HNMT rs11559020: What Your Genotype Means
The genetic variant rs11559020 is a missense single nucleotide polymorphism in the HNMT gene, historically designated as C314T or Thr105Ile. It alters the structure of histamine N-methyltransferase, leading to a modest reduction in the rate at which the body inactivates histamine in peripheral tissues and the central nervous system. As a result, carrying this variant is weakly associated in population studies with differences in histamine clearance and a modest predisposition to atopic conditions.
What each genotype means
Typical enzyme activity
You carry two copies of the common C allele, encoding the standard threonine at amino acid position 105 (Thr105). This genotype is associated with normal histamine N-methyltransferase catalytic function and standard rates of intracellular histamine degradation. Research does not link this baseline genotype with increased susceptibility to atopic symptoms or altered histamine clearance.
Found in approximately 80% to 85% of individuals of European descent and more than 90% of East Asian populations.
Moderately reduced enzyme activity
You carry one copy of the T variant allele, which substitutes isoleucine for threonine (Thr105Ile) and leads to decreased enzyme stability and activity. Published studies report modest associations with increased susceptibility to atopic conditions, including allergic rhinitis and atopic dermatitis, though evidence across broader populations remains limited and variable. If you take medications influenced by histamine pathways, discuss any treatment or symptom concerns with your clinician or pharmacist.
Carried by roughly 15% to 20% of people of European ancestry, but is less common in East Asian and African ancestries.
Significantly reduced enzyme activity
You carry two copies of the T variant allele, which is associated in biochemical studies with substantially lower histamine N-methyltransferase activity and protein levels. While some research links this reduced clearance capacity to higher rates of atopic dermatitis or histamine intolerance symptoms, overall clinical evidence is limited and varies across different ethnic backgrounds. If you have concerns about histamine sensitivity or responses to antihistamines, consult your healthcare provider or pharmacist.
Rare across all major ancestral groups, occurring in roughly 1% or fewer of individuals of European ancestry and exceptionally rarely in Asian populations.
What is rs11559020 and Where Is It Located?
The single nucleotide polymorphism (SNP) rs11559020 is situated within exon 4 of the HNMT gene on chromosome 2. At the DNA level, it involves a substitution of cytosine (C) for thymine (T), often referred to in classical biomedical literature as the C314T transition. This nucleotide change directly modifies the resulting protein code: the standard threonine amino acid at position 105 is swapped for an isoleucine (p.Thr105Ile). Because this change alters the amino acid sequence of the final protein product, it is classified as a nonsynonymous missense variant. Protein structural analyses indicate that the substitution destabilizes the tertiary folding of the enzyme, leading to accelerated degradation of the protein within human cells and lower overall concentrations of functional enzyme in renal, bronchial, and skin tissues.
Biological Function of the HNMT Gene
Histamine is a critical signaling molecule involved in inflammatory responses, gastric acid secretion, bronchoconstriction, and neurotransmission. The body clears histamine primarily via two distinct biochemical pathways: oxidation by diamine oxidase (DAO, encoded by AOC1) predominantly in the extracellular digestive tract, and methylation by histamine N-methyltransferase (HNMT) primarily inside cells. The HNMT enzyme specifically transfers a methyl group from S-adenosylmethionine (SAMe) to histamine, producing N-tele-methylhistamine, which is subsequently converted into methylimidazole acetic acid and excreted. Because HNMT is widely expressed in human bronchial epithelium, kidney, liver, skin, and throughout the central nervous system, it represents the primary pathway for terminating histamine signals within these specific peripheral tissues and the brain.
Research Findings and Evidence Strength
Biochemically, in vitro and ex vivo tissue studies reliably demonstrate that the isoleucine (T allele) variant exhibits approximately 30% to 50% lower enzymatic activity and decreased immunoreactive protein levels compared to the reference C allele. Clinically, however, evidence linking rs11559020 directly to complex disease endpoints remains limited and mixed. Candidate-gene studies have explored associations between the T allele and phenotypes such as atopic dermatitis, allergic asthma, migraine severity, and variable sensitivity to histaminergic medications. While some clinical studies observe a small increase in atopic vulnerability among T allele carriers, genome-wide association studies (GWAS) often fail to replicate large effect sizes for broad allergic diseases. Consequently, scientific consensus views rs11559020 as a functional modifier of baseline enzymatic capacity rather than an independent, definitive driver of disease.
Population Distribution and Global Diversity
The frequency of the rs11559020 T allele varies substantially across worldwide populations. Large genomic reference repositories, such as the Genome Aggregation Database (gnomAD), indicate that the variant allele frequency (MAF) hovers between 0.08 and 0.12 among individuals of European ancestry, meaning roughly 15% to 20% of European-descended populations carry at least one copy of the variant. In contrast, the minor allele is markedly less prevalent in East Asian populations, where allele frequencies often drop below 0.02. Intermediate frequencies are documented across Admixed American, South Asian, and African populations. These population-level differences emphasize the importance of interpreting genetic test results within the appropriate ancestral genetic context, as background frequency heavily impacts statistical associations.
Interpreting Results: What You Can and Cannot Do
Understanding your rs11559020 genotype can provide personal context about your baseline rate of tissue histamine metabolism, but it does not represent a medical diagnosis of histamine intolerance, mast cell disease, or allergies. A reduced-function genotype does not guarantee you will develop allergic symptoms, nor does a standard-function genotype make you immune to atopic disease. Complex immune and histaminergic conditions depend far more on total allergen burden, gut barrier integrity, dietary intake, and environmental factors than on any single SNP. Readers should never use this genetic data to initiate extreme elimination diets or adjust prescribed pharmaceutical therapies. Any questions regarding histamine sensitivity, allergy testing, or medication tolerance should be discussed directly with an allergist, physician, or clinical pharmacist.
How common is this variant?
The minor T allele occurs at a frequency of approximately 0.08 to 0.12 in European populations, making heterozygous CT carriers relatively common. The variant is noticeably less frequent in East Asian ancestral groups (typically below 0.02), while homozygous TT individuals represent roughly 1% of most Western cohorts.
Frequently asked questions
Does having the rs11559020 T allele mean I have histamine intolerance?
No, having one or two copies of the T allele does not diagnose histamine intolerance. While the variant reduces cellular HNMT enzyme activity, clinical symptoms depend on total histamine exposure, diamine oxidase (DAO) enzyme performance, diet, and immune status. Many individuals with the T allele never develop symptomatic histamine intolerance.
What is the difference between DAO and HNMT?
Diamine oxidase (DAO, encoded by the AOC1 gene) is primarily secreted into the gut lumen and extracellular spaces to break down histamine absorbed from food. Histamine N-methyltransferase (HNMT) is an intracellular enzyme that degrades histamine inside tissues throughout the body, including the skin, lungs, and central nervous system.
Can I take supplements to overcome an HNMT gene variant?
There are no clinically proven genetic 'cures' for reduced HNMT enzyme activity, though HNMT relies on S-adenosylmethionine (SAMe) as a methyl donor cofactor. You should consult a physician before taking high-dose methyl donors or dietary supplements, as self-treating based solely on a single genetic variant can disrupt other metabolic pathways.
Does this SNP affect how I respond to antihistamines?
Some exploratory research suggests that individuals with altered HNMT activity might experience subtle differences in histamine clearance and antihistamine response. However, clinical guidelines do not currently endorse altering allergy medications or dosages based on rs11559020. Always speak with your pharmacist or doctor before changing any medication regimen.
Sources & further reading
Educational information only, last refreshed 9/11/2026. Not medical advice — these associations describe population statistics, not individual predictions.
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