MOLECULAR-BASIS OF THE HUMAN DIHYDROPYRIMIDINE DEHYDROGENASE-DEFICIENCY AND 5-FLUOROURACIL TOXICITY

Dihydropyrimidine dehydrogenase (DPD) deficiency constitutes an inborn error in pyrimidine metabolism associated with thymine-uraciluria in pediatric patients and an increased risk of toxicity in cancer patient s receiving 5-fluorouracil (5-FU) treatment. The molecular basis for D PD deficiency in a British family having a cancer patient that exhibit ed grade IV toxicity 10 d after 5-FU treatment was analyzed. A 165-bp deletion spanning a complete exon of the DPYD gene was found in some m embers of the pedigree having low DPD catalytic activity. Direct seque ncing of lymphocyte DNA from these subjects revealed the presence of a G to A point mutation at the 5'-splicing site consensus sequence (GT to AT) that leads to skipping of the entire exon preceding the mutatio n during pre-RNA transcription and processing. A PCR-based diagnostic method was developed to determine that the mutation is found in Caucas ian and Asian populations. This mutation was also detected in a Dutch patient with thymine-uraciluria and completely lacking DPD activity. A genotyping test for the G to A splicing point mutation could be usefu l in predicting cancer patients prone to toxicity upon administration of potentially toxic 5-FU and for genetic screening of heterozygous ca rriers and homozygous deficient subjects.