Inhibition of herpes simplex thymidine kinase gene expression by DNA methylation is an indirect effect.

The biological activity of in vitro methylated HSV-TK DNA was analysed after microinjection into thymidine kinase negative rat 2 cells. It was found that the fully methylated DNA (Hpa II methylase) was as active as the non methylated control DNA for about 48 hours after injection. DNA reextraction experiments and blot analysis showed that DNA demethylation was not the reason for the observed TK activity. With prolonged cultivation time the methylated DNA becomes rapidly inactive and 100 hrs after injection thymidine incorporation was no longer detectable in the recipient cells. In transformed cells, obtained after coinjection with SV40 DNA, the HSV-DNA was partially demethylated and inactive. Addition of 5-azacytidine to the culture medium induced further demethylation and reactivation of the thymidine kinase gene.