MTHFR and Alzheimer disease: Given that AD is a polygenic disorder, it is a matter of debate whether the interactions between MTHFR and APOE genes are related to the pathogenesis of AD (Tysoe et al., 1997; Chapman et al., 1998; Brunelli et al., 2001; Seripa et al., 2003; Wang et al., 2005; Keikhaee et al., 2006; Styczyńska et al., 2008; Bi et al., 2009; Mansouri et al., 2013).