Impact of DNA repair on the dose-response of colorectal cancer formation induced by dietary carcinogens

- Colorectal cancer (CRC) formation is causally linked to dietary habits.
- Heterocyclic aromatic amines (HCAs) and N-nitroso compounds (NOC) are potent dietary carcinogens.
- HCAs and NOC induce carcinogenic DNA adducts in the colorectum.
- Recent studies provide evidence for non-linear dose-response relationships in NOC- and HCA-induced CRC formation.
- DNA repair by MGMT is crucial for the existence of a carcinogenic threshold at low DNA alkylation levels.

Colorectal cancer (CRC) is one of the most frequently diagnosed cancers, which is causally linked to dietary habits, notably the intake of processed and red meat. Processed and red meat contain dietary carcinogens, including heterocyclic aromatic amines (HCAs) and N-nitroso compounds (NOC). NOC are agents that induce various N-methylated DNA adducts and O6-methylguanine (O6-MeG), which are removed by base excision repair (BER) and O6-methylguanine-DNA methyltransferase (MGMT), respectively. HCAs such as the highly mutagenic 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) cause bulky DNA adducts, which are removed from DNA by nucleotide excision repair (NER). Both O6-MeG and HCA-induced DNA adducts are linked to the occurrence of KRAS and APC mutations in colorectal tumors of rodents and humans, thereby driving CRC initiation and progression. In this review, we focus on DNA repair pathways removing DNA lesions induced by NOC and HCA and assess their role in protecting against mutagenicity and carcinogenicity in the large intestine. We further discuss the impact of DNA repair on the dose-response relationship in colorectal carcinogenesis in view of recent studies, demonstrating the existence of 'no effect' point of departures (PoDs), i.e. thresholds for genotoxicity and carcinogenicity. The available data support the threshold concept for NOC with DNA repair being causally involved.