The total levels of DNA 5-methylcytosine were determined in a series of related highly metastatic cell lines which had been isolated from the poorly metastatic human melanoma tumor line MeWo. The procedure used to quantitate DNA 5-methylcytosine involved labeling of DNA with [6-3H]uridine, hydrolysis with formic acid, and separation of cytosine and 5-methylcytosine bases by high-performance liquid chromatography. The DNA 5-methylcytosine content of the parental MeWo tumor was 3.07 +/- 0.16%. Metastatic variants of MeWo which had been isolated using a combination of in vitro and in vivo selection procedures had DNA 5-methylcytosine levels between 2.45 +/- 0.15% and 2.75 +/- 0.09%. Metastatic variants which were isolated using a single step in vivo selection procedure exhibited DNA 5-methylcytosine levels of 1.92 +/- 0.02% to 2.68 +/- 0.09%. No consistent decreases in DNA 5-methylcytosine content were found in "artificial" metastases arising in athymic "nude" mice following the i.v. inoculation of MeWo or its cloned sublines. These observations provide further support for the hypothesis that alterations in the cytosine methylation of DNA may play an important role in the generation of tumor cell heterogeneity and the progression of tumors from relatively benign to highly malignant states.
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