Natural polymorphism of cytomegalovirus DNA polymerase lies in two nonconserved regions located between domains delta-C and II and between domains III and I

CMV-induced Hearing Loss

Congenital cytomegalovirus (cCMV) infection is the most common fetal viral infection and contributes to about 25% of childhood hearing loss by the age of 4 years. It is the leading nongenetic cause of sensorineural hearing loss (SNHL). Infants born to seroimmune mothers are not completely protected from SNHL, although the severity of their hearing loss may be milder than that seen in those whose mothers had a primary infection. Both direct cytopathic effects and localized inflammatory responses contribute to the pathogenesis of cytomegalovirus (CMV)-induced hearing loss. Hearing loss may be delayed onset, progressive or fluctuating in nature, and therefore, a significant proportion will be missed by universal newborn hearing screening (NHS) and warrants close monitoring of hearing function at least until 5-6 years of age. A multidisciplinary approach is required for the management of hearing loss. These children may need assistive hearing devices or cochlear implantation depending on the severity of their hearing loss. In addition, early intervention services such as speech or occupational therapy could help better communication, language, and social skill outcomes. Preventive measures to decrease intrauterine CMV transmission that have been evaluated include personal protective measures, passive immunoprophylaxis and valacyclovir treatment during pregnancy in mothers with primary CMV infection. Several vaccine candidates are currently in testing and one candidate vaccine in phase 3 trials. Until a CMV vaccine becomes available, behavioral and educational interventions may be the most effective strategy to prevent maternal CMV infection.

Genotypic and phenotypic study of antiviral resistance mutations in refractory cytomegalovirus infection

This study describes the genotypic and phenotypic characterisation of novel human cytomegalovirus (HCMV) genetic variants of a cohort of 94 clinically-resistant HCMV patients. Antiviral-resistant mutations were detected in the UL97, UL54 and UL56 target genes of 25/94 (26.6%) patients. The genotype-phenotype correlation study resolved the status of 5 uncharacterised UL54 DNA polymerase (G441S, A543V, F460S, R512C, A928T) and 2 UL56 terminase (F345L, P800L) mutations found in clinical isolates. A928T conferred high triple-resistance to ganciclovir, foscarnet and cidofovir, and A543V had 10-fold reduced susceptibility to cidofovir. Viral growth assays showed G441S, A543V, F345L and P800L impaired viral growth capacities compared with wild-type AD169 HCMV. 3D modelling predicted A543V and A928T phenotypes but not R512C, reinforcing the need for individual characterisation of mutations by recombinant phenotyping. Extending mutation databases is crucial to optimize treatments and to improve the assessment of patients with resistant/refractory HCMV infection.

Cytomegalovirus Infection and the Implications of Drug-Resistant Mutations in Pediatric Allogeneic Hematopoietic Stem Cell Transplant Recipients: A Retrospective Study from a Tertiary Hospital in China

INTRODUCTION

Drug-resistant cytomegalovirus (CMV) infection remains a challenge in the management of pediatric recipients of hematopoietic stem cell transplantation (HSCT). In this study, we retrospectively reviewed the clinical data on pediatric recipients of HSCT and identified known and unknown drug-resistant CMV variants.

METHODS

A total of 221 children underwent allogeneic HSCT between October 2017 and November 2019 at Shenzhen Children's Hospital; of these, 35 patients were suspected of having drug-resistant CMV infections and were tested for drug-resistant mutations in the UL97 and UL54 genes by Sanger sequencing.

RESULTS

Mutations in UL97 or UL54, or in both, were detected in 11 patients. Most of these mutations have not been previously reported. The UL97 mutation (A582V) was detected in only one patient who also harbored two UL54 mutations (T760X and R876W). One patient with both the G604S and T691A mutations in the UL54 gene died of CMV pneumonia. We investigated the risk factors associated with the development of drug-resistant CMV infection. Patients in whom both the donor and recipient had positive CMV serostatuses were less likely to have drug-resistant mutations (Fisher's exact test, p < 0.05).

CONCLUSION

Newly and previously detected CMV mutations in UL97 and UL54 may be associated with the development of drug-resistant CMV infection. The detection of these mutations may provide guidance for the management of post-transplant CMV infections.

Human cytomegalovirus UL54 and UL97 mutations for detection of ganciclovir resistance in congenital infection

Aim: Ganciclovir (GCV) is used as an antiviral drug for the treatment of human cytomegalovirus infection. The aim of this study was to demonstrate GCV-resistant human cytomegalovirus in congenitally infected neonates. Patients & methods: DNA of CMV positive newborn samples was extracted and UL97 and UL54 genes were amplified by PCR and real-time PCR. Sequencing of UL97 and UL54 genes were performed and analyzed. Results: UL97 GCV resistance mutation C603W was detected in one newborn. D605E was the most common polymorphism in UL97 observed in 7/13 (53.8%) of samples. N685S, A688V, A885T and N898D were four known common UL54 polymorphisms. Conclusion: The rate of GCV resistance in congenital CMV is low. Common polymorphisms in UL97 and UL54 genes are also reported as new mutations.

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.

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.

Functional characterization of novel mutations in UL54 of ganciclovir resistant HCMV strain using structural analysis

This study reports the probable impact of the coupled mutations observed in our clinical isolate of HCMV UL54 polymerase, through structural bioinformatics approaches. The reported variant was found to be resistant to Ganciclovir (GCV) as per the clinical records. The presence of Glutamine deletion at 639 (Glu639) and a mis sense mutation of Serine 655 Leucine (Ser655Leu) in UL54 were identified by DNA sequencing and were predicted to lie in the DNA polymerase type-II domain. Docking simulation studies of the phosphorylated forms of Ganciclovir (GCV), Cidofovir (CDV) and Foscarnet (PFA) with the reported mutants showed significant variation in terms of binding affinity and inhibitory constant (Ki) in comparison to wild type UL54. The findings of this study revealed that the observed coupled mutation could potentially induce allosteric effects in the binding pockets of UL54 and thereby alter the drug binding affinity. In specific, it was observed that this coupled mutation could confer changes in the binding affinity of GCV and PFA by altering the binding energies and inhibitory constants to -0.88Kcal/mol and 226.71mM, -5.81Kcal/mol and 54.83µM, respectively, in comparison to Wild Type. On the other hand, CDV showed increased susceptibility for the reported mutant with a binding energy of -6.16Kcal/mol and inhibitory constant of 30.47µM.

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.

Drug-resistant cytomegalovirus in transplant recipients: a French cohort study

OBJECTIVES

Cytomegalovirus (CMV) drug resistance is a therapeutic challenge in the transplant setting. No longitudinal cohort studies of CMV resistance in a real-life setting have been published in the valganciclovir era. We report findings for a French multicentre prospective cohort of 346 patients enrolled at initial diagnosis of CMV infection (clinical trial registered at clinicaltrials.gov: NCT01008540).

PATIENTS AND METHODS

Patients were monitored for detection of CMV infection for ≥2 years. Real-time detection of resistance by UL97 and UL54 gene sequencing and antiviral phenotyping was performed if viral replication persisted for >21 days of appropriate antiviral treatment. Plasma ganciclovir assays were performed when resistance was suspected.

RESULTS

Resistance was suspected in 37 (10.7%) patients; 18/37 (5.2% of the cohort) had virological resistance, associated with poorer outcome. Most cases involved single UL97 mutations, but four cases of multidrug resistance were due to UL54 mutations. In solid organ transplant recipients, resistance occurred mainly during primary CMV infection (odds ratio 8.78), but also in two CMV-seropositive kidney recipients. Neither CMV prophylaxis nor antilymphocyte antibody administration was associated with virological resistance.

CONCLUSIONS

These data show the feasibility of surveying resistance. Virological resistance was frequent in patients failing antiviral therapy. More than 1/5 resistant isolates harboured UL54 mutations alone or combined with UL97 mutations, which conferred a high level of resistance and sometimes were responsible for cross-resistance, leading to therapeutic failure.

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.

Testing the susceptibility of human herpesviruses to antivirals

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

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.

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.

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.

Putative Functional Domains of Human Cytomegalovirus pUL56 Involved in Dimerization and Benzimidazole D-Ribonucleoside Activity

Background

Benzimidazole d-ribonucleosides inhibit DNA packaging during human cytomegalovirus (HCMV) replication. Although they have been shown to target pUL56 and pUL89 (the large and small subunits of the HCMV terminase, respectively) their mechanism of action is not yet fully understood. We aimed here to better understand HCMV DNA maturation and the mechanism of action of benzimidazole derivatives.

Methods

The HCMV pUL56 protein was studied by sequence analysis of the HCMV UL56 gene and herpesvirus counterparts combined with primary structure analysis of the corresponding amino acid sequences.

Results

The UL56 sequence analysis of 45 HCMV strains and counterparts among herpesviruses allowed the identification of 12 conserved regions. Moreover, comparison with the product of gene 49 (gp49) of bacteriophage T4 suggested that the pUL56 zinc finger is localized close to the dimerization site of pUL56, providing a spatial organization of the catalytic site that allows recognition and cleavage of DNA.

Conclusions

This study provides a basis to investigate the mechanism of concatemeric DNA cleavage and a biochemical basis for DNA packaging inhibition by benzimidazole derivatives.

Identification of a highly conserved region in the human cytomegalovirus glycoprotein H gene and design of molecular diagnostic methods targeting the region

Genomic polymorphism of human cytomegalovirus (HCMV) leads to difficulties in the design of molecular diagnostic systems; therefore, a suitable target region was determined in the glycoprotein H (gH) gene, which has been reported to be the most conserved gene. A highly conserved region was identified from codon 1,282 to 1,988 of the gH gene by alignment of 23 nucleotide sequences (14 registered in the DNA Data Bank of Japan and 9 sequenced in this laboratory). Diagnostic methods based on nested PCR, real-time PCR and loop-mediated isothermal temperature amplification (LAMP) were designed for this region. Primers and a probe for nested and real-time PCR were designed for the completely conserved sequences in all HCMV strains. However, a few mismatched nucleotides could not be excluded from the LAMP primers due to the need for eight primer-binding sites in a 200bp-region. The sensitivities of the nested PCR, real-time PCR and LAMP reactions were 5, 10 and 100 copies/tube, respectively. An analysis of clinical specimens showed that both nested and real-time PCR detected HCMV with greater sensitivity than did a pp65 antigenemia assay and were expected to minimize the incidence of false-negative results, whereas the sensitivity of the LAMP reaction was comparable with that of the antigenemia assay.

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.

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.

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.

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.