HUMAN HISTAMINE N-METHYLTRANSFERASE PHARMACOGENETICS - COMMON GENETICPOLYMORPHISMS THAT ALTER ACTIVITY

Histamine N-methyltransferase (HNMT) catalyzes a major pathway in hist amine metabolism. Levels of HNMT activity in humans are regulated by i nheritance. We set out to study the molecular basis for this genetic r egulation. Northern blot analysis showed that HNMT is highly expressed in the kidney, so we determined levels of enzyme activity and thermal stability in 127 human renal biopsy samples. DNA was isolated from 12 kidney samples with widely different HNMT phenotypes, and exons of th e HNMT gene were amplified with the polymerase chain reaction. In thes e 12 samples, we observed a C314T transition that resulted in a Thr105 Ile change in encoded amino acid, as well as an A939G transition withi n the 3'-untranslated region. All remaining renal biopsy samples then were genotyped for these two variant sequences. Frequencies of the all eles encoding Thr105 and Ile105 in the 114 samples studied were 0.90 a nd 0.10, respectively, whereas frequencies for the nucleotide A939 and G alleles were 0.79 and 0.21, respectively, Kidney samples with the a llele encoding Ile105 had significantly lower levels of HNMT activity and thermal stability than did those with the allele that encoded Thr1 05. These observations were confirmed by transient expression in COS-1 cells of constructs that contained all four alleles for these two pol ymorphisms. COS-1 cells transfected with the Ile105 allele had signifi cantly lower HNMT activity and immunoreactive HNMT protein than did th ose transfected with the Thr105 allele. These observations will make i t possible to test the hypothesis that genetic polymorphisms for HNMT may play a role in the pathophysiology of human disease.