Polymorphisms in the Enhancer Region of the Thymidylate Synthase Gene Are Associated with Thymidylate Synthase Levels in Normal Tissues but Not in Malignant Tissues of Patients with Colorectal Cancer

Abstract

The enhancer region of the thymidylate synthase (TS) gene (TSER) contains a polymorphic tandem repeat sequence (2 or 3 repeats, 2R or 3R) and a single-nucleotide polymorphism (G > C) within the second repeat of the 3R alleles which might influence TS expression/activity and response to fluoropyrimidines. However, clinical studies in patients with colorectal cancer (CRC) failed to find a consistent relationship between TSER polymorphisms and protein levels as well as with clinical outcome. The analysis of the relationship between TSER genotype and TS mRNA and activity in normal and malignant tissues might explain the previous controversial data and help in the selection of useful markers to predict drug response and/or toxicity. To address this issue, we studied TSER genotype, TS expression, and activity with specific polymerase chain reaction and activity assays (TS catalytic activity and FdUMP binding) in normal (liver, mucosa) and malignant (primary tumor and liver metastasis) tissues from 83 patients with CRC. No correlation between TSER genotype and TS mRNA and protein levels was observed in malignant tissues. In contrast, normal tissues harboring one or two 3RG alleles were characterized by higher TS protein levels (2.4-fold; P = .008) and catalytic activity (P < .05) compared with the other TSER genotypes. These results suggest that TSER polymorphisms do not predict tumoral TS levels possibly depending on altered TS regulation in cancer tissues, and might explain the lack of clear correlation with clinical outcome after chemotherapy with fluoropyrimidines. However, the relationship between TS phenotype and TSER genotype in normal tissues warrants further investigations in large-scale prospective studies evaluating TS genotype and fluoropyrimidine tolerability.