CYP3A4 is the dominant drug-metabolizing enzyme in the human liver and intestine, processing approximately half of all prescription medications¹¹ approximately half of all prescription medications
Including statins, immunosuppressants, benzodiazepines, calcium channel blockers, HIV antiretrovirals, antifungals, and many anticancer agents. The CYP3A4*11 allele, defined by rs67784355 (c.1088C>T on the coding strand; p.Thr363Met), is one of the rarest functionally characterized CYP3A4 variants — but when it occurs, it produces a measurably compromised enzyme.

Threonine at position 363 sits within the substrate recognition site 5 (SRS-5)²² substrate recognition site 5 (SRS-5)
One of six substrate-recognition sites lining the CYP3A4 active site; SRS-5 contributes to the hydrogen-bonding network that stabilizes the active site architecture and positions substrates for catalysis. Replacing this threonine with a bulkier methionine disturbs the local protein structure, reducing both how much enzyme is produced and how stable it is once made.

The Mechanism

CYP3A4 is encoded on the minus (antisense) strand of chromosome 7. The *11 allele introduces a C→T change at position 1088 of the coding sequence (NM_017460.6:c.1088C>T), which on the genomic plus strand corresponds to a G→A substitution at rs67784355. The encoded threonine (polar, hydroxyl side chain) at codon 363 is replaced by methionine (hydrophobic, larger side chain). This is not a conservative substitution — methionine cannot fulfill the hydrogen-bonding role of threonine in the SRS-5 network.

In vitro expression studies³³ In vitro expression studies
Using baculovirus-insect cell and mammalian expression systems to produce recombinant CYP3A4 protein consistently find that CYP3A4*11 produces substantially less functional apoprotein than wild-type CYP3A4*1. The carbon monoxide-difference spectra⁴⁴ carbon monoxide-difference spectra
A spectrophotometric test of functional CYP content that measures correctly folded heme-containing protein of *11-expressing cells show reduced signal — meaning fewer active enzyme molecules, not just impaired ones.

The Evidence

Four independent in vitro studies characterize CYP3A4*11 across different substrates:

Loperamide: Wang et al. (2019)⁵⁵ Wang et al. (2019)
Functional characteristics of CYP3A4 allelic variants on the metabolism of loperamide in vitro found that CYP3A4*11's intrinsic clearance (CLint) for N-demethylation of loperamide was approximately 5.7-fold lower than wild-type, placing it among the most functionally impaired alleles tested. The authors attributed the deficit to protein instability from the bulky methionine substitution disrupting the tertiary structure at position 363.

Quinine: Gao et al. (2019)⁶⁶ Gao et al. (2019)
Enzymatic activities of CYP3A4 allelic variants on quinine 3-hydroxylation in vitro showed that CYP3A4*11 expressed at particularly low apoprotein levels compared to wild-type, consistent with reduced protein stability, and displayed reduced quinine hydroxylation activity. Five variants in total showed this pattern of low expression — *11 was among the most severely affected.

Midazolam and testosterone: Kato et al. (2021)⁷⁷ Kato et al. (2021)
Functional characterization of 40 CYP3A4 variants by assessing midazolam 1'-hydroxylation and testosterone 6β-hydroxylation confirmed reduced T363M expression in mammalian cells, noting that this substitution affects either protein expression or stability. The kinetic analysis placed *11 among the variants with meaningfully reduced catalytic efficiency.

Original identification: Sata et al. (2000)⁸⁸ Sata et al. (2000)
CYP3A4 allelic variants with amino acid substitutions in exons 7 and 12 first identified *11 from human liver samples and demonstrated its altered catalytic activity, establishing residue 363 as functionally important.

The limitation of all current evidence is that it is exclusively in vitro. The 2023 joint consensus genotyping recommendation⁹⁹ 2023 joint consensus genotyping recommendation
From AMP, CPIC, CAP, DPWG, ESPT, and PharmGKB explicitly excluded CYP3A4*11 from its Tier 1 and Tier 2 clinical assay recommendations because of this limitation — no pharmacokinetic studies in human carriers have been published. The variant is too rare to recruit sufficient carriers for in vivo trials under normal circumstances.

Practical Implications

Because clinical pharmacokinetic data are absent, prescribers cannot rely on published dosing guidelines for *11 carriers as they can for CYP3A4*22 or CYP3A5*1/*3. However, the consistent direction of the in vitro evidence — reduced enzyme levels, reduced clearance across multiple chemically distinct substrates — makes it reasonable to apply the same precautions used for other CYP3A4 reduced-function alleles.

The drugs of greatest concern are those with narrow therapeutic windows metabolized primarily by CYP3A4: tacrolimus and cyclosporine (transplant immunosuppressants), statins particularly atorvastatin and simvastatin, midazolam and other benzodiazepines, and cardiotoxic drugs like loperamide (at high doses). For each of these, standard doses may produce higher-than-expected plasma concentrations in *11 carriers.

Importantly, CYP3A4 inhibitors (grapefruit, clarithromycin, ketoconazole, ritonavir) compound an already-reduced enzyme capacity — the additive effect of genetic reduction plus pharmacological inhibition can push drug levels into the toxic range.

Interactions

CYP3A4*11 exists in the same genetic neighborhood as the more common and better-studied CYP3A4 variants. The CYP3A4*22 allele (rs35599367)¹⁰¹⁰ CYP3A4*22 allele (rs35599367)
An intronic splice variant causing ~50% reduced CYP3A4 mRNA, with established clinical guidelines for tacrolimus dosing is the most clinically relevant partner. CYP3A4*38 was defined precisely because rs67784355 was found in cis with the CYP3A4*3-defining variant in some haplotypes, meaning the same chromosome can carry multiple reduced-function alleles simultaneously.

CYP3A5 status (rs776746) is critical context for CYP3A4*11 carriers. CYP3A5 is a closely related enzyme that partially compensates for reduced CYP3A4 activity. If a *11 carrier is also a CYP3A5 non-expresser (*3/*3, the case for ~85% of Europeans), total CYP3A enzyme capacity is substantially diminished. A CYP3A5 expresser who carries *11 may show near-normal overall CYP3A clearance because CYP3A5 compensates — the opposite is true in non-expressers.

Gene-drug interactions are equally important. Strong CYP3A4 inhibitors added to an already-impaired metabolizer create additive metabolic suppression. Any patient carrying CYP3A4*11 who starts an azole antifungal, macrolide antibiotic, or ritonavir-boosted HIV regimen while on a narrow-therapeutic-index CYP3A4 substrate should be monitored intensively.

On chromosome 11q13.5 sits a stretch of regulatory DNA that, in people carrying the G allele at rs7130588, subtly tips the balance of the immune system toward allergic reactivity. The gene controlled by this region, LRRC32, encodes GARP (Glycoprotein A Repetitions Predominant)¹¹ GARP (Glycoprotein A Repetitions Predominant)
GARP is a surface receptor expressed on activated regulatory T cells (Tregs); it docks latent TGF-β on the Treg cell surface, where TGF-β can be released in a controlled fashion to suppress nearby immune cells — a critical molecule in the suppressive machinery of regulatory T cells (Tregs). When GARP function is altered, the Treg-mediated braking on allergic inflammation weakens, and atopic diseases become more likely.

The Mechanism

GARP is a transmembrane receptor expressed exclusively on activated Tregs (and platelets). Its core job is to anchor the latent TGF-β1 complex — the "sleeping" form of this immunosuppressive cytokine — on the cell surface. Surface-tethered latent TGF-β can be activated locally, delivering targeted suppression to nearby effector T cells, dendritic cells, and B cells without flooding systemic circulation with active TGF-β. This spatial precision is what makes GARP-mediated tolerance so important: it allows Tregs to dampen Th2-skewed responses in peripheral tissues like the airway and skin while leaving systemic immunity intact.

rs7130588 lies in a regulatory (non-coding) region approximately 10 kb upstream of the LRRC32 promoter. A closely linked causal candidate, rs6592645 (in complete linkage disequilibrium with rs7130588), falls within an enhancer element that physically loops to the LRRC32 promoter. Luciferase reporter assays²² Luciferase reporter assays
A standard lab test where the regulatory region controls a bioluminescent reporter gene; more light = more transcriptional activity from that sequence confirmed that the A allele of rs6592645 (the rs7130588-G-linked haplotype) drives higher enhancer activity than the G allele, via preferential binding of the transcription factor TCF3. LRRC32 mRNA levels are correspondingly elevated in lung tissue of asthma patients in a genotype-dependent manner.

The counterintuitive implication: higher GARP expression might be expected to enhance Treg suppression and protect against allergy. The biology, however, is more complex. Elevated GARP may alter the balance between membrane-bound and soluble GARP isoforms, the ratio of latent-to-active TGF-β at the Treg surface, or Treg activation thresholds in ways that impair rather than enhance tolerance in allergic tissue contexts. The net clinical observation is unambiguous: the G-risk allele associates with more atopic disease, not less.

The Evidence

The sentinel discovery came from Ferreira et al. in the Lancet³³ Ferreira et al. in the Lancet
Australian Asthma Genetics Consortium GWAS, 57,800 participants including 2,669 asthmatics and 4,528 controls in the discovery stage plus 25,358 in replication (2011): rs7130588 reached genome-wide significance (OR 1.09, p=1.8×10⁻⁸) for overall asthma, with a stronger signal for atopic asthma specifically (OR 1.33 for atopic status among asthmatics, p=7×10⁻⁴). The authors concluded the locus "directly increases the risk of allergic sensitisation which, in turn, increases the risk of subsequent development of asthma."

The GWAS Catalog records two subsequent associations at this variant: OR 1.29 (95% CI 1.20–1.38, p=4×10⁻¹³) and OR 1.242 (95% CI 1.17–1.31, p=2×10⁻⁹) in larger replication efforts, confirming and strengthening the original signal.

Beyond asthma, the LRRC32 locus is a shared risk locus across the atopic triad. Weidinger et al.⁴⁴ Weidinger et al.
GWAS of childhood atopic dermatitis, 1,563 European cases and 4,054 controls with replication in a second cohort (2,286 cases, 3,160 controls) (2013) confirmed the proximal LRRC32 region at genome-wide significance for atopic dermatitis. Most tellingly, Marenholz et al.⁵⁵ Marenholz et al.
Meta-analysis of 12 populations, 2,428 cases of the "atopic march" phenotype (eczema followed by asthma), 17,034 controls (2015) confirmed LRRC32/C11orf30 as one of seven loci driving the progression from infantile eczema to childhood asthma — the classic atopic march. The locus sits at the intersection of all three major atopic phenotypes: eczema, allergic asthma, and (via the atopic march pathway) allergic rhinitis.

The functional direction — that GARP is genuinely causal, not just a passenger — is supported by a humanized mouse model study⁶⁶ humanized mouse model study
Mice engrafted with human PBMCs from birch-pollen sensitized donors; treatment with soluble GARP reduced airway hyperresponsiveness, neutrophil and macrophage influx, and mucus-producing goblet cells; effect abolished by anti-TGF-βRII antibody in which exogenous soluble GARP reduced allergic airway inflammation in a TGF-β receptor-dependent manner. Separately, a clinical study found that children who outgrow food allergy have significantly higher GARP expression on peripheral blood mononuclear cells compared with those who do not (p=0.005), establishing GARP as a marker — and potentially a driver — of immune tolerance acquisition.

Practical Actions

The G allele's risk is primarily expressed in the context of atopic sensitization. G-allele carriers who already have one atopic condition (eczema, food allergy, or allergic rhinitis) face meaningfully elevated risk of asthma development — the atopic march phenotype. Early allergist evaluation and proactive management of existing atopic conditions can interrupt this progression. Aeroallergen immunotherapy, which directly raises functional Treg activity and GARP expression, is the intervention most biologically aligned with this locus's mechanism.

For GG homozygotes (approximately 11% of people of European descent), the cumulative risk elevation is additive and warrants baseline spirometry and exhaled nitric oxide (FeNO) measurement to detect subclinical airway inflammation before clinical asthma develops.

Interactions

The LRRC32 locus partially overlaps with C11orf30 (EMSY), which is itself an independent asthma and eczema risk gene; fine-mapping distinguishes the two signals, but in population data they travel together on 11q13. rs6592645, the likely causal variant in full LD with rs7130588, is the preferred candidate for functional follow-up. rs3781700 (nearby in this region) has been identified with OR ~1.054 for atopic dermatitis in large GWAS, representing additional independent variation at the locus.

Compound effects with PTPN22 rs2476601 (R620W), a T-cell activation threshold variant, are plausible given both genes influence regulatory T cell function, but no formal gene-gene interaction study has yet been published.

Complement receptor 1 (CR1, also known as CD35) is a molecular garbage collector. It sits on the surface of red blood cells, monocytes, and dendritic cells, binding to complement-tagged debris — including amyloid-beta oligomers¹¹ amyloid-beta oligomers
the toxic protein fragments that accumulate in Alzheimer's disease — and shuttling them to the liver and spleen for destruction. The rs6656401 variant in the CR1 gene was identified in 2009 as one of the first genome-wide significant Alzheimer's risk loci beyond APOE, and it remains among the most robustly replicated genetic risk factors for late-onset disease.

The Mechanism

CR1 mediates immune adherence²² immune adherence
the process by which complement-opsonized particles are bound to erythrocytes and carried out of circulation. When complement proteins C3b or C4b coat amyloid-beta aggregates in the bloodstream or cerebrospinal fluid, CR1 on circulating red blood cells acts as a capture receptor, escorting the tagged debris to the reticuloendothelial system. The CLU gene (clusterin), identified in the same 2009 paper, performs a complementary amyloid-chaperoning role — lending biological plausibility to both genes as amyloid-clearance machinery.

The rs6656401 A risk allele sits within an intron of CR1 and functions as a regulatory variant³³ regulatory variant
it alters CR1 expression or splicing rather than changing the amino acid sequence. A 2023 study measuring plasma protein levels showed that the A allele is significantly associated with increased soluble CR1 (sCR1) concentrations — suggesting the risk mechanism may involve a shift from membrane-bound CR1 (the functional form on erythrocytes) to its soluble, shed isoform. Elevated sCR1 in plasma is believed to act as a decoy receptor, competing with and potentially inhibiting the erythrocyte-bound CR1 that performs physiological clearance.

The Evidence

Lambert et al. (2009) in Nature Genetics⁴⁴ Lambert et al. (2009) in Nature Genetics
first GWAS to identify CR1 as an AD risk locus beyond APOE reported an odds ratio of 1.21 (95% CI 1.14–1.29, P=3.7×10⁻⁹) for the rs6656401 A allele in a combined European dataset of over 9,000 individuals. The association was validated across five independent European cohorts, establishing CR1 as a genuine LOAD susceptibility locus.

A 2015 meta-analysis of 85,939 samples (30,100 cases; 55,839 controls) across 24 studies⁵⁵ A 2015 meta-analysis of 85,939 samples (30,100 cases; 55,839 controls) across 24 studies
the largest pooled analysis specifically of rs6656401 confirmed the association with OR=1.18 (95% CI 1.15–1.22, P=1.82×10⁻²⁶) in combined populations, and OR=1.31 in East Asian samples — a finding that highlights population-specific effect sizes likely driven by differences in genetic background and LD patterns.

Liu et al. (2021), synthesizing 30 studies with over 68,000 participants⁶⁶ Liu et al. (2021), synthesizing 30 studies with over 68,000 participants
the most comprehensive systematic review of CR1 variants in LOAD, confirmed OR=1.23 (95% CI 1.10–1.36) overall and OR=1.26 in European populations specifically, with the A allele as the risk allele in all models tested. The evidence level is strong: the association has been independently replicated across populations and study designs, and a plausible biological mechanism has been characterised. Clinical-guideline-level evidence (CPIC/DPWG) does not yet exist for CR1 because there are no genotype-specific approved interventions.

Practical Implications

Unlike pharmacogenomic variants with direct drug-dose consequences, the CR1 rs6656401 variant informs risk stratification and motivates a specific biological target: complement-mediated amyloid clearance. For A-allele carriers, the actionable focus is on supporting the complement and immune pathways that handle amyloid burden, and on early monitoring.

Erythrocyte CR1 density varies by a HindIII RFLP polymorphism⁷⁷ HindIII RFLP polymorphism
the CR1 high-density (H) and low-density (L) allele determining copy number on red cells independently of rs6656401, and the relationship between rs6656401, sCR1 levels, and erythrocyte CR1 density is an active area of research. What is clear is that the A allele shifts complement biology in a direction associated with impaired amyloid clearance and increased disease risk.

The most actionable steps for A-allele carriers center on: (1) monitoring cognitive function early to catch any changes before they become irreversible, (2) managing the modifiable complement- and inflammation-activating factors (metabolic health, sleep quality, cardiovascular fitness) that amplify amyloid accumulation, and (3) awareness that participation in prevention trials is particularly valuable given the established, replicated risk.

Interactions

The most important interaction is with APOE genotype (rs429358 + rs7412). APOE ε4 affects amyloid-beta production and aggregation via lipoprotein pathways, while CR1 affects its clearance via complement. Carrying both CR1 risk alleles and APOE ε4 may represent compounding impairment of two independent amyloid-handling systems — accumulation plus impaired removal.

BIN1 rs744373 and CLU rs11136000 also influence amyloid and tau biology through different mechanisms. While no single study has formally quantified the combined effect of all four loci, polygenic risk scores incorporating CR1, BIN1, CLU, and APOE together predict Alzheimer's risk substantially better than any single variant alone, supporting the additive model that guides action in high-risk carriers.

CYP2C9 is one of the most clinically important liver enzymes, responsible for metabolizing roughly 15% of all prescription drugs. It sits in the wall of the endoplasmic reticulum¹¹ endoplasmic reticulum
Endoplasmic reticulum: the intracellular membrane system where CYP450 enzymes reside and where drug oxidation occurs and carries out oxidative biotransformation of warfarin, phenytoin, many NSAIDs, and several oral hypoglycemics. Variants that impair its activity require dose adjustments to avoid toxicity. This variant, p.Leu19Ile, sits at the very N-terminus of the protein in the membrane-anchoring region — an extremely rare change with no assigned star allele and no published ClinVar classification.

The Mechanism

Position 19 falls within or immediately adjacent to the N-terminal signal-anchor sequence²² N-terminal signal-anchor sequence
Signal-anchor: a short hydrophobic stretch at the start of CYP450 proteins that lodges the enzyme in the ER membrane of CYP2C9 (approximately residues 3–23). This region does not participate directly in substrate binding or catalysis; its job is to embed the enzyme in the ER membrane so that the catalytic domain is correctly positioned in the ER lumen. The variant substitutes leucine for isoleucine at position 19 — a conservative change³³ conservative change
Conservative change: both Leu and Ile are aliphatic, hydrophobic amino acids with very similar physicochemical properties between two aliphatic, hydrophobic residues. Ensemble VEP rates the impact as MODERATE with SIFT "tolerated_low_confidence" (score 0.05, borderline), and PolyPhen data are insufficient. This is consistent with the absence of any ClinVar submission despite the variant having been observed in large sequencing cohorts.

The Evidence

The massively parallel CYP2C9 variant study by Amorosi et al.⁴⁴ massively parallel CYP2C9 variant study by Amorosi et al.
Amorosi CJ et al. Massively parallel characterization of CYP2C9 variant enzyme activity and abundance. Am J Hum Genet, 2021 measured enzymatic activity for 6,142 CYP2C9 missense variants and found that almost two-thirds showed decreased activity. Position 19 was among variants assayed in that high-throughput screen; however, the specific activity score for p.Leu19Ile was not highlighted in published reports. The Daly et al. review⁵⁵ Daly et al. review
Daly AK et al. Pharmacogenomics of CYP2C9: Functional and Clinical Considerations. J Pers Med, 2017 notes that rare variants of uncertain significance are increasingly detected by exome and genome sequencing and require systematic functional cataloging before clinical application. There are no published case reports, association studies, or CPIC classifications specifically addressing rs67807361. The variant has been observed at a frequency of approximately 0.002% globally (gnomAD v4), with marginal enrichment in South/East Asian populations (~0.02%) relative to European (~0.001%).

Practical Context

Because this variant has no established functional classification, it is best treated as a variant of uncertain significance (VUS) within the CYP2C9 pharmacogenomic framework. If you carry this variant alongside a well-characterized reduced-function allele (*2 or *3), the combined phenotype is determined primarily by those established alleles. Without functional evidence, prescribers should apply standard CYP2C9 clinical annotations from well-characterized alleles. If you are being assessed for warfarin or phenytoin therapy, the CPIC guidelines for CYP2C9 remain the authoritative reference — and this variant should be flagged for the prescribing pharmacogeneticist as unclassified.

Interactions

This variant sits in the same gene as the well-characterized *2 (rs1799853) and *3 (rs1057910) alleles. If rs67807361 were to reduce CYP2C9 expression or activity, compound heterozygosity with *2 or *3 could produce a more pronounced intermediate or poor metabolizer phenotype. However, without direct functional evidence, this potential interaction remains speculative.

Atopic dermatitis (eczema) is the most common chronic inflammatory skin disease, driven by an immune system that responds inappropriately to harmless environmental triggers. One of its key drivers is T-helper cell dysregulation¹¹ T-helper cell dysregulation
T helper cells coordinate immune responses by releasing cytokines; subtypes Th2, Th17, and Th22 all contribute to eczema flares through different inflammatory signals, particularly an imbalance that tips immune activation too far toward inflammation. The rs71625130 variant sits at a genetic locus — chromosome 1q21.3 — that has emerged as one of the most statistically robust atopic dermatitis risk signals ever identified, with an association p-value of 2×10⁻⁸⁹.

The SNP lies within an intron of SNX27 (Sorting Nexin 27), a scaffolding protein that governs how signalling molecules are trafficked within T cells. The 1q21.3 locus also harbours RORC approximately 150 kb downstream — the gene encoding RORγt²² RORγt
RAR-related orphan receptor gamma-t; the master transcription factor that drives Th17 cell differentiation and IL-17 production — making this a biologically coherent stretch of immune regulation DNA.

The Mechanism

SNX27 operates at the immune synapse³³ immune synapse
The organised interface formed between a T cell and an antigen-presenting cell; SNX27 localises here in an activation-dependent manner to regulate signalling kinetics — the contact zone where T cells recognise foreign antigens and decide whether to mount an immune response. SNX27's PDZ domain physically escorts diacylglycerol kinase ζ (DGKζ)⁴⁴ diacylglycerol kinase ζ (DGKζ)
An enzyme that phosphorylates the second messenger diacylglycerol (DAG), reducing its concentration and dampening downstream TCR signalling through PKC and Ras/ERK pathways back to the membrane. This acts as a tuning mechanism: SNX27 limits how strongly a T cell fires.

Beyond DGKζ recycling, SNX27 directly controls the abundance of Lck and CD4⁵⁵ Lck and CD4
Lck is the tyrosine kinase that phosphorylates the T cell receptor complex; CD4 is the co-receptor that stabilises and amplifies the TCR signal. Together they set the threshold for T cell activation at the cell surface. When SNX27 expression is reduced, CD4 and Lck levels fall in resting T cells, yet paradoxically the cells become hyperresponsive: NF-κB activity rises, CXCR4 is upregulated, and lytic enzymes and proinflammatory cytokines pour out. The result is a T cell that activates too easily and produces too much inflammation.

The intronic rs71625130-A allele most likely acts as a regulatory variant⁶⁶ regulatory variant
A non-coding change that alters how much SNX27 is made in immune cells, rather than changing the protein sequence itself; eQTL databases show expression-level effects at this locus, reducing SNX27 expression in T cells and thereby lowering the threshold for inflammatory T cell activation. The nearby RORC gene, whose protein product drives Th17 cell differentiation⁷⁷ Th17 cell differentiation
Th17 cells produce IL-17A, IL-17F, and IL-22, cytokines that trigger epidermal barrier disruption and neutrophil recruitment; RORC is the transcription factor that commits naïve T cells to the Th17 fate, shares long-range regulatory elements at 1q21.3 and may be co-regulated with SNX27 in immune tissue contexts.

The Evidence

The key evidence comes from the largest atopic dermatitis GWAS conducted to date. Budu-Aggrey et al. 2023⁸⁸ Budu-Aggrey et al. 2023
European and multi-ancestry genome-wide association meta-analysis; discovery cohort 1,086,394 individuals; replication cohort 3,604,027 individuals — by far the largest AD genetic study identified 81 loci in Europeans (29 novel) and 10 additional multi-ancestry loci, with rs71625130-A emerging as one of the strongest signals at OR=1.17 and an extraordinary p-value of 2×10⁻⁸⁹. The A allele frequency of ~4% in Europeans means it is carried heterozygously by roughly 1 in 15 Europeans — a meaningful population burden for such a strong effect.

The biological plausibility rests on independent functional studies. Martinez-Martin et al. 2024⁹⁹ Martinez-Martin et al. 2024
Nat Commun; demonstrated SNX27 regulation of Lck/CD4 at the T cell immune synapse and downstream proinflammatory consequences of SNX27 loss showed that impaired SNX27 expression contributes directly to CD4 T cell dysfunction with inflammatory features consistent with atopic disease. Cosen-Binker et al. 2017¹⁰¹⁰ Cosen-Binker et al. 2017
J Immunol; showed SNX27-DGKζ axis controls TCR signalling amplitude and T cell metabolic programming established the mechanistic link between SNX27, signal calibration, and the transcriptional programmes that determine whether a T cell triggers inflammation. Together these studies support rs71625130-A as a functional eQTL that biases the 1q21.3 locus toward lower SNX27 expression, hyperactivated T cells, and elevated atopic inflammation.

The shared genetics of the IL-23/Th17 axis across atopic dermatitis, psoriasis, inflammatory bowel disease, and ankylosing spondylitis — diseases all amenable to IL-17 or IL-23 blockade¹¹¹¹ IL-17 or IL-23 blockade
Biologic therapies targeting IL-17A (secukinumab, ixekizumab) or IL-23p19 (guselkumab, risankizumab) are approved across psoriasis, PsA, IBD, and AS, validating the Th17 axis as a genuine shared therapeutic target — adds strong contextual plausibility to the 1q21.3 locus as a genuine autoimmune-axis risk region.

Practical Actions

Carriers of the A allele have a modestly elevated baseline tendency toward Th17-axis overactivation. This translates to increased lifetime susceptibility to atopic dermatitis and possibly other Th17-mediated conditions. The variant's effect is additive, so heterozygous carriers (AG) have one dose of risk and homozygous carriers (AA) — extremely rare, under 0.05% — carry two.

Monitoring atopic disease activity and considering the triggers that amplify Th17 responses (gut dysbiosis, vitamin D insufficiency, prolonged stress) is relevant for carriers. The same IL-23/Th17 pathway that underlies this SNP is the therapeutic target of dupilumab and several approved biologics, meaning carriers who develop moderate-to-severe AD have access to precisely targeted therapies.

Interactions

The 1q21.3 locus acts within the broader IL-23/Th17 immune axis. Variants in IL23R (rs11209026), IL12B, and TYK2 that modulate this same signalling cascade are documented co-risk loci across atopic disease. Carriers of rs71625130-A who also carry risk variants in barrier genes (FLG, SPINK5) face a dual insult: impaired skin barrier combined with a hyperactivated Th17 immune response, which is the classic setup for severe, early-onset atopic dermatitis.

Motion sickness strikes roughly one in three people and is among the most common neurological complaints during travel. The core mechanism is the sensory conflict model¹¹ sensory conflict model
also called the neural mismatch theory: the brain receives discordant signals from the vestibular (balance), visual, and proprioceptive systems during passive motion. In a car or on a ship, the inner ear signals movement while the eyes — focused on a book or a screen — signal stillness. The brain interprets this mismatch as potential poisoning and triggers nausea as a protective reflex. The genetics of motion sickness susceptibility points, surprisingly, to the visual side of this equation.

rs66800491 is an intergenic variant located approximately 1.1 megabases upstream of PVRL3 (now called NECTIN3)²² PVRL3 (now called NECTIN3)
the nectin cell adhesion molecule 3; nectins are immunoglobulin-like proteins that mediate cell-cell adhesion at adherens junctions and are essential for organogenesis. In the 2015 23andMe motion sickness GWAS, rs66800491 was the strongest association out of 35 genome-wide significant loci — the variant with the most statistical evidence for influencing how prone a person is to motion sickness.

The Mechanism

NECTIN3 encodes a cell adhesion protein required for normal ocular development. Loss of PVRL3 expression in both humans and mice causes congenital ocular defects including lens abnormalities³³ congenital ocular defects including lens abnormalities
mutations in PVRL3 are associated with autosomal recessive congenital cataracts and other structural eye defects in humans. The GWAS authors proposed that the PVRL3 locus influences motion sickness susceptibility through the visual channel of the sensory conflict system: if visual processing or the structural integrity of the visual apparatus is subtly altered, the fidelity of visual motion signals sent to the brain changes, amplifying the mismatch with vestibular input and increasing susceptibility to nausea.

The rs66800491 variant itself is intergenic — it does not change an amino acid. It likely acts as a [regulatory variant | variants in non-coding regions can alter gene expression levels by affecting enhancers, promoters, or long-range chromatin contacts] that modulates PVRL3 expression in developing or mature neural or ocular tissue. The A allele (risk allele, frequency ~29% globally) is associated with increased motion sickness susceptibility, with each A allele adding approximately 0.08 points on the 0–3 motion sickness severity scale. The effect is 1.5× larger in women than in men.

The Evidence

The primary evidence is Hromatka et al. 2015⁴⁴ Hromatka et al. 2015
Genetic variants associated with motion sickness point to roles for inner ear development, neurological processes and glucose homeostasis. Human Molecular Genetics, the first genome-wide association study of motion sickness, surveying 80,494 individuals from the 23andMe database who reported car sickness on a four-point scale (never, occasionally, sometimes, frequently). rs66800491 reached P = 4.2×10⁻⁴⁴ — far beyond the genome-wide significance threshold of 5×10⁻⁸ — making it the single strongest genetic signal for motion sickness susceptibility identified to date. The 35 identified loci together explain approximately 2.9% of the variance in motion sickness susceptibility. The PVRL3 locus was explicitly named among the top candidate genes because of its known role in eye and ear development.

The overall architecture of motion sickness genetics confirmed the sensory conflict model: the genome-wide significant loci cluster near genes for inner ear development (TSHZ1, MUTED), eye development (PVRL3), and craniofacial morphogenesis (HOXB3, HOXD3). This convergent signal across visual and vestibular development genes is biologically coherent — it is not a single pathway but rather the two endpoints of the sensory conflict axis.

Practical Actions

Motion sickness management is well-studied clinically and applies with particular force to people carrying the A allele at rs66800491. The most actionable interventions target the visual channel directly — which is the proposed biological pathway for this locus — or use pharmacological agents that dampen the brain's response to sensory conflict.

Behavioral strategies: fixing gaze on the true horizon (not a nearby fixed point inside the vehicle) provides the visual system with a stable motion reference that matches vestibular input, reducing the mismatch signal. Closing the eyes or lying down removes visual input entirely. Avoiding reading and screens during motion is particularly relevant given the PVRL3 visual mechanism — screens provide high-contrast stationary visual input that maximally conflicts with inner-ear motion signals.

Pharmacological: [transdermal scopolamine | a muscarinic acetylcholine receptor antagonist; brand name Transderm Scop; applied as a patch behind the ear] should be applied several hours before anticipated travel for full effect — it prevents nausea but is not effective once symptoms have started. First-generation antihistamines (dimenhydrinate, cinnarizine) are effective prevention alternatives but cause sedation.

Interactions

The motion sickness GWAS identified 35 loci, suggesting polygenic susceptibility. The PVRL3 locus is the strongest single signal but does not account for the full genetic architecture. The strongest comorbidities are with migraine, postoperative nausea/vomiting, vertigo, and morning sickness — all conditions involving the vestibular-visual-nausea axis. Individuals with genetic susceptibility to migraine (e.g., TRPM8 or PRDM16 variants) may have compounded motion sickness risk.

The GLP-1 receptor (GLP1R) is the target of some of the most widely prescribed medications for weight loss and type 2 diabetes, including semaglutide (Ozempic, Wegovy), liraglutide (Saxenda, Victoza), and tirzepatide (Mounjaro). The rs6923761 variant causes a glycine-to-serine substitution at position 168 of the receptor protein, which sits in the extracellular domain¹¹ extracellular domain
the part of the receptor that protrudes outside the cell and binds the drug where GLP-1 and its pharmaceutical analogs dock.

The Mechanism

The Gly168Ser substitution reduces GLP-1 receptor binding affinity by approximately 30%²² reduces GLP-1 receptor binding affinity by approximately 30%
Integrated pharmacogenomic analysis, p=3.2x10^-5 and decreases receptor expression in adipose tissue. Paradoxically, carriers of the A allele (serine) appear to have higher basal GLP-1 levels and better baseline metabolic profiles, suggesting the variant may cause constitutive activation³³ constitutive activation
a state where the receptor is partially "on" even without a drug binding to it of the receptor. This means carriers get some GLP-1 signaling benefit at baseline but respond less strongly when pharmacologic agonists are added.

This constitutive activation hypothesis explains a striking dual pattern: carriers lose more weight on GLP-1 agonists but get less improvement in blood sugar control. The weight loss likely comes from enhanced gastric emptying delay⁴⁴ gastric emptying delay
slowed stomach emptying, which increases fullness and reduces caloric intake, while the blunted metabolic response reflects diminished beta-cell stimulation by the drug.

The Evidence

The largest pharmacogenomic study to date, a GWAS of 4,571 adults with type 2 diabetes⁵⁵ GWAS of 4,571 adults with type 2 diabetes
Dawed AY et al. Pharmacogenomics of GLP-1 receptor agonists. Lancet Diabetes Endocrinol, 2023, found that each copy of the A allele was associated with 0.9 mmol/mol (0.08%) less HbA1c reduction on GLP-1 receptor agonist therapy (p=6.0x10^-5). While modest per allele, this translates to meaningful differences: the 4% of the population with the least favorable genotype combination (including ARRB1 variants) had 30% less HbA1c reduction than the 9% with the best response.

A randomized controlled trial of 83 obese adults with prediabetes⁶⁶ randomized controlled trial of 83 obese adults with prediabetes
Mashayekhi M et al. Effects of a GLP-1 receptor polymorphism on responses to liraglutide. J Endocrinol, 2025 demonstrated a dose-dependent weight loss effect with liraglutide: GG carriers lost 2.05 kg, AG carriers lost 2.89 kg, and AA carriers lost 4.80 kg. However, only GG carriers showed significant improvements in fasting insulin, HOMA-IR, and glucagon levels. Variant carriers did show significant reductions in PAI-1⁷⁷ PAI-1
plasminogen activator inhibitor-1, a marker of cardiovascular and thrombotic risk prior to any weight change, suggesting a weight-independent cardiovascular benefit.

A pilot pharmacogenetics study of 60 obese individuals⁸⁸ pilot pharmacogenetics study of 60 obese individuals
Chedid V et al. Allelic variant in GLP1R associated with greater effect on gastric emptying. Neurogastroenterol Motil, 2018 showed that A allele carriers had approximately 50% greater gastric emptying delay with liraglutide (129 vs 61 minutes) and exenatide (118 vs 96 minutes), providing a mechanistic explanation for the enhanced weight loss.

An oral semaglutide study of 210 T2D patients⁹⁹ oral semaglutide study of 210 T2D patients
Acta Diabetologica, 2025 found no significant association between rs6923761 and HbA1c or BMI response, though the cohort had lower baseline HbA1c (<7.5%), which may have limited the ability to detect differences.

Practical Implications

This variant creates a pharmacogenomic paradox: if your primary goal is weight loss, carrying the A allele may actually be advantageous on GLP-1 agonists. If your primary goal is blood sugar control, your response may be somewhat blunted. This distinction is clinically relevant as GLP-1 agonists are increasingly prescribed for weight management in people without diabetes.

The DPP-4 inhibitor sitagliptin also shows reduced glucose-lowering efficacy in A allele carriers¹⁰¹⁰ reduced glucose-lowering efficacy in A allele carriers
Mashayekhi M et al. Diabetes Obes Metab, 2021, suggesting the effect extends beyond injectable GLP-1 agonists to the broader incretin drug class.

Interactions

The Lancet GWAS identified an interaction between GLP1R rs6923761 and ARRB1 rs140226575 (beta-arrestin 1). Beta-arrestin mediates GLP-1 receptor internalization and biased signaling. Carriers of both variants had the smallest HbA1c reduction on GLP-1 agonist therapy. The combination of reduced receptor binding (GLP1R) and altered receptor trafficking (ARRB1) may compound the blunted metabolic response. Another GLP1R variant, rs2268641, has been associated with obesity parameters in the same Polish cohort study and may have additive effects on GLP-1 receptor function.

At the heart of every allergic reaction lies a decision made long before the first exposure: which proteins should the immune system tolerate, and which should trigger a response? That decision is governed by the HLA class II system¹¹ HLA class II system
Human Leukocyte Antigen class II molecules are protein complexes on the surface of antigen-presenting cells that display peptide fragments to CD4+ T cells, shaping immune tolerance vs activation. rs7192 introduces a missense change — Leucine to Valine at codon 242 (Leu242Val) — in the HLA-DRA alpha chain, one of the two protein subunits that form HLA-DR molecules. This single amino acid change alters the structural conformation of the antigen-presenting groove and helps tag HLA class II haplotypes associated with peanut allergy, drug hypersensitivity, and broader immune-mediated conditions.

The Mechanism

HLA-DR molecules are heterodimers — an alpha chain (encoded by HLA-DRA) paired with one of several beta chains (encoded by HLA-DRB1, DRB3, DRB4, or DRB5). The peptide-binding groove formed by the two chains determines which protein fragments the immune system is presented for T-cell education during thymic development and peripheral tolerance maintenance. Leucine 242 sits in the alpha-1 domain²² alpha-1 domain
The membrane-distal domain of the HLA-DRA alpha chain that forms one wall of the peptide-binding groove, a region critical to groove shape and peptide-binding specificity.

The Val substitution introduced by the T allele at rs7192 (the missense G→T change at the DNA level, c.724T>G calling the protein change p.Leu242Val) is predicted to alter alpha-chain conformation. This structural change could shift which peptide fragments are preferentially gripped and displayed to T cells, biasing the early immune response toward sensitization for certain protein antigens — including peanut storage proteins — when the environmental context favors allergic programming over tolerance induction.

Additionally, rs7192 acts as a quantitative trait locus for DNA methylation³³ quantitative trait locus for DNA methylation
A genetic variant that influences how nearby genes are methylated, affecting their expression levels at CpG sites in both HLA-DRB1 and HLA-DQB1. This epigenetic effect — modulating the expression of HLA class II genes beyond just the structural change in the protein — likely amplifies the functional consequence of this variant for antigen-presentation capacity.

The Evidence

Hong et al. (2015)⁴⁴ Hong et al. (2015)
Genome-wide association study identifies peanut allergy-specific loci and evidence of epigenetic mediation in US children. Nature Communications, 2015 conducted the first GWAS of well-characterised food allergy in 2,197 US participants of European ancestry. rs7192 reached genome-wide significance (p=5.5×10⁻⁸) in the discovery phase and was independently replicated in an additional European cohort. The per-allele odds ratio for peanut allergy was approximately 1.6, placing this SNP among the strongest common genetic risk factors identified for the condition. Crucially, the association was accompanied by genome-wide significant differential DNA methylation at multiple CpG sites within HLA-DQB1 and HLA-DRB1, suggesting that the variant exerts its effect through both protein structural change and epigenetic modulation of HLA class II gene expression.

A complementary line of evidence comes from Guéant et al. (2015)⁵⁵ Guéant et al. (2015)
HLA-DRA variants predict penicillin allergy in genome-wide fine-mapping genotyping. Journal of Allergy and Clinical Immunology, 2015, which fine-mapped the HLA-DRA locus in 387 beta-lactam allergy cases and 1,124 controls from Spain (replicated in an Italian cohort). The rs7192 missense variant was significantly associated with allergy to penicillins and amoxicillin (p=7.4×10⁻⁶ discovery; p=6.0×10⁻⁴ replication). The study specifically noted that the variant is "predicted to influence alpha-chain conformation," providing direct functional validation that Leu242Val alters HLA-DRA structure in a clinically meaningful way.

Asai et al. (2018)⁶⁶ Asai et al. (2018)
Canadian peanut allergy GWAS meta-analysis. Journal of Allergy and Clinical Immunology, 2018 integrated data from 8 studies (>7,800 subjects total) and confirmed the HLA-DQB1/ DRB1 region as the primary peanut allergy locus — the same locus tagged by rs7192 and its companion SNP rs9275596. Importantly, the association was independent of asthma genetic architecture, establishing peanut allergy as a genetically distinct trait rather than a subset of atopy.

Practical Actions

The clinical relevance of this variant is clearest in three contexts: peanut allergy risk assessment, early-life peanut introduction for infants, and beta-lactam drug hypersensitivity.

Because rs7192 tags the same HLA risk block as rs9275596 (the primary peanut allergy GWAS signal), the practical implications are closely aligned: individuals carrying one or two T alleles carry a modestly elevated prior probability of peanut allergy through HLA class II-mediated sensitization mechanisms. The landmark LEAP trial showed that early peanut introduction (before 11 months) can completely override this HLA-mediated genetic risk by driving protective IgG4 rather than sensitizing IgE responses. For individuals who have already developed peanut allergy, HLA class II haplotype status predicts superior response to peanut oral immunotherapy — carriers of the risk haplotype achieve higher rates of desensitization than non-carriers.

For drug hypersensitivity, the Leu242Val change confers measurably increased risk for penicillin-class antibiotic allergy. This does not represent a contraindication to penicillin use, but it does mean that any reported penicillin reaction should be taken seriously, formally evaluated, and documented.

Interactions

rs7192 is part of the same HLA-DR/DQ risk block as rs9275596, its companion tag SNP from the peanut allergy GWAS. These two variants operate within the extended HLA class II haplotype architecture spanning chromosome 6p21.32. The HLA region also harbours rs7454108 (tagging HLA-DQ8, associated with celiac disease and type 1 diabetes) and rs2187668 (tagging HLA-DQ2.5, the primary celiac disease haplotype). Together, these tag SNPs define the major HLA class II susceptibility landscape for immune-mediated conditions — a person's full HLA class II profile across these loci provides a more complete picture of their antigen-presentation capabilities than any single SNP alone.

Migraine is not simply a brain pain disorder — it involves a coordinated failure in how blood vessels regulate their tone, particularly in the meninges and cranial arteries. FHL5 (four and a half LIM domains 5) encodes a transcriptional coactivator that regulates cAMP-responsive element modulator (CREM)¹¹ cAMP-responsive element modulator (CREM)
CREM is a transcription factor that controls gene expression programs downstream of cyclic AMP signaling; in vascular smooth muscle cells, CREM-dependent programs govern vessel tone and contractility. The FHL5 protein controls vascular disease-associated gene programs in smooth muscle cells, and common intronic variants in this gene — including rs67338227 — are among the most robust genetic determinants of migraine risk discovered in human genetics.

rs67338227 sits in an intron of FHL5 on chromosome 6q16. The T alternate allele is globally rare (approximately 0.3–1.5% depending on ancestry group) but its GWAS signal is exceptionally strong: the T allele carries an odds ratio of 1.09 per allele (95% CI 1.08–1.11) for migraine at p=2×10⁻²⁷, one of the most significant hits in the entire migraine genetics literature. The effect follows an additive model — each copy of T shifts risk upward — consistent with this variant tagging altered FHL5 expression levels in relevant tissues rather than a protein-disrupting change.

The Mechanism

FHL5 is a member of the LIM domain protein family, whose characteristic zinc-finger modules mediate protein-protein interactions in transcriptional complexes. FHL5 acts as a coactivator of CREM²² coactivator of CREM
CREM drives expression of genes involved in spermatid differentiation and has established roles in smooth muscle cell gene programs downstream of cAMP signaling. When cAMP levels rise in vascular smooth muscle — as occurs in response to vasoactive signals such as CGRP (calcitonin gene-related peptide, the primary trigger molecule in migraine attacks) — FHL5-CREM complexes regulate the transcriptional response that controls vessel relaxation and contractility.

An intronic variant such as rs67338227-T likely alters FHL5 expression levels or splicing in arterial smooth muscle cells. Supporting this, Jiang et al. 2021³³ Jiang et al. 2021
Common variants in KCNK5 and FHL5 genes contributed to the susceptibility of migraine without aura in Han Chinese population. Scientific Reports detected significant eQTL signals at the FHL5 locus in migraine patients, confirming that this genomic region influences FHL5 gene expression rather than protein structure. Altered FHL5 expression would dysregulate CREM-dependent gene programs in cranial smooth muscle cells, impairing the normal vasomotor response to migraine triggers.

The most direct functional evidence comes from Knol et al. 2020⁴⁴ Knol et al. 2020
Migraine Genetic Variants Influence Cerebral Blood Flow. Headache, which showed that the rs67338227 risk allele is associated with higher carotid flow velocity and smaller carotid cross-sectional area (p=3.7×10⁻⁸) in 4,665 participants. This is a direct measurement of altered vascular tone and structure in carriers — the biological intermediate between a gene expression change and the clinical migraine phenotype.

The Evidence

The FHL5 migraine locus has been independently replicated across four large genetic studies spanning over a decade:

Anttila et al. 2013⁵⁵ Anttila et al. 2013
Genome-wide meta-analysis identifies new susceptibility loci for migraine. Nature Genetics first identified rs67338227 at the FHL5 locus in a meta-analysis of 23,285 cases and 95,425 controls across 29 studies. The FHL5 region was one of five newly discovered migraine loci reaching genome-wide significance.

Gormley et al. 2016⁶⁶ Gormley et al. 2016
Meta-analysis of 375,000 individuals identifies 38 susceptibility loci for migraine. Nature Genetics substantially replicated and refined the FHL5 association using 59,674 migraine cases and 316,078 controls, pinpointing rs67338227-T at OR=1.09 (95% CI 1.08–1.11), p=2×10⁻²⁷. Notably, this study found that migraine loci as a group were enriched for expression in vascular and smooth muscle tissues — directly implicating FHL5's vascular role in the biology.

Hautakangas et al. 2022⁷⁷ Hautakangas et al. 2022
Genome-wide analysis of 102,084 migraine cases identifies 123 risk loci. Nature Genetics conducted the largest migraine GWAS to date and confirmed the FHL5 locus among 123 independent risk signals, with the FHL5 region meeting criteria as a high-confidence locus.

A 2025 multi-omics integration study (Wei et al. 2025, Journal of Headache Pain⁸⁸ Wei et al. 2025, Journal of Headache Pain) identified FHL5 as one of a small number of "high-confidence cross-subtype genes" supported by multiple analytical approaches, including colocalization with expression and chromatin data.

Practical Actions

Carrying the T allele at rs67338227 does not cause migraine — it increases susceptibility within a complex polygenic architecture. However, understanding the vascular mechanism points to specific, actionable strategies.

CGRP is the primary vasoactive peptide mediating migraine attacks, and FHL5 regulates smooth muscle cell responses to cAMP signaling downstream of CGRP receptors. This makes FHL5 T allele carriers especially relevant candidates for modern CGRP-targeting therapies: monoclonal antibodies targeting CGRP or its receptor (erenumab, fremanezumab, galcanezumab, eptinezumab) are now first-line preventive treatments for episodic and chronic migraine. The biological rationale for these drugs directly overlaps with the FHL5 pathway.

Additionally, the carotid hemodynamic changes documented in T allele carriers — higher flow velocity, smaller vessel diameter — suggest that cerebrovascular tone is measurably altered at baseline. Traditional migraine prophylactics that act on vascular smooth muscle tone (beta-blockers such as propranolol and metoprolol; calcium channel blockers such as flunarizine and verapamil) have established efficacy for migraine prevention and operate through overlapping pathways with FHL5's vascular smooth muscle regulatory function.

Interactions

The FHL5 locus has been studied in the context of migraine with and without aura. The Jiang 2021 study was specific to migraine without aura (MO) in a Han Chinese cohort and identified strong eQTL signals at the FHL5 region for this subtype. The broader European GWAS studies (Gormley 2016, Hautakangas 2022) analysed both migraine subtypes together.

rs11172113 (near TSPAN2) is another replicated migraine GWAS locus with evidence of vascular mechanism, and rs1835740 (near MTDH/SLC2A5) is an established migraine susceptibility variant that may compound the FHL5 vascular effect. The largest migraine polygenic risk scores incorporate both FHL5 and these pathway partners. Carriers of multiple migraine risk alleles at independent vascular loci may derive the greatest benefit from preventive therapy targeting the vascular component of migraine.

CYP2D6 is responsible for metabolizing approximately 20–25% of all clinically prescribed medications, including opioids, antidepressants, antipsychotics, and beta-blockers. Most clinically significant CYP2D6 variants are defined by named star alleles — 4 (the most common null allele), *10 (reduced function), *6 (frameshift), and *41 (splice-impaired). rs71328650 is the legacy identifier for what is now catalogued in dbSNP as **rs28371702*, an intronic variant at position c.181-41 in the CYP2D6 gene. Understanding this variant requires distinguishing between its independent functional significance and its role as a haplotype tag.

The Mechanism

The rs71328650/rs28371702 A>C variant lies in intron 1 of CYP2D6, 41 bases upstream of exon 2 (c.181-41). CYP2D6 is encoded on the minus strand of chromosome 22; the plus-strand reference allele at GRCh38 position 42,129,950 is A, corresponding to a T on the coding strand[| Minus-strand genes: the genomic plus strand runs 5'-to-3' in the opposite direction from the coding sequence. What genome files report as A at this position is T in the mRNA/protein coding context]. The alternate C allele on the plus strand corresponds to G on the coding strand, placing it at c.181-41T>G in HGVS notation.

Importantly, this intronic position is not the c.985+39G>A (2988G>A) causal variant that defines CYP2D6*41[| CYP2D6*41 is defined by a different intronic SNP — c.985+39G>A — which causes aberrant exon 6 splicing]. The *41 variant lies in intron 6, not intron 1, and is mechanistically characterized as increasing nonfunctional splice variants lacking exon 6 by up to 7.3-fold¹¹ 7.3-fold
Toscano C et al. Pharmacogenet Genomics, 2006. rs28371702 does not have a published equivalent mechanistic study demonstrating a specific splicing effect at the c.181-41 position.

The Evidence

Two published studies have associated rs28371702 with CYP2D6 drug metabolism outcomes:

In a 2025 pediatric study of aripiprazole pharmacokinetics²² Xin Y et al. BMC Pediatr, 2025, rs28371702 was among six CYP2D6 variants where the steady-state metabolic ratio (AUC-based) was significantly associated with rs28371702 genotype, suggesting this intronic variant tags a haplotype affecting how aripiprazole is cleared. Aripiprazole is a partial dopamine agonist metabolized primarily by CYP2D6 and CYP3A4, and its active metabolite (dehydro-aripiprazole) accumulates differently depending on CYP2D6 activity.

In a 2022 African study of praziquantel treatment for schistosomiasis³³ Zdesenko G et al. Front Genet, 2022, the rs28371702 CC genotype was the sole CYP2D6 variant significantly associated with treatment failure (odds ratio 2.36, p = 0.024). Praziquantel is primarily metabolized by CYP3A4, but CYP2D6 contributes to its biotransformation; this finding suggests the CC haplotype may tag a reduced-function CYP2D6 background in African populations, where the C allele frequency reaches ~64%.

Neither study provides direct mechanistic evidence that the intronic c.181-41 position itself causes functional change — the most parsimonious interpretation is that rs28371702 serves as a linkage disequilibrium tag for one or more functional CYP2D6 variants that have not been fully resolved in the haplotype structure. Evidence quality is emerging — two independent association signals without mechanistic characterization.

Practical Actions

The clinical actionability of rs28371702 alone is limited by the absence of CPIC or DPWG guideline designations. If you carry one or two C alleles at this position, the most important step is ensuring your overall CYP2D6 phenotype has been assessed through the complete set of characterized star alleles (*2, *4, *6, *10, *41), since the Gaedigk activity score⁴⁴ Gaedigk A et al. CYP2D6 activity score. Clin Pharmacol Ther, 2008 summarizes cumulative CYP2D6 function across all haplotypes. Isolated interpretation of rs28371702 without the full haplotype context may be misleading.

Interactions

CYP2D6 function is determined by the combined activity score of both alleles. The clinically characterized variants in the platform — rs3892097 (*4), rs1065852 (*10), rs16947 (*2), rs28371725 (*6), and rs5030655 (*3) — each have established activity scores and CPIC/DPWG guideline support. rs71328650/rs28371702 does not yet have an activity score assignment. In populations where the C allele is common (African, East Asian), this variant may co-segregate with reduced-function haplotypes that are not fully captured by the European-derived star allele panel — an important consideration for ancestry-aware pharmacogenomics.