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，其表达仅限于造血细胞中的肥大细胞和嗜碱性细胞。我们发现该基因启动子区域的甲基化抑制了转录，并且这些细胞系（肥大细胞和嗜碱性细胞）在启动子区域周围呈排他性去甲基化状态。Azacytidine是一种去甲基化试剂，在P815细胞中产生HDC基因启动子区域的去甲基化并诱导HDC表达，这表明甲基化对该基因的细胞特异性表达具有重要的机械作用。我们将寻找除了我们已经发现的位点之外的其他DNase I超敏位点。我们希望这些位点包含细胞特异性顺式元件，有助于细胞特异性去甲基化，这将导致未来的研究。我们还证明了74 kDa的新生HDC蛋白在细胞质中被翻译并靶向内质网产生53 kDa的加工蛋白。我们发现，74 kDa蛋白不仅可以转化为53 kDa蛋白，还可以在泛素化过程中进行分解代谢，并且这种分解代谢速度比53 kDa蛋白快得多。采用同源重组技术制备HDC基因敲除小鼠，证实敲除小鼠体内无组胺（无组胺能小鼠）。这些老鼠似乎生长正常，而且有生育能力。这种小鼠的增殖模式遵循孟德尔定律，因此似乎没有胚胎死亡。我们正计划用基因敲除小鼠来评估其行为和过敏。一旦转向组胺分解代谢酶，我们克隆了人组胺n -甲基转移酶（HMT）基因。HMT基因由6个外显子组成，位于第2染色体q13-21上。我们也开始为转基因小鼠构建质粒，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.