Study on the structure of gene of histamine inactivating enzyme and its regulation of expression

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1.Gene structure of human histamine N-methyltransferase (HMT)We obtained several genomic clones by screening human genomic library (EMBL3) with human HMT cDNA and determined its structure. In addition, we analyzed a transcriptional point by a primer extension method with nRNA extracted from human kidney. As a result, the experiment of primer extension method revealed that 2 major bands was detected. According to an analysis of HMT gene structure based on the transcriptional point locating downstream, human HMT gene consists of 6 exons, and the first exon contains a 137b coding region and a realtively long 383b noncoding region. While TATA-like sequence did not exist in 2kb region upstream to a transcriptional point, 3SP-1,5 GATA,2 LRF-1,5 LBP-1binding sequence were detected. On the other hand, since another major transcriptional point located about 100b upstream to other one, it is possible to expect a new first exon. As reported in other genes, it was speculated that possible alternat … More ive transcriptional points were involved in a tissue-specific expression.2.Polymorphism of HMT geneTo date, sccording to our previous report and the report by R Weishilboum, two changes of nucleotide sequence in coding region. Nucleotide 314 within codon 105 is a T or a C,resulting in codons that encode isoleucine or threonine. Nucleotide 595 within codon 199 is an A or G,resulting isoleucine or valine. In addition, we found clones which lack of 8 nuleotides (CTAACAAA) in ncleotide 89 of first exon. Furthermore CA repeat sequense near 6th exon was polymorphic and we analyzed the number of CA repeat among 100 normal individuals. As a result, CA repeat number distributed from 15 to 30 and was thought to be a useful marker of polymorphism of HMT gene.3.Genetic linckage of HMT gene to bronchial asthmaThe airway hyperresponsiveness, a critical factor of bronchial asthma, has been recognized as one of physical characteristics inherited genetically. The level of HMT activity is closely associated with airway contractile response to histamine in guinea pigs. In this regard, to examine whether HMT gene is linked to bronchial asthma, we compared polymorphism of asthmatics to that of normal individuals. As a result, the distribution of CA repeat number was significantly different from that of normal individuals. Especially, CA repeat size smaller than 20 seemed to be a risk factor for bronchial asthma. Less