ABSTRACT
The transformation of a normal cell into a metastasizing cancer involves changes in cellular growth control, in the way cells interact with their neighbors, and in biochemical pathways. Deoxyribonucleic acid (DNA) methylation seems to have an important effect on the program of gene expression. The instability of cancer DNA methylation may in part be explained by the elevated levels of DNA methyltransferase found in cancer cells. The elevated rate of transmethylation seems to be related to the frequent occurrence of methionine dependence in cancer cells. Methionine dependence is the inability of cells to grow when methionine is replaced by its immediate precursor, homocysteine, in the cell-culture medium, a condition that allows the growth of normal cells. That methionine metabolism and transmethylation play an important role in cancer is also seen in the fact that giving animals a methyl-deficient diet is carcinogenic.
