MTRR and optic papillitis: In another study, single-nucleotide polymorphisms involved in vitamin B9 and B12 metabolism (5-methyltetrahydrofolate-homocysteine methyltransferase reductase (MTRR) 66 G and serine hydroxymethyltransferase 1 (SHMT1) 1420C alleles) were associated with the magnitude of optic disc edema during strict HDTBR (61).