Regulation of histamine synthesis and degradation at the gene level

组氨酸脱羧酶（HDC）是一种独特的催化组氨酸合成组胺的酶，其表达局限于造血细胞中的肥大细胞和嗜碱性粒细胞。我们发现该基因启动子区的甲基化抑制了其转录，而肥大细胞和嗜碱性粒细胞仅在启动子区附近存在去甲基化状态。去甲基化试剂氮杂胞苷（Azacytidine）可使HDC基因启动子区去甲基化，并诱导P815细胞表达HDC，提示甲基化对该基因的细胞特异性表达具有重要的机制作用。除了已经发现的位点外，我们还将寻找其他的DNase I超敏感位点，希望这些位点含有细胞特异性的顺式元件，有助于细胞特异性的去甲基化，这将引导未来的研究。我们还证明了74 kDa的新生HDC蛋白在胞质溶胶中翻译并靶向ER以产生53 kDa的加工蛋白。我们发现，74 kDa蛋白质不仅转化为53 kDa蛋白质，而且还被分解为viaubiquitinization，并且这种分解速度比53 kDa蛋白质快得多。我们利用同源重组技术制备了HDC基因敲除小鼠，并证明该基因敲除小鼠体内无组胺（无组胺能小鼠）。这只老鼠看起来生长正常而且有生育能力.该小鼠的增殖模式遵循孟德尔定律，因此似乎没有胚胎死亡。我们计划使用基因敲除小鼠评估行为和过敏。本研究以组胺的分解代谢酶为研究对象，克隆了人组胺N-甲基转移酶（HMT）基因。HMT基因由6个外显子组成，位于第二染色体q13-21。我们还开始构建转基因小鼠的质粒，以产生大量的大鼠HMT。

Histidine decarboxylase (HDC) is a unique enzyme catalyzing synthesis reaction of histamine from histidine and its expression is restricted in mast cell and basophil lineage among the hematopoietic cells. We found that the methylation of promoter region of this gene suppress the transcription and those cell lineages (mast cell and basophil) have exclusively demethylated state around promoter region. Azacytidine, an demethylating reagent, produce the demethylation of promoter region of HDC gene and induce HDC expression in P815 cells, suggesting that the methylation is mechanically very important for the cell specific expression of this gene. We are going to seek for other DNase I hypersensitive sites other than the site we already found. We hope these sites contain cell specific cis-element to contribute for cell-specific demethylation which will lead the future investigation. We also proved that 74 kDa nascent HDC protein are translated in cytosol and targeted into ER to produce 53 kDa processed protein. We found that the 74 kDa protein is not only converted to 53 kDa protein but also catabolyzed viaubiquitinization and this speed of catabolism is much faster than that of the 53 kDa protein. We made HDC knockout mice by homologous recombination technique and proved that the knockout mice possess no histamine in the body (ahistaminergic mice). This mice Seemed to grow normally and were fertile. The proliferation pattern of this mice follows Mendelian law, therefore there seems to be no embryonic death. We are planning to assess the behavior and allergy using knockout mice. Once turned to catabolizing enzyme of histamine, we cloned human histamine N-methyltransferase (HMT) gene. HMT gene consists of 6 exons and resides in 2nd Chromosome q13-21. We also started to construct a plasmid for transgenic mice to produce high amount of rat HMT.