Mouse histamine N-methyltransferase: cDNA cloning, expression, gene cloning and chromosomal localization

Objective: Histamine N-methyltransferase (HNMT) catalyzes the N-tau-methyla tion of histamine. We set out to clone a mouse liver HNMT cDNA and the mous e HNMT gene as steps toward characterizing molecular genetic mechanisms inv olved in the regulation of this important histamine-metabolizing enzyme. Design: A PCR-based strategy was used to clone both the mouse HNMT cDNA and the gene encoding that cDNA, Hnmt. The cDNA was used both to express recom binant mouse HNMT and to determine the chromosomal localization of Hnmt. Results: The mouse liver HNMT cDNA was 1657 bp in length with an 888 bp ope n reading frame (ORF) that encoded a 296 amino acid protein with a predicte d Mr value of approximately 32.5 kDa. The amino acid sequence of the encode d protein was 84% identical to that of human kidney HNMT. Mouse HNMT was ex pressed in COS-1 cells, and its apparent K-m values for histamine and S-ade nosyl-L-methionine (Ado-Met), the two cosubstrates for the reaction, were 5 .3 and 5.8 muM, respectively. The mouse HNMT gene, Hnmt, spanned approximat ely 25 kb and had 7 exons. Its structure differed from that of the human ge ne primarily by the presence of an additional exon at the 5'-terminus. Hnmt mapped to mouse chromosome 2 in an area of conserved synteny to human chro mosome 2q, the location of the human gene (2q22) on the basis of fluorescen ce in situ hybridization. Conclusions: Cloning and functional characterization of the mouse HNMT cDNA and gene will now make it possible to study in the mouse molecular genetic mechanisms involved the regulation of this important histamine-metabolizin g enzyme.