Influence of Genetic variation of the β2-Adrenergic receptor on lung diffusion in patients with cystic fibrosis

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Abstract

Rationale

Cystic fibrosis (CF) is a disease that adversely affects the lung resulting in a reduction in lung diffusion. Stimulation of the β2-adrenergic receptors mediates mucociliary clearance and bronchodilation. We sought to determine the influence of an inhaled β-agonist on the diffusing capacity of the lungs for carbon monoxide (DLCO), alveolar-capillary membrane conductance (DM), pulmonary capillary blood volume (Vc), and peripheral oxygen saturation (SaO2) in subjects with CF, when compared to matched healthy subjects, according to genetic variation of the β2-adrenergic receptor (ADRB2).

Methods

To determine this we recruited 18 subjects with CF and 20 healthy subjects (age = 23 ± 7 vs. 24±4years; ht = 168 ± 8 vs. 174 ± 12 cm; wt = 64 ± 16 vs. 70 ± 13 kg; BMI = 23 ± 4 vs. 23±3 kg/m2; FEV1 = 72 ± 27 vs. 92 ± 12%pred; VO2peak = 45 ± 25 vs. 99 ± 24%pred, p < 0.05 for FEV1 and VO2peak, mean ± SD, for CF and healthy, respectively). The study involved measurement of DLCO, DM, VC and SaO2 before and 30, 60, and 90 min following the administration of inhaled albuterol. Subjects were stratified according to genetic variation of ADRB2, Gln27Gln vs. Glu27Glu/Gln27Glu.

Results

Within the healthy group, there were no differences in DLCO, DM, VC, DM/VC at baseline or in response to albuterol according to genetic variation of the ADRB2 at amino acid 27. Within the CF group, the Glu27Glu/Gln27Glu group had higher DM/VC and SaO2 when compared to the Gln27Gln group at baseline (p < 0.05). Both genotype groups demonstrated a significant decline in VC and an improvement in DM/VC and SaO2 in response to albuterol. Subjects with the Glu27 genotype experienced a greater improvement in DM/VC with albuterol when compared to subjects homozygous for Gln at amino acid 27.

Conclusion

These results suggest that there are differences in lung diffusion and peripheral SaO2 according to genetic variation of the ADRB2 at position 27 which could play a potential role in dosing options or adjustments that may be required according to genotype.

Highlights

► Albuterol improved alveolar conductance in CF patients with the Glu27 allele of ADRB2. ► This increase in alveolar conductance with albuterol did not occur in healthy control subjects. ► The increase in alveolar conductance following albuterol administration resulted in an improvement in SaO2.

Introduction

Cystic fibrosis (CF) is an inherited autosomal-recessive disease that affects the lung, pancreas, liver and reproductive organs. Over 30,000 people in the United States and over 70,000 people worldwide are affected by CF. Although more than 80% of patients are diagnosed by the age of three years, there is an increasing number of patients that are living into adulthood, and now more than 40% of the CF population is over 18 years old [1]. Cystic fibrosis is a complicated disease caused by mutations in the cystic fibrosis transmembrane regulator (CFTR) gene on chromosome 7. The most common mutation is the deletion of the amino acid phenylalanine at position 508 (ÄF508). The CFTR protein is a cyclic adenosine monophosphate (cAMP) dependent chloride (Cl) channel that, when mutated, becomes non-functional or mislocalized resulting in decreased Cl channel activity. Further, the loss of CFTR-mediated inhibition of epithelial Na+ channels (ENaC) leads to increased sodium (Na+) and water reabsorption across the airway epithelial cells [1], [2], [3]. This abnormal movement of ions results in decreased airway surface liquid depth and disrupted mucociliary clearance, which is believed to play a primary role in the development of progressive lung damage involving infection, inflammation, and airway obstruction.

β-agonists are primarily prescribed for bronchodilation along with airway clearance therapy and prevention of bronchospasm from other aerosolized therapies; however, their action on β2-adrenergic receptors (ADRB2) can also influence ion and fluid regulation. In the lungs, alveolar fluid clearance is regulated by the activity of ENaC on the apical membrane. Stimulation of the ADRB2 by an endogenous or exogenous agonist has been shown to increase alveolar fluid clearance as ADBR2 mediates an increase in activity and expression of ENaC [4], [5], [6]. Additionally, there is a clear relationship between CFTR and ADRB2 receptors that is quite complex [7]. Stimulation of ADRB2 results in apical localization and activation of the CFTR. Over-expression of CFTR has been shown to increase alveolar fluid clearance [8]. Similarly, mice with ADRB2 knockout and normal CFTR have altered fluid regulation, further illustrating the important relationship between CFTR and ADRB2 [9].

The daily use of a β-agonist is common for patients with CF with 80% of subjects receiving therapy [10]. Stimulation of ADRB2 through an exogenous β-agonist (i.e. albuterol or salmeterol) is thought to be beneficial in patients with CF primarily by facilitating bronchodilation. β2-adrenergic receptor stimulation also increases mucociliary clearance in subjects with CF. However, the evidence regarding the degree of mucociliary clearance is conflicting, with the dose and type of β-agonist appearing to influence the level of clearance significantly [11]. Interestingly, the administration of β-blockers reduces mucociliary clearance in healthy subjects [12].

Previous research has shown that the treatment of severe small airway disease in children with CF is quite variable [13]. Aside from the day to day variability within an individual, only one-half of CF patients demonstrate improvements in pulmonary function with bronchodilator therapy, 10–20% demonstrate a decline in pulmonary function, and one-third will have no response [13], [14]. This suggests that there is a genetic component in the bronchodilatory response to albuterol. Although data supporting the use of short-acting β-agonists is limited, many patients are using these medications on a regular basis.

Polymorphisms of ADRB2 have been identified at several sites, including amino acids 16 and 27. Previous research has primarily focused on the polymorphic alleles Arg16Gly, Gln27Glu, and Thr164Ile in the ADRB2 gene, with conflicting results. In an age-, sex-, and ethnicity-matched study, CF subjects were 16% more likely than controls to be homozygous for glycine (Gly) at position 16 of the ADRB2 and 19% more likely to be homozygous for glutamic acid (Glu) at position 27 of the ADRB2 [15]. Previous work has found that subjects with Gly16/Glu27 variants of the ADRB2 have enhanced response to bronchodilators, and a slower decline in CF disease, when compared to the Arg16/Gln27 genotype group [14]. In another study, patients heterozygous or homozygous for Gly at amino acid 16 had a forced expiratory volume in 1 s (FEV1) that was significantly lower when compared to patients with the Arg16 allele [15]. Additionally, these researchers found that those who carried a Gly16 allele had lower forced vital capacity (FVC) (p < 0.05) and a greater five-year decline in pulmonary function than the Arg16 subjects (p < 0.01). The researchers concluded that these polymorphic alleles in particular may be used to predict severity and progression of CF.

The purpose of this study was to determine the influence of an inhaled β-agonist, albuterol, on pulmonary function, lung diffusion, and alveolar-capillary membrane conductance in healthy and CF subjects according to genetic variation of the ADRB2. We hypothesized that CF subjects with at least one glutamic acid at position 27 of the ADRB2 (Glu27Glu or Gln27Glu) would have a greater improvement in alveolar-capillary membrane conductance when compared to CF subjects homozygous for glutamine at position 27 (Gln27Gln).

Section snippets

Methods

The study design was a case–control study that compared 18 CF subjects to a matched group of 20 healthy subjects in their pulmonary response to albuterol. Healthy subjects were recruited locally using advertisements in Tucson, Arizona. Cystic fibrosis subjects were recruited through the Arizona Respiratory Center in Tucson, Arizona. The study protocol was reviewed and approved by the University of Arizona Institutional Review Board, all participants provided informed written consent prior to

Statistical analysis

All statistical analyses were performed using the SPSS statistical software package (v.16.0, Chicago, IL). Data analysis for this project includes repeated measures ANOVA with a between genotype analysis in both healthy and CF groups. The data includes the two groups, control and CF, each incorporating baseline and three post-albuterol measurements at 30, 60, and 90 min. All data were assessed for normality prior to the ANOVA using a Mauchley’s test and Tukey post-hoc analysis was performed

Results

Eighteen subjects with CF and 20 age-, gender-, and height-matched healthy subjects completed all aspects of the study (Table 2). As expected, subjects with CF had lower aerobic capacity and pulmonary function when compared to healthy subjects (p < 0.05 for VO2peak, FEV1, FEF25-75). Within the CF group, there were no differences in age, height, weight, VO2peak or pulmonary function according to genetic variation of the ADRB2 at amino acid 27 (Table 3).

Within the healthy control subjects, there

Discussion

We found that DM is differentially affected by ADRB2 genotype at amino acid 27 in response to albuterol individuals with CF. Specifically, CF subjects with at least one Glu27 allele had a greater increase in DM/VC when compared to the Gln27Gln genotype following the administration of a β-agonist. Within the CF subjects, both genotype groups demonstrated a significant increase in oxygen saturation following albuterol inhalation; however, baseline and post-albuterol SaO2 was higher in the subject

Limitations

In the present study SaO2 was assessed by pulse oximetry which is not as sensitive as direct arterial blood draws to determine SaO2. However, a previous meta-analysis demonstrated a strong relationship between SaO2 measures using a pulse oximeter and direct blood gas analysis [46]. All subjects were of Caucasian or Hispanic ethnicity so the accuracy of pulse oximetry would not have been reduced due to darker skin color [47]. Also, this is a relatively small study for exploring the phenotypic

Conclusions

Our findings demonstrate that subjects from both CF genotype study groups benefit from the administration of albuterol as shown by increased in DM/VC, which translates into improved SaO2. Whereas in healthy subject there was no significant improvement in lung diffusion. Further, the CF group containing a Glu27 allele, had a significant increase in DM/VC after inhaled albuterol compared to their Gln27Gln counterparts, which did improve, but not to the same degree. These findings are important

Disclosures

The authors of this study have no conflicts of interest to disclose.

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