Discrepancies between the gene expression, protein expression, and enzymatic activity of thymidylate synthase and dihydropyrimidine dehydrogenase in human gastrointestinal cancers and adjacent normal mucosa.

The relationships between gene expression, protein expression, and enzymatic activity of thymidylate synthase (TS) and dihydropyrimidine dehydrogenase (DPD) have not been clarified in tumor and non-tumor tissues of human. In this study, we compared three different parameters by evaluating TS and DPD levels, mRNA expression assessed by RT-PCR, protein expression evaluated by immunohistochemical examination, and enzymatic activity measured by biochemical assay, in the tumor tissue and adjacent normal mucosa of 43 patients with gastrointestinal cancer. TS enzymatic activity in the tumor tissue was significantly higher than in normal tissue in both the stomach (median activity: 81.0 and 38.0 pmol/mg protein, respectively, p=0.012) and the colorectum (49.8 and 30.8, respectively, p=0.023). Similarly, TS mRNA expression in the tumor tissue was significantly higher than in normal tissue in both the stomach (median TS/GAPDH ratio: 6.0 and 2.0, respectively, p=0.009) and the colorectum (3.20 and 0.91, respectively, p=0.001). But no significant differences in DPD activity were observed between the tumor and normal tissue in either stomach (median activity: 41.3 and 41.6 pmol/min/mg protein, respectively) or colorectum (34.9 and 49.0, respectively). On the other hand, DPD mRNA levels in normal tissue were significantly higher than in tumor tissue only in the colorectum (DPD/GAPDH ratio: 0.83 and 0.20, respectively, p=0.003). No linear relationships were found between the enzymatic activity and mRNA expression of TS or DPD either in tumor or normal tissue. Nor were any correlations found between protein expression and either mRNA expression or enzymatic activity for either TS or DPD. These results suggest that tissue TS and DPD levels may vary with differences in the methods used to measure them. These discrepancies must be taken into account when interpreting correlation between TS and DPD levels and clinical outcome.