Mechanism of ganciclovir-induced chain termination revealed by resistant viral polymerase mutants with reduced exonuclease activity

Nucleoside analogues include important drugs that target DNA polymerases and cause chain termination. However, how ganciclovir, the first line of treatment for human cytomegalovirus infections, does this is unknown. Ganciclovir resistance is a serious clinical problem. Many ganciclovir-resistant isolates contain substitutions in the 3'-5' exonuclease domain of the catalytic subunit of viral DNA polymerase. How these mutations confer resistance is a long-standing question. This study shows that both wild-type and exonuclease mutant polymerases incorporate ganciclovir into DNA and continue synthesis, but whereas the wild-type enzyme excises nucleotides two positions downstream of incorporated ganciclovir, the mutant enzymes do not, permitting chain extension. These results show how a therapeutically important drug causes chain termination and explain an unusual mechanism of drug resistance.