A 3-NUCLEOTIDE DELETION IN THE UL97 OPEN READING FRAME IS RESPONSIBLEFOR THE GANCICLOVIR RESISTANCE OF A HUMAN CYTOMEGALOVIRUS CLINICAL ISOLATE

Multiple human cytomegalovirus (HCMV) strains frequently coexist in pa tients with AIDS, and chronic ganciclovir treatment may favor the emer gence of ganciclovir-resistant viral mutants. We report the molecular and biochemical characterization of a HCMV ganciclovir-resistant strai n (VR3480) previously recovered from a patient with AIDS who was under going multiple courses of ganciclovir treatment (G. Gerna, F. Baldanti , M. Zavattoni, A. Sarasini, E. Percivalle, and M. G. Revello, Antivir al Res, 19:333-345, 1992). Ganciclovir resistance of strain VR3480 was related to impaired ability to monophosphorylate the drug, as indicat ed by the finding that ganciclovir phosphorylation values for VR3480 w ere 30% of those shown by the HCMV reference strain AD169 in an in vit ro activity assay. Sequencing of the UL97 gene of VR3480, which encode s the viral kinase responsible for ganciclovir phosphorylation, showed an in-frame deletion of three nucleotides resulting in the loss of a leucine at position 595 in the polypeptide. Mutant VR3480 UL97 DNA was able to transfer resistance to the AD169 strain in marker rescue expe riments. Analysis of virus isolates and blood polymorphonuclear leukoc yte samples spanning the 2-year follow-up period of the patient showed that ganciclovir-resistant strain VR3480 arose ex novo during prolong ed antiviral treatment and accounted for the majority of virus load ci rculating in blood during the period of clinical resistance to gancicl ovir treatment.