MTHFR and rheumatoid arthritis: Summary of previous studies in patients with psoriasis and rheumatoid arthritis (RA) of two single nucleotide polymorphisms (C677T/rs1801133 and A1298C/rs1801131) in the gene methylenetetrahydrofolate reductase (MTHFR) assessing efficacy and toxicity of methotrexate (adapted from Hider et al. 13)