Vol. 62, Issue 3, 714-721, September 2002

Role of the Human Herpesvirus 6 U69-Encoded Kinase in the Phosphorylation of Ganciclovir

Leen De Bolle, Detlef Michel, Thomas Mertens, Chaysavanh Manichanh, Henri Agut, Erik De Clercq, and Lieve Naesens

Rega Institute for Medical Research, Katholieke Universiteit Leuven, Leuven, Belgium (L.D.B., E.D.C., L.N.); Abteilung Virologie, Institut für Mikrobiologie, Universität Ulm, Ulm, Germany (D.M., T.M.); and Laboratoire de Virologie, Groupe Hospitalier Pitié-Salpêtrière, Paris, France (C.M., H.A.)

The human herpesvirus 6 (HHV-6) U69 gene product (pU69) is the presumed functional homolog of the human cytomegalovirus (HCMV) UL97-encoded kinase (pUL97), which converts ganciclovir to its monophosphate metabolite in HCMV-infected cells. It has been reported that insertion of U69 into baculovirus confers sensitivity to ganciclovir in insect cells (J Virol 73:3284-3291, 1999). Our metabolic studies in HHV-6-infected human T-lymphoblast cells indicated that the efficiency of ganciclovir phosphorylation induced by HHV-6 was relatively poor. Recombinant vaccinia viruses (rVVs), expressing high levels of pU69 from two HHV-6 strains (representing the A and B variant), were constructed and used to compare the ganciclovir-phosphorylating capacity of pU69 and pUL97 in human cells. Metabolic studies with [8-³H]ganciclovir showed that ganciclovir was phosphorylated in human cells infected with pU69-expressing rVVs, although the levels of phosphorylated ganciclovir metabolites were approximately 10-fold lower than those observed with pUL97. We also demonstrated that pU69, like pUL97, is expressed as a nuclear protein. Our results indicate that the limited phosphorylation of ganciclovir by pU69 may contribute to its modest antiviral activity against HHV-6 in certain cell systems.