rs61816766 — FLG Filaggrin second variant

FLG Locus — Filaggrin Deficiency Without a Stop Codon

Filaggrin is the structural protein that builds the waterproof outer layer of your skin, retains moisture, and keeps allergens out. The FLG gene encodes profilaggrin¹¹ profilaggrin
a massive precursor protein cleaved into 10–12 filaggrin monomers during terminal differentiation of the epidermis, which then aggregate keratin filaments into the dense matrix of the stratum corneum. Most studied FLG variants are loss-of-function null alleles that introduce premature stop codons (R501X, 2282del4), but the FLG locus contains multiple independent signals that influence filaggrin levels through other mechanisms. The rs61816766 C allele is one of these: an intronic variant in FLG-AS1 (the FLG antisense RNA), located at chromosome 1q21.3, that emerged as one of the four most significant signals at the FLG locus in the largest atopic dermatitis genome-wide association study ever conducted.

Unlike R501X and 2282del4, which directly truncate the filaggrin protein, rs61816766 lies in an intronic region of FLG-AS1. The skin-specific nature of its effect — no evidence of FLG regulation through blood-based analyses was found in the GWAS — is consistent with a regulatory mechanism operating only in differentiating keratinocytes, where FLG expression normally peaks. Whether the variant directly regulates FLG transcription, splicing, or acts as a tag for a causal variant in strong local LD with a functional FLG element remains to be resolved. The biological outcome, however, is clear: reduced functional filaggrin in the stratum corneum, impaired skin barrier, and elevated atopic disease risk.

The Mechanism

Filaggrin deficiency, however caused, triggers the same downstream cascade. The stratum corneum depends on filaggrin to aggregate and flatten keratinocytes into the compressed, interlocking corneocyte layer that forms the physical barrier. Filaggrin is then broken down into natural moisturizing factor (NMF)²² natural moisturizing factor (NMF)
a hygroscopic mixture of amino acids, urocanic acid, pyrrolidone carboxylic acid, urea, and inorganic ions that maintains stratum corneum hydration and acidic pH (4.5–5.5). Without adequate NMF, transepidermal water loss (TEWL) increases³³ transepidermal water loss (TEWL) increases
TEWL measures the rate of water evaporating through the outer skin layer; elevated TEWL is a quantitative marker of barrier failure, measured in g/m²/h, skin pH rises, serine protease activity becomes dysregulated, and microscopic gaps between corneocytes allow environmental allergens — house dust mite, pollen, food proteins — to penetrate the viable epidermis and trigger IgE sensitization. This percutaneous sensitization pathway is the molecular basis of the atopic march from eczema to food allergy to asthma.

Th2 immune cytokines (IL-4 and IL-13) further suppress FLG transcription, creating a self-amplifying cycle: barrier deficiency → allergen penetration → Th2 skewing → further FLG suppression → worse barrier. Dupilumab (anti-IL-4Rα) breaks this cycle and restores barrier function irrespective of FLG genotype⁴⁴ Dupilumab (anti-IL-4Rα) breaks this cycle and restores barrier function irrespective of FLG genotype
Bissonnette et al. 2025 BALISTAD trial: open-label phase 4 study confirming dupilumab normalizes TEWL and FLG expression equally in patients with and without FLG null mutations, confirming that Th2 immune skewing is the proximal therapeutic target regardless of which upstream genetic variant initiated the barrier defect.

The Evidence

The association between rs61816766 and atopic dermatitis was established in the Budu-Aggrey et al. 2023 mega-GWAS⁵⁵ Budu-Aggrey et al. 2023 mega-GWAS
European and multi-ancestry genome-wide association meta-analysis of atopic dermatitis (PMID 37794016); discovery N=1,086,394; replication N=3,604,027 via 23andMe; 81 European loci and 10 additional multi-ancestry loci identified. At the FLG locus, rs61816766 reached OR 1.66 (95% CI 1.58–1.74, p=6.44×10⁻⁸⁹) in European discovery and replicated with OR 1.41 (95% CI 1.39–1.43, p=1.4×10⁻²²⁸) in the 23andMe European replication cohort. This effect size is larger than the classical R501X signal (rs12123821, OR 1.40 in the same study), which is notable given that R501X is a hard loss-of-function mutation that eliminates filaggrin from the affected allele entirely.

The C allele frequency in the ALSPAC birth cohort (3.06%) and gnomAD Europeans (2.37%) exceeds the 1000 Genomes global frequency (0.56%), confirming European enrichment consistent with a North/West European FLG locus variant — the same ancestral pattern seen for R501X and 2282del4. The variant is essentially absent in East Asian populations, where different FLG null alleles (3321delA, K4022X) predominate.

Practical Actions

The practical implications of rs61816766 C allele carriage are the same as for other filaggrin-deficiency variants: external barrier support to compensate for reduced NMF production, elimination of ingredients that disproportionately disrupt a compromised barrier, and early allergen introduction strategies for at-risk infants. For heterozygous TC carriers, the skin barrier is operating at reduced capacity and is more vulnerable to environmental stressors. Homozygous CC carriers (rare) have substantially elevated risk for moderate-to-severe atopic dermatitis with earlier onset and higher IgE sensitization.

Ceramide-dominant topical emollients directly compensate for reduced NMF by providing physiologic lipid replacement. Daily emollient from birth reduces atopic dermatitis cumulative incidence by ~50% in high-risk infants⁶⁶ Daily emollient from birth reduces atopic dermatitis cumulative incidence by ~50% in high-risk infants
BEEP RCT: Simpson et al. 2014, N=124 high-risk neonates randomized to daily full-body emollient vs. standard care; 6-month cumulative AD incidence 22% vs. 43%. For adults with established eczema, dupilumab (anti-IL-4Rα biologic) is the most evidence-based systemic option and is effective regardless of FLG genotype.

Interactions

rs61816766 interacts additively with other FLG locus variants. The Budu-Aggrey 2023 GWAS identified four independent FLG locus signals: rs12123821 (OR 1.40), rs61816766 (OR 1.66), rs72702900 (OR 1.28), and rs61815704 (OR 1.78). Compound carriership — especially of rs61816766 C with rs12123821 T or with FLG null alleles (rs61816761 R501X, rs558269137 2282del4) — likely amplifies barrier deficiency beyond single-variant effects. Proposed compound action for supervisor: when a user carries rs61816766 TC or CC AND rs12123821 CT or TT (both FLG locus risk alleles), the combined recommendation should emphasize twice-daily ceramide emollient, dermatology evaluation for eczema management, early allergen introduction in offspring, and dupilumab discussion if eczema is moderate-to-severe. Evidence level: moderate (independent GWAS signals at the same locus; compound carriership not directly studied but additive biological logic is clear).