The Molecular Perspective: Methotrexate

Abstract

of the poisons used in modern chemotherapy rely on the observation that most cells in the body are not actively dividing, so substances that block some step in cell reproduction will have a disproportionately severe effect on growing cancer cells relative to most healthy tissues. Virtually every step of cell growth and division is targeted by some chemotherapeutic drug today. The enzyme dihydrofolate reductase (DHFR), shown in Figure 1, was the first enzyme to be targeted by chemotherapy. It plays a supporting role, but an essential role, in the synthesis of thymine nucleotides. The leading role is played by the enzyme thymidylate synthase, which adds a methyl group onto uracil to form thymine. This may seem like a small change, but the extra hydrophobic bulk of this methyl group is essential for proper discrimination of thymine from the other three bases by various transcription factors, repressors, enhancers and other DNA-binding proteins. The methyl group is delivered to thymidylate synthase by folate, a small cofactor molecule that is an essential vitamin for human beings. In the course of the reaction, as the methyl is transferred from folate to uracil, the folate molecule is oxidized. This is where DHFR makes its entry: DHFR restores folate to its reduced state, ready for the next round of thymine synthesis. If the action of DHFR is blocked, the cell dies. When treated with methotrexate, the level of reduced folate plummets, and synthesis of thymine and purine nucleotides grinds to a halt. (There is evidence that methotrexate also blocks other steps in nucleotide synthesis, magnifying these effects.) As the available thymine nucleotides are depleted, the normally low levels of uracil nucleotides grow, because thymidylate synthase cannot perform its job. Uracil is then erroneously added to growing DNA chains in place of thymine. Ultimately, this halts DNA synthesis, and promotes fragmentation of the DNA as repair enzymes unsuccessfully attempt to remove the many faulty nucleotides. The Molecular Perspective: Methotrexate