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%11 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 50422 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 effect33 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 tetramer44 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
ClinVar55 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 carcinogen66 Group 1 carcinogen
IARC Working Group on the Evaluation of Carcinogenic Risks to Humans. Lancet Oncol, 2009
by the IARC77 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. showed88 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.