Single-nucleotide polymorphisms might be useful in predicting the efficacy of anticancer therapy, but evidence of exactly how these small changes in DNA can have such important consequences is lacking. Kyoung-Jin Sohn and colleagues have focused on the C667T polymorphism in methylenetetrahydrofolate reductase (MTHFR), which is present in about 35% of the North-American population. They have discovered that the altered enzyme affects the concentration and distribution of folates in cancer cells, which, in turn, alters the chemosensitivity to antifolate drugs.

MTHFR catalyses the conversion of 5,10-methylene THF to 5-methyl THF. The 667T variant of MTHFR is less active, so 5,10-methylene THF accumulates. The authors tested whether the polymorphism altered the effectiveness of the antifolate drugs 5-fluorouracil (5-FU) and methotrexate (MTX) — which are used widely to treat breast and colon cancer — and correlated with their known mechanisms of action.

The authors took a colon cancer and a breast cancer cell line with endogenous 667C MTHFR and transfected them with 667T variant MTHFR. Enzyme activity was 35% lower in the 667T MTHFR cells and the intracellular distribution of folates had also changed: levels of 5-methyl THF were about 12% lower and 5,10-methylene THF levels were about 10% higher. These cells grew faster than 667C variant cells, probably because 5,10-methylene THF is required for DNA synthesis.

So, how sensitive are the 667T variant MTHFR cells to antifolate drugs? The authors postulated that the effect of 5-FU, which acts by forming an inhibitory complex with thymidylate synthase and 5,10-methylene THF, would be increased by the presence of the polymorphism in cancer cells, and this was the case both in vitro and in xenograft models. By contrast, the authors thought that the effect of MTX, which acts by decreasing levels of 5,10-methylene THF, might be compromised by the polymorphism, and this also proved true for the breast cancer cell line. There was no difference in chemosensitivity to MTX in the 667T variant colon cancer cells, but as colon cancer does not usually respond to MTX, this was not that surprising. Interestingly, if MTHFR expression and activity was more completely inhibited by transfecting with antisense to MTHFR, the colon cancer cells were more resistant to MTX.

These data provide evidence of a functional consequence of the C677T MTHFR polymorphism in response to chemotherapy and support the view that this polymorphism might be a useful pharmacogenetic marker for providing tailored chemotherapy to patients with cancer.