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Heritable Filaggrin Disorders: The Paradigm of Atopic Dermatitis

      The Profilaggrin/Filaggrin Protein

      Mutations in the filaggrin gene (FLG) are one of the most common pathogenic single-gene defects identified to date. FLG encodes an important epidermal protein abundantly expressed in the outer layers of the epidermis (
      • Sandilands A.
      • Sutherland C.
      • Irvine A.D.
      • et al.
      Filaggrin in the frontline: role in skin barrier function and disease.
      ). Approximately 10% of individuals of European ancestry are heterozygous carriers of a loss-of-function mutation in FLG resulting in a 50% reduction in expressed protein (
      • Irvine A.D.
      • McLean W.H.
      Breaking the (un)sound barrier: filaggrin is a major gene for atopic dermatitis.
      ).
      As a protein, filaggrin is an unusual beast in biology. It is expressed as a giant, inactive, polymeric protein, profilaggrin, which is a major constituent of keratohyalin granules. At the interface between the outermost granular layer keratinocytes and the stratum corneum, this proprotein is cleaved into several identical filaggrin subunits. It has been postulated that these monomers condense the cytoskeleton and contribute to squame formation. In the cornified layers per se, filaggrin undergoes further posttranslational modifications and is ultimately proteolyzed into a hygroscopic pool of amino acids and chemical derivatives thereof, termed natural moisturizing factor (NMF). Thus, filaggrin has a central role in the biogenesis and subsequent hydration of the stratum corneum (Irvineet al., 2011).

      Disease Associations

      The identification of an association of filaggrin with atopic dermatitis (AD) was a slow burning one. Shortly after the identification of this protein in the late 1970s (
      • Dale B.A.
      Purification and characterization of a basic protein from the stratum corneum of mammalian epidermis.
      ), its role in keratin filament aggregation was rapidly established (
      • Dale B.A.
      • Holbrook K.A.
      • Steinert P.M.
      Assembly of stratum corneum basic protein and keratin filaments in macrofibrils.
      ). Some 7 years later, the first evidence that filaggrin was of potential causal interest in ichthyosis vulgaris (IV) emerged (
      • Sybert V.P.
      • Dale B.A.
      • Holbrook K.A.
      Ichthyosis vulgaris: identification of a defect in synthesis of filaggrin correlated with an absence of keratohyaline granules.
      ). It was to take a further two decades to fully disclose the molecular genetics of IV. That delay was due, in large part, to the technical difficulty of sequencing this highly repetitive gene and also due to the confusing inheritance of IV, which it transpired, which was best explained by semi-dominant Mendelian inheritance, rather than by traditionally understood dominant or recessive inheritance (
      • Smith F.J.
      • Irvine A.D.
      • Terron-Kwiatkowski A.
      • et al.
      Loss-of-function mutations in the gene encoding filaggrin cause ichthyosis vulgaris.
      ).
      Once
      • Smith F.J.
      • Irvine A.D.
      • Terron-Kwiatkowski A.
      • et al.
      Loss-of-function mutations in the gene encoding filaggrin cause ichthyosis vulgaris.
      had cracked the technical aspects of sequencing FLG in carefully chosen clinical pedigrees, the first two recurrent loss-of-function mutations (R501X and 2282del4) were identified. Identification of further recurrent mutations swiftly followed (
      • Sandilands A.
      • Terron-Kwiatkowski A.
      • Hull P.R.
      • et al.
      Comprehensive analysis of the gene encoding filaggrin uncovers prevalent and rare mutations in ichthyosis vulgaris and atopic eczema.
      ). Surprisingly, these mutations proved to be prevalent in European populations, with ~9% of these populations carrying at least one FLG mutation (
      • Irvine A.D.
      • McLean W.H.
      • Leung D.Y.
      Filaggrin mutations associated with skin and allergic diseases.
      ). An ethno-specific mutation profile was seen with population-specific mutation spectra reported in well-studied populations such as the Singapore Chinese (
      • Chen H.
      • Common J.E.
      • Haines R.L.
      • et al.
      Wide spectrum of filaggrin-null mutations in atopic dermatitis highlights differences between Singaporean Chinese and European populations.
      ). In this population, multiple very low-frequency mutations collectively contribute to the overall total, compared with the Irish population where five recurrent mutations constitute most of the total mutations (
      • Chen H.
      • Common J.E.
      • Haines R.L.
      • et al.
      Wide spectrum of filaggrin-null mutations in atopic dermatitis highlights differences between Singaporean Chinese and European populations.
      ).
      Once the molecular genetics had been established, rapid genotyping of large patient collections was possible and the association with AD was established initially in small collections of Irish children with AD and in Scottish children with AD and asthma (
      • Palmer C.N.
      • Irvine A.D.
      • Terron-Kwiatkowski A.
      • et al.
      Common loss-of-function variants of the epidermal barrier protein filaggrin are a major predisposing factor for atopic dermatitis.
      ). These findings have since been replicated many times in varying disease collections and population cohorts; the association is one of the strongest between any gene and a complex disease (
      • Rodriguez E.
      • Baurecht H.
      • Herberich E.
      • et al.
      Meta-analysis of filaggrin polymorphisms in eczema and asthma: robust risk factors in atopic disease.
      ). Further analysis of additional disease and population collections revealed that AD associated with FLGmutations is, in general, more severe, more persistent, more ‘atopic’, more likely to be associated with asthma, and more likely to have herpes infections (
      • Irvine A.D.
      • McLean W.H.
      • Leung D.Y.
      Filaggrin mutations associated with skin and allergic diseases.
      ). Overall, collections of AD show a prevalence of FLG mutations of between 20 and 50% (
      • Irvine A.D.
      Fleshing out filaggrin phenotypes.
      ). In addition, FLG mutations are associated with peanut allergy (
      • Brown S.J.
      • Asai Y.
      • Cordell H.J.
      • et al.
      Loss-of-function variants in the filaggrin gene are a significant risk factor for peanut allergy.
      ). The totality of the contribution of filaggrin to human disease is not limited to low prevalence loss-of-function mutations, as more prevalent intragenic copy number mutations also contribute substantially to risk, with the total filaggrin repeat number (the filaggrin index) significantly predicting AD risk with a lower odds ratio but a higher frequency and therefore a significant overall contribution to disease in addition to the lower frequency null alleles (
      • Brown S.J.
      • Kroboth K.
      • Sandilands A.
      • et al.
      Intragenic copy number variation within filaggrin contributes to the risk of atopic dermatitis with a dose-dependent effect.
      ).

      The Role of Filaggrin in Epidermal Barrier Function

      The filaggrin story placed a primary epidermal barrier defect firmly in the forefront of AD pathogenesis and has led to multiple avenues of inquiry to elucidate early pathogenic pathways in AD. Theflaky tail mouse, a double mutant for murine Flg and the matted gene (ma), shows a primary epidermal barrier defect compounded massively by allergic inflammation (
      • Fallon P.G.
      • Sasaki T.
      • Sandilands A.
      • et al.
      A homozygous frameshift mutation in the mouse Flg gene facilitates enhanced percutaneous allergen priming.
      ) findings consistent with ex vivo studies that show evidence for modulation of filaggrin expression by the Th2 cytokines, IL-13 and IL-4 (
      • Howell M.D.
      • Kim B.E.
      • Gao P.
      • et al.
      Cytokine modulation of atopic dermatitis filaggrin skin expression.
      ). Furthermore, filaggrin may have pleiotropic effects on multiple pathways relevant to the pathogenesis of AD including on the growth of Staphylocccus aureus (
      • Miajlovic H.
      • Fallon P.G.
      • Irvine A.D.
      • et al.
      Effect of filaggrin breakdown products on growth of and protein expression by Staphylococcus aureus.
      ), skin pH (
      • Jungersted J.M.
      • Scheer H.
      • Mempel M.
      • et al.
      Stratum corneum lipids, skin barrier function and filaggrin mutations in patients with atopic eczema.
      ), lipid lamellae secretion and stratum corneum structure (
      • Gruber R.
      • Elias P.M.
      • Crumrine D.
      • et al.
      Filaggrin genotype in ichthyosis vulgaris predicts abnormalities in epidermal structure and function.
      ), and the expression of pro-inflammatory cytokines (
      • Kezic S.
      • O’Regan G.M.
      • Lutter R.
      • et al.
      Filaggrin loss-of-function mutations are associated with enhanced expression of IL-1 cytokines in the stratum corneum of patients with atopic dermatitis and in a murine model of filaggrin deficiency.
      ). FLG-null alleles lead directly to decreased stratum corneum NMF (
      • Kezic S.
      • Kemperman P.M.
      • Koster E.S.
      • et al.
      Loss-of-function mutations in the filaggrin gene lead to reduced level of natural moisturizing factor in the stratum corneum.
      ); careful assessment of these levels demonstrates three separate subpopulations of AD based on FLG genotype, viz FLG +/+, FLG +/–, and FLG–/– (
      • O’Regan G.M.
      • Kemperman P.M.
      • Sandilands A.
      • et al.
      Raman profiles of the stratum corneum define 3 filaggrin genotype-determined atopic dermatitis endophenotypes.
      ).

      Therapeutic Targets

      Filaggrin expression is downregulated in all cases of moderate-to-severe AD (
      • Kezic S.
      • O’Regan G.M.
      • Yau N.
      • et al.
      Levels of filaggrin degradation products are influenced by both filaggrin genotype and atopic dermatitis severity.
      ), making it an appropriate therapeutic target for AD in the broadest sense, rather than being confined to AD associated with FLG-null alleles. The filaggrin index effect, where a modest 20% increase in filaggrin copy number leads to a 40% reduction in AD susceptibility (
      • Brown S.J.
      • Kroboth K.
      • Sandilands A.
      • et al.
      Intragenic copy number variation within filaggrin contributes to the risk of atopic dermatitis with a dose-dependent effect.
      ), is highly suggestive that filaggrin upregulation therapies will be of potential wide utility in AD. Comparatively little is known about the regulation of filaggrin gene expression, although this is now an area of active interest by academia and pharma alike. The fact that filaggrin expression has been shown to be regulated by environmental humidity (
      • Katagiri C.
      • Sato J.
      • Nomura J.
      • et al.
      Changes in environmental humidity affect the water-holding property of the stratum corneum and its free amino acid content, and the expression of filaggrin in the epidermis of hairless mice.
      ) indicates that there are signaling pathways in the skin that actively control filaggrin expression and skin barrier homeostasis in general. These are tractable problems that will lead to specific therapeutic approaches once these pathways are fully understood and targeted pharmacologically.

      Conflict of Interest

      The authors state no conflict of interest.

      References

        • Brown S.J.
        • Asai Y.
        • Cordell H.J.
        • et al.
        Loss-of-function variants in the filaggrin gene are a significant risk factor for peanut allergy.
        J Allergy Clin Immunol. 2011; 127: 661-667
        • Brown S.J.
        • Kroboth K.
        • Sandilands A.
        • et al.
        Intragenic copy number variation within filaggrin contributes to the risk of atopic dermatitis with a dose-dependent effect.
        J Invest Dermatol. 2012; 132: 98-104
        • Chen H.
        • Common J.E.
        • Haines R.L.
        • et al.
        Wide spectrum of filaggrin-null mutations in atopic dermatitis highlights differences between Singaporean Chinese and European populations.
        Br J Dermatol. 2011; 165: 106-114
        • Dale B.A.
        Purification and characterization of a basic protein from the stratum corneum of mammalian epidermis.
        Biochimica Et Biophysica Acta. 1977; 491: 193-204
        • Dale B.A.
        • Holbrook K.A.
        • Steinert P.M.
        Assembly of stratum corneum basic protein and keratin filaments in macrofibrils.
        Nature. 1978; 276: 729-731
        • Fallon P.G.
        • Sasaki T.
        • Sandilands A.
        • et al.
        A homozygous frameshift mutation in the mouse Flg gene facilitates enhanced percutaneous allergen priming.
        Nat Genet. 2009; 41: 602-608
        • Gruber R.
        • Elias P.M.
        • Crumrine D.
        • et al.
        Filaggrin genotype in ichthyosis vulgaris predicts abnormalities in epidermal structure and function.
        Am J Pathol. 2011; 178: 2252-2263
        • Howell M.D.
        • Kim B.E.
        • Gao P.
        • et al.
        Cytokine modulation of atopic dermatitis filaggrin skin expression.
        J Allergy Clin Immunol. 2007; 120: 150-155
        • Irvine A.D.
        Fleshing out filaggrin phenotypes.
        J Invest Dermatol. 2007; 127: 504-507
        • Irvine A.D.
        • McLean W.H.
        Breaking the (un)sound barrier: filaggrin is a major gene for atopic dermatitis.
        J Invest Dermatol. 2006; 126: 1200-1202
        • Irvine A.D.
        • McLean W.H.
        • Leung D.Y.
        Filaggrin mutations associated with skin and allergic diseases.
        N Engl J Med. 2011; 365: 1315-1327
        • Jungersted J.M.
        • Scheer H.
        • Mempel M.
        • et al.
        Stratum corneum lipids, skin barrier function and filaggrin mutations in patients with atopic eczema.
        Allergy. 2010; 65: 911-918
        • Katagiri C.
        • Sato J.
        • Nomura J.
        • et al.
        Changes in environmental humidity affect the water-holding property of the stratum corneum and its free amino acid content, and the expression of filaggrin in the epidermis of hairless mice.
        J Dermatol Sci. 2003; 31: 29-35
        • Kezic S.
        • Kemperman P.M.
        • Koster E.S.
        • et al.
        Loss-of-function mutations in the filaggrin gene lead to reduced level of natural moisturizing factor in the stratum corneum.
        J Invest Dermatol. 2008; 128: 2117-2119
        • Kezic S.
        • O’Regan G.M.
        • Lutter R.
        • et al.
        Filaggrin loss-of-function mutations are associated with enhanced expression of IL-1 cytokines in the stratum corneum of patients with atopic dermatitis and in a murine model of filaggrin deficiency.
        J Allergy Clin Immunol. 2012; 129: 1031e1-1039e1
        • Kezic S.
        • O’Regan G.M.
        • Yau N.
        • et al.
        Levels of filaggrin degradation products are influenced by both filaggrin genotype and atopic dermatitis severity.
        Allergy. 2011; 66: 934-940
        • Miajlovic H.
        • Fallon P.G.
        • Irvine A.D.
        • et al.
        Effect of filaggrin breakdown products on growth of and protein expression by Staphylococcus aureus.
        J Allergy Clin Immunol. 2010; 126: 1184e3-1190e3
        • O’Regan G.M.
        • Kemperman P.M.
        • Sandilands A.
        • et al.
        Raman profiles of the stratum corneum define 3 filaggrin genotype-determined atopic dermatitis endophenotypes.
        J Allergy Clin Immunol. 2010; 126: 574e1-580e1
        • Palmer C.N.
        • Irvine A.D.
        • Terron-Kwiatkowski A.
        • et al.
        Common loss-of-function variants of the epidermal barrier protein filaggrin are a major predisposing factor for atopic dermatitis.
        Nat Genet. 2006; 38: 441-446
        • Rodriguez E.
        • Baurecht H.
        • Herberich E.
        • et al.
        Meta-analysis of filaggrin polymorphisms in eczema and asthma: robust risk factors in atopic disease.
        J Allergy Clin Immunol. 2009; 123: 1361e7-1370e7
        • Sandilands A.
        • Sutherland C.
        • Irvine A.D.
        • et al.
        Filaggrin in the frontline: role in skin barrier function and disease.
        J Cell Sci. 2009; 122: 1285-1294
        • Sandilands A.
        • Terron-Kwiatkowski A.
        • Hull P.R.
        • et al.
        Comprehensive analysis of the gene encoding filaggrin uncovers prevalent and rare mutations in ichthyosis vulgaris and atopic eczema.
        Nat Genet. 2007; 39: 650-654
        • Smith F.J.
        • Irvine A.D.
        • Terron-Kwiatkowski A.
        • et al.
        Loss-of-function mutations in the gene encoding filaggrin cause ichthyosis vulgaris.
        Nat Genet. 2006; 38: 337-342
        • Sybert V.P.
        • Dale B.A.
        • Holbrook K.A.
        Ichthyosis vulgaris: identification of a defect in synthesis of filaggrin correlated with an absence of keratohyaline granules.
        J Invest Dermatol. 1985; 84: 191-194