Familial atopy in Australian pedigrees: adventitious linkage to chromosome 8 is not confirmed nor is there evidence of linkage to the high affinity IgE receptor

5. Genetics of hypersensitivity

Genetics provides the basis for the host response to a variety of environmental factors that can play a role in the generation of complex genetic diseases, such as asthma and atopy. An understanding of the genetic bases for these conditions is therefore essential to understand their pathophysiology. Studies of the genetics of asthma and atopy have suffered from several daunting challenges. These include the recognition that these are conditions caused by numerous genes, with each gene assuming variable roles in different individuals. In addition, each gene presumably contributes only a small percentage to a given individual's genetic risk of asthma. This has led to the current situation, in which studies often demonstrate a lack of replication that can be explained by their being insufficiently powered. Furthermore, the pathophysiologies of asthma and atopy are incompletely understood, and the lack of clearly defined phenotypes also contributes to the inadequacies of the current literature. Nonetheless, regions of the human genome have been reproducibly associated with asthma and atopy. These regions have undergone intense study, and many genetic variants within them have been implicated as asthma and allergy genes. In addition, through candidate gene approaches, several genetic polymorphisms have been convincingly linked to increased risks for the development of asthma and atopy. Many of these genes are associated with alterations in responsiveness to therapeutic agents used in the treatment of these conditions. These genetic studies have an exciting potential for individually tailoring the therapeutic regimen to a given subject's genotype. It is to be hoped that they will also define new targets for the next generation of asthma and allergy therapeutic agents.

Associations of Fc epsilon R1-beta polymorphisms with immunoglobin E antibody responses to common inhalant allergens in a rural population

BACKGROUND

Polymorphisms within the beta subunit of the high-affinity receptor for IgE (Fc epsilon R1-beta ) on chromosome 11q13 have been related to atopy and asthma and the lymphotoxin alpha (LT alpha) gene on chromosome 6 is implicated in asthma.

OBJECTIVE

To elucidate the association of polymorphisms in the Fc epsilon R1-beta and LT alpha genes to IgE responses and asthma in a family-orientated rural population.

METHODS

A total of 461 adult farmers, who participated in an epidemiological follow-up study on respiratory symptoms among farmers on the Swedish island of Gotland, were examined. The traits assessed included serum total IgE, IgE antibody responses to 21 common inhalant allergens and asthma.

RESULTS

The 237G mutation was only detected in seven persons. Atopy was found to be associated with the RsaI-ex7 AB-genotype (OR = 1.9; P = 0.04). The RsaI-ex7 B allele had a significant influence on IgE responses to pollens and dust mites (OR = 5.5; P = 0.03 and OR = 5.2; P = 0.049, respectively). The influence of this allele was stronger when the association towards single dust mite species (Lepidoglyphus destructor) was estimated (OR = 7.1, P = 0.03) and the association increased even more when the major allergen of L. destructor (rLep d 2) was analysed (OR = 11.2, P = 0.02). These associations were independent of sex, age and smoking, and the estimates of RsaI-in2 independent of RsaI-ex7. RsaI-in2, RsaI-ex7 and LT alpha genotypes were unassociated with total serum IgE. No significant difference in the distribution of RsaI-in2, RsaI-ex7 and LT alpha genotypes was found among subjects with atopy or asthma compared to healthy controls.

CONCLUSION

This study supports the notion that polymorphisms in the Fc epsilon R1-beta gene have significant effects on IgE responsiveness. Secondly, dust mites in rural populations influence the expression of genes on chromosome 11q13.

Genomics and proteomics of allergic disease

The world-wide effort to identify susceptibility genes for allergic diseases is motivated by the conviction that the identification of disease genes may permit the design of new classes of anti-inflammatory compounds. Molecules concerned with the allergic reaction, such as cytokines, chemokines, their receptors, major histocompatibility complex molecules, and transcription factors, could provide the candidate genes of the allergic diseases. On the basis of genetic studies, multiple research groups have attempted to identify a susceptibility gene for allergy using the candidate gene approach and/or genome-wide screening. Both of these approaches suggest genetic heterogeneity of allergic diseases. Many variants of candidate genes are or are not associated with particular diseases in different ethnic groups and the function of variants is now being investigated. Based on the information accumulated thus far and the information on the human genome sequence, future advances in research on genetic factors for allergic diseases will be likely lead to the establishment of more effective prophylaxis and therapy for these diseases.

Linkage analysis of markers on chromosome 11q13 with asthma and atopy in a United Kingdom population

Previous studies have suggested that atopy is linked to the beta chain of the high affinity IgE receptor (Fcepsilon R1-beta) on chromosome 11q13. Fcepsilon R1-beta polymorphisms, I181L, V183L, and E237G, are reported to be associated with asthma and atopy. The aim of this study was to investigate linkage to Fcepsilon R1-beta in a UK population and to assess the frequency of the polymorphisms and their association with asthma and atopy. A sample of 131 families was recruited at random with a sample of 109 families ascertained via an asthmatic proband. Each subject completed a written and video-assisted questionnaire and underwent bronchial challenge and skin prick testing. Serum total and specific IgE levels were measured. Quantitative scores were derived for asthma and atopy using principal component analysis. Four microsatellite markers were genotyped, including Fcepsilon R1-beta. The frequency of the I181L and V183L polymorphisms were determined by sequencing, and the E237G polymorphism was determined using the amplification refractory mutation system. We found no evidence for linkage to Fcepsilon R1-beta and only weak evidence for linkage to the less informative marker E237G. We found no examples of the I181L/V183L polymorphism in our population sample. Our study has failed to strengthen the evidence for a candidate gene on chromosome 11q13.

Molecular genetics of allergic diseases

Allergic diseases affect approximately one third of the general population. This class of disease, characterized by elevated serum IgE levels and hypersensitivity to normally innocuous antigen, can manifest in practically any mucosal tissue or as a systemic response. A few examples of serious allergic diseases include asthma, dermatitis, bee sting allergy, food allergy, conjunctivitis, and severe systemic anaphylaxis. Taken together, allergic diseases constitute one of the major problems of modern day medicine. A considerable portion of the healthcare budget is expended in the treatment of allergic disease, and morbidity rates of inner city asthmatics are rising steadily. Due to the enormity of the problem, there has been a worldwide effort to identify factors that contribute to the etiology of allergic diseases. Epidemiologic studies of multigeneration families and large numbers of twins clearly indicate a strong genetic component to atopic diseases. At least two independently segregating diseasesusceptibility genes are thought to come together with environmental factors to result in allergic inflammation in a particular tissue. On the basis of the strong genetic studies, multiple groups have attempted to identify disease-susceptibility genes via either a candidate gene approach or by genome-wide scans. Both of these approaches have implicated multiple regions in the human and mouse genomes, which are currently being evaluated as harboring putative atopy genes.

Mapping oligogenes for atopy and asthma by meta-analysis

Meta-analysis is presented for published studies on linkage or allelic association that have in common only reported significance levels. Reporting is biassed, and nonsignificance is seldom quantified. Therefore meta-analysis cannot identify oligogenes within a candidate region nor establish their significance, but it defines candidate regions well. Applied to a database on atopy and asthma, candidate regions are identified on chromosomes 6, 5, 16, 11, 12, 13, 14, 7, 20, and 10, in rank order from strongest to weakest evidence. On the other hand, there is little support for chromosomes 9, 8, 18, 1, and 15 in the same rank order. The evidence from 156 publications is reviewed for each region. With reasonable type I and II errors several thousand affected sib pairs would be required to detect a locus accounting for 1/10 of the genetic effect on asthma. Identification of regions by a genome scan for linkage and allelic association requires international collaborative studies to reach the necessary sample size, using lod-based methods that specify a weakly parametric alternative hypothesis and can be combined over studies that differ in ascertainment, phenotypes, and markers. This has become the central problem in complex inheritance.

The high-affinity IgE receptor (Fc epsilon RI): from physiology to pathology

The high affinity receptor for immunoglobulin E (designated Fc epsilon RI) is the member of the antigen (Ag) receptor superfamily responsible for linking pathogen-or allergen-specific IgEs with cellular immunologic effector functions. This review provides background information on Fc epsilon RI function combined with more detailed summaries of recent progress in understanding specific aspects of Fc epsilon RI biology and biochemistry. Topics covered include the coordination and function of the large multiprotein signaling complexes that are assembled when Fc epsilon RI and other Ag receptors are engaged, new information on human receptor structures and tissue distribution, and the role of the FcR beta chain in signaling and its potential contribution to atopic phenotypes.

Chromosome 11q13 and atopic asthma

Asthma is a complex syndrome in which bronchial inflammation and smooth muscle hyperactivity lead to labile airflow obstruction. The commonest form of asthma is that due to atopy, which is an immune disorder where production of IgE to inhaled antigens leads to bronchial mucosal inflammation. The ultimate origins of asthma are interactive environmental and genetic factors. The genetics is acknowledged to be heterogeneous, and one chromosomal region of interest and controversy has been 11q13. To clarify the nature of the chromosome 11q13 effect in atopy and asthma, we conducted a genetic association study in subjects with marked atopic asthma and matched controls, which incorporated the study of 13 genetic variants over a distance of 10-12 cM and which took account of detailed immune and clinical phenotyping. Association with high IgE levels was limited to the interval flanked by D11S1335 and CD20 in a 0.8-Mb interval and was greatest for variants of Fc epsilonRIbeta and HTm4; these variants also associated with asthma (recurrent wheeze with labile airflow obstruction and need for regular inhaler treatment). At the more telomeric marker, D11S480, variants associated with asthma, but not with high IgE levels. The data might support the possibility of multiple loci relevant to atopic asthma on chromosome 11q13.

Genetics of allergy and asthma

LEARNING OBJECTIVES

This article summarizes the latest information regarding the role of genetic influences in the development of allergic disorders and asthma and reviews our current information on some of the most likely genes responsible for these conditions. After reading this article, the reader will have a better understanding of the current molecular biologic techniques that are being used to understand complex genetic disorders such as allergies and asthma. The reader should understand the value of how this genetic insight will lead to the recognition of the presence of specific subtypes of these disorders that require unique therapeutic interventions. This information can also be used to identify genetically at risk children and thereby offer earlier intervention. Finally, understanding the genetic causes of allergies and asthma will lead to the development of the next--hopefully curative--generation of asthma and allergy therapeutics.

DATA SOURCES

A detailed literature search was conducted. Studies considered relevant, well performed, and appropriately controlled were used. Only human studies are included and only the English language literature was reviewed. Some of the information presented is based on the author's own research experience.

STUDY SELECTION

Material was only taken from peer-reviewed journals and appropriate reviews.

RESULTS AND CONCLUSIONS

Asthma and allergic diseases are examples of disorders having an unmistakable genetic predisposition, but in the absence of a classic Mendelian inheritance pattern. These "complex" genetic disorders are caused by the interactions of multiple interacting genes some having protective value and some contributing to disease development and with each gene having its own variable tendency to be expressed. In addition, these disorders require the presence of appropriate environmental triggers for their expression. One approach to identifying the genetic basis for these conditions is to perform a genome-wide search in which the location of the disease-causing gene on a human chromosome is identified and nearby genes that may be responsible are subsequently identified. An alternative approach to identifying heritable components to asthma and allergy is to evaluate disordered structure or regulation within genes known to be involved in these disorders. Using these approaches, studies have suggested that genes within the cytokine gene cluster on chromosome 5 (including interleukins-3, -4, -5, -9, and -13), chromosome 11 (the beta chain of the high affinity IgE receptor), chromosome 16 (the IL-4 receptor), and chromosome 12 (stem cell factor, interferon-gamma, insulin growth factor, and Stat 6 [IL-4 Stat]) may contribute to asthma and allergy development. In addition, data support involvement of genes involved in antigen-presentation (MHC class II genes) and T cell responses (the T cell receptor alpha chain). Finally, disease-contributing alleles may be present on genes for the beta-adrenergic receptor, 5-lipoxygenase, and leukotriene C4 synthase.

Mutational analysis of the high affinity immunoglobulin E receptor beta subunit gene in asthma

BACKGROUND

The gene for the beta subunit of the high affinity receptor for immunoglobulin E (FcepsilonRI-beta) on chromosome 11q13 is linked with clinical asthma and certain mutations have been identified. A study was undertaken to identify DNA variation in the FcepsilonRI-beta gene in a population sample in which linkage between 11q13 and asthma was explained by bronchial hyperreactivity (BHR) but not atopy.

METHODS

DNA samples from 71 subjects with asthma, atopy, or BHR were analysed. The complete coding region, some of the introns, and some of the 5' untranscribed region of the FcepsilonRI-beta gene were sequenced.

RESULTS

In the subjects studied there were no deviations from the published sequence in any of the seven coding exons of the FcepsilonRI-beta gene. In particular, the three previously reported mutations (Ile181, Leu183, Glu237) were not detected. Two new polymorphisms were discovered, one at position 243 in the 5' untranscribed region and one at position 4390 in intron III. Neither of these variants showed significant association with asthma, atopy, or BHR.

CONCLUSIONS

These results suggest that, in the population studied, linkage of asthma and BHR to 11q13 is not explained by mutations in the FcepsilonRI-beta gene. Other mutations in the non-coding region of this gene or in adjacent genes must explain the linkage findings in this study.

Genetic factors in lung disease: atopy and bronchial asthma

Atopy defined as high IgE responsiveness has now been subject to genetic studies at the molecular level owing to the development of a great number of DNA markers over the human genome. Either by linkage analysis or by association study strong candidate genes of atopy have been proposed to be located on chromosome 11q13 and 5q31 where high-affinity IgE Fc receptor beta subunit and allergy-associated cytokines, respectively, have been mapped. Meanwhile, we found a novel association between one of alleles of D11S97, an anonymous DNA marker on 11q13, and high total serum IgE in a large number of Japanese general population and atopic family members. However, failure to replicate linkage or association studies by different investigators suggest polygenic nature of atopy. In addition to the genes regulating IgE synthesis, the requirement of local (pulmonary) genetic factors in the development of bronchial asthma have been speculated. Linkage analysis suggested possible existence of gene(s) regulating susceptibility and/or clinical characteristics of bronchial asthma also on chromosome 5q. One of the candidate is beta 2-adrenergic receptor gene polymorphism. Mutated gene transfection studies suggested functional significance of some polymorphisms and clinical evaluations have revealed their contribution to airway responsiveness and severity of asthma.

Atopy and the ectopic immune response

A hypothesis if formulated for the mode of action of the 'atopy gene': the gene(s) causing an increased tendency to mount an IgE immune response to trivial environmental antigens. It is postulated that some antigens are not efficiently transported to the lymphoid organs, the location of the 'eutopic' immune system; migratory lymphoid cells may occasionally react and start an inefficient immune response in sites not so destined. This is called an ectopic immune response. This response lacks the regulatory influences prevailing in the lymphoid organs, which results in approximately equal quantities of the various immunoglobulin isotypes, including IgE. The atopy gene(s) work by increasing the risk of such an ectopic immune response.

Linkage of high-affinity IgE receptor gene with bronchial hyperreactivity, even in absence of atopy

Asthma is a manifestation of bronchial hyperreactivity (BHR) and forms part of the spectrum of atopic disease. Some pedigree studies of atopy have suggested linkage with the high-affinity IgE receptor (Fc epsilon RI beta) gene on chromosome 11q13, but others find no linkage. The molecular genetics of asthma and BHR have not been studied in the general population. We examined the genetic linkage of the Fc epsilon RI beta gene with clinical asthma and the underlying phenotypes of BHR (to methacholine) and atopy (defined by skinprick testing) in 123 affected sibling-pairs recruited from the general population. We found evidence of significant linkage of a highly polymorphic microsatellite marker in the fifth intron of the Fc epsilon RI beta gene to a diagnosis of asthma (18.0% excess of shared alleles, p = 0.002) and to BHR (21.7% excess of shared alleles, p = 0.001). Significant linkage was also observed in siblings sharing BHR when those with atopy were excluded (32.8% excess of shared alleles, p = 0.004). Atopy in the absence of BHR did not show significant linkage to the Fc epsilon RI beta gene (7.2% excess of shared alleles, p = 0.124). These findings suggest that mutations in the Fc epsilon RI beta gene or a closely linked gene influence the BHR underlying asthma, even in the absence of atopy.

HLA genetics and allergic disease

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