Integration of foreign DNA into mammalian genome can be associated with hypomethylation at site of insertion.

The methylation patterns in the genome of mammalian cells are remarkably stable, although occasional changes are observed. In mammalian cells, the non-methylated DNA of human adenovirions (Günthert et al., 1976) undergoes de novo methylation after integration into the host hamster genome (Sutter et al., 1978). The establishment of these specific patterns of methylation in the integrated adenovirus sequences (Sutter and Doerfler, 1979, 1980) requires a considerable number of cell divisions after integration (Kuhlmann and Doerfler, 1982, 1983). Recently, we have reported the analysis of the site of linkage between the left terminus of adenovirus type 12 (Ad12) DNA and unique hamster DNA in the Ad12-induced tumor T1111(2) (Lichtenberg et al., 1987). In what way, if any, are the methylation patterns of the adjacent cellular DNA affected by the insertion of unmethylated foreign (adenoviral) DNA? In normal hamster kidney and spleen DNA and in several Ad12-transformed hamster cell lines, this preinsertion sequence is completely methylated at the 5'-CCGG-3' (HpaII) and 5'-GCGC-3' (HhaI) sequences. The same preinsertion sequences in the DNA of cell line BHK21 and on the non-occupied chromosome in the tumor cell line H1111(2) in passage 9 (p9) are almost completely methylated. In contrast, the same sequence on the chromosome, that carries the integrated Ad12 DNA sequence in the tumor T1111(2), is unmethylated at the 5'-CCGG-3' and 5'-GCGC-3' sequences, as are the abutting Ad12 DNA sequences. Thus, the insertion of unmethylated foreign DNA can lead to the hypomethylation of the flanking cellular DNA in the target sequences.