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

Nicotinamide N-methyltransferase (NNMT) catalyses the N-methylation of nico tinamide and structurally related pyridines. NNMT enzymatic activity in hum an liver Varies over a five-fold range with a bimodal frequency distributio n - 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 live r, eight human hepatic biopsy samples, four each with 'low' or 'high' level s of activity, were used to perform quantitative Western blot analysis. The re was a highly significant correlation (r(s) = 0.96, P < 0.0001) between N NMT activity and immunoreactive protein in these samples. We nest determine d that a potent promoter was located within the initial 700 bp of the 5'-fl anking region of the human NNMT gene. That gene consists of 3 exons, with a n 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 wi th 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 t he NNMT gene were amplified from each of these samples with the polymerase chain reaction, followed by DNA sequencing to identify genetic polymorphism s that might correlate with 'NNMT phenotype', No single nucleotide polymorp hisms (SNPs) or insertion/deletion events were detected within either the e xons or 5'-flanking regions of NNMT for these 27 samples. Although there we re eight SNPs within intron 1, none were systematically related to level of NNMT activity. These results indicate that the exons and 5'-flanking regio n of the NNMT gene display little or no sequence variation. Therefore, poly morphisms within these areas of the gene are unlikely to be related to wide individual variations in the lever of this enzyme activity in the human li ver. Pharmacogenetics 9:307-316 (C) 1999 Lippincott Williams & Wilkins.