Lack of evidence for a polymorphism at codon 160 of human O-6-alkylguanine-DNA alkyltransferase gene in normal tissue and cancer

O-6-benzylguanine (BG) is a potent, specific inactivator of the DNA repair protein O-6-alkylguanine-DNA alkyltransferase (AGT), which enhances sensiti vity to nitrosoureas in cells and tumor-bearing animals. BG is presently un dergoing clinical trials for development as an agent to enhance the therape utic index of alkylating agent chemotherapy, It has been reported that a po lymorphism exists in the human agt gene, with about 15% of the Japanese pop ulation having arginine at codon 160 instead of glycine on the polypeptide (Y, Imai ef al,, Carcinogenesis, 16: 2441-2445, 1995), The resultant mutant AGT protein is equally effective against both methylated DNA as compared w ith wild type protein. However, this mutant AGT protein was less sensitive to inactivation by BG with a 20-fold increase in the ED50 value. This obser vation raised the possibility that a subpopulation of patients may be resis tant to BG due to a single base change. We have demonstrated that this alte ration also reduces the sensitivity to O-6-benzyl-8-oxoguanine, an equally potent, yet much longer-lived human metabolite of BG, To test the possibili ty that this germ-line mutation of the agt gene might explain resistance to BG and O-6-benzyl-8-oxoguanine of patients on our Phase I clinical trials, we evaluated the DNA from lymphocytes of 18 patients. The G160R mutation w as not found in any of the 18 patients. To determine the frequency of this mutation in the United States population, DNA from 181 healthy individuals were investigated and, again, the mutation was not observed in this cohort. Therefore, if the mutation exists, it is in statistically <1.6% of the Uni ted States noncancerous population. To investigate the possibility that thi s mutation might be somatic, we evaluated genomic DNA samples from 94 human primary cancers of four different histological subtypes (brain, colon, eso phageal, and head and neck). Again, none were found to have the G160R mutat ion.