Prevalence

doi:10.1136/bmj.331.7514.443

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John Rees

Genetics

Our understanding of the genetics of asthma has recently advanced considerably. A familial link has been recognised for some time together with an association with allergic rhinitis and allergic eczema. The familial link with atopic disorders is strongest in childhood asthma and with the link to maternal atopy. Earlier investigations were helped by studies of isolated communities—such as in Tristan da Cunha, where the high prevalence of asthma can be traced to three women among the original settlers.

Family tree of an atopic family

Genetic studies

Early studies of genetic links within families with more than one member with asthma suggested a strong link to certain genetic regions of interest, particularly 5q and llq. Further studies in different populations did not replicate all the early findings, and it became evident that, as in other common conditions, the genetic links were not simple. Several issues have been identified, and there seem to be differences in the links between ethnic groups. Susceptibility seems to be determined by several genes that have an effect in different aspects of asthma. There has been steady progress moving from areas of broad linkage to candidate asthma genes. Again, findings have not always been replicated in different populations. Genes have been identified that are linked to the Th2 cytokine signalling pathway, Th2 cell differentiation, airway remodelling, innate and adaptive immune responses, and IgE levels. Most of the work relates to the presence of asthma. Different asthma phenotypes, the natural course and severity of asthma, and interaction with environmental influences are being investigated.

Increase in prevalence of wheeze in children aged 8-10 in two towns in New South Wales between 1982 and 1992. The counts of house dust mite in domestic dust increased substantially over the same period (Peat JK, et al, BMJ 1994 308:1591-6)