It has been known for several decades that diets deficient in methyl donors (methionine, folic acids, and choline) cause cancer and that methylation deficiency is a characteristic of cancer. However, a large gap remains in linking metabolic changes with recognized cancer-causing events such as alterations in oncogenes and tumor suppressor genes. Recent genetic studies from the applicant's laboratory and others have uncovered a new class of tumor suppressor genes that function as histone methyltransferases to transfer methyl group from S-adenosylmethionine to lysine residues on histones. Because the level of S-adenosylmethionine, and in turn, the activity of methyltransferases, depends on dietary methyl intake, the discovery of tumor suppressor genes among methyltransferases provides a direct link between dietary and genetic causes of cancer. The convergence of a multitude of independent findings in the past several decades strongly suggests that carcinogenesis can result from decreased histone methyltransferase activity, caused either by disabling of the genes for these enzymes or by the accumulation of abnormal levels of metabolic intermediates of the one carbon pathway. This application aims to further establish the hypothesis that dietary methyl donor deficiency causes cancer by directly affecting the activity of histone methyltransferases. The specific aims are to: 1. study the regulation of histone methyltransferases and histone methylation by methyl donor deficiency in tissue culture. 2. study in animal models the regulation of histone methylation and histone methyltransferases by diets deficient in methyl donors. 3. study the effects of dietary methyl donor deficiency on tumorigenesis in transgenic mice with reduced levels of a methyltransferase tumor suppressor, RIZ1. 4. study the potential tumor suppressor role of a new member of the histone methyltransferase family, PRDM16, that was recently isolated in the applicant's laboratory. These studies address several key predictions of the hypothesis to be tested in this grant application. The new understanding that will emerge from these studies will help clarify the role of nutrient metabolism in carcinogenesis. The results may have important public health implications.