Expression of Thymidylate Synthase in Primary Colorectal Adenocarcinoma

Thymidylate synthase (TS) is a key regulatory enzyme in the cellular pathway of de novo pyrimidine synthesis and a target enzyme for 5-fluorouracil (5-FU). Most clinical studies have shown that high levels of TS in tumors are associated with decreased sensitivity to 5-FU treatment. In this study TS expression was assessed at DNA, RNA, and protein levels. The study included 69 tumors from patients with primary colorectal adenocarcinoma. At the DNA level TS enhancer polymorphism was measured on whole blood by PCR. At the RNA level TS mRNA expression was measured on formalin-fixed paraffin-embedded tumor tissue and on fresh-frozen tumor tissue by real-time RT-PCR. Protein expression was assessed by IHC. Correlation was found between TS mRNA expression in fresh-frozen tumor tissue and formalin-fixed paraffin-embedded tissue (R=0.71). TS enhancer 3/3 had significantly higher protein levels as assessed by IHC than the TS enhancer 2/2 (P=0.02), although there was no statistically significant correlation between TS enhancer polymorphism and TS mRNA expression. An interesting observation not previously reported is that the predominant IHC reaction pattern in tumors from patients with the TS enhancer genotype 3/3 is different in tumors from patients with genotypes 2/2 and 2/3. The results indicate that clinical studies of the significance of TS with regard to 5-FU-based chemotherapy should be based on assessment of TS activity at DNA, RNA, and protein levels.