| DNA mismatch repair (MMR) is an efficient system for the detection and repair of mismatched and unpaired bases in DNA. Deficiencies in MMR are commonly found in both hereditary and sporadic colorectal cancers as well as in cancers of other tissues. Since fluorinated thymidine analogs (which through their actions might generate lesions recognizable by MMR) are widely used in the treatment of colorectal cancer, we have investigated the role of MMR in cellular responses to 5-fluoro-2′-deoxyuridine (FdUrd).; We found that MLH1-deficient human colon carcinoma (HCT116) cells showed decreased toxicity and a concomitant deficiency in G2 cell cycle checkpoint arrest after treatment with FdUrd compared with genetically matched, MMR-corrected (HCT116 3-6) cells. In contrast, there was little significant difference in survival in these cells following treatment with ionizing radiation. Similar responses to FdUrd were noted in murine MLH1 knockout compared to wild-type primary embryonic fibroblasts. We then extended these findings to human and murine cells deficient in the other major MMR protein, MSH2. This phenomenon, which presumably reflects the cell's reduced ability to recognize and process certain types of DNA lesions, is a hallmark of MMR deficiency and is referred to as damage tolerance.; To further clarify the involvement of MMR in FdUrd-mediated toxicity, we found that the incorporation of [3H]FdUrd into the DNA of MLH1-deficient HCT116 cells and MSH2-deficient embryonic stem cells was greater than that of their MMR-competent counterparts. Additionally, examination of the ATPase activity of the MSH2-MSH6 heterodimer indicated that 5-fluorouracil (5-FU):G, but not 5-FU:A, base pairs were recognized by MMR.; MLH1-mediated G2 cell cycle delay may be important for DNA damage detection and repair prior to chromosome segregation to eliminate carcinogenic lesions in daughter cells. MMR-dependent responses to fluoropyrimidine treatment, which may involve the action of p53 and the formation of DNA double strand breaks, clearly have clinical relevance for the use of this class of drugs in the treatment of tumors with MMR deficiencies. |
