rs403016 — FCGR3A

FCGR3A rs403016 — A Rare Lupus Susceptibility Variant in CD16a

Your immune system deploys natural killer (NK) cells¹¹ natural killer (NK) cells
Innate immune lymphocytes that patrol for antibody-coated targets and kill them directly without prior sensitization as front-line defenders. On the surface of NK cells sits a receptor called FcgammaRIIIa (CD16a), encoded by the FCGR3A gene. This receptor grabs the tail end (Fc region) of IgG antibodies already bound to target cells — triggering antibody-dependent cellular cytotoxicity (ADCC)²² antibody-dependent cellular cytotoxicity (ADCC)
The killing process by which NK cells destroy antibody-flagged targets, including infected cells and cancer cells. The rs403016 variant introduces a rare missense change, replacing arginine with serine at position 36 of the mature CD16a protein. This Arg36Ser substitution sits in the extracellular immunoglobulin-like domain and may alter the receptor's structure or IgG binding characteristics. Chinese family-based genetic studies identified this variant as a susceptibility factor for systemic lupus erythematosus (SLE), though it is distinct from — and much rarer than — the well-characterized V158F polymorphism (rs396991)³³ V158F polymorphism (rs396991)
The established FCGR3A variant affecting IgG binding affinity and monoclonal antibody therapy response, present in ~30% of alleles globally in the same gene.

The Mechanism

FCGR3A is located on chromosome 1q23, a region that harbors multiple Fc gamma receptor genes (FCGR2A, FCGR2B, FCGR3A, FCGR3B) in close proximity. This region is a well-established susceptibility locus for SLE⁴⁴ susceptibility locus for SLE
Multiple independent variants in the 1q23 Fc receptor cluster have been linked to autoimmune disease through GWAS and family-based studies.

The rs403016 variant changes codon 36 from CGG (Arg) to AGC (Ser) in the canonical FCGR3A transcript (NM_000569.8). Arginine carries a positive charge while serine is uncharged and polar. This change at position 36 in the first extracellular immunoglobulin-like domain may disrupt a salt bridge or charge interaction that contributes to receptor folding or the IgG-binding interface. The downstream consequence — if the variant affects IgG binding — would be altered ADCC efficiency, potentially weakening clearance of immune complexes. In SLE, defective immune complex clearance allows IgG-coated nuclear fragments to accumulate, driving the autoantibody cascade and tissue inflammation.

NK cells in active SLE patients already show decreased CD16a surface expression compared to healthy controls⁵⁵ decreased CD16a surface expression compared to healthy controls
Hervier et al. 2011 found significantly reduced FcgammaRIIIa/CD16a expression on CD56dim NK cells in patients with active SLE, suggesting that reduced receptor function is a feature of active lupus. Whether rs403016 contributes to this phenotype by altering receptor stability or expression has not been directly tested.

The Evidence

Three Chinese family-based studies specifically examined rs403016. The original report by Ye et al. (2006)⁶⁶ Ye et al. (2006)
95 nuclear SLE families, 435 total subjects; family-based association using TDT and genotype relative risk analysis identified this as a novel FCGR3A SNP associated with SLE susceptibility in Chinese populations, with the R allele showing preferential transmission to affected offspring⁷⁷ the R allele showing preferential transmission to affected offspring
TDT chi-square = 9.30, P = 0.0032; additive model Z = 2.5444, P = 0.011.

A confirmatory study by Pan et al. (2007)⁸⁸ Pan et al. (2007)
119 SLE patients from 95 nuclear families plus 316 family members; additive and recessive genetic models tested replicated the association with SLE in the same population, finding significant results under both additive (P = 0.011) and recessive (P = 0.028) models. A subsequent haplotype analysis Pan et al. (2008)⁹⁹ Pan et al. (2008)
Examined haplotype structure across FCGR2B and FCGR3A; found no significant inter-gene haplotypes, suggesting FCGR3A-72 and FCGR2B variants act independently further confirmed single-marker association of rs403016 with SLE susceptibility.

Importantly, this evidence is limited to small Chinese family cohorts, with no independent replication in European, African, or other East Asian populations. The G allele is exceedingly rare globally — approximately 0.14% in Japanese populations (the highest documented frequency) and nearly absent in European and African populations (gnomAD v4). This extreme rarity means most large SLE GWAS studies, which are powered for common variants, would not detect it. The evidence level is therefore emerging — the association is statistically significant in the studied population but lacks broad replication.

Note: A marked discrepancy exists between the allele frequencies in the 2006-2008 Chinese studies (~39% "R allele") and current gnomAD data (<0.14% G allele globally). This may reflect changes in rs number assignment between genome builds, differences in how alleles were coded in early studies, or population-specific variation not captured by current databases. The plus-strand orientation used here (C = reference = Arg = common; G = alternate = Ser = rare risk allele) is consistent with current dbSNP annotation.

Practical Implications

Carrying the G allele at rs403016 is rare outside East Asian populations. In the context of SLE susceptibility, the G allele was associated with excess transmission to affected offspring in Chinese families — a statistically significant finding. However, SLE is a complex disease with many genetic and environmental contributors; no single rare variant is deterministic. The primary actionable implication for G allele carriers is heightened awareness of SLE warning signs and appropriate monitoring.

For individuals with this variant who are already monitored for autoimmune conditions (or who have family members with SLE), this variant adds to the overall genetic context. The FCGR3A gene is also relevant to NK cell-mediated immunity, so carriers might benefit from attention to immune health more broadly.

Interactions

The Fc gamma receptor cluster on chromosome 1q23 contains multiple interacting variants. rs403016 in FCGR3A may act in concert with the FCGR2A H131R variant (rs1801274), which affects IgG2 binding and is independently associated with SLE. The 2008 Pan et al. haplotype study found no significant cross-gene haplotype combining FCGR2B and FCGR3A variants in SLE susceptibility in their Chinese cohort, suggesting these loci act independently.

The more extensively studied FCGR3A V158F polymorphism (rs396991) — which affects IgG1 and IgG3 binding affinity and is present in ~30% of alleles globally — operates through a distinct mechanism at a different position in the same protein. These two FCGR3A variants could theoretically compound in a trans configuration, but no direct compound analysis has been published. The broader implication is that individuals with rs403016-G carrying additional Fc receptor risk alleles may have enhanced autoimmune susceptibility, though this remains hypothetical without direct study.