Thymidylate Synthase Gene and Protein Expression Correlate and Are Associated with Response to 5-Fluorouracil in Human Colorectal and Gastric Tumors

Thymidylate synthase (TS) is the target enzyme for 5-fluorouracil (5-FU). We have correlated TS protein and gene expression with the response in patients with colorectal ( n = 9) and gastric cancer ( n = 12) treated with infusional 5-FU plus leucovorin (LV) or infusional 5-FU/LV and cisplatin, respectively. TS protein expression was analyzed by Western blot using TS106 monoclonal antibody and densitometry scanning. TS gene expression was measured by PCR analysis using β-actin as an internal standard and expressed as a TS:β-actin mRNA ratio. A close linear relationship was noted between TS protein expression and TS gene expression ( r 2 = 0.60) for the 21 tumor samples analyzed. TS immunohistochemical staining on 15 of the 21 samples revealed that the TS staining intensity correlated closely with TS protein and mRNA expression. In two biopsy samples, TS protein levels and TS gene expression did not correlate; however, one of these exhibited a focal TS staining pattern. Both the TS protein level and TS gene expression were significantly associated with response to 5-FU-based therapy. Patients with responsive disease had a mean TS protein level of 0.17 ± 0.03 arbitrary units (range, 0.05 to 0.38), whereas in patients whose tumors did not respond, the mean TS protein level was significantly higher 0.60 ± 0.09 (range, 0.06 to 1.01; P < 0.01). A similar pattern was noted with TS gene expression. In patients with responsive disease, the mean TS:β-actin gene ratio was 1.36 ± 0.3 (range, 0.5–3.3 × 10-3). In contrast, biopsies from patients with unresponsive disease had a mean TS:β-actin gene ratio of 15.4 ± 2.6 × 10-3 (range, 2.7–35.9; P < 0.01). TS protein and TS mRNA expression are highly correlated, and each predict for response to 5-FU/LV-based chemotherapy in patients with colorectal and gastric cancer.