Attenuation of genome-wide 5-methylcytosine level is an epigenetic feature of cutaneous malignant melanomas

Epigenetic modification of DNA, namely covalent changes of cytosine residues, plays a key role in the maintenance of inactive chromatin regions, both in health and in disease. In the vast majority of malignant melanomas, the most notable known epigenetic abnormality is the attenuation of 5-hydroxymethylcytosine (5-hmC) residues. However, it remains unknown whether a decrease in 5-hmC represents a primary defect of melanoma cancer epigenome or whether it is secondary to the loss of 5-methylcytosine (5-mC), a chemical substrate for 5-hmC. Here, we evaluated 5-mC levels in a spectrum of melanocytic proliferations. To study the epigenetic features of melanocytic nuclei, we began by measuring 5-mC levels in histologic specimens semiquantitatively by immunohistochemistry. We next treated established melanoma cell lines with S-adenosyl methionine (SAM), a universal methyl group donor, in an effort to cause changes in 5-mC levels. We detected a marked reduction in 5-mC levels in both primary and metastatic melanomas compared with 5-mC levels in benign melanocytic nevi. We also empirically induced changes in 5-mC in melanoma cell lines by incubation with SAM. To our surprise, we observed a significant cytoreductive effect of SAM on all melanoma cell lines examined. At subcytotoxic levels, SAM treatment is accompanied by a genome-wide increase in 5-mC. Moreover, we recorded a dose-dependent increase in genome-wide 5-mC levels in melanoma cell lines following SAM treatment. Taken together, we report that genome-wide attenuation of 5-mC is a hallmark of malignant melanomas. We propose that genome-wide attenuation of 5-mC is not merely an epiphenomenon as it is required for melanoma cell growth, albeit by an as of yet undetermined mechanism. Given its potential benefit in slowing down the growth of melanoma cells, SAM should be studied further to determine its role in epigenome modulation.