Molecular basis of altered uracil catabolism in individuals with 5-FU toxicity and normal DPD enzyme activity

Abstract

3070 Background: Dihydropyrimidine dehydrogenase (DPD) deficiency accounts for approximately 43% of grade 3–4 toxicity to 5-Fluorouracil (5-FU). However, a significant number of patients with normal DPD enzyme activity remain with unexplained molecular basis of 5-FU toxicity. It has been suggested by the few cases previously reported that deficiency of dihydropyrimidinase (DHP)enzyme encoded by the DPYS gene and/or beta-ureidopropionase enzyme, encoded by the BUP-1 gene, could also be implicated in 5-FU toxicity. Methods: This study included 40 volunteers with known 13C-UraBT and DPD enzyme activity, 25 cancer patients with 5-FU toxicity despite normal DPD enzyme activity, and 25 liver biopsies from cancer patients with different grades of toxicity. All samples were analyzed for molecular defects in the DPYS and BUP-1 genes, using DHPLC and RT-PCR techniques. Results: Molecular analysis of the DPYS gene revealed the presence of two non-conservative amino acid changes, one frame-shift mutation that leads to a stop codon and premature termination of the DHP protein, five silent mutations, nine intronic sequence variations, two sequence variations in the 5’UTR and one in the non-coding region of exon 10. Molecular analysis of the BUP-1 gene revealed the presence of two non-conservative amino acid changes, one of which (314C>A: A85E) has already been reported to abolish enzyme activity; six silent mutations, four intronic sequence variations, one polymorphism in the 5’upstream sequence, and one sequence variation in each of the 5’UTR and the 3’UTR. Conclusions: The molecular basis of 5-FU toxicity is not limited to DPD deficiency; since molecular defects in genes downstream of DPD can potentially also impair 5-FU catabolism. Genetic testing for molecular defects in DPYS and BUP-1 may predict patients at risk of developing 5-FU toxicity despite having normal DPD enzyme activity. Assessing the integrity of the entire uracil catabolic pathway might be crucial to avoid toxicity in a significant group of patients receiving 5-FU or a related drug (CA62164). No significant financial relationships to disclose.