The filaggrin null genotypes R501X and 2282del4 seem not to be associated with psoriasis: results from general population study and meta-analysis

Decreased TET2/5-hmC reduces the integrity of the epidermal barrier via epigenetic dysregulation of filaggrin in psoriatic lesions

BACKGROUND

TET2 participates in tumor progression and intrinsic immune homeostasis via epigenetic regulation. TET2 has been reported to be involved in maintaining epithelial barrier homeostasis and inflammation. Abnormal epidermal barrier function and TET2 expression have been detected in psoriatic lesions. However, the mechanisms underlying the role of TET2 in psoriasis have not yet been elucidated.

OBJECTIVE

To define the role of TET2 in maintaining epithelial barrier homeostasis and the exact epigenetic mechanism in the dysfunction of the epidermal barrier in psoriasis.

METHODS

We analyzed human psoriatic skin lesions and datasets from the GEO database, and detected the expression of TET2/5-hmC together with barrier molecules by immunohistochemistry. We constructed epidermal-specific TET2 knockout mice to observe the effect of TET2 deficiency on epidermal barrier function via toluidine blue penetration assay. Further, we analyzed changes in the expression of epidermal barrier molecules by immunofluorescence in TET2-specific knockout mice and psoriatic model mice.

RESULTS

We found that decreased expression of TET2/5-hmC correlated with dysregulated barrier molecules in human psoriatic lesions. Epidermal-specific TET2 knockout mice showed elevated transdermal water loss associated with abnormal epidermal barrier molecules. Furthermore, we observed that TET2 knockdown in keratinocytes reduced filaggrin expression via filaggrin promoter methylation.

CONCLUSION

Aberrant epidermal TET2 affects the integrity of the epidermal barrier through the epigenetic dysregulation of epidermal barrier molecules, particularly filaggrin. Reduced TET2 expression is a critical factor contributing to an abnormal epidermal barrier in psoriasis.

Filaggrin gene mutations with special reference to atopic dermatitis

Purpose of review

Mutations in the Filaggrin gene can cause absent or reduced filaggrin protein, leading to impaired keratinization and skin barrier defect, which produce characteristic phenotypes. In this short review, we report current evidence on the topic with special reference to atopic dermatitis, suggest future directions, and discuss therapeutic implications.

Recent findings

Numerous candidate gene association studies, genome-wide association studies, studies on copy number variations and most recently, sequencing studies, have confirmed the robust association of mutations in the Filaggrin gene with atopic dermatitis, and have also linked these mutations with several other disorders.

Summary

Filaggrin gene defects remain the strongest identified genetic risk factors for atopic dermatitis. Taken in conjunction with other genes found to be associated with this condition, genetic screening and identification of individuals at risk for atopic dermatitis could lead to personalized therapy. Manipulation of genetic regulatory elements to increase the amount of filaggrin protein in deficient individuals is an attractive treatment option for the future.

Strengths and limitations of the United Kingdom Working Party criteria for atopic dermatitis in adults

BACKGROUND

The United Kingdom Working Party's (UKWP) criteria were developed to improve epidemiological research in atopic dermatitis (AD), but have not been validated in an exclusively adult European population.

OBJECTIVE

To validate the UKWP criteria for AD in adults.

METHODS

In this cross-sectional study, three independent samples of adult individuals were drawn and interviewed: patients with a hospital diagnosis of AD or plaque psoriasis in adulthood, and general population controls. Various versions of the UKWP criteria for AD were utilized.

RESULTS

A total of 3490 (general population), 3834 (AD) and 4016 (psoriasis) adult individuals were enrolled in the study. The best combination of the UKWP criteria leads to a sensitivity of 0.71 and a specificity of 0.96 in the general population. The criteria better captured 'AD ever' compared with 'AD within the past 12 months' and had a higher sensitivity in patients with moderate (87.2-97.7%) or severe (95.8-100%) AD at the time of interview compared with those who where asymptomatic (12.6-36.8%). The UKWP criteria also captured high proportions of psoriasis patients (19.7-47.7%) when applied in a cohort of unique psoriasis patients.

CONCLUSIONS

It remains a challenge to accurately diagnose a history of AD in adulthood since symptoms are shared with other skin conditions and AD may have resolved or can be waxing and waning, in turn leading to recall bias. The UKWP criteria performed well in the general population for the purpose of determining the prevalence, but should be used cautiously when studying comorbidity.

Epidermal Expression of Filaggrin/Profilaggrin Is Decreased in Atopic Dermatitis: Reverse Association With Mast Cell Tryptase and IL-6 but Not With Clinical Severity

BACKGROUND

A decrease in filaggrin expression contributes to the pathogenesis of atopic dermatitis (AD) and can be modified by inflammatory factors.

OBJECTIVES

The aim of this study was to determine the correlation of (pro)filaggrin (filaggrin and profilaggrin) expression with clinical severity in AD and with mast cell (MC) tryptase, chymase, and IL-6.

METHODS

Punch biopsies were collected from 17 patients with moderate-to-severe AD and from 10 psoriatic patients. Atopic dermatitis severity was measured using different clinical parameters. (Pro)filaggrin, MC tryptase, chymase, and IL-6 were stained using immunohistochemical, enzymehistochemical, and sequential double-staining methods.

RESULTS

(Pro)filaggrin expression was lower in the lesional than in the nonlesional granular layer in AD and was correlated negatively with itch severity but not with other severity parameters. (Pro)filaggrin expression was also decreased in the psoriatic lesions. In AD, (pro)filaggrin expression correlated negatively with the number of tryptase MCs in the nonlesional granular layer and with IL-6 MCs in both the nonlesional and lesional granular layers.

CONCLUSION

(Pro)filaggrin expression is decreased in AD and is reversely associated with MC tryptase and IL-6. However, it does not associate with disease severity, and it was also decreased in psoriasis.

Genetic background of skin barrier dysfunction in the pathogenesis of psoriasis vulgaris

Psoriasis is a common inflammatory skin disease. It is known to be a complex condition with multifactorial mode of inheritance, however the associations between particular pathogenic pathways remain unclear. A novel report on the pathogenesis of psoriasis has recently included the genetic determination of the skin barrier dysfunction. In this paper, we focus on specific genetic variants associated with formation of the epidermal barrier and their role in the complex pathogenesis of the disease.

The role of filaggrin in the skin barrier and disease development

Filaggrin is a structural protein that is fundamental in the development and maintenance of the skin barrier. The function of filaggrin and its involvement in various cutaneous and extracutaneous disorders has been the subject of considerable research in recent years. Mutations in FLG, the gene that encodes filaggrin, have been shown to cause ichthyosis vulgaris, increase the risk of atopic dermatitis and other atopic diseases, and exacerbate certain conditions. The present article reviews the current knowledge on the role of filaggrin in the skin barrier, FLG mutations, and the consequences of filaggrin 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.

A novel multiplex analysis of filaggrin polymorphisms: a universally applicable method for genotyping

BACKGROUND

The filaggrin protein is expressed as profilaggrin mainly in stratum granulosum cells of the epidermis. The profilaggrin gene codes for 10-12 filaggrin repeats. The filaggrin protein is important for skin barrier function. Filaggrin deficiency due to functional null-polymorphisms affects 8-10% of the people in Northern Europe and is a strong risk factor for several diseases. Here, we describe a novel method for efficient, multiplexed genotyping of variations in the profilaggrin gene.

METHODS

Five known techniques were combined: i) allele-specific PCR, ii) PCR with tagged primers, iii) asymmetric PCR, iv) multiplex PCR, and v) hybridization of single-stranded PCR products to spectrally coded microbeads carrying tag sequences as capture probes. Asymmetry of PCR was accomplished by having the tagged and allele-specific forward primers present in limiting concentrations. Asymmetry ensured that the later PCR cycles generated only single-stranded reverse-strand products. This greatly improved the assay sensitivity and allowed for simple optimization.

RESULTS

The specificity of the tags was verified with single PCR in wildtype and homozygous samples. Only the PCR products with the appropriate anti-tag hybridized to the corresponding beads, demonstrating the specificity of the signal. The hybridization signal is strongly dependent on single-stranded PCR products. After 46 PCR cycles, double-stranded products are clearly present, but only the single-stranded products generated in later cycles hybridize to the beads and elicit the strong signals that allow for unambiguous genotyping.

CONCLUSIONS

We have tested 17,000 samples for three filaggrin polymorphisms using this method, with a call rate exceeding 99% and a reagent cost of US $ 0.75 per sample. The method is universally applicable for multiplex genotyping of e.g. hereditary hemochromatosis, lactose intolerance, or cystic fibrosis.

Loss-of-function mutations in filaggrin gene associate with psoriasis vulgaris in Chinese population

Loss-of-function mutations in filaggrin gene (FLG; OMIM #135940) have been reported to cause the semi-dominant keratinizing disorders such as ichthyosis vulgaris (IV; OMIM #146700) and atopic dermatitis (AD; OMIM #605803). Recent linkage analysis and immunohistochemical studies suggest the possible contribution of FLG to psoriatic susceptibility. However, no susceptibility variant in FLG gene associated with psoriasis (OMIM #177900) has been identified. In this study, we identified a non-sense mutation of FLG (p.K4022X) in a Chinese psoriasis/IV coexisting family. The homozygous p.K4022X mutation was detected in a psoriasis patient, whereas the heterozygous p.K4022X mutation was identified in two IV patients and four apparently normal family members. We also genotyped p.K4022X variant in 441 sporadic Chinese psoriasis patients and found homozygous mutation in two patients, while no homozygous variant was found in 500 control individuals. After sequencing the entire coding region of FLG gene in 441 psoriasis patients, we identified another five mutations (p.R826X, p.W2583X, c.7945delA, c.3321delA and p.Q2417X). Although all six FLG mutations as a whole was not significantly associated with psoriasis (P = 0.105), mutation p.K4022X was significantly associated with psoriasis (P < 0.05). Our data thus indicates an association of FLG with psoriasis in Chinese population.