The Base Excision Repair Enzyme MED1 Mediates DNA Damage Response to Antitumor Drugs and Is Associated with Mismatch Repair System Integrity

• Published in: Proceedings of the National Academy of Sciences - PNAS 2003-12, Vol.100 (25), p.15071-15076
• Main Authors: Cortellino, Salvatore, Turner, David, Masciullo, Valeria, Schepis, Filippo, Albino, Domenico, Daniel, Rene, Skalka, Anna Marie, Meropol, Neal J., Alberti, Christophe, Larue, Lionel, Bellacosa, Alfonso
• Format: Article
• Language: English
• Subjects: Animals; Antineoplastic Agents - pharmacology; Apoptosis; base excision repair; Base Pair Mismatch; Biological Sciences; Blotting, Western; Cell cycle; Cell lines; Cell Separation; Cells, Cultured; Deoxyribonucleic acid; DNA; DNA Damage; DNA Methylation; DNA mismatch repair; DNA Repair; Endodeoxyribonucleases - genetics; Endodeoxyribonucleases - physiology; Enzymes; Exons; Fibroblasts - metabolism; Flow Cytometry; G2 Phase; Genotype; Genotypes; Guanine; irinotecan; Measels mumps rubella vaccine; MED1 protein; Medical research; Methylnitronitrosoguanidine; Mice; Mice, Transgenic; mismatch repair; Mitosis; Models, Genetic; Retroviridae - genetics; Reverse Transcriptase Polymerase Chain Reaction; Thymine DNA Glycosylase - metabolism; Time Factors; Tumors
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.2334585100

Cytotoxicity of methylating agents is caused mostly by methylation of the O6position of guanine in DNA to form$O^6\!-\!methylguanine\>(O^6\!-\!meG).\>O^6\!-\!meG$can direct misincorporation of thymine during replication, generating O6-meG:T mismatches. Recognition of these mispairs by the mismatch repair (MMR) system leads to cell cycle arrest and apoptosis. MMR also modulates sensitivity to other antitumor drugs. The base excision repair (BER) enzyme MED1 (also known as MBD4) interacts with the MMR protein MLH1. MED1 was found to exhibit thymine glycosylase activity on O6-meG:T mismatches. To examine the biological significance of this activity, we generated mice with targeted inactivation of the Med1 gene and prepared mouse embryonic fibroblasts (MEF) with different Med1 genotype. Unlike wild-type and heterozygous cultures, Med1-/-MEF failed to undergo G2-M cell cycle arrest and apoptosis upon treatment with the methylating agent N-methyl-N′-nitro-N -nitrosoguanidine (MNNG). Similar results were obtained with platinum compounds' 5-fluorouracil and irinotecan. As is the case with MMR-defective cells, resistance of Med1-/-MEF to MNNG was due to a tolerance mechanism because DNA damage accumulated but did not elicit checkpoint activation. Interestingly, steady state amounts of several MMR proteins are reduced in Med1-/-MEF, in comparison with Med1+/+and Med1+/-MEF. We conclude that MED1 has an additional role in DNA damage response to antitumor agents and is associated with integrity of the MMR system. MED1 defects (much like MMR defects) may impair cell cycle arrest and apoptosis induced by DNA damage.