rs35018800 — TYK2 TYK2 Ala928Val

TYK2 Ala928Val — A Rare, Potent Brake on Autoimmune Signaling

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.