Epigenetic status of an adenovirus type 12 transgenome upon long-term cultivation in hamster cells

The epigenetic status of integrated adenovirus type 12 (Ad12) DNA in hamster cells cultivated for about 4 decades has been investigated. Cell line TR12, a fibroblastic revertant of the Ad12-transformed epitheloid hamster cell line T637 with 15 copies of integrated Ad12 DNA, carries one Ad12 DNA copy plus a 3.9-kbp fragment from a second copy. The cellular insertion site for the Ad12 integrate, identical in both cell lines, is a >5.2-kbp inverted DNA repeat. The Ad12 transgenome is packaged around nucleosomes. The cellular junction is more sensitive to micrococcal nuclease at Ad12-occupied sites than at unoccupied sites. Bisulfite sequencing reveals complete de novo methylation in most of the 1,634 CpGs of the integrated viral DNA, except for its termini. Isolated unmethylated CpGs extend over the entire Ad12 integrate. The fully methylated transgenome segments are characterized by promoter silencing and histone H3 and H4 hypoacetylation. Nevertheless, there is minimal transcriptional activity of the late viral genes controlled by the fully methylated major late promoter of Ad12 DNA.