Abstract
Dear Editor,Methotrexate (MTX) inhibits several enzymes in folatemetabolic pathway, including dihydrofolate reductase,thymidylate synthase (TYMS), and methylenetetrahydrofo-late reductase (MTHFR), causing defects in DNA and RNAsyntheses and methylation process [1]. Genetic polymor-phisms of genes encoding proteins in folate metabolismaffect their structures and functions. MTHFR C677T andA1298C are associated with decreased enzyme activity andhyperhomocysteinemia [1]. There are 28 nucleotide-tandemrepeat variations in 5’untranslated region of the TYMSgene. The most common variations of this polymorphismare two and three repeats (2R and 3R). TYMS gene with3R polymorphism has a higher expression level than theone with 2R [1, 2]. Reduced folate carrier (RFC) is anessential carrier protein for folate and MTX. RFC G80Apolymorphism affects MTX level in patients treated withhigh dose of MTX. MTX level has been found to besignificantly higher in patients with RFC 80 AA genotypethan other genotypes [1, 3].Our institute normally uses two courses of high-doseMTX (1.5 g/m
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