One of the most studied regions in Crohn's disease genetics sits on chromosome 5q31 — a 250-kilobase stretch called the IBD5 locus¹¹ IBD5 locus
IBD5 was the fifth inflammatory bowel disease susceptibility locus identified by genome-wide linkage analysis; multiple studies from 2001–2006 refined its boundaries and candidate genes. Within this locus, two functional variants in adjacent carnitine transporter genes form a two-allele haplotype that raises Crohn's disease risk by two- to fivefold. rs2631367 is one of those two variants: a single nucleotide change in the promoter of SLC22A5 (encoding OCTN2) that reduces how much carnitine transporter the intestine makes — with significant consequences for mucosal immune homeostasis.

The Mechanism

SLC22A5 encodes OCTN2, a high-affinity sodium-dependent carnitine transporter²² high-affinity sodium-dependent carnitine transporter
Carnitine is an amino acid derivative essential for shuttling long-chain fatty acids across the inner mitochondrial membrane for beta-oxidation; without adequate OCTN2 activity, cells cannot efficiently import carnitine from the circulation expressed prominently in intestinal epithelium, heart, liver, and skeletal muscle. At position -207 in the SLC22A5 promoter, the G allele creates a heat shock element³³ heat shock element
HSEs are short DNA sequences recognized by heat shock transcription factors (HSFs); HSF binding increases gene transcription, especially under stress conditions binding site for heat shock transcription factors. The C allele disrupts this binding site, reducing OCTN2 transcriptional activity. Luciferase reporter assays and lymphoblastoid cell line studies confirmed that haplotypes carrying the -207G allele produce higher OCTN2 mRNA than haplotypes carrying -207C.

In the intestine, this matters beyond fatty acid metabolism. OCTN2-deficient mice⁴⁴ OCTN2-deficient mice
A neonatal mouse model with homozygous OCTN2 deletion showing early intestinal inflammation develop villous atrophy, early lymphocytic and macrophage infiltration, spleen and thymus atrophy via apoptosis, and a pro-inflammatory cytokine profile — a gut and immune phenotype that shares features with Crohn's disease histology. During active inflammatory bowel disease, OCTN2 expression in the terminal ileum is decreased further, suggesting a vicious cycle where inflammation suppresses the transporter and reduced transporter activity fails to protect the epithelial barrier.

rs2631367 forms the promoter component of the IBD5 TC haplotype⁵⁵ TC haplotype
TC stands for the two risk alleles: T at position +1672 in SLC22A4 (OCTN1, rs1050152) and C at position -207 in SLC22A5 (OCTN2, rs2631367); the haplotype was named for the two alleles present simultaneously alongside rs1050152 (L503F in the related OCTN1 transporter). Individually each variant reduces transporter function; together they appear to act additively on intestinal carnitine handling and mucosal barrier integrity.

The Evidence

The landmark study identifying the TC haplotype was published by Peltekova et al. in Nature Genetics⁶⁶ Peltekova et al. in Nature Genetics
Peltekova VD et al., 2004, 36:471–475. They showed that individuals heterozygous for the TC haplotype had odds ratios of 2.1–2.56 for Crohn's disease, while those homozygous for TC/TC had odds ratios of 3.43–5.14 — among the strongest common variant effects reported for IBD at the time. The association was independent of CARD15 variants (the IBD16 locus), and combined TC homozygosity with CARD15 mutations further amplified risk.

Waller et al. (Gut, 2006)⁷⁷ Waller et al. (Gut, 2006)
Waller S et al., 55(6):809–814 confirmed in 1,104 IBD patients and 750 controls that rs2631367 CC genotype carried OR 1.69 (95% CI 1.29–2.22) versus GG, and that the association extended beyond Crohn's disease to ulcerative colitis with comparable effect sizes (P = 0.0001 for overall IBD). The C allele frequency was 52.5% in IBD cases versus 46.3% in controls.

The functional basis of rs2631367 was confirmed by Tahara et al. (J Pharmacol Exp Ther, 2009)⁸⁸ Tahara et al. (J Pharmacol Exp Ther, 2009)
Tahara H et al., 329(1):262–271, who demonstrated that the -207G allele increases OCTN2 transcriptional activity through a heat shock element, while the -207C allele lacks this enhancing element. The functional consequence of lower OCTN2 expression directly reduces intestinal carnitine uptake capacity.

Noble et al. (Gastroenterology, 2005)⁹⁹ Noble et al. (Gastroenterology, 2005)
Noble CL et al., 129(5):1516–1523 extended the analysis to show that OCTN TC haplotype carriers not only had increased Crohn's disease susceptibility but also greater disease severity — homozygous TC/TC carriers were more likely to require intestinal surgery than TC/non-TC patients, suggesting a gene-dosage effect on clinical course.

One important caveat: the entire IBD5 locus is in high linkage disequilibrium¹⁰¹⁰ linkage disequilibrium
LD describes the tendency for nearby variants on the same chromosome to be inherited together more often than random chance; high LD makes it difficult to determine which variant in a region is the true functional cause, and the question of whether the TC haplotype SNPs are causal or merely tagging unknown causal variants remains scientifically debated. The functional data for -207C>G directly affecting OCTN2 transcription strengthens the causal argument considerably.

Practical Implications

For CC homozygotes (highest risk): the evidence supports optimizing carnitine status through dietary sources and monitoring. Red meat and dairy are the richest dietary carnitine sources; individuals with lower OCTN2 expression may benefit from increased intake of carnitine-rich foods. Given the mucosal barrier implications, maintaining intestinal epithelial health through butyrate-producing dietary fiber is also specifically relevant to OCTN2-mediated intestinal dysfunction. Gastrointestinal symptoms — particularly right lower quadrant abdominal pain, changes in stool frequency, bloody stools, or unexplained weight loss — warrant prompt investigation in CC carriers.

For CG heterozygotes: the intermediate risk elevation (approximately 45% increased OR vs GG) is moderate but clinically notable. Awareness of Crohn's disease early symptoms and family history discussion is appropriate.

Interactions

rs2631367 functions as one half of the IBD5 TC haplotype. Its partner, rs1050152 (SLC22A4 L503F, T allele), is an independent OCTN1 missense variant that alters ergothioneine transport. The combined TC genotype at both loci produces the full IBD5 risk signal; carriers of the C allele at rs2631367 without the T allele at rs1050152 may have somewhat reduced risk compared to TC haplotype homozygotes.

The IBD5 locus interacts with the CARD15/NOD2 locus (chromosome 16): Peltekova et al. showed that TC haplotype homozygosity combined with CARD15 mutations substantially elevated Crohn's disease risk beyond either alone, consistent with independent pathway effects (bacterial sensing via NOD2 + intestinal barrier via OCTN2).

Every time you climb stairs, sprint for a bus, or carry groceries, your muscle cells burn through their internal glycogen stores in the first few seconds of exertion. The enzyme responsible for unlocking those stores is myophosphorylase¹¹ myophosphorylase
muscle-specific glycogen phosphorylase, encoded by PYGM on chromosome 11q13 — and in McArdle disease, it doesn't work. Glycogen accumulates unused inside muscle fibers while the cells starve for the glucose they need.

The rs267606993 variant (c.1A>G, p.Met1Val) eliminates the start codon of PYGM entirely. The coding sequence begins with ATG — methionine — and the A>G substitution on the coding strand converts that to GTG (valine). In theory this creates a missense change, but in practice the next available in-frame methionine is at codon 92, meaning any protein produced from the alternate start site is severely truncated and non-functional. The gene effectively produces nothing useful. ClinVar classifies this variant²² ClinVar classifies this variant
ClinVar VCV000002309, pathogenic, multiple submitters no conflicts as pathogenic for glycogen storage disease type V (GSD-V, McArdle disease).

The Mechanism

In healthy muscle, glycogen phosphorylase cleaves glucose-1-phosphate from glycogen chains at the start of exercise, feeding glycolysis to produce ATP. This process is especially critical in the first 8–10 minutes of sustained exertion, before fatty acid oxidation and increased cardiac output can meet energy demand. Without myophosphorylase, muscle cells cannot access their glycogen reserve at all. They rely entirely on blood glucose and fatty acids from the start of exercise — supplies that arrive too slowly in early exertion, producing the characteristic cramping and weakness of McArdle disease.

The "second wind" phenomenon — one of the most distinctive features of McArdle disease — reflects this metabolic bottleneck: after 8–10 minutes of moderate activity, blood glucose delivery, fat mobilization, and heart rate adaptation catch up, and exercise becomes much more tolerable. Patients who learn to pace themselves through the initial difficulty often exercise reasonably well once the second wind arrives.

The Evidence

The Met1Val variant was first identified by Vorgerd et al.³³ Vorgerd et al.
Vorgerd M et al. Mutation analysis in myophosphorylase deficiency (McArdle's disease). Ann Neurol, 1998 in a homozygous Turkish patient. The same research confirmed that R50X (p.Arg50*) is the dominant PYGM mutation in European white populations (allelic frequency ~60%), while start-codon and other loss-of-function variants account for a smaller but well-documented share of cases.

A larger European cohort study by Vieitez et al.⁴⁴ Vieitez et al.
Vieitez I et al. Molecular and clinical study of McArdle's disease in a cohort of 123 European patients. Neuromuscul Disord, 2011 identified 20 novel PYGM mutations across 123 patients and found no genotype-phenotype correlation — the specific PYGM mutation carried does not predict disease severity. This means a patient with Met1Val can have mild or severe disease independent of which variant they carry.

The Met1Val variant emerged as the most prevalent PYGM mutation in Turkish patients in a study by Inal-Gültekin et al.⁵⁵ Inal-Gültekin et al.
Inal-Gültekin G et al. Myophosphorylase (PYGM) mutations determined by next generation sequencing in a cohort from Turkey with McArdle disease. Neuromuscul Disord, 2017, found in 27 of 67 patients across 11 families, illustrating the population-specific distribution of PYGM alleles.

McArdle disease affects approximately 1 in 100,000–170,000 people globally. Life expectancy is normal, though about 11% of patients develop permanent proximal weakness after age 40 and half experience episodes of myoglobinuria that can threaten kidney function if severe.

Practical Actions

The key interventions for McArdle disease are specific to myophosphorylase deficiency and would not be recommended to anyone without a genetic test:

Pre-exercise sucrose: Consuming 25–40 g of sucrose (a glass of orange juice, a banana, or a sports drink) 5 minutes before exercise significantly improves exercise tolerance by elevating blood glucose before myophosphorylase is needed. This is the single most validated pharmacological/nutritional intervention in McArdle disease, specific to the metabolic block.

The warm-up strategy: Starting exercise at very low intensity for 10 minutes allows the second wind to arrive before high-intensity effort begins. Patients trained in this strategy show dramatically reduced exercise-related cramps and myoglobinuria risk compared to sudden high-intensity bursts.

Myoglobinuria recognition: Dark, cola-colored urine after exercise signals rhabdomyolysis — acute muscle breakdown releasing myoglobin, which is filtered by the kidneys. Any episode requires immediate rest, high fluid intake, and medical evaluation; severe episodes cause acute kidney injury.

Regular aerobic exercise: Supervised aerobic training (starting at 30% of VO2max, increasing gradually) improves mitochondrial capacity and thus reduces dependence on early glycogenolysis. This is a genotype-specific rationale for structured aerobic conditioning that would not be recommended to unaffected individuals.

Interactions

This is an autosomal recessive variant. Compound heterozygosity — inheriting Met1Val on one chromosome and a different pathogenic PYGM variant (such as R50X, Gly205Ser, or any of the ~180 known pathogenic alleles) on the other — produces full McArdle disease phenotype equivalent to homozygosity. The diagnostic implication for carriers is primarily reproductive: if both parents carry any pathogenic PYGM variant, each child has a 25% chance of inheriting biallelic loss-of-function alleles and developing GSD-V.

Note on multi-allelic representation: rs267606993 is a multi-allelic site at the PYGM start codon with three pathogenic alternate alleles on the plus strand (T>A = p.Met1Leu; T>C = p.Met1Val; T>G = p.Met1Leu via different codon). This entry represents the T>C (Met1Val) variant as the primary risk allele (C), consistent with ClinVar VCV000002309. The other two alternates (ClinVar 553271, 156341) have identical clinical significance and are captured under the same rsID.

The GABRA2 gene encodes the alpha-2 subunit of the GABA-A receptor, the brain's primary inhibitory neurotransmitter system.

The alpha-2 subunit participates in transporting chloride ions into neurons, causing hyperpolarization and inhibitory effects .

This subunit is found primarily in the hippocampus and forebrain , and

GABA-A receptors can be modulated by benzodiazepines and other agents that bind to the receptor .

Rs279858 is a synonymous SNP in exon 5 of GABRA2 , meaning it doesn't change the amino acid sequence¹¹ it doesn't change the amino acid sequence
This is called a silent mutation - the DNA changes from T to C, but the protein remains unchanged because both codons specify lysine (K132K).

This variant lies within a 140 kb haplotype block that has been reproducibly associated with alcohol dependence across multiple populations .

The Mechanism

Although rs279858 is synonymous, it has functional consequences²² it has functional consequences
Synonymous variants can affect gene expression through multiple mechanisms: mRNA stability, splicing, translation efficiency, and linkage to regulatory variants.

Research using induced pluripotent stem cells found significantly lower levels of GABRA2 mRNA in neural cell cultures derived from C-allele carriers .

C-allele carriers show a low-expression cluster pattern for all four chromosome 4p12 GABA-A genes , suggesting the variant or linked variants affect expression of the entire gene cluster³³ the variant or linked variants affect expression of the entire gene cluster
The chromosome 4p12 cluster includes GABRG1, GABRA2, GABRA4, and GABRB1.

Reduced GABRA2 expression in the temporal and prefrontal cortex has been linked to higher anxiety levels in rodents . The reduced inhibitory signaling may explain why C-allele carriers experience altered subjective responses to alcohol⁴⁴ altered subjective responses to alcohol
The "high" and stimulation from alcohol and increased anxiety-related traits⁵⁵ increased anxiety-related traits
Including nervous feelings and reduced risk-taking behavior.

The Evidence

Alcohol dependence:

Variants in this haplotype block have been replicated in multiple studies across different populations .

A meta-analysis combining multiple datasets found P=5×10⁻⁶ (OR=1.18) for association with alcohol dependence in Europeans .

In a validation study of 1,032 heroin users and 2,863 controls, the G-allele frequency was significantly higher in substance users (p<0.001, OR=0.84) .

The direction of effect varies by context.

One study found that C-allele carriers reported greater feelings of "high" and liking alcohol's effects . However, among already-dependent drinkers, T-allele homozygotes showed greater stimulation, suggesting the influence differs by stage of dependence .

Higher recent drinking was associated with reduced acute tolerance in risk allele carriers .

Anxiety and behavioral traits:

A phenome-wide association study found the C-allele associated with anxiety-related phenotypes, including reduced risk-taking behavior, increased nervous feelings, and reduced number of lifetime sexual partners .

These traits may be related to anxiety or behavioral inhibition identified as a risk factor for alcohol use disorders .

Neurocircuitry:

The G-allele was associated with heightened nucleus accumbens activation during adolescence , a critical period for addiction vulnerability.

In healthy controls, G-allele carriers showed significantly decreased reward network connectivity compared to A-allele carriers .

Aggression:

In patients with alcohol dependence, carriers of a specific A-C haplotype (rs567926-rs279858) were more likely to demonstrate aggressive behavior , and this rare haplotype (1.6%) was more frequent in Cloninger's type II alcoholism , characterized by early onset and aggression.

Practical Implications

This variant influences how you experience alcohol and your vulnerability to developing problematic drinking patterns. The C-allele is associated with enhanced subjective effects of alcohol — feeling more "high," stimulated, and euphoric from drinking. This heightened response can paradoxically increase risk for alcohol dependence, as the rewarding effects may drive continued use⁶⁶ the rewarding effects may drive continued use
This is called incentive-sensitization theory of addiction.

The C-allele also predisposes to anxiety-related traits.

Reduced GABRA2 expression has been linked to higher anxiety in animal models, and anxiolytic drugs increase GABRA2 expression . If you carry one or two C-alleles, you may benefit from non-pharmacological anxiety management strategies⁷⁷ non-pharmacological anxiety management strategies
These work through multiple mechanisms including HPA axis regulation and neuroplasticity like regular exercise, mindfulness practices, and adequate magnesium intake⁸⁸ adequate magnesium intake
Magnesium acts as a natural GABA-A receptor modulator.

For those with the CC or CT genotype and a family history of alcohol problems, awareness of enhanced alcohol sensitivity is protective. Studies show that education about genetic risk can motivate harm-reduction behaviors⁹⁹ Studies show that education about genetic risk can motivate harm-reduction behaviors
This is called personalized prevention.

Interactions

Rs279858 is in strong linkage disequilibrium with other GABRA2 variants including rs567926, rs279826, and rs279871. These variants form two major haplotypes that differ in addiction risk. Additionally, the chromosome 4p12 region contains a cluster of related GABA-A receptor genes (GABRG1, GABRA2, GABRA4, GABRB1) whose expression appears coordinated. Variants affecting this entire cluster may have compounded effects on GABAergic neurotransmission.

Environmental factors strongly moderate genetic effects. Studies show that GABRA2 variants interact with parental monitoring, peer deviance, and stressful life events to influence externalizing behavior and substance use. The genetic vulnerability is expressed primarily in high-risk environments, consistent with a diathesis-stress model.

Deep beneath the skin, an invisible infrastructure of lymphatic vessels works alongside your veins to return fluid to circulation. The gene PROX1 — prospero homeobox 1¹¹ prospero homeobox 1
a transcription factor named for the Drosophila gene prospero, which controls cell identity — is the master regulator of lymphatic endothelial cell identity. Without PROX1, lymphatic vessels fail to form correctly; without properly formed lymphatic vessels and their valves, venous pressure rises and varicose veins develop. The rs340875 variant lies within an intron of PROX1, likely influencing how much of this transcription factor is produced in the endothelial cells that line lymphatic and venous valves.

The Mechanism

PROX1 sits at the apex of a transcriptional cascade that determines whether an endothelial progenitor cell becomes a lymphatic endothelial cell (LEC) rather than a blood vascular cell. It does this by forming a complex with β-catenin and TCF7L1²² forming a complex with β-catenin and TCF7L1
components of the Wnt signaling pathway, which translates mechanical and biochemical cues into gene expression changes to amplify expression of two subordinate transcription factors, FOXC2 and GATA2, that are essential for the assembly and maintenance of lymphatic valve leaflets.

When PROX1 is deleted from valvular endothelial cells³³ When PROX1 is deleted from valvular endothelial cells
Ho et al. 2023 Circulation Research, the consequence is progressive valve degeneration: aortic and mitral valves become thick and myxomatous, FOXC2 expression falls, and extracellular matrix composition is disrupted with excess proteoglycan accumulation. PROX1 suppresses PDGF-B signaling to maintain healthy valve architecture; without it, PDGF-B runs unchecked and the valve structure breaks down.

rs340875 is an intronic variant — it does not change the PROX1 protein sequence, but intronic variants in transcription factor genes can alter splicing efficiency, mRNA stability, or regulatory element activity, affecting the total amount of functional protein produced in endothelial tissues. The C allele is the risk-associated allele identified at the PROX1 locus in varicose veins genetics.

The Evidence

Ahmed et al. 2022⁴⁴ Ahmed et al. 2022
Genome-wide association analysis and replication in 810,625 individuals with varicose veins, Nature Communications conducted the largest varicose veins GWAS to date: 135,514 cases and 675,111 controls drawn from UK Biobank (401,656 individuals) and the 23andMe research cohort (408,969 individuals). The study identified 49 independent genetic signals across 46 susceptibility loci, mapping 237 genes. Pathway analysis confirmed enrichment in lymphangiogenesis — implicating genes like PROX1 that regulate lymphatic vessel formation and valve integrity.

The PROX1 gene's role in valve disease extends beyond varicose veins. PROX1 is classified as required for the development of both lymphatic and venous valves; its loss from endothelial cells is sufficient to cause progressive myxomatous degeneration⁵⁵ sufficient to cause progressive myxomatous degeneration
myxomatous meaning the valves accumulate abnormal proteoglycan-rich matrix that makes them floppy and incompetent of aortic and mitral valves. High PROX1 expression in specific endothelial cells is also the initiating signal for lymphatic valve specification⁶⁶ initiating signal for lymphatic valve specification
Qu et al. 2015 Developmental Biology; cells that fail to upregulate PROX1 do not enter the valve-forming program.

The evidence level for rs340875 specifically is moderate: the PROX1 locus is robustly identified in a very large GWAS, and the biological mechanism is well-established, but the specific functional effect of rs340875 on PROX1 expression or splicing has not been independently characterized at the molecular level.

Practical Actions

For people carrying one or two C alleles at rs340875, the most directly relevant interventions address the lymphatic-venous drainage impairment that underlies varicose veins and venous insufficiency. Compression therapy is the first-line evidence-based intervention for venous reflux regardless of cause; for carriers of a lymphatic gene variant, it directly compensates for the drainage deficit that PROX1 reduction creates. Elevation reduces hydrostatic pressure in the dependent leg veins. Avoiding prolonged static postures — standing or sitting without movement — is specifically important when lymphatic pump function is genetically compromised.

Interactions

PROX1 operates at the top of the lymphatic transcriptional hierarchy. Its downstream targets include FOXC2 (encoded by FOXC2 at chromosome 16q24.1 — mutations in FOXC2 cause lymphedema- distichiasis syndrome) and GATA2. Variants in FOXC2, GATA2, or other lymphatic valve genes interacting with PROX1 may compound the risk of venous insufficiency in carriers of the rs340875 C allele, although specific gene-gene interaction data for this variant are not yet published.

Among the three independent protein-coding protective variants in TYK2, Ala928Val stands apart for the strength of its individual effect. While rs12720356 (Ile684Ser) confers roughly 14% protection per allele and rs34536443 (Pro1104Ala) roughly 24% per allele, the A928V variant confers an odds ratio of 0.53¹¹ odds ratio of 0.53
OR 0.53 means approximately 47% lower odds of rheumatoid arthritis per allele — one of the strongest individual coding-variant effects documented for any autoimmune disease at this sample scale for RA per allele in the largest fine-mapping study conducted at this locus. It is also the rarest of the three, with a minor allele frequency of approximately 0.8% in Europeans and near-zero frequency in East Asian and African populations.

TYK2²² TYK2
Tyrosine kinase 2, a Janus kinase (JAK) family member that transduces signals from cell-surface receptors for IL-12, IL-23, and type I interferons (IFN-α/β) into intracellular gene expression changes driving T cell activation and inflammatory amplification controls the signaling intensity of three of the most important inflammatory cytokine axes in autoimmune disease. Its pseudokinase (JH2) domain is a regulatory scaffold — a non-catalytic structure that modulates the adjacent kinase (JH1) domain and determines how strongly each cytokine signal is amplified.

The Mechanism

TYK2 mediates downstream signaling for the IL-12 receptor (activating STAT4, driving Th1 differentiation), the IL-23 receptor (activating STAT3/STAT4, driving Th17 responses), and the type I interferon receptors IFNAR1/2 (activating JAK1-STAT1/STAT2, driving antiviral and lupus-relevant responses). The JH2 pseudokinase domain acts both as an autoinhibitory brake on basal activity and as a positive regulator that amplifies signal intensity when cytokine receptors are engaged.

The Ala928Val substitution occurs within the JH2 domain at a position that contributes to the intradomain contacts that maintain the regulatory architecture. Replacing the small, non-polar alanine with the bulkier, branched valine introduces steric constraints that partially disrupt the JH2 domain's ability to positively regulate JH1 catalytic function. The result is a [hypomorphic TYK2 | Hypomorphic means partially reduced function rather than complete abolition — the protein is present and active but with reduced signal amplification capacity] that retains sufficient activity for antiviral and homeostatic signaling but measurably dampens the inflammatory amplification loops most relevant to autoimmune tissue damage.

All three TYK2 missense variants (P1104A, I684S, and A928V) were predicted damaging by both PolyPhen-2 and SIFT³³ predicted damaging by both PolyPhen-2 and SIFT, consistent with their functional impairment of the JH2 domain.

The Evidence

The primary genetic evidence comes from Diogo et al. (2015)⁴⁴ Diogo et al. (2015), who combined dense Immunochip genotyping (23,092 RA case/control samples), Exomechip genotyping (18,409 subjects), and targeted exon sequencing (2,236 samples). Conditional analysis and haplotype analysis confirmed that P1104A, I684S, and A928V each lie on distinct haplotype backgrounds and each contribute an independent signal. The A928V effect (OR 0.53, P=1.2×10⁻⁹) survives conditioning on both other signals, establishing it as a genuine causal variant rather than a tag for its sibling alleles. The same omnibus test combining all three TYK2 variants found joint protection against SLE at P=6×10⁻¹⁸.

The 2021 systematic review and meta-analysis by Pellenz et al.⁵⁵ 2021 systematic review and meta-analysis by Pellenz et al. (34 studies, 8 autoimmune conditions) confirmed rs35018800's protective minor allele association across multiple autoimmune diseases alongside the other four TYK2 protective SNPs.

The biological plausibility of A928V is strengthened by parallel cellular mechanistic work on the TYK2 JH2 variant class. Gorman et al. (2019)⁶⁶ Gorman et al. (2019) showed that JH2-domain impairment specifically limits TYK2 signaling under multi-pathway co-activation — the scenario characteristic of active autoimmune disease — while preserving single-pathway responses needed for infection control. Enerbäck et al. (2018)⁷⁷ Enerbäck et al. (2018) demonstrated directly in human blood that a neighboring JH2 variant (I684S) reduces IL-12- stimulated STAT4 phosphorylation in skin-homing T cells, providing the cellular readout expected from A928V's structurally analogous JH2 impairment.

Notably, unlike TYK2 P1104A (rs34536443), no published study has identified a cancer immune- surveillance trade-off (elevated lung cancer or NHL) associated with the A928V variant. This may reflect the variant's rarity — it is statistically underpowered for cancer-association analysis in existing datasets — but no signal has emerged in the large PheWAS performed by Diogo et al. across more than 500 phenotypes.

Practical Implications

With a European MAF of approximately 0.8%, about 1.6% of Europeans carry at least one A allele at rs35018800 (nearly all heterozygous AG, with AA homozygotes vanishingly rare). The variant is essentially absent in East Asian, South Asian, and African populations, making it primarily a European-ancestry finding in current databases.

The OR 0.53 per allele is among the strongest individual SNP effects documented for common autoimmune diseases. For a heterozygous carrier, this translates to approximately 47% lower odds of RA from this locus alone — comparable in effect size to some drug treatments. This result is relevant in the same clinical contexts as the other TYK2 protective alleles: autoimmune disease workup interpretation, family history counseling for RA and SLE, and biologic therapy discussions. If deucravacitinib or a JAK inhibitor is prescribed, the A928V allele contributes an independent layer of baseline TYK2 JH2 attenuation that the prescriber should be aware of.

Interactions

rs35018800 (A928V) is confirmed by haplotype analysis to reside on a distinct haplotype background from rs34536443 (P1104A, MAF ~4% in Europeans) and rs12720356 (I684S, MAF ~8% in Europeans). An individual who carries protective alleles at more than one of these three loci has multiple independent layers of TYK2 JH2 attenuation — the protections are additive in principle because the structural disruptions occur at different intradomain contacts.

rs2304256 (V362F) in TYK2 operates through a separate mechanism involving exon 8 splicing and FERM domain receptor binding. Its GWAS association signal in SLE and RA has been shown to be largely driven by linkage disequilibrium with the coding variants including I684S, and it is functionally and genetically independent from A928V.

Beyond TYK2, A928V operates within the same autoimmune genetic architecture as rs2476601 (PTPN22 R620W, T cell receptor threshold) and rs3087243 (CTLA4 CT60, costimulation threshold). These variants modulate T cell activation at distinct checkpoints and likely provide additive protection when co-inherited with protective TYK2 alleles, though formal compound heterozygosity studies have not been published.

While rs2275913 acts like a volume dial at the IL-17A promoter — controlling how much mRNA is produced when T cells activate — rs3748067 works at the other end of the gene, in the 3' untranslated region (3'UTR)¹¹ 3' untranslated region (3'UTR)
the section of mRNA after the protein-coding sequence, which contains binding sites for microRNAs and other regulatory molecules that control how much protein is ultimately made. Together, these two variants shape the IL-17A Th17 axis through complementary but distinct mechanisms.

The Mechanism

The rs3748067 variant lies at position c.*1249 in the IL17A 3'UTR, placing it in a region where microRNAs (miRNAs)²² microRNAs (miRNAs)
small non-coding RNA molecules that bind to mRNA 3'UTRs and suppress protein translation or trigger mRNA degradation regulate IL-17A protein output. The common C allele is associated with higher IL-17A protein levels in population studies — this may reflect impaired miRNA-mediated suppression at this site. A parallel study identified that pri-miR-938, which targets the IL17A 3'UTR, is associated with gastric cancer susceptibility through this same regulatory region, suggesting that 3'UTR variants and their miRNA regulators act together on IL-17A expression.

Unlike the promoter variant rs2275913, which has a well-characterized NFAT-binding mechanism, the exact miRNA responsible for 3'UTR regulation at rs3748067 has not been conclusively identified in published functional studies. The clinical associations across multiple diseases nevertheless suggest this 3'UTR position exerts meaningful regulatory influence on post-transcriptional IL-17A output.

This variant is part of the same IL17A linkage disequilibrium (LD) block³³ IL17A linkage disequilibrium (LD) block
a chromosomal region where SNPs tend to be inherited together as rs2275913, and the two variants are studied together in haplotype analyses. Despite being in partial LD, each variant retains independent information, and their combination is the most clinically studied unit for IBD susceptibility.

The Evidence

Ulcerative colitis (UC): The clearest evidence for rs3748067's clinical relevance comes from a Japanese case-control study of 202 UC patients and 475 controls (Kageyama et al., Clin Exp Med 2013)⁴⁴ (Kageyama et al., Clin Exp Med 2013). The haplotype combining rs2275913 AA (high-transcription promoter) and rs3748067 CC (unmodified 3'UTR, higher protein output) conferred a 3.38-fold UC risk compared to the low-risk haplotype (p=0.0007) — substantially higher than either variant alone. This synergy between promoter and 3'UTR variants illustrates how the two variants cooperate to amplify total IL-17A output.

Gastric cancer: A Japanese study examining rs3748067 in 337 gastric cancer cases and 587 controls found the T allele to be protective for intestinal-type GC (OR 0.511, 95%CI 0.272–0.962)⁵⁵ (OR 0.511, 95%CI 0.272–0.962), and inversely correlated with lymph node metastasis. However, a 2018 meta-analysis of 9 studies did not find a significant overall association⁶⁶ did not find a significant overall association, suggesting the gastric cancer finding may be population-specific.

Tuberculosis susceptibility: Two meta-analyses found the TT genotype associated with increased TB susceptibility in Asian populations (OR 1.36, 95%CI 1.03–1.79)⁷⁷ (OR 1.36, 95%CI 1.03–1.79). This apparent paradox — the T allele protective in cancer but harmful in infection — is consistent with IL-17A's dual biology: it drives pathological inflammation in autoimmune disease but provides essential protection against extracellular bacteria and fungi. If T allele reduces IL-17A output, users have lower Th17-mediated mucosal immunity against pathogens like Mycobacterium tuberculosis.

Coronary artery disease: A meta-analysis of 6 studies in 3,542 CAD cases and 3,212 controls found the TT genotype significantly protective in Asians (OR 0.37)⁸⁸ (OR 0.37), consistent with the idea that lower IL-17A output reduces vascular inflammation and atherosclerosis risk.

The collective evidence supports a model where the C allele at rs3748067 is associated with higher IL-17A protein levels, increasing risk for inflammatory and autoimmune conditions but potentially providing stronger Th17-mediated bacterial defenses, while the T allele correlates with lower IL-17A output, reducing inflammatory disease risk but decreasing Th17 immune protection.

Practical Actions

Because rs3748067 and rs2275913 operate through the same IL-17A protein, the practical interventions targeting the Th17 axis apply to both variants. For individuals carrying the CC genotype at rs3748067 — especially in combination with the rs2275913 A allele — the key priority is supporting natural regulation of IL-17A output.

Three interventions with evidence in the Th17 pathway are particularly relevant for the CC genotype: maintaining vitamin D3 status in the 40–60 ng/mL range (active vitamin D suppresses IL-17A transcription at the promoter level, complementing 3'UTR regulation), EPA/DHA omega-3 supplementation (EPA-derived prostaglandin D3 suppresses Th17 differentiation and IL-17A production), and multi-strain probiotics with Lactobacillus and Bifidobacterium strains that shift Th17/Treg balance.

For individuals with the TT genotype, the concern is less about inflammatory overactivation and more about potentially reduced Th17 immune defense. This is most clinically relevant in high-risk infectious environments or in individuals with known TB exposure.

Interactions

The most important interaction is with rs2275913⁹⁹ rs2275913
the IL-17A promoter variant at -197G>A that directly controls IL-17A transcription via NFAT binding. The rs2275913 AA + rs3748067 CC haplotype confers 3.38-fold UC risk — the highest IL-17A output state when both transcriptional (promoter) and post-transcriptional (3'UTR) mechanisms drive high protein production. Check your rs2275913 result alongside this variant for the full picture of your IL-17A axis activity.

IL17F rs763780¹⁰¹⁰ IL17F rs763780
a coding variant that reduces IL-17F bioactivity further modulates the Th17 output profile: individuals with high IL-17A output (rs2275913 AA, rs3748067 CC) and functional IL-17F have the highest combined Th17 cytokine activity.

PTPN22 rs2476601¹¹¹¹ PTPN22 rs2476601
a phosphatase variant that lowers the T cell activation threshold can compound autoimmune susceptibility in individuals who also carry high-IL-17A genotypes.

Cardiac myosin-binding protein C¹¹ Cardiac myosin-binding protein C
a large, modular protein encoded by MYBPC3 on chromosome 11p11.2 that integrates into the thick filament of the cardiac sarcomere, where it fine-tunes contraction by regulating myosin head positioning and cross-bridge cycling rate is the most frequently mutated protein in hypertrophic cardiomyopathy (HCM), accounting for roughly 40% of all genotype-positive cases. Among the hundreds of MYBPC3 pathogenic variants, the p.Arg502Trp missense mutation stands out: it is the single most recurrent HCM-causing point mutation identified in large European-descent cohorts, found in approximately 2.4% of all HCM patients screened. This variant is classified Pathogenic/Likely Pathogenic by 31 of 40 ClinVar submitters, with three-star expert review status.

The Mechanism

The MYBPC3 protein is built from ten immunoglobulin-like (Ig) and fibronectin-type III (FN3) domains²² immunoglobulin-like (Ig) and fibronectin-type III (FN3) domains
barrel-shaped protein folds numbered C0 through C10 from the N-terminus; the central C3-C6 region is one of the most common mutation hotspots in HCM. Arg502 sits on the surface of the C3 domain, positioned on a loop exposed to the cytoplasm. Unlike many HCM missense variants that destabilize the domain itself, the NMR structure of the Arg502Trp mutant C3 domain³³ NMR structure of the Arg502Trp mutant C3 domain
solved by Inchingolo et al. in 2014 using solution NMR spectroscopy, deposited as PDB 2MQ3 reveals preserved immunoglobulin-like folding — the domain remains structurally intact. Instead, replacing the positively charged, polar arginine with a bulky, hydrophobic tryptophan dramatically alters the electrostatic surface of C3. This change is predicted to disrupt binding between MYBPC3 and sarcomeric partner proteins that dock on this domain, corrupting the protein's regulatory role in cross-bridge cycling without affecting its incorporation into the sarcomere. The net result is an unrestrained, hypercontractile state characteristic of HCM: excessive myosin engagement, impaired relaxation, and ultimately cardiomyocyte hypertrophy, myofibrillar disarray, and interstitial fibrosis.

The Evidence

The foundational clinical study by Saltzman et al. (Circ Res, 2010)⁴⁴ Saltzman et al. (Circ Res, 2010) screened 1,414 unrelated HCM probands and identified Arg502Trp in 34 (2.4%) — the highest single-variant frequency in a large, unselected HCM cohort. Extending analysis to 17 families identified a total of 77 carriers. Family segregation yielded a calculated odds ratio of 11,000:1 for co-inheritance of the variant with HCM. Penetrance was age-dependent: roughly 50% of carriers lacked clinical evidence of HCM before age 45. Yet by age 50, approximately 30% of carriers had experienced a major adverse cardiac event (death, cardiac arrest, hemodynamically significant arrhythmia, or heart failure hospitalization). When Arg502Trp occurred alongside a second sarcomere gene mutation, 75% of carriers had a serious event before age 20 — a clinical severity approaching that of the most aggressive MYH7 mutations.

Haplotype analysis across all 17 families revealed at least 4 independent chromosomal backgrounds carrying the same mutation, indicating recurrent de novo origins rather than a single ancient founder. This explains why Arg502Trp appears broadly across European populations rather than clustering in a single ethnic group.

The 2024 AHA/ACC HCM guideline (Ommen et al., Circulation)⁵⁵ 2024 AHA/ACC HCM guideline (Ommen et al., Circulation) establishes the current management framework: all pathogenic sarcomere variant carriers require annual echocardiographic surveillance; symptomatic obstructive HCM should first receive beta-blockers or non-dihydropyridine calcium channel blockers, followed by mavacamten (FDA-approved 2022, Class I recommendation in 2024 guidelines) or disopyramide for refractory cases before considering invasive septal reduction therapy.

Practical Actions

For heterozygous Arg502Trp carriers, the practical priorities are: (1) establish baseline cardiovascular assessment with echocardiography, (2) commence annual surveillance even if currently asymptomatic — penetrance is age-dependent and over 50% of carriers will develop overt HCM by their sixth decade, (3) avoid competitive sport until HCM is excluded or appropriately risk-stratified, and (4) ensure all first-degree relatives are offered targeted genetic testing.

When obstructive HCM develops (resting or provoked left ventricular outflow tract gradient ≥30 mmHg), beta-blockers are the first-line treatment. Mavacamten, a cardiac-specific myosin inhibitor that directly reduces the proportion of force-generating myosin heads, is a guideline-endorsed second-line agent for obstructive HCM and operates upstream of the mutation by normalizing cross-bridge cycling — making it mechanistically well-suited for sarcomere protein variants including MYBPC3. Disopyramide added to beta-blockade is an alternative. Surgical septal myectomy at experienced HCM centres remains the reference standard for drug-refractory outflow obstruction.

ICD implantation is guided by formal sudden cardiac death (SCD) risk scoring (ESC HCM Risk-SCD calculator or AHA/ACC 5-year MACE estimate). A prior cardiac arrest, sustained ventricular tachycardia, family history of HCM-related SCD in first-degree relatives, maximal LV wall thickness ≥30 mm, non-sustained VT on Holter monitoring, and unexplained syncope are all established risk factors that should inform the shared decision regarding ICD.

Interactions

The clinical severity of MYBPC3 Arg502Trp increases substantially when a second pathogenic sarcomere variant is present (compound heterozygosity or double heterozygosity). Saltzman et al. reported that carriers of Arg502Trp plus a second sarcomere gene mutation had 75% major event rates before age 20 — indistinguishable from the most aggressive single-gene mutations. Any first-degree relative found to carry both this variant and an additional sarcomere pathogenic variant should be evaluated urgently by an HCM specialist. Other MYBPC3 pathogenic variants already in this database — including rs187830361 (Trp792Arg), rs193922385 (Arg177Cys), rs397514752 (Gly490Val), and rs36211723 (Asp770Asn) — represent independent HCM-causing mutations in MYBPC3; compound heterozygosity for two such mutations is rare but clinically severe.

Toll-like receptor 3 (TLR3)¹¹ Toll-like receptor 3 (TLR3)
TLR3 is the innate immune system's dedicated sensor for double-stranded RNA (dsRNA) — the molecular signature produced when RNA viruses replicate inside cells stands guard in endosomes and on cell surfaces, scanning for viral invaders. When TLR3 detects dsRNA, it triggers production of type I interferons (IFN-α/β)²² type I interferons (IFN-α/β)
Interferons are signaling proteins that put neighboring cells into an antiviral state, slowing viral spread and activating natural killer cells and cytotoxic T lymphocytes — the body's fastest antiviral alarm. The rs3775291 variant (c.1234C>T), which swaps leucine for phenylalanine at amino acid 412, turns that alarm down significantly. Carriers produce a structurally altered receptor that binds viral dsRNA at roughly half the affinity of the normal form.

This variant is remarkably common in Europeans — the T allele appears in about 34% of European blood donors, with roughly 13% of Europeans carrying two copies (TT). By contrast, the T allele is far rarer in African and East Asian populations (approximately 5% and 10% respectively), a pattern that likely reflects ancient differences in viral exposure history during human migration and settlement.

The Mechanism

The leucine-to-phenylalanine substitution occurs in TLR3's ectodomain³³ ectodomain
The extracellular portion of TLR3 that directly contacts dsRNA ligands, forming the binding interface — specifically in a loop region that contacts the dsRNA phosphate backbone. A landmark functional study found that the L412F mutation reduces TLR3's dsRNA-binding capacity to just 51% of wild-type levels⁴⁴ landmark functional study found that the L412F mutation reduces TLR3's dsRNA-binding capacity to just 51% of wild-type levels
Measured using cell-based binding assays with poly(I:C), a synthetic dsRNA mimic; protein expression and surface distribution were unaffected. Critically, the protein is made in normal amounts and traffics correctly — the defect is purely in binding affinity, not receptor availability.

Downstream consequences include reduced NF-κB activation⁵⁵ NF-κB activation
NF-κB is a master transcription factor that drives expression of hundreds of immune genes including cytokines, adhesion molecules, and antiviral effectors, reduced IFN-β production, and — as more recent work has revealed — impaired macroautophagy⁶⁶ macroautophagy
The cellular process of engulfing and digesting damaged organelles and intracellular pathogens; critical for clearing virally infected material. This autophagy impairment appears particularly important in severe viral infections, potentially slowing the cell's ability to dispose of viral particles.

The Evidence

The clinical consequences of half-normal TLR3 function play out differently across diseases, sometimes strikingly in opposite directions.

Viral infections — increased susceptibility: A meta-analysis of 18 case-control studies (3,118 cases and 4,368 controls) found the T allele associated with a 16% increase in overall infectious disease risk⁷⁷ meta-analysis of 18 case-control studies (3,118 cases and 4,368 controls) found the T allele associated with a 16% increase in overall infectious disease risk
Dominant model OR=1.16, 95% CI 1.04–1.28, p=0.004; diseases included hepatitis B/C, HIV, dengue, tick-borne encephalitis, herpes simplex virus, and Japanese encephalitis. However, results were geographically heterogeneous: in European studies the T allele was actually protective (OR=0.83), while Asian and American studies showed increased risk (OR=1.37 and 1.42 respectively). This likely reflects differences in which pathogens dominate in each region and how TLR3 interacts with other immune loci.

For herpes simplex encephalitis (HSE), rare but serious TLR3 pathway mutations explain approximately 5% of childhood HSE cases⁸⁸ rare but serious TLR3 pathway mutations explain approximately 5% of childhood HSE cases
Among 120 HSE patients studied, 6 (5%) had TLR3 mutations; affected children had 66% HSE recurrence vs 10% in the broader cohort. While rs3775291 is a common hypomorphic variant rather than a complete loss-of-function mutation, it exemplifies the principle that TLR3 pathway competence is non-redundant in the brain during primary HSV infection in children. Separate case studies have documented HSE in adults heterozygous for TLR3 mutations.

For HIV, the T allele appears protective in highly exposed Europeans — 80% of highly exposed HIV-seronegative intravenous drug users carried at least one T allele versus 55% in blood donor controls⁹⁹ 80% of highly exposed HIV-seronegative intravenous drug users carried at least one T allele versus 55% in blood donor controls
Study of 497 blood donors and highly exposed seronegative cohort; OR=0.25 for HIV seropositivity among T allele carriers, 95% CI 0.07–0.87. This paradox (less TLR3 function = better HIV resistance) may relate to HIV's ability to exploit TLR3 signaling for its own replication, making a less responsive receptor protective in this particular context.

Age-related macular degeneration — protective: Perhaps the most consistent finding is that the TT genotype protects against geographic atrophy (advanced dry AMD)¹⁰¹⁰ protects against geographic atrophy (advanced dry AMD)
Meta-analysis of 9 studies with 7,400 AMD cases and 13,579 controls; protective OR=0.88 in recessive model, specifically in Caucasians. The biological rationale is compelling: in the aging retina, TLR3 on retinal pigment epithelial cells responds to debris from dying photoreceptors, triggering inflammatory damage. Reduced TLR3 activation attenuates this harmful cycle, protecting vision. This is a case where weaker receptor function is genuinely beneficial.

Pulmonary sarcoidosis: Two independent Caucasian cohorts (Irish and American) found TLR3 L412F significantly associated with persistent clinical disease¹¹¹¹ Two independent Caucasian cohorts (Irish and American) found TLR3 L412F significantly associated with persistent clinical disease
Irish cohort: n=228 patients, 263 controls; American replication cohort: n=123 patients; fibroblasts from 412F-homozygous patients showed reduced IFN-β and dysregulated fibroproliferative responses. Impaired viral clearance may trigger granuloma formation that fails to resolve, locking the disease into a chronic state.

COVID-19: The L412F polymorphism emerged as a marker of COVID-19 severity, particularly in males¹²¹² a marker of COVID-19 severity, particularly in males
Association strengthened in male sub-cohorts; mechanistic link via impaired autophagy, which slows clearance of viral material. Among COVID-19 patients treated with hydroxychloroquine (which further impairs autophagy), L412F carriers showed significantly reduced 28-day survival (p=0.038), suggesting the autophagy pathway is specifically important in this context.

Practical Implications

The duality of this variant — harmful for some viral infections, protective for macular degeneration — means the practical guidance depends heavily on context. TT homozygotes have the strongest protection against AMD but the most reduced antiviral dsRNA sensing. CT heterozygotes sit in between: partially reduced TLR3 function with correspondingly intermediate risks and benefits.

For viral defense, the most actionable insight is vigilance around prevention — ensuring vaccinations are current, minimizing high-risk exposures, and seeking early medical care for neurological symptoms after herpes virus infections. For long-term eye health, TT carriers have a genuine genetic advantage that does not require any active management.

Interactions

TLR3 works within a broader innate immune network. TLR9 (rs352140) senses CpG-rich bacterial and viral DNA¹³¹³ TLR9 (rs352140) senses CpG-rich bacterial and viral DNA
TLR3 senses viral dsRNA; TLR9 senses microbial DNA — together they cover complementary pathogen signatures, while TLR4 (rs4986790) responds to bacterial endotoxin (LPS)¹⁴¹⁴ TLR4 (rs4986790) responds to bacterial endotoxin (LPS)
These three TLRs collectively provide pattern recognition for the major classes of microbial threats. Carriers of multiple hypomorphic TLR variants may have compoundly reduced innate immune surveillance, particularly relevant for individuals with TLR3 L412F combined with TLR4 Asp299Gly — both reduce first-line pattern recognition.

The rs5743305 variant (promoter region, −926bp) reduces TLR3 transcription and has been co-studied with rs3775291 — carriers of both variants showed the lowest anti-viral antibody responses in measles vaccine studies. rs3775290 (exon 4, Phe459Phe) is a silent SNP often studied alongside rs3775291 in haplotype analyses of TLR3.

HLA-B27¹¹ HLA-B27
Human Leukocyte Antigen B*27 — a class I MHC molecule encoded in the highly polymorphic HLA region of chromosome 6, present in approximately 8% of Europeans and 60–90% of ankylosing spondylitis patients worldwide represents the single strongest genetic association with any common disease known to medicine — individuals carrying this allele face a roughly 60-fold increased risk for ankylosing spondylitis (AS) compared to non-carriers. Direct HLA typing is complex and expensive, but rs4349859 — a common intronic SNP located 41 kb centromeric of the HLA-B gene and 5.4 kb telomeric of MICA — serves as a highly accurate genetic proxy for European HLA-B27 subtypes, making it possible to infer HLA-B27 status directly from standard genotyping arrays.

The A allele of rs4349859 arose on a chromosome bearing HLA-B27 in an ancestor shared by nearly all European and some Asian HLA-B27 carriers. This ancient founder event means the A allele travels with HLA-B27 in strong linkage disequilibrium²² linkage disequilibrium
a statistical association between alleles at nearby loci that persists across generations because recombination has not yet broken apart the ancestral haplotype, allowing the SNP to stand in for direct HLA-B27 typing in research and clinical screening applications.

The Mechanism

rs4349859 is an intronic variant in MICA-AS1 (MICA antisense RNA 1), a non-coding RNA gene located between HLA-B and MICA in the MHC class I region. The SNP itself has no known functional consequence — it does not change any protein or alter gene expression. Its clinical relevance derives entirely from the haplotype it marks: the A allele at rs4349859 is a reliable indicator that the individual carries an HLA-B27 allele³³ HLA-B27 allele
most commonly HLA-B*27:05 in Europeans, the primary AS-associated subtype; also tags B*2702, B*2708, and B*2709 on the same chromosome.

HLA-B27 itself causes disease through several proposed mechanisms. The arthritogenic peptide hypothesis holds that HLA-B27 presents self-peptides (or microbial peptides with structural similarity to self) to CD8+ T cells, triggering an autoimmune cascade directed at joint tissues. Supporting this model, variants in ERAP1 — the enzyme that trims peptides before they are loaded onto HLA-B27 — affect AS risk exclusively in HLA-B27-positive individuals, directly implicating the peptide-HLA-B27 interaction in disease pathogenesis. A complementary hypothesis involves the spontaneous misfolding of HLA-B27 heavy chains into homodimers, which activate NK cells and innate lymphocytes independently of peptide presentation.

The Evidence

rs4349859 as HLA-B27 proxy. In the landmark epistasis study by Evans et al.⁴⁴ Evans et al.
Interaction between ERAP1 and HLA-B27 in ankylosing spondylitis implicates peptide handling in the mechanism for HLA-B27 in disease susceptibility. Nature Genetics, 2011, rs4349859 was used as the HLA-B27 proxy across a discovery cohort of 1,787 British and Australian cases plus 4,800 controls and a replication cohort of 2,111 cases and 4,483 controls. The SNP achieved 98.0% sensitivity and 99.0% specificity for HLA-B27 in the European ancestry subset (538 cases and 741 controls with known HLA-B27 status). The dominant model (A allele present = HLA-B27 positive) was used throughout, with the study confirming that ERAP1 variants have no detectable effect on AS risk in rs4349859 GG individuals (presumed HLA-B27-negative), but powerfully modify risk in A-allele carriers.

Population prevalence. A comprehensive New Zealand population study of 1,220 Caucasian controls found rs4349859 genotype frequencies of GG: 90.7%, AG: 8.8%, AA: 0.4%, corresponding to an A-allele frequency of ~4.7%⁵⁵ corresponding to an A-allele frequency of ~4.7%, consistent with the approximately 8–9% HLA-B27 prevalence in Northern European populations (heterozygotes carry one A allele; both A alleles in homozygotes are rarer). Concordance with direct serological HLA-B27 typing was 98.7–100% in European-ancestry individuals. The SNP tags all major European AS-associated subtypes (B*2702, B*2705, B*2708) but does not tag African (B*2703) or most Asian subtypes (B*2704, B*2706, B*2707), limiting its utility to European-ancestry individuals.

HLA-B27 and AS risk. The overall odds ratio for AS given HLA-B27 positivity is approximately 60, the strongest common-variant disease association documented in human genetics. Despite this extraordinary OR, only 1–2% of HLA-B27-positive individuals develop AS in their lifetime⁶⁶ Despite this extraordinary OR, only 1–2% of HLA-B27-positive individuals develop AS in their lifetime
population attributable risk is nonetheless substantial because B27 prevalence is 8% in Europeans and 60-90% of AS cases carry B27. Risk rises to approximately 20% if a first-degree relative has AS. HLA-B27 accounts for approximately 25% of AS heritability, with 116 additional genetic loci contributing ~30% combined.

Practical Implications

Knowing your rs4349859 genotype provides a genetic proxy for HLA-B27 status without the need for traditional HLA typing. For GG individuals (no A allele), the probability of carrying HLA-B27 is below 2%; residual risk comes from rare B27-positive individuals whose HLA-B27 subtype is not tagged by this SNP. For AG individuals (one A allele), HLA-B27 positivity is overwhelmingly likely — approximately 90% of A-allele carriers in European populations are HLA-B27 positive. For AA individuals (two A alleles), HLA-B27 positivity is essentially certain, and these individuals likely carry two HLA-B27 alleles.

The clinical significance of HLA-B27 positivity depends heavily on context. In isolation, carrying HLA-B27 (A allele at rs4349859) means a modestly elevated lifetime risk of AS (~1–2% absolute) that rises substantially with additional genetic risk (ERAP1 variants) or family history. The most important implication is that inflammatory back pain symptoms in an HLA-B27-positive individual should be evaluated rapidly for axial spondyloarthritis, since early treatment with NSAIDs has disease-modifying effects on radiographic progression.

Interactions

rs4349859 × ERAP1 variants (rs26653, rs30187, rs10050860): Epistasis in ankylosing spondylitis. This is the defining interaction for the entire ERAP1 locus. Evans et al. 2011 demonstrated unequivocally that all ERAP1 AS associations are conditional on HLA-B27 status⁷⁷ Evans et al. 2011 demonstrated unequivocally that all ERAP1 AS associations are conditional on HLA-B27 status
ERAP1 SNPs show zero association with AS in HLA-B27-negative individuals (GG at rs4349859), while in HLA-B27-positive individuals (AG or AA at rs4349859), ERAP1 risk alleles confer 3–4-fold differences in AS risk. The combined interaction p-value was 7.3 × 10⁻⁶. This epistasis is mechanistically coherent: ERAP1 trims peptides that are subsequently loaded onto HLA-B27; HLA-B27 status determines whether the trimmed peptide repertoire reaches a disease-triggering threshold.

rs4349859 × rs12191877 (HLA-C*06:02 proxy): Co-occurring immune susceptibility. HLA-C*06:02 is the primary genetic risk factor for psoriasis. Individuals carrying both HLA-B27 (rs4349859 A allele) and HLA-C*06:02 (rs12191877 A allele) are at risk for both AS and psoriasis and face elevated risk for psoriatic arthritis, a spondyloarthritis subtype overlapping both conditions. ERAP1 variants interact with both HLA alleles, albeit through partially distinct mechanisms and peptide repertoires.

Your hunger, fullness, and metabolic rate are governed by a constant dialogue between hormones in the bloodstream and neurons in the hypothalamus. At the center of this dialogue sits SH2B1, an adaptor protein that amplifies the signals from two of the most critical metabolic hormones: leptin (the long-term satiety signal from fat cells) and insulin (the short-term glucose regulator from the pancreas). rs4788102 is an intronic variant in SH2B1 on chromosome 16p11.2 — part of a genomic region robustly linked to body mass index in multiple large genome-wide association studies¹¹ genome-wide association studies
Studies that simultaneously examine hundreds of thousands of genetic variants across the genome to identify which ones correlate with a trait like BMI across tens of thousands of people. The A allele at rs4788102 tags the risk-increasing haplotype at this locus.

The Mechanism

SH2B1 is an adaptor protein with SH2 and PH domains that integrates multiple upstream hormonal signals. In hypothalamic neurons, it binds to and activates JAK2²² JAK2
Janus kinase 2 — the kinase that is activated when leptin binds its receptor and initiates downstream signaling inside the neuron, the kinase immediately downstream of the leptin receptor. When SH2B1 binds phospho-Tyr813 of JAK2, it stimulates JAK2 kinase activity and recruits insulin receptor substrate 1 (IRS1), feeding the signal into the PI3-kinase pathway that ultimately suppresses appetite and elevates energy expenditure. Without SH2B1, even a normal leptin signal is poorly amplified — the neuron becomes functionally leptin-resistant even with intact receptor expression and normal circulating leptin levels.

A 2020 study³³ 2020 study
Jiang et al. Leptin receptor-expressing neuron Sh2b1 supports sympathetic nervous system and protects against obesity and metabolic disease. Nat Commun, 2020 identified an important circuit: SH2B1 in leptin-receptor-positive neurons activates the sympathetic nervous system and brown adipose tissue⁴⁴ brown adipose tissue
Specialized fat tissue that burns calories as heat rather than storing them — the thermogenic furnace that helps maintain body weight thermogenesis. Mice with SH2B1 deleted from leptin receptor neurons progressively develop obesity, insulin resistance, and fatty liver. The pathway extends further: a 2024 circuit-level study⁵⁵ 2024 circuit-level study
Li et al. SH2B1 Defends Against Energy Imbalance, Obesity, and Metabolic Disease via a Paraventricular Hypothalamus→Dorsal Raphe Nucleus Neurocircuit. Adv Sci, 2024 showed that SH2B1 in paraventricular hypothalamus (PVH) neurons projects to the dorsal raphe nucleus to suppress food intake, with optogenetic activation of this circuit reducing eating and SH2B1 deletion in PVH neurons causing obesity and metabolic disease in both sexes.

SH2B1 also potentiates insulin receptor signaling in peripheral tissues, enhancing IRS1 phosphorylation and protecting it from dephosphorylation — a role that extends its metabolic influence beyond the hypothalamus into muscle, liver, and pancreatic beta cells. The comprehensive mechanistic review⁶⁶ comprehensive mechanistic review
Rui L. SH2B1 regulation of energy balance, body weight, and glucose metabolism. World J Diabetes, 2014 describes how SH2B1 deletion in mice produces the full metabolic syndrome: leptin resistance, insulin resistance, hyperphagia, obesity, and type 2 diabetes.

rs4788102 itself is intronic and does not alter the SH2B1 protein. Its BMI association is attributed to linkage disequilibrium with the nearby functional missense variant rs7498665 (Ala484Thr), which lies in a conserved protein domain of SH2B1 and has been independently associated with serum leptin, body fat, and waist circumference.

The Evidence

The first genetic signal at the SH2B1 locus came from the landmark GIANT consortium GWAS, Willer et al. 2009⁷⁷ Willer et al. 2009
Six new loci associated with body mass index highlight a neuronal influence on body weight regulation. Nature Genetics, 2009. Meta-analysis: Stage 1 n=32,000+; Stage 2 n=59,000+; total >91,000, which identified SH2B1 as one of six newly significant BMI loci (P < 5×10⁻⁸) and noted that it "maps near key hypothalamic regulators of energy balance" alongside MC4R, POMC, and BDNF. The association was confirmed in Speliotes et al. 2010⁸⁸ Speliotes et al. 2010
Association analyses of 249,796 individuals reveal 18 new loci associated with body mass index. Nature Genetics, 2010 in nearly 250,000 individuals.

Beyond BMI, Prudente et al. 2011⁹⁹ Prudente et al. 2011
The SH2B1 obesity locus is associated with myocardial infarction in diabetic patients and with NO synthase activity in endothelial cells. Atherosclerosis, 2011 found the A allele of rs4788102 associated with myocardial infarction in type 2 diabetic patients across three cohorts (OR 1.21, 95% CI 1.04–1.41, p=0.016), with the protective GG genotype preserving insulin-stimulated nitric oxide synthase activity in endothelial cells — a mechanism linking SH2B1 to vascular function. An association with HbA1c in non-diabetic young adults¹⁰¹⁰ HbA1c in non-diabetic young adults
Lange et al. Evidence for Association between SH2B1 Gene Variants and Glycated Hemoglobin. Ann Hum Genet, 2016; n=5,641 European Americans was also identified (P = 2.2×10⁻⁴), suggesting the locus modestly influences glucose metabolism even before diabetes develops. A 2017 GWAS interaction analysis found rs4788102-A associated with BMI in physically active individuals (beta = 0.031 kg/m², P = 1×10⁻⁹), confirming that physical activity does not nullify the genetic signal at this locus.

Not all studies show effects: the SH2B1 locus showed no association with abnormal glucose homeostasis in a European meta-analysis of ~93,000 individuals Prudente et al. 2013¹¹¹¹ Prudente et al. 2013
Nutr Metab Cardiovasc Dis, 2013, and no independent association with central obesity in Chinese children after correction for multiple testing. The BMI signal is most robust in Europeans and in large pooled analyses.

Practical Actions

The SH2B1 pathway governs two parallel defenses against weight gain: satiety amplification (hypothalamus → eat less) and thermogenesis activation (sympathetic nervous system → burn more). Risk allele carriers face reduced efficiency in both. This makes both sides of the energy equation relevant: strategies that enhance peripheral satiety hormone responses and strategies that support sympathetically-mediated thermogenesis are particularly targeted for this genotype.

The modest but established vascular signal at this locus — through impaired endothelial NOS activity — makes metabolic monitoring more important for A allele carriers, especially those with additional cardiometabolic risk factors.

Interactions

rs4788102 is in linkage disequilibrium with rs7498665 (Ala484Thr), the functional missense variant in SH2B1 that directly alters protein structure. The two SNPs are co-associated across multiple studies and likely tag the same functional haplotype.

SH2B1 sits biologically upstream of the leptin receptor effector pathway shared with LEPR rs1137101 (Gln223Arg). When the leptin receptor itself is impaired (rs1137101 risk genotype), the upstream signal is already reduced before it reaches SH2B1. When SH2B1 is reduced in effectiveness (rs4788102 A allele), even a normal receptor signal is poorly amplified. Carriers of risk alleles at both loci face compound impairment at two nodes in the same hypothalamic satiety cascade.

Within the broader GIANT consortium obesity loci — FTO (rs9939609), MC4R (rs17782313), GNPDA2 (rs10938397) — each operates through a distinct mechanism. SH2B1's contribution is specifically through leptin signaling amplification and brown fat thermogenesis, complementing FTO's adipocyte thermogenesis mechanism and MC4R's downstream appetite suppression role. GWAS evidence shows these loci have additive, not multiplicative, BMI effects.