ALDH2*2对胃黏膜的保护作用减弱致胃癌高发的分子机制及生物学特性

Gastric cancer dominantly occurs in Asia. Except the diet habit, the genetic variation is also a causal factor. ALDH2 is the key enzyme for the metabolic pathway of alcohol-acetaldehyde-acetic acid, which protect cellular DNA from damage through degradation of toxic aldehydes. There is a single nucleotide polymorphism on exon 12 (rs671) of ALDH2 between different ethnics. Western people carries ALDH2*1(wild type), while as high as 40% of East Asian peoples carry ALDH2*2 (variant type). The single nucleotide replacement of G>A of ALDH2*2 can cause amino acid substitution (Glu504Lys), and then loss enzyme activity. So, the genotype of ALDH2*2 is also called "oriental variation". We speculate that high proportion of ALDH2*2 genotype may associate with the high incidence of gastric cancer in Asian. However, the hypothesis needs to be validated by multiple scientific experiments. This project intends to explore the effects on mucosa damage and carcinogenesis by means of cell models and Aldh2-knockout mice after we expose cells or mice to chemical carcinogen N-methyl-N-nitrosourea (MNU), helicobacter pylori as well as alcohol. We will observe the extent of DNA injury and malignant transformation on stomach mucosa. The protective effect of ALDH2 to gastric mucosa will be verified when we recover the enzyme activity by administration of ALDH2 agonist Alda-1 (N-[1, 3-benzodioxol-5-ylmethyl]-2, 6- dichlorobenzamide) in vivo. Since some study revealed that cancer cells with ALDH2*1 is resistant to doxorubicin drug, we will analyze the reaction of stomach cancer cells with deficient ALDH2 activity to different chemical drugs. The project is expected to provide a scientific explanation of the special phenomenon that East Asian peoples carry high percentage of ALDH2*2 genotype and present high incidence of stomach cancer. The project will also reveal the drug-responsive characteristics of gastric cancer cells with deficient ALDH2 activity and provide the useful reference for personalized medicine.

胃癌是亚洲人高发肿瘤，环境因素与遗传基因均与其发病有关。ALDH2是乙醇-乙醛-乙酸代谢通路的关键酶，通过降解醛类保护DNA免受损伤。ALDH2第12外显子rs671基因多态性有种族差异，欧美人呈ALDH2*1型（野生型），而东亚人ALDH2*2（变异型）携带比例较高，G>A置换导致编码氨基酸改变及酶活性丧失，又被称为“东方人变异”。我们推测：亚裔人高比例的ALDH2*2基因型与胃癌高发可能存在关联，但尚需实验验证。本项目拟从体外细胞模型与Aldh2基因敲除小鼠层面研究ALDH2失活的胃黏膜对MNU、幽门螺杆菌、酒精等因素暴露的DNA损伤与癌变程度；研究向Aldh2*2基因型小鼠补充酶激动剂恢复酶活性对胃黏膜的保护作用；分析ALDH2缺陷型胃癌细胞对不同化学药物的反应性差异。该课题有望对亚洲人胃癌高发现象提供科学合理解释，还将揭示ALDH2酶缺陷胃癌细胞的化学药物应答特征供个体化用药参考。

人体ALDH2基因的生理功能是降解细胞内的醛类物质, 防止氧化应激性损伤。然而，大约有30～40％的亚洲人ALDH2基因的rs671位点存在多态性，从而导致ALDH2的基因功能几乎完全丧失。胃癌是亚洲人群高发性恶性肿瘤，但与ALDH2基因功能缺失的关系尚缺乏系统性研究。本项目通过体外构建ALDH2基因敲除细胞模型和Aldh2-/-小鼠模型，探讨了该基因在小鼠胃癌发生以及对化疗药物敏感性存在差异的分子机制。该研究发现，Aldh2-/-小鼠在化学致癌剂N-甲基-N-亚硝基脲(MNU)与幽门螺杆菌诱导下的胃癌发生率明显升高；体外敲低胃癌细胞的ALDH2基因表达，肿瘤细胞对阿霉素（Doxorubicin，DOX）化疗药物的敏感性明显增加；对DOX药物的敏感性增加在小鼠移植瘤动物实验也得到证实。分子机制研究显示，细胞内ALDH2失活导致p53介导的细胞凋亡增加，若将p53基因与ALDH2基因同时敲低，则胃癌细胞对DOX敏感性增加可以消失，表明这种效应具有p53依赖性。该研究揭示了ALDH2基因对胃黏膜具有保护作用，其功能丧失是胃癌发病率升高的原因之一。然而，携带ALDH2*2基因（rs671）型胃癌个体可以被视作一个特殊的分子亚型可指导后续药物治疗，DOX是候选的有效药物。