rs558269137 — FLG 2282del4

FLG 2282del4 — The Second Broken Allele

The skin's outermost layer depends on a single protein — filaggrin — to hold it together, retain water, and keep allergens out. The FLG gene encodes profilaggrin¹¹ profilaggrin
a massive 435-kDa precursor protein that is proteolytically cleaved during terminal epidermal differentiation into 10–12 filaggrin monomers, which aggregate keratin filaments into the compact waterproof matrix of the stratum corneum. The 2282del4 variant (c.2282_2285del, deletion of four nucleotides in the coding sequence) shifts the reading frame at codon 762, generating a premature stop codon roughly 36 codons later — eliminating all downstream filaggrin repeats from that allele. This is the second most common FLG loss-of-function allele in European populations²² the second most common FLG loss-of-function allele in European populations
The most common is R501X (rs61816761), which together with 2282del4 accounts for approximately 90% of FLG null alleles in Europeans; their combined allele frequency is approximately 4% in this ancestry.

The Mechanism

2282del4 produces the same downstream consequence as R501X: haploinsufficiency of filaggrin protein when heterozygous, and complete filaggrin absence when combined with a second null allele. A single non-functional copy reduces total filaggrin output by approximately 50%, impairing the formation of natural moisturizing factor (NMF)³³ natural moisturizing factor (NMF)
a hygroscopic mixture of amino acids, urocanic acid, pyrrolidone carboxylic acid, urea, and ions — the metabolic breakdown products of filaggrin — that maintain stratum corneum hydration and the acidic pH (4.5–5.5) essential for normal barrier function. Without adequate NMF, transepidermal water loss (TEWL) increases, skin pH rises into the alkaline range, and structural gaps between corneocytes open — allowing environmental allergens (house dust mite, pollen, food proteins) to penetrate into the viable epidermis and trigger IgE sensitization through the skin rather than through the gut. This percutaneous sensitization pathway is the molecular basis of the atopic march⁴⁴ atopic march
the sequential progression from atopic eczema in infancy, to food allergy, to asthma, and allergic rhinitis — each step driven by IgE sensitization initiated through the defective skin barrier.

Inheritance is semidominant: heterozygotes have intermediate phenotype (often identifiable only by palmar hyperlinearity and subtle dryness), while homozygotes and compound heterozygotes show overt ichthyosis vulgaris⁵⁵ homozygotes and compound heterozygotes show overt ichthyosis vulgaris
fish-scale scaling most prominent on the legs and trunk, palmar hyperlinearity, keratosis pilaris; typically begins in early childhood and may improve in adulthood.

The Evidence

The 2282del4 mutation was identified in 2006 by Smith et al.⁶⁶ Smith et al.
Smith FJ et al. Loss-of-function mutations in the gene encoding filaggrin cause ichthyosis vulgaris. Nat Genet. 2006 as one of two primary causative mutations for ichthyosis vulgaris, and simultaneously by Palmer et al.⁷⁷ Palmer et al.
Palmer CN et al. Common loss-of-function variants of the epidermal barrier protein filaggrin are a major predisposing factor for atopic dermatitis. Nat Genet. 2006 as a strong predisposing factor for atopic dermatitis. Heterozygous carriers show elevated eczema risk with effect sizes comparable to R501X; a meta-analysis of 24 studies⁸⁸ meta-analysis of 24 studies
Rodríguez et al. 2009 found overall odds ratio for eczema of 3.12 (95% CI 2.57–3.79) and for eczema-plus-asthma of 3.29 (95% CI 2.84–3.82) across FLG null alleles as a group. Asthma risk is substantially driven by the eczema-asthma compound phenotype: when eczema is absent, the FLG-asthma association disappears, confirming that skin barrier deficiency — not a direct pulmonary effect — is the primary mechanism.

The functional consequences of compound heterozygosity (one 2282del4 allele plus one R501X allele on the opposite chromosome) are equivalent to homozygosity for either mutation alone: all 37 R501X/2282del4 compound heterozygous individuals examined by Carlsen et al.⁹⁹ Carlsen et al.
Carlsen BC et al. Filaggrin compound heterozygous patients carry mutations in trans position. Exp Dermatol. 2013 carried their mutations in trans, establishing complete filaggrin deficiency. Thyssen et al.¹⁰¹⁰ Thyssen et al.
Thyssen JP et al. Individuals who are homozygous for 2282del4 and R501X filaggrin null mutations do not always develop dermatitis. J Eur Acad Dermatol Venereol. 2012 found that compound heterozygous/homozygous individuals represent 3% of all dermatitis patients but 0.3% of the general adult population, and that penetrance is incomplete — some do not develop clinical eczema, suggesting environmental and genetic modifiers influence the clinical expression of genetic barrier deficiency.

Among complications, FLG null alleles sharply elevate eczema herpeticum risk: the combined null genotype (carrying two null alleles) confers odds ratio of 10.1 (95% CI 4.7–22.1, P = 2×10⁻¹¹) for eczema herpeticum versus atopic dermatitis without this complication, compared to OR 3.4 for R501X heterozygosity alone.

Practical Actions

The actionable framework for 2282del4 parallels R501X, because the mechanism is identical: filaggrin haploinsufficiency creates a barrier that needs ongoing external support. Ceramide- dominant emollients (containing ceramide, cholesterol, and free fatty acids at the physiologic 3:1:1 ratio) compensate for impaired NMF production and barrier lipid assembly. Urea-containing formulations (5–25% depending on body region) are particularly suitable as urea is itself an NMF component and can upregulate residual FLG expression in skin cells. Fragrance, methylisothiazolinone, sodium lauryl sulfate, and propylene glycol at high concentrations disproportionately penetrate compromised barriers and should be eliminated from personal care routines.

For heterozygous carriers currently without eczema: proactive moisturizing during high-stress periods (low humidity, cold, frequent handwashing, chlorinated water) preserves barrier integrity before sensitization can establish. For parents carrying 2282del4: the evidence from the BEEP/LEAP framework¹¹¹¹ BEEP/LEAP framework
Early emollient use from birth reduces AD incidence by ~50% in high-risk infants (Simpson et al. 2014) and early oral introduction of allergenic foods (peanut, egg, fish) provides the highest-impact prevention window before percutaneous sensitization establishes food allergy.

Interactions

2282del4 (rs558269137) and R501X (rs61816761) are the two most common FLG null alleles in Europeans and interact as compound heterozygotes. When one copy of 2282del4 is carried on one chromosome and one copy of R501X on the other, the individual has no functional FLG allele at all — an outcome functionally identical to homozygosity for either mutation. This compound heterozygous state produces the full filaggrin-null phenotype: overt ichthyosis vulgaris, high-penetrance severe atopic dermatitis, dramatically elevated asthma risk, and the highest risk of eczema herpeticum of any single-locus FLG genotype. Because each mutation is relatively common in Europeans (each approximately 2% allele frequency), the compound heterozygous state occurs with meaningful frequency: roughly 1 in 2,500 Europeans. The clinical management of compound heterozygotes is identical to FLG homozygotes — intensive daily emollient therapy, dermatology referral, dupilumab consideration for moderate-to-severe eczema, and vigilance for eczema herpeticum.

Proposed compound action for supervisor: When a user carries rs558269137 with a D allele (DI or DD) AND rs61816761 with an A allele (AG or AA), they are compound heterozygous with complete filaggrin absence. The combined recommendation should address zero filaggrin production: intensive twice-daily ceramide-dominant emollient therapy, dermatology referral for personalized ichthyosis/eczema management, dupilumab evaluation if moderate-to-severe eczema, eczema herpeticum recognition and emergency response, and early allergen introduction in offspring. Evidence level: established.