title: DNA repair by MGMT, but not AAG, causes a threshold in alkylation-induced colorectal carcinogenesis.
abstract: Epidemiological studies indicate that N-nitroso compounds (NOC) are causally linked to colorectal cancer (CRC). NOC induce DNA alkylations, including O (6)-methylguanine (O (6)-MeG) and N-methylated purines, which are repaired by O (6)-MeG-DNA methyltransferase (MGMT) and N-alkyladenine-DNA glycosylase (AAG)-initiated base excision repair, respectively. In view of recent evidence of nonlinear mutagenicity for NOC-like compounds, the question arises as to the existence of threshold doses in CRC formation. Here, we set out to determine the impact of DNA repair on the dose-response of alkylation-induced CRC. DNA repair proficient (WT) and deficient (Mgmt (-/-), Aag (-/-) and Mgmt (-/-)/Aag (-/-)) mice were treated with azoxymethane (AOM) and dextran sodium sulfate to trigger CRC. Tumors were quantified by non-invasive mini-endoscopy. A non-linear increase in CRC formation was observed in WT and Aag (-/-) mice. In contrast, a linear dose-dependent increase in tumor frequency was found in Mgmt (-/-) and Mgmt (-/-)/Aag (-/-) mice. The data were corroborated by hockey stick modeling, yielding similar carcinogenic thresholds for WT and Aag (-/-) and no threshold for MGMT lacking mice. O (6)-MeG levels and depletion of MGMT correlated well with the observed dose-response in CRC formation. AOM induced dose-dependently DNA double-strand breaks in colon crypts including Lgr5-positive colon stem cells, which coincided with ATR-Chk1-p53 signaling. Intriguingly, Mgmt (-/-) mice displayed significantly enhanced levels of γ-H2AX, suggesting the usefulness of γ-H2AX as an early genotoxicity marker in the colorectum. This study demonstrates for the first time a non-linear dose-response for alkylation-induced colorectal carcinogenesis and reveals DNA repair by MGMT, but not AAG, as a key node in determining a carcinogenic threshold.
authors.forename: Jörg
authors.initials: J
authors.lastname: Fahrer
authors.forename: Janina
authors.initials: J
authors.lastname: Frisch
authors.forename: Georg
authors.initials: G
authors.lastname: Nagel
authors.forename: Alexander
authors.initials: A
authors.lastname: Kraus
authors.forename: Bastian
authors.initials: B
authors.lastname: Dörsam
authors.forename: Adam D
authors.initials: AD
authors.lastname: Thomas
authors.affiliation: Institute of Molecular Medicine, University Medical Center Mainz, Obere Zahlbacher Str. 67, D-55131 Mainz, Germany.
authors.forename: Sonja
authors.initials: S
authors.lastname: Reißig
authors.affiliation: Institute of Molecular Medicine, University Medical Center Mainz, Obere Zahlbacher Str. 67, D-55131 Mainz, Germany.
authors.forename: Ari
authors.initials: A
authors.lastname: Waisman
authors.forename: Bernd
authors.initials: B
authors.lastname: Kaina
mesh_terms: D000818:Animals; D063646:Carcinogenesis / Q000139:chemically induced / Q000235:genetics; D015179:Colorectal Neoplasms / Q000139:chemically induced / Q000235:genetics* / Q000473:pathology; D045647:DNA Glycosylases / Q000235:genetics*; D015254:DNA Modification Methylases / Q000235:genetics*; D004260:DNA Repair / Q000187:drug effects / Q000235:genetics*; D045643:DNA Repair Enzymes / Q000235:genetics*; D004195:Disease Models, Animal; D006801:Humans; D051379:Mice; D008822:Mice, Transgenic; D009603:Nitroso Compounds / Q000633:toxicity; D025521:Tumor Suppressor Proteins / Q000235:genetics*
chemical_list: D009603:Nitroso Compounds; D025521:Tumor Suppressor Proteins; D015254:DNA Modification Methylases; C528331:MGMT protein, mouse; C021565:3-methyladenine-DNA glycosylase; D045647:DNA Glycosylases; D045643:DNA Repair Enzymes