The inheritance of immunoglobulin E: genetic linkage analysis

A genome-wide association study of plasma total IgE concentrations in the Framingham Heart Study

BACKGROUND

Atopy and plasma IgE concentration are genetically complex traits, and the specific genetic risk factors that lead to IgE dysregulation and clinical atopy are an area of active investigation.

OBJECTIVE

We sought to ascertain the genetic risk factors that lead to IgE dysregulation.

METHODS

A genome-wide association study (GWAS) was performed in 6819 participants from the Framingham Heart Study (FHS). Seventy of the top single nucleotide polymorphisms (SNPs) were selected based on P values and linkage disequilibrium among neighboring SNPs and evaluated in a meta-analysis with 5 independent populations from the Cooperative Health Research in the Region of Augsburg cohort, the British 1958 Birth Cohort, and the Childhood Asthma Management Program cohort.

RESULTS

Thirteen SNPs located in the region of 3 genes, FCER1A, signal transducer and activator of transcription 6 (STAT6), and IL13, were found to have genome-wide significance in the FHS cohort GWAS. The most significant SNPs from the 3 regions were rs2251746 (FCER1A, P = 2.11 × 10(-12)), rs1059513 (STAT6, P = 2.87 × 10(-8)), and rs1295686 (IL13, P = 3.55 × 10(-8)). Four additional gene regions, HLA-G, HLA-DQA2, HLA-A, and Duffy blood group, chemokine receptor (DARC), reached genome-wide statistical significance in a meta-analysis combining the FHS and replication cohorts, although the DARC association did not appear independent of SNPs in the nearby FCER1A gene.

CONCLUSION

This GWAS of the FHS cohort has identified genetic loci in HLA genes that might have a role in the pathogenesis of IgE dysregulation and atopy. It also confirmed the association of the known susceptibility loci FCER1A, STAT6, and IL13 for the dysregulation of total IgE.

The G protein-coupled receptors: pharmacogenetics and disease

Genetic variation in G-protein coupled receptors (GPCRs) is associated with a wide spectrum of disease phenotypes and predispositions that are of special significance because they are the targets of therapeutic agents. Each variant provides an opportunity to understand receptor function that complements a plethora of available in vitro data elucidating the pharmacology of the GPCRs. For example, discrete portions of the proximal tail of the dopamine D1 receptor have been discovered, in vitro, that may be involved in desensitization, recycling and trafficking. Similar in vitro strategies have been used to elucidate naturally occurring GPCR mutations. Inactive, over-active or constitutively active receptors have been identified by changes in ligand binding, G-protein coupling, receptor desensitization and receptor recycling. Selected examples reviewed include those disorders resulting from mutations in rhodopsin, thyrotropin, luteinizing hormone, vasopressin and angiotensin receptors. By comparison, the recurrent pharmacogenetic variants are more likely to result in an altered predisposition to complex disease in the population. These common variants may affect receptor sequence without intrinsic phenotype change or spontaneous induction of disease and yet result in significant alteration in drug efficacy. These pharmacogenetic phenomena will be reviewed with respect to a limited sampling of GPCR systems including the orexin/hypocretin system, the beta2 adrenergic receptors, the cysteinyl leukotriene receptors and the calcium-sensing receptor. These developments will be discussed with respect to strategies for drug discovery that take into account the potential for the development of drugs targeted at mutated and wild-type proteins.

Mapping susceptibility genes for asthma and allergy

Using family data, linkage analysis has been performed to determine the location in the genome of susceptibility genes for allergy and asthma. It has now become clear that there are multiple regions of the genome that contain susceptibility genes for allergy and asthma. The results from two genome screen studies will be reviewed and compared with results from candidate gene approaches. Results from several studies show evidence for linkage to chromosomes 5, 6, 11, 12, 13 and 14 for atopy, asthma or a related phenotype such as total serum IgE levels. Many of these regions contain candidate genes involved in regulating processes that may be involved in the development or progression of allergy and asthma. Some susceptibility genes may affect the expression of these disorders while others may affect response to therapy. Susceptibility to developing allergy or asthma appears to be due to the interaction of multiple genes with the environment.

Genetic susceptibility to asthma in a changing environment

There is a major interest in investigating the genetic components of allergy and asthma. Four different areas are involved in the study of complex genetic diseases: family studies, assessment of phenotype, segregation analysis and gene mapping. Initial assessment of phenotype must be practical, reproducible and relatively independent of compounding variables. Phenotypes important in allergy and asthma include atopic parameters such as total serum IgE, bronchial hyper-responsiveness and the presence/ absence of clinical asthma. Numerous family and twin studies have suggested the presence of a heritable component for allergy, bronchial hyper-responsiveness and asthma. The number of genes involved in these complex genetic disorders and their mode of inheritance have not been fully determined. Our group has been involved in a collaborative US-Dutch study in which 92 families with over 500 individuals have been phenotyped and DNA has been obtained for genotyping. Initial results of the classification of family members show that approximately 26% of the offspring of families ascertained through a parent with asthma have an asthmatic phenotype. A large number of these offspring with clinical evidence of asthma do not have a prior physician diagnosis of asthma, suggesting that there is a spectrum which ranges from preclinical to symptomatic asthma. The familial aggregation of asthma and other obstructive airway diseases in these families is consistent with a significant genetic component. Initial linkage studies have been performed on two characteristics of the allergic and asthmatic phenotype. Total serum IgE was analysed because this measure correlates with the clinical expression of allergy, bronchial hyper-responsiveness and asthma. Segregation analysis of total serum IgE provided evidence for a recessive mode of inheritance. Sib pair analyses and maximum likelihood scores suggest that a gene regulating IgE production maps to chromosome 5q. Bronchial hyper-responsiveness and total serum IgE are related to asthma in population-based studies. Sib pair analyses for bronchial responsiveness showed significant linkage to markers on chromosome 5q.

Evaluation of atopy through an expert system: description of the database

BACKGROUND

In order to understand the medical decisions taken during the initial visit of a new asthmatic patient, a group of experts designed an expert system which provides conclusions about severity, precipitating factors and treatment. Rules for atopy and the assessment of allergic factors have been discussed and implemented in the expert system. Conclusions about severity have been yet validated using an appropriate methodology.

OBJECTIVE

The aim of this study was to investigate a sample of 471 patients according to conclusions regarding atopy.

METHODS

A total of 471 cases report forms (CRF) was filled in for adult asthmatic outpatients, seen for the first time in our clinic without emergency situations. Data of each CRF were used by the expert system to draw conclusions. The expert system discerns three patterns for atopy, yes, possible or no. The variables known to reflect different features according to the classification of asthma as atopic or not have been studied. The variables used in the rules for atopy, obviously linked to the conclusion, were not compared. For many medical problems no unique objective solution exists and this is why a group of patients with possible atopy was introduced.

RESULTS

Patients with atopy had less severe asthma (P = 0.01), a better FEV1 value (P = 0.0007) and showed their first symptoms of asthma earlier (P = 0.00001) than patients without atopy.

CONCLUSIONS

The characteristics of the group studied here are consistent with the literature. This could be considered as an indirect validation of the expert system. Moreover, patients with possible atopy show intermediate findings for these variables and it is possible to suggest a 'dose-effect' relationship.

Genetics of total serum IgE levels: a regressive model approach to segregation analysis

The genetics of basal total serum IgE levels was investigated in 278 individuals from 42 randomly ascertained nuclear families. The data were analyzed using the regressive model approach to segregation analysis with age, sex, and a measure of skin test responsiveness as covariates in the Class D models. The best fitting model was that of recessive inheritance of high IgE levels with a gene frequency of 0.99 for the "high" allele. Only 3 families showed evidence for segregation of the rare "low" allele, and, if extended further, these families could be useful for molecular genetic linkage studies. These results suggest that there may be a rare allele for very low total serum IgE levels that can be detected even after a measurement of allergic responsiveness (skin test results) is considered as a covariate. Therefore, this major gene for IgE levels appears independent of any similar locus controlling atopy.