HIGHER LEVELS OF THYMIDYLATE SYNTHASE GENE-EXPRESSION ARE OBSERVED INPULMONARY AS COMPARED WITH HEPATIC METASTASES OF COLORECTAL ADENOCARCINOMA

Purpose: It has been observed previously that the pulmonary metastases of colorectal adenocarcinoma are less responsive to therapy with fluo rouracil (FUra) as compared with other sites of metastasis (liver, loc al). To investigate the basis of this chemoresistance, the levels of t hymidylate synthase (TS) mRNA and protein were measured, as TS express ion has been shown to be predictive of response to therapy in colorect al cancer. Materials and Methods: Tumors were obtained from 19 patient s with metastatic colorectal cancer(12 hepatic and seven pulmonary). T S expression was measured by quantitative reverse-transcriptase polyme rase chain reaction (RT-PCR) and TS protein levels were measured by We stern blotting. The presence of TS amplification was assessed by South ern blotting. Levels of p53 protein were determined using immunohistoc hemistry. Results: TS mRNA expression was shown to be significantly hi gher in the pulmonary metastases (mean TS/beta-actin ratio, 19.7; n = 7) as compared with the hepatic metastases (mean TS/beta-actin ratio, 4.7; n = 11) of colorectal cancer. Lower TS expression was observed in patients with hepatic metastases who had received prior FUra versus p atients who had not been treated. High levels of TS expression in some samples was associated with low-level (two to three gene copies) incr eases in TS gene copy numbers and this was observed more frequently in the pulmonary metastatic samples. The increased gene copy numbers occ urred both in samples with wild-type p53 and those with mutant p53 tum or-suppressor gene as determined by immunohistochemistry. Conclusion: High levels of TS enzyme may be the basis of the lack of response of p ulmonary metastases to FUra treatment. (C) 1998 by American Society of Clinical Oncology.