Loss of viral genomes from hamster tumor cells and nonrandom alterations in patterns of methylation of integrated adenovirus type 12 DNA.

The insertion stability and DNA methylation patterns of integrated adenovirus type 12 (Ad12) genomes were investigated in Ad12-induced tumors and in tumor cell lines established from them as a function of time of passage under culture conditions. Upon subcultivation of cells from some of the tumors, the viral genomes were eliminated, apparently in a stepwise process with segments of the left termini of Ad12 DNAs persisting the longest. Morphological variants of these tumor cells lost all viral DNA and yet retained the oncogenic phenotype. All 13 independently isolated clones from one revertant line were devoid of Ad12 DNA. It could not be ruled out that very short sequence elements of viral DNA, such as promoters or enhancing sequences, could have persisted in these variants. The extent of viral DNA methylation was minimal in Ad12-induced tumors, although the viral genome was not extensively expressed, if at all. Upon passage in culture, the levels of viral DNA methylation increased. It was interesting that establishment of the final methylation pattern of integrated Ad12 DNAs required many cell generations after the fixation of foreign DNA in the host genome. The shift in methylation was nonrandom. The late parts of the inserted viral genomes became methylated more extensively than did the early gene segments.