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GENETICS AND EPIDEMIOLOGY OF ATOPIC DERMATITIS

Although research on atopic dermatitis has been going on over the last 40 years, only the last 7 years have witnessed considerable growth in atopic dermatitis epidemiology studies. Epidemiology concerns occurrence and risk factors of the disease and can be of help in genetic counseling.

Epidemiology

To get a precise idea about the risk factors for atopic dermatitis, it is important to have an accurate estimate about the prevalence of the disease. In addition, without developing certain diagnostic criteria, which are standardized, validated and universally applicable, it is not possible to get accurate data on atopic dermatitis prevalence.

Currently, the epidemiology of atopic dermatitis is based on less than perfect data about its prevalence. However, it is now estimated that atopic dermatitis affects 10% to 20% of children worldwide, especially those less than five years old. The prevalence in US is 17% and worldwide it is 10% to 25%. The incidence of atopic dermatitis is increasing and along with the cost of treatment, becoming an increasing burden on society. It can occur anywhere in the world and in any race.

Epidemiological studies have shown that genetic factors carry the highest disk factor. In general it is a disease of the rich; it’s incidence in a family is inversely related to family size i.e. the larger the family, the less the likelihood of atopic dermatitis. It occurs more among the migrants and finally, its increasing incidence strongly points towards the environment as a determining factor of the many clinical expressions of atopic dermatitis.

Effect of genetics

Numerous studies have confirmed the role of genetic factors in the development of atopic dermatitis. They have shown that atopic dermatitis is a genetic disorder based on the interactions of unknown number of genes and environmental factors. In a family with a child affected by atopic dermatitis, there is generally a family history of eczema or any of the other atopic diseases; asthma and rhinitis.

The inheritance pattern is more complex and does not follow the Mendelian pattern. Parental atopy (parents with allergies) carries the highest risk factor. An atopic mother is a higher risk factor than an atopic father. This can be explained by stronger maternal genetic heritability, shared physical environment of mother and child, or environmental events that affect the fetus in utero.

In one study of all atopic diseases, a stronger correlation was found between siblings than between siblings and parents, indicating that environmental factors, especially during childhood, may be responsible for increased incidence of atopic dermatitis. The identification of parental genes may be of great help in studying the pathogenesis of atopic dermatitis.

Atopic dermatitis is often accompanied by asthma and hay fever. A multifactorial cause for atopic dermatitis has been suggested, with genetic and environmental factors influencing disease development.

However, genetic studies on atopic dermatitis show, interestingly, that the suspected gene regions of high risk identified show little overlap with the suspected gene regions of asthma suggesting a strong possibility that different sets of genes may be responsible for these two atopic diseases. Instead, some of the identified regions overlap with suspected regions for psoriasis, another chronic skin disease. The implication being, those genes in the skin involved in psoriasis might be playing an important role in atopic dermatitis pathogenesis. Genetic mutation of the epidermal barrier protein, called, filaaggrin is one predisposing factor for atopic dermatitis, but there are many more unkown genes involved.

One important suspected region is on a chromosome, whose genes are involved in the regulation of epidermal differentiation. Recent genetic studies have identified a cluster of genes on the outermost layer of the skin which may be as important as those which modify atopic dermatitis. Identification of the responsible genes would be of enormous help in diagnosis and treatment of the disease.

To summarize, atopic dermatitis genetics are complex and appear to involve several genes, each with a different function. Some relate to immunoglobin E response, some to the skin barrier function and some to skin inflammation.

However, there are still gaps in our knowledge about gene–environment interactions, better studies of the natural history of atopic dermatitis, and better clinical trials that answer questions that are important to physicians and their patients.

Effect of IgE

Several candidate genes with immunological functions were shown to be associated with atopic dermatitis in at least one study. Some of them were shown to influence the response of the immunoglobin E.

IgE is a monomeric immunoglobin. It binds to receptors on mast cells triggering the release of inflammatory mediators. It normally exists at levels below those of other immunoglobins. However, in patients with atopic dermatitis, particularly children, the levels are significantly higher at 80%. But 10% of patients have normal levels of IgE with no specific response. These observations show that the IgE response is not a primary cause, but a phenomenon of atopic dermatitis. A type of IgE, known as cord blood IgE, is a helpful predictor of atopic dermatitis.