Filaggrin gene variants among Saudi patients with ichthyosis vulgaris

Ichthyoses are a heterogeneous group of cornification disorders. The most common form of ichthyoses is ichthyosis vulgaris (IV) ([OMIM] #146,700), which can be inherited as autosomal semi-dominant mutation in the filaggrin gene (FLG). We present the findings of a study involving 35 Saudi patients with a clinical diagnosis of ichthyosis vulgaris. For identifying the pathogenic mutation of their disease, we used Sanger sequencing analysis of the extracted DNA samples. We also identified the underlying 22 FLG variants, which have been seen before. However, the detected mutations do not involve the common p.R501* c. 2282del4 mutations reported in European populations. Indeed, we did not identify any statistical influence of the homozygous or heterozygous genotypes on the phenotype severity of the disease.

Revisiting the Roles of Filaggrin in Atopic Dermatitis

The discovery in 2006 that loss-of-function mutations in the filaggrin gene (FLG) cause ichthyosis vulgaris and can predispose to atopic dermatitis (AD) galvanized the dermatology research community and shed new light on a skin protein that was first identified in 1981. However, although outstanding work has uncovered several key functions of filaggrin in epidermal homeostasis, a comprehensive understanding of how filaggrin deficiency contributes to AD is still incomplete, including details of the upstream factors that lead to the reduced amounts of filaggrin, regardless of genotype. In this review, we re-evaluate data focusing on the roles of filaggrin in the epidermis, as well as in AD. Filaggrin is important for alignment of keratin intermediate filaments, control of keratinocyte shape, and maintenance of epidermal texture via production of water-retaining molecules. Moreover, filaggrin deficiency leads to cellular abnormalities in keratinocytes and induces subtle epidermal barrier impairment that is sufficient enough to facilitate the ingress of certain exogenous molecules into the epidermis. However, although FLG null mutations regulate skin moisture in non-lesional AD skin, filaggrin deficiency per se does not lead to the neutralization of skin surface pH or to excessive transepidermal water loss in atopic skin. Separating facts from chaff regarding the functions of filaggrin in the epidermis is necessary for the design efficacious therapies to treat dry and atopic skin.

Genetics of Inherited Ichthyoses and Related Diseases

Inherited ichthyoses are classified as Mendelian disorders of cornification (MEDOC), which are defined on the basis of clinical and genetic features and are mainly divided into non-syndromic and syndromic ichthyoses. Numerous genes, which encode for corresponding proteins, are involved in the normal differentiation of keratinocytes (cornification) and participate in the formation of a functional epidermal barrier. To date, mutations in more than 50 genes are known to result in various types of ichthyoses. Thanks to modern genetic diagnostic methods based on targeted next generation sequencing (NGS), approximately 80-90% of cases can be resolved at present. Further sequencing methods covering the whole exome (WES) or whole genome (WGS) will obviously elucidate another portion of the remaining unknown ichthyoses in the future.

X-linked and autosomal dominant forms of the ichthyosis in coinheritance

According to modern classification, there are two forms of inherited ichthyoses: syndromic and non-syndromic, each of them consists of more than ten different nosologies. The commonest types of the ichthyosis are X-linked recessive (prevalence 1/2000-6000 in men) and autosomal dominant, or ichthyosis vulgaris with incomplete penetrance (1/250-1000). The X-linked form is associated with mutations in steroid sulfatase STS gene, it is noteworthy that there is a full deletion of the gene in 90% of cases. Ichthyosis vulgaris is caused by heterozygous mutations in the FLG gene encoding filaggrin. It is important to note that the clinical forms of these diseases are indistinguishable. The aim of this study was to search for pathogenic or likely pathogenic mutations which are associated with various forms of the inherited ichthyosis such as other inherited diseases with similar phenotypic signs. The sequencing was done on a HiSeq 4000 sequencer (Illumina) by paired-end reading (2 × 150 bp). The identified mutation p.Arg2037Ter in the heterozygous condition has been described before in databases as being pathogenic. Also, our patient has a full deletion of the STS gene and it was found that our patient carries two pathogenic mutations which are related to different forms of the inherited ichthyosis.

[ANALYSIS OF ONE-CARBON METABOLISM GENES AND EPIDERMAL DIFFERENTIATION COMPLEX IN PATIENTS WITH ICHTHYOSIS VULGARIS]

The aim of the study was to evaluate the effects of allelic polymorphism of the FLG and MTHFR genes and their associations in gynecological patients with ichthyosis vulgaris. Gynecological disorders are observed in presence of some forms of ichtyosis. From the prospective of improving nation's healthcare, the greatest attention is drawn to reproductive disorders. Based on this, the research was also tasked with studying of the genetic nature of gynecological diseases, as well as the influence of geographical latitude on the frequencies of mutagenic alleles of the FLG gene and heterogeneous carriers of these mutations. The collection of clinical-gynecological history was carried out by the method of single registration of the proband on the basis of the Regional Clinical Dermatological and Venereological Health Center No. 1 and the Dermatovenerological Health Centers of the Kharkiv Region. The diagnosis and form of dermatosis is established on the basis of the analysis of clinical and gynecological data and the results of laboratory tests in accordance with ICD-10: ichthyosis vulgaris (Q 80.1.0, OMIM 146700). The data on 18 women and 20 men from 3 families, aged 26 to 76 years old, suffering from ichthyosis, were analyzed. As a result of the study, a direct correlation was determined between the latitude and frequencies of mutant alleles of the FLG gene, as well as between the geographical latitude and frequency of heterozygous carriers of these mutations. The frequencies of the T allele and the CT genotype according to polymorphic variant C677T of the MTHFR gene demonstrate feedback with the latitude indicators. The frequency distributions of the 2282del4 allele and the CT genotype, the N/2282del4 and CT genotypes, the 2282del4 and T alleles, the N/2282del4 genotype and the T allele have opposite latitudinal zonation. The established connections made it possible to predict the development of gynecological pathologies in women with ichthyosis vulgaris. The prevalence of endometriosis and endometrial cancer in women with ichthyosis vulgaris in the Kharkiv region was 33.3%, while the average for the female population in the region was 0.29-0.35%. The number of children born to women with ichthyosis vulgaris did not differ from the regional index.

[Novel missense mutations of the FLG gene identified in two Chinese families affected with ichthyosis vulgaris]

OBJECTIVE

To identify potential mutations of the FLG gene in two Chinese families affected with ichthyosis vulgaris.

METHODS

All coding exons and exon-intron boundary of the FLG gene were amplified by polymerase chain reaction (PCR) and analyzed by direct sequencing. The results were compared with those of 100 unrelated healthy controls.

RESULTS

Two novel missense mutations, c.1360A>G (p.T454A) and c.10363G>T (p.D3455Y), were detected in all affected individuals from family 1 and family 2 respectively but none of the controls.

CONCLUSION

The c.1360A>G (p.T454A) and c.10363G>T (p.D3455Y) of the FLG gene may lead to alteration of the structure and function of the FLG protein and cause ichthyosis vulgaris in the two families.

An Analysis of the Filaggrin Gene Polymorphism in Korean Atopic Dermatitis Patients

Research of the FLG mutation in various ethnic groups revealed non-overlapping mutation patterns. In addition, Japanese and Chinese atopic patients showed somewhat different mutations. These ethnic differences make the research on Korean patients mandatory; however, no systematic research on Korean atopic dermatitis (AD) patients has been performed. This study aims to investigate the genetic polymorphism of FLG in Korean atopic dermatitis patients. The study was made up of three groups including 9 Ichthyosis vulgaris (IV) patients, 50 AD patients and 55 normal controls: the ichthyosis group was incorporated due to the reported association between the FLG mutation and IV. In comparison to other sequencing methods, the overlapping long-range PCR was used. We revealed the genetic polymorphism of filaggrin in Koreans, and at the same time, we discovered nonsense mutations in p.Y1767X and p.K4022X in Korean AD patients. By using FLG sequencing techniques confirmed in this study, new mutations or genetic polymorphisms with ethnic characteristics would be detected and further larger studies of repeat number polymorphisms could be performed.

Distinguishing ichthyoses by protein profiling

To explore the usefulness of protein profiling for characterization of ichthyoses, we here determined the profile of human epidermal stratum corneum by shotgun proteomics. Samples were analyzed after collection on tape circles from six anatomic sites (forearm, palm, lower leg, forehead, abdomen, upper back), demonstrating site-specific differences in profiles. Additional samples were collected from the forearms of subjects with ichthyosis vulgaris (filaggrin (FLG) deficiency), recessive X-linked ichthyosis (steroid sulfatase (STS) deficiency) and autosomal recessive congenital ichthyosis type lamellar ichthyosis (transglutaminase 1 (TGM1) deficiency). The ichthyosis protein expression patterns were readily distinguishable from each other and from phenotypically normal epidermis. In general, the degree of departure from normal was lower from ichthyosis vulgaris than from lamellar ichthyosis, parallel to the severity of the phenotype. Analysis of samples from families with ichthyosis vulgaris and concomitant modifying gene mutations (STS deficiency, GJB2 deficiency) permitted correlation of alterations in protein profile with more complex genetic constellations.

Spontaneous atopic dermatitis-like symptoms in a/a ma ft/ma ft/J flaky tail mice appear early after birth

Loss-of-function mutations in human profilaggrin gene have been identified as the cause of ichthyosis vulgaris (IV), and as a major predisposition factor for atopic dermatitis (AD). Similarly, flaky tail (a/a ma ft/ma ft/J) mice were described as a model for IV, and shown to be predisposed to eczema. The aim of this study was to correlate the flaky tail mouse phenotype with human IV and AD, in order to dissect early molecular events leading to atopic dermatitis in mice and men, suffering from filaggrin deficiency. Thus, 5-days old flaky tail pups were analyzed histologically, expression of cytokines was measured in skin and signaling pathways were investigated by protein analysis. Human biopsies of IV and AD patients were analyzed histologically and by real time PCR assays. Our data show acanthosis and hyperproliferation in flaky tail epidermis, associated with increased IL1β and thymic stromal lymphopoietin (TSLP) expression, and Th2-polarization. Consequently, NFκB and Stat pathways were activated, and IL6 mRNA levels were increased. Further, quantitative analysis of late epidermal differentiation markers revealed increased Small proline-rich protein 2A (Sprr2a) synthesis. Th2-polarization and Sprr2a increase may result from high TSLP expression, as shown after analysis of 5-days old K14-TSLP tg mouse skin biopsies. Our findings in the flaky tail mouse correlate with data obtained from patient biopsies of AD, but not IV. We propose that proinflammatory cytokines are responsible for acanthosis in flaky tail epidermis, and together with the Th2-derived cytokines lead to morphological changes. Accordingly, the a/a ma ft/ma ft/J mouse model can be used as an appropriate model to study early AD onset associated with profilaggrin deficiency.

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.

Common FLG mutation K4671X not associated with atopic dermatitis in Han Chinese in a family association study

BACKGROUND

Filaggrin gene (FLG) mutations have been identified as the cause of ichthyosis vulgaris (IV) and major predisposing factors for atopic dermatitis (AD). The relationship among AD, IV and FLG mutations has not been clarified yet. Mutations 3321delA and K4671X, two of the most common mutations in Chinese patients, were both statistically associated with AD in case-control studies.

MATERIALS AND METHODS

A group of 100 family trios (a total of 300 members with one affected AD proband and both parents) were recruited and screened for three filaggrin null mutations (3222del4, 3321delA and K4671X). The subjects' manifestations of AD and IV were assessed by two experienced dermatologists and recorded in detail. The relationship of common mutations to AD were assessed using both case-control and family-based tests of association. Filaggrin expression was measured in skin of 3 subjects with K4671X heterozygote and the normal control using quantitative real-time RT-PCR and immunohistochemistry.

RESULTS

Of 100 probands for AD, 22 were carriers for common FLG mutations and only 2 of them were from 40 none-IV family trios (5.00%), consistent with that of the healthy control group (3.99%, P>0.05). Significant statistical associations were revealed between AD and 3321delA (P<0.001, odds ratio 12.28, 95% confidence interval 3.35-44.98) as well as K4671X (P = 0.002, odds ratio 4.53, 95% confidence interval 1.77-11.60). The family-based approach revealed that 3321delA was over-transmitted to AD offspring from parents (T:U = 12∶1, P = 0.003) but failed to demonstrate transmission disequilibrium between K4671X and AD (T:U = 10∶8, P = 0.815). Moreover, compared to the normal control, filaggrin expression at both mRNA and protein levels in epidermis of subjects with K4671X(heter) was not reduced.

CONCLUSIONS

AD patients from none-IV family trios have low probability of carrying FLG mutations. The present family samples confirmed the susceptibility of mutation 3321delA to AD in Han Chinese. K4671X was not a pathogenic mutation.

[Filaggrin gene mutations are frequent and increase the risk of skin disease]

Filaggrin proteins are important for the epidermal differentiation and skin barrier function. About 10% of Europeans carry loss-of-function mutations within the filaggrin gene complex. Mutations are strongly associated with ichthyosis vulgaris and atopic dermatitis as well as dry and fissured skin. No overall association with psoriasis vulgaris has been identified. The filaggrin gene discovery has been a major breakthrough for dermatology in terms of diagnostics and prognostics, but it has not yet been translated into therapeutic advances.

Loss-of-function mutations within the filaggrin gene and atopic dermatitis

Atopic dermatitis (AD) is a chronic relapsing eczematous skin disorder that is frequently associated with elevated serum IgE levels and a personal or family history of AD, allergic rhinitis and/or asthma. Filaggrin is a major constituent of the stratum corneum (SC) and contributes to keratin filament aggregation. Its breakdown products form natural moisturizing factor, which plays a central role in hydration of the SC. Sequence analysis and epidemiological studies indicate that loss-of-function mutations in the filaggrin gene known to cause the autosomal dominant scaly skin disorder ichthyosis vulgaris are major genetic predisposing factors of AD. Mutations in filaggrin are also associated with atopic asthma. These findings established the 'filaggrin hypothesis,' which states that AD can be triggered by the chronic exposure of barrier-disrupted skin to percutaneous antigens due to abnormalities in filaggrin. In this chapter, we summarize the genome-wide screening of AD susceptibility loci, filaggrin biochemistry and recent epidemiological studies on filaggrin mutations and allergic diseases. We also summarize recent advances in the study of skin barrier mechanisms and filaggrin-associated skin barrier abnormalities in animal models. Taken together, these findings provide novel perspectives on the pathophysiology of AD and effective therapeutic methods for the treatment and/or prevention of AD through the modification of skin barrier dysfunction.

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.

[Mutations in the gene encoding filaggrin cause ichthyosis vulgaris]

Ichthyosis vulgaris is a common genetic skin disorder with an estimated prevalence of 1:250 caused by mutations in the gene encoding filaggrin. This disorder manifests itself within the first year of life and is clinically characterized by dry, scaly skin, keratosis pilaris, palmar hyperlinearity and atopic manifestations. Patients with a severe phenotype are homozygous or compound heterozygous for the mutations, whereas heterozygous patients show mild disease, suggesting semidominant inheritance with incomplete penetrance. We present a patient with classic severe ichthyosis vulgaris, atopic eczema and two loss-of-function mutations.

[Clinical features of the nephrotic syndrome associated with ichthyosis vulgaris and analysis of related gene mutation]

OBJECTIVE

To study clinical features of 3 children who presented with nephrotic syndrome (NS) associated with ichthyosis vulgaris (IV), and to detect relationship between NS associated with IV in patients and FLG gene or NPHS2 gene.

METHOD

Clinical and kidney pathological data of the 3 patients were analyzed and progress of pathologic damage in the patient kidney was observed through repeated percutaneous renal biopsy. Using polymerase chain reaction-single strand conformation polymorphism and DNA sequencing, the diversity of the expression of NPHS2 gene in the 3 patients were analyzed, and FLG gene in the 3 patients and parts of their family members with IV was detected.

RESULT

(1) The age of the 3 patients (patient 1 was a girl and patients 2 and 3 were boys) suffering from NS was 3 years and 8 months, 2 years and 6 months, and 5 years and 3 months, respectively. The age of onset of IV was 1 year and 6 months, 10 months, and 2 years and 6 months, respectively. All the 3 patients were resistant to steroid therapy. Despite multi-immunosuppressive therapy, no clinical response was achieved. The patients were followed up for 1.5 to 4.0 years. The patients displayed continuous proteinuria, renal function was normal, but their heights were lower than other children at the same age. (2) The older brother of patient 1 died of uremia. The other patients' family members did not have kidney disease. (3) Renal histopathology showed that the patients 1 and 2 had mild mesangial proliferative glomerulonephritis (MsPGN) and the patient 3 had minimal change disease (MCD). One and a half years after the first renal biopsy, the patients 1 and 2 underwent repeated renal biopsy. Renal histopathology showed that the 2 patients' disease developed to medium MsPGN. (4) None of the 3 patients had NPHS2 gene mutation. All the three patients had R501X and 2282del4 which are the common gene mutation type of the FLG, and all the patients were heterozygote. With the detection of the FLG gene of the part of the patients of the three families, the second patient's grandfather had the R501X homozygote mutation and the others were the R501X heterozygote mutation and 2282del4 heterozygote mutation.

CONCLUSION

The 3 cases of NS associated with IV had no response to steroid and multi-immunosuppressive therapy, the renal damage observed by histopathology progressed fast. The children with NS associated with IV displayed R501X heterozygote mutation and 2282del4 heterozygote mutation of FLG gene, which suggested that the absence of response to steroid and multi-immunosuppressive therapy may be related to the FLG gene.

A homozygous frameshift mutation in the mouse Flg gene facilitates enhanced percutaneous allergen priming

Loss-of-function mutations in the FLG (filaggrin) gene cause the semidominant keratinizing disorder ichthyosis vulgaris and convey major genetic risk for atopic dermatitis (eczema), eczema-associated asthma and other allergic phenotypes. Several low-frequency FLG null alleles occur in Europeans and Asians, with a cumulative frequency of approximately 9% in Europe. Here we report a 1-bp deletion mutation, 5303delA, analogous to common human FLG mutations, within the murine Flg gene in the spontaneous mouse mutant flaky tail (ft). We demonstrate that topical application of allergen to mice homozygous for this mutation results in cutaneous inflammatory infiltrates and enhanced cutaneous allergen priming with development of allergen-specific antibody responses. These data validate flaky tail as a useful model of filaggrin deficiency and provide experimental evidence for the hypothesis that antigen transfer through a defective epidermal barrier is a key mechanism underlying elevated IgE sensitization and initiation of cutaneous inflammation in humans with filaggrin-related atopic disease.

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

Filaggrin is an abundant protein of the outer epidermis that is essential for terminal differentiation of keratinocytes and formation of an effective barrier against water loss and pathogen/allergen/irritant invasion. Recent investigations in Europe and Japan have revealed null mutations in the filaggrin gene (FLG) as the underlying cause of ichthyosis vulgaris (IV), a common skin disorder characterised by dry skin, palmar hyperlinearity and keratosis pilaris. Following the development of a strategy for the comprehensive analysis of FLG, we have identified five unique mutations and one recurrent mutation in Singaporean Chinese IV patients. Mutation 441delA is located in the profilaggrin S100 domain, whereas two additional frameshift mutations, 1249insG and 7945delA, occur in the first partial filaggrin repeat ("repeat 0") and in filaggrin repeat 7, respectively. Both nonsense mutations Q2147X and E2422X are found in filaggrin repeat 6, whereas R4307X was found on one of the longer size variant alleles of FLG, within duplicated repeat 10.2. Mutation E2422X, previously found in a single Dutch patient, was found in one Singaporean IV patient and at a low frequency in Asian population controls. Our study confirms the presence of population-specific as well as recurrent FLG mutations in Singapore.

On the role of the epidermal differentiation complex in ichthyosis vulgaris, atopic dermatitis and psoriasis

Undisturbed epidermal differentiation is crucial for an intact skin barrier function. The epidermal differentiation complex (EDC) is a cluster of genes on chromosome 1q21 encoding proteins that fulfil important functions in terminal differentiation in the human epidermis, including filaggrin, loricrin, S100 proteins and others. Recently, evidence emerged that variation within EDC genes plays an important role in the pathogenesis of three common skin disorders, ichthyosis vulgaris, atopic dermatitis (AD) and psoriasis. Two loss-of-function mutations in the filaggrin (FLG) gene, R501X and 2282del4, were identified as causative for ichthyosis vulgaris in 15 affected European families, and the mode of inheritance was found to be semidominant. As ichthyosis vulgaris and AD often occur concomitantly in affected individuals, these two mutations were subsequently investigated in AD patients and found to be strongly associated with the disease. Following this first report, seven replication studies have been performed that all confirm an association of these two mutations with AD (or AD subtypes) in several European cohorts. Additionally, two unique loss-of-function mutations in the FLG gene were identified in Japanese ichthyosis vulgaris families and found to be associated with AD in a Japanese cohort. Thus, the FLG mutations are among the most consistently replicated associations for AD. Additionally, linkage analysis has suggested that variation within the EDC might also predispose for psoriasis but the exact susceptibility variation(s) have not yet been elucidated. Taken together, these findings convincingly demonstrate the important role of barrier dysfunction in various common skin disorders.

[Mapping of pathogenic genes in a pedigree with autosomal dominant ichthyosis vulgaris]

OBJECTIVE

To elucidate the pathogenic genes in a pedigree with autosomal dominant ichthyosis vulgaris (IV).

METHODS

Linkage analysis was performed by using STR markers in chromosome 1, and mutation detection was used to screen for FLG gene mutation.

RESULTS

A maximum two-point Lod score of 3.46 (theta=0) was obtained at D1S2696. Haplotype analysis placed the critical region in a 15-CM interval defined by D1S2726 and D1S305, but no mutation of FLG was found in our IV patients.

CONCLUSION

The pathologic gene of the IV family locates near D1S2696, and the FLG gene may not ruled out from the pathologic genes.

Comprehensive analysis of the gene encoding filaggrin uncovers prevalent and rare mutations in ichthyosis vulgaris and atopic eczema

We recently reported two common filaggrin (FLG) null mutations that cause ichthyosis vulgaris and predispose to eczema and secondary allergic diseases. We show here that these common European mutations are ancestral variants carried on conserved haplotypes. To facilitate comprehensive analysis of other populations, we report a strategy for full sequencing of this large, highly repetitive gene, and we describe 15 variants, including seven that are prevalent. All the variants are either nonsense or frameshift mutations that, in representative cases, resulted in loss of filaggrin production in the epidermis. In an Irish case-control study, the five most common European mutations showed a strong association with moderate-to-severe childhood eczema (chi2 test: P = 2.12 x 10(-51); Fisher's exact test: heterozygote odds ratio (OR) = 7.44 (95% confidence interval (c.i.) = 4.9-11.3), and homozygote OR = 151 (95% c.i. = 20-1,136)). We found three additional rare null mutations in this case series, suggesting that the genetic architecture of filaggrin-related atopic dermatitis consists of both prevalent and rare risk alleles.

Unique mutations in the filaggrin gene in Japanese patients with ichthyosis vulgaris and atopic dermatitis

BACKGROUND

Filaggrin is a key protein involved in skin barrier function. Recently, mutations in the filaggrin gene, FLG, were identified in European families with ichthyosis vulgaris (IV) and shown to be an important predisposing factor for atopic dermatitis (AD).

OBJECTIVE

To study the role of FLG mutations in IV/AD in Japan.

METHODS

The known filaggrin mutations were studied by genotyping and new mutations identified by DNA sequencing.

RESULTS

The European-specific mutations R501X and 2282del4 were absent from 253 Japanese individuals. We therefore sequenced the FLG gene in 4 Japanese families with IV and identified 2 novel mutations, 3321delA and S2554X. Immunohistologic and ultrastructural observations indicated that both truncation mutations lead to a striking reduction of keratohyalin granules in the epidermis. We screened 143 Japanese patients with AD for these FLG null mutations and identified them in 8 patients with AD (5.6%), including S2554X in 6 patients (4.2%) and 3321delA in 2 patients (1.4%). Both null variants were absent from 156 unrelated Japanese nonatopic and nonichthyotic controls, giving a significant statistical association between the FLG mutations and AD (chi(2)P value, .0015). This is the first report of FLG mutations in a non-European population.

CONCLUSION

Our data indicate that FLG mutations in Japan are unique from those found in European-origin populations.

CLINICAL IMPLICATIONS

Filaggrin null variants are also significant predisposing factors for AD in Japan and, on the basis of the recent European studies, may predict a more severe and persistent form of atopy.

Filaggrin mutations p.R501X and c.2282del4 in ichthyosis vulgaris

Ichthyosis vulgaris (IV) is the most common hereditary disorder of cornification in humans, characterized by generalized fine scaling of the skin, palmar hyperlinearity with or without keratosis pilaris and atopy. Recently, the molecular basis of IV was ascribed to loss-of-function mutations in the gene encoding filaggrin (FLG), namely p.R501X and c.2282del4. Homozygotes and compound heterozygotes were severely affected whereas heterozygotes showed mild disease or were asymptomatic, suggesting semidominant inheritance with incomplete penetrance in heterozygotes. We report the presence of FLG mutations in 15 out of 21 IV patients with a marked generalized scaling phenotype, including eight affected members of a four-generation family. In this group of patients not only homozygous and compound heterozygous, but also heterozygous patients for p.R501X and c.2282del4 display a pronounced phenotype, whereas in none of six individuals these two mutations were detectable despite decreased filaggrin expression on immunohistochemistry in two patients, indicating that other mutations in FLG and/or in other genes remain to be identified. In contrast, two additional p.R501X heterozygotes from the extended family are asymptomatic. In a control population from west-Austria a combined p.R501X and c.2282del4 carrier frequency of 6/110 (5.45%) was observed. We confirm that these FLG variants are common, but our results point to the existence of additional modifiers.

Prevalent and Rare Mutations in the Gene Encoding Filaggrin Cause Ichthyosis Vulgaris and Predispose Individuals to Atopic Dermatitis

Mutations in the filament aggregating protein (filaggrin) gene have recently been identified as the cause of the common genetic skin disorder ichthyosis vulgaris (IV), the most prevalent inherited disorder of keratinization. The main characteristics of IV are fine-scale on the arms and legs, palmar hyperlinearity, and keratosis pilaris. Here, we have studied six Irish families with IV for mutations in filaggrin. We have identified a new mutation, 3702delG, in addition to further instances of the reported mutations R501X and 2282del4, which are common in people of European origin. A case of a 2282del4 homozygote was also identified. Mutation 3702delG terminates protein translation in filaggrin repeat domain 3, whereas both recurrent mutations occur in repeat 1. These mutations are semidominant: heterozygotes have an intermediate phenotype most readily identified by palmar hyperlinearity and in some cases fine-scale and/or keratosis pilaris, whereas homozygotes or compound heterozygotes generally have more marked ichthyosis. Interestingly, the phenotypes of individuals homozygous for R501X, 2282del4, or compound heterozygous for R501X and 3702delG, were comparable, suggesting that mutations located centrally in the filaggrin repeats are also pathogenic.

Epidermal differentiation complex yields a secret: mutations in the cornification protein filaggrin underlie ichthyosis vulgaris

Ichthyosis vulgaris (IV), characterized by mild scaling on limbs and lower abdomen, has an incidence of 1 in 250. Smith, McLean, and colleagues demonstrate that common mutations in filaggrin underlie IV. Filaggrin aggregates keratin intermediate filaments and is cross-linked into the cornified envelope to form the epidermal barrier. These findings reinforce the importance of the epidermal barrier in pathogenesis of skin diseases.

Loss-of-function mutations in the gene encoding filaggrin cause ichthyosis vulgaris

Ichthyosis vulgaris (OMIM 146700) is the most common inherited disorder of keratinization and one of the most frequent single-gene disorders in humans. The most widely cited incidence figure is 1 in 250 based on a survey of 6,051 healthy English schoolchildren. We have identified homozygous or compound heterozygous mutations R501X and 2282del4 in the gene encoding filaggrin (FLG) as the cause of moderate or severe ichthyosis vulgaris in 15 kindreds. In addition, these mutations are semidominant; heterozygotes show a very mild phenotype with incomplete penetrance. The mutations show a combined allele frequency of approximately 4% in populations of European ancestry, explaining the high incidence of ichthyosis vulgaris. Profilaggrin is the major protein of keratohyalin granules in the epidermis. During terminal differentiation, it is cleaved into multiple filaggrin peptides that aggregate keratin filaments. The resultant matrix is cross-linked to form a major component of the cornified cell envelope. We find that loss or reduction of this major structural protein leads to varying degrees of impaired keratinization.

Mixed sclerosing dysplasia of the bone associated with ovarian and skin problems

The clinical, radiologic, histopathologic, and genetic features of a sclerosing bone dysplasia found in three members of a family are described. The bone disorder affected all metaphyseal-diaphyseal regions of all long bones, the skull, and three metacarpals. All affected members also had ichthyosis vulgaris and premature ovarian failure. These features have not been reported before, indicating a new syndrome.

Hypoplastic thyroid, growth hormone deficiency, corneal opacities, cataract and hyperkeratotic skin disease: a possible new ichthyosis syndrome associated with endocrinopathies

A 56 year old man presented with ichthyosis vulgaris since early childhood, clinically characterised by fine scaling of the trunk and hyperkeratotic scales on the exterior surfaces of the upper and lower extremities. The patient also showed hypothyroidism due to hypoplastic thyroid, cataract, hypercholesterinemia with concommitant arcus cornealis and biliary concrements. Renal lithiasis caused by calcio-oxalate was additionally present. Endocrinological screening revealed growth hormone deficiency in the 1.55 m tall man-(secondary) osteoporosis was observed. The clinical symptomatology indicates that this case cannot be considered as a subtype of the inherited ichthyosis group, but suggests a new syndrome as a separate nosologic entity.

Linkage analysis suggests a locus of ichthyosis vulgaris on 1q22

Ichthyosis vulgaris (IV) is an inherited scaling skin disorder with a prevalence estimated at 2.29% in China. The gene responsible for this disorder has not been elucidated. To find the disease gene, we ascertained two Chinese IV families. Linkage analysis identified an IV locus on chromosome 1q22 with a maximum two-point Lod score of 2.47 at D1S1653 (theta=0.00). Haplotype analysis placed the critical region in a 7-cM interval defined by D1S1653 and D1S2675. These results provide the basis for further identifying the gene responsible for IV disorder.

Mapping of the associated phenotype of an absent granular layer in ichthyosis vulgaris to the epidermal differentiation complex on chromosome 1

Ichthyosis vulgaris (IV) is a mild to severe scaling disorder of uncertain etiology estimated to affect as many as 1 : 250 in the population. Family studies have shown that in many cases IV follows an autosomal dominant inheritance pattern, but gene mapping studies have not been reported. To investigate the genetic basis for inherited IV, we have performed gene linkage studies in two multigenerational families where affected individuals have clinical features of IV but distinct histological features. The epidermis in this disorder characteristically displays non-specific orthohyperkeratosis. Notably, a subset of IV patients with a reduced or absent granular epidermal layer (AGL) have been reported, and decreased filaggrin levels have been described in others. The prominent role of profilaggrin in human keratohyalin suggests that defects in the gene for profilaggrin (FLG), its processing of profillagrin to filaggrin, or a gene involved in profilaggrin regulation may underlie or modify the pathology in IV. Family 1 had seven individuals with IV, severe heat intolerance and epidermis with 1-3 granular layers (consistent with normal epidermal histology). Ichthyosis vulgaris in this family did not segregate with FLG or other genes in the epidermal differentiation complex. In contrast, five of the six IV patients in Family 2, all siblings, had epidermis with no granular layer. Significant evidence was obtained for linkage of IV with the associated AGL phenotype to the epidermal differentiation complex (which includes FLG) assuming either a recessive (max Lod 3.4) or dominant (max Lod 3.6) inheritance model. Sequence analysis of FLG did not reveal a mutation in the amino or carboxyl terminal portions of the coding sequence adjacent to filaggrin repeats. The AGL may represent an endophenotype for IV, and the presence of a modifier of IV pathology at this locus is discussed.

Absence of the granular layer and keratohyalin define a morphologically distinct subset of individuals with ichthyosis vulgaris

The clinical diagnosis of ichthyosis vulgaris (IV) can be difficult. Abnormalities in the granular layer and the ultrastructure of keratohyalin granules (KHG) suggest that morphology may be helpful. To clarify morphologic findings in IV, 41 clinically affected individuals and 21 unaffected family members or age- and sex-matched controls were studied by light microscopy. In these, the granular layer was totally absent in approximately 50% of affected individuals, while present in all controls. Forty-seven individuals in the light microscopy group were also studied by electron microscopy. Keratohyalin granules were absent in all affected individuals lacking the granular layer by light microscopy. Clinical severity usually correlated with the lack of a granular layer and KHG. Absence of the granular layer was consistent in different anatomic sites and in serial biopsies taken over a 1-3-year period. In a subset of clinically affected, unrelated subjects with moderate to severe involvement, four out of 11 (36%) had similar findings. Keratinocytes cultured from affected individuals with no KHG expressed virtually no detectable profilaggrin protein in vitro. The data suggest that a subset of individuals with moderate to severe IV have a consistently absent granular layer and KHG. Absence of the granular layer and lack of KHG correlated almost perfectly; thus light microscopy offers a convenient means of identifying this subtype of IV. However, both morphologic types of IV were observed within single families. Therefore, the relationship between granular layer abnormalities and IV is complex and requires the study of more affected families. One interpretation of the data is that IV is a multigenic disorder in which one of the genes alters profilaggrin expression. We propose this clinical and histologic phenotype as useful for identifying the gene(s) involved and also for determining whether it represents a modifier or a major locus of the disorder.

Reduced stability and bi-allelic, coequal expression of profilaggrin mRNA in keratinocytes cultured from subjects with ichthyosis vulgaris

Ichthyosis vulgaris (IV) is an inherited scaling skin disorder in which expression of profilaggrin is reduced. Previous studies have indicated that the reduction is caused by defective post-transcriptional control of gene expression. Here we present evidence that profilaggrin mRNA in keratinocytes cultured from subjects with IV is intrinsically unstable and has a shorter half-life compared with that in normal cells. When IV-affected keratinocytes were treated with the protein synthesis inhibitor cycloheximide, the steady-state level of profilaggrin mRNA was increased due to stabilization of the transcript. In addition, the number of filaggrin repeats within the profilaggrin gene was studied. The number of filaggrin repeats (10-12) in individuals with IV did not differ from that of unaffected subjects. Expression of the gene was bi-allelic and coequal in both control and affected individuals. Our results suggest a model in which a labile ribonuclease and a stabilizing factor may modulate the profilaggrin mRNA steady-state level in normal cells, whereas the stabilizing factor may be absent or functionally inactive in IV-affected keratinocytes.

A study of the steroid sulfatase gene in families with X-linked ichthyosis using polymerase chain reaction

We have studied the steroid sulfatase (STS) gene in three Japanese families with X-linked ichthyosis (XLI), using polymerase chain reaction (PCR). PCR was performed using three sets of intraexonic primers covering exons 1, 5 and 10. In affected individuals from two of the families, DNA was not amplified in any of the three exons, suggesting that XLI in these families was due to the complete deletion of the STS gene. In affected individuals in the remaining family, DNA was amplified in predicted sizes in exons 1 and 5, but not in exon 10, suggesting that XLI in this family was due to partial deletion of the STS gene including exon 10. These results suggested that STS gene deficiency is heterogeneous in Japanese families with XLI. PCR is useful for the rapid diagnosis of XLI, the differentiation of XLI from ichthyosis vulgaris, and genetic counseling of XLI families. The PCR method was not applicable for carrier detection.

Decreased profilaggrin expression in ichthyosis vulgaris is a result of selectively impaired posttranscriptional control

Ichthyosis vulgaris is an autosomal dominant disorder of keratinization characterized by mild hyperkeratosis and reduced or absent keratohyalin granules in the epidermis. Profilaggrin, a major component of keratohyalin granules, is reduced or absent from the skin of individuals with ichthyosis vulgaris. In this report, we have further characterized the molecular basis of low profilaggrin expression, which occurs in this disease. In situ hybridization revealed little profilaggrin mRNA in ichthyosis vulgaris-affected epidermis. In keratinocytes cultured from the epidermis of affected individuals, the abundance of profilaggrin was reduced to less than 10% of normal controls, while the mRNA level was decreased to 30-60% of controls. Expression of K1 and loricrin, other markers of epidermal differentiation, were not affected. Nuclear run-on assays indicated that the decrease in mRNA levels was not caused by aberrant transcription. Nucleotide sequencing of 5'-upstream, 3'-non-coding, and flanking regions of the profilaggrin gene from ichthyosis vulgaris-affected individuals revealed only minor changes, probably due to genetic polymorphisms. Our results indicate that defective profilaggrin expression in ichthyosis vulgaris is a result of selectively impaired posttranscriptional control.