Epistasis between FLG and IL4R Genes on the Risk of Allergic Sensitization: Results from Two Population-Based Birth Cohort Studies

Incident Asthma, Asthma Exacerbations, and Asthma-Related Hospitalizations in Patients With Atopic Dermatitis

BACKGROUND

Atopic dermatitis (AD) is thought to induce asthma via the "atopic march," but the effects of AD on incident asthma and asthma severity have not been fully characterized.

OBJECTIVE

To determine risk of asthma, asthma exacerbations, and asthma-related hospitalizations among patients fwith AD.

METHODS

A cohort study was conducted using electronic health records data from UK general practices from 1994 to 2015. Children (<18 years old) and adults (≥18 years) with AD were matched on age, practice, and index date to patients without AD. AD severity was categorized using treatments and dermatologist referrals. Outcomes were incident asthma among all patients and asthma exacerbation or hospitalization among patients with asthma.

RESULTS

On comparing 409,341 children with AD (93.2% mild, 5.5% moderate, 1.3% severe) with 1,809,029 unaffected children, those with AD were found to be associated with a 2-fold greater risk of asthma compared with those without AD (hazard ratio, 1.96; 95% CI, 1.93-1.98). On comparing 625,083 adults with AD (65.7% mild, 31.4% moderate, and 2.9% severe) with 2,678,888 unaffected adults, AD was found to be associated with a 38% higher risk of asthma (hazard ratio, 1.38; 95% CI, 1.36-1.40). Asthmatic patients with AD also had a 21% to 63% greater risk of asthma exacerbations and a 20% to 64% greater risk of asthma-related hospitalizations compared with asthmatic patients without AD. Risk of asthma, asthma exacerbation, or asthma-related hospitalization increased with AD severity in a dose-dependent manner in both the pediatric and adult cohorts.

CONCLUSIONS

AD, especially in children and when more severe, is associated with greater risk of asthma as well as greater risk of asthma exacerbations and hospitalizations among asthmatic patients.

Might biologics serve to interrupt the atopic march?

The atopic march was described more than 20 years ago on the basis of initial observations, and it is now seen in prospective studies. The concept has evolved and is now considered to be the progression of atopic dermatitis to other atopic conditions, including asthma, allergic rhinitis, food allergy, and eosinophilic esophagitis in a nonlinear fashion. The progression can include some or all of the aforementioned atopic conditions. The pathogenesis is part of the classic type 2 inflammatory process involving IL-4, IL-5, and IL-13 preceded by induction of the alarmins (thymic stromal lymphopoietin, IL-33, and IL-25), leading to production of IgE in a genetically predisposed individual. The development of new biologics that interact with T2 pathway represent possible ways to prevent or modify the atopic march.

The link between atopic dermatitis and asthma- immunological imbalance and beyond

Atopic diseases are multifactorial chronic disturbances which may evolve one into another and have overlapping pathogenetic mechanisms. Atopic dermatitis is in most cases the first step towards the development of the atopic march and represents a major socio-economic burden in the industrialized countries. The treatment of atopic diseases is often long-lasting and in some cases with lower effectiveness than expected.

In order to prevent the development of the atopic march, the links between the atopic diseases have to be understood. The aim of this review is to present some major points outlining the link between atopic dermatitis and asthma, through a research in the medical literature from recent years.

Stratifying patient populations according to the clinical phenotype of their disease and according to specific measurable values (biomarkers) can help to establish the main etiopathogenetic mechanisms of the disease in these populations. This will add predictive value for the evolution of the disease, and will allow the use and research of more targeted therapy in order to stop this evolution and comorbidities.

Longitudinal atopic dermatitis endotypes: An atopic march paradigm that includes Black children

BACKGROUND

The atopic march has been studied mostly in White populations, biasing our current paradigms.

OBJECTIVE

We sought to define the atopic march in Black and White children and explore mechanisms for racial differences.

METHODS

Utilizing the Mechanisms of Progression of Atopic Dermatitis to Asthma in Children (MPAACH) cohort (n = 601), we assessed longitudinal sensitization, food allergy (FA), allergic rhinitis, risk of asthma development (through the Pediatric Asthma Risk Score), Scoring for Atopic Dermatitis (SCORAD), transepidermal water loss, skin filaggrin (FLG) expression, exposures, and genetic heritability to define AD progression endotypes in Black and White children.

RESULTS

White MPAACH children were more likely to be sensitized to aero and food allergens (P = .0001) and over 3 times more likely to develop FA and/or allergic rhinitis (AR) without asthma risk (P < .0001). In contrast, Black children were over 6 times more likely to proceed to high asthma risk without FA, sensitization, or AR (P < .0001). White children had higher lesional and nonlesional transepidermal water loss (both P < .001) as well as decreased nonlesional keratinocyte FLG expression (P = .02). Black children had increased genetic heritability for asthma risk and higher rates of exposures to secondhand smoke and traffic-related air pollution.

CONCLUSIONS

Black and White children with AD have distinct allergic trajectories defined by different longitudinal endotypes. Black children exhibit higher asthma risk despite a more intact skin barrier and less sensitization, FA, and AR. White children have less asthma risk, despite a more dysfunctional skin barrier, and more FA, AR, and sensitization. The observed racial differences are likely due in part to increased genetic heritability for asthma risk and harmful environmental exposures in Black children. Collectively, our findings provide a new paradigm for an atopic march that is inclusive of Black children.

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.

Early-life inhalant allergen exposure, filaggrin genotype, and the development of sensitization from infancy to adolescence

Background

We hypothesized that filaggrin (FLG) loss-of-function mutations modify the effect of allergen exposure on the development of allergic sensitization.

Objective

We sought to determine whether early-life exposure to inhalant allergens increases the risk of specific sensitization and whether FLG mutations modulate these odds.

Methods

In a population-based birth cohort we measured mite, cat, and dog allergen levels in dust samples collected from homes within the first year of life. Sensitization was assessed at 6 time points between infancy and age 16 years. Genotyping was performed for 6 FLG mutations.

Results

In the longitudinal multivariable model (age 1-16 years), we observed a significant interaction between FLG and Fel d 1 exposure on cat sensitization, with the effect of exposure being significantly greater among children with FLG mutations compared with those without (odds ratio, 1.36; 95% CI, 1.02-1.80; P = .035). The increase in risk of mite sensitization with increasing Der p 1 exposure was consistently greater among children with FLG mutations, but the interaction did not reach statistical significance. Different associations were observed for dogs: there was a significant interaction between FLG and dog ownership, but the risk of sensitization to any allergen was significantly lower among children with FLG mutations who were exposed to a dog in infancy (odds ratio, 0.16; 95% CI, 0.03-0.86; P = .03).

Conclusions

FLG loss-of-function mutations modify the relationship between allergen exposure and sensitization, but effects differ at different ages and between different allergens.

Blood Biomarkers of Sensitization and Asthma

Biomarkers are essential to determine different phenotypes of childhood asthma, and for the prediction of response to treatments. In young preschool children with asthma, aeroallergen sensitization, and blood eosinophil count of 300/μL or greater may identify those who can benefit from the daily use of inhaled corticosteroids (ICS). We propose that every preschool child who is considered for ICS treatment should have these two features measured as a minimum before a decision is made on the commencement of long-term preventive treatment. In practice, IgE-mediated sensitization should be considered as a quantifiable variable, i.e., we should use the titer of sIgE antibodies or the size of skin prick test response. A number of other blood biomarkers may prove useful (e.g., allergen-specific IgG/IgE antibody ratios amongst sensitized individuals, component-resolved diagnostics which measures sIgE response to a large number of allergenic molecules, assessment of immune responses to viruses, level of serum CC16, etc.), but it remains unclear whether these can be translated into clinically useful tests. Going forward, a more integrated approach which takes into account multiple domains of asthma, from the pattern of symptoms and blood biomarkers to genetic risk and lung function measures, is needed if we are to move toward a stratified approach to asthma management.

The atopic march and atopic multimorbidity: Many trajectories, many pathways

The atopic march recognizes the increased occurrence of asthma, allergic rhinitis, or both after atopic dermatitis (AD) onset. Mechanisms for developing atopic comorbidities after AD onset are poorly understood but can involve the impaired cutaneous barrier, which facilitates cutaneous sensitization. The association can also be driven or amplified in susceptible subjects by a systemic T_H2-dominant immune response to cutaneous inflammation. However, these associations might merely involve shared genetic loci and environmental triggers, including microbiome dysregulation, with the temporal sequence reflecting tissue-specific peak time of occurrence of each disease, suggesting more of a clustering of disorders than a march. Prospective longitudinal cohort studies provide an opportunity to explore the relationships between postdermatitis development of atopic disorders and potential predictive phenotypic, genotypic, and environmental factors. Recent investigations implicate disease severity and persistence, age of onset, parental atopic history, filaggrin (FLG) mutations, polysensitization, and the nonrural environment among risk factors for development of multiple atopic comorbidities in young children with AD. Early intervention studies to repair the epidermal barrier or alter exposure to the microbiome or allergens might elucidate the relative roles of barrier defects, genetic locus alterations, and environmental exposures in the risk and sequence of occurrence of T_H2 activation disorders.