MTHFR and major depressive disorder: Therefore, a meta-analysis is conducted to investigate the correlations between two prevalent MTHFR single nucleotide polymorphisms (SNPs), MTHFR C677T, A1298C, severe psychological disorders (schizophrenia, major depression, bipolar disorder).<h4>Methods</h4>A total of 81 published studies were screened and selected by a search of electronic databases up to April 2022.