Clinical and Immunohistochemical Features of Ichthyosis Vulgaris.

Ichthyosis vulgaris (Figure 2Figure 2

Clinical and Immunohistochemical Features of Ichthyosis Vulgaris.) is one of the most common mendelian (single-

gene) diseases, and it is the most prevalent disorder of keratinization, with a population prevalence of at least 1 case in 250 persons.40 It is characterized by the postnatal appearance of dry, flaking skin, the severity of which varies seasonally; roughening of skin around the hair follicles (keratosis pilaris), particularly on the upper outer arms, thighs, and outer cheeks; and increased skin markings (termed hyperlinearity) on the palms and soles. Ichthyosis vulgaris, which has a well-recognized association with atopic dermatitis,41,42 is inherited in an autosomal semidominant pattern; that is, carriers of a single mutation have no disease or mild disease, whereas carriers of two mutations have more marked disease manifestations.6 The discovery that loss-of-function mutations in FLG underlie ichthyosis vulgaris clarified the inheritance pattern and opened the way to further explanation of the role of FLG in complex diseases.6

Since the identification of the 2 original mutations (R501X and 2282del4), an additional 47 mutations Figure 3

Variation in Filaggrin Mutations among Ethnic Groups and Other Populations.

have been reported, all of which predict loss of function with no translated protein (Figure 3Figure 3

Variation in Filaggrin Mutations among Ethnic Groups and Other Populations.).42 In

contrast to the vast majority of variants linked to complex traits, which are very often single-nucleotide polymorphisms with no clear function, all filaggrin variants are nonsense or frameshift mutations in a protein-encoding exon and are therefore readily identified as causative variants without functional analysis.9 The complex genetic architecture of the FLG locus has become apparent, and it includes a combination of recurrent and family-specific mutations. The recurrent mutations are also specific to certain ethnic groups, with distinct profiles seen in the European and Asian populations that have been well studied thus far (Figure 3).43

In addition to having a causal role in ichthyosis vulgaris, FLG mutations appear to have a modifying effect on the single-gene disorder X-linked recessive ichthyosis. In small studies, persons who carry a mutation in FLG in addition to a mutation in the STS gene appear to have a more severe phenotype than do their affected siblings who have wild-type FLG.44 A similar modifier effect may also be relevant for the keratinization disorder pachyonychia congenita.45 Given the high population frequency of FLG null alleles and the importance of the filaggrin protein in the stratum corneum, it is likely that FLG mutations will prove to be modifiers of other disorders of keratinization.

Atopic Dermatitis

Atopic dermatitis is an itchy inflammatory skin disease that affects approximately 11% of children in the United States46 and up to 25% in the United Kingdom,47,48 making it the most common chronic inflammatory skin disease of early childhood. Atopic dermatitis can cause considerable illness,49 with substantial associated family stress and social and financial burden.50 A systematic review estimated the annual direct and indirect costs of atopic dermatitis in the United States at $364 million to $3.8 billion.51 The disorder is strongly associated with food allergies, asthma, and allergic rhinitis in later life (i.e., the so-called atopic march).52

FLG mutations were strongly associated with atopic dermatitis in an Irish population and with atopic dermatitis plus asthma in a Scottish population.53 These findings have been widely replicated.54-58 Meta-analyses of studies involving many thousands of patients have confirmed these associations,56,59 with an overall odds ratio for atopic dermatitis ranging from 3.12 to 4.78.57,59 Stronger associations have been observed for dermatologist-diagnosed and moderate-to-severe atopic dermatitis, suggesting that patients with atopic dermatitis who carry FLG mutations may have a predisposition for a distinct profile of disease as compared with patients without such mutations. This interpretation is supported by additional studies showing that patients with atopic dermatitis who carry FLG mutations have more persistent disease,60 a higher incidence of skin infections with herpes virus (eczema herpeticum),61 and a greater risk of multiple allergies57,59,60 and asthma60,62 than patients with atopic dermatitis without such mutations. Further work has shown that in addition to having a primary effect on atopic dermatitis, FLG mutations are associated with both irritant contact dermatitis63 and nickel allergy64 (Table 1 in the Supplementary Appendix). In alopecia areata, FLG mutations appear to have a modifier effect associated with more severe outcomes.65

Asthma, Allergic Rhinitis, and Food Allergy

In addition to their role in atopic dermatitis, filaggrin loss-of-function mutations confer genetic risk for Figure 4

Filaggrin Haploinsufficiency and Increased Risk of Several Complex Traits.

several other complex diseases (Figure 4Figure 4

Filaggrin Haploinsufficiency and Increased Risk of Several Complex Traits., and Table 1 in the Supplementary

Appendix). The association of FLG mutations with asthma is complex. In large population studies, FLG loss-of-function mutations have conferred an overall asthma risk ranging from 1.48 to 1.79,60,66 but this effect was limited to subjects with atopic dermatitis or a history of it. Filaggrin is not expressed in the respiratory epithelia.67 The assumption is therefore that atopic dermatitis is a causal risk factor for asthma and systemic allergen sensitization in the context of FLG mutations.68 The mechanisms of this relationship are as yet unclear. Increased allergen and pathogen penetration through the stratum corneum, followed by stimulation of keratinocyte-derived thymic stromal lymphopoietin, an interleukin-7–like cytokine, in inflamed epidermis may lead to distal effects in the lung.69

The relationship between atopic dermatitis, asthma, and FLG mutations is complex; it is now clear that among atopic dermatitis patients, those with FLG mutations have a much greater risk of asthma than do those without FLG mutations,52,60,66 whereas among asthma patients, those with FLG mutations have a more difficult course and more frequent exacerbations (Figure 4).70 The complex and possibly distinct phenotype of “asthma plus atopic dermatitis” thus requires further examination.71 A strong association with allergic rhinitis has also been noted in some population studies.59,66

Peanut allergy is a strongly heritable condition, with a monozygotic concordance of 64% as compared with 7% in dizygotic twins.72 Our recent studies show that FLG mutations confer an overall odds ratio of 5.3 for peanut allergy (defined by a positive food challenge), with a residual odds ratio of 3.8 when corrected for atopic dermatitis.73 These data suggest a barrier defect that facilitates enhanced exposure of peanut allergen to antigen-presenting cells, even in the absence of atopic dermatitis.

Disease Mechanisms of Filaggrin Mutations

The mechanisms by which these genetic defects lead to human disease have been intensely investigated. Ichthyosis vulgaris, the exemplar mendelian disease of filaggrin deficiency, is characterized by dry, flaking skin (Figure 2). FLG mutations are the major determinants of the levels of the hygroscopic and humectant filaggrin breakdown products, pyrrolidone carboxylic acid and urocanic acid, and other components of NMF.32,33 Thus, the mechanism through which filaggrin Figure 5

Filaggrin Deficiency and Possible Mechanisms of Disease.

deficiency causes abnormally dry skin in ichthyosis vulgaris is clear (Figure 5Figure 5

Filaggrin Deficiency and Possible Mechanisms of Disease.). The mechanisms

through which filaggrin deficiency may lead to the disordered epithelial differentiation manifested clinically as dry and flaky skin are discussed below.

Atopic dermatitis, the primary complex disease associated with filaggrin deficiency, is characterized by dry skin, a cutaneous barrier defect, enhanced allergen priming, susceptibility to cutaneous bacterial colonization and infection (especially Staphylococcus aureus infection), and cutaneous inflammation driven by type 2 helper T (Th2) cells.74 Although atopic dermatitis was, for many years, considered to be a primarily immunologically driven disease with a secondary barrier defect (the so-called inside-outside hypothesis), some investigators had hypothesized that the primary defect was in the skin barrier (the outside-inside hypothesis).75,76 Abnormal barrier function of the skin has long been noted in ichthyosis vulgaris, even in the absence of atopic dermatitis.77,78 The remarkable association of FLG mutations with atopic dermatitis has validated the outside-inside hypothesis.

FLG mutations may play a role in the development of each of the key features of atopic dermatitis (Figure 5). Although it is widely held that these mutations lead to a functional barrier defect,79 the mechanisms of this defect are still being elucidated. Important insights have been gained from complementary studies of human subjects and murine models. These studies have shown that filaggrin deficiency may contribute to disease pathogenesis through a variety of differentiation-specific effects (Figure 5). Although studies of mice have limitations in their relevance to complex human disease (because of the potential influence of strain, immunologic background, epidermal differentiation complex haplotype, and the fact that murine skin is much hairier than human skin), useful insights have been gained from murine studies. All studies of the flaky tail mouse, which carries a directly analogous loss-of-function mutation in Flg, have shown abnormalities in barrier function, with lowered irritability thresholds and enhanced cutaneous allergen penetration.80-82 This minor baseline abnormality is greatly enhanced by exposure to allergens and to standard irritants such as sodium lauryl sulfate.80

The integrity of the stratum corneum barrier is primarily conferred by extracellular lipid lamellae. Filaggrin deficiency may contribute to defective lipid lamellae through a number of mechanisms. In the transitional zone of the stratum corneum, filaggrin deficiency impairs filament aggregation, which in turn impairs the maturation and excretion of extracellular lamellar bodies.36 Tight junctions are critical in sealing epidermal cell-to-cell integrity83; these junctions appear to be reduced in number in filaggrin-deficient persons.36 Similarly, such persons have a decreased density of corneodesmosin, the major protein component of corneodesmosomes (organelles that are critical for stratum corneum cell-to-cell adhesion).36

The acidifying effect of filaggrin breakdown products84 is probably important. The elevation in skin-surface pH that is observed in FLG-deficient persons33,85 suggests an alternative mechanism for the barrier defect. Neutral or alkaline pH is optimal for several serine proteases.86 Activation of kallikrein serine proteases has major downstream consequences, including activation of the plasminogen activator type 2 receptor–mediated blockade of lamellar-body secretion.87,88 Activation of serine proteases may also directly drive kallikrein 5–mediated Th2 inflammation, even in the absence of allergen priming.89

An elevation in the pH of the stratum corneum may lead to enhanced S. aureus adhesion and multiplication.31 In addition, urocanic acid and pyrrolidone carboxylic acid may contribute a specific antistaphylococcal effect by directly inhibiting bacterial expression of iron-regulated surface determinant protein A, which promotes bacterial adhesion to squames.31

Although FLG mutations lead to the primary abnormalities as outlined above, it is important to note that atopic dermatitis develops in only approximately 42% of all FLG heterozygotes60; thus, both genetic modifiers and environmental factors must be important. Epidemiologic studies suggest that early exposure to cats90 may be an important environmental factor. In addition, the presence of elder siblings increases the risk of atopic dermatitis in FLG mutation carriers.91 Thus, different causal pathways between genes and the environment may be important in patients with atopic dermatitis who carry such mutations as opposed to those who do not. Epistatic genetic effects, including effects of the plausible candidate genes SPINK5 and KLK7, have not yet been identified.92

Immunomodulation of Filaggrin

Although FLG mutations are strong genetic determinants of atopic dermatitis, the majority of children with atopic dermatitis who carry FLG mutations outgrow the skin disease after 12 years of age, suggesting that there may be immunologic determinants of low FLG expression.60 The barrier function of the skin appears to be reduced in all patients with atopic dermatitis, and decreased filaggrin expression is commonly found even in those who do not carry FLG mutations.33 Consistent with these observations is the finding that the Th2 cytokines interleukin-4, interleukin-13, and interleukin-25, which are involved in allergic responses, reduce FLG expression by keratinocytes in vitro.93 The in vivo significance of these findings is supported by the observation that mice with overexpression of STAT6, a transcription factor that mediates the action of interleukin-4 and interleukin-13, have reduced expression of filaggrin.94 Interleukin-4 and interleukin-13 may inhibit filaggrin expression through down-regulation of keratinocyte differentiation by modulating the calcium-sensing protein S100A11.95 Increased protease activity in the skin's inflammatory responses can lead to excessive degradation of filaggrin by affecting the processing of profilaggrin.20 Diverse immune and inflammatory responses probably modulate filaggrin expression and contribute to skin-barrier dysfunction.

Therapeutic Considerations and Personalized Medicine

The discovery of the prominent role of inherited filaggrin deficiency in ichthyosis vulgaris and atopic disease has brought the barrier function of the skin to the center stage in efforts to develop treatments for these common conditions.96,97 The first approach is immunomodulation. Topical antiinflammatory agents have been shown to reverse the reduced filaggrin expression found in lesional atopic dermatitis,98 a finding that is consistent with the concept that inflammation can reduce filaggrin expression. Given that Th2 cytokines can inhibit filaggrin expression, interventions that interfere with allergic immune responses may also enhance filaggrin expression in the skin of patients with atopic dermatitis and thereby ameliorate the inherent barrier defect.

Second, high-throughput screening is being used to identify compounds that up-regulate filaggrin expression, by targeting regulatory pathways that control the expression of filaggrin and other skin-barrier proteins. However, FLG is a late-differentiation–specific gene that is not normally expressed in monolayer keratinocyte cultures, making it difficult to develop a high-throughput screening assay.

A third strategy is to target the protein-translation machinery in order to fool the cell into “ignoring” nonsense mutations and thereby restore protein expression. This is particularly applicable to persons who are homozygotes or compound heterozygotes for FLG, who have the most severe cases of ichthyosis vulgaris or atopic dermatitis (about 1% of the general population). This approach, already in development for cystic fibrosis and Duchenne's muscular dystrophy, has the potential to treat a subset of patients with many other genetic disorders and therefore has wider applicability.99

Pharmacologic interventions that directly target filaggrin are a long way from clinical application, but in the future, one might envision using inexpensive, rapid genetic testing for filaggrin variants, coupled with an appropriate treatment regimen designed to enhance the barrier function of the skin, as an early intervention or preventive measure in persons at risk for atopic dermatitis. It remains to be seen whether restoration or augmentation of filaggrin expression in patients with preexisting atopic dermatitis will be therapeutic. Nevertheless, the filaggrin discovery has given rise to the new field of skin-barrier enhancement therapy, which represents an exciting opportunity to tackle common diseases.

Disclosure forms provided by the authors are available with the full text of this article at EJM.org.

Drs. Irvine and McLean contributed equally to this article.

We thank the patients and families who have contributed so importantly to our studies, Boyd Jacobson for superb assistance with earlier versions of the figures, and Maureen Sandoval for assistance in preparation of an earlier draft of this manuscript.

Date: 2015-02-16; view: 901