Mismatch repair proficiency and in vitro response to 5-fluorouracil

Background & Aims: The DNA mismatch repair (MMR) system recognizes certain DNA adducts caused by alkylation damage in addition to its role in recogniz ing and directing repair of interstrand nucleotide mismatches and slippage mistakes at microsatellite sequences. Because defects in the MMR system can confer tolerance to acquired DNA damage and, by inference, the toxic effec ts of certain chemotherapeutic agents, we investigated the effect of 5-fluo rouracil (5-FU) on colon cancer cell lines. Methods: We determined growth s election by cell enrichment assay and cloning efficiency after treatment wi th 5 mu mol/L 5-FU, assayed nucleic H-3-5-FU incorporation, and analyzed th e cell cycle by flow cytometry. Results: 5-FU treatment provided a growth a dvantage for MMR-deficient cell lines, indicating a relative degree of tole rance to 5-FU by the MMR-deficient cell lines. Enhanced survival was statis tically significant after 5 days of growth, and a 28-fold reduction in surv ival was noted in the MMR-proficient cells by clonagenic assays after 10 da ys of growth. Differences in nucleotide uptake of 5-FU did not account for the observed growth differences, and specific cell cycle checkpoint arrest was not detected. Conclusions: Intact DNA MMR seems to recognize 5-FU incor porated into DNA but may do so in a different manner than other types of al kylation damage. Defective DNA MMR might be one mechanism for tumor resista nce to 5-FU.