| Summary: | The majority of herpes simplex virus (HSV) isolates resistant to acyclovir (ACV) have a deficiency of thymidine kinase (TD[sup D]). Such mutants are able to establish latency in animal models but are unable to reactivate
which has contributed to the clinical dogma that, following effective eradication, such outbreaks are inevitably followed by ACV-susceptible (ACV[sup S]) reactivations. In addition, genotypic information regarding
mutations conferring the ACV-resistant (ACV[sup R]) phenotype onto HSV is very limited.
HSV type 2 strains 1737 and 89-063, ACV[sup R] and uniformly TD[sup D] by all conventional assays, clinically reactivated i n AIDS patients in the absence of antiviral drug pressure. Investigation of their neurovirulence and latency characteristics in mouse models, however, yielded neurovirulent TK-wild
type (TK[sup WT]) and TK-low producer (TK[sup LP]) populations. Two further ACV[sup R] reactivated isolates (89-353 and 90-150) contained mixed TK[sup WT]/TD[sup D] populations by plaque autoradiography while one (2370) likely exhibited a TK-altered phenotype.
Mutations conferring the ACV[sup R] phenotype were assessed by plaque purification, amplification of the tk gene by polymerase chain reaction (PCR) and cycle sequencing. Three of 8 plaque isolates, 1737-14, 90-150-3 and 89-650-5 contained a guanosine insertion within a stretch of 7 guanosines. Correlation by immunoprecipitation and Western blot confirmed the predicted truncated 28 kd protein. Two further isolates, 89-063-1 and 89-353-1, produced truncated proteins 32 and 28 kd in size respectively while one isolate, 90-110-4, produced no detectable product. Reactivation is dependent on TK activity but may occur with "Ultralow" levels and may dissociate from the ACV[sup R] phenotype. The likely
mechanism is in vivo complementation between heterogeneous TK populations containing TD[sup D] or TK[sup LP] virus which may be detectable by
conventional means or masked. Eradication of TD[sup D] outbreaks does not ensure subsequent reactivations to be ACV[sup S] and alternating antivirals may be required for effective therapy. Mutations in the tk gene occur preferentially at homopolymer hot spots and the majority of mutants produce truncated
products. Such homopolymer stretches may facilitate reactivation by high reversion frequencies or ribosomal frameshifting and may offer novel targets for detection and therapy of ACV[sup R] HSV isolates.
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