CPG methylation reduces genomic instability

Hypomethylation of DNA in tumor cells is associated with genomic instabilit y and has been suggested to be due to activation of mitotic recombination. We have studied the methylation patterns in two 650 kb double minute chromo somes present in two mouse tumor cell lines, resistant to methotrexate. Mul tiple copies of the double minute chromosomes amplifying the dihydrofolate reductase gene are present in both the cell lines. In one of the cell lines (Mut F), two unmethylated CpG islands in the double minute chromosomes are readily cleaved by methylation-sensitive rare-cutting restriction endonucl eases. In the other cell line (Mut C), the cleavage sites in the double min ute chromosomes are partially methylated and resistant to cleavage. The dou ble minute chromosomes with the two unmethylated CpG islands undergo rapid dimerization, whereas the double minute chromosomes with the partially meth ylated CpG islands are unchanged in size for over a year in continuous cult ure. The partially methylated CpG islands can be demethylated by azacytidin e treatment or naturally by extended time in culture, and become sensitive to cleavage with the rare-cutting restriction endonucleases. The Mut C doub le minute chromosomes, with the newly demethylated CpG islands, but mot the double minute chromosomes with the partially methylated CpG islands, under go deletions and dimerizations. These results suggest a role for CpG island methylation controlling mitotic recombination between and within large DNA molecules.