Unique and Recurrent Mutations in the Filaggrin Gene in Singaporean Chinese Patients with Ichthyosis Vulgaris

Increased loss-of-function filaggrin gene mutation prevalence in atopic dermatitis patients across northern latitudes indicates genetic fitness: A systematic review and meta-analysis

Loss-of-function (LoF) mutations in the filaggrin gene (FLG) constitute the strongest genetic risk for atopic dermatitis (AD). A latitude-dependent difference in the prevalence of LoF FLG mutations was systematically evaluated. A systematic review and meta-analysis were performed to estimate the prevalence of LoF FLG mutations in AD patients and the general population by geography and ethnicity. Risk of bias was assessed by Newcastle-Ottawa Scale and Jadad score. StatsDirect, version 3 software was used to calculate all outcomes. PubMed and EMBASE were searched until 9th December 2021. Studies were included if they contained data on the prevalence of LoF FLG mutations in AD patients or from the general population or associations between AD and LoF FLG mutations and were authored in English. Overall, 248 studies and 229 310 AD patients and individuals of the general population were included in the quantitative analysis. The prevalence of LoF FLG mutations was 19.1% (95% CI, 17.3-21.0) in AD patients and 5.8% (95% CI, 5.3-6.2) in the general population. There was a significant positive association between AD and LoF FLG mutations in all latitudes in the Northern hemisphere, but not in all ethnicities. The prevalence of LoF FLG mutations became gradually more prevalent in populations residing farther north of the Equator but was negligible in Middle Easterners and absent in most African populations. FLG LoF mutations are common and tend to increase with northern latitude, suggesting potential clinical implications for future AD management. The existence of possible genetic fitness from FLG LoF mutations remains unknown.

Genetic analysis of seven patients with inherited ichthyosis and Nagashima‑type palmoplantar keratoderma

Inherited ichthyosis comprises a series of heterogeneous dermal conditions; it mainly manifests as widespread hyperkeratosis, xerosis and scaling of the skin. At times, overlapping symptoms require differential diagnosis between ichthyosis and several other similar disorders. The present study reports seven patients with confirmed or suspected to be associated with ichthyosis by conducting a thorough clinical and genetic investigation. Genetic testing was conducted using whole‑exome sequencing, with Sanger sequencing as the validation method. The MEGA7 program was used to analyze the conservation of amino acid residues affected by the detected missense variants. The enrolled patients exhibited ichthyosis‑like but distinct clinical manifestations. Genetic analysis identified diagnostic variations in the FLG, STS, KRT10 and SERPINB7 genes and clarified the carrying status of each variant in the respective family members. The two residues affected by the detected missense variants remained conserved across multiple species. Of note, the two variants, namely STS: c.452C>T(p.P151L) and c.647_650del(p.L216fs) are novel. In conclusion, a clear genetic differential diagnosis was made for the enrolled ichthyosis‑associated patients; the study findings also extended the mutation spectrum of ichthyosis and provided solid evidence for the counseling of the affected families.

Keratosis pilaris and filaggrin loss-of-function mutations in patients with atopic dermatitis - Results of a Finnish cross-sectional study

Keratosis pilaris (KP) associates with epidermal barrier defects in atopic dermatitis (AD) but its role in disease severity and concomitant atopic diseases seems to vary between populations. We performed a cross‐sectional observational study with 502 randomly selected AD patients of a Finnish tertiary health care center. At a single clinical examination, disease severity (Rajka Langeland severity score and EASI), clinical signs and patient history were evaluated and total IgE levels and frequent filaggrin (FLG) loss‐of‐function mutations were investigated. There was no link with disease severity (p = 0.649, 95% CI 0.569–0.654), asthma (p = 0.230, 95% CI 0.206–0.281) or atopic sensitization (p = 0.351, 95% CI 0.309–0.392). Keratosis pilaris was significantly associated with palmar hyperlinearity (p < 0.000, 95% CI 0.000–0.006, OR 4.664, 95% CI 2.072–10.496) and the filaggrin loss‐of‐function mutation 2282del4 (p < 0.000, 95% CI 0.000–0.009, OR 4.917, 95%CI 1.961–12.330). The prevalence of KP in the cohort was generally low and KP seems to be infrequent in Finnish AD patients. This may be explained by the fact that the tested FLG loss‐of‐function mutations are rarer in the Finnish population compared for example, with central Europe or Asia. Mutations in other locations of the FLG gene or other genes of the epidermal barrier may play a more important role.

Cracking the Skin Barrier: Liquid-Liquid Phase Separation Shines under the Skin

Central to forming and sustaining the skin’s barrier, epidermal keratinocytes (KCs) fluxing to the skin surface undergo a rapid and enigmatic transformation into flat, enucleated squames. At the crux of this transformation are intracellular keratohyalin granules (KGs) that suddenly disappear as terminally differentiating KCs transition to the cornified skin surface. Defects in KGs have long been linked to skin barrier disorders. Through the biophysical lens of liquid-liquid phase separation (LLPS), these enigmatic KGs recently emerged as liquid-like membraneless organelles whose assembly and subsequent pH-triggered disassembly drive squame formation. To stimulate future efforts toward cracking the complex process of skin barrier formation, in this review, we integrate the key concepts and foundational work spanning the fields of LLPS and epidermal biology. We review the current progress in the skin and discuss implications in the broader context of membraneless organelles across stratifying epithelia. The discovery of environmentally sensitive LLPS dynamics in the skin points to new avenues for dissecting the skin barrier and for addressing skin barrier disorders. We argue that skin and its appendages offer outstanding models to uncover LLPS-driven mechanisms in tissue biology.

Moisturisers from birth in at-risk infants of atopic dermatitis - a pragmatic randomised controlled trial

Background

Atopic dermatitis (AD) is a common, chronic dermatosis, with onset of disease often manifesting in early infancy. Past studies evaluating the early use of moisturisers in the prevention of AD had mixed results.

Objectives

To compare the incidence of moderate or severe AD and total incidence of AD in a cohort of ‘at‐risk’ infants treated with moisturisers from the first 2 weeks of life, to a similar group without moisturisers.

Methods

We performed a single‐centre, prospective, parallel‐group, randomised study in infants with at least 2 first‐degree relatives with atopy. Subjects were randomised into either a treatment group with moisturisers or a control group without moisturisers. Participants were assessed at 2, 6, and 12 months for AD and if present, the severity was assessed using SCORAD index. We also compared the overall incidence of AD, trans‐epidermal water loss (TEWL), stratum corneum (SC) hydration, pH, and incidence of food and environmental sensitisation and allergies between both groups. Genotyping for loss‐of‐functions mutations in the FLG gene was conducted.

Results

A total of 200 subjects were recruited, with 100 subjects in each arm. There was no significant difference in incidence of moderate or severe AD, and total incidence of AD at 12 months between the treatment and control groups. There was a lower mean SCORAD in the treatment group than in the control group, but no significant difference in TEWL, SC hydration, and skin pH. No significant side‐effects were reported.

Conclusions

The early use of moisturisers in ‘at‐risk’ infants does not reduce the incidence of moderate‐to‐severe AD and overall incidence of AD in infancy.

A loss of function mutation in the filaggrin gene associated with ichthyosis vulgaris and rheumatoid arthritis

Introduction

Mutations in the filaggrin ( FLG) gene are known to cause ichthyosis vulgaris.

Methods

We used whole-genome sequencing (WGS) technology to investigate the genetic causes of rare and complex inherited diseases including rheumatoid arthritis, ichthyosis, and congenital fibrosis of the extraocular muscles type 1 (CFEOM1) in a Chinese family. WGS was performed in four topics, and the identified candidate mutations were further verified through Sanger sequencing.

Results

We identified a mutation in FLG gene (g.152280098 C>A, p.E2422∗) that may be associated with ichthyosis and arthritis. Moreover, a mutation in KIF21A (g.39726207 G>A, p.R954 W) was also determined in affected members as the cause of CFEOM1. The gene interaction network demonstrated an interesting correlation between FLG and genes associated with arthritis and ichthyosis. Functional enrichment analysis of these interacting genes revealed several possible pathways that might be linked to arthritis and ichthyosis.

Conclusion

In general, we confirmed a loss of function mutation in the FLG gene associated with ichthyosis vulgaris and rheumatoid arthritis in this family.

Whole exome sequencing identified two point mutations of COL7A1 and FLG in a Chinese family with dystrophic epidermolysis bullous pruriginosa and ichthyosis vulgaris

We report a 21-year-old man with recurrent bullous eruptions and severe itching on the lower legs and feet since 5 years of age. Dry, dirty brown, tile-like scales covered his lower legs with dystrophic toenails. Nodular prurigo-like lesions, scarring papules and milia remitted after the bullous eruptions. His father and another two family members had similar but mild presentations with recurrent bullae on the lower legs. Whole exome sequencing detected the heterozygous variants of COL7A1 c.6698G>A and FLG c.7249C>T in this pedigree. COL7A1 c.6698G>A was reported in bullous dermolysis of the newborn and FLG c.7249C>T was reported in ichthyosis vulgaris. Thus, the diagnosis of dystrophic epidermolysis bullosa pruriginosa associated with ichthyosis vulgaris was made.

Natural history and risk factors of atopic dermatitis in children

Atopic dermatitis (AD) is one of the most common inflammatory allergic diseases with pruritic skin lesions particularly in infancy. It is considered to be the first step of atopic march and has variable disease courses. Many children with AD may resolve their AD symptoms with increasing age and may develop respiratory allergies such as asthma and rhinoconjunctivitis at certain ages. Natural course of AD has been supported by many cross-sectional and longitudinal studies in many countries. In general, atopic dermatitis tends to be more severe and persistent in young children, particularly if they have some risk factors including genetic factors. It appears that approximately 40%-70% of childhood AD will get resolved when they reach the age of 6-7 years. However, it is also observed that over half of the children with AD developed respiratory allergy during late childhood.

Filaggrin loss-of-function mutations are not associated with atopic dermatitis that develops in late childhood or adulthood

BACKGROUND

The influence of filaggrin gene (FLG) mutations on early- vs. late-onset development of atopic dermatitis (AD), allergic contact dermatitis (ACD) and chronic irritant contact dermatitis (CICD) is not completely understood.

OBJECTIVES

To assess the association between FLG mutations and development of AD, ACD and CICD.

METHODS

This study assessed 241 patients with AD. AD developed during infancy in 85 patients, during childhood in 79 patients (32 early and 47 late) and during adulthood in 77 patients. We also included 100 patients with ACD and 44 with CICD, as well as 164 healthy controls. Four prevalent FLG loss-of-function mutations were genotyped (R501X, 2282del4, R2447X and S3247X).

RESULTS

The 2282del4 mutation was significantly associated with a greater risk of AD in the entire group [odds ratio (OR) 4·33, 95% confidence interval (CI) 1·26-14·96]. However, the 2282del4 mutation was associated only with AD that developed during infancy or in early childhood (≤ 8 years: OR 20·91, 95% CI 2·73-159·9), not with AD development in late childhood or adulthood (> 8 or > 18 years), or ACD or CICD. Similar associations were also observed for the combined 2282del4 or R501X genotype. Carriers of FLG mutations also experienced a longer duration of AD and required hospitalization more often.

CONCLUSIONS

FLG mutations are associated with only the early onset of AD, not late onset. Other factors should receive attention in patients with late-onset AD.

Hair keratin mutations in tooth enamel increase dental decay risk

Tooth enamel is the hardest substance in the human body and has a unique combination of hardness and fracture toughness that protects teeth from dental caries, the most common chronic disease worldwide. In addition to a high mineral content, tooth enamel comprises organic material that is important for mechanical performance and influences the initiation and progression of caries; however, the protein composition of tooth enamel has not been fully characterized. Here, we determined that epithelial hair keratins, which are crucial for maintaining the integrity of the sheaths that support the hair shaft, are expressed in the enamel organ and are essential organic components of mature enamel. Using genetic and intraoral examination data from 386 children and 706 adults, we found that individuals harboring known hair disorder-associated polymorphisms in the gene encoding keratin 75 (KRT75), KRT75(A161T) and KRT75(E337K), are prone to increased dental caries. Analysis of teeth from individuals carrying the KRT75(A161T) variant revealed an altered enamel structure and a marked reduction of enamel hardness, suggesting that a functional keratin network is required for the mechanical stability of tooth enamel. Taken together, our results identify a genetic locus that influences enamel structure and establish a connection between hair disorders and susceptibility to dental caries.

The importance of free fatty acid chain length for the skin barrier function in atopic eczema patients

An important feature of atopic eczema (AE) is a decreased skin barrier function. The stratum corneum (SC) lipids - comprised of ceramides (CERs), free fatty acids (FFAs) and cholesterol - fulfil a predominant role in the skin barrier function. In this clinical study, the carbon chain length distribution of SC lipids (FFAs and CERs) and their importance for the lipid organization and skin barrier function were examined in AE patients and compared with control subjects. A reduction in FFA chain length and an increase in unsaturated FFAs are observed in non-lesional and lesional SC of AE patients. The reduction in FFA chain length associates with a reduced CER chain length, suggesting a common synthetic pathway. The lipid chain length reduction correlates with a less dense lipid organization and a decreased skin barrier function. All changes are more pronounced in lesional SC compared with non-lesional skin. No association was observed between lipid properties and filaggrin mutations, an important predisposing factor for developing AE. The results of this study demonstrate an altered SC lipid composition and signify the importance of these changes (specifically regarding the CER and FFA chain lengths) for the impaired skin barrier function in AE. This provides insights into epidermal lipid metabolism as well as new opportunities for skin barrier repair.

Ichthyosis vulgaris: the filaggrin mutation disease

Ichthyosis vulgaris is caused by loss-of-function mutations in the filaggrin gene (FLG) and is characterized clinically by xerosis, scaling, keratosis pilaris, palmar and plantar hyperlinearity, and a strong association with atopic disorders. According to the published studies presented in this review article, FLG mutations are observed in approximately 7·7% of Europeans and 3·0% of Asians, but appear to be infrequent in darker-skinned populations. This clinical review article provides an overview of ichthyosis vulgaris epidemiology, related disorders and pathomechanisms. Not only does ichthyosis vulgaris possess a wide clinical spectrum, recent studies suggest that carriers of FLG mutations may have a generally altered risk of developing common diseases, even beyond atopic disorders. Mechanistic studies have shown increased penetration of allergens and chemicals in filaggrin-deficient skin, and epidemiological studies have found higher levels of hand eczema, irritant contact dermatitis, nickel sensitization and serum vitamin D levels. When relevant, individuals should be informed about an increased risk of developing dermatitis when repeatedly or continuously exposed to nickel or irritants. Moreover, with our current knowledge, individuals with ichthyosis vulgaris should be protected against neonatal exposure to cats to prevent atopic dermatitis and should abstain from smoking to prevent asthma. Finally, they should be advised against excessive exposure to factors that decrease skin barrier functions and increase the risk of atopic dermatitis.

Mutations in the Filaggrin are Predisposing Factor in Korean Children With Atopic Dermatitis

Purpose

Filaggrin (FLG) is a key protein that facilitates the terminal differentiation of the epidermis and the formation of the skin barrier. Recent studies showed that atopic dermatitis (AD) associates closely with loss-of-function mutations in the FLG gene. Asian and European populations differ in the frequencies of FLG mutations. Several FLG mutations, including 3321delA, E2422X, K4671X, S2554X, and R501X, occur frequently in Chinese and Japanese populations. The association between three FLG null mutations and AD in Korean children was investigated.

Methods

The FLG mutations in 1,430 children (aged 0-18 years) with AD and 862 control subjects were genotyped by using the TaqMan assay.

Results

The FLG null mutation E2422X was not detected in any patients with AD or control subjects. The R501X null mutation was detected in only one child with AD (0.1%). Children with AD had the 3321delA deletion significantly more frequently (2.4%) than the control subjects (0.0%, P<0.001). Children with AD also had a significantly higher combined allele frequency of the three FLG null mutations (2.6%) than the controls (0.0%, P<0.001). The 3321delA null mutation did not associate significantly with AD severity (P=0.842). When the patients with AD were divided into allergic AD and non-allergic AD patient groups, these two groups did not differ in terms of the frequency of 3321delA.

Conclusions

The Korean children had a lower frequency of FLG mutations than European populations. FLG null mutations may be associated with the development of AD in Korean children.

One remarkable molecule: filaggrin

The discovery, in 2006, that loss-of-function mutations in the filaggrin (FLG) gene are the cause of ichthyosis vulgaris-the most common disorder of keratinization-and also a strong genetic risk factor for atopic eczema, marked a significant breakthrough in the understanding of eczema pathogenesis. Subsequent investigations of the role of FLG-null mutations have identified a series of significant associations with atopic disease phenotypes, including atopic asthma, allergic rhinitis, and peanut allergy. However, many questions remain to be answered in relation to the precise mechanisms by which deficiency of an intracellular protein expressed primarily in the differentiating epidermis may contribute to the development of cutaneous and systemic pathology. This review aims to highlight the key milestones in filaggrin research over the past 25 years, to discuss the mechanistic, clinical, and therapeutic implications, and to consider possible future directions for ongoing investigation.

Wide spectrum of filaggrin-null mutations in atopic dermatitis highlights differences between Singaporean Chinese and European populations

BACKGROUND

Null mutations in the filaggrin gene (FLG) cause ichthyosis vulgaris (IV) and predispose to atopic dermatitis (AD). Cohort studies in Europe and Japan have reported an FLG mutation carrier frequency of between 14% and 56%, but the prevalent European FLG mutations are rare or absent in Chinese patients with IV and AD.

OBJECTIVES

To investigate further the spectrum of FLG-null mutations in Chinese patients and to compare it with that in other populations.

METHODS

We conducted comprehensive FLG genetic analysis in a discovery cohort of 92 Singaporean Chinese individuals with IV and/or moderate-to-severe AD. All detected FLG mutations were then screened in a cohort of 425 patients with AD and 440 normal controls. Results In total, 22 FLG-null mutations, of which 14 are novel, were identified in this study; the combined null FLG genotype of 17 mutations detected in cases and controls showed strong association with AD [Fisher's exact test; P = 5·3 × 10⁻⁹; odds ratio (OR) 3·3], palmar hyperlinearity (Fisher's exact test; P = 9·0 × 10⁻¹⁵; OR 5·8), keratosis pilaris (Fisher's exact test; P = 0·001; OR 4·7) and with increased severity of AD (permutation test; P = 0·0063).

CONCLUSIONS

This study emphasizes the wider genetic landscape of FLG-null mutations in Asia that is slowly emerging.

Exacerbation of X-linked ichthyosis phenotype in a female by inheritance of filaggrin and steroid sulfatase mutations

BACKGROUND

X-linked ichthyosis (XLI) is a relatively common, recessive condition caused by mutations in the steroid sulfatase (STS) gene. Common loss-of-function mutations in the filaggrin gene (FLG) cause ichthyosis vulgaris and predispose individuals to atopic eczema.

OBJECTIVE

To test the hypothesis that co-inheritance of FLG mutations can act as a genetic modifier in XLI.

METHODS

An unusually severe XLI phenotype in addition to eczema and mild childhood asthma was investigated in a female Indian patient by fluorescent in situ hybridization (FISH) for the common STS gene deletion. Direct sequencing of the entire FLG gene was also performed.

RESULTS

FISH analysis revealed that the proband was homozygous for the common STS genomic deletion mutation. Further investigation revealed a frame-shift mutation 3672del4 in the gene encoding filaggrin (FLG), leading to premature termination of profilaggrin translation. Interestingly, her father, who had a very typical mild presentation of XLI, did not carry this FLG mutation in addition to his STS deletion. Her mother was a heterozygous carrier of the FLG mutation and consistent with this, had mild symptoms of ichthyosis vulgaris; she was also a heterozygous carrier of the STS deletion.

CONCLUSION

This is the second reported case of the modifying effects of FLG null alleles on XLI and strengthens the hypothesis that filaggrin defects can synergize with STS deficiency to exacerbate the ichthyosis phenotype.

Genetic Polymorphism of FLG in Korean Ichthyosis Vulgaris Patients

BACKGROUND

Filaggrin is a key protein that facilitates the formation of skin barrier by forming a stratum corneum. Mutations in the gene encoding filaggrin (FLG) have recently been reported in patients with ichthyosis vulgaris (IV). Interestingly, there are ethnic differences between FLG mutations identified in Asians and Europeans, and few FLG mutations are overlapping between Chinese and Japanese IV patients.

OBJECTIVE

The aim of this study was to investigative the genetic polymorphism of FLG in Korean IV patients.

METHODS

Genomic DNA was extracted from whole venous blood specimen of Korean patients with IV and a control group, and the full sequence of FLG was determined via overlapping long-range polymerase chain reaction method.

RESULTS

Analysis of base sequence previously unreported reveal new nonsense mutation p.Y1767X in a Korean IV patient, and additional new single nucleotide polymorphisms.

CONCLUSION

On the basis of this study, it is anticipated that analysis of FLG gene sequence be extended to other dermatoses associated with FLG, such as atopic dermatitis.

Mutations in the filaggrin gene in Han Chinese patients with atopic dermatitis

BACKGROUND

Recently, loss-of-function mutations in the filaggrin gene (FLG) have been identified as the cause of ichthyosis vulgaris (IV) and also predisposing factors for atopic dermatitis (AD) and AD-associated phenotypes. Until now, over thirty mutations in FLG have been identified in patients with IV and AD, but the FLG mutation spectrum is not clear for Chinese Han patients with AD. This study aimed to investigate the role of FLG mutations in AD and AD-associated phenotypes in a Chinese population.

METHODS

We carried out a comprehensive sequencing of the entire FLG coding region in 261 Han AD patients.

RESULTS

In our research, we identified ten novel mutations (R826X, 3222del4, R1140X, 4271delAA, Q1790X, 5757del4, 6834del5, 6950del8, S2706X and K4671X) and eight reported mutations (441delA, R501X, 3321delA, R1474X, Q2417X, E2422X, 7945delA and R4306X) in FLG. FLG mutations were present in 31.4% of our AD patients. Mutations 3321delA and K4671X, with a frequency of 14.6% and 9.2%, respectively, were two of the most common mutations in this AD cohort. FLG null alleles (compound genotypes) were significantly associated with AD (P < 0.001) and AD associated with food sensitization (P = 0.012). However, we did not observe a positive association between FLG mutations and other AD-associated phenotypes, including asthma, allergic rhinitis and elevated total serum IgE level.

CONCLUSION

Our study increases the total number of reported FLG mutations. Moreover, we further confirm that FLG mutations are strong predisposing factors for AD in Han Chinese.

Filaggrin gene defects and the risk of developing allergic disorders

Filaggrin is a key protein that facilitates terminal differentiation of the epidermis and formation of the skin barrier. Mutations in the gene encoding filaggrin (FLG) have been identified as the cause of ichthyosis vulgaris (IV) and have been shown to be major predisposing factors for atopic dermatitis (AD). Approximately 40 loss-of-function FLG mutations have been identified in patients with ichthyosis vulgaris (IV) and/or atopic dermatitis (AD) in Europe and Asia. Major differences exist in the spectra of FLG mutations observed between different ancestral groups. Notably, prevalent FLG mutations are distinct between European and Asian populations. Many cohort studies on FLG mutations in AD have revealed that approximately 25-50% of AD patients harbour filaggrin mutations as a predisposing factor. In addition, FLG mutations are significantly associated with AD-associated asthma. The risk for developing allergic rhinitis is also significantly higher with a FLG mutation, both with and without accompanying AD. Recent studies have hypothesized that skin barrier defects caused by FLG mutations allows allergens to penetrate the epidermis and to interact with antigen-presenting cells, leading to the development of atopic disorders including asthma. The restoration of skin barrier function seems a feasible and promising strategy for prophylactic treatment of AD patients with FLG mutations.

The role of filaggrin in the atopic diathesis

Atopic dermatitis (AD) is an inflammatory disease characterized by pruritic skin lesions, immunodysregulation, disrupted epidermal barrier function and IgE-mediated sensitization to food and environmental allergens. Identification of the aetiology of AD has become increasingly a priority, as it is clear that the disease burden exceeds AD alone, with many children suffering severe, multi-system and occasionally life-threatening allergic disease. Previous approaches to understanding AD have centred on mechanisms in the adaptive immune system, often with an emphasis on the Th1-Th2 paradigm. Recently, the conceptual focus has increasingly shifted to include a primary defect in the epithelial barrier as a threshold event in moderate-to-severe AD. Familial aggregation of the disease is well established through many family studies of AD, asthma and allergic rhinitis, suggesting a significant heritable component. The identification of loss-of-function mutations in the filaggrin (FLG) gene, whose product is a key structural protein in the outermost layer of the epidermis in up to 50% of patients with AD, provides a significant insight into explaining disease initiation and points to a complex secondary interplay of environmental and immunological sequelae once barrier disruption is established. The elucidation of the environmental, genetic and immunobiological modifiers of this structural molecule may also direct our understanding of the pathomechanisms and endotypes central to the atopic diathesis. The recent identification of a murine model for FLG-AD, with the detection of a homozygous frame-shift mutation in the Flg gene in flaky-tail (ft/ft) mice, stands to rapidly accelerate our understanding of mechanisms and therapeutic intervention points in AD. Refining the molecular understanding of AD and its subtypes will allow for specific diagnostic, treatment and ultimately, preventative algorithms, and has opened an exciting new world of investigative challenges and collaborations.

Novel and recurrent mutations in the filaggrin gene in Chinese patients with ichthyosis vulgaris

BACKGROUND

Ichthyosis vulgaris (IV) is a common inherited skin disorder, and the filament aggregating protein (filaggrin) is a key protein involved in skin barrier function. Mutations in the filaggrin gene (FLG) have recently been identified as the cause of IV. However, there have been no reports of FLG mutations in mainland Chinese families with IV.

OBJECTIVES

To identify FLG mutations in Chinese patients with IV.

METHODS

Eleven unrelated Chinese families with IV were examined for FLG mutations with denaturing high-performance liquid chromatography prescreening and sequencing. SNaPShot was employed to obtain a high-throughput screening for the identified mutations.

RESULTS

Three mutations - one novel mutation (Q1256X) and two known mutations (3321delA and E2422X) - were identified in these families. The novel mutation, Q1256X, found in a Chinese family with IV, was located in filaggrin repeat 3. Mutation 3321delA, previously found in Japanese patients, was present in eight Chinese families with IV. Mutation E2422X, previously found in a Dutch patient of Chinese origin, was present in two Chinese families with IV. Neither of the null mutations, Q1256X and E2422X, were found in 100 unrelated control cases from the Chinese population; however, the mutation ratio for 3321delA was 3% in these controls.

CONCLUSIONS

Our study suggests that each population may have a unique and prevalent set of FLG mutations.

Filaggrin in the frontline: role in skin barrier function and disease

Recently, loss-of-function mutations in FLG, the human gene encoding profilaggrin and filaggrin, have been identified as the cause of the common skin condition ichthyosis vulgaris (which is characterised by dry, scaly skin). These mutations, which are carried by up to 10% of people, also represent a strong genetic predisposing factor for atopic eczema, asthma and allergies. Profilaggrin is the major component of the keratohyalin granules within epidermal granular cells. During epidermal terminal differentiation, the approximately 400 kDa profilaggrin polyprotein is dephosphorylated and rapidly cleaved by serine proteases to form monomeric filaggrin (37 kDa), which binds to and condenses the keratin cytoskeleton and thereby contributes to the cell compaction process that is required for squame biogenesis. Within the squames, filaggrin is citrullinated, which promotes its unfolding and further degradation into hygroscopic amino acids, which constitute one element of natural moisturising factor. Loss of profilaggrin or filaggrin leads to a poorly formed stratum corneum (ichthyosis), which is also prone to water loss (xerosis). Recent human genetic studies strongly suggest that perturbation of skin barrier function as a result of reduction or complete loss of filaggrin expression leads to enhanced percutaneous transfer of allergens. Filaggrin is therefore in the frontline of defence, and protects the body from the entry of foreign environmental substances that can otherwise trigger aberrant immune responses.

FLG mutation p.Lys4021X in the C-terminal imperfect filaggrin repeat in Japanese patients with atopic eczema

BACKGROUND

Mutations in the gene encoding filaggrin (FLG) have been shown to predispose to atopic eczema (AE).

OBJECTIVES

Further to establish population genetics of FLG mutations in the Japanese population and to elucidate effects of FLG mutations to filaggrin biosynthesis in skin of patients with AE.

METHODS

We searched for FLG mutations in 19 newly recruited Japanese patients with AE. We then screened 137 Japanese patients with AE and 134 Japanese control individuals for a novel mutation identified in the present study. In addition, we evaluated FLG mRNA expression by real-time reverse transcription-polymerase chain reaction and profilaggrin/filaggrin protein expression by immunohistochemical staining in the epidermis of the patients carrying the novel mutation.

RESULTS

We identified a novel FLG nonsense mutation c.12069A>T (p.Lys4021X) in one patient with AE. Upon further screening, p.Lys4021X was identified in four patients with AE (2.9% of all the patients with AE). In total, there are at least eight FLG variants in the Japanese population. Here we show that about 27% of patients in our Japanese AE case series carry one or more of these eight FLG mutations and these variants are also carried by 3.7% of Japanese general control individuals. There is a significant statistical association between the eight FLG mutations and AE (chi(2) P = 6.50 x 10(-8)). Interestingly, the present nonsense mutation is in the C-terminal incomplete filaggrin repeat and is the mutation nearest the C-terminal among previously reported FLG mutations. Immunohistochemical staining for filaggrin revealed that this nonsense mutation leads to remarkable reduction of filaggrin protein expression in the patients' epidermis.

CONCLUSIONS

We clearly demonstrated that FLG mutations are significantly associated with AE in the Japanese population. The present results further support the hypothesis that the C-terminal region is essential for proper processing of profilaggrin to filaggrin.

Filaggrin haploinsufficiency is highly penetrant and is associated with increased severity of eczema: further delineation of the skin phenotype in a prospective epidemiological study of 792 school children

Background

Null mutations within the filaggrin gene (FLG) cause ichthyosis vulgaris and are associated with atopic eczema. However, the dermatological features of filaggrin haploinsufficiency have not been clearly defined.

Objectives

This study investigated the genotype–phenotype association between detailed skin phenotype and FLG genotype data in a population-based cohort of children.

Methods

Children (n= 792) aged 7–9 years were examined by a dermatologist. Features of ichthyosis vulgaris, atopic eczema and xerosis were recorded and eczema severity graded using the Three Item Severity score. Each child was genotyped for the six most prevalent FLG null mutations (R501X, 2282del4, R2447X, S3247X, 3702delG, 3673delC). Fisher’s exact test was used to compare genotype frequencies in phenotype groups; logistic regression analysis was used to estimate odds ratios and penetrance of the FLG null genotype and a permutation test performed to investigate eczema severity in different genotype groups.

Results

Ten children in this cohort had ichthyosis vulgaris, of whom five had mild–moderate eczema. The penetrance of FLG null mutations with respect to flexural eczema was 55·6% in individuals with two mutations, 16·3% in individuals with one mutation and 14·2% in wild-type individuals. Summating skin features known to be associated with FLG null mutations (ichthyosis, keratosis pilaris, palmar hyperlinearity and flexural eczema) showed a penetrance of 100% in children with two FLG mutations, 87·8% in children with one FLG mutation and 46·5% in wild-type individuals (P< 0·0001, Fisher exact test). FLG null mutations were associated with more severe eczema (P= 0·0042) but the mean difference was only 1–2 points in severity score. Three distinct patterns of palmar hyperlinearity were observed and these are reported for the first time.

Conclusions

Filaggrin haploinsufficiency appears to be highly penetrant when all relevant skin features are included in the analysis. FLG null mutations are associated with more severe eczema, but the effect size is small in a population setting.

Ichthyosis vulgaris: novel FLG mutations in the German population and high presence of CD1a+ cells in the epidermis of the atopic subgroup

BACKGROUND

Ichthyosis vulgaris (IV) is a genetic disorder with a prevalence of 1:250-1000 caused by filaggrin (FLG) mutations, which also predispose to atopic diseases.

OBJECTIVES

To study the genotype/phenotype relationship in IV and to analyse whether the suggested skin barrier defect is associated with differences of epidermal dendritic cells.

PATIENTS/METHODS

We evaluated a cohort of 26 German patients with IV, established an IV severity score and analysed epidermal ultrastructure, histology, filaggrin and CD1a antigens. Mutations were screened by restriction enzyme analysis. Particular sequencing techniques allowed the complete FLG analysis to reveal novel mutations.

RESULTS

The combined null allele frequency of R501X and 2282del4 was 67.3%. Patients also showed the mutations S3247X and R2447X as well as five novel FLG mutations: 424del17 and 621del4 (profilaggrin S100 domain), 2974delGA (repeat 2), R3766X (repeat 10(1)) and E4265X (repeat 10(2)). Their combined allele frequency in controls was <0.7%. No mutation was found in one IV patient, all in all approximately 27% were heterozygous, and the majority (approximately 69%) showed two null alleles. The IV severity score and ultrastructure showed a significant correlation with genotypes. Interestingly, CD1a cell counts showed a significant difference between nonatopic and atopic IV patients both with eczema and without eczema.

CONCLUSIONS

We confirm that the mutations R501X and 2282del4 represent the most frequent genetic cause in German IV patients. The novel mutations are probably population and family specific. The observed differences of CD1a cells support the hypothesis that there is a barrier defect that predisposes to atopic manifestations, possibly independent of atopic eczema.

Atopic eczema and the filaggrin story

The discovery that null mutations in the filaggrin gene (FLG) are associated with atopic eczema represents the single most significant breakthrough in understanding the genetic basis of this complex disorder. The association has been replicated in multiple independent studies during the past 2 years with the use of various methodologies, from populations in Europe, the United States, and Japan. Filaggrin plays a key role in epidermal barrier function, and its association with atopic eczema emphasizes the importance of barrier dysfunction in eczema pathogenesis. This review aims to summarize the current state of knowledge regarding the role of FLG mutations in ichthyosis vulgaris, atopic eczema, and other skin disorders, with an emphasis on potential clinical applications. Further research is needed to clarify the precise role of filaggrin in skin and systemic atopic disease, to pave the way for novel therapeutic interventions.

The role of filaggrin loss-of-function mutations in atopic dermatitis

PURPOSE OF REVIEW

To provide a comprehensive summary of recent genetic advances as they relate to the pathogenesis of atopic dermatitis.

RECENT FINDINGS

Atopic dermatitis is a common inflammatory skin disease with a complex cause, resulting from an elaborate interplay between environmental, immunological and genetic factors. The disease is often the prelude to an atopic diathesis that includes asthma and other allergic diseases. The identification of mutations in the barrier protein filaggrin as conferring major susceptibility to atopic dermatitis and atopic dermatitis related asthma has reconfigured our understanding of disease mechanisms and highlights the importance of epidermal barrier disruption as a primary event in the disease.

SUMMARY

In this review we highlight recent advances in our understanding of how filaggrin might influence the environmental-immune interface, impacting disease penetrance, severity and trajectory, and the implications for both research and therapeutics in this field. Focusing on the downstream biological consequences of altered filaggrin expression and the sequence of immunological and environmental triggers that ensue will provide the rationale for targeted therapeutics capable of restoring or preventing disruption of barrier function.