Human T-lymphotropic virus type I transcriptional regulation by methylation.

While human T-lymphotropic virus type I (HTLV-I) proviral genome is readily detected in leukemic lymphocytes from adult T-cell leukemia patients, viral antigens or viral RNA are not expressed unless these cells are cultured. To address the problem of possible restriction mechanism of HTLV-I replication, we studied the methylation state of provirus in four HTLV-I transformed cell lines. These cell lines are chronically infected with HTLV-I and carry several copies of proviruses but are characterized by different viral expression. Molecular analysis showed that in MT4 cell line, which expresses a low level of viral RNA and proteins, not only are the HTLV-I proviruses heavily methylated but methylation also involves large regions around and within the long terminal repeats. MT4 cells treatment with 5-azacytidine led to an increase in both total viral RNA and p24 gag protein expression. These findings indicate that proviral methylation is responsible for the low viral expression in MT4 cells and also suggest that this phenomenon may be relevant to HTLV-I in vivo latency.