Variable 5-methylcytosine levels in human tumor cell lines and fresh pediatric tumor explants

DNA methylation was measured in human tumor cell lines and freshly explanted pediatric tumors by [3H]uridine labeling followed by DNA hydrolysis and high-performance liquid chromatography. The percentage of cytosine residues modified to 5-methylcytosine in 4 cell lines derived from pediatric tumors was increased or similar to the levels of 3.45% found in two strains of diploid human fibroblasts. However, a fibrosarcoma line derived from an adult (HT1080) contained decreased levels of 5-methylcytosine. Four low-passage retinoblastomas had uniform 5-methylcytosine levels of approximately 3.9%, whereas the levels of the modified base in two freshly explanted rhabdomyosarcomas, one Wilms' tumor, and one medulloblastoma were 3.39, 3.56, and 3.45%, respectively. Two neuroblastomas had substantially lower levels (2.63 to 2.83%) of 5-methylcytosine in their DNAs compared to the retinoblastomas, which are also derived from neural tissue. The results show that while tumors of the same histological type had similar 5-methylcytosine values, discrepancies existed between values obtained for fresh tumors and their corresponding cell lines. Tumor cell lines may therefore be inadequate models for studies on the relationship between DNA methylation and cancer. The results also indicate that generalized statements on DNA methylation levels in pediatric tumors cannot be made, although substantial variations may occur between different tumors types.