Gene polymorphism of thymidine kinase and DNA polymerase in clinical strains of herpes simplex virus

Variant Analysis of the Thymidine Kinase and DNA Polymerase Genes of Herpes Simplex Virus in Korea: Frequency of Acyclovir Resistance Mutations

The thymidine kinase (TK) and DNA polymerase (pol) genes of the herpes simplex viruses type 1 (HSV-1) and type 2 (HSV-2) are two important genes involved in antiviral resistance. We investigated the genetic polymorphisms of the HSV-TK and pol genes in clinical isolates from Korean HSV-infected patients using next-generation sequencing (NGS) for the first time in Korea. A total of 81 HSV-1 and 47 HSV-2 isolates were examined. NGS was used to amplify and sequence the TK and pol genes. Among the 81 HSV-1 isolates, 12 and 17 natural polymorphisms and 9 and 23 polymorphisms of unknown significance in TK and pol were found, respectively. Two HSV-1 isolates (2.5%) exhibited the E257K amino acid substitution in TK, associated with antiviral resistance. Out of 47 HSV-2 isolates, 8 natural polymorphisms were identified in TK, and 9 in pol, with 13 polymorphisms of unknown significance in TK and 10 in pol. No known resistance-related mutations were observed in HSV-2. These findings contribute to our understanding of the genetic variants associated with antiviral resistance in HSV-1 and HSV-2 in Korea, with frequencies of known antiviral resistance-related mutations of 2.5% and 0% in HSV-1 and HSV-2, respectively.

Novel UL23 and UL30 substitutions in HSV1 and HSV2 viruses related to polymorphism or drug resistance

Data on herpes simplex virus (HSV) polymorphism as well as acyclovir (ACV) and foscarnet (FOS) resistance mutations are not exhaustive and may hinder accurate diagnosis by next-generation sequencing (NGS). Here, we report novel UL23 and UL30 substitutions for HSV1 and HSV2 identified in immunocompromised patients treated for hematological malignancies during the last 6 years of HSV resistance surveillance at the University Hospital of Lyon. For HSV1, 35 novel UL23 substitutions and 52 novel UL30 substitutions were identified. For HSV2, 2 novel UL23 substitutions and 12 novel UL30 substitutions were identified. These results allow to complete the database of HSV1 and HSV2 substitutions, related either to polymorphism or to ACV and FOS resistance.

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.

HSV susceptibility to acyclovir - genotypic and phenotypic characterization

BACKGROUND

Infections due to drug-resistant herpes simplex viruses (HSV) represent an important clinical concern, especially in immunocompromised patients. The present study was aimed at detecting acyclovir (ACV) susceptibility in HSV clinical samples.

METHODS

A total of 13 HSV-positive clinical samples (5 HSV-1 and 8 HSV-2) recovered from patients (1 immunocompromised and 12 of unknown immune status) were included in the study. The genotypic analysis involved an initial UL23 (thymidine kinase) gene sequencing, followed by a confirmatory phenotypic assay using plaque reduction technique.

RESULTS

Two novel amino acid changes, A37V and H283N, were detected in HSV-1 positive clinical samples, which were found to be susceptible to acyclovir (half maximal effective concentration = 1.5 µM) by plaque reduction assay.

CONCLUSIONS

These two novel amino acid changes could be therefore considered as natural polymorphisms, a phenomenon widely associated with the HSV-UL23 gene.

Relevance of non-synonymous thymidine kinase mutations for antiviral resistance of recombinant herpes simplex virus type 2 strains

Therapy or prophylaxis of herpes simplex virus type 2 (HSV-2) infections with the nucleoside analog aciclovir (ACV) can lead to the emergence of drug-resistant HSV-2 strains, particularly in immunocompromised patients. In this context, multiple amino acid (aa) changes can accumulate in the ACV-converting viral thymidine kinase (TK) which hampers sequence-based diagnostics significantly. In this study, the so far unknown or still doubted relevance of several individual aa changes for drug resistance in HSV-2 was clarified. For this purpose, ten recombinant fluorescent HSV-2 strains differing in the respective aa within their TK were constructed using the bacterial artificial chromosome (BAC) pHSV2(MS)Lox. Similar TK expression levels and similar replication behavior patterns were demonstrated for the mutants as compared to the unmodified BAC-derived HSV-2 strain. Subsequently, the resulting strains were tested for their susceptibility to ACV as well as penciclovir (PCV) in parallel to a modified cytopathic effect (CPE) inhibition assay and by determining the relative fluorescence intensity (quantified using units, RFU) as a measure for the viral replication capacity. While aa changes Y53N and R221H conferred ACV resistance with cross-resistance to PCV, the aa changes G25A, G39E, T131M, Y133F, G150D, A157T, R248W, and L342W maintained a susceptible phenotype against both antivirals. The CPE inhibition assay and the measurement of relative fluorescence intensity yielded comparable results for the phenotypic testing of recombinant viruses. The latter test showed some technical advantages. In conclusion, the significance of single aa changes in HSV-2 TK on ACV/PCV resistance was clarified by the construction and phenotypic testing of recombinant viral strains. This was facilitated by the fluorescence based method.

Highly conserved intragenic HSV-2 sequences: Results from next-generation sequencing of HSV-2 UL and US regions from genital swabs collected from 3 continents

INTRODUCTION

Understanding the variability in circulating herpes simplex virus type 2 (HSV-2) genomic sequences is critical to the development of HSV-2 vaccines.

METHODS

Genital lesion swabs containing ≥ 107log10 copies HSV DNA collected from Africa, the USA, and South America underwent next-generation sequencing, followed by K-mer based filtering and de novo genomic assembly. Sites of heterogeneity within coding regions in unique long and unique short (UL_US) regions were identified. Phylogenetic trees were created using maximum likelihood reconstruction.

RESULTS

Among 46 samples from 38 persons, 1468 intragenic base-pair substitutions were identified. The maximum nucleotide distance between strains for concatenated UL_US segments was 0.4%. Phylogeny did not reveal geographic clustering. The most variable proteins had non-synonymous mutations in < 3% of amino acids.

CONCLUSIONS

Unenriched HSV-2 DNA can undergo next-generation sequencing to identify intragenic variability. The use of clinical swabs for sequencing expands the information that can be gathered directly from these specimens.

Stepwise characterization of non-synonymous mutations in the HSV-1 thymidine kinase gene by different functional assays

Twenty amino acid substitutions in the thymidine kinase (TK) of clinical herpes simplex virus type 1 strains were assessed for conferring acyclovir (ACV) resistance. Site-directed mutagenesis, cell-free protein synthesis and protein expression in Escherichia coli were performed to obtain recombinant TK proteins, which were authenticated by Western blotting. A modified enzyme-linked immunosorbent assay (ELISA) was carried out to determine the phosphorylation activity of the mutants towards 5-bromo-2'-deoxyuridine (BrdU). The activity against ACV and deoxythymidine (dT) was analyzed by high performance liquid chromatography/ultraviolet spectroscopy (HPLC/UV) following incubation of recombinant TK with ACV and dT. Using ELISA, seven substitutions (G61E, A93V, M121K, R163G, P173del, V238F, G264V) showing negative activity could be classified likely as resistance-related, eleven (Q15K, R20C, R32H, E43A, E43D, R89H, A156V, P269S, G271V, S276N, I326V) with high activity as natural polymorphisms, and two (N244H and N376stop) with low phosphorylation activity. Since the N244H protein did not show any activity towards ACV, but activity towards dT using HPLC/UV, it was classified as TK with altered substrate specificity. In conclusion, the ELISA determining activity towards BrdU is suitable for the characterization of substitutions regarding their significance for resistance. Ambiguous results can be re-assessed by HPLC/UV, which classifies TK with altered substrate specificity.

Worldwide circulation of HSV-2 × HSV-1 recombinant strains

Homo sapiens harbor two distinct, medically significant species of simplexviruses, herpes simplex virus (HSV)-1 and HSV-2, with estimated divergence 6-8 million years ago (MYA). Unexpectedly, we found that circulating HSV-2 strains can contain HSV-1 DNA segments in three distinct genes. Using over 150 genital swabs from North and South America and Africa, we detected recombinants worldwide. Common, widely distributed gene UL39 genotypes are parsimoniously explained by an initial >457 basepair (bp) HSV-1 × HSV-2 crossover followed by back-recombination to HSV-2. Blocks of >244 and >539 bp of HSV-1 DNA within genes UL29 and UL30, respectively, have reached near fixation, with a minority of strains retaining sequences we posit as ancestral HSV-2. Our data add to previous in vitro and animal work, implying that in vivo cellular co-infection with HSV-1 and HSV-2 yields viable interspecies recombinants in the natural human host.

Herpesvirus DNA polymerases: Structures, functions and inhibitors

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

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.

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.

Herpes simplex virus drug-resistance: new mutations and insights

PURPOSE OF REVIEW

Acyclovir (ACV) is the first-line treatment for the management of herpes simplex virus 1 (HSV-1) and 2 (HSV-2) diseases. Long-term administration of the drug for the treatment of chronic infections in the immunocompromised host can lead to the development of ACV-resistance. This review provides an update of the mutations linked to drug-resistance and issues to be considered in the management of HSV infections refractory to antiviral therapy.

RECENT FINDINGS

Recent data have shown that HSV drug-resistance should be taken into account not only in immunocompromised individuals but also in immunocompetent persons when HSV infections involve 'immune-privileged sites'. Thus, drug-resistance typing is recommended in cases of ACV unresponsive herpetic keratitis and herpes simplex encephalitis. Several issues regarding HSV drug-resistance were highlighted by recent studies. Phenotypic and genotypic antiviral resistance may vary not only from different compartments but also over time, highlighting the importance of characterizing longitudinal HSV isolates from all sites. Combination therapy should be considered when viruses with distinct phenotype/genotype are identified at one or at distinct body sites.

SUMMARY

Surveillance of HSV drug-resistance is highly recommended in immunocompromised patients and in immunocompetent individuals with infections implicating 'immune-privileged sites' to rationally adapt antiviral treatment.

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.

Acyclovir-resistant herpes simplex virus type 1 in intra-ocular fluid samples of herpetic uveitis patients

BACKGROUND

Acyclovir (ACV) is the antiviral drug of choice to treat patients with herpes simplex virus type 1 (HSV-1) uveitis. The prevalence of intra-ocular ACV-resistant (ACV(R)) HSV-1 in herpetic uveitis is unknown and may have clinical consequences. In addition to its predictive value on ACV susceptibility, the polymorphic HSV-1 thymidine kinase (TK) gene facilitates differentiation between HSV-1 strains.

OBJECTIVES

The objective of this study was to determine the genetic composition and ACV susceptibility of the causative virus in intra-ocular fluid samples (IOF) of HSV-1 uveitis patients.

STUDY DESIGN

The intra-ocular HSV-1 pool from 11 HSV-1 uveitis patients was determined by sequencing IOF-derived viral TK genes. The ACV susceptibility profile of the cloned intra-ocular TK variants was defined by mass spectrometry. In addition, the ganciclovir (GCV) susceptibility of the ACV(R) HSV-1 TK variants was defined.

RESULTS

Intra-ocular fluid samples of HSV-1 uveitis patients contain HSV-1 quasispecies, principally consisting of one major and multiple genetically related minor patient-specific TK variants. Four of 10 patients analyzed had an intra-ocular ACV(R) HSV-1 of which 3 were cross-resistant to GCV. The ACV(R) profile of intra-ocular HSV-1 did not correlate with symptomatic ACV treatment.

CONCLUSIONS

Affected eyes of HSV-1 uveitis patients are commonly infected with a patient-specific HSV-1 quasispecies, including one major and multiple genetically related minor variants. A relatively high prevalence of intra-ocular ACV(R) HSV-1, mainly ACV/GCV cross-resistant viruses, was detected in HSV-1 uveitis patients.

Impact of novel mutations of herpes simplex virus 1 and 2 thymidine kinases on acyclovir phosphorylation activity

The acyclic analogue of guanosine acyclovir (ACV) constitutes the first-line drug for the treatment of herpes simplex virus (HSV) infections. ACV activation requires primophosphorylation by virus-encoded HSV thymidine kinase (TK). In 95% of cases, HSV resistance to ACV is associated with mutations located in TK. The aim of this work was to address the question of the potential involvement of novel HSV-1 and HSV-2 TK mutations in reduced susceptibility to ACV using a novel nonradioactive method, based on luminescent quantitation of ADP, for the evaluation of in vitro phosphorylation activity of TK. All recombinant TKs tested exhibited significantly lower ACV phosphorylation activities in comparison with those of reference KOS or gHSV-2 TKs (p<0.015), therefore indicating that amino acid changes Y53D, L170P, R176W, A207P (HSV-1) and S66P, A72S, I101S, M183I (HSV-2) were likely to be involved in HSV resistance to ACV.

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.