Human nicotinamide N-methyltransferase pharmacogenetics: gene sequence analysis and promoter characterization.

Nicotinamide N-methyltransferase (NNMT) catalyses the N-methylation of nicotinamide and structurally related pyridines. NNMT enzymatic activity in human liver varies over a five-fold range with a bimodal frequency distribution - raising the possibility of regulation by a genetic polymorphism. We set out to characterize molecular genetic mechanisms that might be involved in the regulation of individual variation in human liver NNMT activity. After Northern blot analysis confirmed that NNMT is highly expressed in the liver, eight human hepatic biopsy samples, four each with 'low' or 'high' levels of activity, were used to perform quantitative Western blot analysis. There was a highly significant correlation (r(s) = 0.96, P < 0.0001) between NNMT activity and immunoreactive protein in these samples. We next determined that a potent promoter was located within the initial 700 bp of the 5'-flanking region of the human NNMT gene. That gene consists of 3 exons, with an initial 1240 bp intron and a second intron that is approximately 14 kb in length. We subsequently isolated DNA from 27 human liver biopsy samples with low, intermediate or high levels of NNMT activity. The three exons, all 1240 bp of intron 1 and approximately 700 bp of the 5'-flanking region of the NNMT gene were amplified from each of these samples with the polymerase chain reaction, followed by DNA sequencing to identify genetic polymorphisms that might correlate with 'NNMT phenotype'. No single nucleotide polymorphisms (SNPs) or insertion/deletion events were detected within either the exons or 5'-flanking regions of NNMT for these 27 samples. Although there were eight SNPs within intron 1, none were systematically related to level of NNMT activity. These results indicate that the exons and 5'-flanking region of the NNMT gene display little or no sequence variation. Therefore, polymorphisms within these areas of the gene are unlikely to be related to wide individual variations in the level of this enzyme activity in the human liver.