ALDH2 - The Alcohol Flush Gene
ALDH2 (aldehyde dehydrogenase 2) is the mitochondrial enzyme responsible for
converting acetaldehyde to acetate during alcohol metabolism. Acetaldehyde is the
toxic intermediate that causes many of the unpleasant effects of excessive
drinking. The ALDH2*2 variant (rs671) is one of the most clinically significant
pharmacogenomic variants known, primarily affecting East Asian populations where
it reaches frequencies of 30-50% | Brooks PJ et al. The alcohol flushing response. PLoS Med, 2009.
The Mechanism
The rs671 variant causes a glutamic acid-to-lysine substitution at position 504| Amino acid change: glutamic acid to lysine at position 504 (E504K)
of the ALDH2 protein. This change occurs in the active site and has a
dominant-negative effect| Dominant-negative: a single defective copy sabotages the protein complex even when a normal copy is present - even one copy of the *2 allele dramatically reduces
enzyme activity because ALDH2 functions as a tetramer| A tetramer is a protein complex assembled from four subunits, and
incorporating even one defective subunit impairs the entire complex. Heterozygous
carriers retain only about 6% of normal activity, while homozygous carriers have
essentially zero activity. This variant is classified as pathogenic by
ClinVar | VCV000018390.
The Flush Reaction
When ALDH2 activity is impaired, acetaldehyde accumulates rapidly after drinking
alcohol. This triggers the characteristic "Asian flush" or "alcohol flush reaction":
facial flushing, rapid heartbeat, nausea, headache, and general discomfort. These
symptoms are caused by acetaldehyde's direct toxic effects on blood vessels and
tissues. The reaction is the body's warning that a carcinogenic compound is
accumulating.
The Cancer Connection
The most serious consequence of ALDH2 deficiency is cancer risk. Acetaldehyde
is classified as a Group 1 carcinogen | IARC Working Group on the Evaluation of Carcinogenic Risks to Humans. Lancet Oncol, 2009
by the IARC| Group 1: sufficient evidence of carcinogenicity in humans, the highest IARC classification. Heterozygous carriers who
drink regularly have a 6-10 fold increased risk of esophageal squamous cell
carcinoma. This risk is so significant that the World Health Organization has
identified ALDH2 deficiency combined with alcohol consumption as a major
preventable cause of cancer in East Asia.
Nitroglycerin Interaction
ALDH2 is also involved in the bioactivation of nitroglycerin (glyceryl trinitrate),
a medication used for angina chest pain. Li et al. showed | Li Y et al. ALDH2 bioactivation of nitroglycerin. Arterioscler Thromb Vasc Biol, 2006
that ALDH2*2 carriers had a reduced vasodilatory response to nitroglycerin,
which could be clinically important during cardiac emergencies.
Practical Implications
If you carry the *2 allele, the flush reaction is not just an inconvenience - it
is a cancer warning signal. The safest approach is to avoid or severely limit
alcohol consumption. If you do not experience flushing (GG genotype), standard
alcohol guidelines apply, though moderation remains advisable for overall health.
Note that in European populations, this variant is extremely rare (less than
0.01%), while in East Asian populations it is the most common pharmacogenomic
variant, affecting nearly 1 in 3 people.
All Genotypes
Normal alcohol metabolism
You have normal ALDH2 activity and can metabolize acetaldehyde (a toxic intermediate in alcohol breakdown) efficiently. Over 92% of the global population shares this genotype, though the proportion varies by ancestry.
ALDH2 deficiency - alcohol flush reaction
You carry one copy of the ALDH2*2 variant. About 7% of the global population shares this genotype (but up to 35% in East Asian populations). When you drink alcohol, toxic acetaldehyde builds up, causing facial flushing, rapid heartbeat, nausea, and headache. More importantly, if you drink despite this reaction, you have significantly increased risk of esophageal cancer.
ALDH2 deficiency - severe alcohol intolerance
You have two copies of the ALDH2*2 variant. About 1% of the global population shares this genotype (up to 5-8% in some East Asian populations). You have severe alcohol intolerance with intense reactions to even small amounts. This is actually protective - the unpleasant reaction discourages drinking, which prevents the esophageal cancer risk.