title: O6-methylguanine-induced cell death involves exonuclease 1 as well as DNA mismatch recognition in vivo.
abstract: Alkylation-induced O(6)-methylguanine (O(6)MeG) DNA lesions can be mutagenic or cytotoxic if unrepaired by the O(6)MeG-DNA methyltransferase (Mgmt) protein. O(6)MeG pairs with T during DNA replication, and if the O(6)MeG:T mismatch persists, a G:C to A:T transition mutation is fixed at the next replication cycle. O(6)MeG:T mismatch detection by MutSalpha and MutLalpha leads to apoptotic cell death, but the mechanism by which this occurs has been elusive. To explore how mismatch repair mediates O(6)MeG-dependent apoptosis, we used an Mgmt-null mouse model combined with either the Msh6-null mutant (defective in mismatch recognition) or the Exo1-null mutant (impaired in the excision step of mismatch repair). Mouse embryonic fibroblasts and bone marrow cells derived from Mgmt-null mice were much more alkylation-sensitive than wild type, as expected. However, ablation of either Msh6 or Exo1 function rendered these Mgmt-null cells just as resistant to alkylation-induced cytotoxicity as wild-type cells. Rapidly proliferating tissues in Mgmt-null mice (bone marrow, thymus, and spleen) are extremely sensitive to apoptosis induced by O(6)MeG-producing agents. Here, we show that ablation of either Msh6 or Exo1 function in the Mgmt-null mouse renders these rapidly proliferating tissues alkylation-resistant. However, whereas the Msh6 defect confers total alkylation resistance, the Exo1 defect leads to a variable tissue-specific alkylation resistance phenotype. Our results indicate that Exo1 plays an important role in the induction of apoptosis by unrepaired O(6)MeGs.
authors.affiliation: Department of Biological Engineering and Center for Environmental Health Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
authors.forename: Joanna
authors.initials: J
authors.lastname: Klapacz
authors.forename: Lisiane B
authors.initials: LB
authors.lastname: Meira
authors.forename: David G
authors.initials: DG
authors.lastname: Luchetti
authors.forename: Jennifer A
authors.initials: JA
authors.lastname: Calvo
authors.forename: Roderick T
authors.initials: RT
authors.lastname: Bronson
authors.forename: Winfried
authors.initials: W
authors.lastname: Edelmann
authors.forename: Leona D
authors.initials: LD
authors.lastname: Samson
mesh_terms: D000478:Alkylation; D000818:Animals; D017209:Apoptosis / Q000235:genetics*; D020137:Base Pair Mismatch; D001854:Bone Marrow Cells / Q000166:cytology; D049109:Cell Proliferation; D004268:DNA-Binding Proteins / Q000502:physiology*; D005090:Exodeoxyribonucleases / Q000502:physiology*; D005347:Fibroblasts / Q000166:cytology; D006147:Guanine / Q000031:analogs & derivatives* / Q000494:pharmacology; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D019853:O(6)-Methylguanine-DNA Methyltransferase; D013154:Spleen / Q000166:cytology; D013950:Thymus Gland / Q000166:cytology
chemical_list: D004268:DNA-Binding Proteins; C102997:Msh6 protein, mouse; D006147:Guanine; C008449:O-(6)-methylguanine; D019853:O(6)-Methylguanine-DNA Methyltransferase; D005090:Exodeoxyribonucleases; C028450:exodeoxyribonuclease I