Human Herpesvirus 8-Encoded Thymidine Kinase and Phosphotransferase Homologues Confer Sensitivity to Ganciclovir

Abstract

Human herpesvirus 8 (HHV-8) sensitivity to the nucleoside analog ganciclovir (GCV) suggests the presence of a virally encoded kinase that catalyzes the initial phosphorylation of GCV. Analysis of the HHV-8 genome identified two candidate kinases: proteins encoded by open reading frame (ORF) 21, with homology to the herpesvirus thymidine kinases (TK), and ORF 36, with homology to the herpesvirus phosphotransferases (PT). Experiments presented here show that both ORF 21 and ORF 36 encode GCV kinase activities as demonstrated by GCV phosphorylation and GCV-mediated cell death. In both regards the PT homologue ORF 36 was more active than the TK homologue ORF 21. ORF 21, but not ORF 36, weakly sensitized cells to killing by penciclovir. Neither ORF sensitized cells to killing by (E)-5-(2-bromovinyl)-2′-deoxyuridine.

Antiviral agents active against human herpesviruses HHV‐6, HHV‐7 and HHV‐8

“…In this regard, the ORF-36-encoded protein was the more efficient. ORF-21, but not ORF-36, also sensitised cells to killing by penciclovir, but not brivudin [71]. Ganciclovir-phosphorylating activity of the ORF-21-encoded protein has also been reported by Gustafson et al [70].…”

“…The HHV-8 genome encodes two candidate kinases for the phosphorylation of nucleoside analogues, i.e. (i) the protein encoded by ORF-21, that has homology to the HSV and VZV thymidine kinases and (ii) the protein encoded by ORF-36 which has homology to herpesvirus phosphotransferases [70,71]. Cannon et al [71] demonstrated that both the ORF-21 and ORF-36 encoded kinases confer sensitivity to ganciclovir, as shown by ganciclovir phosphorylation and ganciclovir-mediated cell death.…”

“…Their homologous counterparts in EBV, HHV-6, HHV-7 and HHV-8 have either not been established (HHV-7) or have remained less well defined as far as the substrate specificity is concerned (HHV-6, HHV-8 and EBV). For HHV-6, we have already described the interaction of ganciclovir with the HHV-6 U69-encoded kinase [34], and for HHV-8 we addressed the ORF-21-and ORF-36-encoded kinases also with respect to ganciclovir as substrate [70,71]. The substrate specificity of the EBV BXLF-1-encoded TK is definitely narrower than that of its HSV-1, HSV-2 and VZV homologues: it would have good affinity for brivudin, but only poor activity for the acyclic nucleoside guanosine analogues acyclovir, ganciclovir and penciclovir [77][78][79].…”

Protein kinases conserved in herpesviruses potentially share a function mimicking the cellular protein kinase cdc2

“…EBV, HHV-6 and KSHV encode CHPKs [16,17,19,21]. The HHV-6 and KSHV CHPKs possess both protein kinase activity and kinase activity against nucleotide analogues, as seen for HCMV UL97 [19,59]. Recombinant viruses lacking these CHPKs have not been constructed, leaving the roles of these CHPKs in viral replication unclear at present.…”

Reinitiation after Translation of Two Upstream Open Reading Frames (ORF) Governs Expression of the ORF35-37 Kaposi's Sarcoma-Associated Herpesvirus Polycistronic mRNA

“…S imilarly to other double-stranded DNA (dsDNA) viruses, the vast majority of Kaposi's sarcoma-associated herpesvirus (KSHV) gene expression conforms to the eukaryotic paradigm of cap-dependent translation of a single protein per mRNA. The sole viral protein expressed exclusively from a functionally polycistronic mRNA is the ORF36 protein kinase, a viral factor that activates the c-Jun N-terminal kinase (JNK) signaling pathway and sensitizes KSHV-infected cells to ganciclovir (1)(2)(3)(4)(5)(6). The viral ORF35-37 polycistronic mRNA directs synthesis of both ORF35 and ORF36, whereas ORF37 is translated from an independent monocistronic transcript (7).…”