| Abstract: | Mitochondrial aldehyde dehydrogenase 2 (ALDH2) in the liver removes toxic aldehydes including acetaldehyde, an intermediate of ethanol metabolism. Nearly 40% of East Asians inherit an inactive ALDH2*2 variant, which has a lysine-for-glutamate substitution at position 487 (E487K), and show a characteristic alcohol flush reaction after drinking and a higher risk for gastrointestinal cancers. Here we report the characterization of knockin mice in which the ALDH2(E487K) mutation is inserted into the endogenous murine Aldh2 locus. These mutants recapitulate essentially all human phenotypes including impaired clearance of acetaldehyde, increased sensitivity to acute or chronic alcohol-induced toxicity, and reduced ALDH2 expression due to a dominant-negative effect of the mutation. When treated with a chemical carcinogen, these mutants exhibit increased DNA damage response in hepatocytes, pronounced liver injury, and accelerated development of hepatocellular carcinoma (HCC). Importantly, ALDH2 protein levels are also significantly lower in patient HCC than in peritumor or normal liver tissues. Our results reveal that ALDH2 functions as a tumor suppressor by maintaining genomic stability in the liver, and the common human ALDH2 variant would present a significant risk factor for hepatocarcinogenesis. Our study suggests that the ALDH2*2 allele–alcohol interaction may be an even greater human public health hazard than previously appreciated.Mitochondrial aldehyde dehydrogenase 2 (ALDH2) is essential for alcohol detoxification. It is the second enzyme in the major oxidative pathway of alcohol metabolism, removing acetaldehyde (ACE), a toxic intermediate product from ethanol metabolism (1). More than 500 million people worldwide, mostly in East Asia, have a G-to-A point mutation in their ALDH2 gene (2, 3). This mutation results in a glutamic acid-to-lysine substitution at residue 487 (E487K) of the human ALDH2 protein (designated ALDH2*2). ALDH2*2 has significantly reduced ability to metabolize ACE (4, 5). Importantly, its activity is partially dominant-negative over that of the wild-type ALDH2*1, due to the structural alterations introduced by the mutation to the ALDH2 homotetramer complex (6). As a result, individuals with a heterozygous ALDH2*2/2*1 genotype have less than half the wild-type activity, and ALDH2*2/2*2 homozygotes have very low residual activity (7). Accumulated ACE can cause an alcohol flush reaction, commonly found in Asians with this variant after alcohol consumption (also called “Asian glow”).ACE binds to cellular proteins and DNA, leading to DNA damage and organ injury (8). Specifically, endogenous aldehydes are detrimental to hematopoietic stem cells that are defective in Fanconi anemia DNA repair (9, 10). As a result, Fanconi anemia patients with the ALDH2*2 allele exhibit accelerated disease progression (11). ALDH2*2 can also increase the risk for gastrointestinal cancers, such as gastric carcinoma (12), esophageal cancer (13), and colon cancer (14). Despite the liver being the major organ of ethanol detoxification, the relationship between ALDH2*2 and the risk for liver cancer remains unclear (15, 16).ALDH2 is highly conserved in humans and mice (17, 18), and several mouse models with modified ALDH2 activities have been developed (19). The closest model to the human ALDH2*2 polymorphism is the Aldh2 knockout (KO) mouse, which expresses no protein or enzymatic activity (20). Although Aldh2 KO mice are useful for investigating the impact of complete lack of ALDH2 activity (21), they fail to faithfully reproduce the structural and biochemical properties of ALDH2*2 in human physiology and pathology (21–23). In particular, ALDH2*2 is expressed with reduced but not loss of enzymatic activity and increased protein turnover (24).To provide better mechanistic links between the ALDH2(E487K) mutation and human disease, we generated an ALDH2*2 knockin (KI) mouse. We observed impaired clearance of ACE from hepatocytes in these mutants after acute or chronic alcohol challenges. The ALDH2(E487K) mutation reduced total liver ALDH2 protein levels via a dominant-negative effect on protein stability, as has been documented for human tissues (24, 25). We also revealed a surprising role for ALDH2 as a liver tumor suppressor, raising the concern that this common human polymorphism may expose over 500 million carriers to greater risk of liver cancer. |
