Purpose: To investigate the toxicity of methylenetetrahydrofolate reductase (MTHFR) polymorphism in colorectal cancer patients treated with 5-fluorouracil (5-FU).Methods: A total of 105 patients with colorectal cancer who underwent 5-FU therapy were included in this study. MTHFR C677T polymorphisms were determined using direct sequencing. Physical examination and the results of blood and urine tests were used to evaluate the toxicities, including gastrointestinal toxicity, hematopoietic toxicity, hair-skin toxicity and hand-foot syndrome.Results: In 90.5 % of all patients, 5-FU toxicity was observed. With regard to MTHFR C677T mutation, 45.7 % heterozygote mutants and 19.0 % homozygote mutants were observed. MTHFR C677T polymorphism was statistically related to 5-FU toxicity (p = 0.000). In addition, MTHFR C677T mutation was closely related to hematopoietic toxicity (p = 0.005).Conclusion: MTHFR C677T can be used for the prediction of 5-FU toxicity, and can also predict hematopoietic toxicity in patients with colorectal cancer. Keywords: MTHFR genes, Polymorphism, Colorectal cancer, Biomarker, Toxicity

Purpose: To evaluate the effect of methylenetetrahydrofolate reductase (MTHFR) polymorphism on toxicity in gastric cancer (GC) patients treated with capecitabine. Methods: One hundred and twenty-six GC patients were treated with capecitabine in the study. DNA from GC patients was genotyped for MTHFR A1298C using direct sequencing. Toxicity evaluations were graded. Clinical response was assessed. Results: In 87.3 % of the patients, capecitabine toxicity was observed. As for MTHFR A1298C polymorphism, 55.6 % patients who exhibited it were associated with reduced MTHFR activity. MTHFR A1298C was associated with capecitabine-related toxicity (p = 0.008); in addition, MTHFR A1298C was significantly associated with gastrointestinal toxicity (p = 0.026), but not with other types of toxicity. Conclusion: The findings suggest that MTHFR A1298C may be useful for predicting toxicity in GC patients receiving capecitabine treatment, especially gastrointestinal toxicity. Keywords: MTHFR, Polymorphism, Gastric cancer, Toxicity