- Meta-analysis recently suggested that a 5 μM increase in homocysteine is associated with a 70% higher risk for schizophrenia. Elevated homocysteine is reported to alter macromolecule methylation. We studied whether elevated plasma homocysteine levels in schizophrenia are associated with altered leukocyte global DNA methylation. DNA was extracted from peripheral blood leukocytes of 28 schizophrenia patients vs. 26 matched healthy controls. Percent of global genome DNA methylation was measured using the cytosine-extension method. Homocysteine levels were higher in schizophrenia patients than in controls. No difference in global DNA methylation between schizophrenia patients and control subjects was found (74.0% ± 14.8 vs. 69.4 ± 22.0, p = 0.31). A significant interaction between diagnosis and smoking on DNA methylation was obtained ( F = 6.8, df = 1,47, p = 0.032). Although leukocytes may be a useful cell model to evaluate epigenetic changes such as global DNA methylation in brain, future studies should compare global DNA methylation in peripheral tissue vs. brain in laboratory animals.