Inhibition of human T cell leukaemia virus type I long terminal repeat expression by DNA methylation: implications...

Human T cell leukaemia virus type I (HTLV-I) provirus DNA was found to be methylated in patients with adult T cell leukaemia. We have therefore examined the possibility that DNA methylation might contribute to HTLV-I latency. In vitro methylation of HTLV-I long terminal repeat (LTR)-chloramphenicol acetyltransferase or LTR-Luciferase constructs at eight HpaII sites, a subset of the eukaryotic methylation site CpG, resulted in a three- to fourfold inhibition of transcription in transfected cells. Inhibition of transcription by methylation of all CpG methylation sites using SssI methylase was much more pronounced (50- to 80-fold). As partial methylation of the LTR showed, methylation of the promoter region was responsible for most of the effect. Whereas cellular stimulation by a combination of phorbol 12-myristate 13-acetate and Tax was able to reverse the HpaII methylation effect, the inhibition by SssI methylation was not suppressible under these conditions. The results are in line with a possible function of DNA methylation in HTLV-I latency.