Methylation pattern of human T-cell leukemia virus in vivo and in vitro: pX and LTR regions are hypomethylated in vivo.

The methylation patterns of the gag, pol, env, pX and LTR regions of proviral DNA of human T-cell leukemia/lymphoma virus type I (HTLV) in fresh leukemic cells and established cell lines were examined using HpaII/MspI endonuclease. Peripheral blood lymphocytes (PBL) isolated from patients with adult T-cell leukemia/lymphoma (ATL) did not express viral antigens of HTLV, but PBL that had been cultured for 2 days did express these viral antigens. Most parts of the gag, pol and env regions of the HTLV provirus in PBL isolated from 12 ATL patients and PBL cultured for 2 days were hypermethylated as reported by others. In contrast, in 10 established cell lines that harbored HTLV genomes and expressed viral antigens, HTLV proviruses were hypomethylated. In one cell line, ATL-IK, which harbored an HTLV genome but did not produce viral antigens, the gag, pol and env regions were hypermethylated. However, two HpaII sites, one in the middle of the gag region and the other in the middle of the pol region, were not methylated even in PBL from most ATL patients. Furthermore, the pX and LTR regions were hypomethylated not only in established cell lines but also in PBL of ATL patients. The hypomethylation of the pX and LTR regions detected in fresh leukemic cells of ATL patients may have some etiological significance in cell transformation by controlling the level of transcription of these regions, or modulating the binding of some factors to these regions.