Thymidylate synthase expression and genotype have no major impact on the clinical outcome of colorectal cancer patients treated with 5-fluorouracil

Thymidylate synthase (TS) expression levels appear to be related to response to 5-fluorouracil-(5-FU)-based chemotherapy in colorectal cancer (CRC) patients. Three polymorphisms have been proposed as modulators of TS expression: a tandemly repeated sequence (2R/3R) in the 5′ UTR, a SNP (G>C) within the 3R allele and a 6bp deletion in the 3′ UTR. To evaluate the influence of TS expression and polymorphisms on clinical outcome of 5-FU-treated patients we performed a comprehensive genetic analysis on 63 CRC patients. TS expression levels were analyzed in normal and tumor tissues. TS coding sequence and UTR polymorphisms were investigated on DNA from normal tissue. LOH analysis was performed to determine tumor genotype. A difference in disease-free survival (DFS), although not statistically significant, was observed between high and low mRNA expression levels: patients with low levels showed longer DFS. The 2R2R genotype showed significantly lower expression than the 3R3R and 2R3R genotypes in normal tissue. No other TS polymorphism was associated with mRNA expression or clinical outcome. The results obtained in this pilot study indicate that the number of 5′ UTR repeats is the major genetic determinant of TS expression. The lack of association with other polymorphisms might be partially explained by the existence of linkage disequilibrium in the TS gene. Our data support the growing evidence that TS control may require multiple mechanisms acting in close coordination with one another and suggest that TS genotyping alone in tumor samples is not sufficient to accurately predict response to 5-FU.