Thymidylate Synthetase (TS) Genotype and TS/dihydropyrimidine Dehydrogenase mRNA Level as an Indicator in Determining Chemosensitivity to 5-Fluorouracil in Advanced Gastric Carcinoma

Background: One of the target enzymes of 5-fluorouracil (5-FU) is thymidylate synthetase (TS). The DNA sequence of the TS gene includes either double or triple tandem 28-bp repeats within the promoter region, such that TS genotypes can be classified as homozygous 3R/3R, heterozygous 2R/3R or homozygous 2R/2R. Several recent studies have shown that TS genotype affects mRNA expression, with 3R/3R homozygotes showing higher TS mRNA expression compared to the other genotypes. Purpose: We analyzed the TS genotype and TS and dihydropyrimidine dehydrogenase (DPD) mRNA expression levels in 22 advanced gastric carcinoma patients, and analyzed results with respect to patient 5-FU chemosensitivity, as detected by the tetrazolium-based colorimetric (MTT) assay and survival outcome. Patients and Methods: Between September 2001 and April 2002, 22 Japanese patients with advanced gastric carcinoma were evaluated. Informed written consent was obtained from all patients and the study was approved by the ethical committee at our University Hospital. Fresh surgical specimens from carcinoma lesions were enzymatically dissociated and incubated with 5-FU at a concentration of 50 μg/ml for 48 hours to determine the inhibition rate as detected by MTT assay. Normal and tumor tissue and peripheral blood samples were collected and stored at -80°C until assay for TS genotype and TS and DPD mRNA level. The TS genotype was assessed by PCR assay using peripheral monocytes, since monocyte genotypes represent the genotype of normal and tumor tissues. Quantification of TS and DPD mRNA levels was performed using real-time PCR. Survival outcome was assessed according to the disease-free survival period in cases with similar clinical backgrounds. Results: TS genotyping revealed 19 3R/3R homozygotes and 3 2R/3R heterozygotes. After analysis of normal and tumor tissues, samples from homozygote 3R/3R cases showed higher TS mRNA expression than heterozygote 2R/3R cases, which was statistically significant at p