Introduction: Decreased activity in pyrimidine catabolic enzymes dihydropyrimidine dehydrogenase (DPD), dihydropyrimidinase (DHP), and β-ureidopropionase (BUP-1) predispose cancer patients to severe 5-fluorouracil (5-FU) toxicity. An earlier clinical study of colorectal cancer patients observed significantly increased 5-FU toxicity in African American patients compared to Caucasian patients. Recently, we utilized a novel 2-13C-uracil breath test (UraBT) which demonstrated that African Americans have significantly reduced pyrimidine catabolism compared with Caucasians which may in turn increase their risk for 5-FU toxicity [Clin Cancer Res. 2006; 12:5491-5]. To identify the molecular basis for reduced pyrimidine catabolism in subjects with a reduced UraBT but normal DPD enzyme activity, we screened the DPYS and UPB1 genes (which encode the DHP and BUP-1 enzymes) for sequence variations. Subjects: 194 cancer-free subjects (96 Caucasians, 98 African Americans) participated in this study. Individuals were divided into three groups: NN (normal DPD and UraBT), DD (deficient DPD and UraBT), and ND (normal DPD but deficient UraBT). Methods: Denaturing high performance liquid chromatography was used to screen the DPYS and UPB1 genes for sequence variations. A radioassay measured DPD enzyme activity and the UraBT assessed the integrity of the entire catabolic pathway. Statistical genetic analyses were completed for Caucasian and African American subjects and stratified by race. Linkage disequilibrium was assessed for all non-monomorphic sequence variations for the Caucasian and African American populations separately using both the D and r2 measures implemented in the Haploview software package. In addition to estimation of linkage disequilibrium, Hardy Weinberg equilibrium tests and minor allele frequency estimates were also computed. Results: Nine African American and four Caucasian subjects demonstrated reduced pyrimidine catabolism resulting from reduced DPD activity. Two African American women with reduced pyrimidine catabolism, but normal DPD activity, had novel DPYS sequence variations which were subsequently shown to significantly decrease DHP activity. One African American woman, with reduced DPD enzyme activity, also had a UPB1 sequence variation which was previously shown to abolish BUP-1 activity. In African American subjects, a significant association between decreased DPD activity, reduced pyrimidine catabolism, and UPB1 haplotype was observed (p=0.01). Conclusions: In the current study, UPB1 haplotype was found to be significantly associated with reduced pyrimidine catabolism and DPD deficiency in African Americans, providing a potentially valuable pharmacogenetic marker for 5-FU toxicity resulting from reduced drug catabolism. Large clinical studies of cancer patients receiving 5-FU chemotherapy are warranted to validate these genetic associations with 5-FU toxicity. This study was supported by NIH grants CA 116964 and 062164-2.
First AACR International Conference on the Science of Cancer Health Disparities-- Nov 27-30, 2007; Atlanta, GA