Genotype-phenotype correlation between the polymorphic UGT2B17 gene deletion and NNAL glucuronidation activities in human liver microsomes

The nicotine-derived tobacco-specific nitrosamine, 4-(methylnitrosamino)-1- (3-pyridyl)-1-butanone (NNK), is one of the most potent and abundant procarcinogens found in tobacco and tobacco smoke, and glucuronidation of its major metabolite, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL), is an important mechanism for NNK detoxification. In cigarette smokers and tobacco chewers, there is a wide variation in the urinary levels of the ratio of NNAL to NNAL glucuronide (NNAL-Gluc). To determine whether genetic variation plays a potential role in this interindividual variability, NNAL-glucuronidating activities were analysed in a series of human liver microsomal specimens and compared with UGT2B17 deletion genotypes in the same subjects. Assays performed in vitro demonstrated that over-expressed UGT2B17 exhibits high O-glucuronidating activity against NNAL. When stratifying subjects by UGT2B17 genotype, a significant or near-significant decrease in NNAL-O-Gluc formation was observed in liver microsomes from individuals who were either heterozygous [(+ /0), P = 0.07] or homozygous [(0/0), P = 0.016] for the UGT2B17 deletion compared to liver microsomes from individuals with intact UGT2B17 alleles [(+/+)]. There was a significant (P < 0.01) association between the level of liver microsomal NNAL-O-glucuronide formation and increasing numbers of the UGT2B17 null alleles in the liver microsomal specimens examined in this study, and a significant decrease in NNAL-O-Gluc formation was observed when comparing liver microsomes from individuals who had at least one UGT2B17 allele deleted [(+/0) + (0/ 0)] versus microsomes from UGT2B17 (+/+) subjects (P = 0.004). When stratifying by the median value of NNAL-O-Gluc formation activity, a significantly (P = 0.015) higher number of subjects with liver microsomes having low NNAL-O-Gluc formation activity contained the UGT2B17 null genotype compared to subjects with liver microsomes exhibiting high NNAL-O-Gluc formation activity. When stratifying by UGT2B7/UGT2B17 haplotypes, the association between the level of liver microsomal NNAL-O-glucuronide formation and increasing numbers of the UGT2B17 null allele was at the level of statistical significance for subjects with the UGT2B7 (*1/*2) (P = 0.05) or UGT2B7 (*2/*2) (P < 0.02) genotypes. These data suggest that the UGT2B17 deletion polymorphism is associated with a reduced rate of NNAL detoxification in vivo and may increase individual susceptibility to tobacco-related cancers.