Levels of DNA methylation in diploid and SV40 transformed human fibroblasts.

Total 5-methylcytosine (5mC) has been measured in the DNA of cultured human cells by HPLC chromatography. In agreement with other reports, the level of 5mC declines continuously during the serial passaging of diploid fibroblasts, strain MRC-5. On the contrary, fibroblasts infected with SV40, which have acquired most of the properties of transformed cells, maintain a constant level of 5mC, prior to dying out in the crisis period. MRC-5 and Werner's syndrome fibroblasts fully transformed and immortalised by SV40, also maintain a constant level of 5mC. These results indicate that the ability of cells to maintain DNA methylation is a necessary but not a sufficient step for immortalisation.