Microsatellite instability (MSI) due to mutation or epigenetic silencing is associated with increased cytotoxicity to irinotecan (CPT-11) in human colorectal cancer (CRC) cell lines

Abstract

10527 Background: MSI is a phenomenon found in tumor DNA of individuals with dysfunction of the mismatch repair system (MMR). Epigenetic inactivation by promoter hypermethylation of hMLH1 causes 15 to 20% of sporadic CRC. In vitro studies have suggested an increased sensitivity to CPT-11 of MMR-deficient human CRC cell lines. Chemosensitivity evaluation in preclinical models of human CRC cell lines according to the MMR status could help in the design of specific studies in the clinical setting. Methods: We have performed drug cytotoxicity assays to compare sensitivity to CPT-11 in several human CRC cell lines with different MMR gene status that resemble the most common clinical situations in CRC patients. HCT116, HCT15, SW48 and RKO are MSI-High (MSI-H), being HCT116 due to a homozygous nonsense mutation of hMLH1 gene, HCT15 due to MSH6 mutation (both of them similar to hereditary cases) and both SW48 and RKO due to methylation of hMLH1 promoter (as MSI-H sporadic CRC cases). HT29 expresses normal levels of MMR proteins (as microsatellite stable (MSS) CRC cases). Drug concentrations resulting in 50% growth inhibition (IC50) were determined by a curve-fitting analysis and cell cycle analyses in order to characterize the cytotoxicity of cell lines were performed. Results: IC50 values and 95% confidence intervals (CI) are show in Table 1 . hMLH1-deficient cell lines due to either epigenetic silencing or mutation showed very similar IC50 and were 5- to 8-fold more sensitive to CPT-11 than the MSS line. hMSH6- deficient cell line HCT15 has sensitivity closer to MSS than MSI cell lines. Treatment with CPT-11 induced a G2/M arrest. Conclusions: Lack of hMLH1 protein due to either genetic alteration or epigenetic silencing correlates with increased sensitivity to CPT-11. MSI-H CRC cell lines are more sensitive to CPT-11 than MSS. Future clinical trials tailoring chemotherapy regimens based on microsatellite status are warranted. [Table: see text] No significant financial relationships to disclose.