HUMAN POLYMORPHISM IN DRUG-METABOLISM - MUTATION IN THE DIHYDROPYRIMIDINE DEHYDROGENASE GENE RESULTS IN EXON SKIPPING AND THYMINE URACILUREA

A condition called thymine uracilurea has been described that is due t o a lack of dihydropyrimidine dehydrogenase (DPD) activity. Cancer pat ients experiencing acute 5-fluorouracil toxicity also have lower-than- normal DPD activities. However, to date, the molecular basis of this d isorder has not been addressed. In this study, the phenotype and genot ype of a family that presents a patient showing no DPD activity was de termined. Fibroblast mRNAs from the patient and four family members we re subjected to reverse transcriptase polymerase chain reaction (RT-PC R) using primers generated from the human DPD cDNA sequence. DPD mRNA from the patient was found to lack a segment of 165 nucleotides that r esults from exon skipping. DPD mRNA from the parents and a sibling wer e found to be heterozygous for the deleted and the normal mRNA, while a brother had two normal transcripts. DPD activities and levels of DPD protein correlated with genotype; the deficient patient had no detect able DPD protein. PCR analysis of the genomic DNA from this family rev ealed that the defective mRNA is not due to a deletion of a portion of the gene that contains the exon, thus implying that the mutation is t he result of an as yet nonidentified point mutation that causes faulty splicing.