Genetic polymorphism of human O-6-alkylguanine-DNA alkyltransferase: identification of a missense variation in the active site region

O-6-Alkylguanine-DNA alkyltransferase (AGT, EC 2.1.1.63) is a principle DNA repair protein in repairing O-6-alkylguanine in DNA, a major premutagenic lesion produced by environmental and therapeutic alkylating agents. AGT pla ys a critical role in protecting cells against mutation and cytotoxicity in duced by these alkylating agents. The existence of a large interindividual variation in human AGT activity level has been observed and we hypothesize that genetic polymorphism of AGT could be an important determinant for this variation. The present study reports the identification of a novel missens e polymorphism in the human AGT gene. The polymorphic alteration occurs at codon 143 in exon 5, converting isoleucine (ATC) to valine (GTC). Because I le(143) is adjacent to the alkyl acceptor Cys(145) Of the AGT active site a nd is conserved among mammalian AGTs, amino acid substitution at this posit ion may affect the function of AGT The codon 143 polymorphism appears to be linked to another new polymorphic alteration at codon 178, which converts lysine (AAG) to arginine (AGG). Because it has been reported that human AGT can be truncated at position 176 without loss of activity, the codon 178 p olymorphism may not affect AGT activity. The codon 143/178 polymorphism was found in two of 90 (2%) esophageal cancer patients residing in a high inci dence area of China, but was not detected in 60 normal individuals residing in the same area. Six of 28 (21%) non-cancer Caucasian individuals, howeve r, were found to carry this polymorphic allele, suggesting a significant et hnic difference in distribution of this codon 143/178 polymorphism between Chinese and Caucasian individuals. In addition, we confirmed the existence of a codon 84 genetic polymorphism previously identified in a Japanese popu lation, which converts leucine (CTT) to phenylalanine (TTT). The distributi on of codon 84 polymorphism was 16%, 20% and 36%, respectively, in the Chin ese esophageal cancer patients, Chinese and Caucasian non-cancer individual s. Coexistence of codons 84 and 143/178 polymorphic alterations was found i n one Caucasian individual. In all the Chinese (n = 150) and Caucasian (n = 28) samples examined, we were unable to detect a previously reported codon 160 polymorphism (Gly to Arg) which occurred in 10-25% of the Japanese ind ividuals and was shown to affect the reaction of AGT with the drug O-6-benz ylguanine. The functional significance of the codon 143/178 genetic polymor phism of human AGT and its role in determining an individual's susceptibili ty to environmental alkylating carcinogens and response to alkylating chemo therapeutic drugs both remain to be studied. Pharmacogenetics 9:81-87 (C) 1 999 Lippincott Williams & Wilkins.