rs641153 — CFB R32Q

CFB R32Q — A Genetic Brake on Complement Inflammation

Deep inside the MHC class III region on chromosome 6 sits a single letter change that quietly alters how aggressively your immune system attacks your own tissues. CFB R32Q (rs641153) replaces an arginine with a glutamine at position 32 of complement factor B — a serine protease that is the lynchpin of the alternative complement pathway¹¹ alternative complement pathway
The alternative pathway runs continuously at low level, sampling surfaces for pathogens and damaged cells; unlike the classical pathway it requires no antibodies to activate. The result is a version of factor B that is less efficient at amplifying complement activation, reducing inflammatory damage in tissues exposed to chronic complement fire — most critically, the ageing retina.

The Mechanism

Factor B circulates as a zymogen. When it encounters C3b²² C3b
C3b is deposited on surfaces after complement activation and serves as an anchor; CFB binds C3b to form the proenzyme C3bB before cleavage by factor D, it binds and is cleaved by factor D into Ba (released) and Bb. The Bb fragment stays bound to C3b, forming the alternative pathway C3 convertase (C3bBb), which in turn cleaves many more C3 molecules in a powerful amplification loop. The Arg-32 residue sits in the Ba fragment at the C3b binding interface.

The Gln-32 substitution (the A allele, Q variant) reduces CFB's affinity for C3b by approximately fourfold compared with the common Arg-32 form. This means the proenzyme C3bB complex forms less readily, the C3 convertase assembles less efficiently, and the amplification loop runs at lower intensity. The consequence is a measurable dampening of alternative pathway activity — not an abolition (which would cause serious immunodeficiency) but a calibration that reduces bystander tissue damage when the pathway fires in settings like the ageing retinal pigment epithelium.

The Evidence

The R32Q protective association with AMD is one of the most robustly replicated findings in ocular genetics. Gold et al. (Nature Genetics, 2006)³³ Gold et al. (Nature Genetics, 2006)
Screened ~900 AMD cases and ~400 matched controls; identified the H7 haplotype first defined the H7 haplotype — a chromosomal block combining the C2 intron 10 variant (rs547154) and CFB R32Q (rs641153) — which reduced AMD risk with OR 0.45 (95% CI 0.33–0.61). This haplotype arises because the two variants are in very strong linkage disequilibrium (r² ≈ 0.92–0.96), meaning they are almost always inherited together.

Grassmann et al. (2011)⁴⁴ Grassmann et al. (2011)
Case-control study of 367 neovascular AMD cases and 251 controls with subsequent meta-analysis provided the mechanistic confirmation and largest effect size: meta-analysis across 2,600 individuals showed per-allele OR of 0.30 (p = 1.8 × 10⁻³¹) — one of the most statistically significant protective effects ever observed in a complex disease. The in vitro biochemistry confirmed fourfold reduced C3b affinity for the 32Q protein.

A meta-analysis of 15 published studies (Wang et al., 2013)⁵⁵ meta-analysis of 15 published studies (Wang et al., 2013) showed consistent protection across ethnicity: homozygous AA vs GG yielded OR 0.26 (95% CI 0.15–0.45); the dominant model (any A allele vs GG) yielded OR 0.49 (95% CI 0.40–0.59). Protection was observed in both Caucasians and Asians. The A allele is also associated with reduced risk of polypoidal choroidal vasculopathy, an AMD subtype common in Asian populations.

Beyond AMD, the alternative complement pathway's role in autoimmune tissue damage has led researchers to examine CFB variants in systemic lupus erythematosus (SLE). The 6p21 region harbours multiple SLE susceptibility signals, and reduced complement activation (as produced by R32Q) is biologically consistent with lower autoimmune inflammation; the companion intronic CFB variant rs1270942 shows strong SLE risk association (OR ~2.5), with the R32Q protective allele pointing in the opposite direction.

Practical Implications

Carrying the A allele (Gln-32) is a genuine genetic advantage: your alternative complement pathway runs with lower amplification, reducing chronic inflammatory damage to tissues prone to complement-mediated injury. The most clinically validated benefit is AMD protection. While AMD is a polygenic disease and no single variant eliminates risk, having one or two A alleles meaningfully shifts lifetime probability of developing late AMD — particularly the neovascular (wet) form, where complement-driven choroidal neovascularization is a central mechanism.

For the roughly 79% of people who carry only G alleles, AMD risk from this locus is at population baseline. Standard AMD-relevant lifestyle measures — not smoking, lutein-rich diet, UV protection, and regular eye exams after 60 — remain equally important regardless of genotype. The R32Q variant does not confer immunity and interacts with other complement variants (especially CFH Y402H, ARMS2, and C3) to determine individual AMD risk profiles.

Emerging complement inhibitors (including CFB-specific drugs like iptacopan and danicopan) are in advanced clinical trials for AMD and other complement-driven diseases. Interestingly, individuals with R32Q are essentially pharmacogenomic previews of what CFB inhibition achieves — suggesting the same pathway that confers genetic protection may respond particularly well or less well to therapeutic complement blockade, though this has not yet been formally studied.

Interactions

The strongest documented interaction is the H7 haplotype, where CFB R32Q (rs641153) co-occurs with C2 IVS10 (rs547154) in near-complete linkage disequilibrium. Because the two variants are almost always inherited together, the protective effect measured for each individually is largely the same haplotype block acting in concert — with C2 IVS10 reducing classical pathway convertase efficiency and CFB R32Q reducing alternative pathway convertase efficiency.

The protective effect of R32Q is most clinically meaningful when considered alongside CFH Y402H (rs1061170): a person who carries CFH risk alleles (higher AMD risk) and also carries CFB R32Q (lower AMD risk) sits at a partially buffered intermediate. Combined haplotype analyses⁶⁶ Combined haplotype analyses
Gold et al. 2006 showed that C2/CFB haplotypes combined with CFH variants predicted clinical outcome in 74% of AMD cases and 56% of controls. Similarly, C3 R102G (rs2230199) and ARMS2 A69S (rs10490924) add independent risk layers that interact with the complement inhibitory effect of R32Q.