CYP2A6 rs28399435: Nicotine Metabolism and Variant Meaning
rs28399435 is a single nucleotide polymorphism located in the human cytochrome P450 CYP2A6 gene. This variant has been investigated for its influence on hepatic enzymatic activity, particularly regarding how the body metabolizes nicotine and xenobiotic compounds like coumarin. In pharmacogenomic research, altered alleles in this gene are associated with slower substrate clearance, though overall clinical evidence for this specific variant remains limited.
What each genotype means
Normal enzyme activity
This genotype indicates you carry two functional copies of the CYP2A6 gene. Your body likely clears nicotine and related substances at a typical rate. If you are taking medications metabolized by this enzyme, discuss your specific treatment plan with your clinician or pharmacist to ensure appropriate dosing.
This is the most common genotype globally, found in the vast majority of individuals across all ancestral populations.
Reduced enzyme activity
This genotype indicates you carry one functional CYP2A6 copy and one CYP2A6*20 copy. You may be classified as an intermediate metabolizer, meaning your body may process nicotine and certain medications more slowly than individuals with two functional copies. Please discuss your medication or nicotine-replacement dosing with your clinician or pharmacist.
Carried by a small percentage of individuals, primarily in African populations where the minor allele frequency is approximately 0.02; it is very rare in non-African populations.
Significantly reduced enzyme activity
This genotype indicates you carry two copies of the CYP2A6*20 allele. Research suggests this status is associated with substantially slower nicotine and coumarin clearance, potentially classifying you as a slow metabolizer. Because this significantly alters how your body processes nicotine and specific drugs, it is important to discuss your clinical needs and medication dosing with your healthcare provider or pharmacist.
This genotype is extremely rare across all known populations.
What is rs28399435 and Where Is It Located?
The single nucleotide polymorphism rs28399435 is positioned within the CYP2A6 gene locus on human chromosome 19. In genomic nomenclature, it represents a base substitution that can influence the structural or regulatory integrity of the resulting cytochrome P450 transcript. While wild-type sequences preserve standard enzymatic processing, variants at this genomic position have been tracked in pharmacogenetic catalogs such as dbSNP and PharmGKB. Some research literature annotates variants in this immediate sequence context as functional splice-site alterations or missense changes associated with numbered star alleles, including historical definitions around altered clearance haplotypes. Because the CYP2A6 region contains high sequence homology with neighboring pseudogenes like CYP2A7, precise molecular typing of rs28399435 requires specific assays to distinguish it from surrounding structural variation and linkage blocks.
The Biological Role of the CYP2A6 Gene
The CYP2A6 gene provides the genetic blueprint for cytochrome P450 2A6, a pivotal heme-thiolate monooxygenase primarily expressed in human liver tissue. CYP2A6 accounts for a substantial fraction of human hepatic drug clearance and is responsible for metabolizing approximately 80 to 90 percent of circulating nicotine into its inactive metabolite, cotinine, as well as converting cotinine to trans-3'-hydroxycotinine. Beyond tobacco compounds, the enzyme handles clinical therapeutics including the anticoagulant coumarin, certain anesthetics, and pharmaceutical inhibitors, while simultaneously activating procarcinogens such as tobacco-specific nitrosamines. Consequently, functional alterations that decrease or eliminate CYP2A6 enzymatic output directly prolong the biological half-life of these chemical substrates in the bloodstream.
Scientific Research and Clinical Evidence Strength
Scientific evaluation of rs28399435 currently reflects a limited strength of evidence in broad populations. Pharmacogenomic studies that evaluate the nicotine metabolite ratio (NMR) often pinpoint CYP2A6 variants as major determinants of slow-metabolizer status. When a functional splice-site defect or loss-of-function change occurs at this site, carriers show markedly reduced enzyme activity and diminished systemic clearance of coumarin and nicotine. However, published findings across diverse ethnic cohorts, including European Americans and Mexican populations, have produced mixed or non-significant statistical signals for rs28399435 specifically when isolated from other well-characterized star alleles like CYP2A6*2, *4, or *9. These variable findings highlight that rs28399435 often operates in tight linkage disequilibrium with other loci, meaning its independent predictive power remains modest.
Population Distribution and Ancestral Patterns
The distribution of rs28399435 exhibits marked ancestral differentiation across global populations. Genomic sequencing repositories and population studies indicate that the minor allele is found at low to moderate frequencies in people of African ancestry, where the minor allele frequency (MAF) hovers around 0.02 (approximately 2 percent). Conversely, it is exceedingly rare or practically absent across East Asian and European populations, where alternative CYP2A6 loss-of-function variants predominate. Because allele distributions are geographically stratified, the predictive utility of testing for rs28399435 is largely concentrated within specific demographic lineages and depends heavily on comprehensive multi-variant panel design.
What This Genetic Information Means for You
Discovering an altered genotype at rs28399435 provides personal insight into your underlying biological variation, but it is not a standalone diagnosis or direct medical verdict. Slower metabolic clearance for CYP2A6 substrates can influence personal nicotine intake patterns, dependence behaviors, or responsiveness to specific medications. However, you should never initiate, discontinue, or alter any medication or dosage based solely on direct-to-consumer genetic data. Therapeutic decisions must always be guided by qualified healthcare providers, such as a physician or clinical pharmacologist, who can integrate full clinical context, lifestyle factors, and comprehensive pharmacogenomic panels to ensure safe care.
How common is this variant?
The minor allele occurs at an estimated frequency of approximately 0.02 (~2%) in populations of African ancestry, while remaining exceptionally rare or undetectable in European and Asian cohorts.
Frequently asked questions
Does having the rs28399435 variant make it easier or harder to quit smoking?
Individuals categorized as slow nicotine metabolizers tend to retain nicotine in their circulation longer, which some studies link to smoking fewer cigarettes per day and lower overall nicotine consumption. However, cessation success depends on complex psychological, environmental, and physiological factors, meaning a genetic result alone cannot predict quitting outcomes.
Can rs28399435 affect my reaction to prescription medications?
Because CYP2A6 processes specific prescription agents such as coumarin derivatives and selected therapeutics, diminished enzyme activity could theoretically lead to altered drug exposure. If you are taking CYP2A6-metabolized pharmaceuticals, always consult your physician or clinical pharmacist before considering any changes to your therapy.
Why is this variant categorized as having limited evidence?
While molecular assays confirm functional impact on enzyme processing, large-scale clinical replication across diverse populations remains modest. Many clinical association studies are underpowered due to the low frequency of the allele outside of specific ancestral groups.
How does rs28399435 differ from common alleles like CYP2A6*2 or *4?
Common alleles like CYP2A6*2 or *4 represent missense changes or complete gene deletions that are well-established across Caucasian and Asian populations. In contrast, rs28399435 is an ancestral-specific variant that primarily appears in African lineages and contributes to rare haplotype combinations.
Sources & further reading
Educational information only, last refreshed 9/5/2026. Not medical advice — these associations describe population statistics, not individual predictions.
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Defining functional variant for the CYP2A6*2 allele (L160H) causing defective nicotine C-oxidation and slower metabolic nicotine clearance.
Defines the non-functional CYP2A6*2 allele (p.Leu160His) conferring absent nicotine C-oxidation activity and slow nicotine metabolism.
Functional variant in CYP2A6 defining the *9 allele with decreased transcriptional activity and reduced rate of nicotine metabolism.
Associated with the CYP2A6 locus and variation in rate of nicotine metabolism and cigarette consumption.
Functional polymorphism defining the CYP2A6*9 reduced-function promoter variant causing slower nicotine and cotinine metabolism.
Defining coding polymorphism for CYP2A6*8 (p.Arg365Gln), resulting in significantly decreased catalytic activity towards substrate medications including tegafur.