Comprehensive thymidylate synthase genotyping in adjuvant therapy of CRC

3588 Background: Adjuvant chemotherapy treatment, mainly based in the inhibition of thymidylate synthase (TS) enzyme with 5-FU (5-fluorouracil) is offered to all Dukes C and indications for Dukes B2 cases are being evaluated. TS levels of expression as well the presence of different TS gene polymorphisms have been postulated as a potential marker of 5-FU response. The aim of this study was to evaluate TS genotyping as a predictor of 5-FU response in a series of primary resected CRC. Methods: 213 patients resected of stage II-IV colorectal tumors [63 Dukes B2, 121 Dukes C and 29 Dukes D] were treated with 5-FU plus levamisole or leucovorin as a standard treatment. TS protein expression was immunohistochemically assessed using commercial and a newly generated polyclonal antibody. TS polymorphisms [TSER, the G/C polymorphism based in the second repeat of the 3R allele of TSER, and TS 1494del6], point mutations and allelic imbalances were analyzed in 128 of the 213 cases after tumor cell enrichment. Results: 174 tumors (81.7.%) had high intratumoral TS expression and associated with poorer 5-year overall survival (76.97% vs. 69.77% at 5 years, p=0.079). No correlation was observed between TSER and TS1494del6 polymorphisms and TS protein immunostaining. The presence of the TS1494del6 polymorphism was associated to a better 5-year OS (p=0.006). TSER genotype did not predict for longer DFS (p=0.341) or OS (p=0.255). Inclusion of the presence of the G/C allele in the 3R allele did not modify these results. Allelic imbalances were present in 35% of all informative tumors. No point mutations were detected in the 128 tumors analyzed. Conclusions: A 6-bp deletion in the 3'-untranslated region of the TS and high TS protein expression are effective in predicting outcome of adjuvant chemotherapy in patients with resected CRC. TS allelic imbalances are a frequent event in human colorectal tumorigenesis of unknown significance. No significant financial relationships to disclose.