rs10800309 — FCGR2A FCGR2A intronic variant

FCGR2A Upstream Variant — The Receptor Quantity Control Switch

Your immune system uses antibodies as molecular flags — tagging pathogens and cellular debris for removal. The actual removal work is done by macrophages, neutrophils, and dendritic cells carrying a surface receptor called FcγRIIa, encoded by FCGR2A. This receptor is the cell's sensor for IgG antibody complexes: when an immune complex¹¹ immune complex
A cluster of antigens bound by multiple IgG antibodies, forming a molecular aggregate that signals phagocytes to engulf and destroy the target lands on a macrophage's FcγRIIa receptor, the cell ingests and destroys it. rs10800309 sits in a regulatory region of FCGR2A that controls how many of these receptors appear on the cell surface — influencing the immune system's overall capacity to clear immune complexes before they accumulate in tissues and trigger inflammation.

When immune complex clearance is impaired — whether through reduced receptor affinity (the well-studied rs1801274 H131R variant) or through reduced receptor quantity (the mechanism implicated by rs10800309) — immune complexes can deposit in the kidneys, joints, and small vessels, triggering the complement cascade and the chronic inflammation that characterizes diseases like lupus nephritis and rheumatoid arthritis.

The Mechanism

rs10800309 is located approximately 3.1 kilobases upstream of the FCGR2A transcription start site, within a region identified by CRISPR-based regulatory mapping²² CRISPR-based regulatory mapping
Researchers used CRISPR interference (CRISPRi) to systematically silence 1.7 Mb of open chromatin around FCGR2A, identifying upstream subregions that reduce transcript levels when silenced as harboring important enhancer activity for FCGR2A expression in myeloid cells. It forms a tight [haplotype block | A haplotype block is a chromosomal region where alleles co-segregate due to limited historical recombination; variants in the same block often tag the same underlying functional change] with rs4657039 and rs6696854, suggesting that all three variants together mark the same functional change in the upstream regulatory region.

The A allele at rs10800309 is associated with higher FcγRIIa surface expression on myeloid cells including dendritic cells and monocytes. Carriers of the AA genotype show statistically increased receptor density, while GG homozygotes show the lowest expression. This is consistent with the variant acting as an expression quantitative trait locus (eQTL)³³ expression quantitative trait locus (eQTL)
An eQTL is a DNA variant that predicts how much of a gene product is made — not what the protein looks like, but how many copies are produced for FCGR2A in immune cells. The downstream consequence is quantitative: fewer receptors means slower, less efficient capture and clearance of IgG immune complexes.

The Evidence

The most direct evidence for rs10800309's functional importance comes from a cohort study of HIV controllers⁴⁴ cohort study of HIV controllers
251 HIV controllers vs 250 HIV progressors from predominantly Caucasian cohorts; controller status defined as viral load below 400 copies/mL without antiretroviral therapy (Roederer et al., Genes & Immunity, 2020) that genotyped five FCGR2A SNPs in 501 participants. The AA genotype of rs10800309 was associated with natural HIV-1 control with an odds ratio of 2.84 (95% CI 1.20–6.89, P=0.033) even after adjusting for HLA-B57 and HLA-B27 — the dominant genetic determinants of HIV control. Crucially, the same AA genotype was independently confirmed to predict increased FcγRIIa surface expression on myeloid dendritic cells in a flow cytometry experiment (P=0.0032), establishing that the genotype operates through receptor quantity, not receptor structure.

In the context of autoimmune disease, rs10800309 was investigated as part of a five-SNP FCGR2A haplotype study⁵⁵ five-SNP FCGR2A haplotype study
422 UC patients and 710 healthy controls from southeastern China; haplotype analysis using PHASE software in ulcerative colitis. While the individual genotype frequencies of rs10800309 did not reach significance between patients and controls, the haplotype block it anchors (rs4657039-rs6696854-rs10800309) is an independently inherited segment that collectively influences FCGR2A expression. Separately, the FCGR2A locus — including the broader region containing this haplotype block — has been replicated as an ulcerative colitis susceptibility locus⁶⁶ ulcerative colitis susceptibility locus
GWAS-identified locus with genome-wide significant p-values in Japanese and Korean IBD cohorts.

The biological mechanism linking reduced FcγRIIa expression to autoimmunity is well-established through work on the companion missense variant rs1801274. Individuals with lower-functioning FcγRIIa fail to efficiently clear IgG-opsonized immune complexes; these complexes accumulate in tissues, activate complement, and drive the chronic inflammation underlying lupus nephritis, rheumatoid arthritis, and inflammatory bowel disease. The rs10800309 expression-level effect operates in parallel to this receptor-function effect — compounding risk when the G allele co-occurs with the R131 (G allele of rs1801274) functional variant.

Practical Actions

The GG genotype at rs10800309 reflects reduced FcγRIIa receptor quantity on myeloid cells. For most people, this is a background predisposition rather than a deterministic disease signal — particularly if the companion rs1801274 genotype indicates adequate receptor affinity. The most evidence-backed strategies target the downstream consequences of suboptimal immune complex clearance: supporting immune resolution through long-chain omega-3 fatty acids and vitamin D, and monitoring for early signs of immune complex deposition in susceptible individuals with family history.

Omega-3 fatty acids (EPA and DHA) promote the production of specialized pro-resolving mediators (SPMs)⁷⁷ specialized pro-resolving mediators (SPMs)
Lipid compounds derived from EPA and DHA that actively terminate inflammatory responses by promoting clearance of cellular debris and inhibiting pro-inflammatory cytokine production, compensating downstream for upstream impairments in receptor-mediated immune complex clearance.

Interactions

rs10800309 belongs to the second haplotype block of FCGR2A (block 1: rs4657039, rs6696854, rs10800309) distinct from the block containing rs1801274 (the well-studied H131R missense variant). The two blocks are independently inherited, meaning a person can carry the expression-reducing G allele at rs10800309 AND the affinity-reducing R131 allele at rs1801274 simultaneously — a combination that impairs both receptor quantity and receptor function simultaneously. This dual impairment creates the most pronounced deficit in immune complex clearance and may represent the highest-risk combination in the FCGR2A locus.

rs396991 (FCGR3A, F158V) encodes a separate but related Fc receptor expressed on NK cells and macrophages. Compound carriage of reduced-expression rs10800309 G alleles with reduced-function FCGR3A F158 alleles broadly depresses Fc-receptor-mediated immune surveillance across multiple cell lineages — an interaction relevant to both autoimmune clearance and antibody-dependent cellular responses to infections and biologics.