rs9332739 — C2 E318D

C2 E318D — A Complement Haplotype That Protects the Aging Eye

Complement component C2 is a serine protease that sits at the entry point of the classical complement pathway¹¹ classical complement pathway
the arm of innate immunity triggered by antibody-antigen complexes and debris from dying cells. When the pathway fires, C2 is cleaved to form the C4b2a complex — the classical C3 convertase — which drives opsonisation, inflammatory signalling, and membrane attack. C2 is encoded in the HLA class III region on chromosome 6p21.3, a remarkably gene-dense immunological locus that also contains complement factor B (CFB), C4A, C4B, and TNF, all packed within a few megabases. The rs9332739 variant (E318D, p.Glu318Asp) swaps glutamic acid for aspartic acid at position 318 of C2 — a conservative missense change that tags a protective haplotype. Carriers of the C allele are substantially protected against age-related macular degeneration²² age-related macular degeneration
AMD, the leading cause of severe vision loss in people over 60, with the protective effect mediated through the haplotype structure rather than any known functional change to C2 protein activity.

The Mechanism

The E318D substitution itself is classified as benign by ClinVar — glutamic acid and aspartic acid are chemically similar (both negatively charged), and the amino acid change does not measurably impair C2 function or serum C2 levels. The protective signal comes instead from the haplotype context³³ haplotype context
the block of genetic variants inherited together in the HLA class III region. The C allele at rs9332739 tags the H10 haplotype, which co-inherits with the L9H variant of complement factor B (CFB rs1270942). Together, these variants mark a segment of the HLA class III region that confers reduced susceptibility to complement-driven retinal damage.

The classical complement pathway contributes to drusen formation in the ageing eye — drusen are subretinal deposits of complement fragments, lipids, and oxidised proteins that accumulate between the retinal pigment epithelium and Bruch's membrane. CFH (complement factor H) normally brakes complement activation; the risk variant CFH Y402H (rs1061170) impairs this brake, allowing chronic complement activity in the retina. The C2/CFB H10 haplotype appears to dampen the proximal step of classical pathway activation, reducing the inflammatory load that drives drusen growth and transition to advanced AMD with vision loss.

The Evidence

The protective association was first defined in 2006 by Gold et al. in Nature Genetics⁴⁴ Gold et al. in Nature Genetics
BF and C2 variation reduces AMD risk; H10 haplotype OR 0.45, analysing approximately 900 AMD cases and 400 matched controls in two independent cohorts. The H10 haplotype (E318D + CFB L9H) reduced AMD odds by 55%. Combined with CFH variant genotypes, the C2/CFB haplotype status predicted clinical outcome in 74% of affected individuals and 56% of controls — a level of predictive accuracy that remains clinically meaningful.

Subsequent meta-analyses have firmly replicated the finding in European populations. Sun et al. (2012)⁵⁵ Sun et al. (2012)
CFB/C2 meta-analysis, 15 case-control studies pooled 15 studies and found the C allele reduces AMD risk with OR 0.474 (95% CI 0.378–0.596, P<0.001) in the dominant model — approximately halving the odds for any C carrier. Thakkinstian et al. (2012)⁶⁶ Thakkinstian et al. (2012)
HuGE review, 19 studies estimated OR 0.55 (95% CI 0.46–0.65), translating to an absolute risk reduction of 2–6% in Caucasian populations where AMD is most prevalent. Lu et al. (2018)⁷⁷ Lu et al. (2018)
updated meta-analysis confirmed OR 0.50 (95% CI 0.45–0.56) in the heterozygote model, with consistently stronger effects in Caucasians than in Asian populations — consistent with the very low C allele frequency in East Asians (~1%).

The variant shows pronounced ethnic variation: the C allele reaches ~3.5% in Europeans and ~3.3% in South Asians, but is rare in East Asians (~1.1%) and very rare in Africans (~0.8%). In some Indian cohorts the C allele is actually the major allele, highlighting that AMD genetic architecture varies meaningfully across ancestries.

The Complement System and Classical Pathway Deficiency

The C2 gene's broader clinical significance extends beyond AMD. C2 deficiency is the most common complement deficiency in people of European descent⁸⁸ C2 deficiency is the most common complement deficiency in people of European descent
estimated prevalence ~1/20,000 in Western countries, and manifests in two ways: recurrent infections with encapsulated bacteria (57% of C2-deficient patients develop invasive pneumococcal disease, meningitis, or septicaemia) and autoimmune disease (43% develop systemic lupus erythematosus or connective tissue disease). The Swedish C2 deficiency cohort⁹⁹ Swedish C2 deficiency cohort
40 homozygous-deficient patients followed over 25 years shows this dual burden clearly. However, homozygous C2 deficiency is caused primarily by a 28-base-pair deletion creating a null allele, not by E318D. The E318D missense change does not cause C2 deficiency, and heterozygous carriers with one C allele have normal C2 function. The rs9332739 C allele should not be confused with loss-of-function C2 variants.

Practical Implications

The C allele of rs9332739 is a protective factor — most action is relevant for people who lack the C allele, representing 93% of Europeans. Homozygous GG individuals carry neither copy of the protective C2/CFB haplotype, and their AMD risk is not modified by this locus. These individuals depend on other genetic factors (particularly CFH variants and lifestyle) for AMD risk modulation. Standard AMD prevention and screening recommendations apply.

For GC heterozygotes (~7% of Europeans), one copy of the protective haplotype confers partial protection but does not eliminate AMD risk. For the rare CC homozygotes (<0.1%), maximum haplotype protection is present — though AMD still develops in some individuals, as many other loci and lifestyle factors contribute.

Regardless of C2/CFB genotype, the most important AMD-modifiable lifestyle factor is smoking — which roughly doubles AMD risk by amplifying oxidative stress and complement activation in the retina. Dietary carotenoids (lutein, zeaxanthin) and omega-3 fatty acids support retinal health across all genotypes.

Interactions

The rs9332739 protective effect is almost entirely haplotype-dependent: it operates through its co-inheritance with CFB rs1270942 (the L9H variant). These two SNPs are in strong linkage disequilibrium in Europeans and act as a unit. Studies that analyse rs9332739 alone (without haplotype context) often find weaker or inconsistent effects.

The C2/CFB protective haplotype acts independently of CFH Y402H (rs1061170), the strongest AMD risk variant. The two loci affect different points in complement activation — CFH regulates complement at the C3 convertase level, while the C2/CFB haplotype modulates classical pathway initiation. Their effects are additive: individuals who are both CFH Y402H risk carriers and lack the C2/CFB protective haplotype face the highest AMD risk, while those with both protective factors (CFH non-risk genotype plus C2/CFB haplotype) face substantially reduced risk. C3 R102G (rs2230199) also independently modifies AMD risk and adds to this multi-locus picture.