Atopic diseases (including atopic dermatitis/eczema, and asthma) have increased in frequency in the last several decades (1) and now affect up to 20% of the population in the developed world. Twin and family studies have shown that predisposition to atopic disease is highly heritable (2). To date most genetic studies have focused on immunological mechanisms behind regulation of IgE and atopy, however, a primary genetic epithelial barrier defect has been anticipated (3). Recently we identified two functional genetic variants in the filaggrin gene (FLG) affecting epithelial barrier formation (Smith FJD et al., Nature Genetics, in press, preprint attached). In a second study, we have shown that these filaggrin variants show clear predisposition to this group of common conditions. Filaggrin is a key protein that facilitates terminal differentiation of the epidermis and formation of the skin barrier. Two independent variants in this gene carried by ~9% of Caucasians, result in haploinsufficiency. We have shown that these null alleles are responsible for a substantial genetic predisposition to asthma, and atopic dermatitis (Palmer C et al, under review, Nature Genetics).

In this study the two null variants were genotyped in 1008 Scottish Caucasian schoolchildren with unknown disease status (population cohort) and in 604 Scottish Children with asthma. The frequency of carriers of mutation R501X was 5.8% and the mutation 2282del4 variant was present in 3.8% of the schoolchildren, giving a combined carrier frequency of 9.6%. Both filaggrin variants were over-represented in the asthmatic cohort (mutation R501X: 9.2%, mutation 2282del4: 7.5%, combined carrier frequency: 15.7%) with carriers of either allele demonstrating a dominant risk for asthma (mutation X dominant OR = 1.6, 95%CI = 1.1-2.3, p = 0.025, mutation del4 dominant OR = 2.1 95%CI= 1.3-3.2, p = 0.002, combined genotype dominant OR = 1.8 95%CI= 1.3-2.5, p = 0.0002).

AD is co-associated with asthma (3) and since this is a major epidermal protein, null alleles should be expected to be more strongly associated with the combined AD/asthma phenotype than with asthma alone. Consistent with this, 72% of all the children in the asthma cohort carrying a null allele had AD, in contrast to only 46% of those without these filaggrin variants. A co-dominant model comparing individuals with asthma and AD to the population controls provided the best fit and the most significant association (p = 4.8x10-11).

Over the past several years, the mechanisms by which allergen exposure through the skin could initiate systemic allergy and predispose to asthma have become clearer. Epicutaneous sensitization with a protein allergen has been shown to induce a Th2-type immune response with concomitant high IgE production in mice (4). In mice sensitized with ovalbumin, cutaneous exposure of this allergen induced a T cell and eosinophil-rich cellular infiltrate along with a cytokine response that closely mirrored those seen in AD. Significantly, in addition to generating allergen-specific IgE, these mice developed airway hyper-responsiveness on intravenous challenge with methacholine after a single inhalation challenge with ovalbumin (5). These observations imply that the presence of a heritable defect in the skin barrier, in a substantial proportion of the asthmatic patients in our dataset, who would have increased susceptibility to transepidermal allergen transfer, therefore amplifying the epidermal-driven Th2 response and provoking the onset of atopic disease.

AIMS AND OBJECTIVES OF THIS PROPOSAL

This proposal seeks to further develop the initial findings detailed above and to specifically study:

1. To accurately delineate the longitudinal health risk of happloinsufficiency in this key epidermal protein with particular reference to phenotypic expression of atopic disease including atopic dermatitis, asthma, and allergic rhinitis and intermediate atopic traits such as skin prick test positivity to house dust mice and grasses and bronchial hyperesponsiveness.
2. To study the frequency of these alleles in subsets within the study population identified as having atopic dermatitis, asthma or rhinitis and all combinations of these conditions. We expect to be able to accurately quantify the population-attributable risk of these functional polymorphisms for each of these atopic phenotypes.
3. To examine possible genetic modifiers of the phenotype (gene:gene interactions) in the approximately 900 individuals we expect to identify as happolinsufficient for this key epidermal protein. Possible candidate genes for this phase of the study include genes involved in formation of the stratum corneum and in the inflammatory cascade. As this aspect of the study develops we will discuss each of these potential candidate genes individually with the committee prior to commencing any studies.
4. To explore possible gene:environment interactions in this cohort of 900 individuals. Relevant environmental parameters would include urban versus rural living, smoking by parents, keeping pets, type of accommodation. This will not form part of the preliminary study and will be discussed in detail with the committee prior to work commencing.