rs2066702 — ADH1B Arg370Cys

The African Flush Gene — ADH1B*3 and Alcohol Protection in African-Ancestry Populations

Alcohol dehydrogenase 1B (ADH1B) catalyzes the first step of alcohol metabolism, converting ethanol into acetaldehyde in the liver. Most people carry the common Arg370 form (ADH1B*1). The ADH1B*2 allele (His48Arg, rs1229984) is common in East Asian populations and encodes an enzyme roughly 100-fold more active than the common form. But there is a third functional variant — ADH1B*3 (Arg370Cys, rs2066702) — that is found almost exclusively in populations of African descent.

ADH1B*3 encodes a superactive enzyme with substantially higher ethanol oxidation kinetics than the common ADH1B*1 form. Like ADH1B*2, it accelerates the conversion of ethanol to acetaldehyde, producing an aversive physiological response that acts as a natural deterrent against heavy drinking. In African American populations, where this allele reaches frequencies of 15–25%, it is one of the strongest genetic predictors of alcohol consumption patterns ever identified.

This variant is functionally and epidemiologically distinct from rs1229984. The two alleles reside at different positions in the ADH1B protein (residue 48 versus residue 370), arise from independent mutational events, are distributed across different populations, and contribute independently to alcohol use disorder risk. A person of African descent may carry ADH1B*3 with no ADH1B*2 allele, and vice versa for East Asians.

The Mechanism

The rs2066702 A allele, on the plus (forward) strand of chromosome 4, corresponds to the Cys370 substitution in the ADH1B protein — the ADH1B*3 allele. The gene is located on the minus strand, so the A allele in genome files is the complement of the T allele in the coding sequence notation used in older literature.

At the protein level, replacing Arginine with Cysteine at position 370 alters the active-site geometry of the enzyme in a way that increases its catalytic efficiency for ethanol oxidation. The result is accelerated production of [acetaldehyde | A reactive aldehyde intermediate; classified as a Group 1 human carcinogen by the IARC; causes flushing, nausea, and tachycardia] after alcohol ingestion — similar in kind to what ADH1B*2 produces in East Asian populations, though the mechanistic basis (different residue, different kinetic parameters) is independent.

Because the downstream enzyme ALDH2, which clears acetaldehyde, operates at a fixed rate, any increase in acetaldehyde production from faster ADH1B activity creates a transient acetaldehyde surplus. Carriers of ADH1B*3 experience faster and more aversive responses to alcohol, which behaviorally reduces both the amount consumed and the likelihood of developing alcohol use disorder.

The Evidence

Genome-wide significance in African Americans:

The first genome-wide association study for maximum number of alcoholic drinks consumed in a 24-hour period in African Americans¹¹ The first genome-wide association study for maximum number of alcoholic drinks consumed in a 24-hour period in African Americans
Xu K et al. Genomewide Association Study for Maximum Number of Alcoholic Drinks in European Americans and African Americans. Alcohol and Alcoholism, 2015 identified rs2066702 as the peak genome-wide significant signal in African Americans (p = 2.50×10⁻¹⁰). Eight SNPs in the region on chromosome 4 reached significance, all tagging the ADH1B*3 haplotype.

A subsequent GWAS of maximum habitual alcohol intake in 17,029 African American veterans from the VA Million Veteran Program²² A subsequent GWAS of maximum habitual alcohol intake in 17,029 African American veterans from the VA Million Veteran Program
Gelernter J et al. Genome-wide Association Study of Maximum Habitual Alcohol Intake in >140,000 U.S. European and African American Veterans. Biological Psychiatry, 2019 confirmed rs2066702 as the lead locus for African Americans (p = 2.3×10⁻¹²), a far stronger signal than any other variant identified in that ancestry group.

Clinical alcohol use metrics:

In a large electronic health record study of 57,677 African American veterans³³ In a large electronic health record study of 57,677 African American veterans
Justice AC et al. Validating Harmful Alcohol Use as a Phenotype for Genetic Discovery Using Phosphatidylethanol and a Polymorphism in ADH1B. Alcohol and Alcoholism, 2018, 34.3% of participants carried at least one A allele (minor allele frequency 19.0%). Carrying the protective A allele was associated with approximately half the odds of high AUDIT-C scores (aOR = 0.54) and an aOR of 0.51 when combining AUDIT-C with ICD-coded alcohol use disorder — demonstrating that the protective effect is clinically meaningful and detectable across multiple measurement approaches.

AUD diagnostic criteria:

Among African American participants in a detailed AUD phenotyping study⁴⁴ Among African American participants in a detailed AUD phenotyping study
Hart AB et al. Which alcohol use disorder criteria contribute to the association of ADH1B with alcohol dependence? Addiction Biology, 2016, individuals homozygous for the major (non-protective) allele GG endorsed significantly more DSM-IV and DSM-5 AUD criteria (p = 1.9×10⁻⁹). The criterion most strongly linked to this variant was tolerance — the need for increasing amounts of alcohol to achieve the desired effect — suggesting that ADH1B*3 carriers develop tolerance more slowly, consistent with the variant's faster acetaldehyde production making high-dose alcohol less pleasant.

Prenatal exposure and developmental protection:

A longitudinal study in African American families⁵⁵ A longitudinal study in African American families
Dodge NC et al. Protective effects of the alcohol dehydrogenase-ADH1B*3 allele on attention and behavior problems in adolescents exposed to alcohol during pregnancy. Alcoholism: Clinical and Experimental Research, 2014 found that maternal ADH1B*3 carrier status shielded adolescent offspring from behavioral and attention consequences of prenatal alcohol exposure, with allele frequencies of 17.6% in mothers and 21.0% in adolescents — consistent with the expected 15–20% frequency in African American populations. The proposed mechanism is that faster maternal alcohol metabolism reduces peak blood alcohol concentration reaching the fetus.

Practical Actions

For GG carriers (most common in non-African populations): Without the ADH1B*3 protective allele, you lack one of the biological deterrents against heavy drinking. If you are of African descent, your GG genotype means you metabolize alcohol at the common rate — higher-tolerance drinking is biologically more accessible to you, which removes a natural brake on consumption. This is relevant when considering whether your drinking patterns are influenced by behavioral versus genetic factors.

For AG and AA carriers (primarily found in people of African descent): Your ADH1B*3 allele accelerates alcohol-to-acetaldehyde conversion, producing faster aversive responses to alcohol. This is one of the strongest inherited protective factors against alcohol use disorder in populations of African ancestry. The biological deterrent is real — but, as with ADH1B*2 in East Asian populations, it can be overridden socially. Environmental factors (peer norms, social offers) can diminish the genetic protection when drinking pressures are high.

Interactions

ADH1B*3 (rs2066702) and ADH1B*2 (rs1229984, His48Arg) reside in the same gene but at different protein positions. They arise independently and distribute across different populations. In African American populations where both alleles may occasionally co-occur, combined diplotype analysis is informative. However, the primary co-variant to consider alongside rs2066702 in African-ancestry individuals is ALDH2 (rs671), which controls the downstream clearance of the acetaldehyde that ADH1B*3 produces more rapidly.

ADH1C variants (rs1693482, rs698) are pathway partners that also affect the rate of alcohol oxidation. Combined diplotype analyses have documented that ADH1B and ADH1C allele combinations affect liver disease risk and alcohol metabolism outcomes beyond either variant alone.