PSEN1 and hyperhomocysteinemia: PSEN1 5′-flanking region has a site-specific (only few CpG moieties) methylation pattern that could change in response to metabolic stimuli, vitB deficiency (resulting in hyperhomocysteinemia in mice) causes PSEN1 overexpression through DNA demethylation which can be prevented by SAM (Fuso et al., 2011).