MAT1A and disorder of methionine catabolism: Hypermethioninemia is caused by the deficiency of glycine N-methyltransferase (GNMT) activity (Augoustides-Savvopoulou et al., 2003; Luka and Wagner, 2003; Luka et al., 2006) and/or mutations of the MAT1A gene (Chou, 2000; Chamberlin et al., 2000), which resulted in the plasma concentration of methionine reaching as high as 1870 μM (normal range 5–35 μM).