Selective loss of unmethylated segments of integrated Ad12 genomes in revertants of the adenovirus type 12-transformed cell line T637

We have studied the stability if integrated adenovirus type 12 (Ad12) DNA sequences and the relation of foreign DNA persistence to the state of methylation of this DNA. In the Ad12-transformed hamster cell line T637, multiple copies of Ad12 DNA are chromosomally integrated. Some of these integrated viral genomes are rearranged in that internal parts of the viral DNA have become juxtaposed to its left terminus. Fluorescent in situ hybridization analyses demonstrate that the Ad12 DNA in cell line T637 and in some of its revertants is located at one site on one of the hamster chromosomes. Major portions of the integrated viral genomes in cell line T637 have become extensively de novo methylated in specific patterns. Most of the rearranged Ad12 DNA sequences in the T637 genome are un- or hypomethylated. In the morphological revertants of the Ad12-transformed hamster cell line T637, the majority of the integrated Ad12 genomes has been lost. Surprisingly, we have found that the un- or hypomethylated rearranged viral sequences have been selectively lost, in contrast to some of the methylated sequences that are stably retained.