Phenotypic and genetic characterization of thymidine kinase from clinical strains of Varicella-Zoster virus resistant to acyclovir

Varicella-zoster virus (VZV) is a common herpesvirus responsible for dissem inated or chronic infections in immunocompromised patients. Effective drugs such as acyclovir (ACV), famciclovir (prodrug of penciclovir), and foscarn et are available to treat these infections. Here we report the phenotypic a nd genetic characterization of four ACV-resistant VZV strains isolated from AIDS patients and transplant recipients. Sensitivity to six antiviral drug s,vas determined by an enzyme-linked immunosorbent assay, viral thymidine k inase (TK) activity was measured by comparing [H-3]thymidine and 1-beta-D-a rabinofuranosyl- [H-3] thymine as substrates, and the TK gene open reading frame was sequenced. Three strains were found to be TK deficient, and the f ourth was a mixed population composed of TK-positive and TK-deficient virus es. Each strain presented a unique TK gene mutation that could account for ACV resistance. In one strain, the deletion of two nucleotides at codon 215 induced a premature stop signal at codon 217, In another strain, a single nucleotide addition at codon 167 resulted in a premature stop signal at cod on 206, In both other strains, we identified amino acid substitutions alrea dy described in other ACV-resistant VZV strains: either Glu-->Gly at residu e 48 or Arg-->Gly at residue 143. According to our work and data previously reported on resistant VZV strains, there are three areas in the TEC gene w here 71% of the mutations described to date are located. These areas are pu tative candidates for a genotypic diagnosis of ACV resistance.