Inactivation of hepatitis C virus cDNA transgene by hypermethylation in transgenic mice

Transgenic mice were produced by microinjection of a partial hepatitis C virus (HCV) genome sequence including the structural protein region, under the control of the albumin promoter and enhancer into fertilized eggs of C57BL/6 and BDF1 mice. Three founders carrying at least five copies of the transgene but not expressing HCV-specific RNA were generated. Methylation analysis indicated that the transgene was extensively methylated. Mapping of methylated cytosine residues of the transgenic mouse DNA showed that all C residues of a particular part of the HCV genome but not all the CpG island like sequences were methylated. Transiently expressed HCV cDNA in COS7 cells and the active endogenous albumin gene were not methylated. Furthermore, 5-azacytidine, a potent demethylating agent, induced HCV gene expression in a line of these transgenic mice. These results suggest that methylation of HCV cDNA is a cause of its inactive expression in transgenic mice, and that this phenomenon may occur in other stable systems for expression of the HCV genome.