High thymidylate synthase expression in colorectal cancer with microsatellite instability: Implications for chemotherapeutic strategies

Colon cancers displaying microsatellite instability (MSI) are clinically less aggressive. Based on in vitro studies and recent clinical data, cancers displaying MSI do not respond to 5-fluorouracil (5-FU), The reasons why MSI tumors are clinically less aggressive and do not respond to 5-FU-based therapies have not been fully elucidated. Purpose: We investigated biomolecular markers in an attempt to explain the different clinical behavior and chemotherapeutic responses of MSI and non-MSI colon cancers. Experimental Design: One hundred ninety-two sporadic colon cancers were tested for MSI with five mononucleotide markers and methylation of the hMLH1 promoter. Slides were stained for thymidylate synthase (TS), p53, MDM2, p21(WAF1/CIP1), beta-catenin, vascular endothelial growth factor, hMLH1, hMSH2, and hMSH6. Tumors were regarded as having wild-type, functional p53 (Fp53) if reduced expression of p53 and positive MDM2 and p21(WAF1/CIP1) expressions were found. Results: Of the cases, 12.5% were MSI-H (at least two markers mutated). Of MSI-H cases, 83.3% were characterized by a complete loss of at least one of the mismatch repair proteins, in particular loss of hMLH1 by promoter hypermethylation. MSI-H colon cancers showed higher expression of TS compared with MSS (no mutated markers)/MSI-L (one mutated marker) colon cancers (66.6% for MSI-H versus 14.8% MSS/MSI-L; P < 0.0001); 20.8% of MSI-H cases showed high expression of the vascular endothelial growth factor, compared with 45.8% MSS/MSI-L colon cancers (P = 0.0005); 45.8% MSI-H cases had Fp53 compared 11.9% MSS/MSI-L cases (P < 0.0001). Conclusions: About 12% of colon cancers display MSI mostly due to lack of hMLH1 resulting from promoter hypermethylation. These tumors have high expression of TS and retain fully functional p53 system. Thus, these data suggest why sporadic hMLH1-defective colon cancers often do not respond to 5-FU.