5-Azacytidine prevents transgene methylation in vivo.

Retroviral sequence can silence transgene expression in vitro and in vivo. We report that this effect can be efficiently prevented by in vivo administration of the demethylating agent 5-azacytidine (aza-C). We engineered the U937 human cell line with a retroviral vector consisting of the thymidine kinase suicide gene (tk), which induces sensitivity to ganciclovir (gcv) and through an IRES sequence, the bacterial beta-galactosidase gene (lacZ) as a marker gene. About 90% of the U937 cells expressed the transgene. By injecting the transduced U937 cells in severe combined immunodeficient disease (SCID) mice, we generated a tumor which, during in vivo treatment with aza-C, maintained the high expression of lacZ and tk genes at the baseline values. LacZ-positive cells in the tumour masses after death was weak (1-2%) in the control group, while in mice treated with aza-C it was maintained at 90%. The delay in tumour onset was significantly longer when animals were treated with both aza-C and gcv (P < 0.0001) compared with animals treated with gcv or with aza-C alone. The prevention of silencing phenomena has important implications for gene therapy, because an efficient transduction associated with appropriate drug therapy, might be a powerful strategy for successful application of gene therapy protocols.