Methylation of human T-cell leukemia virus proviral DNA and viral RNA expression in short- and long-term cultures of infected cells

Leukemic peripheral blood lymphocytes from individuals infected with the human T-cell leukemia/lymphoma virus (HTLV) were found to express little or no viral RNA before being put into tissue culture. Within 24-48 hr, viral RNA expression increased at least four- to eightfold. Established HTLV-infected cell lines constitutively express viral RNA. Southern blots of DNA from HTLV-infected cells digested with the methylation-sensitive restriction enzyme HpaII showed that the proviral DNA was methylated in all of the uncultured peripheral blood cells tested. In contrast, no proviral methylation was detected in any of the cell lines examined, suggesting a functional correlation between methylation and viral RNA expression. However, DNA from HTLV-infected lymphocytes cultured for 48 hr (by which time increases in viral RNA expression are evident) did not differ detectably with respect to proviral DNA methylation from uncultured cells, suggesting that the increase in viral RNA expression after short-term culture is mediated by mechanisms independent of changes in DNA methylation.