Mutations at codon 974 of the DPYD gene are a rare event

Crystal structure of dihydropyrimidine dehydrogenase, a major determinant of the pharmacokinetics of the anti-cancer drug 5-fluorouracil

Dihydropyrimidine dehydrogenase catalyzes the first step in pyrimidine degradation: the NADPH-dependent reduction of uracil and thymine to the corresponding 5,6-dihydropyrimidines. Its controlled inhibition has become an adjunct target for cancer therapy, since the enzyme is also responsible for the rapid breakdown of the chemotherapeutic drug 5-fluorouracil. The crystal structure of the homodimeric pig liver enzyme (2x 111 kDa) determined at 1.9 A resolution reveals a highly modular subunit organization, consisting of five domains with different folds. Dihydropyrimidine dehydrogenase contains two FAD, two FMN and eight [4Fe-4S] clusters, arranged in two electron transfer chains that pass the dimer interface twice. Two of the Fe-S clusters show a hitherto unobserved coordination involving a glutamine residue. The ternary complex of an inactive mutant of the enzyme with bound NADPH and 5-fluorouracil reveals the architecture of the substrate-binding sites and residues responsible for recognition and binding of the drug.

Pharmacogenetics and cancer chemotherapy

Cancer chemotherapy is limited by significant inter-individual variations in responses and toxicities. Such variations are often due to genetic alterations in drug metabolising enzymes (pharmacokinetic polymorphisms) or receptor expression (pharmacodynamic polymorphisms). Pharmacogenetic screening prior to anticancer drug administration may lead to identification of specific populations predisposed to drug toxicity or poor drug responses. The role of polymorphisms in specific enzymes, such as thiopurine S-methyltransferases (TPMT), dihydropyrimidine dehydrogenase (DPD), aldehyde dehydrogenases (ALDH), glutathione S-transferases (GST), uridine diphosphate glucuronosyl-transferases (UGTs) and cytochrome P450 (CYP 450) enzymes in cancer therapy are discussed in this review.