Methylation of milk-borne and genetically transmitted mouse mammary tumor virus proviral DNA.

The 5-methylcytosine content of mouse mammary tumor virus (MMTV)-specific DNA sequences acquired by both milk-borne infection and genetic transmission was determined for both normal and neoplastic tissues of the mouse. Using the restriction endonuclease Msp I, which cleaves at the recognition sequence 5'-CCGG, and its isoschizomer Hpa II, which is inhibited by methylation of the cytosine base of the CpG dinucleotide, it can be demonstrated that MMTV proviruses acquired via germ line infection are extensively methylated at both the site for Msp I-Hpa II cleavage and the site for Hha I (5'-GCGC), which is also inhibited by base modification of the cytosine. The virus-specific sequences acquired via milk-borne infection, however, are not modified at these sites in DNA from either normal infected or transformed tissues. Finally, cellular sequences are nonspecifically hypomethylated in a phenomenon unique to transformed tissue and apparently unrelated to the specific hypomethylation of exogenously acquired MMTV proviruses.