title: Homologous recombination rescues mismatch-repair-dependent cytotoxicity of S(N)1-type methylating agents in S. cerevisiae.
abstract: Resistance of mammalian cells to S(N)1-type methylating agents such as N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) generally arises through increased expression of methylguanine methyltransferase (MGMT), which reverts the cytotoxic O(6)-methylguanine ((Me)G) to guanine, or through inactivation of the mismatch repair (MMR) system, which triggers cell death through aberrant processing of (Me)G/T mispairs generated during DNA replication when MGMT capacity is exceeded. Given that MMR and (Me)G-detoxifying proteins are functionally conserved through evolution, and that MMR-deficient Escherichia coli dam(-) strains are also resistant to MNNG, the finding that MMR status did not affect the sensitivity of Saccharomyces cerevisiae to MNNG was unexpected. Because (Me)G residues in DNA trigger homologous recombination (HR), we wondered whether the efficient HR in S. cerevisiae might alleviate the cytotoxic effects of (Me)G processing. We now show that HR inactivation sensitizes S. cerevisiae to MNNG and that, as in human cells, defects in the MMR genes MLH1 and MSH2 rescue this sensitivity. Inactivation of the EXO1 gene, which encodes the only exonuclease implicated in MMR to date, failed to rescue the hypersensitivity, which implies that scExo1 is not involved in the processing of (Me)G residues by the S. cerevisiae MMR system.
authors.affiliation: Institute of Molecular Cancer Research, University of Zürich, August Forel-Strasse 7, CH-8008 Zürich, Switzerland.
authors.forename: Petr
authors.initials: P
authors.lastname: Cejka
authors.forename: Nina
authors.initials: N
authors.lastname: Mojas
authors.forename: Ludovic
authors.initials: L
authors.lastname: Gillet
authors.forename: Primo
authors.initials: P
authors.lastname: Schär
authors.forename: Josef
authors.initials: J
authors.lastname: Jiricny
mesh_terms: D048868:Adaptor Proteins, Signal Transducing; D017209:Apoptosis / Q000235:genetics*; D020137:Base Pair Mismatch / Q000235:genetics*; D004260:DNA Repair / Q000235:genetics*; D024202:Electrophoretic Mobility Shift Assay; D005090:Exodeoxyribonucleases / Q000378:metabolism; D005656:Fungal Proteins / Q000235:genetics; D018014:Gene Transfer Techniques; D008769:Methylnitronitrosoguanidine / Q000378:metabolism*; D008954:Models, Biological; D000070957:MutL Protein Homolog 1; D051718:MutS Homolog 2 Protein / Q000235:genetics; D009841:Oligonucleotides; D011995:Recombination, Genetic / Q000235:genetics*; D012441:Saccharomyces cerevisiae; D029701:Saccharomyces cerevisiae Proteins
chemical_list: D048868:Adaptor Proteins, Signal Transducing; D005656:Fungal Proteins; C088859:MLH1 protein, S cerevisiae; D009841:Oligonucleotides; D029701:Saccharomyces cerevisiae Proteins; D008769:Methylnitronitrosoguanidine; D005090:Exodeoxyribonucleases; C028450:exodeoxyribonuclease I; D000070957:MutL Protein Homolog 1; D051718:MutS Homolog 2 Protein