Opposite effect of two cytomegalovirus DNA polymerase mutations on replicative capacity and polymerase activity

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Abstract
Background: Human cytomegalovirus (HCMV) infections cause significant morbidity in immunocompromised hosts. The aim of this study was to characterize the role of two HCMV DNA polymerase mutations (K805Q and T821I) found in a ganciclovir- and foscarnet- resistant clinical isolate from an AIDS patient. Methods: The effects of single and dual DNA polymerase mutations on virus susceptibility and replicative capacity, as well as on enzymatic activity, were studied using recombinant viruses generated from overlapping cosmids and DNA polymerase enzymes expressed in rabbit reticulocyte lysates. Results: Recombinant viruses containing mutations K805Q, T821I and K805Q+T821I had 0.8-fold, 5.3-fold and 4.8-fold increases in ganciclovir 50% inhibitory concentration (IC50) values and 0.3-fold, 23.3-fold and 15.6-fold increases in foscarnet IC50 values, respectively, compared with those of the wild-type virus. The recombinant virus T821I had impaired replication in fibroblastic cells on day 2 post-infection with a decrease in viral titres of 3.5-fold, 4.3-fold and 2.6-fold compared to the recombinant wild-type, K805Q and K805Q+T821I viruses, respectively. Enzymatic studies of wild-type and mutant DNA polymerase enzymes in presence of foscarnet resulted in IC50 values that were similar to those of the recombinant viruses. Steady-state kinetic constants K m and V max derived from Michaelis–Menten equations showed that the activity of the mutant T821I enzyme was diminished compared with those of wild-type, K805Q and K805Q+T821I mutant enzymes. Thermodynamic stability of the two single mutant enzymes was opposed as shown by computer- assisted three-dimensional modelling studies. Conclusions: The HCMV DNA polymerase mutation K805Q improved the fitness of the T821I mutation associated with high levels of resistance to foscarnet.