الفهرس 
IntroductionOnly 14 pages are availabe for public view
 |  | from | 55 |  |  |
| | | from | 55 | | |
Abstract
Bronchial hyperresponsiveness is a central feature of asthma, and one of the strategies in the dissection of genetic risks of asthma is to investigate genes that contribute to this intermediate phenotype and gives further insight into the causes and mechanisms of the disease.
β2 adrenergic receptor gene (β2AR), coding for the β2AR plays a key role in tuning airways reactivity. Several single nucleotide polymorphisms (SNPs) have been detected within candidates for asthma genes that may confer alterations in the expression of these genes and the function of their products. The most studied variations are at positions 16 (Arg-Gly), and 27(Gln-Glu) in the β2AR gene. The roles of these genetic variations as disease modifying genes remain uncertain.
It is well known that asthma severity is related to environmental exposures. However, it is possible that β2AR variants influence certain intermediate phenotypes conferring severity of asthma.
The β2 adrenergic agonists are the most potent bronchodilators currently available for asthma treatment. Genetic factors controlling β2AR function may be an important determinant for response to bronchodilator therapy and thus of severity and duration of asthmatic symptoms.
The aim of the current work is to study the possible role of β2AR gene in the pathogenesis of bronchial hyperresponsiveness in asthmatic patients by detecting the frequency of SNPs in β2AR gene, determining the association of SNPs in β2AR gene to bronchial hyperresponsiveness and disease severity, and determining the association of SNPs in β2AR gene to long acting β2 agonists treatment response.
60 asthmatic patients (52 females and 8 males), and 60 controls subjects (47 females and 13 males) were included in this study. All the patients have asthma as defined by American Thoracic Society(1). The patients were classified into mild, moderate, and severe according to their clinical severity score. Thirty patients of the asthmatic subjects had received (12 µg formoterol fumarate) inhaled long acting β2 agonists (LABA) therapy twice daily for 14 days.
The following investigations were carried out:
1- Assessment for airway reactivity score and clinical severity score.
2- Measurement of pulmonary flow rates using Jaeger pneumotach. The pulmonary functions measured included FVC, FEV1, FEV1/FVC%, FEF25%, FEF50%, FEF75%, and MMEF.
3- Methacholine inhalation challenge was carried out through the five-breath dosimeter protocol. A dose response curve was constructed for each subject and the PD20-FEV1 was obtained from the curve.
4- Molecular analysis for β2AR polymorphisms at codons 16&27.
The obtained results were statistically analyzed using Statistical Package for the Social Sciences version 17 (SPSS) for windows.
The distribution of allelic frequencies of β2AR gene at codon 16, and 27 among asthmatic patients and controls were in Hardy-Weinberg equilibrium, with no significant difference among patients and controls groups. Also there was no significant association between β2AR polymorphisms at both codons 16 & 27 and the susceptibility to asthma indicating that β2AR gene is not a major asthma susceptibility locus.
Studying the distribution of β2AR at codon16/codon27 in the present study revealed 9 different combinations. The most commonly occurring genotypes were Arg-Gly/Gln-Glu, Gly-Gly/Gln-Glu, and Gly-Gly/Gln-Gln. However; Arg-Arg/Gln-Gln, and Arg-Arg/Glu-Glu genotypes occurred very rarely.
Also the present study showed no significant association between individual SNP in the β2AR gene at both codons 16 &27 and bronchial hyperresponsiveness in asthmatic patients. Yet, a significant positive association between Gly-Gly/Gln-Glu genotype and PD20FEV1 was observed. This could suggest that polymorphisms at β2AR gene might be a risk moderating factor for BHR, and the combination between SNP of the β2AR genotypes may have a significance influence on asthma phenotypes, and may play an important role in modifying clinical characteristics. Also it could suggest that the combinations of β2AR genotypes are more informative than individual SNPs in their predictive power.
The current study showed that there was no significant difference in SNP of β2AR at codon 16 and asthma groups, and that SNP of β2AR at codon 16 was not associated with asthma severity. However; the present study found that homozygous Gln27 genotype was significantly associated with severe asthma. Moreover; the frequencies of combined genotype Gly/Gly-Gln/Gln were always higher in the severe group than in the moderate and mild groups suggesting that it was Gln27 polymorphism that is associated with asthma severity. This was confirmed as well; by the significant positive association observed in the current study between clinical severity score and Gly-Gly/Gln-Gln genotype.
On the other hand; the present study showed that the presence of Glu27 polymorphism in asthmatic patients renders them to have a lower risk for having severe asthma. This was confirmed by the significant negative association found between clinical severity score and both Gln-Glu, and Gly-Gly/Gln-Glu genotypes, and the airway reactivity score and Gly-Gly/Gln-Glu genotype.
Thus, it could be suggested that polymorphisms in β2AR gene are important in the modulation of asthma severity, and that Gln27 genotype is associated with severe form of asthma, while Glu 27 has a protective effect against the severity of the disease. In addition the effect of Glu 27 variant prevails over the opposite effect of Gln27 when both variants are concomitantly presented either alone or when combined with polymorphism at codon 16.
In 30 asthmatic patients who have received inhaled long acting β2 agonists there was a significant improvement in the pulmonary flow rates, and reduction in the severity of the disease in all asthmatic patients. Also, a significant positive association was found between Glu27Glu genotype and the improvement in bronchial hyperresponsiveness (assessed by percentage of change in PD20FEV1) after long acting β2 agonists. On the other hand; no association was found between β2AR polymorphisms at codon 16 and the response to inhaled long acting β2 agonists.
The current study suggested that β2AR polymorphisms at codon 16 have no effect on the response to inhaled long acting β2 agonists. While; the presence of Glu27 seems to be protective factor against tachyphylaxis, and Glu 27 has significant effects on improving bronchial hyperresponsiveness after inhaled long acting β2agonists treatment. Thus, the predetermination of β2AR genotypes might have a utility for predicting the response to inhaled long acting β2 agonists.
Conclusion
In conclusion the present study revealed that:
• β2adrenegric receptor gene is not a major asthma susceptibility locus.
• Polymorphisms at β2adrenegric receptor were associated with bronchial hyperresponsiveness, with showing a greater power for combined genotypes analyses as opposed to single nucleotide polymorphisms.
• Polymorphisms at β2adrenegric receptor at codon 27, and the combined Gly-Gly/Gln-Gln, and Gly-Gly/Gln-Glu genotypes were associated with asthma severity.
• Glu 27 variant has a protective effect against the severity of the disease and prevails over the opposite effect of Gln27 when both variants are concomitantly presented either alone or when combined with polymorphism at codon 16.
• β2adrenergic receptor polymorphisms at codon 16 have no effect on the response to inhaled long acting β2 agonists.
• Homozygous Glu 27 was associated with improving bronchial hyperresponsiveness after inhaled long acting β2agonists treatment.
• The predetermination of β2adrenergic receptor genotypes might have a utility for predicting the response to inhaled long acting β2agonists.
• Studying β2adrenergic receptor polymorphisms may have profound implications for the understating of the genetic factors determining asthma severity and response to asthma therapy.