Transactivation of methylated HIV-LTR by a frog virus 3 protein.

DNA methylation has been implicated in the suppression of transcription of a large spectrum of eukaryotic genes. Frog virus 3 (FV3) contains genomic DNA that is the most extensively methylated of all known animal viruses. However, FV3 gene expression is tightly regulated in a sequential fashion in infected cells. Therefore, FV3 must have evolved a mechanism(s) to overcome the inhibitory effects of DNA methylation. FV3 has been shown to induce expression of methylated foreign genes in transient transfections. This study was designed to establish if this FV3-induced expression of methylated genes could be demonstrated in stable cell lines which contain integrated foreign genes that are silenced by DNA methylation. Stably transfected simian Vero and human T-cells containing a single copy of the methylated and transcriptionally suppressed HIV-LTR CAT construct were either infected with FV3 or fused with FV3-infected fat head minnow cells. The results from these experiments lead us to conclude that FV3 infection does promote expression of a foreign, stably integrated gene (HIV-LTR), which was previously silenced by DNA methylation. We also observed that stably transformed human T-cells incubated at 30 degrees, unlike at 37 degrees, expressed minute but detectable HIV-LTR-directed CAT activity. Significance of this finding in HIV pathogenesis remains elusive.