Mutation in region III of the DNA polymerase gene associated with dual ganciclovir-foscarnet resistance in cytomegalovirus (CMV) isolates from three subjects receiving prolonged antiviral therapy

Most ganciclovir (GCV) resistant clinical CMV isolates have well defined mutations in the UL97 phosphotransferase sequence. CMV DNA polymerase (Pol) mutations also confer various drug resistance phenotypes but are less well characterized. Buffy coat CMV isolates from 3 HIV-infected subjects showing dual GCV and foscarnet (FOS) resistance (IC₅₀ 9.8-16.7 μM and 530-1020 μM respectively) after prolonged therapy were sequenced. In each case, as expected, a known GCV-resistance mutation was observed in UL97 (V594 and/or S595). A Pol mutation (Ala to Val at codon 809, V809) in conserved region III was also detected in each subject. Serial isolates in 2 cases showed that the appearance of V809 correlated with FOS resistance. A segment of Pol containing V809, from one of the clinical isolates, was transferred to laboratory CMV strain AD169. The recombinant virus showed 2.6-fold increased GCV and 6.3-fold increased FOS resistance (IC₅₀ = 7.8 μM and 410 μM respectively) as compared to parental virus. We conclude that the V809 mutation contributed to resistance to both drugs in isolates from all 3 subjects and appears to be a clinically significant resistance marker.