Mlh1-dependent suppression of specific mutations induced in vivo by the food-borne carcinogen 2-amino-1-methyl-6-phenylimidazo [4,5-b] pyridine (PhIP)

Abstract

Disruption of the DNA mismatch repair (MMR) pathway results in elevated mutation rates, inappropriate survival of cells bearing DNA damage, and increased cancer risk. Relatively little is known about the potential impact of environmentally relevant carcinogens on cancer risk in individuals with MMR-deficiency. We determined the effect of MMR status (Mlh1+/+ versus Mlh1-/-) on mutagenesis induced by the cooked-meat mutagen, 2-amino-1-methyl-6-phenylimidazo [4,5-b] pyridine (PhIP) within cII and supFG1 transgene reporters. Despite being a lymphomagen in mice, PhIP was not mutagenic in thymus. In colon, PhIP exposure induced 3-fold more mutations in Mlh1-deficient mice compared to their Mlh1+/+ littermates. Similar induction was seen in Mlh1-/- small intestine. Analysis of mutational spectra revealed that G/C to T/A transversions, the "signature PhIP mutation", were induced to similar levels regardless of Mlh1 status. In contrast, Mlh1-/- mice exhibited hypermutability to frameshifts, G/C to A/T transitions, and G/C to C/G transversions. Thus, both the level and types of mutation induced by PhIP are influenced by the activity of the MMR system. MMR may suppress PhIP-induced mutation through recognition and processing of specific mispairs (PhIP-G/T, PhIP-G/G, and PhIP-G/loop mispairs). In contrast, the PhIP-G/A mispair is unlikely to be a MMR substrate. In addition, the similar induction of both transversions and transitions in Mlh1-/- mice suggests that mutagenic bypass of PhIP-G is similarly efficient with dATP, dTTP, and dGTP, in contrast to previously published conclusions. Our data suggests that MMR-deficiency would increase the likelihood of PhIP-induced carcinogenic mutations. Further evaluation of the risk that consumption of heterocyclic amines may impart to MMR-deficient individuals therefore is warranted.