Inhibition of O-6-methylguanine-DNA methyltransferase increases azoxymethane-induced colonic tumors in rats

Azoxymethane (AOM) causes O-6-methylguanine adduct formation which leads to G-->A transitions. Their repair is carried out by O-6-methylguanine-DNA me thyltransferase (MGMT), To evaluate the importance of this repair event in AOM-induced carcinogenesis, we examined the effect of O-6-benzylguanine (BG ), a potent inhibitor of MGMT, on colonic tumor development. Rats were trea ted weekly for 2 weeks at 0 and 24 h with BG (60 mg/kg body wt i.p.) or veh icle (40% polyethylene glycol, PEG-400), followed 2 h after the first dose of BG with AOM (15 mg/kg body wt) or vehicle (saline) i.p. Rats were killed 35 weeks later and tumors harvested and DNA extracted, In the AOM-treated groups, BG caused a significant increase in tumor incidence with tumors in 65.9%, versus 30.8% in the AOM/PEG-treated group (P < 0.05), In the BG/AOM group there was also a significant increase in tumor multiplicity, with 2.3 tumors/tumor-bearing rat, versus 1.6 tumors/tumor-bearing rat in the AOM/P EG group (P < 0.05), Since O-6-methylguanine adducts can cause activating m utations in the K-ras and beta-catenin genes, we examined the effects of BG on these mutations. In the BG group there were seven mutations in codon 12 or 13 of exon 1 of the K-ras gene in 51 tumors examined, compared with no K-ras mutations in 17 tumors analyzed in the AOM/PEG group (P = 0.12), In t he BG/AOM group there were 10 mutations in exon 3 of the beta-catenin gene among 48 tumors evaluated, compared with six mutations in 16 tumors analyze d in the PEG/AOM group (P 0,16), In summary, MGMT inhibition increases AOM- induced colonic tumor incidence and multiplicity in rats.