Sequencing of cytomegalovirus UL97 gene for genotypic antiviral resistance testing

Emergence of Letermovir-resistant HCMV UL56 mutant during rescue treatment in a liver transplant recipient with ganciclovir-resistant infection HCMV: a case report

Background

Human Cytomegalovirus (HCMV) still represents a crucial concern in solid organ transplant recipients (SOTRs) and the use of antiviral therapy are limited by side effects and the selection of viral mutations conferring antiviral drug resistance.

Case presentation

Here we reported the case of an HCMV seronegative patient with common variable immunodeficiency (CVID), multiple hepatic adenomatosis, hepatopulmonary syndrome and portal hypertension who received a liver transplant from an HCMV seropositive donor. The patient was treated with Valganciclovir (vGCV) and then IV Ganciclovir (GCV) at 5 week post-transplant for uncontrolled HCMV DNAemia. However, since mutation A594V in UL97 gene conferring resistance to ganciclovir was reported, GCV therapy was interrupted. Due to the high toxicity of Foscarnet (FOS) and Cidofovir (CDV), Letermovir (LMV) monotherapy at the dosage of 480 mg per day was administered, with a gradual viral load reduction. However, a relapse of HCMV DNAemia revealed the presence of mutation C325Y in HCMV UL56 gene conferring resistance to LMV.

Conclusions

In conclusion, even if LMV is an effective and favorable safety molecule it might have a lower genetic barrier to resistance. A warning on the use of LMV monotherapy as rescue treatments for HCMV GCV-resistant infections in transplant recipients is warranted.

Prevalence of UL97 gene mutations and polymorphisms in cytomegalovirus infection in the colon associated with or without ulcerative colitis

Cytomegalovirus (CMV) reactivation in the colon is common in patients with severe ulcerative colitis (UC). Ganciclovir (GCV) resistance conferring CMV UL97 gene mutations have been reported in recent years. However, the prevalence of UL97 gene mutations in GCV-naive CMV infection in the colon remains unknown. We investigated the prevalence of CMV UL97 gene mutations in patients with colonic CMV infection associated with or without UC. Twenty-two GCV-naive patients with colonic CMV infection, 15 with UC and 7 with other diseases, were enrolled. Frozen biopsy samples or formalin-fixed paraffin-embedded samples were used for nested polymerase chain reaction (PCR) amplification of the UL97 gene. Sanger DNA sequencing was performed. In comparison with AD169 reference strain, natural polymorphisms were frequently detected in codons N68D (100%), I244V (100%), and D605E (86.4%). Seven polymorphisms were detected infrequently (< 10%) outside the kinase domain. However, no known GCV resistance mutations were found. There seemed to be no difference between the ratio of polymorphisms in patients with and without UC. In conclusions, we did not detect UL97 gene mutations associated with GCV resistance in GCV-naive patients with or without UC. Consistent with previous reports, D605E polymorphism may be used as a genetic marker for CMV in East Asian countries.

The Cytomegalovirus Protein Kinase pUL97:Host Interactions, Regulatory Mechanisms and Antiviral Drug Targeting

Human cytomegalovirus (HCMV) expresses a variety of viral regulatory proteins that undergo close interaction with host factors including viral-cellular multiprotein complexes. The HCMV protein kinase pUL97 represents a viral cyclin-dependent kinase ortholog (vCDK) that determines the efficiency of HCMV replication via phosphorylation of viral and cellular substrates. A hierarchy of functional importance of individual pUL97-mediated phosphorylation events has been discussed; however, the most pronounced pUL97-dependent phenotype could be assigned to viral nuclear egress, as illustrated by deletion of the UL97 gene or pharmacological pUL97 inhibition. Despite earlier data pointing to a cyclin-independent functionality, experimental evidence increasingly emphasized the role of pUL97-cyclin complexes. Consequently, the knowledge about pUL97 involvement in host interaction, viral nuclear egress and additional replicative steps led to the postulation of pUL97 as an antiviral target. Indeed, validation experiments in vitro and in vivo confirmed the sustainability of this approach. Consequently, current investigations of pUL97 in antiviral treatment go beyond the known pUL97-mediated ganciclovir prodrug activation and henceforward include pUL97-specific kinase inhibitors. Among a number of interesting small molecules analyzed in experimental and preclinical stages, maribavir is presently investigated in clinical studies and, in the near future, might represent a first kinase inhibitor applied in the field of antiviral therapy.

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

BACKGROUND

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

AIMS

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

MATERIAL AND METHODS

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

RESULTS

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

DISCUSSION

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

CONCLUSION

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

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.

Cytomegalovirus

Cytomegalovirus (CMV), the largest of the herpesviruses, causes a wide range of clinical syndromes, from asymptomatic infection to severe disease in immunocompromised hosts. Laboratory methods for diagnosis include molecular testing, antigenemia, culture, serology, and histopathology. Treatment of CMV infection and disease is indicated in selected immunocompromised hosts, and preventive approaches are indicated in high-risk groups. This chapter reviews the epidemiology, clinical aspects, and the laboratory diagnosis and management of CMV in immunocompromised hosts.

How I treat resistant cytomegalovirus infection in hematopoietic cell transplantation recipients

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

Analysis of ganciclovir-resistant human herpesvirus 6B clinical isolates using quenching probe PCR methodology

Quenching probe PCR (QP-PCR) analysis was used to determine the frequency of ganciclovir (GCV) resistance among clinical isolates of human herpesvirus 6B (HHV-6B) obtained from patients with primary viral infection and viral reactivation. Forty-two HHV-6B clinical isolates were repeatedly recovered from 15 hematopoietic stem cell transplant (HSCT) recipients, and 20 isolates were recovered from 20 exanthem subitum (ES) patients. Of the 15 HSCT recipients, 9 received GCV during the observation period; however, none of the ES patients were treated with GCV. Two established laboratory strains, Z29 and HST, were used as standards in this study. Regions 1 and 2 of the U69 gene of all of the clinical isolates demonstrated the same melting temperature as regions 1 and 2 of the Z29 strain. For region 3, the melting temperatures of all clinical isolates fell between the melting temperature of the plasmid containing the A462D single nucleotide polymorphism (SNP) and the melting temperature of the Z29 strain, and the melting temperatures profiles of all clinical isolates were similar to the melting temperature profile of the Japanese HST strain. As expected, none of the 20 clinical isolates recovered from the ES patients and the 14 isolates recovered from the HSCT recipients who did not receive GCV treatment carried the six known SNPs associated with GCV resistance. Interestingly, these six SNPs were not detected in the 28 clinical isolates recovered from the 9 HSCT recipients who received GCV. Additional sequence analysis of the U69 gene from the 15 representative isolates from the 15 HSCT recipients identified other SNPs. These SNPs were identical to those identified in the HST strain. Therefore, the rate of emergence of GCV-resistant HHV-6B strains appears to be relatively low, even in HSCT recipients treated with GCV.

Differential properties of cytomegalovirus pUL97 kinase isoforms affect viral replication and maribavir susceptibility

The human cytomegalovirus (HCMV)-encoded kinase pUL97 is required for efficient viral replication. Previous studies described two isoforms of pUL97, the full-length isoform (M1) and a smaller isoform likely resulting from translation initiation at codon 74 (M74). Here, we report the detection of a third pUL97 isoform during viral infection resulting from translation initiation at codon 157 (isoform M157). The consistent expression of isoform M157 as a minor component of pUL97 during infection with clinical and laboratory-adapted HCMV strains was suppressed when codon 157 was mutagenized. Viral mutants expressing specific isoforms were generated to compare their growth and drug susceptibility phenotypes, as well as pUL97 intracellular localization patterns and kinase activities. The exclusive expression of isoform M157 resulted in substantially reduced viral growth and resistance to the pUL97 inhibitor maribavir while retaining susceptibility to ganciclovir. Confocal imaging demonstrated reduced nuclear import of amino-terminal deletion isoforms compared to isoform M1. Isoform M157 showed reduced efficiency of various substrate protein interactions and autophosphorylation, whereas Rb phosphorylation was preserved. These results reveal differential properties of pUL97 isoforms that affect viral replication, with implications for the antiviral efficacy of maribavir.

IMPORTANCE

The HCMV UL97 kinase performs important functions in viral replication that are targeted by the antiviral drug maribavir. Here, we describe a naturally occurring short isoform of the kinase that when expressed by itself in a recombinant virus results in altered intracellular localization, impaired growth, and high-level resistance to maribavir compared to those of the predominant full-length counterpart. This is another factor to consider in explaining why maribavir appears to have variable antiviral activity in cell culture and in vivo.

Rapid detection of human cytomegalovirus UL97 and UL54 mutations directly from patient samples

Human cytomegalovirus (CMV) is a significant contributor to morbidity and mortality in immunocompromised patients, particularly in the transplant setting. The availability of anti-CMV drugs has improved treatment, but drug resistance is an emerging problem. Here, we describe an improved, rapid, sequencing-based assay for the two genes in CMV where drug resistance occurs, the UL97 and UL54 genes. This assay is performed in 96-well format with a single master mix and provides clinical results within 2 days. It sequences codons 440 to 645 in the UL97 gene and codons 255 to 1028 in the UL54 gene with a limit of detection of 240 IU/ml. With this assay, we tested 43 specimens that had previously been tested for UL97 drug resistance and identified 3 with UL54 mutations. One of these patients had no concurrent UL97 mutation, pointing toward the need for an assay that facilitates dual UL97/UL54 gene testing for complete resistance profiling.

Rapid detection and quantitation of ganciclovir resistance in cytomegalovirus quasispecies

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

Successful ganciclovir treatment of primary cytomegalovirus infection containing the UL97 mutation N510S in an intestinal graft recipient

In solid organ transplantation, human cytomegalovirus (HCMV) is considered to be the most important viral pathogen. We report a case of a CMV R-/D+ small intestine transplant recipient with a primary CMV infection on valganciclovir prophylaxis. Sequencing of the HCMV DNA for drug resistance-associated mutations revealed the UL97 mutation N510S. This mutation has been initially reported to confer ganciclovir resistance. Based on in vitro recombinant phenotyping, this assumption has recently been questioned. Switching the antiviral treatment to an intravenous regimen of ganciclovir eliminated HCMV DNAemia, showing the in vivo efficacy of ganciclovir for the UL97 mutation N510S. Hence, knowledge of drug efficacy is crucial for an adequate choice of antiviral medication, carefully balancing antiviral potency versus the risk of harmful side effects.

Effects on maribavir susceptibility of cytomegalovirus UL97 kinase ATP binding region mutations detected after drug exposure in vitro and in vivo

Resistance to the experimental human cytomegalovirus (CMV) UL97 kinase inhibitor maribavir has been mapped to UL97 mutations at codons 353, 397, 409 and 411, in the kinase ATP-binding region, and to mutations in the UL27 gene. We studied the maribavir susceptibility phenotypes of additional UL97 mutations observed in vitro and in clinical trials, and the effect of simultaneous mutation in both UL97 and UL27. In vitro selection under maribavir identified a new locus of UL97 mutation within the conserved kinase p-loop (L337M), which conferred low grade maribavir resistance (3.5-fold increased EC50) without ganciclovir cross-resistance. During maribavir Phase III CMV prevention clinical trials, three previously unknown UL97 sequence variants were detected in plasma samples after 27-98 days of drug exposure (I324V, S334G and S386L). These variants did not confer any drug resistance despite proximity to mutations that confer maribavir resistance. The UL27 resistance mutation R233S, when added to strains containing UL97 mutations L337M or V353A, doubled their maribavir EC50s. These results expand the range of UL97 maribavir-resistance mutations into another part of the kinase ATP-binding region, but offer no genotypic evidence that development of drug resistance affected the outcomes of Phase III maribavir clinical trials after drug exposure of up to 14 weeks. There is a potential for increased maribavir resistance in UL27-UL97 double mutants.

[Prevention of cytomegalovirus disease in lung transplantation]

Lung transplant recipients, more than other organ transplant recipients, are at particular risk for cytomegalovirus (CMV) infection and disease. CMV prevention avoids the indirect effects of this virus, such as opportunistic fungal infections and obliterative bronchiolitis, the latter being the major limiting factor in the long-term success of lung-transplantation. CMV prevention strategies have significantly reduced CMV disease and CMV-related mortality. Two major strategies are commonly used for CMV prevention: universal prophylaxis and preemptive therapy. In lung transplant recipients, the efficacy and safety of preemptive treatment have not been studied and therefore, cannot be recommended. Universal prophylaxis is the best strategy for preventing CMV disease in lung transplant recipients. There is no consensus on the optimal duration of prophylaxis, but the recently published GESITRA-SEIMC/REIPI 2011 Guidelines for the management of CMV infection in solid-organ transplant patients recommend 6 months posttransplantation. In D+/R- recipients, this period can be prolonged to 12 months if there are difficulties in monitoring at 6 months posttransplantation. The future of prevention will probably depend on immunoguided strategies.

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.

Does Cytomegalovirus Develop Resistance following Antiviral Prophylaxis and Treatment in Renal Transplant Patients in Kuwait?

The resistance of cytomegalovirus (CMV) to ganciclovir or valganciclovir is a factor in therapeutic failure and disease progression. CMV strains resistant to ganciclovir or valganciclovir have been associated with specific mutations in the UL97 and UL54 genes. Sequencing of both CMV UL97 and UL54 genes was performed to detect the presence of CMV antiviral resistance in six patients who received ganciclovir (and/or valganciclovir) and had prolonged detectable CMV DNA in their blood during antiviral treatment. Sequencing results showed no specific mutations in either UL97 or UL54 gene of CMV and therefore the CMV strains in kidney transplant patients who received ganciclovir either prophylactically or therapeutically were from the wild type. Our results suggest that CMV management and immunosuppression protocols for kidney transplant patients followed in the Organ Transplant Centre, Kuwait, is very effective in reducing the opportunity of developing CMV antiviral resistance.

Mass spectrometry-based comparative sequencing to detect ganciclovir resistance in the UL97 gene of human cytomegalovirus

BACKGROUND

Persistent infections with herpesviruses such as human cytomegalovirus (HCMV) frequently occur after solid organ or stem cell transplantation, and are due to either failure of the host to immunologically control the virus or emerging resistance of the virus to the antiviral drug(s) used. Antiviral therapy can be guided by viral drug susceptibility testing based on screening for known resistance-inducing mutations in the viral genome. Mass spectrometry-based comparative sequence analysis (MSCSA) might be advantageous for this purpose because of its suitability for semi-automation.

OBJECTIVES

The applicability of MSCSA to detect sequence polymorphisms and drug resistance-inducing mutations in the HCMV genome was investigated.

STUDY DESIGN

We analyzed the 3' part of the HCMV UL97 gene, which encodes the kinase that is activated by the commonly used anti-HCMV drug ganciclovir. Sequences obtained by MSCSA of material from HCMV-infected patients (43 samples) and the HCMV type strain were compared to conventional cycle sequencing results.

RESULTS

In 94.1% of all samples the results obtained by MSCSA of the UL97 gene were identical to those from conventional cycle sequencing. The threshold to detect mutant sequences in a mixture with wild-type material was 20% using either technique. Furthermore, MSCSA was successfully applied to study the development of drug resistance in a patient who developed encephalitis due to ganciclovir-resistant HCMV.

CONCLUSIONS

MSCSA was found to be equally accurate compared to conventional cycle sequencing in the analysis of UL97 of HCMV.

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

PURPOSE

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

DESIGN

Literature review.

METHODS

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

RESULTS

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

CONCLUSIONS

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

Antiviral drug resistance of human cytomegalovirus

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

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.

Stereoselective phosphorylation of cyclopropavir by pUL97 and competitive inhibition by maribavir

Human cytomegalovirus (HCMV) is a widespread pathogen that can cause severe disease in immunologically immature and immunocompromised individuals. Cyclopropavir (CPV) is a guanine nucleoside analog active against human and murine cytomegaloviruses in cell culture and efficacious in mice by oral administration. Previous studies established that the mechanism of action of CPV involves inhibition of viral DNA synthesis. Based upon this action and the structural similarity of CPV to ganciclovir (GCV), we hypothesized that CPV must be phosphorylated to a triphosphate to inhibit HCMV DNA synthesis and that pUL97 is the enzyme responsible for the initial phosphorylation of CPV to a monophosphate (CPV-MP). We found that purified pUL97 phosphorylated CPV 45-fold more extensively than GCV, a known pUL97 substrate and the current standard of treatment for HCMV infections. Kinetic studies with CPV as the substrate for pUL97 demonstrated a Km of 1,750+/-210 microM. Introduction of 1.0 or 10 nM maribavir, a known pUL97 inhibitor, and subsequent Lineweaver-Burk analysis demonstrated competitive inhibition of CPV phosphorylation, with a Ki of 3.0+/-0.3 nM. Incubation of CPV with pUL97 combined with GMP kinase [known to preferentially phosphorylate the (+)-enantiomer of CPV-MP] established that pUL97 stereoselectively phosphorylates CPV to its (+)-monophosphate. These results elucidate the mechanism of CPV phosphorylation and help explain its selective antiviral action.

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

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

Recombinant phenotyping of cytomegalovirus UL97 kinase sequence variants for ganciclovir resistance

A strain of human cytomegalovirus, T2211, modified from standard laboratory strain AD169 to contain a secreted alkaline phosphatase reporter gene for rapid viral quantitation, was cloned as a bacterial artificial chromosome, BA1, and then mutagenized to create recombinant viruses containing viral UL97 kinase sequence variants found in clinical specimens after ganciclovir treatment, but with no phenotypic data to determine their role in drug resistance. Seven control strains and 14 other recombinant strains were phenotyped for ganciclovir resistance and compared with similar strains created using prior technology to show a good concordance of findings. Sequence changes V466M, H469Y, A478V, N510S, A588V, K599R, L600I, G623S, T659I, and V665I were found to confer no significant ganciclovir resistance, while mutations L405P, M460T, A594E, and C603R conferred 3- to 9-fold increases in ganciclovir 50% inhibitory concentrations. Different mutations at codons 594 (A594V, A594E) and 603 (C603W, C603S) conferred varied amounts of ganciclovir resistance. Advances in recombinant phenotyping make it easier to show that many uncharacterized UL97 sequence variants do not confer ganciclovir resistance, but some are newly confirmed as resistance associated, including one (L405P) which is outside the codon range where such mutations are usually found. This information should improve the interpretation of genotypic data generated by diagnostic laboratories.

PCR with quenching probes enables the rapid detection and identification of ganciclovir-resistance-causing U69 gene mutations in human herpesvirus 6

A single-nucleotide polymorphism detection assay using PCR with quenching probes (QP-PCR) was developed for the rapid detection of antiviral drug-resistance mutations of human herpesvirus 6 (HHV-6). The mutations examined were in the HHV-6 U69 gene, and were single-base mutations in sequences known to be associated with ganciclovir (GCV) resistance in HCMV. We previously confirmed that they conferred GCV resistance to recombinant baculoviruses (Nakano et al., J. Virol. Methods 161:223-230, 2009). Six characterized mutations, including a previously reported one that encodes a GCV-sensitive kinase-activity mutant (Isegawa et al., J. Clin. Virol. 44:15-19, 2009), were used. The six mutations were separated into three groups based on their location in the U69 protein, and detected by the hybridization of three probes. We developed and validated a set of assays for these mutations using PCR followed by differential melting of a fluorescently labeled oligo probe, on a Roche Light Cycler platform. Nucleobase quenching was used to detect the hybridized probe. The optimized assay could distinguish the different mutants, and easily detected mutants representing 30% of the DNA in a mixed sample. This QP-PCR assay permitted the rapid (1.5 h), objective, and reproducible detection of drug-resistant mutations of HHV-6.

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

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

Detection and identification of U69 gene mutations encoded by ganciclovir-resistant human herpesvirus 6 using denaturing high-performance liquid chromatography

A denaturing high-performance liquid chromatography (dHPLC) assay was developed to detect antiviral drug-resistance mutations of human herpesvirus 6 (HHV-6). Recombinant baculoviruses were created that contained wild-type and mutant forms of the HHV-6 U69 gene, which determines sensitivity to the antiviral drug ganciclovir (GCV). The mutations causing GCV resistance in HHV-6 U69 were single-base mutations adapted from known GCV-resistant DNA sequences of HCMV, and their ability to confer GCV resistance on recombinant baculoviruses was confirmed. Six characterized mutant sequences, including one reported previously that encodes a GCV-sensitive kinase-activity mutant, were used. DNA was extracted, and the levels of homoduplex and heteroduplex DNA in the PCR products from mixed wild-type and mutant viral DNAs were determined using dHPLC. The optimized assay could distinguish the different mutants, and could detect mutants representing only 10% of the DNAs. The new assay with dHPLC readout permitted the rapid (4 h), objective, and reproducible detection of HHV-6 drug-resistance mutations.

Successful valganciclovir treatment of post-transplant cytomegalovirus infection in the presence of UL97 mutation N597D

Mutations in the human cytomegalovirus (CMV) UL97 protein kinase are the most common mechanism of ganciclovir (GCV) resistance in the clinical setting. A CMV strain with a previously unrecognized UL97 mutation N597D was identified in the blood of a heart transplant recipient who experienced a persistent CMV infection with high viral loads accompanying pain and fever while receiving valganciclovir (valGCV) therapy. The N597D mutation was transferred by mutagenesis to an antiviral sensitive CMV strain for analysis of antiviral susceptibility by standardized phenotypic assay. Recombinant phenotyping showed N597D conferred a less than twofold increase in GCV IC(50) compared to the sensitive control strain. Despite the presence of this mutation, valGCV eventually resolved the infection after 6 weeks of therapy. A subsequent CMV reactivation was also responsive to valganciclovir. This case illustrates the diversity of UL97 mutations in the codon segment 590-607 usually associated with GCV resistance, with some mutations producing minimal levels of resistance that do not preclude a therapeutic response to the drug. Accurate interpretation of genotypic test results ultimately requires experimental determination of the level of resistance conferred by newly discovered UL97 mutations.

Valganciclovir as pre-emptive therapy for cytomegalovirus infection post-allogenic stem cell transplantation: implications for the emergence of drug-resistant cytomegalovirus

OBJECTIVES

Valganciclovir is a well established drug for the management of cytomegalovirus (CMV) infection in haematopoietic stem cell transplantation (HSCT). Data concerning its safety regarding the development of drug resistance are required. The aim of the present study was to retrospectively investigate CMV drug resistance in a group of HSCT patients experiencing relapses of CMV infection after a first-line pre-emptive antiviral therapy.

METHODS

Thirteen adult HSCT patients out of 26 with asymptomatic CMV infection, experiencing relapsing infections 45-155 days after either intravenous (iv) ganciclovir (2 patients) or valganciclovir (11 patients), were studied. Genotypic assays for mutations in the viral phosphotransferase (UL97) and DNA-polymerase (UL54) genes were directly applied on patient specimens. Baseline CMV sequences were compared with those at the time of relapses to identify drug-resistant strains.

RESULTS

UL97 mutations A594V and M460V known to confer drug resistance developed in one relapsing patient who received iv ganciclovir as first-line therapy, corresponding to a rate of 7.7% of relapses due to drug-resistant strains and an overall 3.8% rate of infections due to CMV drug-resistant strains. UL54 drug resistance mutations were absent. No evidence of drug resistance was found in patients on valganciclovir either as first-line therapy or as treatment for relapses.

CONCLUSIONS

The safety profile of valganciclovir as anti-CMV pre-emptive therapy was confirmed, as well as that monitoring CMV drug resistance with genotypic tests on sequential isolates over the time-course of therapy offers guidance to tailor antiviral treatment in a clinically relevant time frame.

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.

Case report: persistent cytomegalovirus (CMV) infection after haploidentical hematopoietic stem cell transplantation using in vivo alemtuzumab: emergence of resistant CMV due to mutations in the UL97 and UL54 genes

Addition of in vivo alemtuzumab to the conditioning regimen enabled 2- or 3-locus-mismatched hematopoietic stem cell transplantation with an acceptable incidence of graft-versus-host-disease. However, the procedure was associated with a high incidence of cytomegalovirus (CMV) reactivation. Although preemptive therapy with ganciclovir prevented successfully severe CMV diseases and CMV-related mortality, a patient developed persistent positive CMV antigenemia for more than 1 year after transplantation and CMV disease, despite the use of ganciclovir and foscarnet. The in vitro susceptibility assay showed that the clinical isolate was resistant to foscarnet, moderately resistant to ganciclovir, but sensitive to cidofovir. Therefore, cidofovir was administered. CMV antigenemia became negative within 2 weeks and never developed again. Nucleotide sequence of the UL54 and UL97 of the clinical isolate showed 4 amino acid substitutions (V11L, Q578H, S655L, and G874R) in UL54 and 2 mutations (A140V and A594V) in UL97 compared with the Towne and AD169 strains. Ganciclovir resistance was suspected to be caused by both A594V of UL97 and Q578H of UL54, whereas foscarnet resistance was due mainly to Q578H of UL54. In conclusion, the in vitro susceptibility assay as well as nucleotide sequence of clinical isolate is important to choose appropriate antiviral agents for patients who have persistent CMV reactivation after stem cell transplantation.

Cytomegalovirus UL97 mutations in the era of ganciclovir and maribavir

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

Viral monitoring and successful treatment of a ganciclovir-resistant cytomegalovirus infection in a heart transplant recipient

We reported a ganciclovir (GCV)-resistant cytomegalovirus (CMV) infection in a heart transplant recipient. Genotypic and phenotypic susceptibility assays demonstrated an A594V mutation in the UL97 phosphotransferase gene and GCV IC(50)>96 microM. Low GCV concentration exposure, immunosuppressive treatment, donor-positive/recipient-negative CMV serostatus, viral reactivations within antiviral prophylaxis or treatment, contributed to GCV-resistant strain selection.

Rapid detection directly from patient serum samples of human cytomegalovirus UL97 mutations conferring ganciclovir resistance

Ganciclovir-resistant cytomegalovirus can cause disease and death in transplant recipients. We describe here a rapid PCR- and sequencing-based assay for ganciclovir resistance that can be performed in 1 to 2 working days directly from patient specimens, without the need for amplification of the virus by cell culture. An evaluation of 120 sequential samples submitted for clinical testing revealed a variety of silent and amino acid mutations.

Prevalence of human cytomegalovirus UL97 D605E mutation in transplant recipients in China

Human cytomegalovirus (CMV) resistance to gancyclovir (GCV) occurs via mutation in the UL97 gene, ethylene diamine tetraacetic acid blood samples were obtained from 23 transplant recipients who received a GCV implant. A nested polymerase chain reaction amplifying UL97 codons 450 to 672 was performed. Nested amplifications were sequenced directly. No known UL97 GCV resistance mutations were found. Eighteen of 23 patients (78%) had revealed mutations at codon 605 (D to E). Mutant D605E may reverse to wild-type during the follow-up treatment. We conclude that human CMV UL97 D605E mutation occurred in Chinese transplant recipients. This mutation may be regarded as a natural sequence variant.

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

BACKGROUND

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

OBJECTIVE

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

STUDY DESIGN

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

RESULTS

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

CONCLUSION

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

Multidrug resistance conferred by novel DNA polymerase mutations in human cytomegalovirus isolates

The emergence of antiviral-resistant cytomegalovirus (CMV) strains is a continuing clinical problem, with increased numbers of immunocompromised patients given longer-duration antiviral prophylaxis. Two previously unrecognized CMV DNA polymerase mutations (N408K and A834P) identified separately and together in at-risk lung and kidney transplant recipients and a third mutation (L737M) identified in a liver transplant recipient were characterized by marker transfer to antiviral-sensitive laboratory strains AD169 and Towne. Subsequent phenotypic analyses of recombinant strains demonstrated the ability of mutation N408K to confer ganciclovir (GCV) and cidofovir (CDV) resistance and of mutation A834P to confer GCV, foscarnet, and CDV resistance. Mutation L737M did not confer resistance to any of the antiviral agents tested. A recombinant strain containing both N408K and A834P demonstrated increased GCV and CDV resistance compared to the levels of resistance of the virus containing only the A834P mutation. The addition of mutation N408K in combination with A834P also partially reconstituted the replication impairment of recombinant virus containing only A834P. This suggests that perturbation of both DNA polymerization (A834P) and exonuclease (N408K) activities contributes to antiviral resistance and altered replication kinetics in these mutant strains. The identification of these multidrug-resistant CMV strains in at-risk seronegative recipients of organs from seropositive donors suggests that improved prophylactic and treatment strategies are required. The additive effect of multiple mutations on antiviral susceptibility suggests that increasing antiviral-resistant phenotypes can result from different virus-antiviral interactions.

Phenotyping of cytomegalovirus drug resistance mutations by using recombinant viruses incorporating a reporter gene

A new recombinant phenotyping method was developed for the analysis of drug resistance mutations in human cytomegalovirus (CMV). CMV strain T2211 was derived from strain AD169 by inserting unique restriction sites and a secreted alkaline phosphatase (SEAP) reporter gene for rapid viral quantitation. Specific viral UL97 and pol gene mutations were transferred by recombination into T2211, and their drug resistance phenotypes (for ganciclovir, foscarnet, or cidofovir) were determined by the drug concentrations required to reduce supernatant SEAP activity by 50% (IC50). Changes in the IC50 conferred by the mutations tested (UL97 M460V, C592G, A594V, and L595S and pol del981-2) were similar to those previously reported in marker transfer and conventional plaque reduction assays. The combination of UL97 C592G and pol del981-2 conferred much higher ganciclovir resistance than either mutation alone. The UL97 polymorphism D605E had no measurable effect on ganciclovir susceptibility, alone or in combination with common UL97 resistance mutations. Transfer into strain T2211 facilitates the phenotyping of newly observed mutations, combinations of mutations, and clinical CMV sequences without an accompanying viral isolate.

How evolution of mutations conferring drug resistance affects viral dynamics and clinical outcomes of cytomegalovirus-infected hematopoietic cell transplant recipients

Infection with cytomegalovirus (CMV) remains a significant cause of morbidity and mortality among hematopoietic cell transplant (HCT) recipients. We describe two pediatric HCT recipients who developed persistent and severe drug-resistant CMV infections. CMV resistance to foscarnet and ganciclovir was detected after only 6 and 11 weeks of therapy, respectively. Viral pol mutations associated with drug resistance in these patients included T838A (a novel mutation) and D588N, which were shown by marker transfer to confer foscarnet and multidrug resistance, respectively. Each of these mutations significantly reduced in vitro replication of CMV, suggesting that they may decrease viral fitness. This finding was further supported by the disappearance of mutations upon withdrawal of antiviral pressure in one patient. Novel antivirals or combination therapy may be required for the treatment of drug-resistant CMV in HCT recipients and perhaps in other severely immunocompromised patients.

Emergence and compartmentalization of fatal multi-drug-resistant cytomegalovirus infection in a patient with autosomal-recessive severe combined immune deficiency

The authors describe a patient with autosomal-recessive severe combined immunodeficiency (SCID) with severe, multiorgan cytomegalovirus (CMV) disease. In the face of appropriate therapy, the patient developed a 100-fold gradient in viral load across the blood-brain barrier. Disseminated disease, including pneumonitis, contributed to a fatal outcome. Serial genotypic analyses revealed multiple UL97 and UL54 (DNA polymerase) mutations that conferred phenotypic resistance to all currently licensed systemic CMV antivirals.

Cytomegalovirus UL97 mutations associated with ganciclovir resistance in immunocompromised patients from Argentina

BACKGROUND

Prolonged therapy with ganciclovir (GCV) can result in the development of GCV-resistant strains due to mutations in the viral phosphotransferase (UL97 gene) and/or in the viral DNA polymerase (UL54 gene).

OBJECTIVES

The purpose of this study was to detect by molecular methods the most prevalent UL97 mutants which confer ganciclovir-resistance in immunocompromised populations.

STUDY DESIGN

Patients from two populations were selected: (a) renal transplant patients with active cytomegalovirus (CMV) infection and more than one cycle of GCV; (b) HIV-infected patients with retinitis due to CMV, who were under GCV induction, maintenance therapy or withdrawal. Patients were followed up by pp65 antigenemia and by viral isolation from blood or/and urine samples. Two fragments (133 and 255pb) of the UL97 gene were amplified by polymerase chain reaction (PCR) from CMV isolates.

RESULTS

Nine from 12 isolates obtained were sequenced, three from two renal transplant patients and six from five HIV-infected patients. A UL97 mutation, known to confer GCV resistance, was found in two isolates from a renal transplant patient. A methionine to valine mutation at codon 460 (M460V) was detected. These isolates exhibited another mutation at codon 605, whose amino acid changed from aspartic acid (D) to glutamic acid (E). These findings were observed after treatment with IV-GCV/ O-GCV/ IV-GCV for 151 days. The 605 mutation was also detected in leukocytes from the same patient previous to the beginning of the treatment with GCV.

CONCLUSIONS

Although a known resistant mutation appeared in a renal transplant patient, it was not associated with CMV disease. We suggest that the D605E mutation could "partially or totally compensate" for the effect of GCV resistance conferred by the 460 mutation. Further studies should be performed to confirm this hypothesis.