Highly reliable heterologous system for evaluating resistance of clinical herpes simplex virus isolates to nucleoside analogues

CRISPR/Cas9-mediated genome editing of the thymidine kinase gene in a clinical HSV-1 isolate identifies F289S as novel acyclovir-resistant mutation

Herpes simplex virus type 1 (HSV-1) is a neurotropic alphaherpesvirus that establishes a lifelong infection in sensory neurons of infected individuals, accompanied with intermittent reactivation of latent virus causing (a)symptomatic virus shedding. Whereas acyclovir (ACV) is a safe and highly effective antiviral to treat HSV-1 infections, long-term usage can lead to emergence of ACV resistant (ACVR) HSV-1 and subsequently ACV refractory disease. Here, we isolated an HSV-1 strain from a patient with reactivated herpetic eye disease that did not respond to ACV treatment. The isolate carried a novel non-synonymous F289S mutation in the viral UL23 gene encoding the thymidine kinase (TK) protein. Because ACV needs conversion by viral TK and subsequently cellular kinases to inhibit HSV-1 replication, the UL23 gene is commonly mutated in ACVR HSV-1 strains. The potential role of the F289S mutation causing ACVR was investigated using CRISPR/Cas9-mediated HSV-1 genome editing. Reverting the F289S mutation in the original clinical isolate to the wild-type sequence S289F resulted in an ACV-sensitive (ACVS) phenotype, and introduction of the F289S substitution in an ACVS HSV-1 reference strain led to an ACVR phenotype. In summary, we identified a new HSV-1 TK mutation in the eye of a patient with ACV refractory herpetic eye disease, which was identified as the causative ACVR mutation with the aid of CRISPR/Cas9-mediated genome engineering technology. Direct editing of clinical HSV-1 isolates by CRISPR/Cas9 is a powerful strategy to assess whether single residue substitutions are causative to a clinical ACVR phenotype.

Phenotypic and genotypic acyclovir resistance surveillance of genital herpes simplex virus 2 in South Africa

Acyclovir (ACV) is currently included in the syndromic management algorithm for genital ulcer disease in South Africa, and is the recommended first-line treatment for herpes simplex virus 2 (HSV-2). In the majority of cases, HSV-2 resistance to ACV is due to amino acid changes within the viral thymidine kinase (TK). Phenotypic and genotypic ACV resistance surveillance of HSV-2 derived from genital ulcer disease swab specimens was conducted at a primary healthcare facility in Johannesburg between 2018 and 2020. The objectives of this surveillance were to identify ACV resistance-associated mutations and polymorphisms in HSV-2 TK, and to determine the phenotypic ACV resistance profiles of the corresponding clinical HSV-2 isolates. Genotypic analysis of TK from 67 HSV-2 positive genital ulcer swabs revealed 48 specimens with TK mutations, conferring 113 nucleotide changes. No resistance-associated mutations were found, however, we identified nine known natural polymorphisms (R26H, A27T, S29A, G39E, N78D, L140F, T159I, R220K and R284S) and five amino acid changes of unknown significance (R18C, G39K, M70R, P75S and L263P). Phenotypic susceptibility testing of 52 cultivable HSV-2 isolates revealed all to be susceptible to ACV with IC₅₀ values of <2 μg/ml. The five amino acid changes of unknown significance identified by genotypic testing were not correlated to phenotypic ACV resistance, and therefore grouped as natural polymorphisms. We did not detect any unknown or resistance-associated mutations in specimens that could not be phenotypically tested for ACV resistance. Our findings will supplement existing databases of HSV antiviral resistance-associated mutations and polymorphisms that could be used for genotypic ACV resistance screening.

Virulence of herpes simplex virus 1 harbouring a UAG stop codon between the first and second initiation codon in the thymidine kinase gene

Herpes simplex virus 1 (HSV-1)-TK(8UAG) expresses a truncated thymidine kinase (TK) translated from the second initiation codon due to a stop UAG codon at the 8th position (counted from the first initiation codon). Here, we showed that the sensitivity of HSV-1-TK(8UAG) to acyclovir (ACV) is similar to that of control HSV-1 wild-type (WT), which expresses an intact TK protein. However, HSV-1-TK(44UAG), which expresses a truncated TK due to a UAG codon at position 44, showed lower sensitivity to ACV. A mouse infection model was used to compare the virulence of HSV-1-TK(8UAG) and HSV-1-TK(44UAG) with that of HSV-1 wild-type (WT). The 50% lethal dose (LD₅₀) value of HSV-1-TK(44UAG) was 7.8-fold higher than that of HSV-1-TK(8UAG), whereas the LD₅₀ value of HSV-1-TK(8UAG) was the same as that of the parental HSV-1 WT. There were no statistically significant differences between HSV-1-TK(44UAG), HSV-1-TK(8UAG), and HSV-1 WT with respect to replication capacity and viral TK mRNA expression in mouse brain. Thus, the virulence of HSV-1 expressing a truncated viral TK translated from the second initiation codon might depend on the position of the UAG stop codon.

Susceptibility Evaluation of Clinically Isolated HSV-1 Strains to Acyclovir: A Phenotypic and Genotypic Study

Background: Mutations in herpes simplex virus Thymidine kinase (TK, UL23) and DNA polymerase (pol, UL30) genes may confer resistance to acyclovir (ACV). Phenotypic resistance must be determined along with genotypic resistance to achieve complete acyclovir susceptibility. Objectives: The present study aimed to characterize HSV-1 clinical isolates from outpatients and organ transplant recipients in terms of phenotypic ACV resistance. Moreover, genotypic resistance to ACV was assessed through sequencing the viral TK and pol genes amplified from virus-infected cell DNA. Methods: Twenty-six HSV-1 clinical isolates collected between 2016 and 2019 were examined for drug susceptibility. The samples were collected from eyes, oropharyngeal, facial, and other skin parts of immunocompetent and immunocompromised individuals. Phenotypic susceptibility was determined by using three different concentrations of ACV. The results were expressed based on the ability of ACV in reducing viral plaques by 50%. Genotyping was carried out by polymerase chain reaction and sequencing of TK and pol genes. Results: All the strains were characterized as sensitive at 0.01 and 0.05 µg.ml-1 concentrations to ACV. Seventy percent inhibition was observed at ≥ 0.1 µg.mL-1 of ACV for three isolates (two from patients who received transplants and one from an outpatient). Nine natural polymorphisms were detected in the TK gene and 31 in the Pol gene. Furthermore, four susceptible-associated mutations in the DNA pol gene were analyzed. A substitution was encoded in the conserved region of the pol Exo III motif (M553L), and nine amino acid substitutions in TK were detected. The phylogenetic analysis of partial genome sequences revealed high diversity in the TK and pol genes of HSV-1. Conclusions: A higher number of mutations were observed in patients who received transplants and underwent long-term treatment compared with outpatients. The high genetic variability of HSV-1 TK and DNA pol was not associated with phenotypic resistance.

Evaluation of Genotypic Antiviral Resistance Testing as an Alternative to Phenotypic Testing in a Patient with DOCK8 Deficiency and Severe HSV-1 Disease

Antiviral resistance frequently complicates the treatment of herpes simplex virus (HSV) infections in immunocompromised patients. Here we present the case of an adolescent boy with dedicator of cytokinesis 8 (DOCK8) deficiency, who experienced recurrent infections with resistant HSV-1. We used both phenotypic and genotypic methodologies to characterize the resistance profile of HSV-1 in the patient and conclude that genotypic testing outperformed phenotypic testing. We also present the first analysis of intrahost HSV-1 evolution in an immunocompromised patient. While HSV-1 can remain static in an immunocompetent individual for decades, the virus from this patient rapidly acquired genetic changes throughout its genome. Finally, we document a likely case of transmitted resistance in HSV-1 between the patient and his brother, who also has DOCK8 deficiency. This event demonstrates that resistant HSV-1 is transmissible among immunocompromised persons.

Phenotypic and Genotypic Testing of HSV-1 and HSV-2 Resistance to Antivirals

Resistance testing of antivirals to herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2) can be done by phenotypic and genotypic methods. The determination of a resistant phenotype is based on the calculation of inhibitory concentrations for the antiviral drug, which should be tested. The main advantage of this resistance test is a clear interpretation of laboratory findings, but the method is time-consuming and a considerable experience is required by handling infectious virus. Genotypic resistance testing is based on the detection of resistance-related mutations in viral genes encoding the thymidine kinase and DNA polymerase, which need to be amplified and sequenced. This approach has the advantage of being faster, but only frameshift mutations, stops of translation, and amino acid substitutions described in the literature can be interpreted without doubt. By contrast, numerous novel amino acid substitutions are diagnostically less conclusive.

Alpha-Herpesvirus Thymidine Kinase Genes Mediate Viral Virulence and Are Potential Therapeutic Targets

Alpha-herpesvirus thymidine kinase (TK) genes are virulence-related genes and are nonessential for viral replication; they are often preferred target genes for the construction of gene-deleted attenuated vaccines and genetically engineered vectors for inserting and expressing foreign genes. The enzymes encoded by TK genes are key kinases in the nucleoside salvage pathway and have significant substrate diversity, especially the herpes simplex virus 1 (HSV-1) TK enzyme, which phosphorylates four nucleosides and various nucleoside analogues. Hence, the HSV-1 TK gene is exploited for the treatment of viral infections, as a suicide gene in antitumor therapy, and even for the regulation of stem cell transplantation and treatment of parasitic infection. This review introduces the effects of α-herpesvirus TK genes on viral virulence and infection in the host and classifies and summarizes the current main application domains and potential uses of these genes. In particular, mechanisms of action, clinical limitations, and antiviral and antitumor therapy development strategies are discussed.

Pharmacokinetics and Safety of Amenamevir in Healthy Subjects: Analysis of Four Randomized Phase 1 Studies

Introduction

Amenamevir (ASP2151) is a nonnucleoside antiherpesvirus compound available for the treatment of varicella–zoster virus infections. In this article we summarize the findings of four phase 1 studies in healthy participants.

Methods

Four randomized phase 1 studies investigated the safety and pharmacokinetics of single and multiple doses of amenamevir, including the assessment of age group effect (nonelderly vs elderly), food effect, and the relative bioavailability of two formulations. Amenamevir was administered orally at various doses as a single dose (5–2400 mg) or daily (300 or 600 mg/day) for 7 days.

Results

Following single and multiple oral doses, amenamevir demonstrated a less than dose proportional increase in the pharmacokinetic parameters area under the plasma drug concentration versus time curve from time zero to infinity (AUC_inf) and C_max. After single and multiple oral 300-mg doses of amenamevir, no apparent differences in pharmacokinetics were observed between nonelderly and elderly participants. In contrast, with the amenamevir 600-mg dose both the area under the plasma drug concentration versus time curve from time zero to 24 h and C_max were slightly increased and renal clearance was decreased in elderly participants. The pharmacokinetics of amenamevir was affected by food, with AUC_inf increased by about 90%. In the bioavailability study, AUC_inf and C_max were slightly lower following tablet versus capsule administration (decreased by 14 and 12%, respectively), with relative bioavailability of 86%. The different amenamevir doses and formulations were safe and well tolerated; no deaths or serious adverse events were reported.

Conclusion

Amenamevir had less than dose proportional pharmacokinetic characteristics. Age may have an influence on amenamevir pharmacokinetics; however, the effect was considered minimal. The pharmacokinetics of amenamevir were affected by food, with AUC_inf almost doubling when amenamevir was administered with food. The concentration versus time profile of the tablet was slightly lower than that of the capsule; the relative bioavailability of the tablet versus the capsule was 86%. Amenamevir was safe and well tolerated in the dose range investigated.

Funding

Astellas Pharma.

Trial registration

ClinicalTrials.gov identifiers NCT02852876 (15L-CL-002) and NCT02796118 (15L-CL-003).

Electronic supplementary material

The online version of this article (10.1007/s12325-017-0642-4) contains supplementary material, which is available to authorized users.

Association between sensitivity of viral thymidine kinase-associated acyclovir-resistant herpes simplex virus type 1 and virulence

Background

Acyclovir (ACV)-resistant (ACVr) herpes simplex virus type 1 (HSV-1) infections are concern in immunocompromised patients. Most clinical ACVr HSV-1 isolates have mutations in the viral thymidine kinase (vTK) genes. The vTK-associated ACVr HSV-1 shows reduced virulence, but the association between the level of resistance and the virulence of the vTK-associated ACVr HSV-1 is still unclear.

Methods

The virulence in mice of 5 vTK-associated ACVr HSV-1 clones with a variety of ACV sensitivities, when inoculated through intracerebral and corneal routes, was evaluated in comparison with ACV-sensitive (ACVs) parent HSV-1 TAS.

Results

Although all the 5 ACVr HSV-1 clones and ACVs HSV-1 TAS showed a similar single-step growth capacity in vitro, the virulence of ACVr HSV-1 clones significantly decreased. A 50% lethal dose (LD₅₀) of each clone was closely correlated with 50% inhibitory concentrations (IC₅₀), demonstrating that the higher the ACV-sensitvity, the the higher the virulence among the ACVr clones. One of the ACVr HSV-1 clones with a relatively low IC₅₀ value maintained similar virulence to that of the parent TAS. The infection in mice with ACVr HSV-1 due to a single amino acid substitution in vTK induced local diseases, keratitis and dermatitis, while vTK-deficient clone did not.

Conclusions

A statistically significant correlation between the virulence and susceptibility to ACV among ACVr HSV-1 clones was demonstrated.

Recombinant herpes simplex virus type 1 strains with targeted mutations relevant for aciclovir susceptibility

Here, we describe a novel reliable method to assess the significance of individual mutations within the thymidine kinase (TK) gene of herpes simplex virus type 1 (HSV-1) to nucleoside analogue resistance. Eleven defined single nucleotide polymorphisms that occur in the TK gene of clinical HSV-1 isolates and a fluorescence reporter were introduced into the HSV-1 strain 17⁺ that had been cloned into a bacterial artificial chromosome. The susceptibility of these different strains to aciclovir, penciclovir, brivudin, and foscarnet was determined with a modified cytopathic effect reduction assay. The strains were also tested for their aciclovir susceptibility by measuring the relative fluorescence intensity as an indicator for HSV-1 replication and by quantifying the virus yield. Our data indicate that the amino acid substitutions R41H, R106H, A118V, L139V, K219T, S276R, L298R, S345P, and V348I represent natural polymorphisms of the TK protein, whereas G61A and P84L mediate broad cross-resistance against aciclovir, penciclovir, brivudin, and susceptibility to foscarnet. This method allows the definition of the resistance genotype of otherwise unclear mutations in the TK gene of HSV-1. Thus, it provides a scientific basis for antiviral testing in clinical isolates of patients suffering from serious diseases and will facilitate testing of new antivirals against HSV-1.

Prevalence of Intrathecal Acyclovir Resistant Virus in Herpes Simplex Encephalitis Patients

Herpes simplex encephalitis (HSE) is a life-threatening complication of herpes simplex virus (HSV) infection. Acyclovir (ACV) is the antiviral treatment of choice, but may lead to emergence of ACV-resistant (ACV^R) HSV due to mutations in the viral UL23 gene encoding for the ACV-targeted thymidine kinase (TK) protein. Here, we determined the prevalence of intrathecal ACV^R–associated HSV TK mutations in HSE patients and compared TK genotypes of sequential HSV isolates in paired cerebrospinal fluid (CSF) and blister fluid of mucosal HSV lesions. Clinical samples were obtained from 12 HSE patients, encompassing 4 HSV type 1 (HSV-1) and 8 HSV-2 encephalitis patients. HSV DNA load was determined by real-time PCR and complete HSV TK gene sequences were obtained by nested PCR followed by Sanger sequencing. All HSV-1 HSE patients contained viral TK mutations encompassing 30 unique nucleotide and 13 distinct amino acid mutations. By contrast, a total of 5 unique nucleotide and 4 distinct amino acid changes were detected in 7 of 8 HSV-2 patients. Detected mutations were identified as natural polymorphisms located in non-conserved HSV TK gene regions. ACV therapy did not induce the emergence of ACV^R-associated HSV TK mutations in consecutive CSF and mucocutaneous samples of 5 individual patients. Phenotypic susceptibility analysis of these mucocutaneous HSV isolates demonstrated ACV-sensitive virus in 2 HSV-1 HSE patients, whereas in two HSV-2 HSE patients ACV^R virus was detected in the absence of known ACV^R-associated TK mutations. In conclusion, we did not detect intrathecal ACV^R-associated TK mutations in HSV isolates obtained from 12 HSE patients.

Database on natural polymorphisms and resistance-related non-synonymous mutations in thymidine kinase and DNA polymerase genes of herpes simplex virus types 1 and 2

The use of genotypic resistance testing of herpes simplex virus types 1 and 2 (HSV-1 and HSV-2) is increasing because the rapid availability of results significantly improves the treatment of severe infections, especially in immunocompromised patients. However, an essential precondition is a broad knowledge of natural polymorphisms and resistance-associated mutations in the thymidine kinase (TK) and DNA polymerase (pol) genes, of which the DNA polymerase (Pol) enzyme is targeted by the highly effective antiviral drugs in clinical use. Thus, this review presents a database of all non-synonymous mutations of TK and DNA pol genes of HSV-1 and HSV-2 whose association with resistance or natural gene polymorphism has been clarified by phenotypic and/or functional assays. In addition, the laboratory methods for verifying natural polymorphisms or resistance mutations are summarized. This database can help considerably to facilitate the interpretation of genotypic resistance findings in clinical HSV-1 and HSV-2 strains.

Genotypic Characterization of Herpes Simplex Virus Type 1 Isolates in Immunocompromised Patients in Rio de Janeiro, Brazil

Herpes simplex virus type 1 (HSV-1) is a prevalent human pathogen that causes a variety of diseases, including an increased risk of developing more severe disease in HIV-infected individuals. In Brazil, there is no information about the molecular epidemiology of HSV-1 infection, especially in HIV-infected individuals. The aim of this study was to perform the genotypic characterization of HSV-1 among HIV-infected patients. A total of 214 serum samples from HIV-positive patients without HSV infection symptoms were enrolled in one of two reference hospitals for HIV infection managing in Rio de Janeiro. The gG and gI genes were analyzed by restriction fragment length polymorphism (RFLP) and full nucleotide sequencing of the US8 (1601 bp), UL44 (1996 bp), and UL23 (1244 bp) regions was performed. A total of 38.3% (82/214) and 32.7% (70/214) of the serum samples tested positive for gG and gI genes, respectively. RFLP analysis classified the HSV-1 as belonging to genotype A. Phylogenetic analysis of the Brazilian samples for the US8, UL44, and UL23 regions demonstrated that the nucleotide identity between Brazilian samples was higher than 97% for all genes. No acyclovir mutation was detected in the patients. The shedding of HSV in the serum samples from HIV-positive patients who were asymptomatic for HSV infection was detected in this work. This is the first report of molecular characterization of HSV-1 in Brazilian samples since there is no previous data available in the literature concerning the genotypic classification and stable distribution of Brazilian strains of HSV-1 in Rio de Janeiro, Brazil.

Sequence Analysis of Herpes Simplex Virus 1 Thymidine Kinase and DNA Polymerase Genes from over 300 Clinical Isolates from 1973 to 2014 Finds Novel Mutations That May Be Relevant for Development of Antiviral Resistance

A total of 302 clinical herpes simplex virus 1 (HSV-1) strains, collected over 4 decades from 1973 to 2014, were characterized retrospectively for drug resistance. All HSV-1 isolates were analyzed genotypically for nonsynonymous mutations in the thymidine kinase (TK) and DNA polymerase (Pol) genes. The resistance phenotype against acyclovir (ACV) and/or foscarnet (FOS) was examined in the case of novel, unclear, or resistance-related mutations. Twenty-six novel natural polymorphisms could be detected in the TK gene and 69 in the DNA Pol gene. Furthermore, three novel resistance-associated mutations (two in the TK gene and one in the DNA Pol gene) were analyzed, and eight known but hitherto unclear amino acid substitutions (two encoded in TK and six in the DNA Pol gene) could be clarified. Between 1973 and 2014, the distribution of amino acid changes related to the natural gene polymorphisms of TK and DNA Pol remained largely stable. Resistance to ACV was confirmed phenotypically for 16 isolates, and resistance to ACV plus FOS was confirmed for 1 isolate. Acyclovir-resistant strains were observed from the year 1995 onwards, predominantly in immunosuppressed patients, especially those with stem cell transplantation, and the number of ACV-resistant strains increased during the last 2 decades. The data confirm the strong genetic variability among HIV-1 isolates, which is more pronounced in the DNA Pol gene than in the TK gene, and will facilitate considerably the rapid genotypic diagnosis of HSV-1 resistance.

A mutation in the DNA polymerase accessory factor of herpes simplex virus 1 restores viral DNA replication in the presence of raltegravir

Previous reports showed that raltegravir, a recently approved antiviral compound that targets HIV integrase, can inhibit the nuclease function of human cytomegalovirus (HCMV terminase) in vitro. In this study, subtoxic levels of raltegravir were shown to inhibit the replication of four different herpesviruses, herpes simplex virus 1 (HSV-1), HSV-2, HCMV, and mouse cytomegalovirus, by 30- to 700-fold, depending on the dose and the virus tested. Southern blotting and quantitative PCR revealed that raltegravir inhibits DNA replication of HSV-1 rather than cleavage of viral DNA. A raltegravir-resistant HSV-1 mutant was generated by repeated passage in the presence of 200 μM raltegravir. The genomic sequence of the resistant virus, designated clone 7, contained mutations in 16 open reading frames. Of these, the mutations F198S in unique long region 15 (UL15; encoding the large terminase subunit), A374V in UL32 (required for DNA cleavage and packaging), V296I in UL42 (encoding the DNA polymerase accessory factor), and A224S in UL54 (encoding ICP27, an important transcriptional regulator) were introduced independently into the wild-type HSV-1(F) genome, and the recombinant viruses were tested for raltegravir resistance. Viruses bearing both the UL15 and UL32 mutations inserted within the genome of the UL42 mutant were also tested. While the UL15, UL32, and UL54 mutant viruses were fully susceptible to raltegravir, any virus bearing the UL42 mutation was as resistant to raltegravir as clone 7. Overall, these results suggest that raltegravir may be a valuable therapeutic agent against herpesviruses and the antiviral activity targets the DNA polymerase accessory factor rather than the nuclease activity of the terminase.

IMPORTANCE

This paper shows that raltegravir, the antiretrovirus drug targeting integrase, is effective against various herpesviruses. Drug resistance mapped to the herpesvirus DNA polymerase accessory factor, which was an unexpected finding.

Single nucleotide polymorphisms of thymidine kinase and DNA polymerase genes in clinical herpes simplex virus type 1 isolates associated with different resistance phenotypes

The role of mutations in the thymidine kinase (TK, UL23) and DNA polymerase (pol, UL30) genes of herpes simplex virus (HSV) for development of different resistance phenotypes has to be exactly determined before genotypic resistance testing can be implemented in patient's care. Furthermore, the occurrence of cross-resistance is of utmost clinical importance. In this study, clinical HSV-1 isolates obtained between 2004 and 2011 from 26 patients after stem cell transplantation were examined in parallel by phenotypic and genotypic resistance testing. Thirteen isolates, which were phenotypically cross-resistant to acyclovir (ACV), penciclovir (PCV) and brivudin (BVDU), exhibited consistently frameshift or non-synonymous mutations in the TK gene known to confer resistance. One of these mutations (insertion of C at the nucleotide positions 1061-1065) has not been described before. Seven strains, phenotypically resistant to ACV and PCV and, except one each, sensitive to BVDU and resistant to foscarnet (FOS), carried uniformly resistance-related substitutions in the DNA pol gene. Finally, 3 isolates, resistant to ACV, PCV and 2 out of these also resistant to BVDU, had known but also unclear substitutions in the TK and DNA pol genes, and 3 isolates were completely sensitive. In conclusion, clinical ACV-resistant HSV-1 isolates, carrying resistance-associated mutations in the TK gene, can be regarded as cross-resistant to other nucleoside analogs such as BVDU. In contrast, clinical FOS-resistant HSV-1 strains which are cross-resistant to ACV may be sensitive to BVDU. This has to be considered for drug changes in antiviral treatment in case of ACV resistance.

Phenotypic and genotypic testing of HSV-1 resistance to antivirals

Resistance testing of antivirals to herpes simplex virus type 1 can be done by phenotypic and genotypic methods. The determination of a resistant phenotype is based on the calculation of inhibitory concentrations for the antiviral drug, which should be tested. The main advantage is a clear interpretation of laboratory findings, but the method is time consuming and a considerable experience is required for handling infectious virus. Genotypic resistance testing is based on the detection of resistance-related mutations in viral genes encoding the thymidine kinase and DNA polymerase by means of amplification and sequencing. This approach has the advantage of being faster, but only frameshift mutations and stops of translation can be interpreted without doubt and numerous amino acid substitutions are diagnostically less conclusive.

Multiple viral infections after haploidentical hematopoietic stem cell transplantation in a child with acute lymphoblastic leukemia

After allogeneic hematopoietic stem cell transplantation (HSCT), viral infections/reactivations are a frequent complication, sometimes with fatal outcome. Thus, early diagnosis is recommended by screening of whole blood or plasma preparations using highly sensitive molecular techniques that test for the most common viral pathogens, such as Epstein-Barr virus, cytomegalovirus, and adenoviruses (ADVs). Despite this approach, not every reactivation/infection can be adequately detected or excluded, even with highly sensitive polymerase chain reaction. Particularly after toxic treatment, uncommon infections or infections resistant to first-line treatment can occur, even in unusual locations. Herein, we present the case of a child with Philadelphia chromosome-positive acute lymphoblastic leukemia after allogeneic HSCT who suffered from 5 different viral reactivations/infections, including acyclovir-resistant herpes simplex virus type 1 esophagitis, human herpesvirus 6 encephalitis, rotavirus gastroenteritis, respiratory syncytial virus pneumonia, and ADV esophagitis, despite routinely performed blood examinations for viral pathogens remaining unrevealing at all times.

Longitudinal Characterization of Herpes Simplex Virus (HSV) Isolates Acquired From Different Sites in an Immune-Compromised Child: A New HSV Thymidine Kinase Mutation Associated With Resistance

BACKGROUND

Herpes simplex virus resistance to acyclovir is well described in immune-compromised patients. Management of prolonged infection and recurrences in such patients may be problematic.

METHODS

A patient with neuroblastoma developed likely primary herpes gingivostomatitis shortly after starting a course of chemotherapy, with spread to the eye during treatment with acyclovir. Viral isolates were serially obtained from separate sites after treatment was begun and tested for susceptibility to acyclovir and foscarnet by plaque reduction and plating efficiency assays. The thymidine kinase and DNA polymerase genes from each isolate were sequenced.

RESULTS

Initial isolates from a throat swab, an oral lesion, and conjunctiva were resistant to acyclovir within 13 days of treatment. Subsequent isolates while on foscarnet were initially acyclovir-susceptible, but reactivation of an acyclovir-resistant isolate was subsequently documented while on acyclovir suppression. Genotypic analysis identified a previously unreported UL23 mutation in some resistant isolates. None of the amino acid changes identified in UL30 were associated with resistance.

CONCLUSIONS

Phenotypic and genotypic antiviral resistance of herpes simplex isolates may vary from different compartments and over time in individual immune-compromised hosts, highlighting the importance of obtaining cultures from all sites. Phenotypic resistance testing should be considered for isolates obtained from at-risk patients not responding to first-line therapy. Empiric combination treatment with multiple antivirals could be considered in some situations.

Novel resistance-associated mutations of thymidine kinase and DNA polymerase genes of herpes simplex virus type 1 and type 2

BACKGROUND

Studies to verify correlations between phenotypes and genotypes of herpes simplex virus (HSV) are an important tool to establish a database of resistance-associated mutations.

METHODS

In this study, 32 acyclovir (ACV)-resistant clinical HSV-1 and 4 ACV-resistant clinical HSV-2 isolates were examined in parallel by both phenotypic and genotypic resistance testing. Additionally, five non-viable HSV-1 strains and two non-viable HSV-2 strains with clinical resistance were included in genotypic resistance analysis.

RESULTS

All ACV-resistant HSV isolates showed cross-resistance to brivudin and penciclovir, and were sensitive to foscarnet and cidofovir. Acyclovir resistance was assigned to frameshift and single non-synonymous mutations of the thymidine kinase (TK) gene in 32 out of 37 HSV-1 strains and in 4 out of 6 HSV-2 strains. In three HSV-1 isolates, there were resistance-associated amino acid substitutions of the DNA polymerase (pol). Six substitutions in the TK and two in the DNA pol gene could not be attributed without doubt to either ACV resistance or natural gene polymorphism. Altogether, 10 resistance-related mutations in the TK and 1 in the DNA pol gene have not been reported previously.

CONCLUSIONS

The novel non-synonymous mutations found in this study enrich the knowledge about the genetic alterations of TK and DNA pol genes in ACV-resistant clinical HSV strains. Together with data from the literature, the findings justify the generation of a HSV database that contains resistance mutations associated with ACV resistance phenotype.

Expression of herpes simplex virus type 1 recombinant thymidine kinase and its application to a rapid antiviral sensitivity assay

Antiviral-resistant herpesvirus infection has become a great concern for immunocompromised patients. Herpes simplex virus type 1 (HSV-1) infections are treated with viral thymidine kinase (vTK)-associated drugs such as acyclovir (ACV), and most ACV-resistance (ACV(r)) is due to mutations in the vTK. The standard drug sensitivity test is usually carried out by the plaque reduction assay-based method, which requires over 10 days. To shorten the time required, a novel system was developed by the concept, in which 293T cells transiently expressing recombinant vTK derived from the test sample by transfection of the cells with an expression vector were infected with vTK-deficient and ACV(r) HSV-1 (TAR), and then cultured in a maintenance medium with or without designated concentrations of ACV, ganciclovir (GCV) and brivudine (BVdU). The replication of TAR was strongly inhibited by ACV, GCV and BVdU in 293T cells expressing recombinant vTK of the ACV-sensitive HSV-1, whereas replication was not or slightly inhibited in cells expressing the recombinant vTK of highly resistant or intermediately resistant HSV-1, respectively. An inverse correlation was demonstrated in the 50% effective concentrations (EC(50)s) and inhibitory effects of these compounds on the replication of TAR among ACV(s) and ACV(r) HSV-1 clones. These results indicate that the EC(50)s of the vTK-associated drugs including ACV can be assumed by measuring the inhibitory effect of drugs in 293T cells expressing recombinant vTK of the target virus. The newly developed antiviral sensitivity assay system for HSV-1 makes it possible to estimate EC(50) for vTK-associated drugs, when whole vTK gene is available for use by gene amplification directly from lesion's samples or from virus isolates.

Genotypic characterization of UL23 thymidine kinase and UL30 DNA polymerase of clinical isolates of herpes simplex virus: natural polymorphism and mutations associated with resistance to antivirals

The molecular mechanisms of herpes simplex virus (HSV) resistance to antiviral drugs interfering with viral DNA synthesis reported so far rely on the presence of mutations within UL23 (thymidine kinase [TK]) and UL30 (DNA polymerase) genes. The interpretation of genotypic antiviral resistance assay results requires the clear distinction between resistance mutations and natural interstrain sequence variations. The objectives of this work were to describe extensively the natural polymorphism of UL23 TK and UL30 DNA polymerase among HSV-1 and HSV-2 strains and the amino acid changes potentially associated with HSV resistance to antivirals. The sequence analysis of the full-length UL23 and UL30 genes was performed. Ninety-four drug-sensitive clinical isolates (43 HSV-1 and 51 HSV-2) and 3 laboratory strains (KOS, gHSV-2, and MS2) were studied for natural polymorphism, and 25 clinical isolates exhibiting phenotypic traits of resistance to antivirals were analyzed for drug resistance mutations. Our results showed that TK and DNA polymerase are highly conserved among HSV strains, with a weaker variability for HSV-2 strains. This study provided a precise map of the natural polymorphism of both viral enzymes among HSV-1 and HSV-2 isolates, with the identification of 15 and 51 polymorphisms never previously described for TK and DNA polymerase, respectively, which will facilitate the interpretation of genotypic antiviral-resistant testing. Moreover, the genotypic characterization of 25 drug-resistant HSV isolates revealed 8 new amino acid changes located in TK and potentially accounting for acyclovir (ACV) resistance.

Acyclovir resistance in herpes simplex encephalitis

Herpes simplex virus type 1 is a common cause of severe sporadic encephalitis. Treatment with acyclovir is highly effective in this disease. We report the case of a 27-year-old, immunocompetent woman with acyclovir-resistant herpes simplex encephalitis. Although she had not been treated before, herpes simplex virus type 1 DNA from the cerebrospinal fluid showed a non-synonymous mutation in the thymidine kinase gene, which is likely to have caused resistance to acyclovir. Herpes simplex encephalitis resolved after treatment with foscarnet. To our knowledge, this is the first report of acyclovir-resistant herpes simplex virus encephalitis in an immunocompetent, previously therapy-naive adult.

Phenotypic and genotypic characterization of acyclovir-resistant clinical isolates of herpes simplex virus

Sixteen herpes simplex virus type 1 (HSV-1) and four type 2 (HSV-2) isolates resistant to acyclovir (ACV) were characterized retrospectively for drug resistance. Phenotypic testing was performed by means of tetrazolium reduction assay and genotypic analysis was carried out by sequencing of thymidine kinase (TK) and DNA-polymerase (pol) genes. All strains were characterized as cross-resistant to penciclovir, brivudin and susceptible to cidofovir. In addition, three strains were resistant to foscarnet. Genotypic analysis revealed two to seven non-synonymous mutations in the TK gene of HSV-1 and one to seven non-synonymous mutations in the DNA pol gene of HSV-1 and 2 associated with the gene polymorphism. Seventeen strains contained at least one non-synonymous resistant-related mutation in the TK gene and three strains, which were additionally foscarnet-resistant, revealed one resistance-associated mutation in the DNA pol gene. In most strains, resistant-related mutations in TK gene represented frameshift mutations and single non-synonymous nucleotide substitutions of conserved gene regions. However, numerous amino acid changes could not be interpreted clearly as accounting for resistance. In conclusion, further studies, e.g. site-directed mutagenesis experiments are required to characterize mutations of the TK and DNA pol genes in ACV-resistant viral strains as part of viral gene polymorphism or as cause of drug resistance.

The Epstein-Barr virus (EBV)-encoded protein kinase, EBV-PK, but not the thymidine kinase (EBV-TK), is required for ganciclovir and acyclovir inhibition of lytic viral production

Ganciclovir (GCV) and acyclovir (ACV) are guanine nucleoside analogues that inhibit lytic herpesvirus replication. GCV and ACV must be monophosphorylated by virally encoded enzymes to be converted into nucleotides and incorporated into viral DNA. However, whether GCV and/or ACV phosphorylation in Epstein-Barr virus (EBV)-infected cells is mediated primarily by the EBV-encoded protein kinase (EBV-PK), the EBV-encoded thymidine kinase (EBV-TK), or both is controversial. To examine this question, we constructed EBV mutants containing stop codons in either the EBV-PK or EBV-TK open reading frame and selected for stable 293T clones latently infected with wild-type EBV or each of the mutant viruses. Cells were induced to the lytic form of viral replication with a BZLF1 expression vector in the presence and absence of various doses of GCV and ACV, and infectious viral titers were determined by a green Raji cell assay. As expected, virus production in wild-type EBV-infected 293T cells was inhibited by both GCV (50% inhibitory concentration [IC(50)] = 1.5 microM) and ACV (IC(50) = 4.1 microM). However, the EBV-PK mutant (which replicates as well as the wild-type (WT) virus in 293T cells) was resistant to both GCV (IC(50) = 19.6 microM) and ACV (IC(50) = 36.4 microM). Expression of the EBV-PK protein in trans restored GCV and ACV sensitivity in cells infected with the PK mutant virus. In contrast, in 293T cells infected with the TK mutant virus, viral replication remained sensitive to both GCV (IC(50) = 1.2 microM) and ACV (IC(50) = 2.8 microM), although susceptibility to the thymine nucleoside analogue, bromodeoxyuridine, was reduced. Thus, EBV-PK but not EBV-TK mediates ACV and GCV susceptibilities.

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.

Genotypic detection of acyclovir-resistant HSV-1: characterization of 67 ACV-sensitive and 14 ACV-resistant viruses

Infections due to herpes simplex virus (HSV) resistant to acyclovir (ACV) represent an important clinical concern in immunocompromised patients. In order to switch promptly to an appropriate treatment, rapid viral susceptibility assays are required. We developed herein a genotyping analysis focusing on thymidine kinase gene (TK) mutations in order to detect acyclovir-resistant HSV in clinical specimens. A total of 85 HSV-1 positive specimens collected from 69 patients were analyzed. TK gene could be sequenced directly for 81 clinical specimens (95%) and 68 HSV-1 specimens could be characterized as sensitive or resistant by genotyping (84%). Genetic characterization of 67 susceptible HSV-1 specimens revealed 10 polymorphisms never previously described. Genetic characterization of 14 resistant HSV-1 revealed 12 HSV-1 with either TK gene additions/deletions (8 strains) or substitutions (4 strains) and 2 HSV-1 with no mutation in the TK gene. DNA polymerase gene was afterwards explored. With this rapid PCR-based assay, ACV-resistant HSV could be detected directly in clinical specimens within 24 h.

[Genotyping diagnosis of acyclovir resistant herpes simplex virus]

Herpes simplex virus resistant to acyclovir (ACV) is a major concern among immunocompromised patients. ACV resistance might be due to mutations located in one of the two genes involved in ACV mechanism of action, the thymidine kinase gene (TK, involved in 95% of the cases) and the DNA polymerase gene. TK gene mutations consist, in half of the cases, in nucleotide insertion or deletion, occurring most of the time in G or C homopolymers considered as hot spots. Half of the other cases involves nucleotide substitutions leading to amino acids substitutions. Studies of sensitive strains revealed a high degree of TK polymorphism, many mutations being not implied in ACV resistance. At the present time, resistance detection can be performed by phenotypic tests that require virus culture and results cannot be given to the physician before 7 to 10 days. Genotyping diagnosis performed directly from clinical samples would allow to detect resistance more rapidly, in order to switch quickly to an appropriate treatment by foscarnet or cidofovir.

DNA Polymerase Mutations in Drug-Resistant Herpes Simplex Virus Mutants Determine In Vivo Neurovirulence and Drug-Enzyme Interactions

Mutations in the thymidine kinase and DNA polymerase genes of herpes simplex virus (HSV) might confer resistance to antiviral drugs, particularly in immunocompromised patients who suffer from chronic and/or disseminated lesions. The patterns of cross-resistance and neurovirulence in mice of several DNA polymerase mutants selected under pressure of foscarnet (PFA) and different acyclic nucleoside phosphonates (ANPs), including ( S)-3-hydroxy-2-phosphonylmethoxypropyl (HPMP) derivatives of adenine (HPMPA) and cytosine (HPMPC, cidofovir) and 2-phosphonylmethoxyethyl (PME) derivatives of adenine (PMEA) and 2,6-diaminopurine (PMEDAP), were investigated. The mutants were derived from the HSV-1 strain KOS following either single or multiple steps of selection with PFA (V714M, A719V, S724N and T821M), PMEA (S724N, L802F and R959H), PMEDAP (Q618H, S724N, S724N+D1070N), HPMPC (V573M, R700M and K960R) or HPMPA (W998L, L1007M and I1028T). These amino acid substitutions were located in different subdomains of the HSV-1 DNA polymerase, either in conserved or non-conserved regions. The sensitivity of the mutants to a new class of ANPs, the 6-(2-[phosphonomethoxy]alkoxy)pyrimidines HPMPO-DAPy and PMEO-DAPy, was investigated. Cross-resistance between the HPMP derivatives and HPMPO-DAPy, on the one hand, and between the PME derivatives and PMEO-DAPy, on the other hand, was observed. Different degrees of cross-resistance between PME derivatives, PMEO-DAPy, PFA and acyclovir were noticed. The mutants ranged from exhibiting near wild-type neurovirulence (V714M, A719V, S724N and L1007M) to significant attenuation (Q618H, S724N+D1070N, L802F, R700M, K960R, W998L and I1028T) or higher levels of attenuation (V573M). It appears that drug-resistant mutants arising under the pressure of HPMP derivatives have the lowest levels of neurovirulence.

Resistance of herpes simplex virus type 1 to acyclovir: Thymidine kinase gene mutagenesis study

By site-directed mutagenesis, we investigate the role of six mutations of herpes simplex virus type 1 thymidine kinase (TK) gene in the acquisition of resistance to acyclovir (ACV). TK activity was not impaired by substitutions located at codons 17, 161 and 374 and these mutations were thus related to TK gene polymorphism. Mutations His105Pro, Leu364Pro and Asp162Ala lead to the loss of TK activity that could result in ACV-resistance.

Thymidine kinase mutations conferring acyclovir resistance in herpes simplex type 1 recombinant viruses

Contributions of thymidine kinase (TK) mutations to acyclovir (ACV) resistance were evaluated in herpes simplex virus type 1 recombinant viruses generated using a set of overlapping cosmids and plasmids. Alterations in both conserved and nonconserved regions of the TK gene were shown to confer high levels of resistance to ACV.

Herpes simplex virus thymidine kinase mutations associated with resistance to acyclovir: a site-directed mutagenesis study

Mutations in the thymidine kinase (TK) gene of herpes simplex virus (HSV) may confer resistance to acyclovir (ACV). Because of the high genetic polymorphism of this gene, discriminating between mutations related to resistance and mutations related to gene polymorphism can be difficult, especially when no sensitive strain has been previously isolated from the same patient. To assess the role of the mutations located at codons 51, 77, 83, and 175, previously detected in HSV-1 clinical isolates (F. Morfin, G. Souillet, K. Bilger, T. Ooka, M. Aymard, and D. Thouvenot, J. Infect. Dis. 182:290-293, 2000), in the acquisition of resistance to ACV, four mutants with site-directed mutations at these respective codons were constructed. The enzymatic activity of the proteins, produced using both a reticulocyte lysate system and a bacterial system, was evaluated using [(3)H]thymidine as substrate. This site-directed mutagenesis revealed that mutations at codons 51, 83, and 175 induce a loss of HSV-1 TK activity and are thus clearly involved in the acquisition of resistance to ACV. On the other hand, the mutation at codon 77 does not affect enzyme activity.

Survey of acyclovir-resistant herpes simplex virus in the Netherlands: prevalence and characterization

BACKGROUND

Widespread and frequent use of acyclovir (ACV) for treatment, suppressive therapy and prophylaxis of herpes simplex virus (HSV) infections and its over the counter availability may be associated with emergence of HSV resistance.

OBJECTIVES

To determine the prevalence of ACV-resistant HSV isolates in different patient groups between 1999 and 2002 in the Netherlands.

STUDY DESIGN

A total of 542 isolates, 410 HSV-1 and 132 HSV-2, from 496 patients were screened for reduced susceptibility to ACV. A newly developed ELVIRA HSV screening assay was used that allowed a high throughput screening. The genotypic analysis of the HSV thymidine kinase gene was performed to identify resistance-associated mutations.

RESULTS

Thirteen isolates, 8 HSV-1 and 5 HSV-2, from 10 patients (2%) were found resistant to ACV. A single ACV-resistant strain was identified among isolates from 368 immunocompetent patients (0.27%; 95% confidence interval [CI], 0.007%-1.5%), whereas in nine isolates from 128 immunocompromised patients resistant HSV was identified (7%; 95% CI, 3.26%-12.93%). The highest frequency of ACV-resistant HSV was associated with bone marrow transplantation: four patients out of 28 (14.3%) shed resistant virus. In addition, resistant virus was obtained from two HIV-positive patients, one patient with a hematological malignancy and two patients on immunosuppressive drugs. Further testing showed that none of the isolates was resistant to foscarnet. Several new mutations were identified in the thymidine kinase gene of these resistant isolates, and their effect on ACV-resistance is discussed.

CONCLUSIONS

Our study shows that the prevalence of ACV resistance is low in immunocompetent patients (0.27%), whereas ACV-resistant HSV infections occur relatively frequently in immunocompromised patients (7%; P < 0.0001). This emphasizes the need for drug susceptibility monitoring of HSV infections in immunocompromised patients with persisting infections despite antiviral therapy.

Genotypic and Phenotypic Characterization of Acyclovir-Resistant Herpes Simplex Viruses Isolated from Haematopoietic Stem Cell Transplant Recipients

Thirty-one herpes simplex virus type one (HSV-1) isolates from 12 haematopoietic stem cell transplant recipients with persistent HSV infections despite acyclovir (ACV) prophylaxis or treatment, were genotypically and phenotypically characterized. The relationship between drug susceptibility of the isolates and mutations in thymidine kinase (TK) and DNA polymerase (DNA pol) genes was examined. In all 12 patients, HSV infections were due to ACV-resistant, foscarnet-sensitive viruses. Out of 31 isolates examined, 23 were resistant and eight were sensitive to ACV; eight patients carried viruses with frameshift mutations in the TK gene (due to addition or deletion of single nucleotides in homopolymeric repeats). These mutations were found at codon 61 (G deletion, one patient), 146 (G insertion, five patients) and 153 or 185 (C deletion, one patient each). In four patients, viruses were selected during ACV therapy that contained novel amino acid substitutions in the TK gene (H58R, G129D, A189V, R216H, R220C). Their possible role in ACV resistance was further confirmed phenotypically and by the absence of any resistance-associated mutations in the DNA pol gene. These substitutions were located in ATP- or nucleoside-binding sites or in conserved regions of the TK gene. In addition, a single mutation, Q570R, in the δ-region C of the DNA pol gene, was identified in an isolate from a single patient with resistance to ACV. Our study confirms and expands previous data on genotypic changes associated with ACV resistance of HSV-1 clinical isolates.

Molecular analysis of clinical isolates of acyclovir resistant herpes simplex virus

We characterised the antiviral phenotype and genotype of 41 herpes simplex virus (HSV) strains from patients clinically resistant to acyclovir (ACV). Our results confirm recognised mutational sites as being major determinants of thymidine kinase (tk)-associated ACV resistance, in particular insertions and/or deletions at homopolymer stretches of Gs and Cs (59% of all isolates). Previously described amino acid substitutions in functional sites of the tk were also identified (7% of all isolates). In addition, we identified several stop codons in novel locations on the amino acid sequence (7% of all isolates) and amino acid substitutions (15% of all isolates) likely to be directly responsible for conferring resistance to ACV. When there were no mutations detected in the tk gene (12% of all isolates), mutations in the DNA polymerase gene likely to be important in the generation of resistant virus were identified.

Sequential Switching of Dna Polymerase and Thymidine Kinase-Mediated Hsv-1 Drug Resistance in An Immunocompromised Child

Sequential herpes simplex virus type 1 (HSV-1) isolates were obtained from a paediatric haematopoietic stem cell transplant (HSCT) patient who received prolonged therapy with acyclovir (ACV) followed by foscarnet (PFA) and topical cidofovir (HPMPC) for severe persistent mucocutaneous HSV-1 infection. The isolates were retrospectively studied for drug resistance. The first resistant isolate associated with clinical failure of antiviral therapy emerged 44 days post-ACV treatment initiation. Susceptibility testing revealed an ACV-resistant HSV strain that demonstrated cross resistance to PFA in the absence of any previous PFA treatment. The observed cross resistance was conferred by a single amino acid substitution, Ser724Asn, in the HSV DNA polymerase (DNA pol) gene. During the subsequent course of ACV therapy, the ACV/PFA-cross-resistant isolates were replaced by ACV-resistant, PFA-sensitive isolates. These isolates carried no DNA pol mutations, but had an Arg163His substitution in the thymidine kinase gene. Upon subsequent switching of antiviral therapy from ACV to PFA, the original ACV/PFA-cross-resistant DNA pol mutant re-appeared. Our study shows the emergence of different drug-resistant HSV variants during ongoing, unchanged ACV therapy. Furthermore, a rapid re-selection of the original resistant variant was observed after switch. For optimal antiviral management of HSV infections in HSCT recipients, therapeutic decisions should be guided by drug susceptibility results whenever therapeutic failure is observed and/or when changes in antiviral treatment are considered.

Drug resistance patterns of recombinant herpes simplex virus DNA polymerase mutants generated with a set of overlapping cosmids and plasmids

Herpes simplex virus (HSV) DNA polymerase (Pol) mutations can confer resistance to all currently available antiherpetic drugs. However, discrimination between mutations responsible for drug resistance and those that are part of viral polymorphism can be difficult with current methodologies. A new system is reported for rapid generation of recombinant HSV type 1 (HSV-1) DNA Pol mutants based on transfection of a set of overlapping viral cosmids and plasmids. With this approach, twenty HSV-1 recombinants with single or dual mutations within the DNA pol gene were successfully generated and subsequently evaluated for their susceptibilities to acyclovir (ACV), foscarnet (FOS), cidofovir (CDV), and adefovir (ADV). Mutations within DNA Pol conserved regions II (A719T and S724N), VI (L778M, D780N, and L782I), and I (F891C) were shown to induce cross-resistance to ACV, FOS, and ADV, with two of these mutations (S724N and L778M) also conferring significant reduction in CDV susceptibility. Mutant F891C was associated with the highest levels of resistance towards ACV and FOS and was strongly impaired in its replication capacity. One mutation (D907V) lying outside of the conserved regions was also associated with this ACV-, FOS-, and ADV-resistant phenotype. Some mutations (K522E and Y577H) within the delta-region C were lethal, whereas others (P561S and V573M) induced no resistance to any of the drugs tested. Recombinants harboring mutations within conserved regions V (N961K) and VII (Y941H) were resistant to ACV but susceptible to FOS. Finally, mutations within conserved region III were associated with various susceptibility profiles. This new system allows a rapid and accurate evaluation of the functional role of various DNA Pol mutations, which should translate into improved management of drug-resistant HSV infections.

Herpes simplex virus resistance to antiviral drugs

Herpes simplex virus (HSV) infections are efficiently treated with antiviral drugs such as acyclovir (ACV). However, resistance has been reported, mainly among immunocompromised patients (prevalence around 5%) and particularly allogeneic bone marrow transplant patients (prevalence reaching 30%). Resistance to ACV is associated with mutations on one of the two viral enzymes involved in the ACV mechanism of action: thymidine kinase (TK) and DNA polymerase. In 95% of the cases, ACV resistance is associated with a mutation in the TK gene as this enzyme is not essential for viral replication, unlike viral DNA polymerase, which is rarely involved in resistance. Strains resistant to ACV are almost always cross-resistant to other TK-dependent drugs such as penciclovir and famciclovir. Resistant infections can be managed by foscarnet or cidofovir but both are more toxic than ACV. These drugs also inhibit viral DNA polymerase but they are active on most ACV-resistant HSV as they do not depend on TK; nevertheless virus resistant to ACV because of a mutation in the DNA polymerase may be cross-resistant to these molecules. Published data on genetic characterization of resistant clinical isolates point out hot spots in viral TK and DNA polymerase genes. TK mutations associated with resistance are either insertion or deletion (codons 92 and 146 of TK gene) or substitution (codon 176-177, 336 of TK gene). DNA polymerase mutations are mainly located in conserved sites of the enzyme. A high level of gene polymorphism has also been reported for these genes, especially for TK. These results are useful for the development of rapid genotypic assays for the detection of mutations associated with resistance.

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.

Elimination of the truncated message from the herpes simplex virus thymidine kinase suicide gene

Introduction of the Herpes simplex virus thymidine kinase (HSV-tk) gene into target cells renders them susceptible to killing by ganciclovir (GCV). We are studying the use of HSV-tk-transduced T lymphocytes in the context of hematopoietic stem cell transplantation. We have previously shown, in vitro and in vivo, the occurrence of transduced cells resistant to GCV due to a deletion within HSV-tk. This deletion, a consequence of the presence of cryptic splice donor and acceptor sites, originates in the retroviral producer cell. Here we adopt two different methods that introduce third-base degenerate changes at the cryptic splice sites and so prevent splicing. Consequently, the HSV-tk protein is unaltered and the sensitivity of the target cells to GCV is preserved. The use of this mutated HSV-tk should reduce the likelihood of the development of resistant genetically modified cells during clinical trials.