Astronauts with specific single nucleotide polymorphisms (SNPs) in one-carbon pathway genes, namely, the G allele for methionine synthase reductase (MTRR) A66G and the C allele for serine hydroxymethyltransferase (SHMT1) C1420T had a higher incidence of SANS pathologies (e.g., optic disc edema, globe flattening, choroidal folds) (5, 6). Here, MTRR is linked to optic papillitis.