In addition to genetic risk factors, analysis of AD brain tissue has taken the veil off the incidence of DNA hypomethylation in low levels of SAM [34]; it not only leads to the reduction of folic acid coupled with an increase in homocysteine in the PB samples of patients [35] but also elevates the expression of genes involved in the amyloid beta (Aβ) pathway, such as APP, PSEN1, and BACE1 in the brain [36]. This evidence concerns the gene PSEN1 and Alzheimer disease.