Characterization of mutations induced by 2-amino-1-methyl-6-phenylimidazo [4,5-b]pyridine in the colon of gpt delta transgenic mouse: novel G : C deletions beside runs of identical bases

Mutations induced by one of the typical dietary mutagens/carcinogens, 2-ami no-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), were characterized using gpt delta transgenic mice. This transgenic mouse model has two selection m ethods to efficiently detect different types of mutations, i,e, 6-thioguani ne selection for point mutations and Sp(-) selection for deletions. The mic e were fed with a diet containing 400 p,p,m, PhIP for 13 weeks and gpt and Spi(-) mutations were analyzed from the colon, where the highest mutant fre quencies were detected. Concerning the types of gpt mutations from PhIP-tre ated mice, 81% were single base pair substitutions and G:C --> T:A transver sions predominated; single base pair deletions at G:C base pairs were also observed. In untreated mice G:C --> A:T transitions predominated and >80% o f these events involved 5'-CpG-3' sites. Concerning Spi(-) mutants from PhI P-treated mice, 76% were G:C base pair deletions and more than half of thes e events occurred in monotonic G or C run sequences. Interestingly, a novel type of frameshift motif, i.e, G:C base pair deletions beside run sequence s, was observed. The most frequently observed mutation in this class was th e 5'-TTTTTTG-3' --. 5'-TTTTTT-3' event. These results suggest that PhIP ind uces point mutations, such as base substitutions and single base pair delet ions, rather than larger deletions irt vivo and that run sequences may play an important role in PhIP-induced G:C base pair deletions.