Genetic polymorphism of thymidine kinase (TK) and DNA polymerase (pol) of clinical varicella-zoster virus (VZV) isolates collected over three decades.

BACKGROUND: Genotypic resistance testing of varicella-zoster virus (VZV) strains to antivirals is of high relevance in immunocompromised patients with VZV reactivations unresponsive to therapy. However, the knowledge on mutations associated with natural gene polymorphism or resistance is limited.

OBJECTIVES: To examine the genotype of the thymidine kinase (TK) and DNA polymerase (pol) of unselected clinical VZV isolates collected between 1984 and 2014 and to verify the phenotype related to novel amino acid (aa) substitutions.

STUDY DESIGN: The TK and DNA pol genes of 169 VZV isolates were analyzed by amplification and sequencing. Sequences were compared to that of the reference strain Dumas. The phenotype to acyclovir and other antivirals was examined in isolates with novel aa substitutions using modified plaque reduction assay.

RESULTS: In the TK of four strains, four different aa substitutions were detected, apart from the known change S288L that was present in all strains compared to Dumas. All four substitutions have hitherto not been described in the literature and were phenotypically classified as natural gene polymorphisms although two out of them (S51L, K186R) were localized in conserved gene centers. The DNA pol of 34 isolates exhibited 19 different substitutions, 14 out of them were novel, and two (R753K, V777I) were within conserved gene regions. Again, these changes were characterized as natural gene polymorphisms.

CONCLUSIONS: Non-synonymous mutations in VZV TK or DNA pol conferring natural gene polymorphism are rare events. Nevertheless, the phenotypic characterization of 18 novel polymorphisms can help to provide a better identification of resistance mutations.