Impact of postreplicational DNA mismatch repair on radiosensitization by the halogenated thymidine analogues iododeoxyuridine and bromodeoxyuridine

The halogenated thymidine (dThd) analogues iododeoxyuridine (IdUrd) and bromodeoxyuridine (BrdUrd) have been successful in improving the efficacy of radiation therapy in clinical trials for various rapidly dividing tumor types in slow-growing normal tissues, including malignant brain tumors, colorectal liver metastases, pancreatic cancer, and unresectable sarcomas. Our work now indicates that these dThd analogs may also be used to target tumors which commonly lose their capacity to carry out postreplicational DNA mismatch repair. Our data demonstrates that protein products of the two genes most commonly mutated or epigenetically-silenced in hereditary or sporadically-arising mismatch repair (MMR)-deficient cancers, MSH2 and MLH1, are involved in processing both IdUrd and BrdUrd in DNA. We have shown in vitro that the MutSα MMR recognition complex, a dimer composed of the two mismatch repair proteins MSH2 and MSH6, directly recognizes DNA containing an IdUrd-G mispair, which is the base pair implicated in halogenated thymidine analog-induced mutagenesis. In vivo, the MSH2 protein is redistributed throughout the nucleus following incorporation of IdUrd into DNA, to localize in discrete foci within the nucleus. Following recognition by the MMR proteins, we have shown in multiple cell models that levels of IdUrd and BrdUrd in DNA decrease over time in MMR-proficient cells, but not in MMR-deficient cells. As a result, MMR-deficient cells are much more susceptible to IdUrd- and BrdUrd-induced radiosensitization than the MMR-proficient cells, which would allow for selective radiosensitzation of MMR-deficient tumor cells in MMR-proficient tissue, and decreased dThd analogue-induced mutagenesis at clinically-relevant concentrations of the halogenated thymidine analogues in cells with MMR activity.