DNA repair by MGMT, but not AAG, causes a threshold in alkylation-induced colorectal carcinogenesis

Fahrer, J�rg; Frisch, Janina; Nagel, Georg; Kraus, Alexander; D�rsam, Bastian; Thomas, Adam D; Rei�ig, Sonja; Waisman, Ari; Kaina, Bernd

Abstract

© The Author 2015. Published by Oxford University Press. All rights reserved. Epidemiological studies indicate that N-nitroso compounds (NOC) are causally linked to colorectal cancer (CRC). NOC induce DNA alkylations, including O6-methylguanine (O6-MeG) and N-methylated purines, which are repaired by O6-MeGDNA 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. O6-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.

• Journal Article Type: Article
• Acceptance Date: Jul 27, 2015
• Online Publication Date: Aug 4, 2015
• Publication Date: Oct 1, 2015
• Journal: Carcinogenesis
• Print ISSN: 0143-3334
• Electronic ISSN: 1460-2180
• Publisher: Oxford University Press (OUP)
• Peer Reviewed: Peer Reviewed
• Volume: 36
• Issue: 10
• Pages: 1235-1244
• DOI: https://doi.org/10.1093/carcin/bgv114
• Keywords: alkylation, dna, dna repair, magnetoencephalography, megestrol, o(6)-methylguanine-dna methyltransferase, mice, carcinogenesis
• Public URL: https://uwe-repository.worktribe.com/output/804838
• Publisher URL: http://dx.doi.org/10.1093/carcin/bgv114
• Additional Information: Additional Information : This is a pre-copyedited, author-produced version of an article accepted for publication in Carcinogenesis following peer review. The version of record [Fahrer, Jörg, Frisch, J., Nagel, G., Kraus, A., Dörsam, B., Thomas, A. D., Reißig, S., Waisman, A. and Kaina, B. (2015) DNA repair by MGMT, but not AAG, causes a threshold in alkylation-induced colorectal carcinogenesis. Carcinogenesis, 36 (10). pp. 1235-1244. ISSN 0143-3334] is available online at: http://dx.doi.org/10.1093/carcin/bgv114.