like maribavir, leflunomide and artesunate, which exhibit anti-HCMV activity in vitro and in patients need to be evaluated in clinical studies. Besides these, new therapy approaches like immunotherapy or new diagnostic techniques like pyrosequencing have to be considered in the future.

Cytomegalovirus UL97 mutations in the era of ganciclovir and maribavir

Mutations in the human CMV UL97 kinase gene are a major mechanism of viral resistance to two anti-CMV drugs, ganciclovir (GCV) and maribavir (MBV). GCV, the most widely used and established therapy for CMV, is a substrate for the UL97 kinase. Well-characterised GCV-resistance mutations at UL97 codons 460, 520 and 590-607 impair the phosphorylation of GCV that is necessary for its antiviral activity, presumably by altering substrate recognition. In contrast, MBV is an inhibitor of the UL97 kinase and is the first new CMV therapy to reach later stage clinical trials in many years. No MBV-resistant CMV isolates have yet been detected in clinical trials, but after culture propagation under drug, UL97 mutations that confer moderate to high-level MBV resistance have been identified at codons 353, 397, 409 and 411. These mutations are located upstream of the GCV-resistance mutations and are close to the ATP-binding and catalytic domains common to all kinases, consistent with MBV acting as a small molecule ATP-competitive kinase inhibitor. So far, no UL97 mutations are known to confer resistance to both GCV and MBV.

Rapid Semiquantitative Real-Time PCR for the Detection of Human Cytomegalovirus UL97 Mutations Conferring Ganciclovir Resistance

Background

The development of infections with ganci-clovir (GCV)-resistant human cytomegalovirus (HCMV) remains a serious problem in recipients of stem cell or organ transplants. Nearly all GCV-resistant clinical isolates have mutations in the viral UL97 gene. The rapid detection of GCV-resistant HCMV infections is necessary and the relative proportions of wild-type and mutant strains are predictive for the efficiency of antiviral therapy. To date, genotypical resistance screening has been limited to restriction fragment length polymorphism (RFLP) and sequencing analyses. Here, we present a comprehensive real-time PCR approach for the detection of most frequent mutations in the UL97 gene associated with GCV resistance.

Methods

The laboratory strains AD169 and Towne, different wild-type isolates and plasmids constructed by site-directed mutagenesis and overlap extension with specific point-mutations in the UL97 gene were analysed by LightCycler PCR and compared with UL97 RFLP and sequencing analyses.

Results

A new and comprehensive set of LightCycler PCRs was created using specific hybridization probes with melting-point analysis for the relevant codons 594, 595, 603 and 607. Different wild-type isolates and plasmids containing specific UL97 mutations conferring GCV resistance were investigated in the real-time PCR assay. Total processing time was 80 min per assay, whereas combinations of RFLP and sequencing needed at least 3–4 days. Proportions of co-existing wild-type and mutant strains in mixed viral populations can be obtained.

Conclusions

We established a rapid real-time PCR approach for the detection of most frequent HCMV UL97 mutations associated with GCV resistance. Moreover, the method allows semiquantitative differentiation of the proportions of co-existing wild-type and mutant strains. This approach represents a new alternative for laborious RFLP analysis.

Use of time-saving flow cytometry for rapid determination of resistance of human cytomegalovirus to ganciclovir

There are two ways to assess the susceptibility of human cytomegalovirus (HCMV) to ganciclovir (GCV): one is a genotypic test that detects resistance-related mutations and the other is a phenotypic test that actually assesses susceptibility. The advantages of genotyping the UL 97 gene are its rapidity and accuracy. However, to detect novel mutations or mutations affecting the UL 54 DNA polymerase, a phenotypic test such as the plaque reduction assay (PRA) is also required. To avoid the shortcomings of PRA such as its time-consuming nature and labor-intensiveness, we developed a time-saving fluorescence-activated cell sorting (TS-FACS) technique. We obtained a GCV 50% inhibitory concentration (IC(50)) from five clinical isolates and an HCMV laboratory strain (AD169) and compared the results with those from the PRA. The laboratory strain and three clinical isolates were sensitive to GCV. Although there was a minor discrepancy in the case of one of the three isolates, the GCV IC(50) values obtained by TS-FACS analysis correlated well with the results of the PRA. The remaining two isolates were resistant to GCV; one was GCV resistant due to the mutation M 460 V, and the GCV IC(50) results obtained by TS-FACS analysis and by PRA were also comparable. The advantages of TS-FACS analysis are the shorter time required, the possibility of automation, and its comparability to PRA, considered the gold standard. Thus, TS-FACS analysis may be useful as an alternative to PRA in the clinic.

Geldanamycin, a potent and specific inhibitor of Hsp90, inhibits gene expression and replication of human cytomegalovirus

The effect of geldanamycin (GA), a specific inhibitor of heat shock protein 90 (Hsp90), on gene expression and replication of human cytomegalovirus (HCMV) was studied in human embryonic lung (HEL) fibroblasts. Kinetic analysis indicated that GA delayed synthesis of major immediate early (MIE), early and late viral proteins, and blocked a second tier of the synthesis of these proteins that occurred in untreated cells after 48 h post-infection (pi). Moreover, when HCMV-infected HEL cells were maintained with medium containing 40 nM GA for 6 days, with medium changes at 2-day intervals, the virus yield was reduced to an undetectable level. On a molecular level, the cellular kinase Akt and the transcription factor NFkappaB were activated in HCMV-infected cells within 30 min pi. NFkappaB was shown to be essential for MIE gene expression. However, in GA-treated cells, activation of both Akt and NFkappaB was greatly inhibited. Because LY294002, an inhibitor of cellular phosphatidylinositol 3-kinase (PI3-K), also prohibited HCMV-mediated activation of Akt and NFkappaB and synthesis of the MIE proteins, PI3-K signalling was necessary for expressing the MIE genes. These results suggest that the inhibitory effect of GA on HCMV replication is primarily caused by the disruption of the PI3-K signalling pathway, leading to the activation of NFkappaB, which plays a crucial role in expression of the critical MIE genes.

The UL97 protein kinase of human cytomegalovirus and homologues in other herpesviruses: impact on virus and host

The human herpesviruses, herpes simplex virus 1 (HSV-1), HSV-2, varicella zoster virus (VZV), Epstein-Barr virus (EBV), human cytomegalovirus (HCMV), human herpesvirus 6A (HHV-6A), HHV-6B, HHV-7 and HHV-8, establish persistent infections with possible recurrence during immunosuppression. HCMV replication is inhibited by the nucleoside analogue ganciclovir (GCV), the compound of choice for the treatment of HCMV diseases and preemptive treatment of infections. The viral UL97 protein (pUL97) which shares homologies with protein kinases and bacterial phosphotransferases is able to monophosphorylate GCV. Homologues of pUL97 are found in HSV (UL13), VZV (ORF47), EBV (BGLF4), HHV-6 (U69), HHV-8 (ORF36) as well as in murine CMV (M97) or rat CMV (R97). Several indolocarbazoles have been reported to be specific inhibitors of pUL97. The protein is important for efficient replication of the virus. Autophosphorylation of pUL97 was observed using different experimental systems. Most recently, it has been shown that pUL97 interacts with the DNA polymerase processivity factor pUL44. Indolocarbazole protein kinase inhibitors are promising lead compounds for the development of more specific inhibitors of HCMV.

Mapping ganciclovir resistance in the human herpesvirus-6 U69 protein kinase

Human herpesvirus-6 (HHV-6) is a growing concern in immunocompromised individuals, such as in the transplant setting. Alone, or in concert with human cytomegalovirus (HCMV), infections with HHV-6 are often severe enough to require antiviral therapy, generally in the form of ganciclovir (GCV). GCV resistance in HCMV is well documented, both clinically and in the laboratory, and has been shown to result from mutations in the UL97 protein kinase and/or UL54 DNA polymerase. GCV resistance in HHV-6 has been documented. However, to date, it has only been investigated to a limited extent. The baculovirus system has previously been shown to be useful in studying GCV resistance with respect to herpesvirus protein kinase mutations. Using the baculovirus system, we created recombinant baculoviruses expressing either a wild-type HHV-6 U69 protein kinase or a mutated form containing homologous mutations to those documented in the UL97 protein kinase of GCV resistant HCMV isolates. The recombinant baculoviruses were used to infect Sf-9 cells and cultured in the presence of GCV to determine the effect of the HHV-6 U69 protein kinase mutations on GCV susceptibility. Mutations in the HHV-6 U69 protein kinase, homologous to those in the HCMV UL97 protein kinase documented to cause GCV resistance, result in GCV resistance in the recombinant baculoviruses.

Current and potential therapies for the treatment of herpes-virus infections

Human herpesviruses are found worldwide and are among the most frequent causes of viral infections in immunocompetent as well as in immunocompromised patients. During the past decade and a half a better understanding of the replication and disease-causing state of herpes simplex virus types 1 and 2 (HSV-1 and HSV-2), varicella zoster virus (VZV), and human cytomegalovirus (HCMV) has been achieved due in part to the development of potent antiviral compounds that target these viruses. While some of these antiviral therapies are considered safe and efficacious (acyclovir, penciclovir), some have toxicities associated with them (ganciclovir and foscarnet). In addition, the increased and prolonged use of these compounds in the clinical setting, especially for the treatment of immunocompromised patients, has led to the emergence of viral resistance against most of these drugs. While resistance is not a serious issue for immunocompetent individuals, it is a real concern for immunocompromised patients, especially those with AIDS and the ones that have undergone organ transplantation. All the currently approved treatments target the viral DNA polymerase. It is clear that new drugs that are more efficacious than the present ones, are not toxic, and target a different viral function would be of great use especially for immunocompromised patients. Here, an overview is provided of the diseases caused by the herpesviruses as well as the replication strategy of the better studied members of this family for which treatments are available. We also discuss the various drugs that have been approved for the treatment of some herpesviruses in terms of structure, mechanism of action, and development of resistance. Finally, we present a discussion of viral targets other than the DNA polymerase, for which new antiviral compounds are being considered.

Expression of an RNase P ribozyme against the mRNA encoding human cytomegalovirus protease inhibits viral capsid protein processing and growth

A sequence-specific ribozyme (M1GS RNA) derived from the catalytic RNA subunit of RNase P from Escherichia coli was used to target the mRNA encoding human cytomegalovirus (HCMV) protease (PR), a viral protein that is responsible for the processing of the viral capsid assembly protein. We showed that the constructed ribozyme cleaved the PR mRNA sequence efficiently in vitro. Moreover, a reduction of about 80% in the expression level of the protease and a reduction of about 100-fold in HCMV growth were observed in cells that expressed the ribozyme stably. In contrast, a reduction of less than 10% in the expression of viral protease and viral growth was observed in cells that either did not express the ribozyme or produced a catalytically inactive ribozyme mutant. Further examination of the antiviral effects of the ribozyme-mediated cleavage of PR mRNA indicates that (1) the proteolytic cleavage of the capsid assembly protein is inhibited significantly, and (2) the packaging of the viral genomic DNA into the CMV capsids is blocked. These observations are consistent with the notion that the protease functions to process the capsid assembly protein and is essential for viral capsid assembly. Moreover, our results indicate that the RNase P ribozyme-mediated cleavage specifically reduces the expression of the protease, but not other viral genes examined. Thus, M1GS ribozyme is highly effective in inhibiting HCMV growth by targeting the PR mRNA and may represent a novel class of general gene-targeting agents for the studies and treatment of infections caused by human viruses, including HCMV.

Mutations in the UL97 ORF of ganciclovir-resistant clinical cytomegalovirus isolates differentially affect GCV phosphorylation as determined in a recombinant vaccinia virus system

Mutations in the human cytomegalovirus (HCMV) UL97 phosphotransferase have been associated with ganciclovir (GCV) resistance due to an impairment of GCV monophosphorylation. Vaccinia virus recombinants (rVV) were generated that encoded different HCMV UL97 proteins (pUL97) with mutations previously detected in resistant HCMV clinical isolates at codons 460, 520, 592, 594, 595, 598 and 607. These rVVs allowed quantification of GCV phosphorylation catalyzed by the different mutated pUL97s. When compared to rVV-UL97 wild type, mean levels of residual intracellular GCV phosphorylation differed by a factor of 10 for the mutated UL97 proteins ranging from 5.2 to 51.8%. Mutations M460V (located in a UL97 region homologous to domain VIb of protein kinases) and H520Q (located in a cytomegalovirus-specific, functionally critical domain) were responsible for the lowest levels of residual GCV phosphorylation (9.3 and 5.2%). Mutations in a region homologous to the domain IX had a lower impact on GCV phosphorylation (15.8-51.8%). The relevance of pUL97 mutation G598S in inducing GCV resistance was demonstrated for the first time.

Resistance of herpesviruses to antiviral drugs: clinical impacts and molecular mechanisms

Nucleoside analogues such as acyclovir and ganciclovir have been the mainstay of therapy for alphaherpesviruses (herpes simplex virus (HSV) and varicella-zoster virus (VZV)) and cytomegalovirus (CMV) infections, respectively. Drug-resistant herpesviruses are found relatively frequently in the clinic, almost exclusively among severely immunocompromised patients receiving prolonged antiviral therapy. For instance, close to 10% of patients with AIDS receiving intravenous ganciclovir for 3 months excrete a drug-resistant CMV isolate in their blood or urine and this percentage increases with cumulative drug exposure. Many studies have reported that at least some of the drug-resistant herpesviruses retain their pathogenicity and can be associated with progressive or relapsing disease. Viral mutations conferring resistance to nucleoside analogues have been found in either the drug activating/phosphorylating genes (HSV or VZV thymidine kinase, CMV UL97 kinase) and/or in conserved regions of the viral DNA polymerase. Currently available second line agents for the treatment of herpesvirus infections--the pyrophosphate analogue foscarnet and the acyclic nucleoside phosphonate derivative cidofovir--also inhibit the viral DNA polymerase but are not dependent on prior viral-specific activation. Hence, viral DNA polymerase mutations may lead to a variety of drug resistance patterns which are not totally predictable at the moment due to insufficient information on specific drug binding sites on the polymerase. Although some CMV and HSV DNA polymerase mutants have been found to replicate less efficiently in cell cultures, further research is needed to correlate viral fitness and clinical outcome.

GCV resistance due to the mutation A594P in the cytomegalovirus protein UL97 is partially reconstituted by a second mutation at D605E

A ganciclovir (GCV)-resistant human cytomegalovirus (HCMV) was isolated from an AIDS patient. Molecular analysis of the HCMV UL97 gene revealed two point mutations, A594P and D605E, respectively. In order to evaluate quantitatively the impact of the individual mutations on GCV phosphorylation, recombinant vaccinia viruses (rVVs) were generated carrying either the two mutations (rVV-594/605) or only one mutation (rVV-594 or rVV-605, respectively). In cells infected with the rVV-594/605 double mutant, the GCV phosphorylation was decreased to 50% compared with the phosphorylation in cells infected with the rVV-UL97 wild-type. In cells infected with the rVV-594, however, the GCV phosphorylation was further decreased to 30%. Interestingly, the mutation D605E led to an even better GCV phosphorylation than that measured in cells infected with the rVV-UL97 wild type. These results were confirmed by plaque reduction assays, indicating that rVV-594 was more resistant to GCV than rVV-594/605. In contrast, rVV-605 was more sensitive to GCV than the rVV-UL97 wild type. Therefore, our results demonstrated for the first time that compensatory mutations can also occur in HCMV, as already shown for human immunodeficiency virus type 1.

Engineered RNase P ribozymes inhibit gene expression and growth of cytomegalovirus by increasing rate of cleavage and substrate binding

We have previously employed an in vitro (genetic) selection procedure to select RNase P ribozyme variants for their activity in cleaving a mRNA substrate from a pool of ribozymes containing randomized sequences. In this study, one of the variants was used to target the overlapping region of the mRNAs encoding the major transcription regulatory proteins, IE1 and IE2, of human cytomegalovirus (HCMV). The ribozyme variant exhibited an enhanced substrate binding and rate of chemical cleavage, and was at least 25 times more efficient in cleaving the target mRNA in vitro than the ribozyme derived from the wild-type sequence. Our results provide the first direct evidence that a point mutation at nucleotide 86 of RNase P catalytic RNA from Escherichia coli (A(86)-->C(86)) increases the rate of chemical cleavage while another mutation at nucleotide 205 (G(205)-->C(205)) enhances substrate binding of the ribozyme. Moreover, the variant was also more effective in inhibiting IE1 and IE2 expression and HCMV growth in cultured cells. A reduction of more than 97% in IE1 and IE2 expression and a reduction of 3000-fold in viral growth were observed in cells expressing the variant. Thus, RNase P ribozyme variant is highly effective in inhibiting HCMV gene expression and growth. Our results provide the direct evidence that increasing the rate of chemical cleavage and substrate-binding affinity of the ribozymes should lead to an improvement of their anti-HCMV efficacy. Moreover, our data also suggest that highly effective anti-HCMV ribozyme variants can be developed using genetic engineering approaches including in vitro selection.

Variations in the cytomegalovirus DNA polymerase and phosphotransferase genes in relation to foscarnet and ganciclovir sensitivity

BACKGROUND

Identification of human cytomegalovirus (CMV) genome variation is important for understanding mutations associated with drug resistance.

OBJECTIVES

To investigate the CMV resistance to foscarnet (PFA) and ganciclovir (GCV) in patients treated with antiviral drugs and to identify the DNA polymerase (UL54) and phosphotransferase (UL97) gene mutations inducing resistance.

STUDY DESIGN

Antiviral susceptibility of CMV strains/isolates for PFA and GCV was compared by plaque reduction assay and in situ ELISA. UL54 and UL97 gene mutations were identified by sequencing. Growth phenotype of two CMV recombinants with mutations in UL54 was studied.

RESULTS

Six of seven GCV resistant strains had alterations within the UL97. Five of them also had alterations in the UL54 (F412C, L802M or K513E), previously shown to induce GCV resistance. Seven isolates had no or reduced susceptibility to PFA, which had alterations in the UL54 (D588N, E756K, V781I or L802M). By in vitro mutagenesis, it was shown that a mutation at codon D588N of UL54 conferred 9-fold reduced susceptibility to PFA, while a mutation at codon V781I induced 4-fold reduced susceptibility to PFA and GCV. Both recombinants showed the same kinetics of protein expression (IE, E, and L antigen) and virus yields as the CMV Towne strain.

CONCLUSIONS

The recombinants containing alterations within the UL54 (D588N and V781I) showed a reduced susceptibility to antiviral drugs but no change in the replication rate compared to the CMV Towne.

Sequence analysis of UL54 and UL97 genes and evaluation of antiviral susceptibility of human cytomegalovirus isolates obtained from kidney allograft recipients before and after treatment

The frequency of infections caused by drug-resistant cytomegalovirus (CMV) in solid-organ transplant recipients is not known. Only a few resistant strains have been described in transplant recipients. Antiviral susceptibility to ganciclovir (GCV) and foscarnet (PFA) of CMV isolates from 24 renal transplant patients with CMV viremia and CMV disease before and after therapy were investigated by a solid phase ELISA. The CMV DNA polymerase (UL54) and viral phosphotransferase (UL97) genes were also sequenced. Ten patients did not receive antiviral treatment; five and nine patients were treated with PFA and GCV, respectively. No appearance of drug-resistant viruses was observed in the present study, but one isolate showed a reduced sensitivity to PFA after treatment with GCV. This finding could not be explained by the presence or development of mutations that have been associated with drug resistance in UL54. We found no evidence that short-term treatment of CMV with PFA- or GCV-induced resistance.

Current and potential therapies for the treatment of herpesvirus infections

Human herpesviruses are found worldwide and are among the most frequent causes of viral infections in immunocompetent as well as in immunocompromised patients. During the past decade and a half a better understanding of the replication and disease causing state of herpes simplex virus types 1 and 2 (HSV-1 and HSV-2), varicella-zoster virus (VZV), and human cytomegalovirus (HCMV) has been achieved due in part to the development of potent antiviral compounds that target these viruses. While some of these antiviral therapies are considered safe and efficacious (acyclovir, penciclovir), some have toxicities associated with them (ganciclovir and foscarnet). In addition, the increased and prolonged use of these compounds in the clinical setting, especially for the treatment of immunocompromised patients, has led to the emergence of viral resistance against most of these drugs. While resistance is not a serious issue for immunocompetent individuals, it is a real concern for immunocompromised patients, especially those with AIDS and the ones that have undergone organ transplantation. All the currently approved treatments target the viral DNA polymerase. It is clear that new drugs that are more efficacious than the present ones, are not toxic, and target a different viral function would be of great use especially for immunocompromised patients. Here, we provide an overview of the diseases caused by the herpesviruses as well as the replication strategy of the better studiedmembers of this family for which treatments are available. We also discuss the various drugs that have been approved for the treatment of some herpesviruses in terms of structure, mechanism of action, and development of resistance. Finally, we present a discussion of viral targets other than the DNA polymerase, for which new antiviral compounds are being considered.

Cytomegalovirus treatment options in immunocompromised patients

Cytomegalovirus (CMV) infection was recognised in congenitally infected infants in the first half of the 20th century. Following the increased use of immunosuppressive regimens for bone marrow and solid organ transplantation, various manifestations of CMV disease were recognised. Milder symptoms included fever, anorexia and malaise but severe symptoms included pneumonitis, hepatitis, gastrointestinal ulceration, choreoretinitis and encephalopathy, all with a high morbidity or mortality. With the onset of the AIDS epidemic, manifestations of CMV became evident, predominantly retinitis. Ganciclovir used intravenously has been the principal anti-CMV agent investigated. However, ganciclovir has problems with suboptimal efficacy, toxicity, poor oral bioavailability and evolution of resistant strains. Additional studies have been performed on foscarnet and cidofovir, although the use of both have been limited by their nephrotoxicity. Combination therapy with ganciclovir and foscarnet for resistant strains has been used. There are promising newer drugs like the methylenecyclopropane nucleoside analogues and benzimidazole. The most novel compound is the antisense oligonucleotide fomivirsen that has been evaluated principally in CMV retinitis. The role of immunotherapy with either immunoglobulin prophylaxis or the novel adoptive immunotherapy needs further evaluation.

Emergence of multiple human cytomegalovirus ganciclovir-resistant mutants with deletions and substitutions within the UL97 gene in a patient with severe combined immunodeficiency

Infection with multiple ganciclovir-resistant human cytomegalovirus mutants, containing different substitutions and deletions in the UL97 gene, was found in a patient with severe combined immunodeficiency (SCID) within 3 weeks of ganciclovir therapy. A novel 11-codon deletion at positions 590 to 600 was identified. These unique findings may be related to the nature of the immunodeficiency in the SCID patient.

Current and potential therapies for the treatment of herpesvirus infections

Human herpesviruses are found worldwide and are among the most frequent causes of viral infections in immunocompetent as well as in immunocompromised patients. During the past decade and a half a better understanding of the replication and disease causing state of herpes simplex virus types 1 and 2 (HSV-1 and HSV-2), varicella-zoster virus (VZV), and human cytomegalovirus (HCMV) has been achieved due in part to the development of potent antiviral compounds that target these viruses. While some of these antiviral therapies are considered safe and efficacious (acyclovir, penciclovir), some have toxicities associated with them (ganciclovir and foscarnet). In addition, the increased and prolonged use of these compounds in the clinical setting, especially for the treatment of immunocompromised patients, has led to the emergence of viral resistance against most of these drugs. While resistance is not a serious issue for immunocompetent individuals, it is a real concern for immunocompromised patients, especially those with AIDS and the ones that have undergone organ transplantation. All the currently approved treatments target the viral DNA polymerase. It is clear that new drugs that are more efficacious than the present ones, are not toxic, and target a different viral function would be of great use especially for immunocompromised patients. Here, we provide an overview of the diseases caused by the herpesviruses as well as the replication strategy of the better studied members of this family for which treatments are available. We also discuss the various drugs that have been approved for the treatment of some herpesviruses in terms of structure, mechanism of action, and development of resistance. Finally, we present a discussion of viral targets other than the DNA polymerase, for which new antiviral compounds are being considered.

Dosing of intravenous ganciclovir for the prophylaxis and treatment of cytomegalovirus infection in solid organ transplant recipients

BACKGROUND

The optimal regimen for the prevention and treatment of cytomegalovirus (CMV) disease in solid organ transplant recipients remains to be defined, particularly for patients with abnormal or changing renal function.

METHODS

A prospective trial was conducted in patients receiving i.v. ganciclovir using a standardized dosing nomogram that corrects for renal function. Steady state peak (P) and trough (T) serum levels were determined by high-performance liquid chromatography and correlated with therapeutic outcomes and toxicities attributable to ganciclovir.

RESULTS

Over the study period, 44 individuals received ganciclovir prophylaxis (5 mg(kg/day) and 25 patients were treated (5 mg/kd q12 hr) for symptomatic CMV disease. Ganciclovir levels (microg/ml+/-SD) achieved in prophylaxis were P: 7.98+/-3.34, T: 3.03+/-2.63; and in treatment were P: 9.00+/-3.72, T: 2.65+/-1.82. Despite corrections for renal dysfunction, undialyzed patients with serum creatinine >3.0 mg/dl had trough levels in excess of the population mean (T: range 3-8 microg/ml). Failure of prophylaxis (disease) or therapy (relapse) occurred in 14 patients; 8 of these were at risk for primary infection (donor CMV seropositive, recipient seronegative, P<0.01). Patients at greatest risk for relapse after treatment of CMV disease were liver transplant recipients, patients with ganciclovir-resistant viral isolates, and renal patients with six antigen MHC donor-recipient mismatches.

CONCLUSIONS

This trial demonstrates the efficacy of a nomogram for ganciclovir dosing during renal dysfunction; reduced doses can be used for prophylaxis for undialyzed patients with renal dysfunction (1.25 mg/kg/day for Cr > or =3.0, 1.25 mg/kg QOD for Cr > or =5.0). Some groups of transplant recipients may require more intensive anti-CMV regimens.

Comparison of human cytomegalovirus DNA polymerase activity for ganciclovir-resistant and -sensitive clinical strains

Previously, we found that a ganciclovir (GCV)-resistant clinical human cytomegalovirus (HCMV) isolate had an amino acid substitution at codon 501 (Leu --> Phe) in the delta-region C of the DNA polymerase gene. DNA polymerases have now been (partially) purified from both the GCV-resistant and sensitive parental strains and the activity of DNA polymerase and 3'-5' exonuclease compared. With respect to DNA polymerase activity, the Michaelis constant (Km) and maximum velocity (Vmax) of the GCV-resistant strain for the DNA template were lower than those of the GCV-sensitive strain. With respect to 3'-5' exonuclease activity, the Km and Vmax of the GCV-resistant strain for the DNA substrate in the presence of ammonium sulfate were lower than those of the GCV-sensitive strain, while being similar in the absence of ammonium sulfate. Although the polymerase activity of the two strains showed almost the same sensitivity for the different polymerase inhibitors, the 3'-5' exonuclease activity of the GCV-resistant strain was more resistant to these inhibitors than that of the GCV-sensitive strain.

New strategies for prevention and therapy of cytomegalovirus infection and disease in solid-organ transplant recipients

In the past three decades since the inception of human organ transplantation, cytomegalovirus (CMV) has gained increasing clinical import because it is a common pathogen in the immunocompromised transplant recipient. Patients may suffer from severe manifestations of this infection along with the threat of potential fatality. Additionally, the dynamic evolution of immunosuppressive and antiviral agents has brought forth changes in the natural history of CMV infection and disease. Transplant physicians now face the daunting task of recognizing and managing the changing spectrum of CMV infection and its consequences in the organ recipient. For the microbiology laboratory, the emphasis has been geared toward the development of more sophisticated detection assays, including methods to detect emerging antiviral resistance. The discovery of novel antiviral chemotherapy is an important theme of clinical research. Investigations have also focused on preventative measures for CMV disease in the solid-organ transplant population. In all, while much has been achieved in the overall management of CMV infection, the current understanding of CMV pathogenesis and therapy still leaves much to be learned before success can be claimed.

A nine-codon deletion mutation in the cytomegalovirus UL97 phosphotransferase gene confers resistance to ganciclovir

A deletion mutation (codons 595 to 603) in the cytomegalovirus (CMV) UL97 gene was recently reported after sequence analysis of leukocyte DNA from a patient receiving ganciclovir. The corresponding viral phenotype was examined by transfer of this mutation to a laboratory CMV strain (strain Towne). The recombinant virus was resistant to ganciclovir (8.4-fold increase in the 50% inhibitory concentration), was sensitive to foscarnet, and replicated normally in cell culture.

Human cytomegalovirus: challenges, opportunities and new drug development

In the age of highly active antiretroviral therapy, the incidence of human cytomegalovirus (HCMV) retinitis in AIDS patients has decreased substantially. However, this change does not indicate that HCMV disease in AIDS patients and other immunocompromised patients has abated and is no longer a concern. On the contrary, HCMV disease in graft recipients, newborns, and even in AIDS patients still accounts for considerable morbidity, and drug resistance to the anti-HCMV compounds is a major problem. Furthermore, HCMV may have a role in metabolic diseases, such as atherosclerosis. Fortunately there are novel and potentially very effective new compounds undergoing pre-clinical and clinical evaluation. These developments point the way toward new therapies and also to a clearer understanding of the biology of HCMV replication, infection and disease.

Acyclovir is phosphorylated by the human cytomegalovirus UL97 protein

Acyclovir (ACV) has shown efficacy in the prophylactic suppression of human cytomegalovirus (HCMV) reactivation in immunocompromised renal transplant patients without the toxicity associated with ganciclovir (GCV). The HCMV UL97 gene product, a protein kinase, is responsible for the phosphorylation of GCV in HCMV-infected cells. This report provides evidence for the phosphorylation of ACV by UL97. Anabolism studies with the HCMV wild-type strain AD169 and with recombinant mutants derived from marker transfer experiments performed by using mutant UL97 DNA from both clinical isolates and a laboratory-derived strain resistant to GCV showed that mutations in the UL97 gene cripple the ability of recombinant virus-infected cells to anabolize both GCV and ACV. These mutant UL97 recombinant viruses were less susceptible to both GCV and ACV than was the wild-type strain. A recombinant herpes simplex virus type 1 strain, in which the thymidine kinase gene is deleted and the UL13 gene is replaced with the HCMV UL97 gene, was able to induce the phosphorylation of ACV in infected cells. Finally, purified UL97 phosphorylated both GCV and ACV to their monophosphates. Our results indicate that UL97 promotes the selective activity of ACV against HCMV.

Rapid detection by reverse hybridization of mutations in the UL97 gene of human cytomegalovirus conferring resistance to ganciclovir

BACKGROUND OF STUDY: Diseases due to human cytomegalovirus (HCMV) infection constitute a major threat in marrow and solid organ transplant recipients. Ganciclovir (GCV) is widely used in prophylaxis and pre-emptive therapy of active HCMV infection. Resistance to ganciclovir (GCV) may arise at variable frequency under GCV therapy and is conferred by mutations (i) in the UL97 gene (codons 460, 520, and 591-607) encoding a phosphotransferase which is essential for monophosphorylation of GCV and, to a lesser extent, (ii) in the UL54 gene coding for the DNA polymerase of HCMV.

OBJECTIVE

The purpose was to develop a rapid assay to screen for emerging GCV resistance mutations in the UL97 gene of HCMV whereby avoiding virus isolation and nucleotide sequencing procedures.

STUDY DESIGN

A nested PCR (nPCR) amplifying UL97 codons 450-672 was developed. Nested amplicons were subsequently sequenced directly. Oligonucleotides for use in a reverse hybridization assay were designed to detect relevant non-synonymous mutations at codons UL97 460, 520, 603 and 607. Strain AD169 served as a wild-type control.

RESULTS

UL97-specific nPCR amplicons were obtained from 18 EDTA blood samples of ten transplant recipients receiving GCV for more than 30 days. In three consecutive samples from a single patient a GCV resistance mutation at codon 603 (C-->W) was detected. In addition, two out of four cell culture-adapted HCMV isolates known to exhibit GCV resistance in vitro revealed mutations at codons 460 (M-->V) and 607 (C-->Y), respectively. By reverse hybridization a discrimination of single nucleotide changes at codons 460, 520, 603 and 607 was possible whereby matching exactly the results of the nucleotide sequence analysis for all 23 amplicons examined.

CONCLUSIONS

Reverse hybridization appeared to be a rapid and convenient alternative to nucleotide sequencing when screening the UL97 gene of HCMV for selected markers of GCV resistance.

Antiviral drug resistance in human cytomegalovirus

Drug-resistant cytomegalovirus (CMV) should be considered when viral shedding persists after several weeks of therapy. The problem is most likely to arise in the setting of a severely immunosuppressed host with continuing or relapsing disease. Not all treatment failure can be attributed to drug resistance. The testing of CMV isolates for drug resistance in cell culture is time-consuming and labor-intensive, but recent advances in understanding of the genetics of resistance have resulted in rapid genotypic assays for specific mutations in the viral UL97 phosphotransferase or UL54 DNA polymerase genes that can predict resistance and cross-resistance to specific drugs. This information may help in the selection of alternative therapy.

Inhibitory effect of 4-(4-fluorophenyl)-2-(4-hydroxyphenyl)-5-(4-pyridyl)1H - imidazole on HCMV DNA replication and permissive infection

We found that Human Cytomegalovirus (HCMV) infection of human fibroblasts resulted in a dramatic increase in p38 mitogen-activated protein kinase (MAPK) phosphorylation. Recently, drug mediated inhibition of p38 has been demonstrated to exhibit anti-viral activity against HIV (Shapiro, L., Heidenreich, K.A., Meintzer, M.D. and Dinarello, C.A., 1998. Role of p38 mitogen-activated protein kinase in HIV type 1 production in vitro. Proc. Natl. Acad. Sci. USA. 95, 7422-7426). Therefore, we examined the effect of a specific p38 kinase inhibitor on HCMV infection. Inhibiting p38 activity in HCMV infected cells by treating cells with 4-(4-fluorophenyl)-2-(4-hydroxyphenyl)-5-(4-pyridyl)1H-imidazole; (FHPI), a p38 inhibitor drug, prevented permissive HCMV infection as measured by plaque assay. In the presence of FHPI, HCMV immediate early gene expression was slightly lower at early times of infection, but there was no inhibition of expression of the early gene UL-84, an HCMV protein essential for viral replication. However, FHPI inhibited HCMV DNA replication and late gene expression. The inhibitory effect of FHPI was reversible, as demonstrated by the induction of HCMV replication upon withdrawal of FHPI. Our data describes FHPI as a novel anti-HCMV compound that inhibits synthesis/activation of cellular and/or viral factors required for initiation of HCMV DNA replication.

Resistance of human cytomegalovirus to antiviral drugs

Resistance of cytomegalovirus (CMV) to antiviral agents is a well-recognized phenomenon that has been observed in the laboratory and in the clinical setting. Infections caused by antiviral-resistant CMV have been found exclusively among immunocompromised individuals, including patients with AIDS, bone marrow and solid-organ transplant recipients, and patients with hematologic malignancies, and in individuals with primary immunodeficiencies. The majority of these infections have been described to occur in patients with AIDS receiving prolonged antiviral therapy for CMV end-organ disease. Antiviral agents currently licensed for the treatment of CMV infections include ganciclovir, foscarnet, and cidofovir. Resistance of CMV to ganciclovir is related to mutations in the UL97 region of the viral genome and/or mutations in the viral DNA polymerase. Resistance to foscarnet and cidofovir is associated with mutations in the viral DNA polymerase. Antiviral susceptibility of CMV strains containing DNA polymerase mutations is dependent on the region of the DNA polymerase where the mutations are located. Some DNA polymerase mutant viruses are cross-resistant to ganciclovir, foscarnet, and cidofovir. The recognition that specific UL97 and UL54 mutations are associated with resistance to antiviral agents has led to the development of molecular methods for detection of mutant viruses. This article reviews the mechanisms of resistance of CMV to antiviral agents, the laboratory methods for detection of resistant CMV, and the clinical aspects of infections caused by antiviral-resistant CMV.

Novel mutation in the CMV UL97 gene associated with resistance to ganciclovir therapy

Cytomegalovirus (CMV) strains resistant to ganciclovir have been associated with specific mutations in the UL97 and UL54 genes. The UL97 gene of a CMV strain isolated from a renal transplant recipient before and after 438 days of ganciclovir treatment was amplified by polymerase chain reaction and sequenced. A novel mutation resulting in deletion of codons 595 to 603 was identified in the viral DNA from specimens obtained after, but not before, prolonged ganciclovir therapy. Clinical and virological resolution of CMV disease occurred after switching to foscarnet therapy. Although many ganciclovir resistance mutations have been mapped to the UL97 codon range 591-607, this one is unusual in that it involves deletion of half these codons. Because UL97 seems to be necessary for effective CMV replication, this deletion suggests that much of codons 591-607 can be removed without destroying the biological function of UL97, and that this codon range can be altered in various ways to affect ganciclovir susceptibility. Rapid, flexible genotypic assays directed at this part of UL97 may facilitate the early recognition of ganciclovir resistance.

Human cytomegalovirus infection of human hematopoietic progenitor cells

For a number of years it has been well established that human cytomegalovirus (HCMV) can be transmitted by the cellular components of blood. HCMV is also associated with a number of hematologic disorders. Although HCMV was thought to be present in blood cells in a latent or persistent form, it was not known how the virus was maintained and which cells were the carriers of HCMV. In addition to peripheral blood cells, there has been clinical evidence that HCMV may be associated with specific disorders of the hematopoietic system. Recently, a number of advances in cell and molecular biology have helped to develop a better understanding of the relationship between HCMV and the hematopoietic system. The application of the polymerase chain reaction (PCR) to the study of HCMV infection has revealed that the virus was present in mononuclear cells with only limited transcription of its genome. Studies conducted in our laboratory have demonstrated that both CD34+ progenitor cells and monocytes could be infected with HCMV and virus recovered when the cells were allowed to terminally differentiate. Subsequently, these results have been confirmed in vivo: HCMV DNA and limited RNA transcripts could be detected in in vivo infected hematopoietic progenitor cells and HCMV has been rescued from macrophages derived through in vitro differentiation of monocytes from normal seropositive blood donors. Although our understanding of the relationship between HCMV and the hematopoietic system has been advanced, the mechanisms by which the virus can be maintained in a latent state and how it is reactivated is still unclear. Furthermore, it remains to be determined what HCMV-mediated effect is responsible for the inhibition of hematopoiesis following an in vitro infection and its significance in vivo.