Genetics in asthma: the solution to a lasting conundrum?
Authors: W. Eder, E. von Mutius
Journal: Allergy (2005)

The scientific community has now at its hands the comprehensive findings of the Human Genome Project uncovering approximately 25 000 protein-coding genes and many ill-determined structures within the human genome (1). One might therefore be inclined to think that the ‘code of life’ has been broken and that it will only take some time to relate the erroneous building blocks for the construction of amino acids and thereby proteins to the development of disease. The catchphrase ‘from diagnosis and treatment to prediction and prevention’ reflects the high expectations put into genomics to improve health in the 21st century (2). As a result of these ambitious efforts, we are now almost daily confronted with discoveries of new genetic variations proposed to be associated with increased/decreased risk for some disease, and with promises of a change in diagnosis, treatment, and prevention of that condition. Genetics has already helped us to identify the underlying causes of Down Syndrome, hemoglobinopathies, and some inborn errors of metabolism. Furthermore, in several Mendelian disorders, such as cystic fibrosis, Huntington's disease, and Duchenne muscular atrophy, linkage studies have unraveled the defect in the genetic make-up of affected patients and thereby greatly contributed to the understanding of these illnesses. So why shouldn't we use genetic tools to progress toward a better understanding of such puzzling illnesses as asthma? Well, apart from the unconstrained human drive to explore any new and unknown field, most scientists agree upon the great potential that lies in all the exciting new methods in genetics to improve public health. We eagerly aim to identify susceptible individuals for certain diseases, in such a way as to enable us to modify their environment and by that put primary prevention into action. We also anticipate considerable benefits in tailoring medications to the individual's genetic profile, and eventually we might be able to correct inborn genetic errors. As sound as these visions seem, we still have to overcome substantial obstacles to realize them, especially when dealing with complex diseases. Different to single mutation disorders, the term complex disease already promises the challenge that lurks behind. Current concepts of the pathogenesis of complex diseases propose that several or many genetic and environmental factors need to occur to put forth the phenotype, very likely even in a certain time-dependent manner in the development of the organisms. Such a complex and genetically heterogeneous trait is asthma and the first difficulty already arises when attempting to identify with great certainty the clinical phenotype. Numerous papers have been devoted to the demarcation and classification of asthma phenotypes using temporal criteria, associated traits, clinical features, and triggering factors. A way out of this dilemma may be the use of a clinical diagnosis of asthma, as physicians by experience and intuition may tend to attribute the same label to patients even in the absence of articulation of a written definition. In addition, objectively measured asthma components, such as bronchial hyperresponsiveness or markers of airway inflammation, can be used to reduce genetic heterogeneity, by dividing the study population into phenotypically more homogenous subgroups. This was done using asthma severity and allergy testing in the study by Saçkesen et al. (3) presented in this issue of Allergy. The aim of their Case-only approach was to assess the association between previously described functional polymorphisms in CD14 and TLR4 and asthma-related phenotypes and to evaluate heterogeneity between defined groups of cases. The authors confirmed previously reported associations between serum IgE and CD14/-159 (syn. CD14/-260) in a subset of atopic children with stable asthma and without controller medication. A fairly respectable number of epidemiological studies in different populations is now available suggesting a certain role for CD14 in IgE regulation (4–7), irrespective of some well designed and large studies that showed no association at all (8, 9). There is no doubt that replication of gene-disease associations in independent populations supports causal inference, but differences in studies are sources of heterogeneity. In particular, in populations with a similar genetic background, some conflicting results between studies may be secondary to variation in the prevalence and variability of interacting environmental factors. As Saçkesen et al. address in their article, CD14 is a gene that encodes part of the receptor complex that interacts with endotoxin to initiate innate immune responses. It is biologically plausible to speculate that the association between genetic variation in these genes and markers of atopy are modified by levels of endotoxin in the environment, which by itself is a significant determinant for atopy/IgE in childhood (10). Under these assumptions, one might replicate studies only when carried out under similar environmental exposures. This may be particularly important for TLR4 that codes for a central part of the endotoxin receptor complex. The mutant allele of the TLR4Asp299Gly polymorphism has been associated with bronchial hyporesponsiveness to endotoxin in healthy subjects (11). Remarkably, it was less frequent in moderate/severe than mild atopic asthma in the population studied by Saçkesen et al., whereas the severity of atopy in asthmatics was higher in another study (12), and no association between this polymorphism and asthma or asthma related phenotypes was found in two family-based cohorts (13). We recently studied the same polymorphism in a rural population of children and found that its association with allergic sensitization was significantly modified by levels of endotoxin in the environment (14). We found no association between TLR4Asp299Gly and asthma without taking endotoxin exposure into account. Given the high a priori biologic plausibility for an association between the functional TLR4Asp299Gly polymorphism and phenotypes known to be modified by exposure to the TLR4 ligand lipopolysaccharide (endotoxin), the apparently conflicting results rather encourage to search for environmental or behavioral factors that modify this association than to adhere rigidly to the principle of replication. In studies with a well-defined hypothesis, one might also consider to estimate measures of effect with their confidence intervals instead of testing statistical hypotheses. Another issue needs to be addressed when different studies are compared. There is a strong overlap of traits such as atopy and asthma in populations that potentially obscures the association between a certain genotype and the outcome under study. Is the signal among atopic asthmatics referring to asthma or atopy? As long as no non-asthmatic atopic and asthmatic non-atopic control group of sufficient size is included in the analysis the interpretation of the findings remains incomplete. Apart from the effect of gene–environment interaction on well defined phenotypes, studying complex diseases will have to deal with the role of complex genotypes, i.e. the interaction of different alleles at multiple loci. The use of high throughput genotyping techniques allows us to evaluate associations between thousands of alleles and phenotypes. In this approach the prior probability of a positive finding is low and a high proportion of false-positive findings is expected. However, there is the possibility that alleles other than those researchers believe to be related to the outcome of interest are discovered. Given the large number of possible allelic combinations, it is not surprising that the statistical methodology is still under development. The same holds true for allelic combinations within a gene, haplotypes, which are considered as more powerful in association studies of complex disease. Until we have adequate analytical tools at hand, we might focus on allele combinations that contribute to the largest proportion of a well defined phenotype and regard them as likely candidates for a closer investigation. The completion of sequencing the human genome is an important milestone in the history of medicine. The discovery of associations between genetic variations and phenotypes is only a single step on the long way to understand the pathogenesis of complex diseases and to eventually translate findings from genetic studies into meaningful actions to improve health and prevent disease. Additional information from different medical and public health disciplines is needed, for example, to clarify molecular mechanisms underlying epidemiological observations. Keeping in mind that sequence variation is only part of the genetic variance determining phenotypes, researchers can trust to have a huge field of research and an exciting time for decades to disentangle the interplay between genetic variation and environmental influences.