Methylenetetrahydrofolate reductase (MTHFR) variants and fluorouracil-based treatments in colorectal cancer

5-fluorouracil (5FU)-based treatments remain the main chemotherapy for colorectal cancer. Optimal cytotoxicity of fluoropyrimidines requires elevated CH2FH4 tumoral concentrations, controlled by the methylenetetrahydrofolate reductase (MTHFR) enzyme, which irreversibly converts CH2FH4 into 5-methyltetrahydrofolate. The MTHFR gene is subject to several polymorphisms, of which the 677C > T and 1298A > C SNPs are the two most commonly linked with altered enzyme activity. Since a drop in MTHFR enzymatic activity may theoretically favor an increase in intracellular CH2FH4 concentrations, it can be hypothesized that tumors exhibiting the rare MTHFR variants may be more sensitive to 5FU cytotoxicity. Accordingly, experimental data have shown that rare MTHFR variants in position 677 and 1298 are more sensitive to 5FU. However, results of clinical data do not concord regarding the influence of MTHFR genotype on tumoral CH2FH4 concentration, 5FU responsiveness, patient survival and 5FU-related toxicity. These discrepancies may result from the interpatient variability arising from the individual folate status, as well as from the limited role of fluoropyrimidines in the current chemotherapy regimen administered in colorectal cancer.