TRANSGENIC EXPRESSION OF HUMAN MGMT PROTECTS AGAINST AZOXYMETHANE-INDUCED ABERRANT CRYPT FOCI AND G-TO-A MUTATIONS IN THE K-RAS ONCOGENE OFMOUSE COLON

Transgenic mice over-expressing MGMT, which codes for the human protei n O-6-alkylguanine-DNA alkyltransferase, are protected from methylatin g agent-induced thymic lymphomas. In this study we evaluated the abili ty of transgenic overexpression of MGMT in the colon to protect mice f rom the development of azoxymethane(AOM)-induced aberrant crypt foci ( ACF) and mutations in K-ras. Colonic alkyltransferase in MGMT+ transge nic mice was > 5-fold higher than in nontransgenics: 10.5 +/- 1.1 vs 2 .,2 +/- 1.1 fmol/mu g DNA, P = < 0.0001, Mice received 20 mg AOM/kg i. p. at 6 weeks or 15 mg AOM/kg at 6 and 7 weeks of age, and 8 wks later colons were examined for ACF. A significant protective effect of MGMT was seen in mice given single dose of 20 mg AOM/kg. The incidence of ACF/colon was lower in MGMT+ mice (2.0 +/- 1.2) than in nontransgenic mice (3.9 +/- 1.8, P = 0.02). G to A mutations in codon 12 of K-ras we re detected by PCR-RFLP in ACF and in random samples of normal appeari ng mucosa, The incidence of ACF with mutant K-ras in MGMT transgenic m ice (0.6 +/- 0.7/colon) was significantly reduced compared to nontrans genic mice (2.3 +/- 1.7/colon, P = 0.02), We propose that AOM induces at least two overlapping but not identical premalignant lesions (aberr ant crypt foci and K-ras mutations) which can be prevented by over-exp ression of MGMT. Thus, MGMT may protect colonic mucosa from carcinogen esis involving methylating agents such as AOM.