Methylenetetrahydrofolate reductase (MTHFR) is a crucial enzyme that regulates nucleotide synthesis and DNA methylation. The MTHFR gene includes a common polymorphism (rs1801133 or C677T), which is associated with enzyme activity. The aim of the present study was to assess the possible role of the MTHFR C677T polymorphism in the development of bipolar disorder (BD) and schizophrenia and its association with age at onset. Participants were grouped into control, bipolar, and schizophrenia groups (108, 90, and 96, respectively). The genotype and allele frequencies of the MTHFR C677T polymorphism were studied by PCR and restriction fragment length polymorphism. There was significant association of the MTHFR C677T polymorphism with BD and schizophrenia. The T allele was significantly prevalent in the BD and schizophrenia groups compared with the control group. Age at onset was significantly decreased in patients with schizophrenia carrying one copy of the wild C allele or CC genotype but not in patients with BD. In conclusion, the present findings suggest that the MTHFR C677T polymorphism is likely associated with the risk of developing BD and schizophrenia, and influences the age at onset of schizophrenia but not the age at onset of BD.
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