In Alzheimer’s and Parkinson’s disease, dysregulated DNA methylation affects neuronal genes involved in synaptic plasticity, oxidative stress, and protein aggregation—including APP, SNCA, and MAPT—while aberrant histone deacetylation (e.g., loss of H3K27ac, H4K16ac) silences neuroprotective promoters such as BDNF and CREB. This evidence concerns the gene SNCA and Parkinson disease.